The Evaporation Characteristics of Cylinder Oil of Low-Speed 2-Stroke Marine Engines

The combustion of cylinder lubricating oil (called as cylinder oil for short) is one of the major sources of PM emissions of low-speed 2-stroke marine diesel engines. For pre-mixed combustion low-speed 2-stroke marine gas engines, the auto-ignition of cylinder oil might result in knock or more hazard abnormal combustion - pre-ignition. Evaporation is a key sub-process of the auto-ignition process of cylinder oil droplets. The evaporation behavior has a profound impact on the auto-ignition and combustion processes of cylinder oil droplets, and a great influence on engine combustion performance and emission characteristics. This paper applied an oil suspending apparatus to investigate the evaporation behavior of cylinder oil droplets and base oil droplets. The effects of ambient temperatures on the evaporation process were measured and analyzed. The results indicate that the evaporation of cylinder oil includes heating, evaporating, pyrolysis, and polymerization. Under high temperature conditions, the rapid increase of oil droplet temperature resulted in boiling of light components and generated bubbles inside oil droplets. The bubbles grew steadily and finally caused breaking up of the boundary of the oil droplets. During this process, the oil droplet size variation curves appeared intensive waving. The major composition - BS 150 base oil droplets also appeared such phenomena. But the other major composition- SN 400 base oil did not show such characteristics. The lifetimes of cylinder oil droplets tightly correlated with ambient temperatures. Under high temperature conditions, the lifetimes of cylinder oil droplets rapidly decreased when the ambient temperatures were increased; under the same ambient temperature condition, the lifetimes of cylinder oil droplets rapidly decreased when droplet sizes were decreased.