High pressure fuel injection is a crucial process that impacts the subsequent steps of mixture preparation, ignition and combustion in diesel or gasoline direct injection engines. Experimental studies have revealed the mixing-controlled and local phase equilibrium characteristics of liquid vaporization in high injection pressure engine sprays. In this study an Equilibrium Phase (EP) spray model is explored. The model is developed based on jet theory and the phase equilibrium assumption, without modeling drop breakup, collision and finite-rate interfacial vaporization processes.
The model development is carried out in an advanced version of KIVA-3vr2. The presently proposed EP spray model is validated extensively in simulations of the ECN Spray A, and in an optical diesel engine and in DISI gasoline sprays. The predictions of liquid/vapor penetration, fuel mass fraction distribution, heat release rate and emission formation are in good agreement with experimental data. In addition, good computational efficiency and grid-independency are also seen with the present EP model. The examined operating conditions cover wide ranges that are relevant to IC engines, which are 700?1400 K, 7.6?22.8 kg/m3 for diesel sprays, and 500-900 K, 3?9 kg/m 3 for gasoline DISI sprays, respectively.
Thermal radiation is another topic of interest in this work since it accounts for an important mechanism of heat transfer in engine combustion. The role of thermal radiation in engine combustion is investigated by implementing the Discrete Ordinate Method (DOM) model and the narrowband model into CFD code together with the EP spray model. Engine simulation results show that radiative heat loss accounts for about 9?18% of the total wall heat loss for operating loads from 4 to 14.5 bar gIMEP. A strong correlation is found between the global equivalence ratio and the fraction of radiative heat loss to total wall heat loss, which even applies in low temperature combustion modes. The participating gases of CO2, H2O and CO are found to be the dominant source of thermal radiation due to their abundancy in high temperature regions. Also, the cooling effect of thermal radiation is shown to have significant impact on the prediction of soot emissions.