The modelling of fuel dispersion and concentration in direct injection diesel engines

The presented investigation develops a modelling technique, which allows measurement in a dynamically similar gas jet, to be related to the diesel engine injection process in the presence of air swirl. Modelling experiments have been performed for direct injection processes from both the centre and the circumference of the combustion chamber. A tracer gas technique has been employed, by which, concentration of jet nozzle gas in the simulated air swirl was measured, thus yielding information on model predicted trajectory, spread and local fuel concentration. Gas concentration was measured using a specially developed hot wire anemometry technique which allowed measurement in a transient pulsed jet simulation. The modelling theory was developed on the basis that spray droplet velocity relative to the air entrained into the spray is small after an initial jet disintegration, and droplet formation process. Consequently the fuel spray is assumed to behave as an air jet bearing a mist of liquid droplets. Favourable comparison of model gas jet and engine fuel spray behaviour is initially made with published film data. Further comparison of engine performance and associated high speed photographic results, with the model predicted fuel dispersion and local concentration levels, is made from data obtained on a modified Petter PM test engine. The results indicate that model predicted rich fuel regions, both at the combustion chamber wall and within the Jet core correspond to smoke generation areas recorded on the high speed films. Similarly, experimental engine performance parameters such as exhaust emission levels' rate of pressure rise, and peak pressure are shown to directly relate to the model predicted dispersion of fuel. The conclusions drawn are that the modelling technique has potential in optimising the fuel injection equipment specification at the design stage and effectively represents the behaviour of the modelled engine fuel spray.