Oxidation of hydrocarbons from lubricant oil layers in spark-ignition engines

Abstract

The postflame oxidation of hydrocarbons desorbed from the lubricant oil layer in spark-ignition engines is investigated through simulations and comparison with experimental measurements. A one-dimensional reactive-diffusive model is formulated for the process of desorption and oxidation of hydrocarbons emerging from the lubricant oil layer. Energy, mass, and species conservation equations are solved through the expansion stroke, using simplified assumptions for the turbulent diffusivity, boundary layer growth, and chemical reaction rates. Simulation results show that under the current assumptions both reaction and diffusion rates are controlling. Chemical reaction rates peak over a narrow range of temperatures throughout the expansion stroke. This oxidation zone moves away from the wall during the expansion stroke due to heat transfer and volumetric expansion: Sensitivity analysis shows that the single most important parameter determining the oxidation of hydrocarbons from the oil layer is turbulent diffusion, which must be included for predictions to be of the same order of magnitude as experimental values. Other important parameters are chemical reactivity of the hydrocarbon and the initial boundary layer thickness. Good agreement is obtained regarding the sensitivity of oxidation levels to dopant type and the most sensitive operating parameters: dilution and coolant temperature. This good agreement indicates that oxidation of unburned hydrocarbons in engines may well be represented by such simplified models. However, a factor of 3 discrepancy in the prediction of absolute values for the oxidation levels suggest that substantial refinement is required in the submodels.