气门正时对柴油燃料HCCI燃烧影响的初步实验研究

开发了变进排气正时控制机构,实现气门正时的调节,采用在进气上止点前进行柴油燃料的喷射,利用缸内残余高温废气余热加速燃油蒸发,实现了柴油燃料的HCCI燃烧,同时,研究了不同气门重叠期下HCCI燃烧的燃烧特性,不同负荷的工作稳定性和排放特性,结果表明,对于低温自燃性好的柴油燃料,排气门早关和进气门晚开引起的缸内温度升高比由此引起的残余废气增加对工质的稀释效果更大,使HCCI燃烧的着火始点提前,易引起大负荷工况HCCI燃烧的工作粗暴,但有利于小负荷工况HCCI燃烧的工作稳定性。     

小升程凸轮轴发动机HCCI燃烧特性的研究

为了在发动机的低速低负荷区实现均质充量压缩着火（HCCI）燃烧，设计了气门升程小和气门开启持续期短的进、排气门凸轮轴，并将其安装在Ricardo Hydra单缸汽油机上。试验研究了发动机使用理论空燃比混合气时的燃烧情况，结果表明，使用负气门重叠角可以在低速低负荷区实现HCCI燃烧。在HCCI燃烧方式下运行时的平均指示压力（PIMEP）依赖于气门定时和发动机转速。排气门关闭越早，缸内的残余废气量增加，每循环进气量减少，燃烧持续期变长，PIMEP减小，然而泵气损失减小；进气相位对PIMEP的影响小于排气相位的影响；高的发动机转速对燃烧过程的影响类似于排气门早关．     

汽油HCCI发动机燃烧相位及瞬态过程的控制

由于HCCI燃烧受化学动力学控制,燃烧相位控制一直是HCCI面临的挑战之一.通过缸内直喷方式,可以灵活控制HCCI燃烧相位.研究表明,在HCCI失火界限,在负气门重叠期喷入燃油,能够实现燃油重整,通过调整不同的负气门重叠喷油量,可以控制燃烧相位;在HCCI爆震界限,负气门重叠期的喷油会导致燃烧粗暴,NOx排放大量上升,而在压缩行程喷入燃油形成分层混合气,能够有效地控制燃烧相位,且不引起排放的恶化.通过负气门重叠喷油形成的组分控制和压缩行程喷油形成的浓度控制相结合,能够在整个HCCI运行范围内有效地控制燃烧相位.通过考察燃烧相位调整的瞬态过程,发现通过缸内直喷技术,能够在一个发动机循环内实现燃烧相位的调整,控制灵活,响应迅速.     

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。     

过量空气系数对HCCI汽油机燃烧特性的影响

在一台Ricardo Hydra单缸四气门汽油机上,利用气门重叠负角方法实现了均质充量压缩着火(HCCI)燃烧,并通过试验研究了过量空气系数对HCCI汽油机燃烧特性的影响.研究结果表明,在相同的转速和气门相位角下,随着过量空气系数的增加,平均指示压力减小,缸内残余废气率也减小,但燃油消耗率的变化趋势与转速有关.在大多数工况下,过量空气系数为1.05时,HCCI发动机的着火时刻最早,燃烧持续期最短.过量空气系数对循环波动的影响与转速和气门相位角有关.随着转速的增加,循环波动增大.     

基于可变气门定时策略的HCCI汽油机试验研究

在电控气口喷射四冲程单缸试验机上,利用特殊设计的小包角配气凸轮,通过负气门重叠角实现了由内部残余废气控制的汽油HCCI燃烧,详细研究了气门定时参数对HCCI燃烧的影响.结果表明,就进排气门定时比较而言,排气门关闭时刻对内部EGR率和负荷的影响更大,而进气门开启时刻对HCCI燃烧的影响相对较小.在进排气门相位对称条件下,随着气门重叠负角的减小,最大压力升高率增加,着火时刻提前,负荷也增大.随着转速的增加,内部EGR率增加,排气温度升高,着火时刻也提前.通过调整气门定时,在不需要进气加热的条件下,可在转速880～4 000 r/min,负荷0.25～0.75 MPa（pIMEP）的范围实现HCCI燃烧.     

进气上止点燃油喷射实现柴油HCCI燃烧的试验研究

提出了采用在进气上止点附近进行燃油喷射,通过内部EGR促进燃油蒸发混合的方法,实现柴油燃料的均质压燃(HCCI),并通过副喷嘴喷射引燃柴油解决HCCI发动机的冷启动困难问题．试验表明该方法有利于在缸内形成均质混合气,在常温进气的情况下就可得到较好的HCCI燃烧效果．同时,还对影响HCCI燃烧的发动机负荷和进气温度进行了研究,认为由这些因素引起的缸内温度变化是影响HCCI燃烧的主要原因。     

进排气门相位和升程对汽油机HCCI燃烧控制的试验研究

利用内部高温残余废气加热新鲜充量是实现均质压燃（HCCI）的一种有效方式。设计了几种具有小持续角的特殊进、排气凸轮轴，以满足高内部残余废气率。在Ricardo Hydra 140单缸汽油机上，分别采用不同持续角的凸轮轴研究了进、排气门定时和升程等参数对汽油机HCCI燃烧的影响。试验结果表明，排气门定时对残余废气率和HCCI燃烧有着更强的控制作用。选择合适的气门升程可以拓展HCCI运行范围，升程大的凸轮轴将HCCI运行范围向高速大负荷方向拓展，而升程小的凸轮轴则有利于获得更低的HCCI运行负荷和转速。     

缸内温度对柴油均质燃烧NOx排放影响的试验研究

采用在进气上止点附近进行燃油喷射,通过缸内高温残余废气促进燃油蒸发混合的方法,在单缸135柴油机上实现了柴油燃料的均质压燃(HCCI).对HCCI燃烧的燃烧特性和排放特性进行了测试分析,根据实测示功图计算了HCCI燃烧缸内温度,分析了其对NOx排放的影响,当缸内温度超过1700K时,将使HCCI燃烧的NOx排放显著增长.     

