Auto-Ignition of a Hexadecane Droplet Mixed with Different Octane Number Fuels at Elevated Pressures to Investigate the Pre-Ignition Behavior

Abstract

Pre-ignition is an abnormal combustion event that may occur in boosted direct injection gasoline engines, where one or more auto-ignition events are observed before spark ignition. Due to the direct injection of fuel into the cylinder, some liquid fuel may splash off the walls, pulling lubricating oil with it. The auto-ignition of liquid fuel/lubricant droplets is considered as one of the possible sources of pre-ignition. To assess this stochastic phenomenon in a controlled way, the auto-ignition of a single droplet of a hexadecane-fuel mixture was investigated, with hexadecane serving as a surrogate for the lubricating oil. This experiment included suspending a single hexadecane-fuel mixture droplet on a thermocouple bead in preheated air, at a temperature of 300 ± 4 °C, in a constant volume chamber over a wide range of pressures (4-30 bar). Four different fuels with a range of research octane number (RON) between 70 and 120 were mixed with hexadecane at a volume percentage of 75% hexadecane to 25% fuel to investigate the time to ignition of the droplet, designated as TI. The TI was measured by recording the droplet temperature history simultaneously with high-speed droplet imaging. The droplet ignition is triggered by the auto-ignition of the combustible mixture formed by the hexadecane-fuel mixture's vapor that mixes with the hot ambient air around the droplet. An empirical model was proposed to predict the TI in terms of pressure and the mixture's RON. At constant pressure, the rate of evaporation of the mixture's droplet increases with increasing RON. The time to ignition is seen to increase exponentially as the fuel's RON used in the mixture increases. On the other hand, for a given fuel RON, the time to ignition decreases with increasing ambient pressure. The empirical model shows that at pressures above 20 bar, the dilution of hexadecane by the different fuels has no significant effect on delaying the auto-ignition of the droplet.