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The fuel injection plays a crucial role in determining the mixture formation process in Gasoline Direct Injection (GDI) engines. Pollutant emissions, and soot emissions in particular, as well as phenomena affecting engine reliability, such as oil dilution and injector coking, are deeply influenced by the injection system features, such as injector geometric characteristics (such as injector type, injector position and targeting within the combustion chamber) and operating characteristics (such as injection pressure, injection phasing, etc.). In this paper, a new CFD methodology is presented, allowing a preliminary assessment of the mixture formation quality in terms of expected soot emissions, oil dilution and injector coking risks for different injection systems (such as for instance multihole or swirl injectors) and different injection strategies, from the early stages of a new engine design.

In this work the formation and the evolution of the fuel spray emerging from a hollow-cone swirl injector were investigated. The first aim of the work was to set up a tool for fuel spray simulation in a CFD analysis that can offer a reasonable accuracy with no significant increment in the computational time. The analysis started from a theoretical formulation of the fuel flow inside the injector, based on the potential theory, obtaining an injector model which allows the calculation of the main spray characteristics usually required by the CFD analysis (i.e. droplet velocity, fuel film thickness, droplet size distribution). These parameters can be obtained only from spray cone angle and mass flow rate, which are the data commonly provided by injector manufacturers. Furthermore, a phenomenological approach was also presented, in order to properly simulate in CFD analysis the spray tip penetration in the dense spray zone, without requiring an increase of the spatial grid resolution.

Gasoline direct injection in two-stroke engines has led to even more advantageous results, in comparison with four-stroke engines, as far as unburned hydrocarbon emissions and fuel consumption are concerned. A new electronically controlled injection system has therefore been fitted in a crankcase-scavenged two-stroke engine, previously set up with indirect injection equipment. The comparison between the performance of the two gasoline feeding systems has highlighted the potential of the direct injection strategy. The direct injection system here tested has allowed the optimization of the engine torque characteristic at wide open throttle operating conditions. Moreover, the engine original exhaust system, has been replaced with an expansion-chamber exhaust-pipe system, in order to evaluate the impact of direct gasoline injection also with these optimized exhaust configuration.