ANALYTICAL INVESTIGATION OF INTERACTION BETWEEN AIR MOTION AND FUEL SPRAY IN DIRECT INJECTION DIESEL ENGINE

Abstract

In a small size direct injection diesel engine, fuel injection spray impinges on the combustion chamber wall, thus significantly affecting fuel-air mixing and burning characteristics of the spray. With decrease in engine displacement, air swirl inside the combustion chamber is considered essential to improve fuel-air mixing.'.' Consequently , the understanding of the influence of the wall on the spray development in the combustion chamber is quite important. It is reported in the literature that the decrease in displacement volume of the engine results in a significant increase in brake specific fuel consumption (BSFC). The deterioration in the fuel economy is attributed to the wall wetting due to the spray in addition to higher heat transfer and friction-losses. A proper matching of fuel injection, air motion and piston bowl geometry is, therefore, very important. In literatdre, only a few attempts are seen concerning the modelling of the wall jet development in engine situation. The available models present a simplified analysis in this respect without consideration of the swirl and the wall shear effects. The present study is an attempt to investigate the influence of spray-swirl-wall interaction in direct injection diesel engine. An analytical model has been developed to predict the fuel-air mixing and burning characteristics of small DI diesel engines. The predicted results have been validated with the available experimental data. Besides.comparison with the dublished data of earlier investigators, the' data are also measured on engines. Four mass and momentum conservation equations are written along the tangential and normal directions to the spray for both it free and wall regions based on continuum integral approach. The flow along the smooth and impermeable wall is considered to be two dimensional. The velocity distribution in the spray along the wall is taken by the composite profile developed combining one-seventh law of velocity distribution in turbulent boundary layer at the wall and the velocity profile of Abramovich in the free jet 'region. The influence of the bowl geometry in 'terms of moment of inertia and the wall friction on the instantaneous swirl level during compression is considered. The spray structure is multi-zonal and the simulation of atomization, droplet-size distribution, evaporation turbulent mixing and combustion processes is included. The fuel spray is divided into a number of elemental zones in the axial and radial directions in order to simulate the heterogeneous nature of the diesel spray. It is postulated that following the ignition delay period, combustion occurs at the specified rate in the elements where the actual air-fuel vapour equivalence ratio falls within the limits of inflammability Hence, the mass rate of burned fuel and the rate of heat release is computed. Conservation equations for the mass and energy are formulated for the spray and surrounding fluids. Solution of energy equation and equation of state gives the cylinder pressure, the temperature and the volume of the spray and temperature of the surrounding fluids. Heat transfer from gases to the walls of the cylinder is computed following Woschni's correlation. Moment of inertia of the air, depending upon the shape of bowl-in--piston and the bowl offset, influences the angular swirl near top centre position of the piston at the time of injection. It is found that there is an appreciable variation in the air swirl at the top centre position due to the variation in bowl geometry...