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An Experimental Study on the Effect of Intake Pressure on a Natural Gas-Diesel Dual-Fuel Engine
Abstract
Natural gas-diesel dual-fuel (NDDF) combustion can be a viable method to reduce diesel usage in compression ignition (CI) internal combustion engines. Potential benefits of NDDF engines in comparison to conventional diesel engines include decreases in soot and carbon dioxide emissions. This study focuses on the effect of intake pressure on a dual-fuel engine with intake port injected natural gas (NG) and direct injected diesel at two engine operation conditions – low load-high speed and high load-low speed. The research work was performed on a heavy-duty, single-cylinder engine at a NG-diesel energy ratio of approximately 3:1.
The results show that when the intake pressure was increased, the indicated thermal efficiency (ITE) increased for diesel combustion. The trend was similar at the high load-low speed condition but opposite at the low load-high speed condition for NDDF. ITEs of diesel combustion were generally higher than NDDF combustion due to higher unburned hydrocarbon (HC) emissions associated with the latter. For low load-high speed dual-fuel combustion, increasing intake pressure increased the HC and soot emissions, but decreased the nitrogen oxides (NOx) emissions. For high load-low speed case, increasing intake pressure caused the HC emissions to increase and the NOx and soot emissions to decrease. In-cylinder temperature measured at the tip of the diesel injector showed that the injector tip temperatures were higher for NDDF cases compared to diesel cases. These temperatures could be correlated with the combustion phasing and NOx emissions.
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Diesel fueled compression ignition engines are widely used in power generation and freight transport owing to their high fuel conversion efficiency and ability to operate reliably for long periods of time at high loads. However, such engines generate significant amounts of carbon dioxide (CO 2 ), nitrogen oxides (NOx), and particulate matter (PM) emissions. One solution to reduce the CO 2 and particulate matter emissions of diesel engines while maintaining their efficiency and reliability is natural gas (NG)-diesel dual-fuel combustion. In addition to methane emissions, the temperatures of the diesel injector tip and exhaust gas can also be concerns for dual-fuel engines at medium and high load operating conditions. In this study, a single cylinder NG-diesel dual-fuel research engine is operated at two high load conditions (75% and 100% load). NG fraction and diesel direct injection (DI) timing are two of the simplest control parameters for optimization of diesel engines converted to dual-fuel engines. In addition to studying the combined impact of these parameters on combustion and emissions performance, another unique aspect of this research is the measurement of the diesel injector tip temperature which can predict potential coking issues in dual-fuel engines. Results show that increasing NG fraction and advancing diesel direct injection timing can increase the injector tip temperature. With increasing NG fraction, while the methane emissions increase, the equivalent CO 2 emissions (cumulative greenhouse gas effect of CO 2 and CH 4 ) of the engine decrease. Increasing NG fraction also improves the brake thermal efficiency of the engine though NOx emissions increase. By optimizing the combustion phasing through control of the DI timing, brake thermal efficiencies of the order of ∼42% can be achieved. At high loads, advanced diesel DI timings typically correspond to the higher maximum cylinder pressure, maximum pressure rise rate, brake thermal efficiency and NOx emissions, and lower soot, CO, and CO 2 -equivalent emissions.
Natural gas-diesel dual fuel (NDDF) engine has the potential to significantly reduce carbon dioxide (CO2) and particulate matter (PM) emissions, while retaining diesel engine’s high efficiency and reliability. The operation and performance of NDDF engine depend on natural gas energy fraction (%NG), which ranges from low %NG, where diesel fuel provides most of the combustion energy, to high %NG where diesel fuel is used only to initiate combustion. Although pertaining published studies demonstrated that the effect of %NG on the combustion performance and emissions of NDDF engine depend upon the engine load, none of these studies discussed the reasons behind this dependence. The present study attempts to shed light on this issue by experimentally and numerically investigating the effect of different %NGs on NDDF engine under different load conditions. Both experimental and numerical results revealed that the peak pressure rise rate (PPRR) first increases and then drops with increasing %NG under all load conditions. The calculated local equivalence ratio and charge temperature contours showed that increasing %NG to a certain limit increases local equivalence ratio inside the ignition kernel which implies that more premixed natural gas – air mixture participates in reactions during the premixed combustion stage. This results in an increased flame temperature and higher PPRR right after ignition. However, increasing %NG beyond this range decreases local equivalence ratio inside the ignition kernel which leads to lower PPRR. Increasing %NG retards the combustion phasing at low to medium load conditions. However, increasing %NG generally advances the combustion phasing under medium to high load conditions due to the fact that the flame propagation speed of natural gas – air mixture increases with %NG at medium to high load conditions. Methane emissions grow with increasing %NG under all examined engine load conditions. However, this growth becomes much smaller under medium to high engine load conditions. Increasing %NG significantly increases GHG emissions under lower load conditions. However, using advanced diesel injection timing decreases GHG emissions of NDDF engine under lower load conditions. Increasing %NG decreases GHG emissions by 6 to 11% under medium to high load conditions.
