Auto-ignition Characteristics of Lubricant Droplets under Hot Co-Flow Atmosphere

It has been revealed by researches that lubricant properties have a great effect on the low-speed pre-ignition (LSPI) frequency in downsizing turbocharged direct-injection engines which are developed for better fuel economy. Droplets of lubricant or lubricant-gasoline mixture are considered to be the potential pre-ignition sources. Those droplets fly into the combustion chamber and ignite the gasoline-air mixture. To study lubricant droplets fundamentally, a novel set of droplet auto-ignition system is designed based on a Dibble Burner for this experiment. Influences of metallic additive contents, viscosities, lubricant diluted with gasoline and waste lubricant on the ignition delay of droplets are investigated by testing 12 groups of lubricants or lubricant-gasoline mixture. The equivalent diameter of each droplet generated by micro-syringes is around 2.1 mm. The co-flow temperature varies from 1123 K to 1223 K, and the experiments are carried out at atmospheric pressure. The auto-ignition process of each lubricant droplet in a dark background is captured and recorded by a high-speed camera. The results show that ignition delays of all groups significantly decrease with the increase of co-flow temperature. Besides, increasing calcium or decreasing zinc dialkyldithiophosphates (ZDDP) in lubricants obviously promotes the trend on auto-ignition of droplets. But magnesium content has little influence on the ignition delay of lubricant droplets. In addition, lubricant with a higher viscosity has a longer ignition delay. Compared with undiluted lubricant droplets, droplets of lubricant-gasoline mixture tend to possess a longer ignition delay at high temperature. Although the ambient pressure in this experiment is much lower than that in an automotive engine, the observed results of effects of lubricant properties on auto-ignition are consistent with the regularities summarized by some engine bench tests. The quantitative results of lubricant droplet auto-ignition may be used as a criterion to evaluate the pre-ignition performance related to lubricants in engines.