Remarkable improvement of NOx–PM trade-off in a diesel engine by means of bioethanol and EGR

In order to realize a premixed compression ignition (PCI) engine, the effects of bioethanol–gas oil blends and exhaust gas recirculation (EGR) on PM–NO_x trade-off have been investigated focusing on ignition delay, premixed combustion, diffusion combustion, smoke, NO_x and thermal efficiency. The present experiment was done by increasing the ethanol blend ratio and ethanol and by increasing the EGR ratio in a single cylinder direct injection diesel engine. It is found that a remarkable improvement in PM–NO_x trade-off can be achieved by promoting the premixing based on the ethanol blend fuel having low evaporation temperature, large latent heat and low cetane number as well, in addition, based on a marked elongation of ignition delay due to the low cetane number fuel and the low oxygen intake charge. As a result, very low levels of NO_x and PM, which satisfies the 2009 emission standards imposed on heavy duty diesel engines in Japan, were achieved without deterioration of brake thermal efficiency in the PCI engine fuelled with the 50% ethanol blend diesel fuel and the high EGR ratio. It is noticed that smoke can be reduced even by increasing the EGR ratio under the highly premixed condition.     

Influence of high rates of supplemental cooled EGR on NOx and PM emissions of an automotive HSDI diesel engine using an LP EGR loop

Previous experimental studies on diesel engine have demonstrated the potential of exhaust gas recirculation (EGR) as an in‐cylinder NO_x control method. Although an increase in EGR at constant boost pressure (substitution EGR) is accompanied with an increase in particulate matter (PM) emissions in the conventional diesel high‐temperature combustion (HTC), the recirculation of exhaust gases supplementary to air inlet gas (supplemental EGR) by increasing the boost pressure has been suggested as a way to reduce NO_x emissions while limiting the negative impact of EGR on PM emissions. In the present work, a low‐pressure (LP) EGR loop is implemented on a standard 2.0 l automotive high‐speed direct injection (HSDI) turbocharged diesel engine to study the influence of high rates of supplemental cooled EGR on NO_x and PM emissions. Contrary to initial high‐pressure (HP) EGR loop, the gas flow through the turbine is unchanged while varying the EGR rate. Thus, by closing the variable geometry turbine (VGT) vanes, higher boost pressure can be reached, allowing the use of high rates of supplemental EGR. Furthermore, recirculated exhaust gases are cooled under 50°C and water vapour is condensed and taken off from the recirculated gases. An increase in the boost pressure at a given inlet temperature and dilution ratio (DR) results in most cases an increase in NO_x emissions and a decrease in PM emissions. The result of NO_x–PM trade‐off, while varying the EGR rate at fixed inlet temperature and boost pressure depends on the operating point: it deteriorates at low load conditions, but improves at higher loads. Further improvement can be obtained by increasing the injection pressure. A decrease by approximately 50% of NO_x emissions while maintaining PM emission level, and brake specific fuel consumption can be obtained with supplemental cooled EGR owing to an LP EGR loop, compared with the initial engine configuration (HP moderately cooled EGR). Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental investigation of control of NOx emissions in biodiesel-fueled compression ignition engine

Biodiesel is an alternative fuel consisting of the alkyl esters of fatty acids from vegetable oils or animal fats. Vegetable oils are produced from numerous oil seed crops (edible and non-edible), e.g., rapeseed oil, linseed oil, rice bran oil, soybean oil, etc. Research has shown that biodiesel-fueled engines produce less carbon monoxide (CO), unburned hydrocarbon (HC), and particulate emissions compared to mineral diesel fuel but higher NO_x emissions. Exhaust gas recirculation (EGR) is effective to reduce NO_x from diesel engines because it lowers the flame temperature and the oxygen concentration in the combustion chamber. However, EGR results in higher particulate matter (PM) emissions. Thus, the drawback of higher NO_x emissions while using biodiesel may be overcome by employing EGR. The objective of current research work is to investigate the usage of biodiesel and EGR simultaneously in order to reduce the emissions of all regulated pollutants from diesel engines. A two-cylinder, air-cooled, constant speed direct injection diesel engine was used for experiments. HCs, NO_x, CO, and opacity of the exhaust gas were measured to estimate the emissions. Various engine performance parameters such as thermal efficiency, brake specific fuel consumption (BSFC), and brake specific energy consumption (BSEC), etc. were calculated from the acquired data. Application of EGR with biodiesel blends resulted in reductions in NO_x emissions without any significant penalty in PM emissions or BSEC.     

