A Novel Combustion Chamber to Physically Stratify the Charge in a Gasoline Direct Injection Engine

Abstract

Realizing the potential of the gasoline direct injection (GDI) concept lies in effectively stratifying the charge at different engine operating conditions. This is generally obtained by properly directing the air and fuel through carefully oriented intake port(s) and fuel spray and appropriately changing injection parameters. However, robust methods of charge stratification are essential to extend the lean operating range, particularly in small GDI engines. In this work, a novel piston shape was devel-oped for a 200 cm3, single-cylinder, four-stroke gasoline engine to attain charge stratification. Stratification of charge is achieved even when the fuel was injected early in the intake stroke by a specially shaped wedge on the piston crown that produced twin vortices during compression and physically separated the charge into two sides in the combustion chamber. Computational fluid dynamics (CFD) studies indicated that the spark plug side had a combustible mixture with good homogeneity, whereas the other side that was separated through the wedge on the piston housed a leaner mixture. Experiments indicated that this physical stratification allowed a much leaner opera-tion with good combustion phasing without the dependence of the intake port-generated airflow. The result revealed higher thermal efficiency, particularly at part loads with lower emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The possibility of reducing the output power by leaning the mixture, hence reducing throttling losses is also demonstrated. The engine was found to be more stable for a given air-fuel ratio (AFR) as compared to the base flat piston.