Some Concepts of DISI Engine for High Fuel Efficiency and Low Emissions

Stratified-charge DISI engines have been launched in the market by Mitsubishi, Toyota, and Nissan. This paper discusses the current production stratified-charge DISI systems and some alternative systems, including the system using air-forced fuel injection and a proposed system that uses a swirl flow in the piston bowl with a special shape to separate the fuel-rich mixture layer from the wall surface. New DISI concepts are proposed to overcome some drawbacks of current bowl-in-piston type stratified-charge DISI systems. Charge stratification can be realized by using a soft spray with proper spray penetration, droplet size, and cone angle, as shown by CFD simulation results. The drawbacks of fuel wall wetting, soot limited load with charge stratification, large surface to volume ratio, etc., of the bowl-in-piston type system can be minimized. Bench tests with a conventional DI swirl-type injector showed that a liquid spray impinging on a semispherical target could produce a suitable soft spray fuel cloud. Finally, an engine combining homogeneous-charge DISI and variable valve timing may achieve the same fuel efficiency level as an emission constrained stratified-charge DISI engine. Cost of the engine system can be lower due to elimination of the lean NOx trap and its regeneration requirements.