H2 effects on diesel combustion and emissions with an LPL-EGR system

In this study, we examined H₂ effects on the combustion and emissions of a diesel engine with low-pressure loop (LPL) exhaust gas recirculation (EGR). We converted a 2.2-L four-cylinder direct-injection diesel engine satisfying Euro5 for H₂ supply. An LPL-EGR system replaced the high-pressure loop (HPL) EGR system. For all tests, the brake mean effective pressure (BMEP) was kept at 4 bar and the EGR ratio was varied from 9 to 42%. The H₂ energy percentage was varied from 0 to 7.4% independently to evaluate the H₂ effects and EGR effects separately. The heat release rate was calculated from the measured cylinder pressure. We found that substitution of H₂ for diesel fuel made the premixed burn fraction larger, and reduced the nitrous oxide (NOx) and particulate matter (PM) emissions simultaneously. For example, the NOx emissions were reduced by 36% for an EGR of 42% and an H₂ percentage of 7.4%. PM emissions were reduced by 18% for an EGR of 35% and an H₂ percentage of 7.4% compared with diesel fuel only cases.     

Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine

Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production and is used on most modern high-speed direct injection (HSDI) diesel engines. However EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease of peak heat release, decrease in O₂ concentration (and thus of global air/fuel ratio (AFR)) and flame temperature, increase of lift-off length, etc.), and thus the influence of EGR on NO_x and particulate matter (PM) emissions is not perfectly understood, especially under high EGR rates. An experimental study has been conducted on a 2.0 l HSDI automotive diesel engine under low-load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NO_x/PM emissions. The increase of inlet temperature with EGR has contrary effects on combustion and emissions, thus sometimes giving opposite tendencies as traditionally observed, as, for example, the reduction of NO_x emissions with increased inlet temperature. For a purely diffusion combustion the ROHR is unchanged when the AFR is maintained when changing in-cylinder ambient gas properties (temperature or EGR rate). At low-load conditions, use of high EGR rates at constant boost pressure is a way to drastically reduce NO_x and PM emissions but with an increase of brake-specific fuel consumption (BSFC) and other emissions (CO and hydrocarbon), whereas EGR at constant AFR may drastically reduce NO_x emissions without important penalty on BSFC and soot emissions but is limited by the turbocharging system.     

Effect of exhaust gas recirculation on diesel engine nitrogen oxide reduction operating with jojoba methyl ester

Jojoba methyl ester (JME) has been used as a renewable fuel in numerous studies evaluating its potential use in diesel engines. These studies showed that this fuel is good gas oil substitute but an increase in the nitrogenous oxides emissions was observed at all operating conditions. The aim of this study mainly was to quantify the efficiency of exhaust gas recirculation (EGR) when using JME fuel in a fully instrumented, two-cylinder, naturally aspirated, four-stroke direct injection diesel engine. The tests were carried out in three sections. Firstly, the measured performance and exhaust emissions of the diesel engine operating with diesel fuel and JME at various speeds under full load are determined and compared. Secondly, tests were performed at constant speed with two loads to investigate the EGR effect on engine performance and exhaust emissions including nitrogenous oxides (NO_x), carbon monoxide (CO), unburned hydrocarbons (HC) and exhaust gas temperatures. Thirdly, the effect of cooled EGR with high ratio at full load on engine performance and emissions was examined. The results showed that EGR is an effective technique for reducing NO_x emissions with JME fuel especially in light-duty diesel engines. With the application of the EGR method, the CO and HC concentration in the engine-out emissions increased. For all operating conditions, a better trade-off between HC, CO and NO_x emissions can be attained within a limited EGR rate of 5–15% with very little economy penalty.     

Combining GTL fuel,reformed EGR and HC-SCR aftertreatment system to reduce diesel NOx emissions. A statistical approach

An ultra-low sulphur diesel (ULSD) fuel and a synthetic gas-to-liquid (GTL) fuel, besides different types of standard and reformed EGR, were evaluated in a single-cylinder, direct injection, diesel engine equipped with hydrocarbon-selective catalytic reduction (HC-SCR) aftertreatment system. The results obtained were statistically analysed (at 95% statistical significance) to identify the most significant factors that affect NO_x emissions and to search for the optimum operation conditions in order to minimize these emissions. For that purpose, a fully crossed factorial experimental design was used, including two different engine speeds (1200 and 1500 rpm), two engine loads (25% and 50%), and four EGR/REGR ratios (0%, 10%, 20% and 30%) resulting in almost one hundred tests. An optimal combination of fuel type, REGR type and REGR ratio was proved to reduce around 89–95% of the reference NO_x emissions. In general, at 25% engine load GTL fuelling combined with the reformed EGR with the highest hydrogen content was found the most desirable, as the hydrogen sharply increased the NO_x conversion in the SCR catalyst. Differently, at 50% load standard EGR was sufficient to reach high NO_x reductions. These findings may be used for the implementation of a system on-board capable to switch from EGR to REGR, which will help engine manufacturers to meet the future emission regulations.     

