In this study, the four kinetic reaction mechanisms were developed to simulate the formation of pollutant species in CNG fired IC engine. The reactions were generated using EXGAS and coupled with Leed’s NOx reactions to develop four kinetic mechanisms. These reaction mechanisms described the combustion of natural gas at low (below 800 K) to high (above 1000 K) temperature in combustion chamber. The simulation studies predicted that the maximum cylinder pressure was achieved up to 18.0 atm & 40.0 atm under fuel leaner conditions (φ ≈0.6) and fuel rich conditions (φ=1.13 to 1.3) respectively. The simulation based data was compared with the experimental data (when engine was operated at 3000 rpm, φ=1.0, P<sub>inlet</sub>=0.67 atm). For fuel rich conditions, high concentrations of CO were observed while NO<sub>x</sub> levels were lowered while the fuel leaner mixture produced the lower CO emissions and moderate levels of NO<sub>x</sub> emissions. The NO<sub>x</sub> and CO profiles depicted that Mechanism-I, Mechanism-II and Mechanism III seemed to be inappropriate for predicting the emissions in due to CNG combustion IC engine. The molded data for Mechanism-IV exhibited closer agreement with experimental measurements. The rate of production analysis identified the important reactions in each mechanism which were contributing in the formation of emission concentrations of pollutant species. Although each proposed mechanism illustrated some discrepancies among the profiles, Mechanism-IV (consisting of 208 reactions and 78 species) showed good agreement with experimental data for pressure, temperature and pollutant species profiles.
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