Combustion and emission characteristics of a lean burn natural gas engine

Lean burn is an effective way to improve spark ignition engine fuel economy. In this paper, the combustion and emission characteristics of a lean burn natural gas fuelled spark ignition engine were investigated at various throttle positions, fuel injection timings, spark timings and air fuel ratios. The results show that ignition timings, the combustion duration, the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) and engine-out emissions are dependent on the overall air fuel ratio, spark timings, throttle positions and fuel injection timings. With the increase of the air fuel ratio, the ignition delays and combustion duration increases. Fuel injection timings affect ignition timings, combustion duration, IMEP, and the COV of the IMEP. Late fuel injection timings can decrease the COV of the IMEP. Moreover, the change in the fuel injection timings reduces the engine-out CO, total hydrocarbon (THC) emissions. Lean burn can significantly reduce NOx emissions, but it results in high cyclic variations.     

Combined effect of boost pressure and injection timing on the performance and combustion of CNG in a DI spark ignition engine

There is an increasing interest in supercharging spark ignition engines operating on CNG (compressed natural gas) mainly due to its superior knock resisting properties. However, there is a penalty in volumetric efficiency when directly injecting the gaseous fuel at early and partial injection timings. The present work reports the combined effects of a small boost pressure and injection timing on performance and combustion of CNG fueled DI (direct injection) engine. The experimental tests were carried out on a 4-stroke DI spark ignition engine with a compression ratio of 14. Early injection timing, when inlet valves are still open (at 300°BTDC), and partial injection timing, in which part of the injection occurs after the inlet valves are closed (at 180°BTDC), were varied at each operating speed with variation of the boost pressure from 2.5 to 10 kPa. A narrow angle injector (NAI) was used to increase the mixing rate at engine speeds between 2000 and 5000 rpm. Similar experiments were conducted on a naturally aspirated engine and the results were then compared with that of the boosting system to examine the combined effects of boost pressure and injection timing. It was observed that boost pressure above 7.5 kPa resulted in an improvement of performance and combustion of CNG DI engine at all operating speeds. This was manifested in the faster heat release rates and mass fraction burned that in turn improved combustion efficiency of the boosting system. An increased in cylinder pressure and temperature was also observed with boost pressure compared to naturally aspirated engine. Moreover, the combustion duration was reduced due to concentration of the heat release near to the top dead center as the result of the boost pressure. Supercharging was also found to reduce the penalty of volumetric efficiency at both the simulated port and partial injection timings.     

Effect of a 2-stage injection strategy on the combustion and flame characteristics in a PCCI engine

Recently, to reduce environmental pollution and the waste of limited energy resources, there is an increasing requirement for higher engine efficiency and lower levels of harmful emissions. A premixed charge compression ignition (PCCI) engine, which uses a 2-stage type injection, has drawn attention because this combustion system can simultaneously reduce the amount of NOx and PM exhausted from diesel engines. It is well known that the fuel injection timing and the spray angle in a PCCI engine affect the mixture formation and the combustion. To acquire two optimal injection timings, the combustion and emission characteristics of the PCCI engine were analyzed with various injection conditions. The flame visualization was performed to validate the result obtained from the engine test. This study reveals that the optimum injection timings are BTDC 60° for the first injection and ATDC 5° for the second injection. In addition, the injection ratio of 3 to 7 showed the best NOx and PM emission results.     

TJ376Q二气门汽油机准均质稀混合气燃烧实验研究

本文对夏利TJ376Q汽油机进气系统进行了改造，大大提高了涡流和滚流比，强化空气运动的结果有利于组织燃料在缸内的浓度分布，从而为在二气门汽油机上实现稀燃烧打下基础；原发动机油器式供油系统被改为电控气道内燃油喷射系统。在此基础上，采用了两次燃油喷射技术。通过对这两次喷油时刻、喷油量的分别调节，在缸内形成精细分层的混合气即所谓准均质混合气，从而优化了油耗和排放指标，成功地在产品二气门汽油机上实现了稀薄燃烧。     