Diesel engines have been widely used due to the higher reliability and superior fuel conversion efficiency. However, they still generate significant amount of carbon dioxide (CO2) and particulate matter (PM) emissions. Natural gas is a low carbon and clean fuel that generates less CO2 and PM emissions than diesel during combustion. Replacing diesel by natural gas in internal combustion engines help reduce both CO2 and PM emissions. Natural gas – diesel dual fuel combustion is a practical and efficient way to replace diesel by natural gas in internal combustion engines. One concern for dual fuel combustion engines is the diesel injector tip temperature increase with increasing natural gas fraction.
This paper reports an experimental investigation on the diesel injector tip temperature variation and combustion performance of a natural gas – diesel dual fuel engine at medium and high load conditions. The natural gas fraction was changed from zero to 90% in the experiment. The results suggest that the injector tip temperature increased with increasing natural gas fraction at a given diesel injection timing or with advancing the diesel injection timing at a given natural gas fraction. However, the injector tip temperature never exceeded 250 °C in the whole experimental range. The effect of natural gas fraction on combustion performance depended on engine load and diesel injection timing.
As an inexpensive and low carbon fuel, the combustion of natural gas reduces fuel cost and generates less carbon dioxide emissions than diesel and gasoline. Natural gas is also a clean fuel that generates less particulate matter emissions than diesel during combustion. Replacing diesel by natural gas in internal combustion engines is of great interest for industries. Dual fuel combustion is an efficient way to apply natural gas in internal combustion engines.
An issue that to a certain extent offsets the advantage of lower carbon dioxide emissions in natural gas–diesel dual fuel engines is the higher methane emissions and low engine efficiency at low load conditions. In order to seek strategies to improve the performance of dual fuel engines at low load conditions, an experimental investigation was conducted to investigate the effect of diesel injection split on combustion and emissions performance of a heavy duty natural gas–diesel dual fuel engine at a low load. The operating conditions, such as engine speed, load, intake temperature and pressure, were well controlled during the experiment. The effects of diesel injection split ratio and timings were investigated. The engine efficiency and emissions data, including particulate matter, nitric oxides, carbon monoxide and methane were measured and analyzed. The results show that diesel injection split significantly reduced the peak pressure rise rate. As a result, diesel injection split enabled the engine to operate at a more optimal condition at which engine efficiency and methane emissions could be significantly improved compared to single diesel injection.
Natural gas is an abundant and inexpensive fuel in North America. It produces lower greenhouse gas emissions than diesel fuel when burned in an internal combustion engine. It is also considered to be a clean fuel because it generates lower particulate matter emissions than diesel fuel during combustion.
In this study, an experimental study was conducted to investigate the combustion and emissions performance of a natural gas – diesel dual fuel engine at low and medium loads. A single cylinder direct injection diesel engine was modified to operate as the dual fuel engine. The diesel fuel was directly injected into the cylinder, while natural gas was injected into the intake port. The operating conditions, such as engine speed, load, intake temperature and pressure, were well controlled during the experiment. The effect of natural gas fraction on energy efficiency, cylinder pressure, exhaust temperature, and combustion stability were recorded and analyzed. The emissions data, including particulate matter, nitric oxides, carbon monoxide, and methane at various natural gas fractions and operating conditions were also analyzed. The results showed that natural gas – diesel dual fuel combustion slightly decreased brake thermal efficiency at low and medium load conditions and significantly reduced carbon dioxide and particulate matter emissions. Methane and NOx emissions increased in dual fuel combustion mode compared to diesel operation. The variation of carbon monoxide emissions in dual fuel mode depended on load and speed conditions.
