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Efficiency and Emissions Characteristics of Partially Premixed Dual-Fuel Combustion by Co-Direct Injection of NG and Diesel Fuel (DI 2 ) - Part 2
... Furthermore, Westport innovations Inc. had developed a system for a high-pressure direct injection (HPDI) of NG on a co-direct dual-fuel engine. The fuel system is equipped with a concentric-needle injector; diesel and NG can be sprayed from their respective nozzle holes [20][21][22][23][24][25][26]. A small pilot diesel (PRD = 5-10%) could ignite the NG jet, leading to NG combustion with non-premixed combustion [20,21]. ...
... The results showed that the engine brake thermal efficiency was improved by over 2% compared with the non-premixed NG combustion and reduced the CH 4 emissions by 75% compared to an equivalent fumigated dual-fuel engine. In their another study [26], the NG nozzle spray angle was reduced, so additional CH 4 reductions from the crevice region were realized, significant CH 4 emission reduction was achieved, and high brake thermal efficiency was maintained at a high engine load. ...
... Gas and diesel injection can be independently controlled, and the relative timing and injection duration can be changed. Detail specifications of the engine and fuel system can be seen in the literature [26][27][28]. The diesel and NG were directly injected into the cylinder, and the injection pressure of diesel was 10 bar higher than that of NG. Figure 1 shows the schematic diagram of the engine system. ...
The approach for achieving efficient and clean combustion in a diesel–natural gas (NG) heavy-duty engine at low loads was studied by computational fluid dynamics simulation. This study proposed the concentration and temperature-stratified combustion technology and clarified its mechanism. The results revealed that different stratified combustions can be organized by controlling the pressures, timings, and durations of diesel and NG injections, and stratified combustion can be classified into moderate, lean, and rich stratified combustion modes. Efficient and clean combustion can be realized simultaneously at low engine loads: the gross indicated thermal efficiency (ITEg) of engine breakthrough was improved to 47.9%, and the indicated-specific emissions of unburned hydrocarbon (ISUHC) were greatly reduced to 1.6 g/kWh, while the indicated-specific emissions of nitrogen oxide (ISNOx) remained at 0.6 g/kWh. Moreover, the detailed analysis of three typical stratified combustion modes demonstrates that coupling control of the concentration and temperature of the charge is the key to obtaining excellent engine performance. Most of the NG-stratified mixture should burn in the react ratio range of 0.4 to 0.8 for low unburned hydrocarbon emissions, low nitrogen oxides emissions, and rapid combustion. The proper temperature stratification should ensure that a high-temperature charge is around the over-lean NG mixture. This study can provide the fundamentals of stratified combustion control and feasible solutions for commercial applications of NG engines.
... Faghani et al., 13 McTaggart-Cowan et al. 14,15 ), and (ii) stratifiedpremixed PIDING modes where one or more NG injections are performed during the compression stroke to generate highly premixed conditions (e.g. Florea et al., 16,17 Li et al. 18 ). For both these strategies, latecycle SOI pilot is used for fast-response combustion phasing control. ...
... In a follow-up investigation, a narrower NG injection angle was successfully used to reduce unburned CH 4 emissions and increase efficiency for early SOI NG . 17 Similar observations regarding the importance of the piston position for DI fuel mixing processes have been made for wall-guided DISI engines (e.g. Yadollahi and Boroomand, 20 Baratta and Rapetto 21 ). ...
... For À404SOI NG 4 À 30 CAD aTDC, combustion was unstable and significantly advanced SOI pilot was necessary to further decrease RIT. This is indicated by discontinuous lines for each injection control strategy in Figure 3. Florea et al. 16,17 observed the same abrupt . Injection strategy used to maintain constant combustion phasing (see Table 2) across variation of RIT. ...
