SOOT MODELING FOR DIRECT INJECTION DIESEL, ENGINE

Abstract

The work presented in this thesis aims at the development of.a detailed diesel soot emission model for direct injection engines. Tesner's soot formation mechanism and Magnussen's concept of soot oxidation are taken as the basis. Prior to the, soot predictions, a combustion model is required. Hence, a combustion model capable of predicting histories and spatial distributions of flame tempera-ture and equivalence ratio together with cylinder pressure history is also developed. Assuming that a multi-dimensional model is more suitable in such an investigation, a two dimensional multi-zone model is formulated within the constraints of available information on description of engine processes, and keeping computer storage and time requirements minimal. The synthesis of equations of engine processes and thermodynamic analysis of conservation equations enabl-ed predictions of the instantaneous values of pressure and the temporal and spatial. distributions of temperature and equivalence ratio in the engine cylinder. Diesel engine combustion process is severely dependent on the injection parameters through the processes of atomisation and evaporation. The injected. fuel spray is divided in many elemental zones. The spray has 16 equal radial divisions. The axial divisions are formed in terms of each packet of injection in discretised time steps. Fuel injection rate is Imputed assuming incompressible flow through an orifice of injector nozzle. The atomisation model is based on Simmons' binary droplet division model. The instantaneous Sauter mean diameter is computed from correlation of Elkotb and Abdalla.* The fuel distribu-tion pattern in the spray is established assuming that the droplets of all the sizes are randomly distributed throughout the spray region iii following a well established concentration profile of Abramovich. The evaporation of fuel sprays containing droplets of a wide range of sizes, is computed using simplified approach of spherico-symmetric droplet evaporation model with empirical correlation for convection effect. The spray-swirl interaction model is developed using centre-line velocity vector/continuum approach. The model has three-dimensional feature in terms of the fuel spray motion. The four integral con-servation equations used in the model are for fuel mass, total mass, momentum along spray centre-line and momentum along normal to the centre-line. The solution of these equations gives the spray growth, direction, centre-line velocity and concentration. At the end of the injection period, the air-fuel vapour mixing is postulated to be'controlled by the turbulent kinetic energy dis-sipation rate of the system. The source of turbulence is taken as the instantaneous net energy available in the system due to air motion, viscous losses, fuel injection, combustion and heat transfer. Appro-priate conversion efficiency of the kinetic energy to turbulent energy is used. The stoichiometric combustion assumption is made in the spray elements having air-fuel vapour equivalence ratio within the limits of flammability. Thermodynamic analysis of the system is made by writing mass and energy conservation equations for the spray elements and the whole control volume as well as the equations of state for the spray and the surroundings. Solution of these equations gives the histories of cylinder pressure, the surrounding temperature, and volume and• temperature of the spray.