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Effects of diesel injection strategy on natural gas/diesel reactivity controlled compression ignition combustion
... RCCI also provides a balance between NOx and soot emissions through controlling ignition timing, heat release rate (HRR), PRR, and combustion duration [5,8,9]. Parameters including injection pressure [10,11], diesel fuel fraction [10,11], EGR rate [8], start of injection (SOI) [10,[12][13][14][15][16][17][18], premixed charge ratio [10,11,[13][14][15][16][17]19] as well as the dwell time between multiple injections [10,16,19,20] and mass split fraction of multiple injections [16,20] have been investigated in the past to achieve better control over combustion efficiency and ignition timing. ...
... RCCI also provides a balance between NOx and soot emissions through controlling ignition timing, heat release rate (HRR), PRR, and combustion duration [5,8,9]. Parameters including injection pressure [10,11], diesel fuel fraction [10,11], EGR rate [8], start of injection (SOI) [10,[12][13][14][15][16][17][18], premixed charge ratio [10,11,[13][14][15][16][17]19] as well as the dwell time between multiple injections [10,16,19,20] and mass split fraction of multiple injections [16,20] have been investigated in the past to achieve better control over combustion efficiency and ignition timing. ...
... RCCI also provides a balance between NOx and soot emissions through controlling ignition timing, heat release rate (HRR), PRR, and combustion duration [5,8,9]. Parameters including injection pressure [10,11], diesel fuel fraction [10,11], EGR rate [8], start of injection (SOI) [10,[12][13][14][15][16][17][18], premixed charge ratio [10,11,[13][14][15][16][17]19] as well as the dwell time between multiple injections [10,16,19,20] and mass split fraction of multiple injections [16,20] have been investigated in the past to achieve better control over combustion efficiency and ignition timing. ...
In this study, we investigate the effect of different split injection strategies on ignition delay time (IDT) and heat release rate (HRR) characteristics in Reactivity Controlled Compression Ignition conditions via large-eddy simulation and finite-rate chemistry. A diesel surrogate (n-dodecane) is injected into a domain with premixed methane and oxidiser in two separate injection pulses. Three different split injection strategies are investigated by fixing the amount of total fuel mass: varying the first injection timing, varying the second injection timing, and changing the fuel mass ratio between the two injections at a fixed injection timing. A compression heating mass source term approach is utilised to take compression heating into account. The main findings of the study are as follows: (1) In general, the IDT shifts towards the top-dead centre when the first injection is advanced or the second injection is retarded. The size and spatial pattern of the ignition kernels are shown to depend on the dwell time between the injections. (2) A precisely timed first injection offered the best control over ignition and HRR characteristics. However, advancing the first injection may lead to over-dilution downstream, preventing volumetric ignition and reducing the peak HRR value. (3) Approximately 21% decrease in the maximum HRR value, as well as a factor of 2.8 increase in combustion duration could be achieved by advancing the first injection timing. (4) As indicated by frozen-flow chemistry analysis, in the investigated configurations, the reactivity stratification is controlled by mixture stratification rather than temperature. The findings indicate that the first injection controls the downstream reactivity stratification, offering ignition and HRR control.
... Flow turbulence simulation k-ε model (Han and Reitz, 2007) Separation and pulverization of droplets Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) hybrid model (Beale and Reitz, 1999) Evaporation of droplets Dukowicz model (Dukowicz, 1979) Wall heat transfer Han and Reitz model (Han and Reitz, 1997) Nitrogen formation mechanism Zeldovich model (Maroteaux and Saad, 2015) -Chemistry method combustion model (Gholinia et al., 2018) RCCI combustion SAGE (Paykani et al., 2015) the injection start time and fuel injection angle, to approach the RCCI combustion phase. Based on this, the geometry of the dual-fuel heavy-duty diesel engine called the D87 (Jafari, 2013;Gholinia et al., 2018;Engine Test Report, 2007) has been used for experimental testing and numerical simulations (using CFD Converge commercial software). ...
... The advantage of the SAGE model is that it includes detailed chemistry for combustion applications. With the detailed chemical kinetics, the SAGE model can simulate combustion with a set of CHEMKIN formatted input files; CHEMKIN is the standard format for defining chemical mechanisms (Paykani et al., 2015). In the SAGE combustion pattern, the chemical response mechanism is based on a set of elementary reactions that describe the overall chemical response in detail. ...
About one-third of the energy entering the cylinder of an internal combustion engine is converted into useful work while the rest of the energy is wasted in various ways. Therefore, providing solutions that can recover part of the engine's wasted energy is crucially important. One of the newest techniques of interest in the field of internal combustion engines is low temperature combustion (LTC) methods. The aim of this research is to approach the reactivity control compression ignition (RCCI) combustion phase by changing the effective parameters (time and angle of injection) of a dual-fuel heavy diesel engine (diesel/ methane (CH 4)) To do this, numerical simulation (CONVERGE-CFD) and experimental test have been used. The results illustrate that by advancing the high reactive injection schedule (from-30 • to-50 •), the maximum cylinder temperature increases which occurs closer to the top dead center. This phenomena increases efficiency and output power, while consequently reduces Nitrogen oxides (NOx), Carbon monoxide (CO) and Hydrocarbon (HC) pollutants. For different fuel injection angles, the results show that at 62.5 • , the sprayed diesel fuel droplets are thoroughly mixed with the air inside the cylinder. Hence, the maximum amount of thermal energy released and the minimum amount of emissions occurs at angle of 62.5 • .
... For diesel-biogas (which involves 50% inert N 2 ) RCCI combustion, Wang et al., showed slower CA50 due to mixture homogenisation and reactivity. The findings greatly outperform those obtained by (Paykani et al., 2015) for 50% NG-diesel combustion. ...
... The result revealed that biodiesel's in-built fuel oxygen enhanced BTE while NOx emissions dropped. Paykani et al. 20 examined the effect of split injection techniques of a CH 4 /diesel dual fuel RCCI engine. It was discovered that the pre-injection and pilot mass are critical elements influencing the performance of an RCCI combustion. ...
The study intends to calibrate the compression ignition (CI) engine split injection parameters as efficiently. The goal of the study is to find the best split injection parameters for a dual-fuel engine that runs on 40% ammonia and 60% biodiesel at 80% load and a constant speed of 1500 rpm with the CRDi system. To optimize and forecast split injection settings, the RSM and an ANN model are created. Based on the experimental findings, the RSM optimization research recommends a per-injection timing of 54 °CA bTDC, a main injection angle of 19 °CA bTDC, and a pilot mass of 42%. As a result, in comparison to the unoptimized map, the split injection optimized calibration map increases BTE by 12.33% and decreases BSEC by 6.60%, and the optimized map reduces HC, CO, smoke, and EGT emissions by 15.68%, 21.40%, 18.82, and 17.24%, while increasing NOx emissions by 15.62%. RSM optimization with the most desirable level was selected for map development, and three trials were carried out to predict the calibrated map using ANN. According to the findings, the ANN predicted all responses with R > 0.99, demonstrating the real-time reproducibility of engine variables in contrast to the RSM responses. The experimental validation of the predicted data has an error range of 1.03–2.86%, which is acceptable.
... These engines have found wide applications in automotive transportation, small engine equipment, industrial machinery, and marine transportation. 13,16,18 For instance, RCCI is a type of combustion process in internal combustion (IC) engines that combines the principles of both gasoline spark ignition engines and diesel compression ignition engines. ...
The search for an effective solution to improve performance and emission characteristics of internal combustion (IC) engines used in the commercial sector is regarded as one of the most important and essential issues in recent years due to increasing levels of pollution. Nanoparticles with their additive ability to increase fuel reactivity and atomization, due to their large surface area and high heat transfer coefficient, can improve the performance and emission characteristics of a fuel. This review highlights the use of nanoparticles as fuel additives to enhance the emission and performance characteristics of IC engines. Detailed comparisons of performance, emission, and combustion characteristics of IC engines using fuels blended with nanoparticles have been done. Nanoparticles were observed to be an oxygen buffer for fuel combustion and boost fuel atomization, thus enhancing engine performance. While alumina exhibited a decrease in levels of HC and CO but a considerable increase in NOx, graphene nanoparticles and ceria were found to be particularly effective in enhancing engine performance. Detailed study has been done on other nanoparticles, including metal‐oxide, nonmetal‐oxide as well as carbon nanoparticles. Overall, the use of nanoparticles can enhance the thermophysical characteristics of fuels, improving the emission and performance characteristics of engines. The review suggests that selecting the right dosage and variety of nanoparticles is crucial for optimizing engine performance, and thus directly helps in tackling the ongoing pollution problem.
... Various gaseous fuels have been regarded as substitutes for petroleum-based fuels, including liquefied petroleum gas (LPG), natural gas (NG), liquefied natural gas (LNG), compressed natural gas (CNG), biogas, and syngas [16][17][18][19][20]. These fuels are suitable for higher compression diesel-fueled engines because of their excellent resistance against knock compared to conventional liquid fuels [21,22]. ...
... While CO and HC emissions rose under every operational circumstance. Natural gas was employed as a LRF in a single-cylinder engine by Paykani et al. [14]. The cylinder pressure, HRR, smoke and NOx emissions all decreased as the premixed ratio increased, but CO and UHC emissions rose. ...
The primary source of power in many situations, including backup power, emergencies, isolated locations, construction, etc., is an internal combustion engine. These have higher engine emissions, which is a major drawback. Low temperature combustion engines may prove to be the best option in this case, because they not only produce power with high efficiency but also produce fewer engine emissions. It was investigated how a reactivity-controlled compression ignition engine that runs on liquid petroleum gas performs and produces emissions. The pilot fuel i.e., diesel was directly injected during the compression stroke into the engine cylinder, whereas the main fuel, liquified petroleum gas, was injected during suction stroke into the inlet port via mechanical injection system. At the engine's inlet port, an electronic port injector was mounted. The engine's experimental testing was conducted at a fixed 1500 rotation per minute controlled with the help of governor. The maximum rated power output of the engine was 3.7 kW. The ratio of premixed energy is taken at 95%. Experiments were first carried out on a normal diesel combustion engine before being switched to the reactivity controlled compression ignition (RCCI) engine. The experimental results demonstrate the brake specific fuel consumption (BSFC) and brake power (BP) is reduced up to 83.14% and by 34.65% respectively. The rise in cooling water temperature is reduced by 15.38% and 5.88% at 0% and 100% loading conditions respectively. The exhaust gas temperature is reduced by up to 29.77%. Brake thermal efficiency increased by 19.17%. Smoke opacity is reduced by 81.29% and 69.81% at 0% and 100% loading condition respectively, as compared to the normal diesel combustion engine. According to the findings, a reactivity controlled compression engine may operate efficiently using liquified petroleum gas that contains approximately 95% premixed energy. As a result, there will be less demand for diesel fuel and engine emissions.
... Soot and NOx emissions were also reduced due to leaner equivalence ratios and lower adiabatic flame temperatures [21]. Paykani et al. [22] developed a 3-D CFD model. They varied the pilot and main SoI HRF to characterise the combustion and emissions from a heavyduty RCCI combustion engine fuelled by mineral diesel and natural gas at varying loads and speeds. ...
Amongst all low-temperature combustion (LTC) strategies, reactivity-controlled compression ignition (RCCI)
combustion garners more debate from researchers because of its excellent combustion and performance characteristics and lower emissions over a wide operating range. In this experimental investigation, the consequences
of the start of diesel injection (SoIdiesel) timing using different pilot injection techniques, such as single and
double pilot injection (SPI and DPI, respectively), have been investigated. Tests were performed at a fixed engine
speed and load (1500 rpm and 3 bar BMEP, respectively) using different premixed ratios (rp: 0, 0.25, 0.50, and
0.75) of methanol on an energy basis. Experimental results indicated that the start of combustion (SoC) and
combustion phasing (CP) advanced with advancing SoIdiesel. However, too advanced and retarded SoIdiesel timings resulted in combustion noise and misfire. The use of pilot injection was found suitable for the RCCI mode,
especially at lower rp. However, SPI and DPI strategies didn’t show significant variations in the combustion
characteristics. Exhaust gas temperature (EGT) showed a random trend with SoIdiesel timing, and it decreased
with the increasing number of pilot injections. RCCI mode produced higher hydrocarbon (HC) and carbon
monoxide (CO) and lower oxides of nitrogen (NOx) emissions. All these gaseous emissions decreased with
advancing the SoIdiesel. Particulate matter (PM) analysis showed that the RCCI mode emitted relatively lower PM
than the baseline CI mode, further reducing with increased methanol fuelling. PM emissions also decreased by
retarding SoIdiesel, in which reduction in smaller particles was dominant. The analysis of critical parameters such
as brake thermal efficiency (BTE), HC, NOx, and PM emissions suggested that the optimum rp of methanol and
suitable injection strategy can significantly improve the combustion, performance, and emissions characteristics
of the RCCI mode.
