Ignition delay of diesel sprays is a strong function of ambient temperature and pressure. However, the physical delay has not been modelled satisfactorily in the literature. In this paper, phenomenological calculations of the cooling of spray surface have shown that the physical parameters and fuel type influence the temperature of the mixture of air and the vapour produced by the first parcel of the injected fuel throughout its life upto ignition.A unique thin-ring like zone on the spray surface is postulated where the preflame reactions have reached a critical level beyond which uncontrolled reactions take place. The time at which the spray just touches the ring, ignition is predicted. However, due to turbulence, ignition will take place at only a few points in the neighbourhood of the ring.Decrease in hole-size and increase in injection quantity decrease the drop-size. Reduction in size increases the cooling rate and hence increases the ignition delay. Decrease in volatility decreases fuel vapour concentration and hence decelerates the reaction to increase the delay. Increasing cetane number decreases the activation energy of chemical reaction and hence decreases the delay. It is concluded that detailed consideration of droplet formation, evaporation of fuel and preflame reaction has enabled prediction of delay period and location of the ignition accurately within the experimental errors and errors in the input to the calculations.