Exhaust gas recirculation (EGR) and lean burn utilize the diluents into the engine cylinder to control combustion leading to enhanced fuel economy and reduced emissions. However, the occurrence of excessive cyclic variation with high diluent rates, brings about an undesirable combustion instability within the engine cylinder resulting in the deterioration of both engine performance and emissions. Proper stratification of mixture and diluents could improve the combustion stability under high diluent environment.
EGR stratification within the cylinder was made by adopting a fast-response solenoid valve in the midst of EGR line and controlling its timing and duty. With EGR in both homogeneous mode and stratified mode, in-cylinder pressure and emissions were measured. The thermodynamic heat release analysis showed that the burning duration was decreased in case of stratified EGR. It was found that the stratification of EGR hardly affected the emissions. Almost same amount of nitrogen oxides (NOx) reduction was attained with and without the EGR stratification process.
Planar laser induced fluorescence (PLIF) technique was applied to investigate the stratification of the in-cylinder charge. Acetone as a tracer was seeded into the EGR to get the EGR distribution within cylinder. It was found that EGR occupied the low part of cylinder through the stratification process and this could help reducing the burning duration.
The effects of the diluents on flame propagation characteristics were investigated in an optically accessible single cylinder engine. A series of bottom view flame images were taken from direct visualization using a UV intensified high-speed CCD camera. Flame propagation patterns showed that the flame speed was less decreased by the stratified EGR than homogeneous EGR.