Fuel octane effects on gasoline multiple premixed compression ignition (MPCI) mode

This paper proposes a novel combustion concept namely “multiple premixed compression ignition” (MPCI) in gasoline direct injection compression ignition (GDICI) regime. Its predominant feature is the first premixed and followed quasi-premixed combustion processes in a sequence of “spray-combustion-spray-combustion” around the compression top dead center. The multiple stage premixed combustion decouples the pressure rise with pollutants formation process. With the optimization of operating parameters such as injection timing and split ratio, it can lower the pressure rise rate and emissions simultaneously while achieving a high thermal efficiency at high engine load. The experimental study of the gasoline MPCI mode has been carried out in a single cylinder research engine retrofitted from a light-duty diesel engine with a compression ratio of 18.5. Gasoline with the research octane number (RON) of 66, 76 and 86 were tested under 1400 rpm, 0.8 MPa IMEP conditions as injection timing sweeping, without EGR and intake conditioning. Compared to the single-stage diffusion combustion mode of conventional diesel engines, the RON66 MPCI mode achieves lower emissions of soot, NO, CO, as well as higher thermal efficiency, with a penalty of higher THC emissions. For RON76 and RON86, it is harder to realize the same performance due to a poorer auto-ignition quality than RON66.