Investigation of the effects of hydrogen on cylinder pressure in a split-injection diesel engine at heavy EGR

The distinctive properties of hydrogen have initiated considerable applied research related to the internal combustion engine. Recently, it has been reported that NO_x emissions were reduced by using hydrogen in a diesel engine at low temperature and heavy EGR conditions. As the continuing study, cylinder pressure was also investigated to determine the combustion characteristics and their relationship to NO_x emissions. The test engine was operated at constant speed and fixed diesel fuel injection rate (1500 rpm, 2.5 kg/h). Diesel fuel was injected in a split pattern into a 2-L diesel engine. The cylinder pressure was measured for different hydrogen flow rates and EGR ratios. The intake manifold temperature was controlled to be the same to avoid the gas intake temperature variations under the widely differing levels (2%-31%) of EGR. The measured cylinder pressure was analyzed for characteristic combustion values, such as mass burn fraction and combustion duration.The rising crank angle of the heat release rate was unaffected by the presence of hydrogen. However, supplying hydrogen extended the main combustion duration. This longer main combustion duration was particularly noticeable at the heavy EGR condition. It correlated well with the reduced NO_x emissions.     

Performance and emission characteristics of a diesel engine with Diesel Premixed Compression Ignition and exhaust gas recirculation

In the present work, diesel was used as a premixed fuel along with the conventional injection of diesel with a premixed ratio of 0.25. The premixed charge was burned in the cylinder along with the fuel directly injected into the cylinder by a conventional injection system. To control nitrogen oxide(s) (NO_x) emissions, Exhaust Gas Recirculation (EGR) was adopted and the exhaust gas was varied from 10% to 30% in steps of 10%. The performance and emission characteristics were compared with conventional 100% diesel injection in the main chamber. Based on the experiments conducted on a Compression Ignition Direct Injection (CIDI) engine, it was found that unburnt hydrocarbons, carbon monoxide, and soot emissions increase. Soot emission decreases with up to 20% EGR and increases when EGR was increased beyond 20%. Hence 20% EGR was found to be the optimum use for DPMCI mode with a premixed ratio of 0.25. Due to the lean operation, significant reduction in NO_x was achieved with the DPMCI combustion mode. Brake thermal efficiency was marginally decreased compared to CIDI mode.     

提高化学计量比汽油机的燃油经济性及降低排放的研究

提出了采用排气再循环同时提高压缩比的方法改善化计量比汽油机的经济性、降低排放的新方法,并在单缸汽油机上进行了试验研究。试验中保持空燃比为化学计量空燃比,通过优化压缩比、涡流比及EGR率,发动机压缩比可以提高到11．8而不发生爆燃,燃油经济性提高5．3％,NOx排放下降50％－60％,HC排放有所上升,但（NOx＋HC）排放下降20％－30％。试验结构表明,采用该方法来改善化学计量比汽油机的性能是行之有效的。     

Experimental study on lean-burn characteristics of an SI engine with hydrogen/gasoline combined injection and EGR

Hydrogen has shown potential for improving the combustion and emission characteristics of the spark ignition (SI) dual-fuel engine. To reduce the additional NO_x emissions caused by hydrogen direct injection, in this research, the cooperative control of the addition of hydrogen with exhaust gas recirculation (EGR) in the hydrogen/gasoline combined injection engine was investigated. The results indicate that both the addition of hydrogen and the use of EGR can increase the brake mean effective pressure (BMEP). As the α_H2 value increases from 0% to 25%, the maximum BMEP increases by 9%, 12.70%, 16.50%, 11.30%, and 8.20%, respectively, compared with the value without EGR at λ = 1.2. The CA0-10 tends to increase with increases in the EGR rate. However, the effect of EGR in increasing the CA0-10 can be offset by the addition of 15% hydrogen at λ = 1.2. Measurements of the coefficient of variation of the indicated mean effective pressure (COV_IMEP) indicate that the addition of hydrogen can effectively extend the EGR limit. Regarding gaseous emissions, NO_x emissions, after the introduction of EGR and the addition of hydrogen, are lower than those of pure gasoline without EGR. An 18% EGR rate yields a significant reduction in NO_x, reaching maximum decreases of about 82.7%, 77.8%, and 60% compared to values without EGR at λ = 1.0, 1.2, and 1.4, respectively. As the EGR rate increases, the hydrocarbon (HC) emissions continuously increase, whereas a blend of 5% hydrogen can significantly reduce the HC emissions at high EGR rates at λ = 1.4. Finally, according to combustion and emissions, the coupling of a 25% addition of hydrogen with 30% EGR at λ = 1.2, and the coupling of a 20% addition of hydrogen with an 18% EGR rate at λ = 1.4 yield the best results.     

