Replacement of expansion valves which are used in natural gas pressure reduction stations, with expansion engine, to recycle wasted energy to generate electricity is the main objective of this study. In these engines, ports are used for inlet and outlet the gas. Control valve geometry set on these ports has a great impact on performance of this engine. In this research, simulation and optimization of the optimum opening and closing time of these valves according to two types of valve, piston and spool valves, to maximize the Exergy Efficiency has been done for the first time which Genetic Algorithm is used for this optimization. Simulation has been conducted with regard to the composition of gas stations which to calculate the thermodynamic properties of natural gas, AGA8 standard is used. For optimization constrain of having no back flow has been applied. Results showed that exergy destruction due to outlet processing in piston valve and due to inlet processing in cylinder valve is more than other destruction sources. Overall engine with cylinder valve has better performance than engine with piston valve. Variation of supply line pressure showed that engine with cylinder valve could not be used in inlet pressure of 30 bar, because engine doesn’t produce power.