Abstract
Gasoline partially premixed combustion (PPC) is a promising combustion concept with high indicated thermodynamic efficiency (ITE) and low emission level. This study investigated the effect of gasoline viscosity index improvement on the multi-cylinder engine performance and the dependence of the PPC combustion on injection pressure over a wide range of engine speeds and loads. Results show that a small amount of viscosity improver can improve the viscosity of gasoline effectively without influencing the octane numbers. This is beneficial for elevating the mechanical efficiency of the fuel pump and improving the brake thermal efficiency (BTE) of the engine. For the part and medium load conditions, the requirement of common-rail pressure in PPC mode with engine-out NOx and soot emissions below Euro 6 levels is significantly lower than the calibration of conventional diesel combustion (CDC) mode with tailpipe Euro 6 emissions. For the high-load conditions, the low-speed operations are prone to achieve high premixing. While the high-speed operations are mainly mixing-controlled with similar NOx-soot trade-off relationship compared to CDC mode under moderate injection pressures. The fuel injection pressure becomes the most important driving factor for air-fuel mixing in PPC mode. The local equivalence ratio dominated by the injection pressure is more important than the global equivalence ratio for soot reductions. As a result, gasoline PPC load extension is very sensitive to rail pressure, and its injection pressure requirement is much higher than that of the Euro 6 CDC mode.
