Investigation On The Effects Of Exhaust Gas Recirculation On The Combustion And Emission Characteristics For Direct Injection Spark Ignition Engines

With the increasingly prominent energy and environmental problems,many countries around the world have issued relative laws and regulations to restrict the fuel economy and pollutant emissions of vehicles,many automobile manufacturers are devoted to develop more efficient and clean vehicle powertrains.Presently,direct injection spark ignition(DISI)gasoline engines have been widely applied in light-duty vehicle powertrain systems,while there are still some problems such as higher particulate matter emissions and poor fuel economy at part loads.In this work,experiments are conducted on a 1.8L turbocharged direct injection gasoline engine to systematically investigate the effects of exhaust gas recirculation(EGR)on the combustion characteristics,gaseous emissions and particulate matter emissions of direct injection gasoline engines under different operating parameters.The results showed that:For different loads,the effects of EGR on combustion and emissions characteristics are similar.When EGR is introduced,the cylinder pressure and temperature are reduced,the combustion duration is prolonged and the heat release rate becomes more mild.For the part load,the effect of EGR is more obvious.Moreover,with increasing EGR rate,the nitrogen oxides(NO_x)and the carbon oxides(CO)emissions are decreased,the total hydrocarbon(THC)emissions are slightly increased,and total particle number concentration are decreased.For DISI engines,fuel injection timing and excess air ratio have great impact on particle number concentration.For early injection timings,particulate number concentration significantly increases due to the pool fire caused by more fuel impingement on the piston crown.Retarding fuel injection timings,the time for fuel atomization and mixture formation become shorter and total particle number concentration therefore increases.As the excess air ratio decreases,the mixture through the cylinder becomes richer and particle number emissions increase gradually.Under different operating parameters,the impact of EGR on particulate emissions is quite different.For early injections during the intake stroke,with the increasing EGR rate,the total particle number concentration increases.For the middle-stage and late injections during the intake stroke,as the EGR rate increases,total particle number concentration is gradually decreased.When the excess air ratio is less than 1,total particle number concentration increases significantly with increasing EGR rate.When excess air ratio is greater than 1,however,total particle number concentration would reduce with increasing EGR rate.The effect of tumble ratio on combustion characteristics also showed difference with different injection timings.For early and middle-stage injections during the intake stroke,with the increase of the tumble ratio,the combustion duration is significantly shortened.The combination of high tumble ratio with EGR can further improve fuel economy,due to the effectively shortened combustion duration.However,for late injection timings,the combustion duration with high tumble ratio is longer than that with low tumble flow.If EGR is introduced,the combustion duration is further prolonged,The cycle-to-cycle variation is increased,and the combustion stability is reduced.