Research On The Stratified Stoichiometric Mixture And Exhaust Gas Dilution Combustion Mode Of Gasoline Direct Injection Engine

The gasoline direct injection (GDI) plays an important role in the engineperformace improvement and will be the main technology for gasoline engines. Butthere are also some challenges for GDI engines at present, including the lowcompression ratio limited by kncoking, the pump loss under the stoichiometric-burnmode, the NOXemission problem under the lean-burn mode and the sootemission due to the stratified mixture combustion.The research was carried outfor the GDI engine high efficience and low emission combustion by numericalsimulation and experimental investigation in this paper.The concept of stratified stoichiometric mixture (SSM) was proposed tosuppress knocking at high loads for GDI engines. The SSM combustion canincrease the speed of the flame propagation and also suppresse the end-gas autoignition. The CO, HC and NOXemission can be effectively after-treated using athree-way catalyst due to the stoichiometric mixture.The stratified ratio andhomogeneity of SSM take an important effect on the knocking suppressionmechanism.Another concept of two-zone homogeneity (TZH) was proposed toalleviate combustion deterioration and soot formation in the stratified mixturecombustion condition. SSM can reduce the decrease of power output comparedto the conventional methods of retarding spark timing and exhaust gas recycle(EGR) for suppressing knocking, and have better fuel economy and lessemission than the method of enriching mixture. A compact gasoline chemicalkinetics mechanism including30species and38reactions was developed andused to analyse the knocking phenomenon coupled with three-dimensionalcomputational fluid dynamics (CFD).The species of CHO and HO take animportant effect on the knocking process and intensity.The mode of dilution combustion by exhaust gas trapped was proposed toimprove the part-load efficency of GDI engines. The experimental results show thatthe dilution combustion at part-load can save fuel consumption by5%~16%due toreducing pumping loss and improving combustion efficiency.The dilution effect can beenhanced by increasing the exhaust gas ratio and temperature, and the combinedcombustion mode with flame propagation and compression combustion occurred simultaneously when a higher compression ratio was adopted. The stratified mixture oftwo-zone homogeneity can reduce the engine cyclic variation and shorten thecombustion duration to improve thermal efficiency in the dilution combustionmode.The dilution combustion by exhaust gas trapped (EGT) can improve theexhaust gas tolerance to boost the dilution effect than the exhaust gas recycle(EGR) mode. The dilution combustion with exhaust gas can reduce the pumploss more effectively due to the heat effect contrast with the lean mixturecombustion mode, and can also improve the emission control of GDI engines.Based on the above concepts, a GDI engine with high efficiency wasdeveloped in practise, and the fuel economy in typical operation improved byabove13%than the commercial GDI engine.