Natural gas (NG) is an attractive fuel for heavy-duty internal combustion engines because of its potential for reduced CO 2 , particulate, and NO X emissions and lower cost of ownership. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a main direct injection of NG. Recent studies have demonstrated that increased NG premixing is a viable strategy to increase PIDING indicated efficiency and further reduce particulate and CO emissions while maintaining low CH 4 emissions. However, it is unclear how the combustion strategies relate to one another, or where they fit within the continuum of NG stratification. The objective of this work is to present a systematic evaluation of pilot combustion, NG combustion, and emissions behavior of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions. A sweep of the relative injection timing, [Formula: see text], of NG and pilot diesel was performed in a heavy-duty PIDING engine with [Formula: see text] = 140–220 bar, [Formula: see text] = 0.47–0.71, and a constant NG energy fraction of 94%. Apparent heat release rate and emissions analyses identified interactions between the pilot fuel and NG, and qualitatively characterized the impact of NG stratification on combustion and emissions. Changes in the [Formula: see text] resulted in six distinct PIDING combustion regimes, for all considered injection pressures and equivalence ratios: (i) RIT-insensitive premixed, (ii) stratified-premixed (early-cycle injection), (iii) NG jet impingement transition, (iv) stratified-premixed (late-cycle injection), (v) variable premixed fraction, and (vi) minimally-premixed. Parametric definitions for the bounds of each regime of combustion were valid for the wide range of [Formula: see text] and [Formula: see text] investigated, and are expected to be relevant for other PIDING engines, as previously identified regimes agree with those identified here. This conceptual framework encompasses and validates the findings of previous stratified PIDING investigations, including optimal ranges of operation that provide significantly increased efficiency and lower emissions of incomplete combustion products.
... Neely et al. studied the benefits of partially pre-mixed natural gas spray angle being ignited by a pilot of diesel fuel would ignite a previously direct-injected natural gas using a 3D-CFD model [4]. The benefits included an increase from 46.8 to 47.5% efficiency when modifying the nozzle angle at 1000RPM 12 bar BMEP [4]. ...
... Neely et al. studied the benefits of partially pre-mixed natural gas spray angle being ignited by a pilot of diesel fuel would ignite a previously direct-injected natural gas using a 3D-CFD model [4]. The benefits included an increase from 46.8 to 47.5% efficiency when modifying the nozzle angle at 1000RPM 12 bar BMEP [4]. Another benefit of the study showed DI 2 showed up to 50% decrease in combustion losses compared to a fumigated fully pre-mixed NG dual fuel engine [4]. ...
... The benefits included an increase from 46.8 to 47.5% efficiency when modifying the nozzle angle at 1000RPM 12 bar BMEP [4]. Another benefit of the study showed DI 2 showed up to 50% decrease in combustion losses compared to a fumigated fully pre-mixed NG dual fuel engine [4]. ...
Both experimental and simulation efforts have been employed to further advance the design and build of a single cylinder research engine. The engine will aid in advancing technology and understanding the operation of compression-ignition (CI) engines using natural gas within the heavy-duty engine industry. The basis for the engine is a Cummins 15L ISX engine that has been modified, retrofitted, and instrumented to allow for late-cycle direct-injection of high-pressure compressed natural gas. Along with the engine build, a one-dimensional GT-Power simulation model has been created and used to analyze the engine operation and specify components including the engine compression ratio and charging system. The combustion model was calibrated to a kinetic combustion model at multiple speed load points in effort to understand the effect of compression ratio, temperature, and start of injection, on natural gas compression ignition.
... Recent developments in advanced combustion concepts have focused on using two fuels of different propensities to autoignition to achieve high thermal efficiencies and ultra low NOx emissions [6][7][8][9][10]. These combustion concepts introduce a low reactivity fuel e.g. ...
... These dual fuel combustion concepts using natural gas have immense potential to reduce CO2 emissions, however, there is the added risk of "methane slip", i.e. any amount of unburned natural gas that escapes the tailpipe poses a risk. For example, if the engine-out methane emissions are greater than 0.1 g/bhp-hr, then they must be multiplied by a global warming potential (GWP) factor of 34 [10]. A particular scenario of concern occurs at low engine loads where the co-authors and other researchers have observed instances of high engine-out hydrocarbon and carbon monoxide emissions and high cyclic combustion variations [11][12][13][14][15][16][17][18]. ...
Dual fuel diesel-methane low temperature combustion (LTC) has been investigated by various research groups, showing high potential for emissions reduction (especially oxides of nitrogen (NOx) and particulate matter (PM)) without adversely affecting fuel conversion efficiency in comparison with conventional diesel combustion.
