An Empirical, Mixing-Limited, Zero-Dimensional Model for Diesel Combustion

A zero-dimensional model is introduced that combines recently presented empirical relationships for diesel jet penetration and flame lift-off length in order to produce a realistic heat release rate and predict the temperature and equilibrium species concentrations in five zones within the combustion chamber. The new model describes the compression, combustion and expansion portions of a diesel cycle. During fuel injection and combustion, the temperature, geometry, and composition of five zones are calculated: 1) vaporizing fuel and air, 2) vaporized reactants, 3) premixed products, 4) adiabatic flame sheath, and 5) surrounding charge gas. The apparent heat release rate predicted by the model is compared with data from a constant volume combustion vessel (CVCV) and two single-cylinder direct-injection diesel engines. The rate of charge air entrainment is determined from the correlation of a non-vaporizing, non-reacting jet with no wall impingement. Jet-to-jet interactions are found to be significant in describing heat release and a sub-model was developed for this interaction. The results show that in all cases the initial rise in the apparent heat release rate is over-predicted by the model. Peak heat release rates can be found from either the flame length or the interaction length of the jet. The shape of the apparent heat release in the CVCV, where there is no wall or jet interaction, is well characterized by the model, as is the steady-state apparent heat release rate. The modeled diesel engine results do not produce the correct shape for the “diffusion burn” period without the jet interaction sub-model, but are much improved with it included. Trends in combustion duration, pressure, and temperature are captured by the model. The over-prediction of heat release exhibited by the model is attributed to the fact that the flame location lies within the jet boundary and therefore, not all of the air entrained into the jet has reached the flame.