Diesel engines require atomized fuel injection inside the combustion chamber for better combustion and reduced emissions, which in turn requires a common rail fuel injection system with higher operating pressure capabilities. But, these requirements lead to increased fuel leakage through the working clearance in the pump to the engine lubrication oil chamber and increased lubrication oil leakage to the fuel side of the pump. The fuel leakage to lubrication oil (FtO) affects the lubrication property of the oil, which in turn affects the life of the lubricated components in the engine. The lubrication oil leakage to fuel (OtF) increases the injector nozzle coking and emission. The leakage flow through the clearance gap was generally studied for 1-dimensional cases by using the Couette-Poiseuille equation obtained from the continuity and the incompressible Navier-Stokes equation. The existing analytical approaches do not consider the fluid interactions/mixing in the 2-dimensional domain. The same is addressed in this study using the numerical simulation tool, Ansys CFX, to estimate the volume flow rate of OtF and FtO considering various design parameters such as diametrical clearance (4-6 μm), cylinder bore taper and piston speed. The leakage of fuel and lubrication oil take place between the working clearance of the piston and the cylinder bore. Pressure and drag effects are two important mechanisms that drive the leakage flow. The transient piston wall speed and the transient pressure at fuel side and lubrication oil side were used as the inputs to the simulation. The grid sensitivity analysis using different grid sizes was done to optimize the grid size. Higher computation time and memory for simulation work was reduced by optimizing the various simulation input parameters. The benchmark problem of Couette-Poiseuille flow was solved and the results were cross-checked with the analytical results. The actual two dimensional flow domain was modeled for the simulation of fluid flow with mixing. The mass and volume flow rate of lubrication oil and fuel were captured at the specified boundary with respect to time. The simulation was carried for various clearance values, clearance taper and speed ranges. The OtF and FtO were found to be increasing with respect to increase in clearance and speed. With this analysis, the sensitivity of the leakage flow rate of fuel and lubrication oil with respect to the important parameters was observed. Copyright © 2014 by ASME.