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

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 posi-tions 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, pres-sure 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 cylin-drical 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.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

In this study, the knock phenomenon of a reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel is numerically investigated. The knock mechanism is explained and a strategy to suppress knock is proposed. The knock characteristics are studied by monitoring pressure oscillations at different positions in the cylinder. The results show that under high load conditions, the pressure oscillation amplitude is significant at the center and edge of the cylinder. The knock mechanism is analyzed using various methods such as pressure difference, temperature isosurface, and heat release rate. It is found that the knock in the RCCI engine is mainly caused by end-gas auto-ignition. The effects of pilot oil injection timing and compression ratio on knock are further studied. It is confirmed that diesel knock and end-gas knock can coexist in the same cycle of RCCI engine operation, and proper adjustments can effectively suppress knock.