The role of accurate modelling of heating and evaporation of fuel droplets in predictive simulations of partially premixed compression ignition engines

Predictive simulations of partially premixed compression ignition (PPCI) engines has been a challenging task especially at late fuel injection timings. Fuel physical properties and the rate at which it evaporates play a vital role for the start of combustion. In this paper, the implementation into CONVERGE CFD software of the effective thermal conductivity/effective diffusivity (ETC/ED) heating and evaporation model based on the analytical solutions of the heat conduction and species diffusion equations in liquid phase via User-Defined Function (UDF) is presented. First, model implementation is validated against single droplet evaporation of a mixture of n-heptane and n-decane showing better agreement than the infinite thermal conductivity/infinite diffusivity ITC/ID heating and evaporation model. Then simulations of hollow cone spray of PRF65 are performed and validated against experimental data from the literature. Finally, full cycle PPCI engine simulations of PRF65 are successfully performed with different start of injection (SOI) timings -35, -25 and -20 CAD aTDC exhibiting good agreement with experimental data. The effect of heating and evaporation model used on combustion phasing is investigated highlighting the importance of accurate modelling of physical processes to properly predict PPCI engine performance.