Exhaust emissions and its control methods in compression ignition engines: A review

Extensive usage of automobiles has certain disadvantages and one of them is its negative effect on environment. Carbon dioxide (CO₂), carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NO_x), sulphur dioxide (SO₂) and particulate matter (PM) come out as harmful products during incomplete combustion from internal combustion (IC) engines. As these substances affect human health, regulatory bodies impose increasingly stringent restrictions on the level of emissions coming out from IC engines. This trend suggests the urgent need for the investigation of all aspects relevant to emissions. It is required to modify existing engine technologies and to develop a better after-treatment system to achieve the upcoming emission norms. Diesel engines are generally preferred over gasoline engines due to their undisputed benefit of fuel economy and higher torque output. However, diesel engines produce higher emissions, particularly NO_x and PM. Aftertreatment systems are costly and occupy more space, hence, in-cylinder solutions are preferred in reducing emissions. Exhaust gas recirculation (EGR) technology has been utilized previously to reduce NO_x. Though it is quite successful for small engines, problem persists with large bore engines and with high rate of EGR. EGR helps in reducing NO_x, but increases particulate emissions and fuel consumption. Many in-cylinder solutions such as lower compression ratios, modified injection characteristics, improved air intake system etc. are required along with EGR to accomplish the future emission norms. Modern combustion techniques such as low temperature combustion (LTC), homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) etc. would be helpful for reducing the exhaust emissions and improving the engine performance. However, controlling of autoignition timing and achieving wider operating range are the major challenges with these techniques. A comprehensive review of diesel engine performance and emission characteristics is given in this paper.     

二甲醚/甲醇混合燃料燃烧数值模拟研究

均质充量压缩着火燃烧(HCCI)技术的提出为内燃机的发展开辟了一种更为节能高效、绿色环保的新模式,着火性能差异较大的两种燃料掺混是实现均质混合压燃着火控制的有效方法。文章利用CHEMKIN化学反应动力学模拟软件对二甲醚(DME)/甲醇混合燃料均质混合压燃燃烧过程进行了数值模拟研究,重点分析了燃料掺混比、过量空气系数、发动机转速以及进气温度对HCCI发动机燃烧特性的影响规律。     

喷油控制参数对中置喷油器GDI发动机喷雾及燃烧的影响

针对匹配中置高压喷油器的直喷汽油光学发动机,试验研究了不同喷油时刻及喷油压力下的缸内燃烧及喷雾发展特性,分析了燃油喷射控制参数对直喷汽油机缸内喷雾及燃烧的影响规律。研究结果表明:随第三段喷油时刻(θ_(SOI3))提前,燃烧持续期与滞燃期均先减小后增大,燃烧特征参数均在θ_(SOI3)=120°BTDC时存在明显拐点,此时平均指示压力(p_(mi))的循环变动系数C_(OVpmi)相对较小;第三段喷油时刻过晚,活塞上行距上止点较近,易导致油束冲击活塞表面;提高喷油压力可缩短燃烧持续期,有助于改善燃烧定容度,但喷油压力过大,油束贯穿距进一步延长,油束冲击缸壁的倾向增加,滞燃期及燃烧持续期反而延长。     

车用稀燃增压单一燃料CNG发动机电控系统的研究

为深入研究 CNG发动机稀薄燃烧特性 ,进一步降低天然气发动机的有害排放 ,设计了车用增压天然气发动机电控系统。该系统采用高能点火模块来获得天然气着火所需的点火能量 ,电控喷射单元采用模糊控制器来获得比较精确的空燃比控制 ,实现对天然气发动机的顺序多点喷射。运用该系统对增压单一燃料 CNG发动机的燃烧和排放性能进行了试验研究 ,结果表明了该系统的有效性。     

