Experimental and numerical investigation of the auto-ignition characteristics of cylinder oil droplets under low-speed two-stroke natural gas engines in-cylinder conditions

Auto-ignition of cylinder oil droplets in the cylinder of low-speed two-stroke gas engines may cause the pre-ignition. To verify and understand the mechanism of the auto-ignition of cylinder oil droplets, experimental research and numerical analysis were carried out to investigate the auto-ignition behaviors of cylinder oil droplets under in-cylinder conditions of low-speed two-stroke natural gas engines. The experimental equipment includes an optical rapid compression machine to simulate the engines' in-cylinder conditions and a dual-camera system to capture the cylinder oil droplets' ignition processes. The numerical simulation model contains a transient multi-component evaporation model and a multi-step chemical reaction kinetic mechanism. The evaporation of cylinder oil droplets and the ignition of oil vapor in the cylinder of the rapid compression machine were simulated. The optical experiment verified that breaking a single large-size droplet into the droplet group consisting of the small basic droplets could greatly shorten ignition delay and form multiple ignition hot points intensifying the pre-ignition. The overall ignition delay decreases with the increase of the temperature for large oil droplets, while the ignition delay is not monotonous with the temperature change for small oil droplets. The ignition radius and the vapor/air ratio of ignition moment were obtained by combining experiment and simu-lation. The ignition radius is reduced as the temperature and pressure increase. And the vapor/air ratio of ignition moment could be reduced by decreasing ambient temperature and increasing ambient pressure. The effect of methane on the overall ignition delay of oil droplets depends on the initial droplet diameter. The auto-ignition of small-size single oil droplets could induce natural gas pre-ignition was valid. And these findings are beneficial to understanding the mechanisms of pre-ignition induced by cylinder oil droplets auto-ignition, and the design of low-speed two-stroke gas engines.