Fuel Preparation Requirements for Direct-Injected Spark-Ignition Engines

Calculations were performed to estimate some of the spray characteristics that would be required for low-emissions, direct-injected, gasoline engines. Two criteria were used to estimate the spray characteristics required. First, it was assumed that all of the fuel would need to be evaporated well before the end of the combustion event, and preferably much of it before the spark event. Second, it was assumed that the liquid drops should have minimum contact with the piston or walls of the combustion chamber. Using these criteria and typical drop-size distributions for pressure atomizers, it was determined that Sauter mean diameters (SMDs) of roughly 15 μm or smaller, with Rosin-Rammler N values of 1.9 or higher, are required. For a pressure-swirl atomizer, a differential pressure of roughly 4.9 MPa_diff (710 psid) is required to achieve this SMD. Fine sprays could also be generated with air-assist injectors, although these were not evaluated in this paper. Many other criteria must also be met in the design of fuel injection and mixture preparation systems for direct-injected gasoline engines.