... In this context, the necessity of improving NO and smoke opacity emissions from diesel combustion and increasing fuel efficiency are among today's hot topics [3]. Many studies, such as biofuel use [4], piston modifications [5], various injection applications [6], low combustion temperature technology [7], HCCI [8], and RCCI [9], are carried out to improve emissions from diesel combustion. ...
In this study, the effect of the conversion of the mechanical fuel system of a single-cylinder, four-stroke, direct injection diesel engine to the common rail fuel system on the performance and emission characteristics was investigated. The experiments were first carried out on the original fuel system (mechanical fuel system) of the engine, at five different loads and constant engine speed. Then, the common rail fuel system was installed instead of the mechanical fuel system of the engine, and experiments were carried out under the same conditions. The performance and emission values of the mechanical and common rail fuel systems of a single-cylinder, air-cooled diesel engine were compared with each other. When the test results were examined, it was determined that the common rail fuel system improved the specific fuel consumption and thermal efficiency compared to the mechanical fuel system. It has been observed that HC, CO, and smoke emissions have decreased.
... For the simulation of fuel combustion process, SAGE combustion model based on detailed chemical reaction mechanism calculation is adopted in this study [19]. The fuel combustion process is simulated by directly solving the chemical reaction mechanism of fuel combustion with CHEMKIN format, CHEMKIN is a standard format for defining chemical mechanisms [20]. ...
With the increasing environmental and energy problems, the development of new combustion technology has become a hot topic in the research of internal combustion engine. RCCI combustion mode is the most promising combustion mode at present, which can solve the problems of combustion ignition controllability, and expand the engine load range. In this paper, optical diagnostics and numerical simulation are used to study the diesel / natural gas / hydrogen tri-fuel RCCI engine performance. Results show that hydrogen can significantly improve in-cylinder combustion and emission characteristics. With the increase of hydrogen addition, the combustion efficiency is increased accordingly, and the afterburning is prevented, which makes the exhaust energy loss decreased.
... Double diesel injection provides better control over combustion as the first injection influence the global flame propagation speed, and the second injection dictate the start of ignition [7]. Combustion phasing is mainly controlled by premixing ratio and slightly by diesel start of injection timing (SOI-2) and diesel split ratio [8]. The effects of control variables on efficiency, emissions, and cyclic variability of a CNG-diesel RCCI engine at various engine operating conditions are well documented in the literature, and a brief description is provided in a recent article by the present authors [9]. ...
This study investigates the changing deterministic features of the cycle-resolved location of maximum pressure and its correlation with combustion phasing for dynamical transitions in the combustion of a CNG-Diesel RCCI engine. The investigation is performed using nonlinear dynamical and chaotic methods such as return maps, recurrence, and cross recurrence plots. The experiments are performed on a single-cylinder automotive engine operated in RCCI mode with the aid of the development ECU. The experiments are conducted by running the engine at a fixed engine speed of 1500 rpm and a load of 3 bar BMEP. Diesel fuel is injected directly into the cylinder by following a double injection strategy using an equal amount of fuel in both the pilot and main injections. The effect of the start of main injection timing of diesel on combustion dynamics is investigated at two different masses of CNG. The predominance of deterministic periodic features is discovered in the cycle-resolved dynamics of the engine combustion during the RCCI regime. Results show that with advancement in diesel injection timing, the mode of combustion shifts from conventional dual fuel to RCCI, and this shift is coupled to the onset of noisy periodic-2 bifurcations., The periodic-2 behavior even transforms to periodic-3 and 4 with an increasing advancement in diesel injection timing for engines operating with a higher CNG mass. For most of the operating conditions, the deterministic features in the location of maximum pressure are comparable with that of combustion phasing. Recurrence and cross recurrence plots-based methodology advocates for the existence of strong correlations or at least a phase synchronization between the location of maximum pressure and combustion phasing when the engine operates in the RCCI regime, irrespective of diesel injection timing and amount of port-injected CNG fuel. The presence of similar deterministic features in the location of maximum pressure and combustion phasing and a strong relationship between these two at intermediate diesel injection timings in the RCCI regime for both the CNG masses makes this regime most suitable for using the location of maximum pressure as a feedback/controlled parameter for model predictive control of the engine.
... The significantly large proven reserves of natural gas (200 trillion cubic meters worldwide) combined with the capability to operate a cleaner and more efficient cycle make it an attractive alternative fuel [5]. The use of natural gas as a standalone fuel, or as a blend with diesel or hydrogen, has been extensively studied in traditional spark ignition engines [6][7][8][9] and more advanced cycles such as homogeneous charge compression ignition (HCCI) [10][11][12][13][14] and reactivity controlled compression ignition (RCCI) [15][16][17][18]. ...
Natural gas is known as a relatively clean fossil fuel due to its low carbon to hydrogen ratio compared to other transportation fuels, which yields a reduction of carbon monoxide, carbon dioxide, and unburned hydrocarbons emissions. However, it has a low cetane number, which makes it a difficult fuel for use in compression ignition engines. A potential solution for this issue can be adding small amounts of argon, as a noble gas with a low specific heat to modify the intake conditions. In this numerical study, a commercial compression ignition engine has been modeled to evaluate the auto-ignition of natural gas with the modified intake conditions. Different amounts of argon added to the intake air are examined in order to attain the optimal operating conditions. A detailed chemistry solver is implemented on a 53-species chemical kinetics mechanism to calculate the rate constants. The results show that compression ignition of natural gas can be achieved by adding small amounts of argon to the intake air. It drastically increases the in-cylinder temperature and pressure near TDC, which enables the auto-ignition of the injected natural gas. Moreover, it leads to the reduction in ignition delay and heat release rate, and expands the combustion duration. Emissions analysis indicates that NOx and CO2 can be significantly diminished by increasing the amount of argon in the intake composition. This study introduces an efficient and clean compression ignition engine fueled with natural gas running in optimal operating conditions using argon addition to the intake.
... The RCCI combustion mode showed the tremendous potential of using alternative fuels in the last few years. In RCCI combustion mode, CNG [257][258][259][260], syngas [261], alcohols including methanol [122,250,262,263], ethanol [264,265], and butanol [177,266,267] have been injected into the port as LRF. These LRFs helped in extending the load range of engine operation due to their higher resistance to auto-ignition (less knocking), higher reactivity gradient (optimum combustion phasing), and charge-cooling effect (due to their higher latent heat of vaporization). ...
Several advanced low-temperature combustion (LTC) strategies have been developed to reduce the harmful emissions from diesel engines. These LTC strategies, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and reactivity-controlled compression ignition (RCCI), can reduce engine-out nitrogen oxides (NOx) and soot emissions simultaneously. LTC investigations exhibit several limitations of HCCI and PCCI combustion modes, such as lack of combustion control and other operational issues at higher engine loads, making their application in production-grade engines challenging. RCCI combustion mode exhibited promising results in combustion control, engine performance, and applicability at higher engine loads. The potential of the RCCI concept was demonstrated on different engine platforms, showing engine-out NOx levels below the limits proposed by the emissions regulations, together with ultra-low soot emissions, eliminating the need of after-treatment devices. However, the RCCI combustion mode has several challenges, such as excessive hydrocarbons (HC) and carbon monoxide (CO) emissions at low loads and excessive maximum pressure rise rate (MPRR) at high loads, which limit its effective operating range and practical applications. This review article includes recent advancements in RCCI combustion mode, its potential for using alternative fuels, the effects of different parameters on RCCI combustion mode and its optimization, and the ability of RCCI combustion mode to extend the engine operating limit to reach higher loads, which prevents the application of this concept in commercial applications. The findings of different optical diagnostics have also been included, which have been performed to understand the detailed chemical kinetics of the fuel-air mixtures and the effect of fuel reactivities on the RCCI combustion mode. The first part of this article focuses on these studies, which provide important outcomes that can be used for the practical implementation of RCCI combustion mode in production-grade engines. The second part of this article covers different RCCI combustion mode strategies that can be used to eliminate the restrictions of RCCI combustion mode at high loads. Among the different techniques, dual-mode concepts have been extensively investigated. The dual-mode concept is based on switching between two different combustion modes, typically an LTC mode and conventional compression ignition (CI) combustion mode, to cover the entire operational range of the engine. Many studies showed that the NOx and soot emissions from stationary engines with dual-mode RCCI/CI combustion had substantially improved versus a single-fueled CI combustion mode engine. Results related to the measurements of emissions and performance in transient conditions and driving cycles have also been included, which exhibit promising results for RCCI combustion mode. A comprehensive review on overcoming the challenges and real-world applicability of RCCI combustion mode is not available in the open literature yet. This article includes the results of relevant RCCI combustion mode investigations carried out in single-cylinder and multi-cylinder engines, intending to fill this research gap. Finally, the results from alternative RCCI combustion mode concepts such as the dual-mode, hybrid-RCCI, simulations, and experiments in transient conditions using various driving cycles make this article uniquely relevant for researchers.
... The diesel engines are the main power sources of heavy vehicles (Cunanan et al., 2021;An et al., 2015), due to their higher thermal efficiency, stronger power performance, lower fuel consumption compared to the gasoline engines (Benajes et al., 2015). Compared with the gasoline engines under the same power, the hydrocarbons (HCs) and carbon monoxide (CO) emissions of diesel engine are relative smaller (Benajes et al., 2014) but the particulate matter (PM) emissions are 30-80 times higher in the diesel engines (Paykani et al., 2015), which lead to the increase of the PM2.5 concentration (Hotta et al., 2005). ...
In this paper, a three-dimensional mathematical model of the rotary diesel particulate filter (RDPF) for truck diesel engine is established according to the fluid mechanics and porous media theory. The effects of inlet velocity and structure parameters (diameter ratio, expansion angle and filter length) on the flow uniformity in the RDPF are investigated. Furthermore, the Fuzzy grey relational analysis (FGRA) is employed to make a weight analysis of the influences of structure parameters on the regeneration performance and pressure drop of the RDPF. The results show that the velocity uniformity in the RDPF can be improved by properly reducing the inlet velocity, diameter ratio or expansion angle θ1. The capture-regeneration volume ratio with 8–10 is appropriate range for the structural optimization. Finally, the expansion angle θ1 is the most important structure parameter for the filter regeneration performance (regeneration time R = 0.8467; regeneration efficiency R = 0.6849) and the diameter ratio is the most important structure parameter for the pressure drop at the capture-regeneration “balance point” (R = 0.9352).
... From an ITE perspective, the combustion efficiency of CI engines is superior to that of dual-fuel PCI combustion because low-reactivity fuel decreases ignitability. Thus, dual-fuel combustion cannot promise a higher ITE than CI engines (Paykani et al., 2015;Yang et al., 2016). However, the flame of dual-fuel PCI combustion consists of both flame propagation and a diffusive flame, which are the main characteristics of spark ignition (SI) and compression ignition (CI) engines, respectively (Lee, 2021). ...
In-cylinder flow motion is important for enhancing combustion in internal combustion engines. There are two major flow motions: tumble and swirl. Tumble enhances the flame propagation speed to spread throughout the entire cylinder. Swirl affects the behavior of diesel spray; an enhanced swirl ratio has been widely used in conventional diesel engines. Because dual-fueled combustion has the characteristics of premixed combustion from background fuels and mixing-controlled combustion from directly injected fuels such as diesel, understanding the effect of in-cylinder flow motion on combustion characteristics in dual-fuel combustion is critical. In this research, the effect of swirl in gasoline and diesel dual-fuel combustion was evaluated with different diesel injection timing conditions. As the dual-fuel combustion engine used in this research was driven by diesel spray, swirl was selected as the main flow motion rather than tumble. The effect of swirl with different diesel injection timings was investigated under low-speed and low-load conditions. The results demonstrate that increasing swirl intensity provokes fast-burning caused by the enhanced air-fuel mixture, decreasing the diesel fraction and allowing more exhaust gas recirculation (EGR) to be used for the reduction of engine-out nitrogen oxides (NOx) and smoke emissions, without reducing thermal efficiency.
... RCCI is a fuel-flexible combustion strategy. Apart from conventional diesel and gasoline fuels [10][11][12][13][14][15], gaseous fuel such as CNG in place of gasoline has also been investigated by several researchers [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Due to CNG's lower carbon to hydrogen ratio, its combustion results in lower unburned hydrocarbons and CO emissions [27]. ...