Automotive engineers have proposed various solutions towards the simultaneous reduction of NO and soot emissions from direct injection compression ignition engines, one of which is the use of "natural gas/diesel dual-fuel engines". Natural gas is considered to be a promising alternative fuel for vehicular applications, particularly in urban transit bus fleets due to its clean combustion. It has high auto-ignition temperature compared to other gaseous fuels facilitating thus its use on future and existing compression ignition engine fleets without serious modifications of their structure. According to extended experimental and theoretical investigations conducted in the past by the present research group, natural gas/diesel operation results to curtailment of NO emissions accompanied by significant reduction of soot emissions while it affects negatively engine efficiency and CO emissions compared to normal diesel operation. In an effort to minimize these negative side effects the present research group has investigated various technical solutions and specifically: the quantity of injected liquid fuel and the introduction of exhaust gas recirculation (EGR) to control NOx. In order to be examined the effect of the aforementioned parameters a theoretical investigation has been conducted using a comprehensive two-zone phenomenological model. From the results of the present investigation, important information is derived revealing the applicability of each one of the aforementioned techniques on an existing pilot ignited diesel/natural gas engine. This has been achieved by making a comparative assessment of the relative impact of each technical solution on engine performance characteristics and emitted pollutants. The information derived from the present work is valuable, especially if one wishes to define the optimum combination of examined strategies to improve the behavior of an existing direct injection compression ignition engine miming under natural gas/diesel operating mode. (C) 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of the Programme Committee of the Transport Research Arena 2012
Past research has shown that advancing diesel injection timing is a promising approach to decrease the unburned methane and greenhouse gas (GHG) emissions of natural gas/diesel dual-fuel engines at lower engine loads. However, this benefit may not persist under medium to high load-low speed conditions. To explore this, the present paper uses experiments and detailed computational fluid dynamic (CFD) modeling to investigate the impacts of diesel injection timing on the combustion and emissions performance of a heavy-duty natural gas/diesel dual-fuel engine under four different engine load-speed conditions. The results showed that advancing diesel injection timing increases the peak pressure, thermal efficiency, and NOx emissions for all examined engine load-speed conditions. Advancing diesel injection timing also significantly decreases the unburned methane and CO2-equivalent (GHG) emissions of the dual-fuel engine under low load-low speed and medium load-high speed conditions. The concentration of OH and CH4 revealed that the central part of the combustion chamber is the main source of the unburned methane emissions under low load-low speed and medium load-high speed conditions, and advancing diesel injection timing significantly improves the combustion of natural gas-air mixture in this region. However, advancing diesel injection timing slightly increases the unburned methane emissions trapped in the crevice volume. However, this slight increase in the unburned methane emissions in the crevice volume is much lower than its significant decrease in the central region of the combustion chamber. At medium to high load-low speed conditions, there is almost no unburned methane in the central part of the combustion chamber, and the crevice region is considered as the main source of unburned methane emissions. As a result, advancing diesel injection timing does not improve the combustion of natural gas-air mixture in the central part of the combustion chamber but slightly increases the unburned methane trapped in the crevice region. This is the main reason that advancing diesel injection timing slightly increases the unburned methane emissions under medium to high load-low speed conditions. Overall, advancing diesel injection timing significantly increases thermal efficiency and decreases the unburned methane and GHG emissions under low load-low speed and medium load-high speed conditions. It improves the thermal efficiency under medium to high load-low speed conditions, but comes at the expense of increased methane and unchanged GHG emissions.
