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A new Light Stratified-Charge Direct Injection (LSC DI) spark ignition combustion system concept was developed at Ford. One of the new features of the LSC DI concept is to use a ‘light’ stratified-charge operation window ranging from the idle operation to low speed and low load. A dual independent variable cam timing (DiVCT) mechanism is used to increase the internal dilution for emissions control and to improve engine thermal efficiency. The LSC DI concept allows a large relaxation in the requirement for the lean after-treatment system, but still enables significant fuel economy gains over the PFI base design, delivering high technology value to the customer. In addition, the reduced stratified-charge window permits a simple, shallow piston bowl design that not only benefits engine wide-open throttle performance, but also reduces design compromises due to compression ratio, DiVCT range and piston bowl shape constraints.

Development and research of a stratified-charge direct-injection SI combustion system was carried out at Ford. This paper presents the multidimensional engine CFD modeling analysis of mixture formation and piston surface wetting under the stratified-charge conditions. The effect of various design and operating parameters including piston shape, spray cone angle, swirl ratio, injection timing, engine speed and load on charge stratification and piston-wetting due to spray impingement were studied. The results led to design changes that warranted improvement of engine functions. Of particular interest, the model was shown to be capable to predict piston surface wetting and a qualitative correlation was found between the amount of the remaining liquid fuel on the piston surface and the engine-out smoke number.