稀燃快燃点火电路研究

在电容储能点火系统的基础上,利用火花能量转换原理和能量叠加原理,提出了一种有别于传统发动机点火系的"稀燃快燃点火系"。简要介绍和分析了该点火系的组成及工作原理,对其进行了设计研究,通过试验验证了稀燃快燃点火系比传统点火系具有的优越性。结果表明:该点火系统能够提高点火线圈的次级电压,增加火花持续时间,有效提高点火能量的利用率,改善发动机点火性能。该点火系在进行适当匹配后不仅适用于现代高速、稀燃、高压缩比发动机,而且也适用于传统点燃式发动机。     

Effects of altitude on combustion characteristic during cold start of heavy-duty diesel engine

Altitude has a significant effect on combustion of heavy-duty diesel engines, especially during cold start. An experimental study on a heavy-duty diesel engine operating at different altitudes was conducted. Tests were based on a direct injection (DI) turbocharged diesel engine with intake and exhaust pressure controlled by the plateau simulation test system to stimulate altitude conditions including 0 m, 1000 m, 2000 m, 3000 m and 4000 m. Results indicated that the compression and expansion resistance moment reduced and the speed increased during the cranking period. The peak pressure of several cycles was increased during the start-up period; however, the expansion pressure dropped more and the indicated mean effective pressure (IMEP) reduced as the altitude rose. While at an altitude of over 2000 m, the peak pressure fluctuated obviously during the start-up period. The higher the altitude was, the more the fluctuation amplitude and cycle number increased and combustion instability enhanced, which resulted the start-up period time increasing at high altitude. When the altitude rose, the cycle-to-cycle variation of the peak pressure and speed fluctuation increased during the idle, the ignition and CA50 were delayed and the combustion duration was shortened. The effect of altitude on combustion characteristics of the diesel engine was more significant during the start-up period than during its idle period.     

Development of an on-line model to predict the in-cylinder residual gas fraction by using the measured intake/exhaust and cylinder pressures

The in-cylinder RGF (residual gas fraction) of internal combustion engines for new combustion concepts, such as CAI (controlled auto ignition) or HCCI (homogenous charged compression ignition), is a major parameter that affects the combustion characteristics. Thus, measurement or prediction of the cycle-by-cycle RGF and investigation into the relation between the RGF and the combustion phenomena are critical issues. However, on-line prediction of the cycle-by-cycle RGF during engine testing is not always practical due to the requirement of expensive, fast response exhaust-gas analyzers and/or theoretical models that are just too slow for application. In this study, an on-line model that can predict the RGF of each engine cycle and cylinder during the experiment in the test cell has been developed. This enhanced model can predict the in-cylinder charge conditions of each engine cycle during the test in three seconds by using the measured dynamic pressures of the intake, exhaust, and cylinder as the boundary conditions. A Fortran77 code was generated to solve the 1-D MOC (method of characteristics). This code was linked to Labview DAQ as a form of DLL (dynamic link library) to obtain three boundary pressures for each cycle. The model was verified at various speeds and valve timings under the CAI mode by comparing the results with those of the commercial code, GT-Power.     

垃圾填埋气内燃机的研制与试验

阐述了开发垃圾填埋气内燃机的重要性,分析了垃圾填埋气中CO2成分对垃圾填埋气内燃机有关参数的影响。以2190T天然气发动机为基础,采用提高压缩比、调整点火提前角、预燃室式燃烧室、进气增压等技术措施,初步开发出2190垃圾填埋气内燃机。试验结果表明,采用上述技术措施,可改善和提高垃圾填埋气内燃机的性能指标。     

Applicability of heat transfer equations to hydrogen combustion

Previous research by the authors showed that hydrogen combustion exhibits a higher cooling loss to the combustion chamber wall of an internal combustion engine compared to hydrocarbon combustion because of its higher burning velocity and shorter quenching distance. The high cooling loss means that reduction of the cooling loss is essential to establish a high thermal efficiency in hydrogen combustion engines. This research analyzed the applicability of equations to describe the heat transfer from burning gases to hydrogen combustion. The result shows that equations calculate a lower cooling loss than experimental values, and the use of correction coefficients does not accurately define the actual cooling rate. It is therefore concluded that the derivation of a new heat transfer equation for hydrogen combustion is necessary to improve the thermal efficiency of hydrogen fuelled engines.     