The combustion of natural gas reduces fuel cost and generates less emissions of carbon dioxide and particulate matter (PM) than diesel and gasoline. Replacing diesel by natural gas in internal combustion engines is of great interest for transportation and stationary power generation. Dual fuel combustion is an efficient way to burn natural gas in internal combustion engines. In natural gas-diesel dual fuel engines, unburned hydrocarbon emissions increase with increasing natural gas fraction. Many studies have been conducted to improve the performance of natural gas-diesel dual fuel engines and reported the performance of combustion and emissions of regulated pollutants and total unburned hydrocarbon at various engine operating strategies. However, little has been reported on the emissions of different unburned hydrocarbon components. In this paper, an experimental investigation was conducted to investigate the combustion performance and emissions of various unburned hydrocarbon components, including methane, ethane, ethylene, acetylene, propylene, formaldehyde, acetaldehyde, and benzaldehyde, at a low engine load condition. The operating conditions, such as engine speed, load, intake temperature, and pressure, were well controlled during the experiment. The combustion and emissions performance of pure diesel and natural gas-diesel dual fuel combustion were compared. The effect of diesel injection timing was analyzed. The results show that appropriately advancing diesel injection timing to form a homogeneous charge compression ignition (HCCI)-like combustion is beneficial to natural gas-diesel dual fuel combustion at low load conditions. The emissions of different unburned hydrocarbon components changed in dual fuel combustion, with emissions of some unburned hydrocarbon components being primarily due to the combustion of natural gas, while those of others being more related to diesel combustion.
Natural gas/diesel dual-fuel combustion is currently one of the most promising LTC strategies for the next generation of heavy-duty engines. While this concept is not new and it has been deliberated lengthily in the past two decades, several uncertainties still exist. A major shortcoming of this concept is associated with its low thermal efficiency and high level of unburned methane and CO emissions under low engine load conditions. The present paper reports an experimental and numerical study on the effect of different injection strategies (single and two pulses injection of pilot diesel fuel) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. The results of single diesel injection mode showed that advancing diesel injection timing from 10 to 30 °BTDC reduced unburned methane and CO emissions by 62% and 61% and increased thermal efficiency by 6%; however, NOx emissions increased by 74%. In order to achieve NOx – CH4 and NOx – CO trade-off and increased thermal efficiency at low load conditions, the effect of split injection strategy was experimentally and numerically examined. The results of split injection mode revealed that split injection strategy considerably increases the in-cylinder peak pressure compared to that of single injection (10 °BTDC). The results showed also that the heat release produced by the first injection of diesel fuel considerably increased the in-cylinder charge temperature before the start of the second injection. The flame zone of the split injection mode is markedly higher than that of the single injection due to larger heat release produced during the first injection which promotes the combustion of the second one. When the first injection timing is close to the second injection timing, the MPRR of split injection mode is higher than that of single injection (10 °BTDC). However, further advancing of the first injection timing continuously decreased the MPRR. OH radical analysis showed that for advanced first injection timings (38-50 °BTDC), the overall growth rate of OH radical becomes slower and its distribution is narrower as indicated by the wider non-reactive blue zones compared with those observed at a late first injection timing in the initial stages of combustion. However, OH radicals gradually grow during last stages of combustion in the expansion stroke, indicating that a more premixed combustion takes place in these cases. For very advanced first injection timing of 55 °BTDC, the OH distribution is similar to that of the single injection mode with lower OH intensity at initial stages of combustion and they barely grow during the late expansion stroke. At this condition, the ignition of premixed mixture is mainly controlled by the second diesel fuel injection. The trade-off between NOx – CH4 and NOx – CO is achieved when applying split injection. Compared to single injection (10 °BTDC), the first injection timing of 50 °BTDC decreased unburned methane and CO emissions by 60% and 63%, respectively, and increased the thermal efficiency by 8.9%. However, NOx emissions were maintained at the same level as single injection mode (10 °BTDC).
