Diesel engines find application in a large number of sectors of modern industries, such as the automotive and maritime. The wide adoption of Diesel engines, along with the rise of environmental concerns created the needs for optimization of fuel efficiency and power output as well as minimization of exhaust gas emissions. These targets must be met while retaining the reliability of Diesel engines. One technique that is used to significantly enhance the reliability of Diesel engines is piston cooling. The working fluid is engine oil which is injected in the cylinder towards the lower side of the piston. The objective of the current study is to simulate the process of cooling with the aid of CFD. The numerical methodologies that are used in the current study are the Reynolds-Averaged Navier-Stokes (RANS) equations for the continuous phase, while for the disperse phase the modelling is conducted with the Lagrangian modelling. The numerical model which is built focuses on the effect that the mesh resolution has on the results of the simulation, especially on the disperse phase of the flow. It can be inferred that there is dependence between the quality of the results and the mesh resolution. It is shown that the grid has to be fine enough to produce quality results, but if there is excessive refinement, the quality of results drops because assumptions of the Lagrangian modelling are not satisfied. Following this phase of the study, a CFD model is built to simulate the phenomenon of droplet impingement on a solid wall. In this set of simulations, the goal is to compare the impingement model that is used in the software with published experimental results of droplet impingement. The results indicate agreement between the CFD model and the experimental process. In the final stage of the thesis, there is the numerical study of heat transfer involved in the process of spray impingement on a hot wall. A CFD model is built to compare the heat transfer which is predicted by the software model and results from relevant published experimental studies. It can be concluded that there is divergence between CFD and published experimental results, stemming from the modelling assumptions of the software. The results of the present thesis can be used in future studies of the company, either as a continuation of the spray cooling topic, or for novel studies such as gallery cooling.