Interpreting diesel combustion with a fuel spray computer model

The experimental development and matching of diesel fuel injection equipment (FIE) to a new combustion chamber design is a time consuming procedure, however, significant advances have been made in recent years in developing computer modelling techniques which can assist the experimental processes. This paper describes the development of a phenomenological computer model of a diesel fuel spray in an air swirl. The fuel spray was initially modelled as a turbulent gas jet. This model was then further developed to account for fuel evaporation and two phase flow effects. To validate the model, calculated results have been compared with a wide range of experimental data for gas jets, non-evaporating liquid sprays injecting into a quiescent high pressure chamber, and diesel fuel sprays injecting into an engine fitted with a transparent combustion chamber. A sample of the comparisons are shown in this paper. The fuel spray model has been used as a diagnostic tool to interpret experimental data from an engine on a test bed. Experimental data for injector needle lift, line pressure and engine air swirl are supplied as input data to the computer model. Examples are given to illustrate how the model has been used to identify the reasons for differences in engine exhaust smoke levels at particular engine operating conditions for both truck DI and HSDI combustion systems using electronic unit injectors which give low levels of soot emission. The understanding gained can then be used to propose changes to the FIE or the engine combustion system to achieve even lower levels of soot emission.