However, when operated at low load conditions, dual fuel LTC typically exhibit poor combustion efficiencies. This behavior is mainly due to low bulk gas temperatures under lean conditions, resulting in unacceptably high carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions. A feasible and rather innovative solution may be to split the pilot injection of liquid fuel into two injection pulses, with the second pilot injection supporting the methane combustion once the process is initiated by the first one.
In this work, diesel-methane dual fuel LTC is investigated numerically in a single-cylinder heavy-duty engine operating at 5 bar brake mean effective pressure (BMEP) at 85% and 75% percentage of energy substitution (PES) by methane (taken as a natural gas surrogate). A multidimensional model is first validated in comparison with experimental data obtained on the same single-cylinder engine for early single pilot diesel injection at 310 CAD and 500 bar rail pressure. With the single pilot injection case as baseline, the effects of multiple pilot injections and different rail pressures on combustion emissions are investigated, again showing good agreement with experimental data.
Apparent heat release rate and cylinder pressure histories as well as combustion efficiency trends are correctly captured by the numerical model. Results prove that higher rail pressures yield reductions of HC and CO by 90% and 75%, respectively, at the expense of NOx emissions, which increase by ∼30% from baseline. Furthermore, it is shown that post-injection during the expansion stroke does not support the stable development of the combustion front as the combustion process is confined close to the diesel spray core.
... DF-PCCI provides significant advantages, particularly under high load conditions. Numerous studies have shown that DF-PCCI can operate effectively at load levels comparable to conventional diesel combustion (CDC) [8][9][10]. Furthermore, DF-PCCI demonstrates a substantial potential for reducing CO2 emissions. ...
Internal combustion (IC) engines have long been the dominant technology in transportation, but their environmental impact, particularly in terms of air pollution, has become a growing concern. To address these challenges, advanced combustion technologies like homogeneous-charge compression ignition (HCCI) and dual-fuel premixed-charge compression ignition (DF-PCCI) engines have been developed. This paper explores the potential of HCCI and DF-PCCI engines to serve as more sustainable alternatives to conventional IC engines by analysing their combustion mechanisms, thermal efficiency, and emissions performance. While both engines offer significant environmental benefits, such as lower NOx, particulate matter, and CO2 emissions, they each face unique challenges in terms of fuel injection control, biofuel integration, and combustion stability. DF-PCCI demonstrates advantages in combustion phasing and high-load performance, while HCCI requires further development in fuel injection and exhaust gas recirculation (EGR) systems. Hybridization emerges as a promising solution to improve the flexibility and real-time control of both engines. Ultimately, this paper highlights the potential of HCCI and DF-PCCI engines to significantly reduce transportation-related emissions and contribute to global sustainability goals.
... Research has shown that during the process of engine intake and exhaust, there is an overlap in the intake and exhaust valves; during this process, low reactivity fuels such as natural gas enter the exhaust port without burning, which is known as the blowthrough effect. Neely et al. [22] proposed using a codirect dual-fuel injection to avoid blow-through effect. In dual-fuel engines, premixed low reactivity fuels are more likely to enter the crevice. ...
As a carrier of hydrogen, ammonia has great potential in reducing pollution and carbon. This paper investigates the impact of diesel compression ignition on the combustion and emission performance of a natural gas/ammonia mixture RCCI engine through numerical simulation. The energy fraction of ammonia is set from 0% to 100% in the study to evaluate the feasibility of ammonia application in heavy-duty natural gas/diesel RCCI engines. A natural gas/n-heptane/ammonia reaction mechanism that includes 89 species and 611 reactions was innovatively established, which can accurately simulate the ignition process. The results indicate that under all engine load conditions, as the ammonia energy fraction increases, the cylinder pressure and thermal efficiency of the engine decrease, and the power performance gradually weakens. Under high load conditions, when ammonia is added to premixed natural gas, as the ammonia energy fraction increases, the characteristic frequency near the resonance mode gradually disappears, which is beneficial for reducing the knock trend and ringing intensity of engines. However, due to the increase in incomplete combustion increase, the ammonia energy fraction should not be too large at high loads.
