Stratified exhaust gas recirculation under lean operation of a liquefied petroleum gas spark-ignition engine with liquid phase injection

Abstract

Stratified exhaust gas recirculation (EGR) not only achieves reductions in NOx emissions and fuel consumption but also improves combustion stability for highly dilute mixtures. In this study, EGR stratification within a cylinder was completed by installing a fast-response solenoid valve in an EGR line and controlling its timing and duration. Lean burn of liquefied petroleum gas (LPG) with stratified EGR was examined to attain lower emissions and better fuel economy. The obvious stratification of EGR along the vertical distance from the spark plug was observed through a concentration measurement by a planar laser-induced fluorescence (PLIF) technique. Properly stratified EGR showed faster combustion and improved combustion stability for both stoichiometric and lean mixtures compared to those from homogeneous EGR. With a 30 per cent EGR rate, the peak heat release was 10 per cent higher, accompanied by a shorter combustion duration in the stratified EGR (SEGR) case than in the homogeneous EGR (HEGR) case. Addition of EGR into a lean mixture of λ=1.2 led to a further 3 per cent further reduction in fuel consumption, which was attainable only with stratified EGR but not with homogeneous EGR. More EGR could be admitted and NOx emission further reduced with less increase in hydrocarbon emission with stratified EGR than with homogeneous EGR. Those results are due to the dominance of stratified EGR over homogeneous EGR in combustion speed and stability.