Free piston engine generator (FPEG) in the engine system can be considered as a linear energy conversion system. Cascade control for the stability of piston operation level and PID controllers are proposed for both the outer and inner loop. The free movement of the piston in the cylinder an adjust the compression ratio and optimize the combustion process. In this paper, previous study on the scavenging process in engine has been discovered. Scavenging process in engine can be divided into three types; crossflow scavenging, loop scavenging and uniflow scavenging. The scavenging process is simulated using the combination of dynamic model and computational fluid dynamics (CFD). This study has disclosed the scavenging process in two stroke FPLE and the exchange process of the gas for loop scavenging engine. Experimental study and CFD simulation application by Anysys software described in this article.