Effect of hydrogen/ammonia mixing ratio on combustion and emission performance of hydrogen engine with different injection timing

Hydrogen/ammonia has a good prospect as an alternative fuel to traditional fossil fuels, and is one of the best ways to realize the goal of energy saving and emission reduction. However, due to hydrogen fuel's high flame propagation velocity and low ignition energy, abnormal combustion, such as backfire in port hydrogen injection engines, while ammonia fuel, characterized by high ignition energy and low laminar burning rate, thus the mixing of ammonia fuel as one of the effective methods to realize the hydrogen engine. This study endeavors to investigate the constraints of H2/NH3 by utilizing a combined experimental and simulation approach to investigate the effect of the hydrogen/ammonia mixture ratio on engine combustion and emission performance at different injection timings. The results show that reasonable selection of injection timing can improve the fuel trapping ratio and reduce the fuel residue in the intake tract, which has an important effect on reducing the probability of backfire. For engine performance, the indicated mean effective pressure (IMEP), indicated thermal efficiency (ITE), and indicated power (IP) of the hydrogen internal combustion engine were reduced to 5.7 bar, 30%, and 5.1 kW, respectively, when ammonia was doped at a mass ratio of 50%. For combustion characteristics, ammonia addition reduced the in-cylinder pressure and exothermic rate, and extended the combustion duration. At the same time, chemical kinetic analysis showed that with the addition of ammonia, the concentration of H and OH radicals decreased, which would have a negative effect on the in-cylinder reaction rate, slowing down the reaction rate of the primitives, thus reducing the overall combustion rate and combustion efficiency. In terms of emissions characterisation, an increase in the ammonia content leads to higher nitrogen oxide (NOx) emissions, but these start to decrease once the ammonia content exceeds 40%. The results of the research could advance the development of hydrogen/ammonia engines and contribute to the realization of a zero-carbon combustion model for internal combustion engines.