... Diesel is composed of hundreds or thousands of compositions, including alkanes, cycloalkanes, aromatics, etc. [23]. The compositions of diesel are also influenced by the source and production process. ...
To further understand the influence of n-heptane on the ignition process of ammonia, an isotope labeling method was applied in the current investigation to decouple the influence of the chemical effect, the thermal effect, and the effect of O radical from the oxidation of n-heptane on the ignition delay times (IDTs) of ammonia. An analysis of the time evolution of fuel, analysis of the time evolution of temperature, rate of consumption and production (ROP) analysis, and sensitivity analysis were conducted to gain a further understanding of the mechanism of the influence of the chemical effect, the thermal effect, and the effect of O radical on the ignition of ammonia. The results showed that the negative temperature coefficient (NTC) behavior of n-heptane is mitigated by the blending of ammonia, and this mitigated effect of ammonia is mainly due to the chemical effect. The IDTs of ammonia under low and medium temperatures are significantly shortened by the chemical effect at a n-heptane mass fraction of 10%. The promoting effect of the chemical effect decreases when the n-heptane mass fraction increases. The time evolution of n-heptane for NC7H16/ND3-G can be classified into three stages at 800 K, and the rapid consumption stage is mitigated by an increase in temperature. The rapid consumption stage is suppressed by the chemical effect of ammonia, while O radical has a promoting effect on the rapid consumption stage. The chemical effect will enhance the sensitivities of reactions associated with ammonia. As the n-heptane mass fraction increases, the sensitivities of reactions associated with n-heptane are enhanced. Correspondingly, the effect of reactions associated with ammonia is weakened. When the n-heptane mass fraction is 30%, only reactions related to n-heptane have a great influence on the ignition of ammonia/n-heptane fuel blends under the thermal effect + the effect of O radical or only the thermal effect.
... Diesel is composed of hundreds of compounds/ species, ranging from alkanes, cycloalkanes, alkenes, cycloolefin, aromatic, and other species. 19 Furthermore, the source will lead to great differences in the composition of diesel. Therefore, it is difficult to directly investigate the ignition characteristics on the fundamental experimental facilities. ...
Pilot‐ignited ammonia‐fueled engines have drawn more and more attention for low carbon emissions compared to traditional diesel engines. The ignition processes of NH3/NC7H16 mixtures under compression ignition engine‐like conditions are numerically investigated. By comparing the ignition delay times (IDTs) calculated by six ammonia mechanisms with experimental data, the Glarborg mechanism is selected. Then, the Glarborg mechanism and the Zhang detailed n‐heptane mechanism are merged into a new mechanism, which is adopted in the present study. Results show that the negative temperature coefficient behavior of the IDTs is only observed as the ammonia mass fraction is 70%. Only temperature has a significant effect on IDTs at all research conditions, and the effect of ammonia mass fraction is significant when the temperature is lower than 1000 K. However, the effects of equivalence ratio and pressure are small, especially at high temperatures, high equivalence ratios, and high pressures. Interestingly, the IDTs are categorized into three regions by temperature and ammonia mass fraction. The sensitivity analysis indicates that the sensitivity coefficients of most reactions associated with ammonia decrease with a decrease in ammonia mass fraction, whereas only R4210 is sensitive to ammonia mass fraction for n‐heptane‐related reactions. Rates of production and consumption (ROP) analyses indicate that the ammonia mass fraction mainly affects the ROPs of NC7H16, NH3, and NNH at low and medium temperatures, whereas the ammonia mass fraction affects the ROP of H2NO before the temperature of 2000 K. The ROPs of NC7H16, NH3, and NNH significantly increase with increasing temperature, whereas the ROP of H2NO slightly increases with increasing temperature. The increase of temperature in the early and middle stages is mainly contributed by the oxidation of n‐heptane, while the increase of temperature in the middle and late stages is mainly contributed by the oxidation of ammonia.
... In addition to assisted turbocharging, air cooling, and injection shaping, exhaust gas recirculation (EGR) is a control option that is effective to reduce the NOx engine-out emissions [52,69,70]. The EGR reduces the NOx lowering the oxygen concentration in the combustion chamber, as well as absorbing heat. ...
