Keywords: gasoline direct injection, spark ignition engines, multidimensional modeling, split gasoline injection.

Achievement of optimal charge conditions at all the engine loads and speeds in modern gasoline direct injection (GDI) spark ignition (SI) engines is undoubtedly a challenging task, especially if the so-called mixed mode boosting is to be realized, with homogeneous rich mixtures at the higher loads and stratified lean mixtures at the lower ones. This is the reason why injection splitting is being considered also in SI engines, in analogy with compression ignition ones.

This work is directed towards the numerical investigation of the advantages of split injection in a high performance GDI engine for two wheel applications. The engine considered has a single cylinder, with 638 cc displacement, four strokes and four valves. It is equipped with a new generation high pressure multi-hole injector. This last is first characterized from an experimental point of view, by varying the dwell time (dw) between two successive pulses and the percentages of gasoline mass injected per injection event. The minimum value of dw, below which the opening of the second injection event interferes with the closing of the previous one is found to be equal to 320µs.

A three-dimensional computational fluid dynamics (CFD) study of the in-cylinder processes is then carried out, based on a preliminary assessed computational model for the dynamics of the high pressure gasoline spray. The in-cylinder mixture formation and combustion processes are examined in two main cases, namely a high load-high speed operation and a moderate load-moderate speed one. Advantages and shortcomings deriving from splitting injection are discussed.

The use of split injection determines changes in the engine performance from both an energetic and an environmental point of view, that strongly depends on the instant of time where the second pulse is initiated.

The use of double injection in a high speed-high load engine operating condition is proven to be non effective in improving the engine performance, especially from the energetic point of view. On the other hand the analysis of a moderate speed- moderate load case shows benefits obtained on the whole combustion process, if a proper choice is made of the start of the timing of the two successive pulses. The advantages are an increase of the mean in-cylinder pressure and a reduction of the produced NO and CO for a second injection event initiated before the beginning of the compression stroke. The possibility of resorting to control of the in-cylinder pressure peak position by changing the dwell time between two successive pulses is interesting. This may constitute the basis for control strategies aimed at reducing the tendency of the engine to knock.

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