This study uses nonlinear dynamical and chaotic methods to investigate the cyclic combustion dynamics of a CNG-Diesel reactivity control compression ignition (RCCI) engine. The experiments of RCCI combustion mode are conducted on a single-cylinder automotive diesel engine with the development ECU. Low reactivity fuel (i.e., CNG) is injected into the intake manifold, and high reactivity fuel (i.e., diesel) is injected directly into the engine cylinder during the RCCI experiments. Mass of fuel injection per cycle and their injection events are controlled by using ECU. The engine is tested for fixed engine load and speed of 3 bar BMEP and 1500 rpm, respectively. A double diesel injection strategy is used for injecting the diesel fuel into the engine cylinder. In-cylinder pressure is measured using a piezoelectric pressure transducer on a crank angle basis with a resolution of 0.1 CAD. Combustion parameters are calculated on a cyclic basis from measured in-cylinder pressure. In this study, the effect of diesel injection timing (SOI-2) on the behavior of combustion dynamics for two different masses of port-injected CNG fuel (mc) is investigated. The CA50 time series of 1000 consecutive engine cycles is analyzed for combustion dynamics analysis using symbol sequence analysis, return maps, recurrence plots (RPs), recurrence quantitative analysis (RQA), and 0–1 test. The peaks in symbol sequence histograms and the corresponding symbol series depict the period-2 nature of the cyclic combustion dynamics at the intermediate SOI-2 of 30° and 40° bTDC. Return maps also confirm the onset of noisy period-2 bifurcation at 30° bTDC SOI-2. The dominance of deterministic period-2 characteristics is found in the cyclic combustion dynamics at 30° and 40° bTDC SOI-2 using recurrence plots. The 0–1 test method depicts stronger chaotic cyclic combustion dynamics at 20° and 50° bTDC SOI-2. In summary, the present study supports the existence of a periodic window in between the chaotic combustion for the intermediate values of SOI-2. The dominance of the deterministic characteristics makes this regime of intermediate SOI-2 values suitable for a better cycle resolved control.
... In this regard, advanced modes of combustion, particularly low temperature combustion (LTC) have attracted the attention of researchers more. 1 Various LTC techniques, including homogeneous charge compression ignition (HCCI), 2 premixed charge compression ignition (PCCI), 3 and reactivity controlled compression ignition (RCCI) 4 have been employed to achieve sufficient premixing of air and fuel in order to reduce combustion temperature. It has been realized that injection strategy 5 and injection parameters such as the injection quantity, 6 start of injection (SOI) timing, 7 injection pressure, 8 spray angle, 9 and injection rate shape have significant impact on the performance of internal combustion engines. Since combustion process is different in each mode of combustion, one parameter has different effects on the performance and exhaust emissions of each mode. ...
Injection rate shape has a great influence on the spray evolution, and consequently on the performance and emission characteristics of compression ignition engines. In this study, effect of different ramp injection rate shapes on the performance and exhaust emissions of a natural gas/diesel reactivity controlled compression ignition (RCCI) engine was investigated. 64 numerical experiments were performed to study effect of two characteristic parameters of the ramp injection rate shape, including ramp duration and ramp injection rate on the engine gross indicated efficiency (GIE) and emissions formation. It was realized that for a constant value of the ramp duration, the engine gross indicated efficiency and [Formula: see text] emissions are lower, and CO and unburned hydrocarbons (UHC) emissions are higher for medium values of the ramp injection rate. Moreover, for a constant value of the ramp injection rate, the engine gross indicated efficiency and [Formula: see text] emissions are higher, and CO and unburned hydrocarbons emissions are lower for medium values of the ramp duration. The optimum values of the ramp duration and ramp injection rate were determined and it was revealed that the optimum ramp injection rate shape can improve the engine gross indicated efficiency by 54.78%.
... Wang et al. [20] observe slowed CA50 because of the atomization of direct-injected fuel and mixture reactivity for dieselbiogas (having 50% inert N 2 ) RCCI combustion. The results for this work are far above those observed by [70] for NG-diesel combustion at a 50% fraction. Fig. 7a further indicates that an increased mixing distance speed up the combustion for 35 and 45% CO 2 ratios, which might be due to improved atomization and mixture reactivity [20]. ...
Reactivity-controlled compression ignition (RCCI) combustion burns fuels of varying reaction rates to enhance combustion stratification. Biogas-diesel RCCI combustion requires energy assessment to improve power and reduce emissions. This study investigates the effects of different carbon dioxide (CO2) contents (25, 35, and 45%) and port mixing distances (0, 28.75, 57.5, 86.25, and 115 mm) experimentally, at 6.5 bar IMEP and 1600 rpm in a premixed and port injection at the valve. An exergy analysis facilitates understanding high-CO2 biogas stratification and the cause of emissions trade-off experimentally, unavailable in the literature. Based on energy analysis, a 35% CO2 content demonstrates moderate combustion work and enhanced output power, while 45% CO2 shows lower heat loss across the mixing distances. Injecting biogas at the valve increases combustion work by 9.11 – 27.33% and reduces power output by 0.33 – 4.90% due to increased exhaust loss. Based on exergy analysis, evenly distributed in-cylinder temperatures and sufficient fuel stratification were observed for 35% CO2 and 50% distance. The 35% CO2 increases the internal energy potential recovery by 24.26% over mixing spaces because of reduced destruction. Injection at the valve causes stratified CO2 layers at the bowl centerline and diffused temperature at the cylinder wall, simultaneously decreasing CO2, carbon monoxide, unburned hydrocarbon, nitrogen oxides, and particulate matter emissions by 4.46, 2.04, 3.05, 44.67, and 9.13%, respectively. Therefore, increased compression work allows more heat transfer through the exhaust, while heat outflow through the cylinder wall reduces combustion work. Furthermore, the reduced emissions due to increased CO2 content at higher mixing distances compromised the output power of the biogas-diesel RCCI engine.
... Currently, some new combustion modes (PPCI (partially premixed compression ignition) [3,4], RCCI (reactivity controlled compression ignition) [5][6][7], HCCI (homogeneous charge compression ignition) [8,9], etc.) have been proposed and studied to solve the particulate emissions. However, the diesel particulate filter (DPF) is considered to be one of the most effective and simplest methods for reducing particulate emissions [10,11] and its regeneration technology [12,13] is the key process for actual application of the DPF. ...
... Compression ignition-controlled reactivity is more promising than homogeneous charge CI with lower temperature combustion techniques [16]. The reactivity can be varied by adjusting the ratio of high CN and ON of the fuel in RCCI combustion [17]. NO x emissions were minimized by the introduction of cotton biodiesel in the RCCI ignition combustion. ...
Heterogeneous combustion in a diesel engine is noisier, uncontrolled and more polluting. This can be achieved with a strategic approach of a reactivity-controlled compression ignition (RCCI) mode engine that operates with low and high reactive fuel combinations. In the present work, a diesel engine is operated in RCCI mode with gaseous fuels viz. CNG as a primary fuel and a blend of diesel and Karanja biodiesel (BD20) as pilot fuel. This research aims to determine the operating limits of CNG fuel for less noisy combustion and clean exhaust.
Further, relative air-fuel ratio (λ), cycle to cycle variations, combustion noise and emissions were studied for full load operation. The CRDI engine is optimized for diesel operation with a split injection strategy. The knock limits for CNG as the primary fuel are obtained. The combustion noise increases at a higher energy share by CNG. Also, higher values of HC and CO emissions are observed. This may be due to higher energy share values, flame speed and octane number of CNG fuel.
Further, NOx emissions and smoke are decreased. The CNG induction of 10 ms with 90% ES can be noted as a knock limit for 3.5 kW power. The highest BTHE of 24.2% and least BSFC 0.3 kg/kWhr reported by 60%ES of LRF is better than diesel and KBD20 fuel. An optimum 60% energy share of CNG is observed for clean combustion and emissions strategy using the RCCI mode of a modified diesel engine.
For computational fluid dynamic (CFD) modeling of advanced combustion engines, the cylinder is usually considered a closed system in which the initial conditions are estimated based on the experimental data. Most of these approximations hinder observing the effect of design parameters on engine performance and emissions accurately, and most studies are limited to a few design parameters. An approach is proposed based on the combination of a 1D gas dynamic and a 3D CFD model to simulate the whole engine with as few simplifications as possible. The impact of changing the in-cylinder initial conditions, injection strategy (dual direct injection or multiple pulse injections), and piston bowl geometry on a reactivity controlled compression ignition (RCCI) engine’s performance, emissions, and fuel stratification levels was investigated. It was found that applying the dual direct injection (DDI) strategy to the engine can be promising to reach higher load operations by reducing the pressure rise rate and causing stronger stratification levels. Increasing the number of injection pulses leads to lower Soot/NOx emissions. The best reduction in the pressure rise rate was found by the dual direct strategy (38.36% compared to the base experimental case) and higher exhaust gas recirculation (EGR) levels (41.83% reduction in comparison with the base experimental case). With the help of a novel piston bowl design, HC and CO emissions were reduced significantly. This resulted in a reduction of 54.58% in HC emissions and 80.22% in CO emissions.
div class="section abstract"> A reactivity-controlled compression ignition (RCCI) engine offers ultralow soot and nitrogen oxide (NOx) emission in addition to higher thermal efficiency than diesel or compression ignition (CI) engines. However, the higher emissions of unburned hydrocarbons (HC) and carbon monoxide (CO) from RCCI engines pose a significant challenge that hinders their adoption in the future automotive sector. Additionally, HC includes several hydrocarbons that harm human health and the environment. This study aims to minimize HC and CO formation and emissions by implementing different injection strategies, including adjustments to spray angle configuration, injection timing, and fuel premixing ratio. Additionally, the study examines how different injection strategies affect the spatial and temporal distribution of HC and CO inside the combustion chamber. To achieve this objective, a numerical investigation is conducted on a single-cylinder diesel engine modified to operate in RCCI mode, utilizing a detailed reaction mechanism with ANSYS FORTE. The reaction mechanism comprises 137 species and 1,022 reactions, using n-heptane and CH4 as fuel surrogates. Initially, the computational model is developed using engine geometry and validated against experimental results for conventional diesel and RCCI modes, after which a parametric investigation is conducted. The results demonstrate that, among injection strategies, the spray configuration has the greatest impact on HC and CO emissions. Narrow spray configuration in RCCI combustion leads to a significant decrease in HC and CO emissions. HC and CO emissions increase with advanced injection timing and a higher fuel premixing ratio. RCCI engines exhibit lower acidification potential and eutrophication potential equivalent emissions compared to conventional diesel engines.
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The dual-fuel combustion strategy using compressed natural gas (CNG) and diesel exhibits immense potential for improving conventional diesel engines. To reduce CO2 emissions, diesel should be substituted with CNG to the maximum extent possible. In this study, a numerical investigation was conducted using a three-dimensional computational model for cases with 97 % CNG substitution (CNG97), which has not been explored through experiments previously owing to the technical limitations of diesel injection systems. The simulation model was validated through experiments conducted under operating conditions similar to a diesel engine; the results revealed that ignition was initiated by the diesel fuel regardless of the CNG substitution rate. In the CNG97 case, ignition occurred near the center of the cylinder owing to short spray penetration. Consequently, the CNG remained inside the piston bowl and top crevice area. In addition, the small quantity of diesel weakened the initial stage of combustion. These two effects retarded combustion significantly, which caused substantial phasing loss in CNG97. Based on these findings, it was concluded that the diesel injector should be optimized for a high CNG substitution rate to reduce CO2 emissions effectively.
An internal combustion engine is the prime source of transportation throughout the world like trucks, buses, passenger cars, three-wheeler and two-wheelers. Due to the use of hydrocarbon fuels these IC engines produce lot of engine emission which ultimately create problems to the humans, living organisms and to the environment. The low temperature combustion strategyis likely to help in overcoming this problem and this can be achieved majorly through homogenously charged compression ignition, premixed charged compression ignition and reactivity-controlled compression ignition.The low temperature combustion engine is very effective and has special ability that can be used in place of conventionalinternal combustion engine. The low temperature combustion engine has more than 50% thermal efficiency,reduce fuel consumption and reduce engine emissions level below EROU VI.However, low temperature combustion engines are suffered from some disadvantages such as low control over combustion, low load capacity, cycle-to-cycle variation in combustion, etc. Among all variants of low temperature combustion engine, reactivity-controlled compression ignition has an upper hand due to its number of advantages over others. The reactivity-controlled compression ignition can operate at different loads, use alternate fuels effectively, produce lower emissions, have lower cycle to cycle variations etc. However more research is required to make them commercially viable.