Diesel-LNG internal combustion engines (ICEs) are the most promising light and heavy-duty truck (HDT) powering solution for a transition towards a mixed electric-hydrogen renewable energy economy. The diesel-liquid CH4 ICEs have indeed many commonalities with diesel-liquid H2 ICEs, in the infrastructure, on-board fuel storage, and injection technology, despite the fact H2 needs a much lower temperature. The paper outlines the advantages of dual fuel (2F) diesel-LNG ICEs developed adopting two high-pressure (HP) injectors per cylinder, one for the diesel and one for the LNG, plus super-turbocharging. The diesel-LNG ICEs provide high fuel energy conversion efficiencies, and reduced CO2, PM, and NOx emissions. Super-turbocharging permits the shaping of the torque curve while improving acceleration transients. Diesel-LNG ICEs may also clean up the air of background pollution in many polluted areas in the world. Computational results prove the steady-state advantages of the proposed novel design. While the baseline diesel model is a validated model, the 2F LNG model is not. The perfect alignment of the diesel and diesel-LNG ICE performances proven by Westport makes however the proposed results trustworthy.
... The DI 2 combustion mode is also studied in Neely et al. (2017). The natural gas is injected during the compression stroke prior to the diesel injection ignition. ...
The pros and the cons of lean-burn, compression ignition (CI), direct injection (DI) internal combustion engines (ICE) are reviewed for transport applications. Fueling options considered include diesel only and dual-fuel applications with diesel and a gaseous fuel (CNG, LNG, and LPG). CIDI ICEs have higher fuel conversion efficiencies than stoichiometric, spark ignition (SI) ICEs, whether DI or port fuel injected (PFI). However, diesel-fueled CIDI ICEs have higher particulate matter (PM) and NOx engine-out emissions. The tail-pipe NOx emissions in real-world driving of diesel-powered vehicles have been, in the past, above the limits requested over the simplified cold start driving cycles used for certification. This issue has recently been resolved. The newest diesel-powered vehicles are now compliant with new laboratory test cycles and real-world-driving schedules and have no disadvantages in terms of criteria air pollutants compared to older diesel vehicles, while delivering improvements in fuel economy and CO2 emissions. Dual-fuel CIDI ICEs offer the opportunity for enhanced environmental friendliness. Dual-fuel CIDI ICEs have lower engine-out NOx and PM emissions compared to diesel-only CIDI ICEs. The latest diesel-only vehicles and vehicles with dual-fuel ICEs deliver dramatic reductions in tail-pipe PM emissions compared to older diesel-only vehicles. Moreover, they deliver tail-pipe PM emissions well below the ambient conditions in most city areas that are highly polluted, thereby helping to clean the air. The diesel-fueled CIDI ICEs may be further improved to deliver better fuel economy and further reduced tail-pipe emissions. The dual-fuel CIDI ICE has more room for improvement to produce similar or better steady state and transient performance in terms of torque, power output and fuel conversion efficiency compared to diesel-fueled CIDI ICEs, while drastically reducing CO2 and PM tail-pipe emissions, and improving NOx tail-pipe emissions. This is due to the ability to modulate the premixed and diffusion phases of combustion with a second fuel that is much easier to vaporize and is less prone to auto-ignition. Further development of the fuel injection system for the second fuel will lead to novel dual-fuel CIDI ICE designs with better performance.
... Some other combustion modes have also been studied [93]. A high-pressure direct injection (HPDI) system was developed by Westport Corporation in which diesel was directly injected and combusted before the direct injection of natural gas. ...
Natural gas engines have become increasingly important in transportation applications, especially in the commercial vehicle sector. With increasing demand for high efficiency and low emissions, new technologies must be explored to overcome the performance limitations of natural gas engines such as limits on lean or dilute combustion, unstable combustion, low burning velocity, and high emissions of CH4 and NOx. This paper reviews the progress of research on natural gas engines over recent decades, concentrating on ignition and combustion systems, mixture preparation, the development of different combustion modes, and after-treatment strategies. First, the features, advantages, and disadvantages of natural gas engines are introduced, following which the development of advanced ignition systems, organization of highly turbulent flows, and the preparation of high-reactivity mixtures in spark ignition engines are discussed with a focus on pre-chamber jet ignition, combustion chamber design, and H2-enriched natural gas combustion. Third, the progress in natural gas dual-fuel engines is highlighted, including the exploration of new combustion modes, the development of novel pilot fuels, and the optimization of combustion control strategies. The fourth section discusses after-treatment systems for natural gas engines operating in different combustion modes. Finally, conclusions and future trends in the development of high-efficiency and clean combustion in natural gas engines are summarized.
