Ignition characteristics of nitromethane-doped ethanol in a heated shock tube

Abstract

Ignition delay times of nitromethane-doped ethanol dual fuels are measured under the micro-combustion engine conditions (8 ​× ​105 ​Pa and 904–1477 ​K). The promotion effect of nitromethane on ethanol ignition is more pronounced at a fuel-lean condition; it first increases and then decreases with increasing blending ratio and temperature. The optimal promotion effect is achieved at 950–1150 ​K with an addition of ~50% nitromethane (mole fraction). A chemical kinetic model is proposed to describe the ignition process of nitromethane-doped ethanol, and can reproduce the measured ignition delay times. Reaction pathway, sensitivity, and rate of production analyses are performed to reveal the promotion mechanisms of nitromethane. The consumption of ethanol via the H-abstraction reactions is enhanced due to the pyrolysis of nitromethane, and the produced ethanol radicals are beneficial to the re-establishment of the radical pool after the first-stage ignition of nitromethane. With decreasing temperature, nitromethane decomposes slowly, while the low-temperature oxidation of ethanol becomes more important. At a moderate temperature, both these two reasons result in a shorter ignition delay time.