Impact of ammonia addition on knock resistance and combustion performance in a gasoline engine with high compression ratio

Increasing the compression ratio of gasoline engines is a promising method to increase the engine's fuel economy. However, engine knock caused by auto-ignition is still a large obstacle to improving thermal efficiency and engine load for high compression ratio hybrid engines. Spark induced compression ignition (SICI) is an effective way to utilize auto-ignition to solve the aforementioned problems. Meanwhile, ammonia, a carbon-free fuel, with an outstanding antiknock property, has the great potential to be used in SICI mode. In this study, the effects of ammonia addition on knock suppression, combustion characteristics, thermal efficiency, and emission performance were investigated in a high compression ratio (15.5), four-valve, single-cylinder gasoline engine under SICI combustion mode. In experiments, gasoline was directly injected into the cylinder while ammonia was injected into the intake port. The results show that blending ammonia could resist engine knock and improve thermal efficiency. Within the knock limitation, the duration of flame propagation under ammonia blending conditions could be shortened and meanwhile, the auto-ignition becomes weakened compared with pure gasoline. Benefiting from combustion phase optimization, the thermal efficiency and engine load could be increased or maintained at optimal ammonia blending ratio. The maximum increase of thermal efficiency and engine load is 2.46% and 0.2 MPa respectively. Moreover, the increased engine load can extend the limit of the ammonia blending ratio. For nitrogen emissions, blending ammonia results in NOx emission deterioration due to the formation of fuel-type NOx. NOx emission has a weak dependence on the ammonia blending ratio, and the trend of NOx emission varied with spark timing is opposite to pure gasoline conditions, which is closely related to the pressure sensitivity of fuel-type NOx. Ammonia slip was also detected in the engine exhaust because of the incomplete combustion.