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The internal mixture formation by gasoline direct injection offers a remarkable potential to improve the engine performances and to reduce the pollutant emission, due to the large possibilities of process control. On the other hand, the control mechanisms their selves are more complex and sensitive at speed or load variations than the ones used for external mixture formation. The spray characteristics, as well as the shape of injection rate have to be accurately adapted to every condition of load, speed and surrounding. This paper presents a method for the effective optimization of GDI techniques for SI engines, which is exemplified by a system with direct injection by high pressure modulation. The method is based on the interactive optimization of the processes within the injection system respectively during the spray evolution, by a feed-back strategy between separate numerical simulations of both processes.

The propulsion of future motorcycles and small vehicles will be determined by the reduction of dimensions, weight, fuel consumption and pollutant emission for a considered power output, implicating an improved control of the internal process stages consisting on scavenging, mixture formation and combustion. A main support of such process improvement is the internal mixture formation by gasoline direct injection. However, the compactness and the high-speed range of a small engine for two-wheelers, marine or garden equipment make the application of direct injection more difficult than for automotive engines. On the other hand, after the initially tested wall- and air- guided techniques, it is generally recognized that the only way for a successful large scale utilization of gasoline direct injection is the spray-guided mixture formation.