【作者】 杨志伟；

【导师】 赵华； 张岩；

【作者基本信息】 天津大学 ， 动力工程， 2017， 硕士

【摘要】 汽油机作为使用最为广泛的的动力机械,如何提高其燃油经济性一直是工业界关注的课题。为削弱泵气损失大、压缩比和工质比热比低等问题对汽油机经济性,尤其是部分负荷热效率的制约作用,本文基于配备进气道喷射（Port Fuel Injection,PFI）和缸内直喷（Direct Injection,DI）双喷系统的高压缩比米勒循环汽油机,系统探究了进气门早关策略对汽油机热效率的影响规律。针对高压缩比米勒循环小负荷失稳和中高负荷爆震加剧问题,分别利用分层火焰引燃（Stratified Flame Ignition,SFI）混合燃烧技术和进气道喷水结合缸内汽油直喷的复合喷射策略实现对燃烧过程的优化控制,提升燃烧稳定性和经济性。首先,系统研究了进气门早关策略下米勒循环度、有效膨胀比和点火角等参数对换气过程、燃烧过程、稳定性的影响规律。研究表明,通过调节米勒循环度和有效膨胀比可实现发动机“虚拟小型化”,有效调控歧管压力和泵气损失。通过综合优化米勒循环度、有效膨胀比和点火时刻,相较于原机在2000r/min,IMEP3.6bar、6bar和8bar指示热效率分别提升了15.7%,10.3%和10.9%。其后,针对米勒循环小负荷燃烧失稳问题,系统探究了SFI燃烧模式下直喷比例、直喷正时及火花点火对放热率形态和燃烧稳定性的调控规律。结果表明,在2000r/min,IMEP3.6bar工况,相较于均质混合气燃烧,SFI燃烧可以强化前期火焰传播,提高燃烧稳定性,COV_（IMEP）降幅达68%,而后期自燃的参与使得放热过程更为集中迅速,燃烧等容度和指示热效率分别提升了11.2%和3.8%。最后,针对高压缩比带来的中高负荷爆震加剧问题,探究了进气道喷水结合缸内燃油直喷的复合喷射策略对燃烧过程和最大压升率的调控作用。研究结果表明,1600r/min,IMEP6.0bar工况试验结果表明,进气道喷水结合缸内汽油直喷的策略能在保持较低的压升率和循环波动的前提下,将指示热效率由纯DI模式下的37.4%提升至38.8%。综上所述,米勒循环能有效降低汽油机部分负荷泵气损失,但小负荷燃烧稳定性下降及中高负荷爆震问题限制了其经济性提升和工程化应用。对此,本研究分别采用SFI燃烧模式和气道喷水策略实现对高压缩比米勒循环燃烧过程的优化控制,改善燃烧稳定性和经济性。更多还原

【Abstract】 Apart from high pumping loss at part load and low compression ratio,the thermal efficiency of traditional gasoline engine also suffers from low specific heat ratio for engine has to run under stoichiometric condition to meet legal emissions standards,which leaves much to be desired.In order to improve its thermal efficiency,systematic research regarding early intake valve closure（EIVC）strategy’s influence on fuel economy was conducted on a 4-cylinder Miller cycle gasoline engine equipped with PFI/DI system.Meanwhile,stratified flame ignition（SFI）was achieved through orchestrated fuel injection strategy,which effectively enhanced thermal efficiency and combustion stability of Miller cycle engine at part load.Moreover,study show that combining water injection in the intake port with gasoline direct injection can further optimize the combustion process during medium/high load,which has extra benefits on fuel economy.Firstly,experiments were carried out to study the impact of Miller cycle ratio,effective expansion ratio and spark timing on pumping loss,combustion process and combustion stability.Study show that‘virtual downsizing’can be achieved through adjusting Miller cycle ratio and effective expansion ratio,the interplay between them has strong regulative ability over intake manifold pressure and pumping loss,which have huge potential on improving combustion process when combining optimized ignition timing.At 2000rpm,bench test results show that 15.7%,10.3%and 10.9%improvement in indicated thermal efficiency was achieved at operating load of IMEP3.6bar,IMEP 6.0bar and IMEP 8.0bar respectively.Then,in order to solve the combustion instability that occurs when Miller cycle engine operating at low load,the ratio of fuel injected through DI,injection timing of DI and spark timing’s effects on heat release trace and combustion stability were systematically studied,which leads to successful deployment of SFI.Research indicate that SFI can strengthen and stabilize the early flame propagation process due to fuel stratification,up to 68%decrease in COV_（IMEP）was observed.The Auto-ignition in the second stage of SFI combustion also shortened the combustion duration,which gained extra 3.8%benefits in indicated thermal efficiency,compared to homogeneous stoichiometric combustion.Finally,intake port water injection strategy and its impact on combustion process and maximum PRR was investigated as it has great potential to suppress knock.Study show that it can lower in-cylinder temperature thus making it possible for spark timing to advance without knock,which makes for better CA50 and less heat transfer loss.It can further boost the fuel economy of gasoline engine while keeping the maximum PRR and COV_（IMEP）under control,comparing to GDI engine without intake port water injection.In a nut shell,Miller cycle can effectively reduce the pumping loss as well as heat transfer loss of traditional gasoline engine,leading to better fuel economy,but the combustion instability during low engine load and severe knock during high load still limit its application in engineering fields.However,the deployment of SFI combustion can strength the flame propagation in the early stage of combustion process through fuel stratification,which can stabilize the combustion at low load while enhance the thermal efficiency at the same time.Strategy like intake port water injection provides another way to improve fuel economy at medium/high load as it can effectively control the maximum PRR and optimize the combustion phase.更多还原