Modeling the evolution of composition of the fuel and lubricant mixture on the cylinder wall in internal combustion engines

Abstract

Improvements in engine efficiency are necessary for the advancement of modem engine technology. Fuels and lubricants used in internal combustion (IC) engines play an important role in governing the engine efficiency. In IC engines some amount of fuel spray can eventually end up on the cylinder liner. This fuel spray mixes with the oil (lubricant) present on the liner. Now the liquid layer on the liner consists of fuel and oil (fuel-oil film is formed) which interacts with the piston rings and the up-scraping of the fuel-oil film can cause the release of oil droplets into the combustion chamber. The released oil droplets lower the self-ignition temperature of the fuel vapor which might lead to pre-ignition. Pre-ignition is a phenomenon in which the fuel vapor ignites before the spark plug fires causing huge pressure rise which can be detrimental for the engine. The fuel spray on the liner can also pass through the piston rings during the compression stroke and can cause oil dilution in the crank case. The current work is mainly focused on analyzing the fuel-oil interaction on the cylinder liner. A numerical model has been developed in which fuel was modeled as a mixture of 10 hydrocarbon components and oil was modeled as a single n-alkane hydrocarbon. In this model, diffusion in the film, heat transfer in the film, and vaporization at the film-air interface have been coupled. Moving boundary (due to vaporization) was handled by solving the time required to remove the outermost layer while utilizing regular meshing. Implicit method and Newton's iteration method were used to ensure numerical stability and efficiency. Eventually the model calculates the remaining mixture thickness and content before the piston comes back to the specified location in the compression stroke. Some of the main inputs to the model are timing and location of the fuel droplets depositing on the liner, initial fuel film thickness, initial oil film thickness, liner temperature, and cylinder gas pressure. The results showed that with typical engine operational parameters, substantial portion of the initial film mixture still remains when the piston comes back if the initial fuel film thickness is in the range of 20 pm. Further studies were made to examine the consequences of the remaining mixture on the liner. A brief quantitative study was performed to compare the fuel-oil scraped volume and crevice volume. Additionally, the increase in ring-liner contact force due to local oil dilution on the liner was examined using an existing ring-liner lubrication model.