Mixing Control and Combustion in Gasoline Direct Injection Engines for Reducing Cold-Start Emissions

A two-stage combustion is one of the Mitsubishi GDI^™ technologies for a quick catalyst warm-up on a cold-start. However, when the combustion is continued for a long time, an increase in the fuel consumption is a considerable problem. To solve the problem, a stratified slight-lean combustion is newly introduced for utilization of catalysis. The stratified mixture with slightly lean overall air-fuel ratio is prepared by the late stage injection during the compression stroke. By optimizing an interval between the injection and the spark timing, the combustion simultaneously supplies substantial CO and surplus O2 to a catalyst while avoiding the soot generation and the fouling of a spark plug. The CO oxidation on the catalyst is utilized to reduce the cold-start emissions. Immediately after the cold-start, the catalyst is preheated for the minimum time to start the CO oxidation by using the two-stage combustion. Following that, the stratified slight-lean combustion is performed. The combustion induces the powerful CO oxidation on the catalyst. The oxidation warms up the catalyst and also promotes the HC oxidation. This strategy results in improvement of the fuel economy while reducing the cold-start emissions because the two-stage combustion is almost replaced to the stratified slight-lean combustion, which drastically decreases the fuel consumption. The stratified slight-lean combustion is rapid and stable even when the spark timing is retarded after the TDC. The characteristics are also utilized for the catalyst warm-up without using the two-stage combustion.