The depletion of fossil fuel and the concerns for harmful emissions and global warming has instigated researchers to use alternative fuels. Hydrogen (H2) and natural gas (NG) are attractive fuels for internal combustion engines. The dual-fuel combustion strategy is promising to reduce emissions with efficient engine operation. The concern for using NG in this strategy is the lower efficiency at low load conditions and the emission of exhaust gases like carbon monoxide and unburnt hydrocarbon. Mixing fuel with a wide flammability limit and a faster burning rate with NG is an effective method to compensate for the limitations of using NG alone. Hydrogen (H2) is the best fuel added with NG to cover NG limitations. This study investigates the in-cylinder combustion phenomenon of reactivity-controlled compression ignition (RCCI) engines using hydrogen-added NG as a low-reactive fuel (H2 addition to NG on a 5% energy basis) and diesel as a highly reactive fuel. The numerical study was done on a 2.44 L heavy-duty engine using CONVERGE CFD code. Three low, mid, and high load conditions were analyzed in six stages by varying the diesel injection timing from -11 to -21 O after top dead centre (ATDC). The H2 addition to NG had shown deficient harmful emissions generation like carbon monoxide (CO) and unburnt hydrocarbon with marginal NOx generation. At low load conditions, the maximum imep was achieved at the advanced injection timing of -21OATDC, but with the increase in load, the optimum timing was retarded. The diesel injection timing varied the optimum performance of the engine for these three load conditions.
Axial flow compressors/fans are widely used machines in industrial as well as aircraft gas turbine engines. The performance improvement of such machines has improved significantly due to key efforts of researchers and engineers. The hard work and research inputs require for the development of a single compressor which includes years of careful study and investigations. Rotor is a primary element of any axial compressor/fan which imparts kinetic energy to fluid, and so, the designers have emphasized more on the rotor design. Present study incorporates an effort for the development of a low-speed axial flow compressor/fan rotor blade based on NACA 65-series airfoil. The limiting design parameters are selected from the grounded axial fan test rig available in the department lab. The study initiates with one-dimensional flow parameters calculation based on velocity triangle at mean location and is extended for total 11-radial locations from hub to tip. The blade angles are obtained from correlations available in literatures, and the corrected angles are used to set up selected airfoil profile along spanwise locations using commercial modeling tool. Once the airfoils are stacked on one another, a solid blade model is developed. This is further extended to develop entire rotor with calculated number of blades. The data obtained from the present study are validated with the available literature plots and are in good trend. The obtained blade has high pressure variation and total pressure loss close to tip. The blade has higher camber close to hub and lower camber at higher spanwise positions.KeywordsAxial flow compressorSubsonicRotor bladeNACA 65-airfoilBlade plots
The necessity for reducing pollution due to increased population and its adverse effects on health and the environment has called for worldwide government policies. Researchers and engineers from automobile communities have been working on the advanced design and development of a diesel engine to meet these mandatory emission regulations. The split injection among these new developments has proven to be the key to reduce soot and NOx emissions in diesel engines. The split injection system is augmented with various low-temperature combustion (LTC) modes like homogeneous charge compression ignition engine (HCCI), premixed charge compression ignition engine (PCCI), and reactivity controlled compression ignition engine (RCCI) to counter the drawback of the traditional approach. This review paper provides a compilation of these recent innovations. A comprehensive overview of abundant research carried out for different split injection schemes in these LTC modes has been presented in the current research work. All these modern strategies and advancements in the split injection system are investigated and compared to expose and evaluate their strengths and weaknesses to arrive at a prominent solution.KeywordsCI engineSplit injectionPerformanceEmission controlLow-temperature combustion
Natural Gas (NG) with its high octane number and low C/H ratio is beneficial for reactivity controlled compression ignition (RCCI) engines in controlling the rate of pressure rise and lower the emissions, given that these engines still suffer from these two problems. Therefore, in this study, a detailed 3D combustion model was developed in ANSYS-Forte environment on the scanned image of a real single-cylinder diesel engine for the application of NG to RCCI engines and then validated with the experimental data of the same engine. Afterward, the effect of NG premixed ratio (Rp, 15%, 30% and 45%) on the combustion and emission characteristics of the RCCI engine were analyzed at 20% load and 2400 rpm. Results showed that by increasing the Rp, the heat release rate and cylinder pressure decreased while the ignition delay (ID) period increased. Moreover, NOx and HC emissions were observed to be increased whereas CO and C2H2 emissions tended to be declined as Rp increased. CO emissions were decreased up to 29% when RCCI mode was applied. Also, C2H2 emissions were reduced by about 97% by RCCI compared to CDM. However, engine efficiency started to decrease as Rp increased.
The research objectives of pilot diesel injection (PDI) ignition natural gas technology include high efficiency, clean combustion, and low pilot diesel mass. This study is based on a single-cylinder thermodynamic engine, combined with the CONVERGE simulation model and CHEMKIN chemical reaction kinetics model. The effects and mechanisms of various PDI strategies on the mixture equivalent ratio, temperature, and characteristics of combustion and emissions were investigated. The experimental results showed that the best PDI mass was 8 mg/cycle. The thermal atmosphere and activity in the cylinder were improved with an increase in PDI mass from 2 to 8 mg/cycle, which stabilized the mixture combustion. Further, the effects of different pilot injection timing (PIT) on combustion and emissions were investigated via experiments and simulation by controlling the operating conditions and maintaining a constant PDI total mass. The results show that the diesel had a single low-temperature reaction path when the PIT was close to the top dead center, whereas the PIT at the early stage of the compression stroke (CS) changed the chemical reaction path and accelerated the transformation of CH 3 to CH 2 O, accumulating numerous active groups and accelerating the combustion rate, which is difficult to control the ignition phase. The reaction path of the double PDI strategy was similar to that of the PIT at the early CS stage, and its combustion is closed to premixed combustion; however, the accumulation of active groups was relatively small, and the combustion rate was relatively slow because the ignition phase was controlled by the second PDI, making the combustion phase easy to control. Finally, with the double PDI strategy that had the advantages of efficient combustion and avoidance of knock, the gross indicated thermal efficiency reached 49.3% that involved a −60°crank angle (CA) after top dead center (ATDC) first injection and −4°CA ATDC second injection.
Natural gas/diesel RCCI mode is considered as a high-efficiency clean low-temperature combustion strategy with great application prospects. In order to solve the problem that RCCI engines are prone to knock combustion under high load, the effects of diesel start of main-injection (SOImain), start of pilot-injection (SOIpilot) and pilot-injection quantity (PIQ) on in-cylinder combustion and pollutant emissions of RCCI engine were systematically investigated. Results show that the advance of SOImain effectively reduces CO, HC, soot and CH4 emissions of RCCI engine, but increases knock tendency and NOx emissions. Split injection strategy makes combustion exothermic of RCCI mode present two-stage heat release phenomenon. When SOIpilot is advanced from −15°CA ATDC to −27°CA ATDC, the IMEP and combustion pressure peak increase, the knock tendency significantly decreases, and CO, HC, soot and CH4 emissions are reduced by 55.6%, 43.1%, 28.8% and 39.1% respectively. With the increase of PIQ, the in-cylinder combustion of RCCI engine gradually changes from single-stage to two-stage heat release, the ignition delay is shortened, and the knock tendency is enhanced. Compared with single injection strategy, split injection strategy can simultaneously reduce HC, CO and greenhouse gas emissions, as well as maximum pressure oscillation amplitude and pressure rise rate in RCCI mode.
This paper tries to approach the combustion phase of RCCI by changing the input parameters of the engine, a dual-fuel methane/natural gas engine, to compare the performance of the engine under the influence of modifications in these factors. The effect of the most important factors on the combustion of the RCCI engine including the start time of single-stage injection, two-stage injection schedule, compression ratio, the amount of injection with high reactivity and the angle of injection, numerically (CONVERGE-CFD) and Laboratory test are analyzed. The results show that CO, NOX and HC pollutants increase with a delay in the onset of single-stage injection. The results for different two-stage injection schedules show that by doubling the injection time of the second stage, the temperature inside the chamber and the rate of heat released decrease. But the data for changing the compression ratio from 11.5 to 19 indicate that the temperature of the mixture at the end of the compression phase has increased by more than 700 K, which will be directly related to the ignition start time and the combustion duration. In addition, increasing the compression ratio reduces the production of CO and HC pollutants. For smaller amounts of high reactive fuels, the maximum temperature moves to areas farther away from the top dead center, increasing CO and HC emissions. Also, using a larger injection angle (from 55° to 70°) will reduce CO, NOX and HC pollutants because more diesel fuel is trapped in the gaps and squish regions.
One of the novel methodologies used to increase energy efficiency and reduction of environmental pollutants in internal combustion engines is the idea of low-temperature combustion (LTC) especially reactivity controlled compression ignition (RCCI). Given that the ultimate goal of RCCI combustion is combustion controllability through in-cylinder reactivity stratification by using two different fuels, there are many modifiable factors, which can be improved or adjusted. The aim of this research is to use the concept of RCCI combustion in a biodiesel (C11H22O2) / natural gas heavy-duty engine and the performance and amount of pollutants of engine output is evaluated and compared by modifying the input parameters (different fuel injection strategy). Accordingly, numerical simulations have been carried out to study combustion in the geometry of the Caterpillar 3401E engine with CONVERGE computational fluid dynamic software and the SAGE combustion model. Results show that: By changing the biodiesel injection time (from -40° to -60°), although, the rate of heat released decreases, output work and Indicated mean effective pressure (IMEP) increase. By increasing the lag of time between the first and second injections (in Cases 1-3), the IMEP will increase from 6.9 to 8.2 bar and the work from 1686 to 1997 J. In Cases (7-9), the mass of HC and CO pollutants has drastically decreased with the onset of injection earlier, whereas the mass of NOx pollutants has increased.
For dual-fuel engines, the mixture of dual fuels results in a complex combustion reaction in the combustion chamber compared with diesel engines, and an excessively high piston thermal load may be caused by the complex combustion reaction. Since temperature distribution on the piston crown surface is an important indicator reflecting the piston thermal load, a temperature prediction methodology based on a hybrid neural network is proposed in this study to evaluate whether the piston thermal load meets the requirement of design guidelines. Once the hybrid neural network is learned through datasets generated from a real-data validated dual-fuel engine computational fluid dynamics (CFD) model, temperature distribution under different operating conditions can be rapidly and accurately predicted. In the case study, the results of multiple metric indicators reveal that the predicted and CFD temperature distributions are in good agreement, illustrating that the hybrid neural network possesses a promising generalization performance. Moreover, the comparative analysis indicates that the temperature prediction performance of the hybrid neural network is better than that of two recently published neural networks at almost all operating conditions, especially for the operating conditions with a relatively large in-cylinder temperature gradient, which demonstrates the superiority of the hybrid neural network in predicting temperature distribution on the piston crown surface.
Among the measures that improve the combustion process and reduce emissions in the design of dual-fuel engines is the use of the RCCI combustion strategy. This paper attempts to investigate the effects of biodiesel and diesel fuels, fuel injection time, and fuel injection rate in the RCCI combustion mode. In this study, using CONVERGE CFD commercial software and SAGE combustion model for the simulation. Accordingly, the geometry of a biodiesel (C11H22O2) / natural gas dual-fuel engine (Caterpillar 3401E) was exploited for experimental tests and numerical simulations. Results show that: The pressure inside the combustion chamber is especially greater from -12° to +10° crank angle for biodiesel, in fact, the maximum pressure for biodiesel fuel is approximately equal to 9.12 MPa and for diesel fuel is equal to 8.9 MPa. The mass of HC emissions for the two fuels is almost equal, but the mass of CO emissions for biodiesel is higher than diesel. By reducing the duration of fuel injection from 16° to 8° crank angle, the mass of soot is facing a decreasing trend, and this amount lowers from 0.065 to 0.00086 mg. Moreover, the indicated mean effective pressure (IMEP) and production work have increased from Case A (8 and 16 mg biodiesel in the first and second injection) to C (16 and 8 mg biodiesel in the first and second injection), and the knocking rate is reached from 3.4 to 6.6.
