Effect of intake air temperature on combustion characteristics and operation range of high-pressure ammonia direct injection engine

Abstract

Ammonia is emerging as a promising carbon-free fuel for internal combustion engines due to its high hydrogen content and potential for sustainable production. However, ammonia’s low flame speed and narrow flammability limit pose challenges to combustion stability and efficiency, necessitating research into strategies. This study investigates the effects of intake air temperature control combined with port-injected hydrogen addition on the combustion characteristics of a direct-injection ammonia-fueled spark-ignition engine. Experiments were conducted at engine speeds ranging from 1000 to 1950 rpm under various load conditions. The results demonstrate that elevating the intake air temperature significantly enhances combustion stability at lower engine loads. Specifically, increasing the intake air temperature from 32°C to 109°C reduced the coefficient of variation (COV) of the indicated mean effective pressure by 11.9%. More importantly, intake air heating expanded the operable range of pure ammonia combustion, enabling stable engine operation at speeds exceeding 2000 rpm that were otherwise difficult to sustain without combustion assistance. However, these improvements involved a trade-off; the elevated intake air temperatures resulted in increased NO x emissions due to higher peak combustion temperatures. Unburned NH 3 emissions were closely correlated with efficiency losses under higher loads.