Optical study on the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode

Abstract

Employing high-reactivity fuels such as polyoxymethylene dimethyl ethers (PODE) are effective ways to improve ammonia-based engine performance. However, abnormal combustion (i.e., knocking combustion) may be encountered at reactivity-controlled compression ignition (RCCI) mode. In this study, the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode were studied in a highly-strengthened optical engine at different injection timing conditions, with synchronization measurement of in-cylinder pressure and high-speed photography. The port-injection of ammonia and direct injection of PODE were employed at an energy ratio of 80/20. Results shows that knock intensity is significantly increased with the advancement of injection timings under elevated thermodynamic conditions, accompanied by steep pressure peaks and pressure oscillations. Combustion visualization elucidates the relationship between auto-ignition (AI) progress and knock intensity. Pressure oscillations under knocking combustion are closely correlated to the interplay between different AI flame fronts. Further analysis identifies two distinct combustion characteristics, manifested in the most AI locations and the initial AI flame speed. Numerical results indicate that different AI behaviors are primarily determined by the degree of inhibition in PODE pre-flame reactions by ammonia dehydrogenation, which affects AI kernel development. The slow initial auto-ignition flame results in the formation of a sufficiently active premixed mixture in the unburned region before the initial AI flame front arrives. Once the second AI flame occurs, the active premixed mixture undergoes rapid combustion, leading to strong pressure waves.