多火花塞点火实现快速燃烧的试验研究

使用快速压缩-膨胀机改装的多火花塞点火试验台架,研究了火花塞数目对指示功大小、最高燃烧压力和最大压升率、燃料放热速率及爆震特性的影响。得到结果如下:液压源压力及混合气浓度相同,点火相位从上止点前5~35mm逐渐增大时,指示功先增大后减少,即存在最佳点火相位;随着火花数目的增加,最佳点火时刻后移;多火花塞点火的放热速率、最高燃烧压力和压升率、指示功均比单火花塞点火有明显提升,提升幅度和火花塞数目并不成线性关系;在混合气稀薄时,多火花塞点火对燃烧速率及指示功的提升作用更加明显;随着液压源压力的增大,相同点火相位时,多火花塞比单火花塞点火爆震倾向更加严重。     

活塞运动规律对点燃式HFPE燃烧过程影响的仿真研究

使用仿真软件Converge,建立了点燃式液压自由活塞发动机(HFPE)的三维仿真模型。为了解决HFPE的爆震问题,提升其热效率,研究了不同活塞运动规律对HFPE的燃烧过程、爆震倾向与热功转换效率的影响。结果表明:火花点燃式HFPE的爆震燃烧发生在活塞边缘的末端混合气区域,是由若干个爆震核快速扩展而成,与曲柄连杆式火花点燃发动机爆震的发生地点和机理相同;通过改变活塞的运动规律,使活塞上行速度加快,在上止点附近停留时间变短,可以明显减小发动机的爆震倾向与爆震强度。利用优化的活塞运动规律,加上高达14的高压缩比,可以在一定程度上提升发动机的热功指示效率。     

火花点火式液压自由活塞发动机的模拟试验研究

使用液压快速压缩-膨胀机(RCEM)对火花点火式液压自由活塞发动机(SI-HFPE)的单次工作过程进行了模拟试验研究,研究了在使用甲醇燃料的情况下,液压源压力、点火提前相位、混合气浓度等因素对SI-HFPE活塞运动规律、放热率及指示效率的影响,提出了使用双火花塞同时点火方案。研究结果表明:SI-HFPE的实际压缩比主要受液压源压力和点火提前相位的影响,液压源压力越高或点火提前相位越小,则实际压缩比越大。在10MPa的驱动液压力下,使用双火花塞同时点火,调整合适的点火提前相位,指示效率可以大于45%,此时的实际压缩比达到18。     

点燃式液压自由活塞发动机高增压燃烧过程的试验与仿真研究

在快速压缩-膨胀机上进行试验,模拟液压自由活塞发动机(hydraulic free piston engine,HFPE)在不同缸内初始压力下的单次燃烧做功过程,并利用OpenFOAM和CONVERGE三维计算流体力学(computational fluid dynamics,CFD)仿真平台进行增压仿真研究。结果表明:液压自由活塞发动机随着缸内初始压力的增大,相同压缩比下发动机循环周期缩短,活塞在上止点附近停留时间缩短,爆震极限压缩比增大,抗爆能力增强。适当提高缸内初始压力有利于提高指示效率,当缸内初始压力提高至0.15MPa时,发动机指示效率由0.30提高至0.31,但当初始压力达到0.20MPa后,指示效率又降至0.30。针对缸内初始压力进一步增大后出现的效率降低问题,在仿真研究中发现采用多火花塞点火方案,即使在初始压力0.80MPa的条件下也能得到较高的指示效率而不发生爆震。     

对置活塞二冲程汽油机多点点火燃烧特性研究

由于取消了气缸盖,对置活塞二冲程(OP2S)汽油机的火花塞只能布置于气缸侧壁。因此在单火花塞点火时,缸内火焰传播不对称,导致OP2S汽油机具有较明显的爆震倾向。为了有效地抑制爆震的发生,利用三维CFD软件对OP2S直喷汽油机在不同火花塞数目下缸内的燃烧过程进行了数值仿真,并结合试验研究了火花塞数目对混合气燃烧特性的影响,分析了不同火花塞数目对缸内燃烧压力、燃烧持续期及燃烧放热率的影响规律。研究结果表明,增加火花塞数量可以有效改善缸内的燃烧情况,而对侧布置双火花塞的布置形式为最佳方案。     

