Simulation Research On Application Of EGR To A GDI Engine

Automotive industry develops rapidly in China today.The energy crisis compelsthe automotive manufacturer to improve fuel economy and reduce emissions.Currently,most GDI engines still use the throttle to control load, so the pumping loseis the main factor which contribute to low fuel economy in part load.The gasolinethermal efficiency is also limited by the low compression ratio to restrain knock. EGRcan be used to reduce the pumping lose in part load and to restrain knock tendency inheavy load.According to these issue,this paper studied the application of EGR on a GDIengine by the three-dimensional simulation software AVL-fire.In partial load,theeffect of EGR ratio and temperature of EGR on pumping lose,combustion process isstudied,and meanwhile, the influence of advance angle of ignition to combustion isdiscussed. In heavy load,the effect of EGR ratio,temperature of EGR on knocktendency is discussed and the three way (delayed ignition,high EGR ratio,low EGRtemperature)to restrain knock is compared.Some conclusion is obtained:1.The use of EGR increases the amount of intake gas when the mass of injectedfuel and air-fuel ratio is constant.As a result,the movement speed of gas in cylinder ishigher and it can accelerate the fuel evaporation so it is benefit to form a moreuniform mixture. Improve intake temperature can also shorten the fuel evaporationtime. Therefore,it is benefit to the air-fuel mixture process when the rate andtemperature of EGR is enhanced.2.Along with the increase of EGR ratio,the pumping loss decreases. The pumpingloss decreased34.2%when the EGR ratio rose from0to25%. The similar result canalso achieved by improving the EGR temperature. The pumping loss decreased19.2%when the EGR temperature increased from300K to340K. However,EGR is harmfulto combustion process. With the increase of EGR ratio,the flame spreadslower,ignition delay and combustion duration gets longer and heat release rate decreased. All these negative effects can be reduced at a certain degree by enhance thetemperature of EGR.3.As the sparking time is advanced,the ignition delay and combustion durationbecomes short,the heat release rate and explosion pressure rise. Advancing thesparking time can make the combustion process better with limited effect.4.Higher EGR rate and lower EGR temperature are proved to be effectivemethods to reduce knock tendency. The higher EGR ratio,the lower combustiontemperature. So the end mixture absorb less heat and the knocking tendency decreases.With lower EGR temperature, the end mixture need more time to prepare autoignitionthen the knocking tendency decreases.5.Three way (delayed ignition,high EGR ratio,low EGR temperature)to restrainknock is compared. When the same restrain effect is achieved,ignition delay and highEGR rate made the explosion pressure reduced by2.2MPa.By contrast,the lowtemperature method reduced the explosion pressure by1MPa.Therefore,compared todelayed ignition, low EGR temperature is more efficient to restrain knock.