Cooling effect of methanol on an n-heptane HCCI engine using a dual fuel system

Homogeneous charge compression ignition (HCCI) engines have the potential to raise the efficiency of reciprocating engines during partial load operation. However, the performance of the HCCI engine at high loads is restricted by severe knocking, which can be observed by the excessive pressure rise rate. This is due to the rapid combustion process occurring inside the cylinder, which does not follow the flame propagation that is seen in conventional engines. In this study, a low compression ratio of 9.5:1 for a gasoline engine was converted to operate in HCCI mode with the goal being to expand the stable operating region at high loads. Initially, pure n-heptane was used as the fuel at equivalence ratios of 0.30 to 0.58 with elevated intake charge temperatures of 180 and 90 °C, respectively. The n-heptane HCCI engine could reach a maximum performance at an indicated mean effective pressure (IMEP) of 0.38 MPa, which was larger than the performance found in the literature. To reach an even higher performance, a dual-fuel system was exploited. Methanol, as an anti-detonant additive, was introduced into the intake stream with various amounts of n-heptane at fixed equivalence ratios in the range of 0.42 to 0.52. It was found that the methanol addition cooled the mixture down prior to combustion and resulted in an increased coefficient of variation (COV). In order to maintain stable combustion and keep the pressure rise rate below the limit, the intake charge temperature should be increased. Introduction of 90% and 95% (vol/vol) hydrous methanol showed a similar trend but a lower thermal conversion efficiency and IMEP value. Therefore, a dual fuel HCCI engine could maintain a high thermal conversion efficiency across a wide load and enhance a 5% larger load compared to a pure n-heptane-fuelled HCCI engine. The hydrocarbon (HC) and carbon monoxide (CO) emissions were lower than 800 ppm and 0.10%, respectively. They were less at higher loads. The nitrogen oxides (NO _x ) emissions were below 12 ppm and were found to increase sharply at higher loads to a maximum of 23 ppm.     

Experimental study of performance and emission characteristics of DEE-assisted minimally processed ethanol fuelled HCCI engine

The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for reciprocating engines. This study investigates an HCCI engine fuelled with Diethyl ether (DEE) ignition assisted wet ethanol (ethanol with 20% water content). The direct use of wet ethanol could reduce the associated energy required for distillation and dehydration of ethanol. The HCCI engine offers significant benefits in terms of its high efficiency and ultra low emissions. The experiment is conducted with various DEE flow rates and at different air-fuel ratios, for which stable HCCI combustion is achieved. Incylinder pressure, heat release analysis and exhaust emissions were observed. In this study, the effect of DEE on combustion parameters, thermal efficiency and emissions is analysed and discussed in detail. The experimental results indicate that the DEE flow rates have a significant effect on the maximum in-cylinder pressure and its position, thermal efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operating points, brake thermal efficiency is higher than reference mode at lower loads and almost same at higher loads. The emission parameters such as NO, HC and CO are lower than the dual fuel mode which is considered as a reference model for this experiment.     

Effect of direct in-cylinder CO2 injection on HCCI combustion and emission characteristics

Fuel injection during negative valve overlap period was used to realize diesel homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion, CO₂ in-cylinder injection was used to simulate external EGR. Effects of CO₂ injection parameters (injection timing, quantity, pressure) on HCCI combustion and emission characteristics were investigated. Experimental results revealed that CO₂ in-cylinder injection can control the start of combustion and effectively reduce NO_x emission. Either advancing CO₂ injection timing or increasing CO₂ injection quantity can reduce peak cylinder pressure and mean gas temperature, delay the starts of low temperature reaction (LTR) and high temperature reaction (HTR), and lower pressure rise rate; NO_x emission was reduced, while smoke, HC, and CO emissions increased. Since the combustion phase was improved, the indicated thermal efficiency was also improved. Injection pressure determines the amount of disturbance introduced into the cylinder. Generally, with the same injection quantity, higher injection pressure results in higher momentum flux and total momentum. Larger momentum flux and momentum has a stronger disturbance to air-fuel mixture, resulting in a more homogeneous mixture; therefore, larger injection pressure leads to lower NO_x and smoke emissions.     