Major sources of hydrocarbon emissions in a premixed charge compression ignition engine

Although premixed charge compression ignition (PCCI) combustion engines are praised for potentially high efficiency and clean exhaust, experimental engines built to date emit more hydrocarbons (HCs) and carbon monoxide (CO) than the conventional machines. These compounds are not only strictly controlled components of the exhaust gas of road vehicles but are also an energy loss indicator. The prime objective of this study was to investigate the major sources of the HCs formed in the combustion chamber of an experimental PCCI engine in order to suggest some effective technologies for HC reduction. In this study, to explore the dominant sources of HC emissions in both operation modes, a single cylinder engine was prepared such that it could operate using either conventional diesel combustion or PCCI combustion. Specifically, the contributions of the top-ring crevice volume in the combustion chamber and the bulk quenching of the lean mixture were investigated. To understand the influence of the shape and magnitude of the crevice on HC emissions, the engine was operated with 12 specially prepared pistons with different top-ring crevices installed one after another. The engine emitted proportionally more HCs as the depth of the crevice increased as long as the width remained narrower than the prevailing quench distance. The top-ring-crevice-originated exhaust HCs comprised approximately 31% of the total HC emissions in the baseline condition. In a series of tests to estimate the effects of bulk quench on exhaust HC emissions, intake air was heated from 300K to 400K in steps of 25K. With the intake air heated, HC and CO emissions decreased with a gradually diminishing rate to zero at 375K. In conclusion, the most dominant sources of HC emissions in PCCI engines were the crevice volumes in the combustion chamber and the bulk quenching of the lean mixtures. The key methods for reducing HC emissions in PCCI engines are minimizing crevice volume in the combustion chamber and maximizing intake air temperature allowed based on the permissible NOx level.     

米勒循环发动机缸内气体流动与燃烧分析

在某1．6L发动机的基础上进行纵向开发,以此来实现高压缩比米勒循环发动机。利用大型三维计算流体动力学（CFD）软件STAR‐CD对采用进气门晚关策略的高压缩比米勒循环发动机进行数值模拟,对比分析了采用两种几何形状活塞的米勒循环发动机的缸内气体流动模式及燃烧过程。计算结果表明：经过结构优化的活塞方案相对于原方案可以获得较优的缸内流动及燃烧特性。计算结果将为实际开发中的高压缩比米勒循环发动机的活塞选型及燃烧室优化提供理论及数据支撑。     

基于负阀重叠EGR策略的HCNG发动机仿真

为降低HCNG发动机NOx排放,采用负阀重叠EGR策略,利用AVL-Fire软件对HCNG发动机不同进气门开启角(θIVO)下的进气过程和燃烧过程进行了三维仿真计算,对比分析了采用负阀重叠前后发动机缸内EGR分布和燃烧过程。仿真结果表明:负阀重叠EGR策略下,排气门关闭角(θEVC)固定为340°曲轴转角不变,当θIVO为380°曲轴转角时,既可避免发生回火又能保证一定的进气量及充气效率;采用负阀重叠后,在压缩冲程后期,缸内EGR率呈梯度分布(靠近火花塞位置EGR率较低),更有利于着火及火焰传播;采用负阀重叠可降低缸内最高燃烧压力及最高温度,但会减少进入气缸的新鲜工质,降低发动机功率;通过负阀重叠实现内部EGR可降低NOx排放,但会导致着火困难,燃烧速度变慢;提高点火能量可缩短着火落后期和燃烧持续期,加快燃烧速度。     

