Combustion Characteristics of a Reverse-Tumble Wall-Controlled Direct-Injection Stratified-Charge Engine

Experimentally obtained combustion responses of a typical reverse-tumble wall-controlled direct-injection stratified-charge engine to operating variables are described. During stratified-charge operation, the injection timing, ignition timing, air-fuel ratio, and levels of exhaust gas recirculation (EGR) generally determine the fuel economy and emissions performance of the engine. A detailed heat-release analysis of the experimental cylinder-pressure data was conducted. It was observed that injection and ignition timings determine the thermal efficiency of the engine by controlling primarily the combustion efficiency of the stratified charge. Hence, combustion phasing is determined by a compromise between work-conversion efficiency and combustion efficiency. To reduce nitric-oxide (NOx) emissions, a reduction in overall air-fuel ratio as well as EGR addition is required. With the use of external EGR it is possible to achieve reduction in NOx emissions up to a certain point without a penalty in thermal efficiency. However, further NOx reduction with additional EGR is obtained at the expense of a decrease in thermal efficiency. In this respect, it is determined that this stratified-charge engine, due to its much higher combustion rates, fares much better than a typical premixed, homogeneous-charge engine at high EGR rates. Nevertheless, there is significant potential for improving the stratified-charge engine's thermal efficiency by increasing its combustion efficiency, which allows optimum combustion phasing.