Free-piston linear generator is single axis mechanism operating engine which much simple compare to conventional.This paper focus more on free-piston linear generator gas exchange simulation.A computational study has been conducted under the design of exhaust manifold and simulation of the effect of exhaust manifold on gas exchange process.The engine already had intake port and exhaust port design, but the design of exhaust port was multi hole exhaust port which give a backlash of difficulty of collecting the gas exhaust residual and reduce the combustion pressure performance.Based on study, smaller and compact manifold volume will increase the back pressure but the velocity of air exchange is increase.The back pressure also should less than atmospheric pressure so that the residual still can exit from exhaust port. © 2023, Institute of Technology PETRONAS Sdn Bhd.