Partial Load Thermal Efficiency Optimization Of High Compression Ratio Miller Cycle Gasoline Engine

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.