Visualization study on the effects of pre-chamber jet ignition and methane addition on the combustion characteristics of ammonia/air mixtures

•The effects of pre-chamber jet ignition and methane addition on ammonia combustion were studied.•The jet ignition method could accelerate the ammonia combustion process and improve the combustion stability significantly.•The jet ignition method could advance the timing of the maximum pressure rise rate significantly.•A higher initial pressure hindered the flame propagation but led to a higher average pressure rise rate.•The effects of small amounts of methane addition on ammonia combustion were limited. As a carbon-free green fuel, ammonia has recently attracted more and more attention. However, the application of ammonia as an alternative fuel in internal combustion engines is a challenge due to its low chemical reactivity. Thus, more fundamental researches on the combustion characteristics of ammonia are necessary to enhance the ignition and combustion of ammonia in internal combustion engines. In this study, the effects of pre-chamber jet ignition and different methane additions on the combustion characteristics of ammonia/air mixtures were investigated experimentally. The experimental results showed that pre-chamber jet ignition could improve the combustion performance of the premixed ammonia/air and ensure combustion stability at a low equivalence ratio, compared with spark direct ignition. For the jet ignition mode, the optimal equivalence ratio of the ammonia/air mixtures was between 1.0 and 1.1. The combustion duration was determined by the flame transverse propagation velocity, which was minimally influenced by the jet flame. For both jet ignition and direct ignition modes, a higher initial pressure would hinder the flame propagation resulting in a longer combustion duration, while an increase in the maximum pressure led to an increase in the average pressure rise rate. And for both ignition modes, the promotion of increasing methane addition from 0 to 10% on the pure ammonia combustion was higher than that of increasing methane addition from 10% to 20% on the combustion, and small amounts (10% and 20%) of methane addition had a less promotion on pure ammonia combustion at a higher initial pressure.