Low temperature combustion concepts in internal combustion engines have arisen as one of the solutions that reduce NOx and particulate emissions without a fuel consumption penalty. Among the combustion concepts, reactivity controlled compression ignition (RCCI) has the potential of being able to be achieved in a wider operating range than other low temperature concepts. On the other hand, exhaust gas recirculation (EGR) use in diesel-compression ignition (CI) engines is an effective way to reduce NOx emission. This study experimentally investigated the high-EGR CI fueled with diesel and gasoline/diesel fueled RCCI combustion modes in terms of combustion, performance, and emission at fixed energy inputs corresponding to low load conditions for different engine speeds. In RCCI combustion mode, different premixed ratios were also tested to determine their effects. The results show that CA50 locations in all high-EGR CI combustion occurred later than those of RCCI modes, which led to late peak pressure occurrence. By considering heat release rate curve characteristics, the controlled combustion phase emerged at the highest premixed ratio in RCCI as in the high-EGR CI combustion. RCCI combustion showed an improvement in smoke emissions, and the maximum improvement was attained by about 86%. In RCCI combustion, nitrogen oxide (NO) emissions at 2000 rpm engine speeds are significantly lower than that in high-EGR CI whereas the increasing premixed ratio at 1500 and 1750 rpm led NO emission to increase. Besides, RCCI combustion with the highest premixed ratios resulted in slightly higher brake thermal efficiencies (BTE). The maximum difference in BTE values was obtained by about 3.0% in favor of RCCI combustion.
Reactivity controlled compression ignition (RCCI) is one of the strategies to improve the ignition process and reduce pollutant emissions for bi-fuel engines. This study seeks to get a bi-fuel engine closer to the RCCI phase by changing the fuel injection time to assess the performance and emissions of the engine. Therefore, the numerical study simulated the geometry of a heavy bi-fuel diesel engine by CONVERGE CFD software and SAGE ignition model. The results indicated that delaying the single-stage injection with high reactivity led to a higher NOx, CO, and HC emission while enhancing ignition efficiency. Furthermore, for different fuel injection times, the closer was the diesel fuel injection time to the top dead center (TDC), the lower was the maximum pressure inside the cylinder. Furthermore, longer ignition delay periods resulted in a longer diesel-air mixing time, and consequently, higher pressure inside the cylinder, and thus, more output power.Reactivity controlled compression ignition (RCCI) is one of the strategies to improve the ignition process and reduce pollutant emissions for bi-fuel engines. This study seeks to get a bi-fuel engine closer to the RCCI phase by changing the fuel injection time to assess the performance and emissions of the engine. Therefore, the numerical study simulated the geometry of a heavy bi-fuel diesel engine by CONVERGE CFD software and SAGE ignition model. The results indicated that delaying the single-stage injection with high reactivity led to a higher NOx, CO, and HC emission while enhancing ignition efficiency. Furthermore, for different fuel injection times, the closer was the diesel fuel injection time to the top dead center (TDC), the lower was the maximum pressure inside the cylinder. Furthermore, longer ignition delay periods resulted in a longer diesel-air mixing time, and consequently, higher pressure inside the cylinder, and thus, more output power.
reaktivite kontrollü sıkıştırma ile ateşlemeli bir motorun sayısal analizi
Compressed natural gas (CNG)/diesel reactivity-controlled compression ignition was investigated to understand the combustion and emission characteristics over wide ranges of intake pressure and exhaust gas recirculation (EGR) rate at high load condition. Reactivity-controlled compression ignition is a dual fuel engine combustion strategy that employs in-cylinder fuel blending with two different reactivity fuels, and multiple injections for the in-cylinder fuel reactivity control to optimize the combustion phasing, duration, and magnitude as well as NOx and soot reduction. High CNG substitution was of interest since it could lead to greater CO2 and particulate matter reductions. The present 80% CNG substitution required substantial intake boosting due to the charge air displaced by CNG. The experimental engine is commissioned for the reactivity-controlled compression ignition regime using natural gas and diesel fuel with a two-stage turbocharging system. The intake boosting system achieved the intake pressure and EGR rate of 210 kPa and 25%, respectively, when the EGR valve was fully open. The intake pressure and EGR levels varied in the ranges of 150 to 210 kPa and 0 to 25%, respectively. The corresponding equivalence ratio ranged from 0.85 to 0.5. The intake system achieved the highest turbocharger efficiency of 56% at the maximum boosting operation. The highest CO2 reduction was 23.4% at the maximum boosting condition. The improvements under the enhanced intake condition were significant from the standpoints of both efficiency and emissions, including in terms of the particle concentrations.
In this study, the combustion and emission characteristics of a diesel/methanol/n-butanol blended fuel engine with different pre-injection timings and pre-injection mass ratios were investigated by a computational fluid dynamics (CFD) model. The CFD model was verified by the measured results and coupled with a simplified chemical kinetics mechanism. Firstly, the corresponding three-dimensional CFD model was established by CONVERGE software and the CHEKMIN program, and a chemical kinetic mechanism containing 359 reactions and 77 species was developed. Secondly, the combustion and emission characteristics of the diesel engine with different diesel/methanol/n-butanol blended fuels were analyzed and discussed. The results showed that increases in the pre-injection timing and the pre-injection mass ratio could increase cylinder pressure and cylinder temperature and decrease soot, HC, and CO emissions. At 100% load, the maximum cylinder pressures at the start of pre-injection timing from −15 °CA to −45 °CA, were 7.71, 9.46, 9.85, 9.912, and 9.95 MPa, respectively. The maximum cylinder pressures at pre-injection fuel mass ratios from 0.1 to 0.9 were 7.98, 9.10, 9.96, 10.52, and 11.16 MPa, respectively. At 50% load, with increases of the pre-injection timing and pre-injection fuel mass ratio, the soot emission decreased by 7.30%, 9.45%, 27.70%, 66.80%, 81.80% and 11.30%, 20.03%, 71.32%, 83.80%, 93.76%, respectively, and CO emissions were reduced by 5.77%, 12.31%, 22.73%, 53.59%, 63.22% and 8.29%, 43.97%, 53.59%, 58.86%, 61.18%, respectively. However, with increases of the pre-injection timing and pre-injection mass ratio, NOx emission increased. In addition, it was found that the optimal pre-injection timing and optimal pre-injection mass ratio should be −30 °CA and 0.5, respectively. Therefore, through this study we can better understand the potential interaction of relevant parameters and propose pre-injection solutions to improve combustion and emission characteristics.
Reactivity controlled compression ignition (RCCI) has great potential for a simultaneous reduction in Nitrogen oxides (NOx) and particulate matter (PM) with increase in thermal efficiency. In this experimentation, an attempt is made to investigate the effect of injection pressure on the performance emission and combustion characteristics of single cylinder RCCI engine. Literature reveals that injection pressure has a great influence on the quality of charge preparation, fuel stratification, and incylinder reactivity. Suitably modified engine was operated for 0 to 12 kg loads, for 400 to 700 injection pressure. The blend of ethanol gasoline E20 used as a low reactivity fuel and blend of diesel jatropha biodiesel B20 used as a high reactivity fuel. Experimental results showed that increase in injection pressure enhances the degree of charge homogeneity, reduces the combustion duration, and provides higher rate of energy release. For 12 kg load and 700 bar injection pressure, it is observed that 5% rise in thermal efficiency, 27% reduction in smoke opacity, 2% reduction in HC, 4% reduction in CO and 20% rise in NOx as compared to 400 bar injection pressure.
Driven by the Paris agreement and tightening IMO regulations, the marine sector is focusing on lowering engine emissions and moving to low-carbon fossil or renewable fuel such as methane. The dual-fuel concept allows the usage of methane as main energy-source in diesel engines. A small pilot diesel injection acts as an ignition source for the premixed methane. It was investigated how NOx formation, mainly taking place during combustion of the pilot fuel, could be minimized.
To better understand the process of ignition of a pilot injection in dual-fuel engines, optical research has been performed on a medium speed dual fuel marine engine. A unique Bowditch 200mm single cylinder setup enabled high speed recordings of natural luminescence. Both Reactivity Controlled Compression Ignition (RCCI) and Conventional Dual Fuel (CDF) combustion was investigated. The RCCI combustion was created by an early pilot injection, allowing a long mixing time. The CDF cases had a late injection timing.
In RCCI operation the higher degree of premixing was recognized by combustion luminescence starting further away from the injector, at a varying location. The diluted pilot combustion generated a limited brightness. The heat release profile was Gaussian/bell-shaped, without the typical diesel premixed peak. In CDF operation the recorded images show that combustion follows the shape of the diesel injector jet. The heat release profile was showing a strong initial peak, resembling the premixed peak known from conventional diesel combustion. This heat release peak in CDF combustion, correlated to NOx emissions, is absent in RCCI mode.
Effects of hydroxyl group position (OHP) and carbon chain length (CCL) on combustion and emission characteristics of Reactivity Controlled Compression Ignition (RCCI) engine under different oxygen mass contents (Ro) and start of injections (SOI) were experimentally studied, and real influence factors and sequence of OHP, CCL, Ro and SOI were analyzed quantitatively. Results indicated that: ①Factors having effects on characteristics were not necessarily real influence factors. For example, HC decreased with SOI advance, while they were not correlated. ②Real influence factors and sequence of CA10, CA50 and CA90 were only SOI, ignition delay (ID) was SOI > Ro, indicated mean effective pressure (IMEP) and indicated thermal efficiency (ITE) were CCL > Ro > OHP, HC was Ro > CCL > OHP, CO was CCL > Ro, NOx and particle mass concentration (PMC) were only SOI, while particle number concentration (PNC) and particle average diameter (PAD) were Ro > CCL > SOI. ③OHP and CCL were real influence factors for partial characteristics. IMEP, ITE and HC were correlated to OHP and CCL, and more sensitive to CCL, while CO, PNC and PAD were only correlated to CCL. ④Consumption pathway change aroused by OHP or CCL change was probably primary cause for HC, CO, PNC and PAD change without combustion phase change.
In BSVI diesel engines, the limits of the NOx (oxides of nitrogen) and PM (particulate matter) reduced by 68% and 82% as compared to BSIV engines for the category of the vehicle having gross weight less than 3500kg. It is subjected to implement a complex and costly emission reduction system which reduces fuel economy. Reactivity controlled compression ignition (RCCI) is a duel fuel combustion strategy which has great potential to reduce NOx and PM and the need for an advanced after treatment system with enhanced thermal efficiency. The paper reviews potential of the RCCI strategy, to achieve the emission standards of BSVI norms, which reflects the need for cost assessment of existing engines equipped with advanced after treatment technologies.
In reactivity-controlled compression ignition (RCCI) engines, the ignition and combustion of premixed low reactive fuel (LRF) such as natural gas (NG) is controlled by the injection of high reactive fuel (HRF) such as diesel fuel during the compression stroke. In this study, the effects of six different input parameters on the performance and emissions of the natural gas/diesel fueled RCCI engine are studied using fractional factorial design (FFD) method, which is one of the design of experiment (DOE) methods. In this method, the effects of the interactions of input parameters, referred to as “factors,” on the outputs, referred to as “responses,” are investigated. The factors include premixed ratio (PR), start of first injection (SOI1), spray angle (SA), exhaust gas recirculation (EGR), start of second injection (SOI2), and mass fraction of first injection. Sixteen runs were conducted to evaluate the effects of the interaction between input factors on performance and emissions of a RCCI engine using a validated computational fluid dynamics (CFD) model. DOE results indicate that in order to increase gross indicated efficiency (GIE), higher premixed ratio, 85%, with wider spray angle, 150°, is an effective way. Meanwhile, carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions as well as ringing intensity (RI) are decreased at this condition. To reduce NO x emissions, it is beneficial to raise premixed ratio from 55% to 85% or to use 40% EGR, independently.
Low-temperature combustion is an emerging engine technology that has the ability to yield low NOx and soot emissions while maintaining high fuel efficiency. Low-temperature combustion strategies include homogeneous charge compression ignition, premixed charge compression ignition, reactivity-controlled compression ignition and partially premixed combustion. These low-temperature combustion strategies use early fuel injections to allow sufficient time for air–fuel mixing before combustion. According to the literature, some low-temperature combustion strategies are not promising for future automotive and power generation applications due to difficulties in controlling the heat release rate and the lack of a combustion phasing control mechanism. To mitigate these problems, the reactivity-controlled compression ignition combustion concept was introduced. Reactivity-controlled compression ignition is a dual-fuel partially premixed combustion concept, which uses port fuel injection of a low-reactivity fuel (e.g. gasoline, natural gas and alcohol fuels) and direct injection of a high-reactivity fuel (e.g. diesel and biodiesel) with blending inside the combustion chamber to increase the combustion duration and to provide phasing control. Combustion phasing is controlled by the relative ratios of the two fuels, and the combustion duration is controlled by spatial stratification between the two fuels. This article begins by an overview of the different low-temperature combustion strategies and demonstrates some advantages of reactivity-controlled compression ignition, over homogeneous charge compression ignition and premixed charge compression ignition combustion strategies in regard to fuel flexibility and combustion controllability. A comprehensive review of recent research on various aspects of reactivity-controlled compression ignition and comparisons of thermal efficiency and pollutant emissions over conventional diesel combustion is also presented. This article presents the significance of reactivity-controlled compression ignition strategy as a promising solution for future automotive engines and discusses future research directions.