Effect of operating parameters on diesel/propane dual fuel premixed compression ignition in a diesel engine

In this research, the effects of three operating parameters (Diesel injection timing, propane ratio, and exhaust gas recirculation (EGR) rates) in a diesel-propane dual fuel combustion were investigated. The characteristics of dual-fuel combustion were analyzed by engine parameters, such as emission levels (Nitrogen oxides (NO_x) and particulate matter (PM)), gross indicated thermal efficiency (GIE) and gross IMEP Coefficient of Variance (CoV). Based on the results, improving operating strategies of the four main operating points were conducted for dual-fuel PCCI combustion with restrictions on the emissions and the maximum pressure rise rate. The NOx emission was restricted to below 0.21 g/kWh in terms of the indicated specific NO_x (ISNO_x), PM was restricted to under 0.2 FSN, and the maximum pressure rise rate (MPRR) was restricted to 10 bar/deg. Dual-fuel PCI combustion can be available with low NO_x, PM emission and the maximum pressure rise rate in relatively low load condition. However, exceeding of PM and MPRR regulation was occurred in high load condition, therefore, design of optimal piston shape for early diesel injection and modification of hardware optimizing for dual-fuel combustion should be taken into consideration.     

转速对甲醇HCCI燃烧特性的影响试验研究

在一台双缸4冲程柴油机上实现了甲醇均质压燃的试验,分析了发动机转速对甲醇均质压燃燃烧特性的影响。试验结果表明：过量空气系数和进气温度一定的情况下,随着发动机转速在一定范围内的升高,缸内压力、压力升高率和放热率均有所升高,且存在一个最佳的转速,使得压力、压力升高率和放热率处在较好的水平。     

Reduction of emissions with propane addition to a diesel engine

Recent studies on dual-fuel combustion in compression-ignition (CI) engines, also known as diesel engines, fall into two categories. In the first category are studies focused on the addition of small amounts of gaseous fuel to CI engines. In these studies, gaseous fuel is regarded as a secondary fuel and diesel fuel is regarded as the main fuel for combustion. The objectives of these studies typically involve reducing particulate matter (PM) emissions by using gaseous fuel as a partial substitution for diesel fuel. However, the addition of gaseous fuel raises the combustion temperature, which increases emissions of nitrogen oxides (NO_x). In the second category are studies focused on reactivity-controlled compression-ignition (RCCI) combustion. RCCI combustion can be implemented by early diesel injection with a large amount of low-reactivity fuel such as gasoline or gaseous fuel. Although RCCI combustion promises lower NO_x and PM emissions and higher thermal efficiency than conventional diesel combustion, it requires a higher intake pressure (usually more than 1.7 bars) to maintain a lean fuel mixture. Therefore, in this study, practical applications of dual-fuel combustion with a low air-fuel ratio (AFR), which implies a low intake pressure, were systemically evaluated using propane in a diesel engine. The characteristics of dualfuel combustion for high and low AFRs were first evaluated. The proportion of propane used for four different operating conditions was then increased to decrease emissions and to identify the optimal condition for dual-fuel combustion. Although the four operating conditions differ, the AFR was maintained at 20 (? approximately equal to 0.72) and the 50% mass fraction burned (MFB 50) was also fixed. The results show that dual-fuel combustion can reduce NO_x and PM emissions in comparison to conventional diesel combustion.     

外部热EGR对基于优化动力技术的汽油HCCI燃烧的影响

考察了外部热EGR对基于优化动力技术的汽油HCCI发动机燃烧的影响。试验结果表明:外部热EGR可以推迟HCCI燃烧的着火时刻,减缓放热速率,但对于高辛烷值燃料的HCCI燃烧,它对更高EGR率的兼容能力不强,需要提高进气温度来提高燃烧的稳定性;随着EGR率的增加,燃烧持续期延长,缸内温度和压力峰值均减小,指示热效率也随着减小;NOx排放随着EGR率的增加在经过一个"拐点"后始终维持在一个较低的水平,而CO和HC的排放随着EGR率的增加显著增加,燃烧恶化。     

柴油HCCI燃烧负荷边界判定依据研究

通过对HCCI燃烧负荷边界燃烧变化规律及判定依据的研究,基于单缸HCCI柴油机实测气缸压力,探讨了HCCI燃烧负荷边界处的燃烧变化规律,着重分析负荷、转速对气缸压力、燃烧放热率、循环变动率和最大压升率的影响,通过对负荷上限和下限敏感的最大压力升高率和循环变动率等参数的变化研究,确定负荷边界判定依据。     

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.     

