For recovering the waste potential energy of natural gas during pressure reduction in City Gate Stations (CGS), Reciprocating Expansion Engine (REE) could be utilized instead of throttling valves. The main goal of this theoretical analysis is to optimize ports opening and closing time of REEs. The first and second law analysis of the natural gas inside the cylinder, as a control volume, has been carried out for optimization. The optimization is based on Genetic Algorithm from exergy efficiency concept. The influence of the REE pressure ratio on the exergy efficiency is analyzed by numerical calculations too. In general, the results of the analysis showed that exergy efficiency based on optimization of inlet/outlet ports opening and closing timing has a huge impact on the REE performance. It was found that for optimized timing, exergy destruction due to outlet throttling is two or three times of destruction due to inlet throttling, in one case 14.5 kW against of 6 kW. It is also found that engine size does not have much impact on the port timings. The results showed that exergy destruction due to mixing and heat transfer can be neglected, although friction and inlet/outlet throttling have a significant impact on exergy loss. The portion of friction is about 5-10% in all cases. It is also found that inlet pressure has significant effects on optimized port timing. In cases of inlet pressure of 70 bars, inlet processes should be finished at 63 degrees and in cases of 55 bars it is about 72 degrees. If a REE is optimized for inlet pressure of 70 bar, it should not be utilized for inlet pressure less than 30 bar. (C) 2015 Elsevier Ltd. All rights reserved.