In light aircraft, it is common to find low-displacement diesel engines, which, due to their size and configuration, offer good performance at low cost. The fact that there are volumetric losses of the combustion chamber towards the crankcase (known as the Blow-By) decreases the performance of this thermal machine. In this investigation, a new methodology is proposed in order to predict the amount of mass sent to the crankcase, so that the operational or the design conditions can be defined that minimize the impact of Blow-By on efficiency. Using a CAD model, a Computational Fluid Dynamics (CFD) analysis has been applied in order to determine the mass flow by Blow-by for the specific operational condition. The results of the theoretical model adjusted through Computational Fluid Dynamics have been compared with experimental values, thus obtaining relative errors of less than 2% in the Blow-By prediction. Different case studies have been applied to the engine used, analyzing the pressure and velocity profile in the piston rings. It has been verified that the study of this phenomenon is critical since it has been found out that up to 9% of the initial mass of gas contained in the combustion chamber is lost by Blow-By, which affects the volumetric efficiency. In the same way, the proposed methodology will allow characterizing the volumetric performance of diesel engines without the need for experimental tests and with high predictability.