液压自由活塞发动机活塞运动规律自适应特性的研究

在一台液压自由活塞发动机(HFPE)样机上进行了活塞运动规率的试验。研究表明:活塞的运动规律对于燃烧相位和累积放热量的变动具有自适应性;随着燃烧相位的提前或累积放热量的增大,活塞换向提前,最大升程和压缩比降低;这种自适应性可有效避免均质压燃过程中的爆震与后燃现象,保证缸内最高压力、最大放热速率的稳定,减少指示功的损失。     

点火方式对汽油机燃烧过程的影响

通过基本结构的微小变动,将单火花塞点火(single spark ignition,SSI)改造成双火花塞点火(dual spark ignition,DSI),运用三维仿真软件AVL FIRE模拟仿真,并通过试验验证。再对单火花塞点火、双火花塞同步点火(dual synchronous spark ignition,DSSI)、异步点火(dual asynchronous spark ignition,DASI)3种不同的点火方式进行对比。结果表明:在6500 r/min转速全负荷状态下,空气过量系数为1.00而其他参数调整为最佳时,单火花塞的最佳点火提前角为29°,在空气过量系数为1.15的最佳参数下,双火花塞同步点火的最佳点火提前角为22°,双火花塞异步点火的最佳点火提前角为22°和24°。其中,发动机综合性能在双火花塞异步点火条件下表现最好:相对于单火花塞点火指示功提升8.49%;相对于同步点火,可将最高燃烧压力和压缩负功减小,指示功提升3.60%;同时改善了排放性。上述研究中发动机均控制在未发生爆震情况下。     

活塞式航空煤油发动机性能优化及爆震抑制研究

本文基于一台压缩比可变的单缸热力学发动机,使用自主开发的空气辅助喷射系统,在全负荷条件下,开展了活塞式航空煤油发动机性能优化及爆震抑制的试验研究。探究了采用双点火、降低压缩比以及使用CO2辅助喷射对航空煤油发动机的性能及爆震抑制的影响研究。结果表明,采用双点火可以有效提高航空煤油火焰传播速率,提高燃烧相位,降低循环波动,并且有抑制爆震的作用;通过降低压缩比有效实现了爆震抑制,解决在较高压缩比下航空煤油发动机只能运行在小负荷区间的难题,压缩比降至6,发动机实现全负荷运行,动力性、经济性较好,且不易发生爆震;采用CO2辅助航空煤油喷射时,随着CO2脉宽的增加,同一点火时刻下,发动机的动力性经济性下降,但由于CO2的抑制爆震的作用,MBT点火时刻最大可提前至14 °CA BTDC,使得燃烧相位提前,发动机燃烧效率提高。     

高压缩比下扫气式预燃室湍流射流点火对废气再循环稀释汽油机性能影响研究北大核心CSCD

基于一台可调压缩比(compression ratio,CR)的单缸发动机和自主设计的湍流射流点火(turbulent jet ignition,TJI)系统,开展高压缩比下扫气式预燃室湍流射流点火对废气再循环(exhaust gas recirculation,EGR)稀释汽油机性能影响的研究。研究发现在高EGR率时,扫气式TJI的点火方式燃烧稳定性最高,可以在EGR率超过30%时实现稳定燃烧。提升压缩比对提升TJI在高EGR率下的燃烧稳定性有积极作用,然而对提升高EGR率下火花塞点火(spark ignition,SI)的稳定性作用不大。对于TJI,在低EGR率时提升压缩比会造成发动机强烈爆震,过于推迟点火造成燃烧定容度下降,燃油消耗率上升。在高EGR率时,发动机爆震受到抑制,可以提前点火优化燃烧相位,降低燃油消耗率,在压缩比15时最低燃油消耗率相比压缩比11时降低2.2%。高EGR率时,提升压缩比有利于提升燃烧速率,降低滞燃期和燃烧持续期,提升发动机燃烧稳定性。在EGR率为30%而压缩比为15时,逐渐提前点火时刻会加大末端混合气自燃倾向,放热率出现两阶段高峰。     

