Numerical Investigation of the Influence of Oil Dilution on the Ability to Initiate a Pre-Ignition Combustion 2020-01-0611
Although the root-cause of the initial pre-ignition is not yet proven, there is a general agreement throughout the literature that it is very likely a 2-phase particle or droplet induced phenomenon. In the case of a droplet induced mechanism there are still uncertainties regarding the detached droplet size, velocity and composition. Former research work suggests that heavy wall wetting during injection is increasing the PI-frequency in WOT operation points. Due to the oil dilution the viscosity and the surface tension are reduced which enhances the likelihood of detachment.
The present work is therefore investigating the influence of the oil dilution on the droplet evaporation and the PI-tendency. According to previous work performed at the research group there is an influence of the radical Ketohydroperoxide that has its origin in incomplete oil combustion on the pre-ignition probability of an engine cycle. This underlines the role of oil combustion chemistry and the oil mass fraction in detached droplets.
In order to characterize the droplet evaporation a multicomponent surrogate model was chosen. The selection of the species is based on the distillation curves and the basic chemical composition using a detailed distillation model. In order to capture mixture effects, the vapour-liquid-equilibrium incorporates activity coefficients calculated by the UNIFAC method. The investigated lubricant oil is modelled as a mixture of four n-alkanes based on a GC-analysis. This established model shows an exceptionally good agreement with the measurement data.
The fluid surrogates are transferred to a CFD-Code to predict the evaporation behaviour of different oil-fuel mixtures under engine-like conditions. Different release timings during a compression stroke are investigated. The compression is modelled through a time-dependent change of the ambient gas pressure and temperature. Furthermore a sensitivity analysis of the boundary conditions (T_gas, T_Drop, v_rel) was performed. It can be shown that there is a critical amount of oil dilution at which the ignition of a droplet is suppressed.