混合气分层对HCCI汽油机极限负荷和燃烧特性的影响

对比研究HCCI汽油机在不同空燃比下采用混合气分层策略时的极限负荷、NOx排放量和燃油经济性,考察了在此策略下过量空气系数λ和EGR率对HCCI发动机燃烧特性的影响。结果表明,混合气分层压缩燃烧模式能有效降低HCCI燃烧的压力升高率,具有拓展负荷范围的潜力,但同时也使NOx排放增加;适当的过量空气系数能在一定程度上改善HCCI发动机的燃烧特性,采用9%的EGR率时发动机油耗率最低,具有明显节油效果。     

多次喷射对柴油机燃烧与排放影响的试验研究

基于1台高压共轨涡轮增压柴油机,采用不同的预喷正时、预喷油量与后喷正时等,研究了多次喷射对燃烧放热、排放生成与燃油经济性的影响,以实现均质压燃和低温燃烧过程。研究结果表明:随预喷正时提前,缸内峰值压力降低,主燃阶段的滞燃期缩短,NOx和炭烟排放均降低;随预喷油量增加,预喷阶段燃烧的放热率和最大压力升高率增大,NOx和HC排放增大,而PM和CO排放降低;随后喷始点推迟,缸内压力与主放热率峰值差异变小,NOx排放降低,但炭烟排放先增大后逐渐降低。     

纯柴油、柴油/CNG两种燃烧模式下应用EGR的发动机性能研究

在一台单缸柴油机上加装天然气电控喷射系统，改造成柴油／CNG双燃料发动机。通过示功图、放热率、排放特性和经济性的实验与分析，研究了纯柴油和双燃料两种燃烧模式下应用冷却EGR的效果。研究表明：不管是纯柴油模式还是双燃料模式，使用EGR之后，NOx都有大幅度降低；纯柴油模式使用EGR之后，会带来碳烟排放的恶化，但是对于双燃料模式，当EGR比例在22％以内，负荷小于75％时，不会造成碳烟排放的恶化。随着EGR比例的增大，燃烧放热率曲线由双峰过渡到单峰形状，具有均质压燃(HCCI)燃烧过程的特征。     

柴油机HCCI压缩过程缸内工质状态的模拟研究

分析了均质充量压燃(HCCI)的一个技术难点,即燃烧始点的控制,指出其燃烧两阶段性对应着放热的两阶段性。建立了HCCI柴油机早期缸内燃油喷射的压缩模型,通过对R4102柴油机的模拟计算,获得了压缩过程缸内工质T-p变化历程;从低温放热和高温放热分析了进气温度T0、过量空气系数a以及压缩比cε等因素对HCCI燃烧始点的影响。     

不同辛烷值参比燃料HCCI燃烧特性的试验研究

对不同辛烷值基本参比燃料及其混合物在高速4缸柴油机上进行单缸HCCI燃烧试验,研究了燃料辛烷值、发动机冷却水温度、进气温度以及冷EGR率对HCCI发动机燃烧特性和排放特性的影响。结果表明;在同一当量比下,随燃料辛烷值增大,着火时刻推迟,燃烧放热速率降低,HC和CO排放增大。HCCI燃烧随负荷的增大、EGR牢的减小、进气温度和冷卸水温度的升高,着火时刻提前,燃烧放热速率加快。     

Real-time empirical NO<Subscript>x</Subscript> model based on In-cylinder pressure measurements for light-duty diesel engines

This paper proposes a real-time empirical model of NO_x emissions for diesel engines. The proposed model predicts the level of NO_x emissions using an empirical model developed based on the thermal NO formation mechanism, the extended Zeldovich mechanism. Since it is difficult to consider the exact physical NO formation phenomena in real-time applications, the proposed algorithm adapts the key factors of the NO formation mechanism from the extended Zeldovich mechanism: temperature of the burned gas, concentration of the gas species, and combustion duration where NO is generated. These factors are considered in a prediction model as four parameters: exhaust gas recirculation rate (EGR rate), crank angle location of 50 % of mass fraction burned (MFB50), exhaust lambda value, and combustion acceleration. The proposed prediction model is validated with various steady engine experiments that showed a high linear correlation with the NO_x emission measured by a NO_x sensor. Furthermore, it is also validated for transient experiments.     