... However, unburned methane emissions can significantly offset its greenhouse gas benefits if natural gas does not burn to completion. According to EPA's GHG Phase (I) regulation, the GHG impact value of 25 is assigned to any tailpipe methane emissions when computing the total CO 2 equivalent or GHG emissions from the engine [40]. Thus, in the present study, the factor of 25 is considered for the calculation of GHG emissions. ...
... In contrast, more recent studies have utilized common-rail diesel injection systems that have enabled the realization of a variety of diesel-natural gas dual fuel low-temperature combustion (LTC) strategies at low-to-medium loads (Li et al. 2016;Srinivasan et al. 2006a;Raihan et al. 2014;Guerry et al. 2016) and at high loads, including premixed mixture ignition in the end gas region (PREMIER) (Azimov et al. 2011), advanced injection low pilot-ignited natural gas (ALPING) (Krishnan et al. 2004;Srinivasan et al. 2006b), reactivity-controlled compression ignition (RCCI) (Walker et al. 2015). In addition, simultaneous direct injection of natural gas along with diesel using a specialized high-pressure direct injection (HPDI) system has been explored as a pilot-ignited nonpremixed diesel-natural gas dual fuel combustion strategy by several researchers (McTaggart-Cowan et al. 2007Faghani et al. 2017;Neely et al. 2017) and shown capable of maintaining diesel-like performance while deriving most of the power (>90%) from natural gas. ...
Dual fuel combustion is achieved by using a combination of two fuels with extremely different ignition characteristics. For instance, a low-reactivity fuel such as natural gas is compression-ignited using a calibrated amount of appropriately timed, high-pressure, high-reactivity diesel spray. The ensuing combustion occurs at predominantly fuel-lean conditions and is therefore devoid of soot emissions, and the relatively small amount of diesel fuel used also results in the simultaneous reduction in nitrogen oxide emissions. In addition, the use of natural gas, which is predominantly composed of methane, offers the necessary fuel flexibility required to reduce carbon dioxide emissions from conventional neat diesel fired power trains in transportation and power generation applications. The greatest reductions in carbon dioxide emissions are achieved with highest natural gas substitution. However, this also causes problems with high cyclic combustion variations leading to an increased propensity to misfire and high engine-out hydrocarbon emissions. This chapter reviews the current state of the art in strategies to mitigate cyclic combustion variations in dual fuel natural gas engines and provides substantial insights gleaned from past experimental dual fuel combustion research conducted by the authors. In particular, the chapter discusses opportunities and challenges associated with low-temperature dual fuel combustion engines.
div class="section abstract"> Conventional diesel combustion (CDC) is known to provide high efficiency and reliable engine performance, but often associated with high particulate matter (PM) and nitrogen oxides (NOX) emissions. Combustion of fossil diesel fuel also produces carbon dioxide (CO2), which acts as a harmful greenhouse gas (GHG). Renewable and low-carbon fuels such as renewable diesel (RD) and methanol can play an important role in reducing harmful criteria and CO2 emissions into the atmosphere. This paper details an experimental study using a single-cylinder research engine operated under dual-fuel combustion using methanol and RD. Various engine operating strategies were used to achieve diesel-like fuel efficiency. Measurements of engine-out emissions and in-cylinder pressure were taken at test conditions including low-load and high-load operating points. At each engine condition, advanced injection timing showed a reduction in combustion loss, including reductions in carbon monoxide (CO) and unburned hydrocarbons (UHC). A maximum pressure rise rate (MPRR) was set at 15 bar/CAD which limited the advance of injection timing advance at high load. MPRR was below the limit at low load even for the highest methanol substitution rate. The effect of increased intake temperature was also investigated at low load. Higher intake temperatures resulted in reduced combustion losses but also limited the allowable injection timing advance.