Effects of EGR on performance of engines with spark gap projection and fueled by biogas–hydrogen blends

In this study, the effects of exhaust gas recirculation (EGR) on the behavior of a spark ignition engine fueled by hydrogen-blended low-calorific biogas were investigated, and its performance and emission characteristics were compared with those of the lean burn engine investigated in our previous work. The engine was operated at a constant rotational speed of 1800 rpm under a 60 kW power output condition, and a simulated biogas containing H₂ was used to realize a wide range of gas compositions. The engine test results demonstrate that when less than 20% H₂ was added to the biogas, the EGR operations had inferior fuel economy to the lean burn technique. However, when the H₂ blending ratio was increased, the EGR method achieved higher engine performance with lower NO_x emissions than the legal standard. Analyses of the O₂ fraction and thermal capacity variations of the inlet charge also indicated that a dilution (O₂ replacement) effect rather than a thermal effect was the dominant factor when EGR was introduced in a low-calorific biogas engine. Subsequently, in order to improve the engine efficiency as well as combustion characteristics, the spark gap was projected further into the combustion chamber with EGR engine operations. The engine test results show that repositioning the discharge location improved the thermal efficiency, and the maximum tolerable EGR rate increased because of spatial advantages such as relatively short flame propagation lengths and high electrode temperatures.     

Effect of induction on exhaust gas recirculation and hydrogen gas in compression ignition engine with simarouba oil in dual fuel mode

Biofuels extracted from non-edible oil is sustainable and can be used as an alternative fuel for internal combustion engines. This study presents the performance, emission and combustion characteristic analysis by using simarouba oil (obtained from Simarouba seed) as an alternative fuel along with hydrogen and exhaust gas recirculation (EGR) in a compression ignition (CI) engine operating on dual fuel mode. Simarouba biofuel blend (B20) was prepared on volumetric basis by mixing simarouba oil and diesel in the proportion of 20% and 80% (v/v), respectively. Hydrogen gas was introduced at the flow rate of 2.67 kg/min, and EGR concentration was maintained at 30% of total air introduction. Performance, combustion and emission characteristics analysis were examined with biodiesel (B20), biodiesel with hydrogen substitution and biodiesel, hydrogen with EGR and were compared with neat diesel operation. Results indicate that BTE of the engine operating with biodiesel B20 was decreased when compared to neat diesel operation. However, introducing hydrogen along with B20 blend into the combustion chamber shows a slight increase in the BTE by 1%. NO_x emission was increased to 18.13% with the introduction of hydrogen than that of base fuel (diesel) operation. With the introduction of EGR, there is a significant reduction in NO_x emission due to decrease in in-cylinder temperature by 19.07%. A significant reduction in CO, CO_2, and smoke emissions were also noted with the introduction of both hydrogen and EGR. The ignition delay and combustion duration were increased with the introduction of hydrogen, EGR with biodiesel than neat diesel operation. Hence, the proposed biodiesel B20 with H₂ and EGR combination can be applied as an alternative fuel in CI engines.     

Comparison of the effects of EGR and lean burn on an SI engine fueled by hydrogen-enriched low calorific gas

