废气再循环与燃料辛烷值对均质压燃发动机性能和排放影响的试验研究

在一台单缸直喷式柴油机上研究了废气再循环（EGR）对不同辛烷值燃料均质压燃（HCCI）发动机性能和排放特性的影响。结果表明，混合气较稀，EGR对指示热效率影响较小，其影响和燃料辛烷值有关；混合气变浓，EGR对指示热效率的影响增大。不同辛烷值燃料最高指示热效率出现在高EGR率、混合气较浓的区域，并且靠近爆震燃烧边界，辛烷值为60的燃料最高指示热效率最高，并且覆盖的工况区域最宽。高EGR率区域，EGR对HC排放的影响十分明显，EGR率升高，HC排放急剧增大，而且随着燃料辛烷值增大，这种趋势越明显；CO排放与缸内燃烧温度有较大的相关性，EGR率升高，CO排放升高。NOx排放出现急剧升高的“拐点”是判断HCCI爆震燃烧的一个重要判据，EGR率增大，“拐点”出现的混合气浓度增大，在正常工作范围内，NOx排放极低，EGR对NOx排放几乎没有影响。     

运行工况对基础燃料均质压燃燃烧过程影响的试验研究

在一台经改装的单缸直喷式柴油机上进行了不同辛烷值基础燃料下发动机转速对均质压燃（HCCI）燃烧特性、工况范围和排放特性影响的试验研究。研究结果表明：发动机转速升高,不同辛烷值燃料着火燃烧时刻推迟,以曲轴转角计算的燃烧持续期延长,高辛烷值燃料的缸内最大爆发压力和缸内温度降低;在中间转速,HCCI实现的最高平均指示压力最大,高转速工况,最高平均指示压力降低;对于低辛烷值燃料,转速对燃烧效率影响不大,转速升高,指示热效率增大;对于高辛烷值燃料,转速升高燃烧效率降低,指示热效率在中间转速最高,高转速降低。排放测试表明,转速升高使得HCCI运转的HC和CO排放都升高,NOx排放则逐渐降低。     

不同EGR方法对柴油燃料HCCI燃烧影响的探讨

研究了内部EGR和外部EGR模式对柴油燃料HCCI燃烧的影响。随着内部EGR率的增大,HCCI燃烧的着火始点提前,在相对较小的负荷下,-40°CA气门重叠期的着火始点比-20°CA气门重叠期的着火始点提前5~7°CA,内部EGR的加热作用大于其对混合气的稀释作用;内部EGR增大有利于均质混合气的形成,使柴油燃料HCCI燃烧的烟度排放减小,但使低NOx排放的负荷范围减小;外部EGR起到了推迟着火始点的作用,是一种有效的扩展运行范围负荷上限的方法。     

废气再循环和发动机运转参数对不同辛烷值燃料HCCI燃烧的影响

考察了废气再循环(EGR)、进气温度、冷却水出水温度和转速等发动机运转参数对HCCI发动机燃烧特征和排放特性的影响。实验结果表明：随EGR率提高,各种燃料的两阶段着火时刻推迟,燃烧持续期延长;高十六烷值燃料可以容许较高的EGR率,RON75最高仅可以采用45％的EGR;EGR对高十六烷值燃料的CO和UHC影响不大,对高辛烷值燃料的CO影响明显,并随EGR率增加CO排放升高。在其它运转参数中,进气温度对HCCI燃烧影响最为显著,随进气温度提高、冷却水温度升高,HCCI燃烧的着火时刻提前、燃烧持续期缩短,高辛烷值燃料的UHC和CO显著降低。转速升高,着火延迟,燃烧持续期延长。此外,研究发现,高辛烷值燃料对HCCI发动机的运转参数更为敏感。     

缸内直喷汽油机HCCI燃烧对压缩比和辛烷值的适应性研究

在缸内直喷汽油机（GDI）上采用多次燃油喷射和可变配气技术来控制缸内混合气形成和燃烧,实现了SI/HCCI复合燃烧方式。研究了不同压缩比和辛烷值对均质混合气压燃（HCCI）燃烧排放特性的影响。结果表明,汽油HCCI燃烧呈现单阶段燃烧燃料特性,HCCI着火发生在上止点附近时油耗低。低压缩比下,HCCI燃烧可以在较浓空燃比下工作,NOx排放较高。高辛烷值燃料HCCI燃烧可运行的负荷范围窄。汽油HCCI发动机在偏高压缩比条件下燃用偏低辛烷值汽油可以获得较好的经济性和排放性能。     