Natural gas/diesel dual-fuel combustion compression ignition engine has the potential to reduce NOx and soot emissions. However, this combustion mode still suffers from low thermal efficiency and high level of unburned methane and CO emissions at low load conditions. The present paper reports the results of an experimental and numerical study on the effect of diesel injection timings (ranging from 10 to 50 °BTDC) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. Both experimental and numerical results revealed that advancing the injection timing up to 30 °BTDC increases the maximum in-cylinder pressure. However, with further advancing the injection timing up to 50 °BTDC, the maximum in-cylinder pressure decreases which is mainly due to the lower in-cylinder temperature before SOC. Moreover, the analysis of OH spatial distribution shows that, at very advanced diesel injection timings, the non-reactive zones are much narrower than later injection timings during the last stages of combustion, indicating a more predominant premixed combustion mode. At retarded diesel injection timings, the consumption of premixed fuel in the outer part of the charge is likely to be a significant challenge for dual-fuel combustion engine at low engine load conditions. However, with advancing the diesel injection timing, the OH radical becomes more uniform throughout the combustion chamber, which confirms that high temperature combustion reactions can occur in the central part of the charge. Diesel injection timing of 30 °BTDC appears to be the conversion point of all conventional dual-fuel combustion modes. Further advancing diesel injection timing beyond this point (30 °BTDC) results in noticeable reduction in NOx and unburned methane emissions, while CO emissions exhibit only slight drop. However, at very advanced diesel injection timings of 46 and 50 °BTDC, NOx, and unburned methane emissions continue to drop, whereas and CO emissions tend to increase. The results showed also that the highest indicated thermal efficiency is achieved at these very advanced diesel injection timings of 46 and 50 °BTDC. Finally, the results revealed that, by advancing diesel injection timing from 10 °BTDC to 50 °BTDC, NOx, unburned methane, and CO emissions are reduced, respectively, by 65.8%, 83%, and 60% while thermal efficiency is increased by 7.5%.
This review paper covers potential alternative fuels for automotive engine application for both spark ignition (SI) and compression ignition (CI) engines. It also includes applications of alternative fuels in advanced combustion research applications. The representative alternative fuels for SI engines include compressed natural gas (CNG), hydrogen (H2) liquefied petroleum gas (LPG), and alcohol fuels (methanol and ethanol); while for CI engines, they include biodiesel, di-methyl ether (DME), and jet propellent-8 (JP-8). Naphtha is introduced as an alternative fuel for advanced combustion in premixed charge compression ignition. The production, storage, and the supply chain of each alternative fuel are briefly summarized, and are followed by discussions on the main research motivations for such alternative fuels. Literature surveys are presented that investigate the relative advantages and disadvantages of these alternative fuels for application to engine combustion. The contents of engine combustion basically consist of the combustion process from spray development, air–fuel mixing characteristics, to the final combustion product formation process, which is analyzed for each alternative fuel. An overview is provided for alternative fuels together with summaries of engine combustion characteristics for each fuel, in addition to its current distribution status and future prospects.
Stricter emission limitations for NOx and particulates in mobile applications will require the use of active aftertreatment methods like Diesel Particulate Filters (DPF), Selective Catalytic Reduction (SCR) with urea and Lean NOx Trap (LNT) as combinations in the 2010's. Due to the significant total space and required investments, a lot of efforts have been focused recently on the optimization of the combinatory aftertreatment systems (ATS). In this study the possibilities to intensify the catalytic ATS were analyzed and reviewed by the examples and studies with engines, laboratory reactors and simulations. The focus was on diesel applications, where the number of needed ATS units is the widest. The diesel engine modifications on SCR or EGR engines have to be also designed together with ATS. The intensification includes the principles of down-sizing and the integration of ATS units with control systems. The reaction engineering of catalytic reaction and regenerations (DPF, LNT) was also analyzed. The intensification covered the examples about the integration of DOC, LNT and SCR on DPF, high cell density substrates (metallic) and the integration of hydrolysis and ammonia slip catalysis on the SCR unit. The intensification cases were analyzed by the observed catalytic activities, pressure drop, the volume of systems and costs. The LNT coating on DPF gave a promising possibility to integrate PM, nitrate and sulfate regeneration methods and OBD. The integration of control units and their functions is also a modern way to keep emissions, complexity and costs low. The use of natural gas as fuel results in low PM emissions and no DPF is needed in Euro 6. However, the intensification of ATS may result in a stoichiometric engine with a three-way catalyst instead of a lean combustion engine with oxidation and SCR catalysts in heavy-duty natural gas applications.
