Modeling the Evolution of Fuel and Lubricant Interactions on the Liner in Internal Combustion Engines 2018-01-0279
In internal combustion engines, a portion of liquid fuel spray may directly land on the liner and mix with oil (lubricant), forming a fuel-oil film (~10μm) that is much thicker than the original oil film (~0.1μm). When the piston retracts in the compression stroke, the fuel-oil mixture may have not been fully vaporized and can be scraped by the top ring into the 1st land crevice and eventually enter the combustion chamber in the format of droplets. Studies have shown that this mechanism is possibly a leading cause for low-speed pre-ignition (LSPI) as the droplets contain oil that has a much lower self-ignition temperature than pure fuel. In this interest, this work aims to study the oil-fuel interactions on the liner during an engine cycle, addressing molecular diffusion (in the liquid film) and vaporization (at the liquid-gas interface) to quantify the amount of fuel and oil that are subject to scraping by the top ring, thereby exploring their implications on LSPI and friction. An analytical model is developed by coupling multi-component heat and mass transfer using an implicit, adaptive-time and fixed-space numerical scheme. The results of this model suggest that a substantial fraction of the fuel-oil mixture still remains on the liner when the piston retracts if the initial fuel film thickness is on the order of 20 μm; this fuel-oil mixing also results in a local oil dilution that can lead to a significant increase in the ring-liner contact force.