基于当量燃烧的天然气发动机燃烧室优化研究

在一台采用废气再循环(exhaust gas recirculation,EGR)策略的当量燃烧天然气发动机上开展了不同挤气比、压缩比的活塞对燃烧、热效率和排放影响的对比试验研究。结果表明:在50%负荷与中低转速75%负荷下,增大EGR率拓展了爆震边界,使得主燃烧相位(CA50)提前,指示热效率提高;而在100%负荷及高转速75%负荷下,EGR率的增大对燃烧持续期的延长作用更为明显,且CA50后移,指示热效率降低。增大压缩比和适当增大挤气比有利于增强缸内湍流运动,加快天然气火焰传播速度,使CA50更靠近上止点,热功转换效率提高,最高指示热效率提高了0.24%,NO_x和CH_4排放分别升高了2.30g/(kW·h)、0.55g/(kW·h)。进一步增大挤气比会受到爆震的限制,最佳点火时刻推迟,燃烧定容度小,燃烧持续期延长,最高指示热效率下降了0.51%,NO_x和CH_4排放分别降低了2.20g/(kW·h)、0.44g/(kW·h),CO排放升高了0.36g/(kW·h),因此挤气比存在一个优化的范围。     

Knocking combustion in spark-ignition engines

Knocking combustion research is crucially important because it determines engine durability, fuel consumption, and power density, as well as noise and emission performance. Current spark ignition (SI) engines suffer from both conventional knock and super-knock. Conventional knock limits raising the compression ratio to improve thermal efficiency due to end-gas auto-ignition, while super-knock limits the desired boost to improve the power density of modern gasoline engines due to detonation. Conventional combustion has been widely studied for many years. Although the basic characteristics are clear, the correlation between the knock index and fuel chemistry, pressure oscillations and heat transfer, and auto-ignition front propagation, are still in early stages of understanding. Super-knock combustion in highly boosted spark ignition engines with random pre-ignition events has been intensively studied in the past decade in both academia and industry. These works have mainly focused on the relationship between pre-ignition and super-knock, source analyses of pre-ignition, and the effects of oil/fuel properties on super-knock. The mechanism of super-knock has been recently revealed in rapid compression machines (RCM) under engine-like conditions. It was found that detonation can occur in modern internal combustion engines under high energy density conditions. Thermodynamic conditions and shock waves influence the combustion wave and detonation initiation modes. Three combustion wave modes in the end gas have been visualized as deflagration, sequential auto-ignition and detonation. The most frequently observed detonation initiation mode is shock wave reflection-induced detonation (SWRID). Compared to the effect of shock compression and negative temperature coefficient (NTC) combustion on ignition delay, shock wave reflection is the main cause of near-wall auto-ignition/detonation. Finally, suppression methods for conventional knock and super-knock in SI engines are reviewed, including use of exhaust gas recirculation (EGR), the injection strategy, and the integration of a high tumble - high EGR-Atkinson/Miller cycle. This paper provides deep insights into the processes occurring during knocking combustion in spark ignition engines. Furthermore, knock control strategies and combustion wave modes are summarized, and future research directions, such as turbulence-shock-reaction interaction theory, detonation suppression and utilization, and super-knock solutions, are also discussed.     