Diesel engine emissions of NO X and particulate matter (PM) can be reduced simultaneously through the use of high levels of exhaust gas recirculation (EGR) to achieve low temperature combustion (LTC). These reductions are highly dependent on the oxygen concentration in the combustion chamber. This paper investigates varying the intake pressure to adjust the oxygen concentration and the corresponding impacts on emissions for EGR rates up to 65%. An engine operating condition corresponding to 600 kPa gross indicated mean effective pressure (gimep) at 2500 rpm is investigated using a 0.51 litre single cylinder high speed direct injection (HSDI) diesel engine. This facility is equipped with independent control of the intake pressure and temperature, the EGR rate and the exhaust back pressure. This work focuses on investigating the potential of increasing EGR tolerance at medium load by increasing the boost pressure from 120 kPa to 180 kPa (absolute), while keeping PM and NO X emissions at acceptable levels. This paper also presents an investigation into the effects of injection timing and injection pressure on diesel LTC. The combined effects of EGR rate and intake pressure on PM, NO X and HC emissions are reported along with gross indicated specific fuel consumption (gisfc).
A single-cylinder, light-duty, diesel engine was used to investigate the effect of changes in intake pressure (boost) on engine performance and emissions in low-temperature combustion (LTC) regimes. Two different LTC strategies were examined: a dilution-controlled regime characterized by high rates of exhaust gas recirculation (EGR) with early-injection (roughly 30° BTDC), and a late-injection (near TDC) regime employing moderate EGR levels. For both strategies, moderate (8 bar IMEP) and low (3 bar IMEP) load conditions were tested at intake pressures of 1.0, 1.5, and 2.0 bar. For both LTC strategies, increased intake pressure reduces emissions of unburned hydrocarbons (UHC) and CO, with corresponding improvements in combustion efficiency and indicated specific fuel consumption (ISFC), particularly at high load. Depending on the operating condition, UHC and CO emissions can stem from either over-lean or over-rich mixtures. UHC emissions can be further impacted by fuel from quench layers and liquid films. Increased intake pressure also reduces peak soot emissions at high load and shifts the peak soot emissions (the soot "bump") towards lower oxygen concentrations. Due to this shift, the influence of intake pressure on soot emissions differs for different oxygen concentrations. Soot emissions are reduced with increased intake pressure at high oxygen concentrations, but increased at low oxygen concentrations. Already low NOx levels are reduced further at high intake pressures, though the influence of intake pressure is small compared to the influence of oxygen concentration. Comparisons of the two LTC strategies at a fixed NOx emission index of 0.5 g/kg-fuel show that late-injection LTC offers improvements in engine noise and soot over dilution-controlled LTC. Conversely, dilution-controlled LTC yields lower emissions of UHC and CO and better combustion efficiency and ISFC than late-injection LTC.
In this paper, a common rail diesel research engine was converted to operate in dual-fuel mode and extensive experiments were conducted to investigate the effects of natural gas injection timing on the combustion and emissions performance under different pilot injection pressure and timing at low load conditions. The presented results include the cylinder pressure, heat release rate (HRR), ignition delay, combustion duration and brake thermal efficiency, as well as CO, HC and NOx emissions at different natural gas injection timing under pilot injection pressure (46 and 72 MPa) and pilot injection timing (−8° and −17° ATDC) operation conditions at low load (BMEP = 0.24 MPa). The results indicated that retarded natural gas injection timing can achieve a stratified-like air-fuel mixture in cylinder under the different pilot injection conditions, which provided a method to improve the combustion performance and exhaust emissions at low load. Moreover, under higher pilot injection pressure (72 MPa) conditions, better combustion performance, such as shorter ignition delay and combustion duration, higher brake thermal efficiency, were achieved; however, the exhaust emissions significantly increased compared with those under lower pilot injection pressure (46 MPa). On the other hand, under the advanced pilot injection timing (−17° ATDC), the combustion performance was radically better, THC and CO emissions were lower but the NOx emissions were significantly higher compared with those under the regular pilot injection timing (−8° ATDC). This is attributed to faster flame propagation speed, better combustion phasing and higher volumetric efficiency. Consequently, employing appropriate natural gas injection timing accompanied with reasonable pilot injection parameters is critical to further improve combustion performance and exhaust emissions of a dual-fuel engine at low loads.
