Spray motion visualization, mixture strength measurement, flame spectral analyses and flame behavior observation were performed in order to elucidate the mixture preparation and the combustion processes in Mitsubishi GDI engine. The effects of in-cylinder flow called reverse tumble on the charge stratification were clarified. It preserves the mixture inside the spherical piston cavity, and extends the optimum injection timing range. Mixture strength at the spark plug and at the spark timing can be controlled by changing the injection timing. It was concluded that reverse tumble plays a significant role for extending the freedom of mixing.
The characteristics of the stratified charge combustion were clarified through the flame radiation analyses. A first flame front with UV luminescence propagates rapidly and covers all over the combustion chamber at the early stage of combustion. Then, the combustion of rich mixture proceeds in the reaction zone behind a second flame front with thermal radiation. The second flame front propagates into the post flame zone of the first flame front filled with the products of first flame such as radicals and CO. Soot generated in the rich mixture zone is burned-up in this radical rich zone.
Based on this finding, a new mixing control strategy for knock suppression named “two-stage mixing” was proposed. A first injection is performed during the early stage of the intake stroke to prepare the very lean premixed mixture and a second injection is performed during the later stage of the compression stroke to prepare the distinctively stratified mixture. The premixed mixture is too lean to induce knock and the stratified mixture does not have the enough time for the incubation reaction of knock to proceed. What is interesting is that soot generated in the rich mixture zone is not emitted at all. In the case of the conventional stratified charge rich combustion, soot behind the flame front propagates to the air zone and is cooled to generate cold soot that will not be reburned. In case of the two-stage mixing, soot in the rich mixture zone propagates to the lean premixed mixture zone, where soot plays a role of the ignition site and ignites the lean mixture. Soot in the lean mixture zone is burned-up efficiently utilizing the air, radicals and heat generated by the combustion of the lean mixture.