The paper reports an investigation focusing on the transient evolution of diesel sprays. In order to understand the relationship between fuel–air mixing and spray penetration, a one-dimensional spray model is developed, which is capable of predicting the spray behaviour under transient conditions. The main assumptions of the model are the mixing-controlled hypothesis and the validity of self-similarity for conservative properties. Validation of such concepts is achieved by comparing model predictions with both CFD gas jet simulations and experimental diesel spray measurements. Results show that a reasonable estimation of the spray evolution can be obtained for both non-vaporising and vaporising conditions.