Mixture Preparation for Direct-Injection SI Engines

Meeting future exhaust emission and fuel consumption standards tor passenger cars will require refinements in how the combustion process is carried out in spark ignition engines. A direct injection system decrease fuel consumption under road load cruising conditions, and stratified charge of the fuel mixture is particularly effective for ultra lean combustion. To achieve stable combustion at an ultra lean air fuel ratio, fuel spray speed, angle, droplet diameter and in-cylinder air behavior must be optimized.

In this paper, the engine system which has an injector and spark plug located in the center of combustion chamber, and has a flat type piston was investigated. We adopted a swirl type injector (fuel pressure 5MPa) which gave a droplet diameter of 20 μ m. Swirl air motion was applied in the cylinder to concentrate the fuel mixture at its cylinder center. Fuel spray speed was decreased to under 30m/s to keep the mixture at the top of the cylinder. We achieved a lean limit air fuel ratio of about 40 using the flat type piston.