A study on optical diagnostics and numerical simulation of dual fuel combustion using ammonia and n-heptane

As a zero-carbon fuel, ammonia is considered an ideal alternative for reducing carbon footprints. However, the use of pure ammonia is limited by its poor fuel properties, which can be mitigated by employing a dual fuel approach using high reactivity fuel and ammonia. Nonetheless, the evolution mechanisms of nitrogen oxides (NOx) and unburned ammonia (NH3) remain unclear. In this study, n-heptane was selected as the high reactivity fuel. The characteristics of the dual fuel approach and the evolution mechanisms of NOx and unburned NH3 were investigated using optical diagnostics and numerical simulations, respectively. The results indicate that the combustion characteristics can be improved by adjusting the direct injection strategy. Unconsumed NH3 remains in the low-temperature regions of the late period of combustion, resulting in unburned NH3 emissions. The consumption of unburned NH3 is governed by the reaction NH3 + OH <=> H2O + NH2. The distribution of N2O is primarily concentrated in low-temperature regions. The generation of N2O is controlled by the reaction NH + NO <=> H + N2O. The improvement of the in-cylinder temperature can suppress the generation of N2O and promote the consumption of NH3, resulting in reduced emissions of both N2O and unburned NH3.