Flame characteristics and abnormal combustion of ammonia-diesel dual-fuel engine with considering ammonia energy fractions

Abstract

Carbon neutrality is a significant challenge but also an essential opportunity for internal combustion engines (ICE), and fueled with low-carbon fuels is the key for ICE to reduce carbon emissions. Ammonia is a carbon-free fuel, whereas ammonia-diesel dual-fuel (ADDF) combustion can provide efficient and clean combustion of ammonia. In this study, the flame characteristics and abnormal combustion of ADDF engines are studied by considering ammonia energy fractions and diesel injection timings. The results show that a high ammonia energy fraction led to an extended period of combustion, while the CA05 first advances and then decreases with the decrease of ammonia energy fraction. This phenomenon indicates that an easily ignitable ammonia-air mixture is also important for ammonia-diesel dual-fuel ignition. Besides, an optimal SOI is needed to maximize the promoting effect of ignited fuel on ammonia combustion, and the optimal SOI changes with the ammonia energy fraction. Flame images confirm that the weightiness of the auto-ignition and flame propagation changes with the SOI, and the auto-ignition kernels gradually reduce and flame propagation plays a more and more important role as the SOI advances. At high loads, knocking combustion will also occur for ADDF engines, although the knock intensity is low. Besides, due to the enhancement effect of the knocking pressure wave, the flames show a strong white light at the knocking onset. The current results can help understand the combustion process and thus find the optimization method of ADDF engines.