Study on the mechanism of the ignition process of ammonia/hydrogen mixture under high-pressure direct-injection engine conditions

As environmental problems and energy crisis become more serious, ammonia is one of the potential alternative fuels. In order to better use ammonia as fuel in power equipment, the ignition process was studied under high-pressure direct-injection engine condition. In the paper, the Homogeneous model in Chemkin package was selected for numerical calculation. In the six cases with different hydrogen mixing ratios, the effect of initial temperature, pressure, equivalence ratio and hydrogen mixing ratio on ignition delay time (IDT) were studied. It conducted that IDT could be effectively reduced when adding 10–50% hydrogen to ammonia. Then, after sensitivity analysis of NH₃/H₂ mixtures, the key equations and free radicals affecting combustion characteristics were found. The rate of production (ROP) of the key radicals were carried out. It was found that the hydrogen provided the initial concentration of H radical before the start fire, which greatly improved the ROP of OH radical of R1(H + O₂=O + OH) compared to the original H needed to break the N–H chemical bond in pure ammonia. And the OH radical was related to the consumption of NH₃ by R31(NH₃+OH=NH₂+H₂O).