An improved spray atomization model is presented for use in both diesel and gasoline spray computations. The KH-RT hybrid atomization model consists of two distinct steps: primary and secondary breakup. The Kelvin-Helmholtz (KH) instability model was used to predict the primary breakup of the intact liquid core of a diesel jet. The secondary breakup of the individual drops was modeled with the Kelvin-Helmholtz model in conjunction with the Rayleigh-Taylor (RT) accelerative instability model. A modification was made to the KH-RT hybrid model that allowed the RT accelerative instabilities to affect all drops outside the intact liquid core of the jet. In previous implementations, only the drops beyond the breakup length are affected by RT breakup. Furthermore, a Rosin-Rammler distribution was used to specify the sizes of children drops after the RT breakup of a parent drop. The modifications made to the KH-RT hybrid model were found to give satisfactory results and to improve the temperature dependence of the liquid fuel penetration of the diesel sprays significantly. The KH-RT model was also found to predict the spray shape, penetration, and local SMD of hollow-cone sprays as well as previous gasoline spray models based on the TAB model.
In this paper, a numerical study is performed to provide the combustion and emission characteristics resulting from fuel-reactivity controlled compression ignition (RCCI) combustion mode in a heavy-duty, single-cylinder diesel engine with gasoline and diesel fuels. In RCCI strategy in-cylinder fuel blending is used to develop fuel reactivity gradients in the combustion chamber that result in a broad combustion event and reduced pressure rise rates (PRR). RCCI has been demonstrated to yield low NOx and soot with high thermal efficiency in light and heavy-duty engines. KIVA-CHEMKIN code with a reduced primary reference fuel (PRF) mechanism are implemented to study injection timings of high reactivity fuel (i.e., diesel) and low reactivity fuel percentages (i.e., gasoline) at a constant engine speed of 1300 rpm and medium load of 9 bar indicated mean effective pressure (IMEP). Significant reduction in nitrogen oxide (NOx), while 49% gross indicated efficiency (GIE) were achieved successfully through the RCCI combustion mode. The parametric study of the RCCI combustion mode revealed that the peak cylinder pressure rise rate (PPRR) of the RCCI combustion mode could be controlled by several physical parameters – PRF number, and start of injection (SOI) timing of directly injected fuel
This article covers key and representative developments in the area of high efficiency and clean internal combustion engines. The main objective is to highlight recent efforts to improve (IC) engine fuel efficiency and combustion. Rising fuel prices and stringent emission mandates have demanded cleaner combustion and increased fuel efficiency from the IC engine. This need for increased efficiency has placed compression ignition (CI) engines in the forefront compared to spark ignition (SI) engines. However, the relatively high emission of oxides of nitrogen (NOx) and particulate matter (PM) emitted by diesel engines increases their cost and raises environmental barriers that have prevented their widespread use in certain markets. The desire to increase IC engine fuel efficiency while simultaneously meeting emissions mandates has thus motivated considerable research. This paper describes recent progress to improve the fuel efficiency of diesel or CI engines through advanced combustion and fuels research. In particular, a dual fuel engine combustion technology called “reactivity controlled compression ignition” (RCCI), which is a variant of Homogeneous Charge Compression Ignition (HCCI), is highlighted, since it provides more efficient control over the combustion process and has the capability to lower fuel use and pollutant emissions. This paper reviews recent RCCI experiments and computational studies performed on light- and heavy-duty engines, and compares results using conventional and alternative fuels (natural gas, ethanol, and biodiesel) with conventional diesel, advanced diesel and HCCI concepts.
Achieving simultaneous reduction of NOx
, CO and unburned hydrocarbon (UHC) emissions without compromising engine performance at part loads is the current focus of dual fuel engine research. The present work focuses on an experimental investigation conducted on a dual fuel (diesel-natural gas) engine to examine the simultaneous effect of inlet air pre-heating and exhaust gas recirculation (EGR) ratio on performance and emission characteristics at part loads. The use of EGR at high levels seems to be unable to improve the engine performance at part loads. However, it is shown that EGR combined with pre-heating of inlet air can slightly increase thermal efficiency, resulting in reduced levels of both unburned hydrocarbon and NOx
emissions. CO and UHC emissions are reduced by 24% and 31%, respectively. The NOx
emissions decrease by 21% because of the lower combustion temperature due to the much inert gas brought by EGR and decreased oxygen concentration in the cylinder.
A fuel reactivity controlled compression ignition (RCCI) concept is demonstrated as a promising method to achieve high efficiency – clean combustion. Engine experiments were performed in a heavy-duty test engine over a range of loads. Additionally, RCCI engine experi-ments were compared to conventional diesel engine experiments. Detailed computational fluid dynamics modelling was then used to explain the experimentally observed trends. Specifically, it was found that RCCI combustion is capable of operating over a wide range of engine loads with near zero levels of NO x and soot, acceptable pressure rise rate and ringing intensity, and very high indicated efficiency. For example, a peak gross indicated efficiency of 56 per cent was observed at 9.3 bar indicated mean effective pressure and 1300 rev/min. The comparison between RCCI and conventional diesel showed a reduction in NO x by three orders of magni-tude, a reduction in soot by a factor of six, and an increase in gross indicated efficiency of 16.4 per cent (i.e. 7.9 per cent more of the fuel energy was converted to useful work). The simulation results showed that the improvement in fuel conversion efficiency was due both to reductions in heat transfer losses and improved control over the start-and end-of-combustion.
Reactivity controlled compression ignition (RCCI) combustion employs two fuels with a large difference in auto-ignition properties that are injected at different times to generate a spatial gradient of fuel-air mixtures and reactivity. Researchers have shown that RCCI offers improved fuel efficiency and lower NOx and Soot exhaust emissions when compared to conventional diesel diffusion combustion. The majority of previous research work has been focused on premixed gasoline or ethanol for the low reactivity fuel and diesel for the high reactivity fuel. The increased availability of natural gas (NG) in the U.S. has renewed interest in the application of compressed natural gas (CNG) to heavy-duty (HD) diesel engines in order to realize fuel cost savings and reduce pollutant emissions, while increasing fuel economy. Thus, RCCI using CNG and diesel fuel warrants consideration. A computational study was performed on a 15L HD diesel engine to examine trade-offs of pollutant emissions, fuel consumption, peak cylinder pressure and maximum cylinder pressure rise rate. The results from the model indicated that an RCCI combustion strategy had the potential of 17.5% NOx reduction, 78% soot reduction and a 24% decrease in fuel consumption when compared to a conventional diesel combustion strategy using the same air-fuel ratio (AFR) and exhaust gas recirculation (EGR) rate, at the rated power operating condition. This was attained while meeting peak cylinder pressure and maximum cylinder pressure rise rate constraints.
The advantages of applying Compressed Natural Gas (CNG) as a fuel for internal combustion engines are well known. In addition to a significant operating cost savings due to a lower fuel price relative to diesel, there is an opportunity to reduce the engine's emissions. With CNG combustion, some emissions, such as Particulate Matter (PM) and Carbon Dioxide (CO2), are inherently reduced relative to diesel fueled engines due to the nature of the combustion and the molecular makeup of the fuel. However, it is important to consider the impact on all emissions, including Total Hydrocarbons (THC) and Carbon Monoxide (CO), which can increase with the use of CNG. Nitrogen Oxides (NOx) emission is often reported to decrease with the use of CNG, but the ability to realize this benefit is significantly impacted by the control strategy and calibration applied. FEV has investigated the emissions and performance impact of operating a heavy-duty diesel engine with CNG in a dual fuel mode. The CNG was introduced via injectors mounted to an inlet pipe located upstream of the intake manifold. The fumigation approach included a mixer to improve the distribution of gas prior to delivery to the cylinder. The initial investigations sought to determine how the performance of a heavy-duty diesel engine would be affected by the introduction of CNG. For this effort there was no change to the base engine calibration, and the ability to maximize substitution of diesel with CNG was investigated. It was observed that the ability to maximize substitution of diesel with CNG across the operating map was limited by extremely high THC levels, combustion instability and limitations in peak cylinder pressure and exhaust gas temperature. With the application of a simplified engine calibration with a single diesel injection and Exhaust Gas Recirculation (EGR), timing adjustments allowed higher CNG substitution levels in several areas of the operating map. A further increase in gas substitution along with higher fuel conversion efficiency, improved combustion stability and even lower emissions could be achieved through Reactivity Controlled Compression Ignition (RCCI) combustion. This approach required a unique injection strategy along with a careful balance of EGR rates and boost pressure. Under this combustion regime it was possible to observe a simultaneous reduction of NOx and PM emissions, approaching engine-out emission levels that could avoid, or significantly minimize, aftertreatment of these species. With the desire to quickly apply CNG systems to existing diesel engine architecture in an effort to reap the benefit in fuel cost savings, manufacturers and system developers must be careful to understand the full impact on the engine's performance and emissions. Tests conducted as part of this investigation have revealed that an un-optimized approach to CNG introduction can lead to extreme THC emissions that mostly consist of Methane (CH4). In addition, the maximum gas substitution level is significantly limited in most regions of the engine operating map. Thus, the ability to specifically tune the calibration for operation with CNG is essential to achieving the maximum benefit in fuel cost savings and emission control.
Substitution of diesel fuel with natural gas in heavy-duty diesel engines offers significant advantages in terms of operating cost, as well as NOx, PM emissions and greenhouse gas emissions. However, the challenges of high THC and CO emissions, combustion stability, exhaust temperatures and pressure rise rates limit the substitution levels across the engine operating map and necessitate an optimized combustion strategy. Reactivity controlled compression ignition (RCCI) combustion has shown promise in regard to improving combustion efficiency at low and medium loads and simultaneously reducing NOx emissions at higher loads. RCCI combustion exploits the difference in reactivity between two fuels by introducing a less reactive fuel, such as natural gas, along with air during the intake stroke and igniting the air-CNG mixture by injecting a higher reactivity fuel, such as diesel, later in the compression stroke. Recent studies to optimize dual fuel diesel-CNG RCCI combustion have primarily focused on the simultaneous reduction of NOx and soot emissions. However, further investigation is needed to outline the in-cylinder conditions that are required in order for RCCI combustion to proceed. In addition, the THC emissions produced under dual fuel diesel-CNG RCCI operation need to be analyzed to better understand how to address this limitation of the technology. The current study builds on the dual fuel diesel-CNG study previously presented by the same set of authors by analyzing the experimental RCCI combustion results achieved on a heavy-duty diesel engine at 6 bar BMEP and multiple engine speeds. The study evaluates the impact of various control variables, such as CNG substitution, EGR rate and injection strategy on achieving RCCI combustion at 6 bar BMEP, thereby establishing a general framework for in-cylinder mixture properties required in realizing RCCI combustion. The conclusions at 6 bar BMEP are supported by 3D simulations of the complete combustion chamber using Converge CFD software. CFD results are also used to highlight the causes of high CH4 and CO emissions with dual fuel diesel-CNG RCCI operation. Further, the paper analyzes the experimental RCCI combustion results at 14 bar BMEP and multiple engine speeds to lay out the challenges in achieving RCCI combustion at increased engine load.
For natural gas (NG)-diesel RCCI, a multi-zonal, detailed chemistry modeling approach is presented. This dual fuel combustion process requires further understanding of the ignition and combustion processes to maximize thermal efficiency and minimize (partially) unburned fuel emissions. The introduction of two fuels with different physical and chemical properties makes the combustion process complicated and challenging to model. In this study, a multi-zone approach is applied to NG-diesel RCCI combustion in a heavy-duty engine. Auto-ignition chemistry is believed to be the key process in RCCI. Starting from a multi-zone model that can describe auto-ignition dominated processes, such as HCCI and PCCI, this model is adapted by including reaction mechanisms for natural gas and NOx and by improving the in-cylinder pressure prediction. The model is validated using NG-diesel RCCI measurements that are performed on a 6 cylinder heavy-duty engine. For three different engine operating points, it is operated at various diesel injection timings and NG-diesel blend ratios. The validation is focused on variables that are relevant for engine control, such as CA50, peak cylinder pressure, and engine-out NOx emissions. The validation shows that the multi-zone method with detailed chemistry reproduces the correct trends for important control parameters. From this validated model, real-time, map-based RCCI models are derived, which are considered to be an important step towards model-based NG-diesel RCCI control development.
