An Improved Physics-Based Combustion Modeling Approach for Control of Direct Injection Diesel Engines

Cycle-by-cycle combustion prediction in real time during engine operation can serve as a vital input for operating at optimal performance conditions and for emission control. In this work, a real-time capable physics-based combustion model has been proposed for the prediction of the heat release rate in a direct injection diesel engine. The model extends the approaches proposed earlier in the literature by considering spray dynamics such as spray penetration and Sauter mean diameter in order to calculate the mass of evaporated fuel from the spray. Wall impingement of the liquid spray is predicted by considering the liquid length based on the prevailing in-cylinder conditions. These effects are considered even after the hydraulic end of injection till the last droplet of fuel impinges on the combustion chamber wall. The fuel evaporated from the wall film and its contribution to the kinetic energy of the charge are also considered. The model assumes the heat release rate to be proportional to the mass of fuel available in the vapor phase and the instantaneous turbulent kinetic energy of the charge (which depends on the kinetic energy imparted by the injector and that available in the liquid fuel). The constants of the model were tuned with limited experimental data on a turbocharged, intercooled common rail multicylinder diesel engine. The heat release rate predicted by the model was validated against experimental data at other load conditions from the same engine and from another naturally aspirated common rail diesel engine without any further tuning. The results indicated that the model can predict the heat released during different stages of diffusion combustion viz. free jet, wall jet, and after-burning with good accuracy. Since the model does not involve iterative procedures and uses conventionally available parameter inputs in the ECU, it can be used for real-time combustion control.