A comparative study on the effects of air–fuel mixing quality on combustion characteristics was carried out in both conventional and low temperature diesel combustion (LTC) regimes. The injection pressure and intake pressure were considered as variables as they are important factors which influence the air–fuel mixing process. The intake O2 concentration was varied to realize different combustion regimes. Improved air–fuel mixing with a higher injection pressure enhanced the combustion process in both conventional combustion and LTC regimes, resulting in higher peaks of in-cylinder pressure and heat release rate. The combustion phase in the LTC regime was more influenced by injection pressure due to longer premixing time than that of conventional combustion. A higher injection pressure reduced CO and HC emissions over a wide range of intake O2 concentrations. The reduction of CO and HC emissions in the conventional combustion regime was due to higher combustion temperature, while that in the LTC regime was due to decreased under-mixed fuel by improved air–fuel mixing. Soot emissions at a higher injection pressure were reduced, particularly, in the conventional combustion regime where the soot formation rate is high. The increase of intake pressure was also advantageous in reducing CO, HC and soot emissions due to improved air–fuel mixing as well as enrichment of absolute amount of oxygen, which lead to enhanced combustion process. A direct flame image was taken to observe the flame structure of two different combustion regimes to correlate with the exhaust emission results and combustion characteristics. High flame luminosity was observed around the periphery of the spray jet in the conventional combustion regime, which was a direct indication of soot formation and high temperature combustion; while low luminosity was observed around the piston bowl in the swirl direction in the LTC regime, which indicated a longer air–fuel mixing period and low temperature combustion.
Natural gas is a potential alternative to conventional liquid fuels for utilization in automotive internal combustion engines. Its utilization can significantly reduce the levels of nitrogen oxide and particulate matter emissions from dual-fuel diesel/natural-gas engines. However, this reduction in the particulate matter emissions may induce an increase in the quantity of particles of smaller size, which are harmful to human health. The engine parameters, namely the diesel injection timing and the pilot quantity directly, affect the homogeneity of the fuel–air mixture, the spatial distribution of initial ignition centres and the quantity of initial ignition centres. In this paper, the effects of these two engine parameters on the combustion and the particle emission characteristics of micro-diesel pilot-ignited natural-gas fuel are investigated. The combustion parameters were calculated from the cylinder pressure data. An electrical low-pressure impactor was employed to measure the particle number concentration and particle size distribution. Experimental results indicate that the ignition delay period is prolonged and the particle number concentration progressively increases with advance of the injection timing, while the particle mass concentration undergoes a steady decrease and then a rapid increase. On the other hand, increasing the pilot diesel quantity can shorten the ignition delay period and enhance the burning rate. Meanwhile, the particle mass concentration increases evidently but the particle number concentration decreases unexpectedly. It should be noted that the total number of fine particles is sensitive to advancing injection and decreasing pilot mass, especially above the 20° crank angle before top dead centre injection timing and below the basic pilot mass.
The performance and emissions of a single-cylinder, natural gas fueled engine using a pilot ignition strategy have been investigated. Small diesel pilots (2–3 percent on an energy basis), when used to ignite homogeneous natural gas-air mixtures, are shown to possess the potential for reduced NOx emissions while maintaining good engine performance. The effect of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling was studied. With appropriate control of the above variables, engine-out brake specific NOx emissions could be reduced to the range of 0.07–0.10 g/kWh from the baseline diesel (with mechanical fuel injection) value of 10.5 g/kWh. For this NOx reduction, the decrease in fuel conversion efficiency from the baseline diesel value was approximately 1–2 percent. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. Heat release schedules obtained from measured cylinder pressure data are also presented. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.
Homogeneous charge compression ignition (HCCI) combustion is a spontaneous multi-site auto-ignition of a lean premixed fuel–air mixture, which has high heat release rate, short combustion duration and no evidence of flame propagation. In HCCI engines, there is no direct control method for the time of auto-ignition. Auto-ignition timing should be controlled in order to make heat release process take place at the appropriate time in the engine cycle. Heat release analysis is a diagnostic tool which aids engine experimenters. It facilitates the endeavors being conducted in obtaining a control method by investigating heat release rate and also cumulative heat release. This study can be divided into two parts. First, traditional first law heat release model which is widely used in engine combustion analysis was presented and the applicability of this model in HCCI engines was investigated. Second, a new heat release model based on the first law of thermodynamics accompanying with a temperature solver was developed and assessed. The model was applied in four test conditions with different operating conditions and a variety of fuel compositions, including i-octane, n-heptane, pure NG, and at last, a dual fueled case of NG and n-heptane. Results of this work indicate that utilizing the modified first law heat release model together with a solver for temperature correction will guarantee obtaining a well-behaved and accurate apparent heat release trend and magnitude in HCCI combustion engines.