Detailed chemical kinetics, although preferred due to increased accuracy, can significantly slow down CFD combustion simulations. Chemistry solutions are typically the most computationally costly step in engine simulations. The calculation time can be significantly accelerated using a multi-zone combustion model. The multi-zone model is integrated into the CONVERGE CFD code. At each time-step, the CFD cells are grouped into zones based on the cell temperature and equivalence ratio. The chemistry solver is invoked only on each zone. The zonal temperature and mass fractions are remapped onto the CFD cells, such that the temperature and composition non-uniformities are preserved. Two remapping techniques published in the literature are compared for their relative performance. The accuracy and speed-up of the multi-zone model is improved by using variable bin sizes at different temperature and equivalence ratios. In addition, a general n-dimensional zoning strategy is developed to include other cell variables such as pressure, mass fractions of different species, etc. to improve the performance of the zoning strategy. This paper discusses the savings in computational time achieved and the accuracy of the results using the multi-zone model for a range of scenarios. Gasoline and Diesel engine simulations are performed. Test cases are run for single fuel and multi-component fuels. Exhaust gas recirculation (EGR) scenarios are also tested.
The potential of boosted HCCI for achieving high loads has been investigated for intake pressures (P in) from 100 kPa (naturally aspirated) to 325 kPa absolute. Experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) equipped with a compression-ratio 14 piston at 1200 rpm. The intake charge was fully premixed well upstream of the intake, and the fuel was a research-grade (R+M)/2 = 87- octane gasoline with a composition typical of commercial gasolines. Beginning with P in = 100 kPa, the intake pressure was systematically increased in steps of 20 - 40 kPa, and for each P in, the fueling was incrementally increased up to the knock/ stability limit, beyond which slight changes in combustion conditions can lead to strong knocking or misfire. A combination of reduced intake temperature and cooled EGR was used to compensate for the pressure-induced enhancement of autoignition and to provide sufficient combustion-phasing retard to control knock. The maximum attainable load increased progressively with boost from a gross indicated mean effective pressure (IMEP g) of about 5 bar for naturally aspirated conditions up to 16.34 bar for P in = 325 kPa. For this high-load point, combustion and indicated thermal efficiencies were 99% and 47%, respectively, and NOx emissions were < 0.1 g/kg-fuel. Maximum pressure-rise rates were kept sufficiently low to prevent knock, and the COV of the IMEP g was < 1.5%. Central to achieving these results was the ability to retard combustion phasing (CA50) as late as 19° after TDC with good stability under boosted conditions. Detailed examination of the heat release rates shows that this substantial CA50 retard was possible because intake boosting significantly enhances the early autoignition reactions, keeping the charge temperature rising toward the hot-ignition point despite the high rate of expansion at these late crank angles. Overall, the investigation showed that well-controlled boosted HCCI has a strong potential for achieving power levels close to those of turbo-charged diesel engines.
This work investigates the effects of the direct injection timing and blending ratio on RCCI performance and engine-out emissions at different engine loads using four low reactivity fuels: E10-95, E10-98, E20-95 and E85 (port fuel injected) and keeping constant the same high reactivity fuel: diesel B7 (direct injected). The experiments were conducted using a heavy-duty single-cylinder research diesel engine adapted for dual-fuel operation. All the tests were carried out at 1200 rpm. To assess the blending ratio effect, the total energy delivered to the cylinder coming from the low reactivity fuel was kept constant for the different fuel blends investigated by adjusting the low reactivity fuel mass as required in each case. In addition, a detailed analysis of the air/fuel mixing process has been developed by means of a 1-D in-house developed spray model.
Results suggest that notable higher diesel amount is required to achieve a stable combustion using E85. This fact leads to higher NOx levels and unacceptable ringing intensity. By contrast, EURO VI NOx and soot levels are fulfilled with E20-95, E10-98 and E10-95. Finally, the higher reactivity of E10-95 results in a significant reduction in CO and HC emissions, mainly at low load.
The increased availability of natural gas (NG) in the U.S. has renewed interest in the application to heavy-duty (HD) diesel engines in order to realize fuel cost savings and reduce pollutant emissions, while increasing fuel economy. Reactivity controlled compression ignition (RCCI) combustion employs two fuels with a large difference in auto-ignition properties to generate a spatial gradient of fuel-air mixtures and reactivity. Typically, a high octane fuel is premixed by means of port-injection, followed by direct injection of a high cetane fuel late in the compression stroke. Previous work by the authors has shown that NG and diesel RCCI offers improved fuel efficiency and lower oxides of nitrogen (NOx) and soot emissions when compared to conventional diesel diffusion combustion. The work concluded that NG and diesel RCCI engines are load limited by high rates of pressure rise (RoPR) (>15 bar/deg) and high peak cylinder pressure (PCP) (>200 bar). A high degree of premixing has been found by several researchers to cause excessively high rates of pressure rise thus limiting load. The dual fuel engine proposed in this work employed direct injection of natural gas (DI-NG) (modeled as methane), as the main fuel, during the compression stroke in addition to early and late injections of small quantities of diesel fuel (modeled as n-heptane) to provide the ignition source. The DI-NG concept creates enhanced stratification of the NG fuel portion and avoids excessive premixing, which tempers the RoPR, thus enabling higher load operation. A computational study was performed to examine the trade-offs of fuel consumption, PCP, and peak RoPR, with engine emissions. Several parameters were studied including: relative azimuthal angle between NG and diesel fuel nozzles, diesel pilot injection timing and quantity splits as well as injection timing sweeps. The results from the study indicated that DI-NG was successful in controlling the RoPR to below 10 bar/deg and PCP to less than 180 bar, while improving the NOx, HC and soot emissions to meet engine out targets for engines equipped with modern aftertreatment systems.
Studies have shown that premixed combustion concepts such as PCCI and RCCI can achieve high efficiencies while maintaining low NOx and soot emissions. The RCCI (Reactivity Controlled Compression Ignition) concept use blending port-injected high-octane fuel with early direct injected high-cetane fuel to control auto-ignition. This paper describes studies on RCCI combustion using CNG and diesel as the high-octane and high-cetane fuels, respectively. The test was conducted on a heavy-duty single cylinder engine. The influence of injection timing and duration of the diesel injections was examined at 9 bar BMEP and1200 rpm. In addition, experiments were conducted using two different compression ratios, (14 and 17) with different loads and engine speeds. Results show both low NOx and almost zero soot emissions can be achieved but at the expense of increasing of unburned hydrocarbon emissions which could potentially be removed by catalytic after-treatment. CA50 generally occurred before TDC when using a compression ratio of 17. While the CA50 could be shifted to slightly after TDC by increasing the amount of EGR, this would lead to excessive HC emissions. A lower compression ratio of 14 was therefore used to retard the CA50 while maintaining acceptable UHC emissions.
Many recent studies have shown that the Reactivity Controlled Compression Ignition (RCCI) combustion strategy can achieve high efficiency with low emissions. However, it has also been revealed that RCCI combustion is difficult at high loads due to its premixed nature. To operate at moderate to high loads with gasoline/diesel dual fuel, high amounts of EGR or an ultra low compression ratio have shown to be required. Considering that both of these approaches inherently lower thermodynamic efficiency, in this study natural gas was utilized as a replacement for gasoline as the low-reactivity fuel. Due to the lower reactivity (i.e., higher octane number) of natural gas compared to gasoline, it was hypothesized to be a better fuel for RCCI combustion, in which a large reactivity gradient between the two fuels is beneficial in controlling the maximum pressure rise rate. The multi-dimensional CFD code, KIVA3V, was used in conjunction with the CHEMKIN chemistry tool and a Nondominated Sorting Genetic Algorithm (NSGA-II) to perform optimization for a wide range of engine operating conditions. Engine design parameters that were controlled by the genetic algorithm include the fraction of total fuel that is premixed (methane), the timing of the two diesel injections, the amount of diesel in each injection, the diesel fuel injection pressure, and the EGR percentage. The objective of the optimization was to simultaneously minimize soot, NOx, CO, and UHC emissions, as well as ISFC and ringing intensity. A broad load/speed range was investigated; six operating points from 4 to 23 bar IMEP and 800 to 1800 rev/min were optimized. These load/speed combinations represent typical heavy-duty engine conditions. Using the stock compression ratio of 16.1, it was determined that operation up to 13.5 bar IMEP could be achieved with no EGR, while still maintaining high efficiency and low emissions. The study also examined the sensitivity of RCCI combustion at high load to injection system parameters. The results emphasize that precise injection control is needed for combustion control.
Several studies carried out with the aim of improving the RCCI (reactivity controlled compression ignition) concept in terms of thermal efficiency conclude that the main cause of the reduced efficiency at light loads is the reduced combustion efficiency. The present study used both a 3D computational model and engine experiments to explore the effect of the oxygen concentration and intake temperature on RCCI combustion efficiency at light load. The experiments were conducted using a single-cylinder heavy-duty research diesel engine adapted for dual fuel operation.
Results suggest that it is possible to achieve an improvement of around 1.5% in the combustion efficiency with both strategies studied; the combined effect of intake temperature and in-cylinder fuel blending as well as the combined effect of oxygen concentration and in-cylinder fuel blending (ICFB). In addition, the direct comparison of both strategies suggests that the combustion losses trend is mainly associated to the in-cylinder equivalence ratio stratification, which is determined by the diesel to gasoline ratio in the blend since the injection timing is kept constant for all the tests. Moreover, the combined effect of the intake temperature and ICFB promotes a slight improvement in the combustion losses over the combined effect of the oxygen concentration and ICFB.
Engine experiments and multi-dimensional modeling were used to explore Reactivity Controlled Compression Ignition (RCCI) to realize highly-efficient combustion with near zero levels of NOx and PM. In-cylinder fuel blending using port-fuel-injection of a low reactivity fuel and optimized direct-injection of higher reactivity fuels was used to control combustion phasing and duration. In addition to injection and operating parameters, the study explored the effect of fuel properties by considering both gasoline-diesel dual-fuel operation, ethanol (E85)-diesel dual fuel operation, and a single fuel gasoline-gasoline+DTBP (di-tert butyl peroxide cetane improver). Remarkably, high gross indicated thermal efficiencies were achieved, reaching 59%, 56%, and 57% for E85-diesel, gasoline-diesel, and gasoline-gasoline+DTBP respectively.
Using conditions based on CFD simulations, engine experiments were performed using a heavy-duty test engine and the modeling was further used to explain the experimentally observed results. The experiments confirmed that by optimizing the fuel reactivity based on the specific operating conditions, combustion phasing can be optimized in order to minimize fuel consumption. Additionally, it was found that highly efficient operation (greater than 50% indicated thermal efficiency) could be achieved with all three fuel blending strategies over a wide range of loads. This study showed that, compared to gasoline-diesel, significantly higher quantities of diesel fuel were required to maintain optimal combustion phasing with the E85-diesel fuel blends. This result is due to a combination of the lower reactivity and higher enthalpy of vaporization of ethanol (compared to gasoline) and combustion chemistry effects of ethanol diesel blends. Secondly, the single fuel gasoline-gasoline+DTBP yielded near identical emissions and ISFC results to gasoline-diesel operation. Although the emissions and ISFC of all three strategies were similar, the low temperature heat release (LTHR) were different with all three fuels, and the high temperature heat release (HTHR) was different with E85-diesel blends. Fuel chemistry effects for all three fuels were investigated and their effect on the reactivity gradient was found to be responsible for the combustion differences.
The present experimental study explores the effects of compression ratio and piston design in a heavy-duty diesel engine operated with Reactivity Controlled Compression Ignition (RCCI) combustion. In previous studies, RCCI combustion with in-cylinder fuel blending using port-fuel-injection of a low reactivity fuel and optimized direct-injections of higher reactivity fuels was demonstrated to permit near-zero levels of NOX and PM emissions in-cylinder, while simultaneously realizing high thermal efficiencies. The present study consists of RCCI experiments at loads from 4 to 17 bar indicated mean effective pressure at engine speeds of 1,300 and 1,700 [rev/min]. The experiments used a modified piston to examine the effect of piston crevice volume, squish geometry, and compression ratio on performance and efficiency. Results from a bathtub-style piston with reduced crevice height, short squish length, and a 14.88:1 compression ratio were compared with results from an open-crater-style piston with both 16.1:1 and 11.6 compression ratios, and a piston with a PCCI-style narrow bowl with 15.5:1 compression ratio. Of all the tested pistons, the bathtub-style piston was found to offer the highest brake efficiency and to enable low emissions and low pressure rise rates with practical intake and exhaust gas recirculation (EGR) temperatures (e.g., 70°C and 120°C, respectively). The experiments also demonstrated that engine-out unburned hydrocarbon emissions (HC) were highly correlated with crevice volume, but that optimizing the squish geometry could significantly reduce engine-out HC emissions, even with a large crevice volume. The study also found that the reduced compression ratio, bathtub-style piston decreased peak cylinder pressure, which is expected to increase brake efficiency by lowering friction losses. Engine operation with the stock and bathtub pistons was demonstrated to meet EPA HD 2010 NOX and PM mandates in-cylinder (no after-treatment required) and to provide a 5-15% improvement in the estimated brake efficiency compared to conventional diesel operation with 16.1:1 compression ratio and 50% overall turbocharger efficiency, which requires after-treatment for soot and NOX reduction.
