A Study of Diesel Cold Starting using both Cycle Analysis and Multidimensional Calculations

The physical in-cylinder processes and ignition during cold starting have been studied using computational models, with particular attention to the influences of blowby, heat transfer during the compression stroke, spray development, vaporization and fuel/air mixture formation and ignition. Two different modeling approaches were used. A thermodynamic zero dimensional cycle analysis program in which the fuel injection effects were not modeled, was used to determine overall and gas exchange effects. The three-dimensional KIVA-II code was used to determine details of the closed cycle events, with modified atomization, blowby and spray/wall impingement models, and a simplified model for ignition. The calculations were used to obtain an understanding of the cold starting process and to identify practical methods for improving cold starting of direct injection diesel engines. It was found that, blowby gas flow represents an important source of reductions for the cylinder gas temperature at lower cranking speeds, opposing the squish flow. Overfueling and advanced injection increase the amount of fuel evaporated. The spray intact core is extended at low temperatures, spray wall impingement phenomena are characterized by low impact velocities and the bouncing of liquid drops enhances the limited fuel-air mixture formation. Failure to achieve successful ignition at low initial air temperatures was predicted by the simple kinetics model.