The effect of pressure on soot formation in a laminar ethylene/air coflow diffusion flame was investigated by experiments and numerical simulation at pressures from 1 to 8 atm. Soot volume fraction was measured by the diffuse-light two-dimensional line-of-sight attenuation optical diagnostic method and calculated by moments model and a relatively detailed gas phase chemistry. The numerical model successfully captured the variation of soot volume fraction with increasing pressure. The detailed analysis of numerical simulation results suggests that although the rates of all soot formation sub-processes increase with increasing pressure, the rates of increase of these sub-processes differ, with that of PAH condensation being the fastest, followed by those of inception and acetylene addition, respectively. At atmosphere pressure, acetylene addition contributes most to soot formation in terms of the overall formed soot mass, while the contribution of PAH condensation significantly increases at high pressures. The variation in the soot formation mechanism is due to the different increase rates of the mole concentrations and formation of several key species that significantly affect soot inception, acetylene addition and PAH condensation. Crown
Petroleum resources are finite and, therefore, search for their alternative non-petroleum fuels for internal combustion engines is continuing all over the world. Moreover gases emitted by petroleum fuel driven vehicles have an adverse effect on the environment and human health. There is universal acceptance of the need to reduce such emissions. Towards this, scientists have proposed various solutions for diesel engines, one of which is the use of gaseous fuels as a supplement for liquid diesel fuel. These engines, which use conventional diesel fuel and gaseous fuel, are referred to as 'dual-fuel engines'. Natural gas and bio-derived gas appear more attractive alternative fuels for dual-fuel engines in view of their friendly environmental nature. In the gas-fumigated dual-fuel engine, the primary fuel is mixed outside the cylinder before it is inducted into the cylinder. A pilot quantity of liquid fuel is injected towards the end of the compression stroke to initiate combustion. When considering a gaseous fuel for use in existing diesel engines, a number of issues which include, the effects of engine operating and design parameters, and type of gaseous fuel, on the performance of the dual-fuel engines, are important. This paper reviews the research on above issues carried out by various scientists in different diesel engines. This paper touches upon performance, combustion and emission characteristics of dual-fuel engines which use natural gas, biogas, producer gas, methane, liquefied petroleum gas, propane, etc. as gaseous fuel. It reveals that 'dual-fuel concept' is a promising technique for controlling both NOx and soot emissions even on existing diesel engine. But, HC, CO emissions and 'bsfc' are higher for part load gas diesel engine operations. Thermal efficiency of dual-fuel engines improve either with increased engine speed, or with advanced injection timings, or with increased amount of pilot fuel. The ignition characteristics of the gaseous fuels need more research for a long-term use in a dual-fuel engine. It is found that, the selection of engine operating and design parameters play a vital role in minimizing the performance divergences between an existing diesel engine and a 'gas diesel engine'.
The effects of pressure and gravity on the sooting characteristics and flame structure of coflow methane–air laminar diffusion flames between 1 and 60 atm were studied numerically. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows which include complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which was verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed under both normal- and zero-gravity conditions to help separate and identify the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the sooting behavior and structure of the flames through their effects on buoyancy and temperature. Zero-gravity flames generally have lower temperatures, broader soot-containing zones, and higher soot volume fractions than normal-gravity flames at the same pressure. Buoyancy forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures caused the zero-gravity flames to lengthen. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. In the zero-gravity flames, increasing pressure beyond 20 atm caused the maximum carbon conversion factor to decrease.
Experimental Analysis of Diesel-Ignited Methane Dual-Fuel Low-Temperature Combustion in a Single-Cylinder Diesel Engine
- M S Raihan
- E S Guerry
- U Dwivedi
- K K Srinivasan
- S R Krishnan
Raihan, M. S., Guerry, E. S., Dwivedi, U., Srinivasan, K. K., and Krishnan, S. R.,
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