Optimization of the Stratified-Charge Regime of the Reverse-Tumble Wall-Controlled Gasoline Direct-Injection Engine 2004-01-0037

An optimum combustion chamber was designed for a reverse-tumble wall-controlled gasoline direct-injection engine by systematically optimizing each design element of the combustion system. The optimization was based on fuel-economy, hydrocarbon, combustion-stability and smoke measurements at a 2000 rev/min test-point representation of road-load operating condition. The combustion-chamber design parameters that were optimized in this study included: piston-bowl depth, piston-bowl opening width, piston-bowl-volume ratio, exhaust-side squish height, bowl-lip draft angle, distance between spark-plug electrode and piston-bowl lip, spark-plug-electrode length, and injector spray-cone angle. No attempt was made to optimize the gross engine parameters such as bore and stroke or the intake system, since this study focused on optimizing a reverse-tumble wall-controlled gasoline direct-injection variant of an existing port-fueled injection engine.

The results of this study showed that numerous tradeoffs in engine performance parameters have to be made in selecting the geometric features of the piston bowl, spray angle and spark-plug protrusion in arriving at the optimum design. The results of this study also showed that by preserving certain physical characteristics of the piston, operating characteristics of the engine with that piston could be passed on to other piston designs. A piston bowl that combines the design features of a positive draft-angle lip in the back of the bowl and a negative draft-angle lip on the sides of the bowls generates a hybrid piston bowl that possesses features from both original designs. In the final design, the positive draft-angle lip in the back of the piston bowl as well as its increased bowl depth allowed it to achieve very low smoke. While the negative draft-angle lips on the side of the piston bowl allowed the final piston-bowl design to achieve very low hydrocarbons emissions.