In summary, this work (1) demonstrates the feasibility of operating a heavy-duty diesel engine under dual-fuel (RD and methanol) combustion mode, and (2) provides a pathway towards optimized engine and emissions performance.
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Gaseous fuels for heavy-duty internal combustion engines provide inherent advantages for reducing CO 2 , particulate matter (PM), and NO X emissions. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a late-cycle main direct injection of NG, resulting in significant reduction of unburned CH 4 emissions relative to port-injected NG. Previous works have identified NG premixing as a critical parameter establishing indicated efficiency and emissions performance. To this end, a recent experimental investigation using a metal engine identified six general regimes of PIDING heat release and emissions behavior arising from variation of NG stratification through control of relative injection timing (RIT) of the NG with respect to the pilot diesel. The objective of the current work is to provide comprehensive description of in-cylinder fuel mixing of direct injected gaseous fuel and its impacts on combustion and pollutant formation processes for stratified PIDING combustion. In-cylinder imaging of OH*-chemiluminescence (OH*-CL) and PM (700 nm), and measurement of local concentration of fuel is considered for 11 different [Formula: see text], representing 5 regimes of stratified PIDING combustion (performed with [Formula: see text] MPa and [Formula: see text]). The magnitude and cyclic variability of premixed fuel concentration near the bowl wall provides direct experimental validation of thermodynamic metrics ([Formula: see text], [Formula: see text], [Formula: see text]) that describe the fuel-air mixture state of all 5 regimes of PIDING combustion. The local fuel concentration develops non-monotonically and is a function of RIT. High indicated efficiency and low CH 4 emissions previously observed for stratified-premixed PIDING combustion in previous (non-optical) investigations are due to: (i) very rapid reaction zone growth ([Formula: see text] m/s) and (ii) more distributed early reaction zones when overlapping pilot and NG injections cause partial pilot quenching. These results connect and extend the findings of previous investigations and guide the future strategic implementation of NG stratification for improved combustion and emissions performance.
For n-butanol/diesel dual-fuel combustion, there are two different implementations, i.e., dual direct injection (DI²) strategy and reactivity controlled compression ignition (RCCI) strategy. The DI² strategy is a novel combustion concept, which is expected to address the inefficient combustion and high pressure rise rate dilemmas raised by RCCI. In this work, the full-parameter optimizations of DI² and RCCI were first performed at mid load, and the optimal cases were comprehensively compared. The potential benefits of DI² over RCCI and its optimal operating parameters were determined. The results indicate that DI² can achieve low nitrogen oxides (NOx) emissions below 0.00094 g/kWh and near-zero soot emissions while keeping high fuel economy near 167 g/kWh comparable to that of RCCI, but it usually yields higher NOx, soot, and carbon dioxide (CO2) emissions than RCCI. In the optimal DI² cases, very advanced n-butanol injection timing (before −120°CA ATDC), high n-butanol energy fraction (around 0.94), and early diesel injection timing (near −50 °CA ATDC) are adopted. Meanwhile, the combination of a narrow n-butanol spray angle (30°<αbutanol < 50°) and a wide diesel spray angle (>65°) is preferred for DI². Compared with RCCI, DI² exhibits lower combustion losses, however, it suffers from higher exhaust and heat transfer losses. The higher intake temperature, exhaust gas recirculation (EGR) rate, and lower intake pressure are more beneficial for DI². Moreover, it is suggested that DI² is easier to achieve lower ringing intensity (RI), and the ignition timing and RI of DI² are more sensitive to the n-butanol fraction than RCCI.