Advancing lean combustion of hydrogen-air mixtures by laser-induced spark ignition

We investigate how ignition through laser-induced plasma can improve the application of lean combustion, in particular in environmental conditions relevant to hydrogen internal combustion engines (ICE). Major design goals when developing combustion engines are increasing thermal efficiency and decreasing combustion emissions. High compression ratios, lean combustion and precise ignition timings are contributing factors in ICE optimization. In our studies, several gains from laser spark ignition are investigated. The high energy content of laser-induced ignition kernels are shown to speed up the development of the early flame kernels. These extended ignition kernels transfer into self propagating flames even in lean mixtures. Leaner mixtures are ignited in our experiments using laser spark ignition in comparison to conventional electrical spark plugs. Precise ignition timing is realized. Multi-point ignitions are synchronized on the timescale of microseconds to enhance the progress of combustion. We modified the locus of ignition in a mixture flow to decrease the temporal extent of flame contact with the wall. Therefore, burning duration and heat loss can be reduced.     

不同过量空气系数下射流点火发动机燃烧、排放和爆震特性的试验研究

基于单缸试验机研究了过量空气系数对射流点火发动机性能的影响.通过分析发动机性能曲线、缸内燃烧情况及爆震特性探究射流点火最佳运行区间,并与火花点火燃烧方式进行对比.结果表明,射流点火可以有效提升瞬时放热率并拓展发动机稀燃极限,缩短缸内混合气滞燃期与燃烧持续期,同时燃油经济性有一定提升.在稀燃条件下氮氧化物排放极低.爆震方...     

基于高压直喷航空煤油点燃式发动机燃烧性能优化的试验研究

为提高航空煤油在点然式发动机中的燃烧热效率,改善发动机爆震及拓宽发动机负荷范围,以3号航空煤油(RP-3)为基础燃料,基于一台单缸水冷、压缩比可调、4冲程点燃式发动机结合高压共轨缸内直喷技术,开展了单双点火、不同负荷、压缩比、喷射压力、喷射时刻和两次喷射策略下航空煤油燃烧特性的试验研究。结果表明,在原机压缩比为10的条件下,将直喷汽油改为直喷航空煤油后,由于航空煤油的抗爆性差、雾化困难、燃烧速率慢等理化特性,发动机的动力性损失约50.0%,油耗增加约60.0%,循环波动也大幅增加;相比于单点火,双点火可使缸内平均有效压力提高,燃烧相位提前,循环波动降低;为了抑制高压缩比下的爆震倾向,可通过降低压缩比来拓宽负荷范围,恢复原机功率。随着压缩比的降低,有效平均压力(BMEP)持续增大,当压缩比为6时,最大转矩可达39.5N·m,功率恢复至原机的88.0%。同时耦合高压及两次喷射策略,随着喷射压力的增大,有效燃油消耗率(BSFC)减小约30.0%,经济性有所提高。相比于单次喷射,采用两次喷射策略可降低油耗,提升缸内有效平均压力,提升燃烧效率,最终可实现发动机燃用航空煤油的性能接近原机水平,最大负荷达原机的90.0%且油耗增加量不超过15.0%。     

Part-load characteristics of direct injection spark ignition engine using exhaust gas trap

Internal exhaust gas recirculation (EGR) is an effective strategy to reduce pumping loss and improve fuel economy using mixture dilution than traditional external EGR. In this paper, the internal EGR was obtained by exhaust gas trap (EGT) using the negative valve overlap (NVO) method. The effects of EGT on the part-load characteristics, including energy conversion, combustion and emission characteristics were studied in a direct injection spark ignition (DISI) engine. The experimental results showed that EGT can save fuel consumption by 5–16% due to reduced pumping loss and improved combustion efficiency, while it also can increase the engine cyclic variation and combustion duration. The engine cyclic variation increases with increasing of the EGT level; this can be overcome by advancing spark timing to stabilize the combustion. The flame propagation and compression combustion occurred simultaneously when high EGT level and high compression ratio were adopted; the combined combustion can reduce combustion duration but increase the engine cyclic variation. The stratified mixture using the two-stage injection strategy can reduce the engine cyclic variation and shorten the combustion duration so as to improve the thermal efficiency. Moreover, the second injection mass ratio and timing take an important effect on the combustion and emission characteristics in DISI engines using EGT strategy.