An experimental and numerical study of the evaporation and pyrolysis characteristics of lubricating oil droplets in the natural gas engine conditions

Abstract

The auto-ignition of lubricating oil droplets in cylinder has been confirmed to be one of the major reasons of abnormal combustion in natural gas engines. A serial of fundamental experiments were carried out to elucidate the lubricating oil evaporation and pyrolysis behaviors under different ambient temperatures. An enhanced multi-component evaporation and pyrolysis model for lubricating oil droplets was presented. This model takes into account all key processes during the droplet lifetime, including the finite heat conduction and mass diffusion in the liquid phase, multi-component diffusion in the gas phase, as well as the pyrolysis and polymerization of high molecular-weight hydrocarbons in high temperatures. By comparing with the measurements of the droplet behaviors of lubricating oil and heavy oil, the present model shows good predictions of the droplet behaviors. Furthermore, this model was used to discuss the detailed behaviors of lubricating oil droplets in natural gas engine conditions. It is found that the lubricating oil droplets have significantly different behaviors from those of diesel and gasoline. They show slight pyrolysis and polymerization behaviors due to the high molecular-weight components. The heating period is crucial for the lubricating oil droplets, and the highest ratio of the heating period to the total droplet lifetime is up to 80%. The smaller droplet takes shorter time to get the high concentration of oil vapor surrounding the droplet surface than the larger droplet.