Natural gas is an attractive fuel for internal combustion engines in light of its potential for reduced greenhouse gas and particulate emissions, and significant reserves. To facilitate natural gas use in compression ignition engines, pilot-ignited direct-injection natural gas combustion uses a small pilot injection of diesel to ignite a more significant direct injection of natural gas. Compared to modern diesel combustion, this strategy is a promising technology for the reduction of CO 2 emissions while retaining diesel-like efficiency without a significant CH 4 emission penalty. To further develop this technology, investigation of in-cylinder combustion processes is needed to identify the primary fuel conversion processes. The objective of this work was to provide a framework of conceptual understanding by identifying key processes in a typical pilot-ignited direct-injection natural gas combustion event and characterizing their sensitivity to fuel injection parameters. A parametric sweep of injection pressure, natural gas injection duration, and relative timing of the diesel pilot and natural gas injections was performed in an optically accessible 2 L single-cylinder engine. Combined heat release rate and OH*-chemiluminescence reaction zone analysis was used to demarcate the transition from ignition reactions to primary natural gas heat release. Five distinct combustion processes were identified: (1) pilot auto-ignition; (2) natural gas ignition; (3) rapid, distributed partially premixed natural gas combustion; (4) non-premixed combustion; and (5) late-cycle oxidation. While natural gas ignition was found to be insensitive to injection pressure, it was strongly affected by the time between pilot and natural gas injections. Reducing the relative injection timing from +8° to −6° resulted in the primary natural gas heat release transitioning from non-premixed, to distributed partially premixed, to stratified premixed flame propagation as a result of increasing natural gas premixing. The presented measurements and analysis serve to refine an initial conceptual model of the combustion process and lay the groundwork for future, more focused studies of pilot-ignited, direct-injection natural gas combustion.
Advanced combustion technologies (ACTs), which have great potential for nitrogen oxides (NOx) and particulate matter (PM) reduction simultaneously, have been researched steadily to replace conventional diesel combustion (CDC) over the past 40 years. However, it is still unclear which ACT is suitable as a practical alternative combustion technology. In this study, single-fueled and dual-fueled ACTs, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) and dual-fuel premixed charge compression ignition (DF-PCCI), were implemented to compare with each other in a heavy-duty single cylinder engine under a specified load condition at which all ACTs can be operated stably. The combustion characteristics, emission performance, thermal efficiency (ηT), and combustion efficiency (ηc) were considered as comparative factors in order for the comparative evaluations of among the three ACTs. The results showed that all ACTs could reduce the NOx and PM emissions simultaneously under the EU-VI NOx and PM regulations without after-treatment system. In addition, DF-PCCI combustion has achieved indicated thermal efficiency (ITE) of 45.3%, which was higher than that for CDC, due to the superior controllability of combustion phase and burn duration. However, DF-PCCI combustion produced high amounts of total hydrocarbon (THC) and carbon monoxide (CO) emissions which could deteriorate the combustion efficiency. These results suggest that although the DF-PCCI combustion technology has various strengths, such as high thermal efficiency and superior combustion controllability, the combustion efficiency should be improved through THC and CO reductions in order to become a practical combustion technology.
Universal concerns about degradation in air quality, stringent emissions regulations, energy scarcity, and global warming have prompted research and development of compressed ignition engines using alternative combustion concepts. Natural gas/diesel dual-fuel combustion is an advanced combustion concept for compression ignition diesel engines, which has attracted global attention in recent years. This combustion concept is accomplished by creating reactivity stratification in the cylinder via the use of two fuels characterized by distinctly different reactivities. The low reactivity and main fuel (i.e., natural gas) is firstly premixed with air and then charged into the cylinder through the intake manifold, and the high reactivity fuel (i.e., diesel) is then injected into the charged mixture through a direct injector. This combustion concept offers prominent benefits in terms of a significant reduction of particulate matter (PM) and sometimes nitrogen oxides (NOx) emissions while maintaining comparable fuel efficiency compared to diesel engine. However, low thermal efficiency and high greenhouse gas (GHG) emissions under low load conditions are major challenges which prevented the implementation of dual-fuel concept in commercial automative engines. The present study investigates different combustion approaches with the aim to enhance combustion performance and reduce emissions of unburned methane, CO, NOx, soot, and GHG of natural gas/diesel dual-fuel engines under different engine load-speed conditions. In particular, the main focus of this thesis is on low load conditions where GHG emissions of conventional natural gas/diesel dual-fuel engine is much higher than that of conventional diesel engine. Alongside the experimental study, a computational fluid dynamic (CFD) model is developed to help understand the behaviour of natural gas/diesel dual-fuel combustion process under different engine load-speed conditions. The studied approaches showed that the fuel efficiency and GHG emissions of natural gas/diesel dual-fuel engine can be significantly improved under low engine load conditions compared to diesel engine. II Acknowledgements
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