Combustion stratification for naphtha from CI combustion to PPC

This study demonstrated the change in combustion homogeneity from conventional diesel combustion via partially premixed combustion towards HCCI. Experiments are performed in an optical diesel engine at a speed of 1200 rpm with diesel fuel. Single injection strategy is employed and the fuel is injected at a pressure of 800 bar. The cylinder pressure at TDC is maintained at 35 bar and a high-speed video of the combustion process is captured through optical piston. The high speed video is processed to study the combustion homogeneity based on an algorithm reported in previous studies. Starting from late fuel injection timings, the combustion homogeneity is investigated by advancing to early fuel injection timings. For late fuel injection timings, a direct link between fuel injection timing and combustion phasing is noticed. At advanced fuel injection timings, the start of combustion is independent of fuel injection timing. The combustion homogeneity for the transition from CI via PPC towards HCCI is also investigated for various levels of dilution by displacing oxygen with nitrogen in the inlet. The start of combustion was retarded with the increase in dilution, while the mixture homogeneity is enhanced due to longer ignition delay. To compensate for the retarded combustion phasing with dilution, the inlet air temperature is increased. The experimental results show that the high speed image is initially blue and then turned yellow, indicating soot oxidation. The images are processed to generate the level of stratification based on the image intensity. This study shows better combustion homogeneity for early fuel injection timing and higher level of dilution and temperature in the inlet