Investigation into various strategies to achieve stable ammonia combustion in a spark-ignition engine

Ammonia (NH3) is a carbon-free fuel, which could partially or completely eliminate hydrocarbon (HC) fuel demand. Using ammonia directly as a fuel has some challenges due to its low burning speed and low flammability range, which generates unstable combustion inside the combustion chamber. This study investigated the effect of two different compression ratios (CRs) of 10.5 and 12.5 on the performance of ammonia combustion by using a conventional single spark-ignition (SI) approach. It was found that at a lower CR of 10.5, the combustion was unstable even at advanced spark timing (ST) due to poor combustion characteristics of ammonia. However, increasing the CR to 12.5 improved the engine performance significantly with lower cyclic variations. In addition, this research work also observed the effect of multiple spark ignition strategies on pure ammonia combustion and compared it with the conventional SI approach for the same operating conditions. Multiple flames were generated by four spark plugs, which were mounted at equal intervals on the periphery of a customized metal liner and one additional spark plug was fitted at the top of the cylinder head. The results illustrated that adding more spark ignition sites builds higher in-cylinder pressure and temperature, which further burned the charge rapidly. This produced higher engine efficiency, lower combustion duration, and reduced cycle-to-cycle variations. Additionally, multiple spark plugs together were used to ignite a lean case of air-fuel equivalence ratio, λ: 1.2, and compare it with the stoichiometry condition of λ: 1.0. Furthermore, a Cambustion fast NO and NO2 analyzer was used to precisely record the NO and NO2 concentrations during ammonia combustion. It was observed that firing multiple spark plugs produced higher NOx emissions than the single spark plug case, considered to be due to the higher in-cylinder temperature generated by the creation of multiple flame kernels