A naturally aspirated spark ignition (SI) engine fueled by hydrogen-blended low calorific gas (LCG) was tested in both exhaust gas recirculation (EGR) and lean burn modes. The “dilution ratio” was introduced to compare their effects on engine performance and emissions under identical levels of dilution. LCG composed of 40% natural gas and 60% nitrogen was used as a main fuel, and hydrogen was blended with the LCG in volumes ranging from 0 to 20%. The engine test results demonstrated that EGR operations at stoichiometry showed a narrower dilution range, inferior combustion characteristics, lower brake thermal efficiency, faster nitrogen oxides (NO_x) suppression, and higher total hydrocarbon (THC) emissions for all hydrogen blending rates compared to lean burn. These trends were mainly due to the increased oxygen deficiency as a result of using EGR in LCG/air mixtures. Hydrogen enrichment of the LCG improved combustion stability and reduced THC emissions while increasing NO_x. In terms of efficiency, hydrogen addition induced a competition between combustion enhancement and increases in the cooling loss, so that the peak thermal efficiency occurred at 10% H₂ with excess air ratio of 1.5. The engine test results also indicated that a close-to-linear NO_x-efficiency relationship occurred for all hydrogen blending rates in both operations as long as stable combustion was achieved. NO_x versus combustion duration analysis showed that adding H₂ reduced combustion duration while maintaining the same level of NO_x. The methane fraction contained in the THC emissions decreased slightly with an increase in hydrogen enrichment at low EGR or excess air dilution ratios, but this tendency was diminished at higher dilution ratios because of the combined dilution effects from the inert gas in the LCG and the diluents (EGR or excess air).     

Development of variable timing fuel injection cam for effective abatement of diesel engine emissions

Fossil fuel run diesel engines are being favored in light, medium and heavy duty applications as they exhibit higher fuel conversion efficiencies. Direct injection diesels are still facing challenges to obtain trade-off between oxides of nitrogen and particulate emissions. There are sophisticated strategies such as common rail direct injection, particulate filters with associated sensors and actuators but limited to expensive comfort vehicles. In the present experimental study, a mechanically operated simple component, variable timing fuel injection cam, is designed for a 510 cc automotive type naturally aspirated, water-cooled, direct injection diesel engine. Modifications in the fuel injection cam and gear train are carried out to suit the existing engine configuration. Variable speed tests are carried out for testing the efficacy of component on both engine and chassis dynamometers for performance and emissions. It is observed that the engine which is already retarded could further be retarded with variable timing fuel injection cam. Significant reductions in NO_x and smoke emission levels are achieved. Combined effect of VIC with 7% EGR could reduce CO by about 88%, HC + NO_x by 37% and PM emissions by 90%. The Engine incorporated with the designed component and EGR, successfully satisfied the existing emission norms with improved power and specific fuel consumption.     

Effects of EGR on combustion process of DI diesel engine during cold start

Experiments on the effects of external and internal exhaust gas recirculation (EGR) on combustion and emission performance during a cold start process were investigated in a 135 single-cylinder DI diesel engine. Combustion was improved during the initial ignition cycles by introducing internal or external EGR. The addition of an appropriate amount of internal or external EGR can promote the combustion stability significantly. However, excessive amounts of external EGR could lead to extremely unstable combustion or even misfiring. An appropriate amount of internal or external EGR decreased smoke opacity effectively during a cold start. External EGR reduced NO _x emissions effectively while internal EGR led to an increase in NO _x emissions due to thermal effects. __________ Translated from Transactions of CSICE, 2007, 25(3): 193–201 [译自:内燃机学报]     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.     

Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine

The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly. When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NO_x is reduced without PM penalty. Stratified EGR potentially reduces NO_x by maximum 45%, without penalties of performance and other emissions. A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NO_x by 7% and PM by 23%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

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 (NO_x) 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 NO_x emissions. High NO_x 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 NO_x 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.     

The influence of Exhaust Gas Recirculation (EGR) on combustion and emissions of n-heptane/natural gas fueled Homogeneous Charge Compression Ignition (HCCI) engines

In Homogeneous Charge Compression Ignition (HCCI) combustion, a lean premixed charge combusts simultaneously in multiple sites. Utilizing highly diluted mixtures, and lack of any significant flame propagation, in-cylinder NO_x formation is reduced. Making HCCI engine a feasible alternative to conventional engines requires several challenges to be resolved. Combustion timing control is one of the most important of these items. It should be done in order that heat is released at the most optimum phasing for efficiency and emissions. In this study, a Waukesha Cooperative Fuel Research (CFR) single cylinder research engine was used to be operated in HCCI combustion mode fueled by natural gas and n-heptane. The main goal of the experiments was to investigate the possibility of controlling combustion phasing and combustion duration using various Exhaust Gas Recirculation (EGR) fractions. For the analysis of the results, a modified apparent heat release model was developed. The influence of EGR on emissions was discussed. Results indicate that applying EGR reduces mean charge temperature and has profound effect on combustion phasing, leading to a retarded Start of Combustion (SOC) and prolonged burn duration. Heat transfer rate decreases with EGR addition. Under examined condition EGR addition improved fuel economy, reduced NO_x emissions and increased HC and CO emissions.