The Investigations Of Effects Of Direct-injection Ratios And Control Parameters On Combustion And Particulate Emissions In A Dual-injection Gasoline Engine

Though Gasoline Direct Injection（GDI）engines have better power and fuel economy performances,they tend to generate much more particulate emissions than traditional Port Fuel Injection（PFI）engines.The upcoming ChinaⅥemission standards have set stringent PN regulations(6×10¹¹#/km),posing a great challenge for the particulate emissions control of GDI engines.Dual-injection,which combines port-injection with direct-injection,is able to take full advantages of two different fuel supply modes to ensure good efficiency while reducing PN emissions.In this paper,an electronic control system was developed to control the dual-injection engine which was modified from a GDI engine.The engine bench tests were conducted to investigate the impacts of direct-injection ratios and control parameters on combustion and particulate emissions characteristics.The results of the experiments for direct-injection ratios indicate that,compared with the direct-injection mode,the order of magnitudes of PN emissions of dual-injection with a direct-injection ratio of 50%is reduced from 10⁷#/cm³ to 10⁶#/cm³,with the maximum decreases of particulate matter located at low speed and loads.When the direct-injection ratio is reduced,the peaks of in-cylinder pressure increases,and the heat release is advanced gradually.At the same time,CO emission is significantly reduced,while NOx emission is slightly increased.With the decrease of direct-injection ratio,PN emissions of the engine almost present a linear decline,and when the direct-injection ratio is less than 20%,the PN emission of dual-injection is approaching that of the PFI mode.The reduction of direct-injection ratio makes the engine particle size distributions shift towards smaller size regions.At low coolant temperatures（30℃）for dual-injection mode(R_DI=60%),the combustion phasing is advanced,and PN emissions are decreased by more than 80%compared with that of the direct-injection mode.Both in lower and normal coolant temperature conditions,dual-injection can effectively reduce the oxidation activation energy of exhaust particulates,improving the oxidation activity of soot particles compared with that of the direct-injection mode.The results of the experiments for control parameters indicate that,there exists the best direct-injection timing to reach lowest PN emissions in the direct-injection mode.The best direct-injection timing of dual-injection is closer to the exhaust top dead center than the direct-injection mode.No matter in direct-injection mode or dual-injection mode,increasing the direct-injection pressure can effectively reduce the PN emissions of the engine.In addition,the geometric mean diameter（GMD）of the particles also gradually decreases with the increase of direct-injection pressure.Under the low coolant temperature conditions,as the ignition timing is delayed,the PN emissions of dual-injection decrease first and then increase slightly,while the PN emissions of the direct-injection mode continuously decrease.In addition,the delayed ignition angle tends to reduce combustion stability and accumulation PN fractions.