A HFPE (hydraulic free-piston engine) has been proposed by the SKLE (State Key Library of Engines) in Tianjin University as a mean to significantly improving the IC (Internal Combustion) engines' cycle thermal efficiency and lowering exhaust emissions. As the piston dynamic and the cylinder structure are hardly seen in conventional engine, a 3-D (three-dimensional) CFD (computational fluid dynamics) model was built to investigate the scavenging process. A wide range of structure and operating parameters were investigated including the intake port radial angle, intake port height and operating frequency. We achieved the highest scavenging efficiency 0.907 through optimization, which is 11% higher than that of the original design. (C) 2014 Elsevier Ltd. All rights reserved.