Enhancement of NOx-PM trade-off in a diesel engine adopting bio-ethanol and EGR

For realizing a premixed charge compression ignition (PCCI) engine, the effects of bio-ethanol blend oil and exhaust gas recirculation (EGR) on PM-NOx trade-off have been investigated in a single cylinder direct injection diesel engine with the compression ratio of 17.8. In the present experiment, the ethanol blend ratio and the EGR ratio were varied focusing on ignition delay, premixed combustion, diffusive combustion, smoke, NOx and the thermal efficiency. Very low levels of 1.5 [g/kWh] NOx and 0.02 [g/kWh] PM, which is close to the 2009 emission standards imposed on heavy duty diesel engines in Japan, were achieved without deterioration of the thermal efficiency in the PCCI engine operated with the 50% ethanol blend fuel and the EGR ratio of 0.2. It is found that this improvement can be achieved by formation of the premixed charge condition resulting from a longer ignition delay. A marked increase in ignition delay is due to blending ethanol with low cetane number and large latent heat, and due to lowering in-cylinder gas temperature on compression stroke based on the EGR. It is noticed that smoke can be reduced even by increasing the EGR ratio under a highly premixed condition.     

Measurement of hydrocarbon and carbon monoxide emissions during the starting of automotive DI Diesel engines

Most of hydrocarbon (HC) and carbon monoxide (CO) emissions from automotive DI Diesel engines are produced during the engine warm-up period and are primarily caused by difficulties in obtaining stable and efficient combustion under these conditions. Furthermore, the contribution of engine starting to these emissions is not negligible; since this operating condition is highly unfavorable for the combustion progress. Additionally, the catalytic converter is ineffective due to the low engine temperature. In conjunction with adequate engine settings (fuel injection and fresh air control), either the glow plugs or the intake air heater are activated during a portion of the engine warm-up period, so that a nominal engine temperatures is reached faster, and the impact of these difficulties is minimized. Measurement of gaseous pollutants during engine warm-up is currently possible with detectors used in standard exhaust gas analyzers (EGA), which have response times well-suited for sampling at such transient conditions. However, these devices are not suitable for the measurement of exhaust emissions produced during extremely short time intervals, such as engine starting. Herein, we present a methodology for the measurement of the cumulative pollutant emissions during the starting phase of passenger car DI Diesel engines, with the goal of overcoming this limitation by taking advantage of standard detectors. In the proposed method, a warm canister is filled with an exhaust gas sample at constant volumetric flow, during a time period that depends on the engine starting time; the gas concentration in the canister is later evaluated with a standard EGA. When compared with direct pollutant measurements performed with a state-of-art EGA, the proposed procedure was found to be more sensitive to combustion changes and provided more reliable data.     

Comparison of two injection systems in an HSDI diesel engine using split injection and different injector nozzles

The demand for reduced pollutant emissions has motivated various technological advances in passenger car diesel engines. This paper presents a study comparing two fuel injection systems and analyzing their combustion noise and pollutant emissions. The abilities of different injection strategies to meet strict regulations were evaluated. The difficult task of maintaining a constant specific fuel consumption while trying to reduce pollutant emissions was the aim of this study. The engine being tested was a 0.287-liter single-cylinder engine equipped with a common-rail injection system. A solenoid and a piezoelectric injector were tested in the engine. The engine was operated under low load conditions using two injection events, high EGR rates, no swirl, three injection pressures and eight different dwell times. Four injector nozzles with approximately the same fuel injection rate were tested using the solenoid injection system (10 and 12 orifice configuration) and piezoelectric system (6 and 12 orifice design). The injection system had a significant influence on pollutant emissions and combustion noise. The piezoelectric injector presented the best characteristics for future studies since it allows for shorter injection durations and greater precision, which means smaller fuel mass deliveries with faster responses.     

缸内直喷汽油机多种燃烧模式的试验研究

基于量产增压缸内直喷汽油机,采用可变升程机构实现多种燃烧模式,在低升程下实现了均质燃烧、分层稀燃和均质压燃。试验结果表明:在转速2 000r/min,负荷0.2MPa,0.4MPa下,均质压燃的燃烧持续期最短,放热最快;在转速2 000r/min,负荷0.2MPa下,均质压燃的燃油消耗率最低,达到337g/(kW.h),同时均质压燃的NOx排放远低于均质燃烧和分层稀燃。