VVT对GDI发动机中等负荷燃烧及排放的影响

为了应对日益严格的排放法规,充分挖掘VVT技术对增压直喷汽油机在节能减排方面的潜力,试验研究了VVT技术对车用GDI发动机最常用工况燃烧及排放的影响.结果表明:适度调节进排气VVT,燃油经济性能够得到有效改善,最大改善5.8％;随着气门重叠角的增大,燃烧滞燃期和持续期增大,CA50延后,最大缸内压力、最大放热率降低,T...     

Study of low emission homogeneous charge compression ignition (HCCI) engine using combined internal and external exhaust gas recirculation (EGR)

This paper focuses on the effects of internal and cooled external exhaust gas recirculation (EGR) on the combustion and emission performance of diesel fuel homogeneous charge compression ignition (HCCI). The use of fuel injection before the top center (TC) of an exhaust stroke and the negative valve overlap (NVO) to form the homogeneous mixture achieves low NO_x and smoke emissions HCCI. Internal and external EGR are combined to control the combustion. Internal exhaust gas recirculation (IEGR) benefits to form a homogeneous mixture and reduces smoke emission further, but lower the high load limits of HCCI. Cooled external EGR can delay the start of combustion (SOC) effectively, which is very useful for high cetane fuel (diesel) HCCI because these fuels can easily self-ignited, making the SOC earlier. External EGR can avoid the knock combustion of HCCI at high load, which means it can expand the high load limit. HCCI maintains low smoke emission at various EGR rates and various loads compared with a conventional diesel engine because there are no fuel-rich volumes in the cylinder.     

乙醇燃料HCCI发动机燃烧特性研究

应用调整进排气门相位控制缸内残余废气率策略,在Ricardo Hydra四冲程进气道喷射单缸试验机上实现了无水乙醇燃料的均质压燃,获得了运行工况范围,并分析研究了空燃比、转速和气门相位对乙醇燃料均质压燃的燃烧特性。结果表明:乙醇燃料的均质压燃的可运行范围仍然受到爆震、换气过程及失火的限制,但在高速及稀燃区域得到拓展;其着火时刻及燃烧持续期依赖于气门定时、空燃比及转速。     

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。     

Active fuel design—A way to manage the right fuel for HCCI engines

Homogenous charge compression ignition (HCCI) engines feature high thermal efficiency and ultralow emissions compared to gasoline engines. However, unlike SI engines, HCCI combustion does not have a direct way to trigger the in-cylinder combustion. Therefore, gasoline HCCI combustion is facing challenges in the control of ignition and, combustion, and operational range extension. In this paper, an active fuel design concept was proposed to explore a potential pathway to optimize the HCCI engine combustion and broaden its operational range. The active fuel design concept was realized by real time control of dual-fuel (gasoline and n-heptane) port injection, with exhaust gas recirculation (EGR) rate and intake temperature adjusted. It was found that the cylinderto- cylinder variation in HCCI combustion could be effectively reduced by the optimization in fuel injection proportion, and that the rapid transition process from SI to HCCI could be realized. The active fuel design technology could significantly increase the adaptability of HCCI combustion to increased EGR rate and reduced intake temperature. Active fuel design was shown to broaden the operational HCCI load to 9.3 bar indicated mean effective pressure (IMEP). HCCI operation was used by up to 70% of the SI mode load while reducing fuel consumption and nitrogen oxides emissions. Therefore, the active fuel design technology could manage the right fuel for clean engine combustion, and provide a potential pathway for engine fuel diversification and future engine concept.     

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。     

Imaging studies of in-cylinder HCCI combustion

An optically accessed, single cylinder engine operated in homogenous charge compression ignition (HCCI) mode with negative valve overlap (NVO) strategy was used to perform combustion processes diagnostics under premixed conditions corresponding to the low load regime of the HCCI operational envelope. The aforementioned processes analysis was conducted utilizing synchronized simultaneous combustion event crank-angle resolved images, acquired through piston crown window with in-cylinder pressure recording. This investigation was carried out for one-step ignition fuel—standard gasoline, fuel proceeding single-stage ignition process under conditions studied. The initial combustion stage is characterized by a maximum local reaction spreading velocity in the range of 40–55 m/s. The later combustion stage reveals values as high as 140 m/s in case of stoichiometric combustion. The mixture as well as combustion stages effects are pronounced in these observed analytical results.     

Diethyl ether as an ignition improver for acetylene-fuelled homogeneous charge compression ignition operation: an experimental investigation

In this study, a single-cylinder, water-cooled, direct-injection, compression ignition (CI) engine is operated on the homogeneous charge CI (HCCI) mode using acetylene and diethyl ether (DEE) as fuels. Acetylene is inducted as a main fuel and a small quantity of DEE is injected into the intake manifold for controlling combustion phasing. The engine performance and exhaust emission characteristics are investigated experimentally under various operating conditions. From the results, it is found that brake thermal efficiency of the acetylene-DEE HCCI mode is comparable with that of the conventional CI mode. It has a wide operating range of 0–4.2 bar brake mean effective pressure without exhaust gas recirculation compared with the liquid fuels-HCCI mode. The engine could easily start in the HCCI mode using DEE alone. Nitric oxide and smoke emissions are at ultra-low level. Hydrocarbon emissions are comparatively lower at lower loads and higher at higher load conditions. Carbon monoxide emissions are comparable with that of the conventional CI mode.