Numerical study on knock characteristics and mechanism of a heavy duty natural gas/diesel RCCI engine

Abstract

In this paper, the knock phenomenon of reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel was numerically studied. The knock mechanism of the RCCI engine is explained and the strategy of suppressing knock is put forward. The knock characteristics were studied by setting monitoring points in different spaces positions of the cylinder. The results show that the pressure oscillation amplitude at the center and edge of the cylinder is large under the high load condition of RCCI engine. In addition, the knock mechanism was studied by using pressure difference method, maximum amplitude of pressure oscillation, important components, temperature isosurface, pressure fluctuation and heat release rate. The results show that the knock of RCCI engine is mainly caused by the end-gas auto-ignition. The pressure difference results show that the characteristic frequency is consistent with the natural resonance mode (0,1) of the cylindrical combustion chamber. On this basis, the effects of pilot oil injection timing and compression ratio on engine knock are further studied. It is confirmed that diesel knock and end-gas knock may exist simultaneously in the same cycle when RCCI engine knock occurs. And diesel knock occurs before top dead center, and end-gas knock occurs after top dead center. Proper adjustment of pilot oil injection timing and reduction of compression ratio can effectively suppress engine knock.