This paper presents the analysis on flux and electromotive force (EMF) developed by moving coil and moving iron linear generators using finite element method (FEM). A 6 pole moving coil and 6/4 moving iron linear generator are used to analyze and compare the flux distribution in the air gap. These generators will be used for free piston linear engine. A moving permanent-magnet linear generator has drawbacks of thermal and impact force demagnetization, in addition to requiring complex control strategies. To overcome these limitations, one of the solutions is to place magnets on the stator instead on the translator. Based on the analysis it is proposed to replace the moving permanent-magnet by a moving-coil for a linear generator whose prime mover is a free-piston linear combustion engine.