Simulation Research On EGR Stratification In A GDI Engine Based On Intake Port Design

The EGR technology was originally designed to solve the problem of NOxemissions in engines, it can suppress the combustion temperature through piping theexhaust gas into cylinder in order to reduce NOxemissions. But for gasoline engines,the pumping loss is the main bottleneck, the most effective way to reduce thepumping loss lies in cancelling the throttle, this would require using EGR technologyto increase the substance in cylinder to reduce the pumping loss, furthermore it’sconducive to inhibit the detonation in gasoline engine and improve the efficiency. Butthe piping into of EGR would affect the normal combustion of gasoline engine in acertain extent. Therefore, EGR stratification technology appeared at the historicmoment, it focused on separating the exhaust gas and the air in the cylinder, making itpresent the hierarchical state, ensuring that the area near the spark plug wouldn’t beaffected by the concentration of exhaust gas at ignition moment. And the most idealform of stratification is keeping the exhaust gas near the cylinder wall, and thecombustible mixture distributing in the middle area, thus playing the role of theexhaust gas. So the stratified EGR technology which can meet the future NOxemissions and fuel economy at the same time has shown great superiority, and it’sbecoming a hotspot of EGR technology.This article applied FIRE of the CFD fluid simulation software, simulated andstudied the effects of the GDI engine stratified EGR based on different airway modeldesigns of a direct injection gasoline engine, in order to determine the influence ofdifferent structure forms on layered effect in cylinder, finally provided reference basisfor the experimental studies and necessary technical guidance.This paper studied the effects of different inlet modes, EGR pressures and thecrankshaft stopping phase of EGR on stratified EGR in cylinder. Through thesimulation analysis obtained the following conclusion:1, For the pattem of adding the partition in airways, because of the limitation ofstructures, the retentivity of hierarchical effects in cylinder are worse. In the intake stroke,there can maintain a good hierarchical effect, but the mixing becomes worsewhen reaching TDC on the compression stroke. For the patterns of adding the catheterin airways, the effect of EGR pressures on stratified EGR is larger. When the EGRpressure is120kPa and the crankshaft stopping phase is420°CA, got better effect ofstratified EGR which got about15%of the EGR gradient near the TDC.2, For the pattem of combining tangential airway with spiral airway, to achievethe former EGR gradient, the EGR pressure needs to be higher than the former. Theresults show that when the EGR pressure is130kPa and when the crankshaft stoppingphase is480°CA phase, it can reach25%average EGR rate, and about10%of theEGR gradient, the layered trend is relatively obvious.3, The combustion process in cylinder of homogeneous pattem is more sensitiveto the ignition moment than the stratified EGR pattem, the combustion process ofstratified pattem is more stable. Under the condition of same EGR rates and ignitionadvance angles, stratified pattem can get a significant rising in cylinder pressure,temperature and peak heat release rate, the corresponding crank angle is also morecloser to the TDC on the compression stroke, the duration of heat release becomesshortened significantly. Under the condition of increasing the ignition advance angle,the gap between the temperatures and peak heat release rates of stratified pattem withhomogeneous pattem is decreased. Under the same ignition advance angle, the hightemperature stage of stratified pattem is more closer to the TDC, which cansignificantly reduce the heat loss.