This study investigates the potential of controlling premixed charge compression ignition (PCCI) combustion strategies by varying fuel reactivity. In-cylinder fuel blending using port fuel injection of gasoline and early cycle, direct-injection of diesel fuel was used for combustion phasing control at a medium engine load of 9 bar net IMEP and was also found to be effective to prevent excessive rates of pressure rise. Parameters used in the experiments were guided from the KIVA-CHEMKIN code with a reduced primary reference fuel (PRF) mechanism including injection timings, fuel percentages, and intake valve closing (IVC) timings for dual-fuel PCCI combustion. The engine experiments were conducted with a conventional common rail injector (i.e., wide angle and large nozzle hole) and demonstrated control and versatility of dual-fuel PCCI combustion with the proper fuel blend, SOI and IVC timings. For example, at the 9 bar operating point, NOx and soot were 0.012 g/kW-hr and 0.008 g/kW-hr, respectively. That is, US EPA 2010 heavy-duty NOx and PM emissions regulations are easily met without after-treatment, while achieving 53% net indicated thermal efficiency.
This study investigates the potential of controlling premixed charge compression ignition (PCCI and HCCI) combustion strategies by varying fuel reactivity. In-cylinder fuel blending using port fuel injection of gasoline and early cycle direct injection of diesel fuel was used for combustion phasing control at both high and low engine loads and was also effective to control the rate of pressure rise. The first part of the study used the KIVA-CHEMKIN code and a reduced primary reference fuel (PRF) mechanism to suggest optimized fuel blends and EGR combinations for HCCI operation at two engine loads (6 and 11 bar net IMEP). It was found that the minimum fuel consumption could not be achieved using either neat diesel fuel or neat gasoline alone, and that the optimal fuel reactivity required decreased with increasing load. For example, at 11 bar net IMEP, the optimum fuel blend and EGR rate for HCCI operation was found to be PRF 80 and 50%, respectively. Engine experiments using a dual-fuel PCCI strategy with port fuel injection of gasoline and early cycle multiple injections of diesel fuel with a conventional diesel injector (i.e., wide angle and large nozzle hole) were performed. The experimental results confirmed that an extension of the PCCI operating regime is possible when optimized fuel blends are used. At the 11 bar operating point, NOx and soot were ~0.01 g/kW-hr and ~0.008 g/kW-hr, respectively. That is, US 2010 heavy-duty emissions regulations are easily met without aftertreatment while achieving 50% thermal efficiency.
In the present study, various low temperature combustion strategies were investigated using single cylinder engine experiments. The combustion strategies that were investigated premix the majority of the fuel and do not require exhaust gas recirculation (EGR) to achieve ultra-low NOx and soot emissions for low- to mid-load engine operation. These types of advanced compression ignition combustion strategies have been shown to have challenges with combustion phasing control. The focus of the study was to compare engine performance and emissions, combustion sensitivity to intake conditions, and the ability to control any observed sensitivity through the fuel injection strategy. Even though these are steady state engine experiments, this will demonstrate a given combustion strategies controllability on a cycle-to-cycle basis. The combustion strategies that were investigated are fully premixed dual-fuel homogeneous charge compression ignition (HCCI), dual-fuel reactivity controlled compression ignition (RCCI), and single-fuel partially premixed combustion (PPC). The baseline operating condition was an engine load representative of a light-duty engine: 5.5 bar gross indicated mean effective pressure (IMEP) and 1500 rev/min. At the baseline operating condition, in which the boundary conditions were chosen to yield near optimal engine performance, all three combustion strategies demonstrated high gross indicated efficiency (∼47%) and ultra-low NOx and soot emissions. By perturbing the intake conditions, it was found that all three combustion strategies display similar combustion phasing sensitivities. Both dual-fuel HCCI and RCCI were able to readily correct the observed sensitivities through the global fuel reactivity with no negative implications on the NOx emissions. However, single-fuel PPC was unable to correct for the observed combustion phasing sensitivity and, in some cases, had negative implications on the NOx emissions.
Premixed combustion concepts like PCCI and RCCI have attracted much attention, since these concepts offer possibilities to reduce engine out emissions to a low level, while still achieving good efficiency. Most RCCI studies use a combination of a high-cetane fuel like diesel, and gasoline as low-cetane fuel. Limited results have been published using natural gas as low-cetane fuel; especially full scale engine results. This study presents results from an experimental study of diesel-CNG RCCI operation on a 6 cylinder, 8 l heavy duty engine with cooled EGR. This standard Tier4f diesel engine was equipped with a gas injection system, which used single point injection and mixed the gaseous fuel with air upstream of the intake manifold. For this engine configuration, RCCI operating limits have been explored. In the 1200-1800 rpm range, RCCI operation with Euro-VI engine out NOx and soot emissions was achieved between 2 and 9 bar BMEP without EGR. Corresponding hydrocarbon levels were high, but exhaust temperature levels hold promise for a suitable reduction through catalytic aftertreatment. Thermal efficiency was comparable to or better than diesel operation. In the load ranges tested, gas Methane Number (MN) variations between 70 and 100 have only a small effect on RCCI performance.
An analysis of in-cylinder gas density and oxygen mass concentration (YO2) impact on the mixing and oxidation processes and the final soot emissions in conventional high temperature diffusive Diesel combustion conditions is presented in this paper.Parametrical tests were performed on a single cylinder heavy duty research engine. The density was modified adjusting the boost pressure following two approaches, maintaining the YO2 either before or after the combustion process. The YO2 was modified by diluting fresh air with exhaust gas maintaining a constant density. The possibility of controlling the soot emissions combining both parameters (YO2 and density) is evaluated and, in a final part, the NOX emission results are also addressed.Results show that YO2 has a strong effect on both mixing and oxidation processes while density affects principally the mixing process. Both parameters affect the final soot emissions. The density modification through adjustment of boost pressure modifies the trapped mass and has a strong impact on the evolution of YO2 (thus on the evolution of the mixing process) during combustion. If the density is increased maintaining constant the YO2 at the beginning of the combustion, the NOX-Soot trade-off is enhanced.
Homogeneous charge compression ignition (HCCI) is a promising technique for advanced low-temperature combustion strategies that offers a high fuel conversion efficiency and low nitrogen oxide and soot emissions. One of the major problems associated with HCCI combustion engine application is the lack of direct control for combustion timing. A proposed solution for combustion timing control is to use a binary fuel blend in which two fuels with different autoignition characteristics are blended at various ratios on a cycle-by-cycle basis. Because dual-fuel diesel—natural-gas engines have already been used, a fuel blend of n-heptane (diesel-like fuel) and natural gas (mostly methane) is one of the best available options.
The objective of this study is to optimize the chemical kinetic mechanisms available for n-heptane and natural gas to be used in a binary-fuel blend scenario. Using the genetic algorithm method, a combined mechanism was optimized and modelling results were verified against experimental results. The agreement between experimental and modelling results was found to be acceptable within the examined conditions. As a result, an optimized chemical kinetic mechanism for an n-heptane—natural-gas blend is presented.
The RNG κ-ϵ turbulence model derived by Yakhot and Orszag (1986) based on the Renormalization Group theory has been modified and applied to variable-density engine flows in the present study. The original RNG-based turbulence transport approximations were developed formally for an incompressible flow. In order to account for flow compressibility the RNG ϵ-equation is modified and closed through an isotropic rapid distortion analysis. Computations were made of engine compressing/expanding flows and the results were compared with available experimental observations in a production diesel engine geometry. The modified RNG κ-ϵ model was also applied to diesel spray combustion computations. It is shown that the use of the RNG model is warranted for spray combustion modeling since the ratio of the turbulent to mean-strain time scales is appreciable due to spray-generated mean flow gradients, and the model introduces a term to account for these effects. Large scale flow structures are predicted which are affected by the spray and the squish and are consistent with endoscope combustion images. The effects of flow compressibility on both non-reacting compressing/expanding flows and reacting flows are discussed. It is concluded that predicted combustion parameters, particularly, soot emissions, are significantly influenced by the treatment of flow compressibility in the turbulence model.
A mathematical model was developed for predicting the concentration of exhaust nitric oxide, soot and other emissions in a direct injection diesel engine. In the model, it was emphasized to describe the phenomena occurring in the combustion chamber from the microscopic point of view. The prediction was based on the knowledges concerning a single droplet as well as the droplet size distribution in a fuel spray and the spatial and temporal distribution histories of fuel in a combustion chamber. The heterogeneous field of temperature and equivalence ratio, and uniform pressure in the cylinder were postulated. The heat release model gives the burning rate of injected fuel and pressure and temperature history in the cylinder. The concentration of nitric oxide and soot in the cylinder was predicted by the emission formation model. In order to confirm the validity of the theoretical analysis, the calculated results were compared with the experimental results for typical direct injection diesel engine.
Low temperature combustion (LTC) engines are an emerging engine technology that offers an alternative to spark-ignited and diesel engines. One type of LTC engine, the homogeneous charge compression ignition (HCCI) engine, uses a well-mixed fuel–air charge like spark-ignited engines and relies on compression ignition like diesel engines. Similar to diesel engines, the use of high compression ratios and removal of the throttling valve in HCCI allow for high efficiency operation, thereby allowing lower CO2 emissions per unit of work delivered by the engine. The use of a highly diluted well-mixed fuel–air charge allows for low emissions of nitrogen oxides, soot and particulate matters, and the use of oxidation catalysts can allow low emissions of unburned hydrocarbons and carbon monoxide. As a result, HCCI offers the ability to achieve high efficiencies comparable with diesel while also allowing clean emissions while using relatively inexpensive aftertreatment technologies.
The droplet collision algorithm of O'Rourke is currently the standard approach to calculating collisions in Lagrangian spray simulations. This algorithm has a cost proportional to the square of the number of computational particles, or “parcels”. To more efficiently calculate droplet collisions, a technique applied to gas dynamics simulations is extended for use in sprays. For this technique to work efficiently, it must be able to handle the general case where the number of droplets in each parcel varies. The present work shows how the no-time-counter (NTC) method can be extended for the general case of varying numbers of droplets per parcel. The basis of this improvement is analytically derived. The new algorithm is compared to closed-form solutions and to the algorithm of O'Rourke. The NTC method is several orders of magnitude faster and slightly more accurate than O'Rourke's method for several test cases. The second part of the paper concerns implementation of the collision algorithm into a multidimensional code that also models the gas phase behavior and spray breakup. The collision computations are performed on a special collision mesh that is optimized for both sample size and spatial resolution. The mesh is different every time step to further suppress the artifacts that are common in the method of O'Rourke. The parcels are then sorted into cells, so that a list of all the parcels in a given cell are readily available. Next, each cell is individually checked to see if it is so dense that a direct collision calculation is cheaper than the NTC method. The cheaper method is applied to that cell. The final result is a method of calculating spray droplet collisions that is both faster and more accurate than the current standard method of O'Rourke.
Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NOx) emissions, while producing lower emissions of carbon dioxide (CO2), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NOx emissions. High NOx emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas–hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NOx and CO2 emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.
A numerical technique has been developed for computing the behavior of atomized non-evaporating liquid sprays injected into a gaseous environment. The same technique, however, is applicable to general incompressible flows containing particles or droplets. The method consists of a fully-interacting combination of Eulerian fluid and Lagrangian particle calculations. The interaction calculations are performed simultaneously and noniteratively. A Lagrangian description of the particles avoids numerical diffusion, and allows individual attributes, such as particle size, composition, etc., to be statistically assigned for each particle. Numerical calculations and comparisons with experimental data are given for some sprays typical of diesel engine fuel injection.
- A Paykani
A. Paykani et al. / Energy 90 (2015) 814e826