Ignition and Combustion Characteristics of Diesel-Ignited Ammonia/Methane Premixed Gas

An optical rapid compression and expansion machine (RCEM) platform was used to establish a flame natural luminosity visualization system. The effects of pilot diesel injection timing, injection quantity, and injection pressure on the ignition and combustion characteristics of ammonia–methane premixed mixtures were investigated. Flame propagation images, ignition delay, flame projection area, in-cylinder pressure, and heat release rate were analyzed to reveal the combustion evolution under different injection conditions. The results show that the combustion process can be divided into two stages: a diesel-dominated first stage and a premixed-gas-dominated second stage. Advancing the injection timing increases the peak in-cylinder pressure and heat release efficiency. The optimal injection timing is from 12 to 9 crank angle degrees before top dead center. Increasing the injection quantity shortens the ignition delay and increases the flame area and peak cylinder pressure. The optimal injection quantity is 10 to 12 milligrams. Increasing the injection pressure strengthens the heat release in the first stage at low equivalence ratios. The optimal injection pressure is 100 to 120 megapascals. Adding 40% methane significantly increases the chemical reactivity of the mixture which shifts the combustion mode from diesel-dominated ignition to synergistically promoted combustion.