内部EGR和燃料辛烷值对小负荷低温燃烧的影响

将不同比例的正庚烷和异辛烷掺混得到3种不同辛烷值的混合燃料PRF70、PRF80和PRF90,在单缸直喷柴油机上研究了燃料辛烷值和基于排气门两次开启的内部EGR策略对高辛烷值燃料小负荷低温燃烧的影响.结果表明,小负荷工况(平均指示压力小于0.3,MPa)下,适当增大内部EGR率能提高缸内温度,改善高辛烷值燃料的燃烧稳定性和HC、CO排放,但内部EGR率过大时由于废气的稀释作用增强,对高辛烷值燃料小负荷燃烧带来不利影响;燃料辛烷值对高辛烷值燃料的燃烧有影响,随着辛烷值增大,着火始点推迟,放热速率降低,HC和CO排放升高,燃烧稳定性下降.通过采用合适的喷油时刻、内部EGR策略结合辛烷值优化能够实现高辛烷值燃料在小负荷工况的稳定燃烧.     

不同进气温度条件下燃料特性对均质压燃燃烧过程影响的试验研究

为了研究不同进气温度(Tin)条件下燃料特性对均质压燃(HCCI)燃烧特性、工况范围和排放特性的影响,在一台改装的4缸直喷柴油机上进行了不同辛烷值(RON)基础燃料(PRF)、汽油和含氧燃料的进气加热发动机试验.结果表明,Tin较低时,PRF着火时刻最早,缸内最大爆发压力和峰值放热率最高,其次是汽油和含氧燃料,但Tin较高时,则是汽油燃料着火时刻最早,燃烧效率和能够实现的最高指示效率最高;Tin升高,所有燃料主燃烧着火时刻提前,缸内最大爆发压力和燃烧效率升高,HCCI正常运转工况范围向小负荷区域拓展,但Tin对指示效率的影响与燃料的RON有关.排放测试表明,Tin升高,所有燃料的HC和CO排放均降低,Tin较高时,燃料特性对两者的影响减小;Tin高于363 K时,汽油的HC和CO排放均最低;Tin和RON都相同,汽油燃料的Nox排放值相对较高,Tin对所有燃料的Nox排放的影响均不明显.     

采用快速压缩机对甲醇燃料的HCCI燃烧特性的研究

为进一步了解高辛烷值燃料的均质压燃燃烧特性,利用自主研发的快速压缩机,研究了边界条件对甲醇燃料HCCI燃烧特性的影响.试验表明:随着充量温度的升高,燃烧始点提前,燃烧持续期缩短,燃烧温度最大值增加,放热率最大值增加,最大压力升高率增大,最大压力升高率出现时刻提前;随着可燃混合气过量空气系数的增加,可燃混合气的浓度减小,燃烧始点延迟,燃料的最高燃烧温度降低,放热率最大值降低,最大压力升高率降低,着火温度升高,同时反应速率减慢,所以燃烧持续期增加,最大压力升高率减小,最大压力升高率出现时刻延迟.     

双燃料对汽油机HCCI燃烧与排放的影响北大核心CSCD

采用双燃料喷射系统在进气道分别喷入汽油和正庚烷,结合优化动力技术手段,实现了4缸汽油机的均质混合气压燃(HCCI)稳定燃烧。研究了在不同EGR条件下,燃料比例对汽油机HCCI燃烧和排放的影响。研究结果表明:随着汽油比例的增加,燃烧相位推迟,燃烧持续期延长,HC和CO排放随之增加,最优工况点对应的EGR率降低。提高正庚烷的比例,HCCI燃烧对EGR敏感性降低,可以提高EGR率上限。在同一燃料比例下,随着EGR率的变化,CA10与CA50有线性关系,其斜率与燃料中汽油比例正相关。     

利用快速压缩机研究辛烷值对HCCI燃烧特性的影响

在一台燃烧边界条件可控的快速压缩机上对均质压燃燃烧方式(HCCI)下燃料辛烷值对燃烧特性的影响进行了研究.以甲醇、乙醇、异辛烷、PRF90和正庚烷等燃料为研究对象,在不同的燃烧边界条件下,通过对燃烧起始时刻、燃烧持续期、燃烧放热率以及燃烧压力变化历程等参数的分析,得出了燃料辛烷值对HCCI燃烧特性的影响规律.试验结果表明:随着燃料辛烷值的升高,燃烧始点延迟,燃烧持续期延长,放热率最大值和最高燃烧温度同时降低,压力升高率最大值降低、出现时刻延迟,着火温度升高.     

