Nowadays, governments exert extra pressures upon auto manufacturers to reduce emission of their products, especially emissions that affect Ozone layer. Among the emissions, CO2 has an important role in destruction of Ozone layer. In comparison with other fuels such as: diesel and gasoline, compressed natural gas (CNG) has better properties (in the case of CO2 production). Therefore, CNG is a good alternative for liquid fuels. One of the most economic methods is cycle conversion from diesel to SI. In this method, injectors should be replaced by spark plugs, so that, air and CNG mixture enters combustion chamber from intake valves, because CNG ignited in less pressure related to diesel fuel. In order to reduce pressure ratio and reach the maximum flame speed, we have to change combustion chamber geometry. In this project, some different geometry for combustion chamber is developed. By fluid flow analysis into combustion chamber, the effect of variation upon the flow velocity and intensity of turbulent before combustion initiation are investigated. The goal of this work is to determine combustion chambers geometry with the maximum efficiency and performance and minimum emission production