Ignition delay time measurements and kinetic modeling of methane/diesel mixtures at elevated pressures

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in ‘air’ mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness.