Fundamental analysis on auto-ignition condition of a lubricant oil droplet for understanding a mechanism of low-speed pre-ignition in highly charged spark-ignition engines

Abstract

This article presents a study of the mechanism that the lubricant oil droplet initiates low-speed pre-ignition in highly boosted downsized gasoline engines. Low-speed pre-ignition is a phenomenon that the fuel–air mixture ignites before the spark timing, leading to flame propagation that results in a heavy knock. The ignition of lubricant oil droplets is thought to be one possible mechanism for low-speed pre-ignition. However, the oil droplet ignition conditions are not yet well understood. First, the conditions under which a single oil droplet initiates the combustion of a fuel–air mixture were investigated using a rapid compression and expansion machine. When an initial droplet temperature was above 250 °C, the vaporized oil ignited before the gasoline–air mixture, in which case the combustion of the gasoline–air mixture around the droplet was initiated. The numerical results showed that the oil droplet temperature increases above 250 °C if the droplet is heated by burned gas remaining in the combustion chamber from the previous cycle. A direct-injection single-cylinder research engine was operated under the condition that no residual gas exists in the combustion chamber. In this case, no low-speed pre-ignition occurred even if gross indicated that mean effective pressure was 2.5 MPa. These results indicate that an oil droplet does not cause low-speed pre-ignition if any droplet flies into the combustion chamber unless it remains in the chamber over the exhaust stroke.