Autoignition behavior of methanol/diesel mixtures: Experiments and kinetic modeling

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650-1450 K, pressures of 6-20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (similar to 1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (similar to 940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates center dot OH, whereas center dot OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, center dot OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via center dot HO2 while CH3OH consumes a large percentage of center dot OH. Consequently, the competition between methanol and diesel for center dot OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study investigates the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios under various conditions. The research reveals the crossover of IDTs at different temperatures for varying equivalence ratios and the non-linear mixing effect of methanol addition on diesel ignition. Simulation results indicate the competition between methanol and diesel for center dot OH radicals during the ignition process.