Fuel design and management for the control of advanced compression-ignition combustion modes

Due to concerns regarding the greenhouse effect and limitations on carbon dioxide emissions, the possibility of a next-generation combustion mode for internal combustion engines that can simultaneously reduce exhaust emissions and substantially improve thermal efficiency has drawn increasing attention. The most prominent characteristic of new combustion modes, such as Homogenous-Charge Compression-Ignition (HCCI), Stratified-Charge Compression-Ignition (SCCI), and Low-Temperature Combustion (LTC), is the requirement of creating a homogenous mixture or controllable stratified mixture prior to ignition. To this end, a lean fuel/air mixture and/or a controllable high level of exhaust gas recirculation (EGR) are employed to prolong the timescale of the ignition chemistry and port fuel injection or early in-cylinder injection is used to lengthen the mixing period. The mixture then undergoes controlled self-ignition near the top dead center (TDC) position due to the compression effect of the piston’s upward movement. It is worth noting that the entire combustion process lacks a direct method for the control of ignition timing and combustion rate, which are instead controlled primarily by chemical kinetics and, to a lesser extent, by turbulence and mixing. Because of the significant impacts of fuel physical–chemical properties on the ignition and combustion process, fuel design and management has become the most common approach for the control of ignition timing and combustion rate in such advanced combustion modes.This paper summarizes the concepts and methods of fuel design and management and provides an overview of the effects of these strategies on ignition, combustion, and emissions for HCCI, LTC, and SCCI engines, respectively. From part 2 to part 4, the paper focuses on the effect of fuel design on HCCI combustion. A fuel index suitable for describing ignition characteristic under HCCI operating conditions is first introduced. Next, the proposed fuel design concept is described, including principles and main methodologies. Strategies based on the fuel design concept (including fuel additives, fuel blending, and dual-fuel technology) are discussed for primary reference fuels (PRF), alternative fuels, and practical gasoline and diesel fuels. Additionally, the effects of real-time fuel design on HCCI combustion fueled with PRFs and dimethyl ether/liquefied petroleum gas (DME–LPG) are evaluated. Diesel HCCI combustion has suffered from difficulties in homogenous mixture formation and an excessively high combustion rate. Therefore, LTC, which concentrates on local combustion temperature and a balance of mixture formation timescale and ignition timescale, has been proposed by many researchers. In Part 5, this paper provides an overview of the major points and research progress of LTC, with a preliminary discussion of the fundamental importance of fuel properties and fuel design strategy on the LTC process and emissions. Due to the stratification strategy has the capable of extending the HCCI operation range to higher loads, SCCI combustion, which incorporates HCCI combustion into a traditional combustion mode, has the potential to be used in commercial engines. Thus, this paper discusses the principles and control strategies of fuel design and management and also summarizes recent progress and future trends. The effect of fuel design and management on SCCI combustion is assessed for high cetane number fuels and high octane number fuels as well as the in SCCI combustion of gasoline–diesel dual-fuel and blends.     

Progress and recent trends in homogeneous charge compression ignition (HCCI) engines

HCCI combustion has been drawing the considerable attention due to high efficiency and lower nitrogen oxide (NO_x) and particulate matter (PM) emissions. However, there are still tough challenges in the successful operation of HCCI engines, such as controlling the combustion phasing, extending the operating range, and high unburned hydrocarbon and CO emissions. Massive research throughout the world has led to great progress in the control of HCCI combustion. The first thing paid attention to is that a great deal of fundamental theoretical research has been carried out. First, numerical simulation has become a good observation and a powerful tool to investigate HCCI and to develop control strategies for HCCI because of its greater flexibility and lower cost compared with engine experiments. Five types of models applied to HCCI engine modelling are discussed in the present paper. Second, HCCI can be applied to a variety of fuel types. Combustion phasing and operation range can be controlled by the modification of fuel characteristics. Third, it has been realized that advanced control strategies of fuel/air mixture are more important than simple homogeneous charge in the process of the controlling of HCCI combustion processes. The stratification strategy has the potential to extend the HCCI operation range to higher loads, and low temperature combustion (LTC) diluted by exhaust gas recirculation (EGR) has the potential to extend the operation range to high loads; even to full loads, for diesel engines. Fourth, optical diagnostics has been applied widely to reveal in-cylinder combustion processes. In addition, the key to diesel-fuelled HCCI combustion control is mixture preparation, while EGR is the main path to achieve gasoline-fuelled HCCI combustion. Specific strategies for diesel-fuelled, gasoline-fuelled and other alternative fuelled HCCI combustion are also discussed in the present paper.     

辛烷值对均质压燃发动机燃烧特性和性能的影响

通过不同比例的正庚烷和异辛烷混合得到不同辛烷值的混合燃料，在一台单缸直喷式柴油机上研究燃料辛烷值对均质压燃发动机燃烧特性、性能和排放特性的影响．研究结果表明，燃料辛烷值增加，着火始点推迟，燃烧反应速率降低，缸内爆发压力降低．燃料辛烷值增高，均质压燃向大负荷工况拓宽，燃料辛烷值较高时，存在极限转速，辛烷值增加，极限转速降低．对于每一工况，存在一个最佳经济性的燃料辛烷值，负荷增大，最佳辛烷值增高；随着燃料辛烷值增高，发动机NO、HC和CO排放增加，尤其是HC排放增加更为明显．对于均质压燃发动机，低负荷工况适合燃用低辛烷值燃料，高负荷工况适合燃用高辛烷值燃料。     

HCCI发动机着火过程控制参数的研究

为了研究HCCI发动机着火控制时刻影响因素,建立了模拟HCCI发动机燃烧的计算模型,以甲烷/丙烷混合物和正庚烷/异辛烷混合物作为燃料,考察了十六烷值、辛烷值、压缩比、燃空当量比、进气温度和压力等因素对HCCI发动机着火时刻的影响.计算结果表明:随着燃料十六烷值的减小或辛烷值的增加,相同条件下燃料的着火延迟期增加;压缩比、燃空当量比和进气温度的变化会引起燃料着火时刻的显著变化;进气压力的变化对燃料着火延迟期的影响较小;气体十六烷值越低,辛烷值越大,着火延迟期受上述参数变化影响越大.研究结果为HCCI发动机的优化设计和燃烧控制提供指导依据.     

废气再循环对二甲基醚均质压燃燃烧过程影响的试验研究

在一台单缸发动机上进行了废气再循环(EGR)对二甲基醚(DME)均质压燃(HCCI)燃烧过程影响的试验研究。结果表明，EGR比例小于20％对运行最大负荷工况范围影响不大；采用高比例EGR可以拓宽DME均质压燃运行工况范围，随着EGR率增大，HCCI运行的最大负荷工况增大，着火燃烧时刻推迟，燃烧放热率降低，缸内最大爆发压力降低，发动机热效率增大；EGR率小于75％，HC排放略有降低或相当，EGR率为75％时，HC排放显著增加；EGR率大于25％，随着EGR率增加，CO排放增大，小负荷工况尤其明显，在中高负荷工况，EGR率对CO排放影响较小。     

废气再循环和进气加热实现汽油机HCCI燃烧的性能对比

废气再循环和进气加热是实现汽油机HCCI燃烧的两种不同方式,其对HCCI燃烧性能的影响也不同,为此,在同一台汽油机上分别采用废气再循环和进气加热实现HCCI燃烧,并分析了其在HCCI燃烧性能上存在差异的机理.试验结果表明,相对于进气加热,废气再循环的工质比热容高,但由于稀释比较小,使得其工质总热容反而低,从而缸内燃烧温度高.废气再循环HCCI燃烧的未燃HC排放比进气加热的排放值低41%～59%;NOx排放是后者的2～20倍;而CO排放与负荷有关;其燃烧效率比进气加热HCCI的值高0.8%～14%.然而,由于进气加热的PMEP低,缸内工质比热比大,传热损失小,最终使得进气加热HCCI燃烧的ISFC比废气再循环HCCI燃烧的值低6.6%～16.4%.     

双燃料HCCI燃烧的优化策略

对有废气再循环(EGR)的情况下单一二甲基醚(DME)、DME／甲醇(Methanol)和DME/天然气(CNG)双燃料的均质压燃(HCCI)燃烧进行了实验研究．研究结果表明,单一DME的HCCI只能在小负荷下实现．采用DME／甲醇双燃料后,HCCI的负荷范围达到了原柴油机中高负荷水平．EGR能扩大双燃料HCCI的可控燃烧范围,但对扩展双燃料HCCI燃烧的负荷范围作用不大．分层燃烧技术有扩大双燃料发动机的负荷范围到大负荷的潜力．DME／甲醇双燃料HCCI的指示热效率要优于DME／CNG．在低负荷工况,采用单一DME加EGR的HCCI燃烧能获得更好的经济性指标．