缸内直喷醇类燃料发动机的燃烧与排放特性

根据所测的示功图和排放,分析了一台采用火花点火、缸内直喷周向分层燃烧系统的发动机在燃用甲醇和乙醇时的性能和燃烧特性。研究表明,醇类燃料发动机的燃烧由预混燃烧与扩散燃烧组成,具有非常快的燃烧速率,而且非常稳定,ATDC(3°CA～6°CA)就燃烧完50%燃料,循环变动小于6%。与燃用乙醇相比,燃用甲醇时滞燃期较短,燃烧速率较快。由于采用分层燃烧,醇类燃料发动机具有与直喷柴油机相当的热效率,在负荷特性上,燃用醇类燃料时的NOx排放仅为柴油机的10%～40%,且能实现无烟燃烧,CO排放的增加低于1%,HC排放高于柴油机。     

内部EGR及增压拓展柴油HCCI燃烧负荷范围试验研究

在135单缸柴油机上对比了传统燃烧模式和HCCI燃烧模式的负荷特性,优化了HCCI燃烧模式的喷油始点,分析了内部EGR率及增压压力对HCCI燃烧负荷范围及排放的影响。试验结果表明:对于负气门重叠期喷油的HCCI燃烧模式,1 500r/min下,最佳喷油始点为370°BTDC,气门重叠期为-30°时既保证了较低的NOx排放,又可以获得较佳的负荷范围;提高增压压力不仅可以拓展HCCI燃烧的负荷上限,对负荷下限的燃烧稳定性也有利;将增压压力提高到0.18MPa时,负荷上限从传统燃烧的0.594MPa上升到0.723MPa,但负荷下限较传统燃烧模式要高,CO排放、烟度和燃油经济性都较差。     

Evaluation and visualization of stratified ultra-lean combustion characteristics in a spray-guided type gasoline direct-injection engine

To comply with reinforced emission regulations for harmful exhaust gases, including carbon dioxide (CO₂) emitted as a greenhouse gas, improved technologies for reducing CO₂ and fuel consumption are being developed. Stable lean combustion, which has the advantage of improved fuel economy and reduced emission levels, can be achieved using a sprayguided-type direct-injection (DI) combustion system. The system comprises a centrally mounted injector and closely positioned spark plugs, which ensure the combustion reliability of a stratified mixture under ultra-lean conditions. The aim of this study is to investigate the combustion and emission characteristics of a lean-burn gasoline DI engine. At an excess air ratio of 4.0, approximately 23% improvement in fuel economy was achieved through optimal event timing, which was delayed for injection and advanced for ignition, compared to that under stoichiometric conditions, while NO_x and HC emissions increased. The combustion characteristics of a stratified mixture in a spray-guided-type DI system were similar to those in DI diesel engines, resulting in smoke generation and difficulty in three-way catalystutilization. Although a different operating strategy might decrease fuel consumption, it will not be helpful in reducing NO_x and smoke emissions; therefore, alternatives should be pursued to achieve compliance with emission regulations.     

二甲醚均质压燃燃烧特性试验研究

介绍了在一台单缸柴油机上进行的二甲醚(DME)均质压燃燃烧过程的试验研究,DME的燃烧保持了低温反应与高温反应两个阶段的特征,着火时刻较早。试验结果表明,混合气燃空当量比的变化对低温反应开始时刻影响不大,但对高温反应开始时刻有较大影响,随着燃空当量比的增大,高温反应开始时刻逐步提前,当燃空当量比为0.21时,出现轻微爆震现象。随着发动机转速的增加,低温反应开始时刻提前,高温反应阶段的放热率略有增大。进气温度和冷却水温度升高,低温反应和高温反应时刻都有较大的提前。在进气温度(T)大于300 K后,缸内最大压力出现在上止点前,进气温度是控制着火时刻的重要参数。     

二甲醚发动机的燃烧与排放研究

在一台2135直喷柴油机上，对燃油供给系统进行了适当的改造，测试了燃用二甲醚发动机的燃料喷射时刻、气缸压力和有害排放，计算分析了放热规律及滞燃期变化规律。研究结果表明，在供油提前角相同的情况下，二甲醚的喷射延迟比柴油长而滞燃期比柴油短，着火时刻落后于柴油；二甲醚的最大放热速率小，而燃烧持续期短；二甲醚发动机接近无烟排放，N0x排放浓度显著低于柴油机。