A mathematical model based on turbulent gas jet theory was used to study spray-swirl interaction in direct injection diesel engine. Vectorial approach was used to predict the momentum change in two directions (i) along the spray centre-line and (ii) normal to the centre-line. Effect of changes in air density and swirl velocity during compression was accounted for. Effect of different injection parameters viz. nozzle size, injection rates, duration of injection, shape of injection rate curve, location of injector in the cylinder head and spray angle was studied on spray penetration, spread, air entrainment and momentum ratio of spray to air etc. Initial rate of injection and mass averaged injection pressure play significant role in air-fuel mixing for spray injected from side of the cylinder, whereas the initial rate of injection dominates the mixing for central injection. Nozzle orifice size does affect fuel-air mixing but it is predominantly controlled by the injection pressure.