Optimization of stratification combustion in a spark ignition engine by double-pulse port fuel injection

Abstract

The potential of lean burn in a spark-ignition (SI) engine with optimized fuel injection was experimentally investigated and numerically simulated. The experiments were carried out on a production SI engine which has a port fuel injection (PFI) system. The previous port electronic fuel injection system was modified and the technique of double-pulse fuel injection (DFI) was employed. By regulating injection timings and proportions of DFI, the air-fuel mixture stratification was significantly improved and the expected lean burn was implemented. The experimental results showed that the reduction of fuel consumption with DFI could be above 10 per cent over quite a wide load range, compared to single fuel injection. With optimized fuel injection timings and double-pulse proportions, the ideal engine performance and emissions can be achieved with a two to three times higher air-fuel ratio (AFR) than single fuel injection. With numerical simulation, the effects of mixture stratification formed by different fuel injection amounts and timings were analysed using a phenomenological model. The mixture in the cylinder was divided into different regions that distribute spherically around the spark plug and consist of a central region of stoichiometric air-fuel mixture and a gradually leaner outside region. Simulation results demonstrated that the improvements in fuel economy and emissions with DFI were mainly attributed to increased stratification zones and a reduced AFR gradient in the stratification zones.