The results from the experimental diagnosis of the combustion process taking place in a 0.5 litre single cylinder D.I. Diesel engine with common rail injection system are presented and analysed in this paper. Some parameters characterising the injection process were modified during the experimental study, such as the start of injection, the pressure in the common rail and the injection hole diameter. Fuel delivery was kept constant in all cases. The tests were scheduled with two objectives: to determine the separate effect of each of the mentioned parameters, and to analyse the effect of the spray characteristics, for which injection pressure and hole diameter values were combined to provide similar injection rate and injection duration.The combustion diagnosis was carried out using a thermodynamic model based on the instantaneous thermodynamic properties of the gas in the chamber, which are affected in the model by the injection law input. This study is focused on the effect of the mentioned parameters relative to the injection process on the engine performance and emissions and on the parameters describing the heat release law. The latter parameters are very useful for justifying the observed emissions tendencies.