Study on auto-ignition characteristics of N-heptane/methanol/ammonia mixed fuel

An investigation was carried out on the auto-ignition properties of n-heptane/ammonia/methanol fuel mixtures, characterized by a small amount of n-heptane concentration (2 %). The experiment was conducted using a rapid compression machine (RCM) within a temperature bracket of 790-1112 K, under pressures of 1.5 and 3.05 MPa. The fuel incorporates a methanol content of 0 to 98 %, with mixture equivalence ratios set at 0.5, 1.0, and 2.0. Observations indicate that the fuel component and the equivalence ratio influence the ignition delay time (IDT). It was found that the reactivity of the mixtures increases upon the induction of n-heptane. For the 1 % CH3OH mixture, the IDT is the shortest with stoichiometry. When the methanol content increases, the IDT decreases with increased equivalence ratios. In addition, a detailed mechanism has been modified to improve the predictive performance of the experimental mixture. The chemical reaction kinetic analysis shows that n-heptane promotes the combustion reaction. The consumption of fuels in the combustion process follows a sequential pattern, with n-heptane being consumed first, followed by methanol and ammonia. Both n-heptane and methanol experience nearly complete consumption before the combustion. In contrast, the consumption of ammonia proceeds at a significantly slower rate and is primarily consumed in substantial quantities at the combustion stage. The dehydrogenation reactions compete for key radicals, which inhibit the oxidation process of the system. The extraction of H from methanol by HO2 to produce H2O2 is a sensitive reaction to promote combustion.