Impact of small-amount diesel addition on methane ignition behind reflected shock waves: Experiments and modeling

Abstract Autoignition characteristics of methane/diesel mixtures with diesel fractions of 5%, 15%, and 30% were investigated in a heated shock tube over temperatures of 960–1500 K, pressures of 6–20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. Generally, ignition delay times (IDTs) decrease with increasing reflected-shock pressure or diesel content. The total equivalence ratio exhibits a slight influence on IDTs, whereas a crossover for IDTs occurs at a higher temperature from fuel-lean to fuel-rich conditions. Besides, three mechanisms in conjunction with a well-validated diesel surrogate were used to predict the IDTs of dual-fuel mixtures. Mech-3 (CRECK-2003) demonstrates the best performance under all test conditions based on quantitative error analysis. Moreover, there is a nonlinear mixing effect for methane/diesel mixtures. The IDT-reducing effect of diesel to methane is more pronounced compared to n-heptane, especially at intermediate-temperatures. Furthermore, the impact of diesel-addition on methane ignition was explored with kinetic analyses including species evolution, brute-force sensitivity analysis and rate of production analysis. These active radicals produced at the initial stage, especially for H radicals rapidly generated through the decomposition of diesel, readily trigger H-abstraction reactions on CH4, resulting in the continuous production of H from HCO decomposition and eventually triggering the hot-ignition via the chain-branching reactions of H + O2 = O + OH and H2 + O = H + OH. Consequently, the main reason for the IDT-reduction by diesel-addition at the high temperature is the rapid decomposition of diesel to produce H during the initial ignition period.