Investigation of evaporation and auto-ignition of isolated lubricating oil droplets in natural gas engine in-cylinder conditions

The auto-ignition of isolated lubricating oil droplets in cylinder conditions is one of main causes of abnormal combustion of natural gas engines. The evaporation behavior of less volatile lubricating oil droplets is the key parameter controlling the auto-ignition event. The suspended droplet technique was used and a fully transient multi-component model was developed to study the evaporation and auto-ignition behaviors of oil droplets. The transient model is closed against mass, species, and energy conservation, and the global one-step chemistry is used to estimate the chemical reaction source term. The boundary conditions at the gas-liquid interface are based on mass, species, and energy flux continuity, as well as the fugacity equilibrium. The model predictions were validated against experimental results, including the evaporation rate of binary-component droplets (n-heptane/n-decane), as well as the overall ignition delay of single-component (n-dodecane) and base oil droplets. The results show that the overall ignition delay of based oil droplets is very sensitive to the initial size and ambient temperature. The ignitable diameter limit is decreased with an increase in ambient pressure and temperature, and the ignitable temperature limit decreases as the ambient pressure increases. The dependence of ignition delay on pressure is determined by the ambient temperature. The ignition radius is reduced as the ambient temperature, pressure, and the initial droplet size increase. The involved methane concentration in natural gas engine in-cylinder conditions shows moderate effects on the ignition delay time of oil droplets depending on the initial size.