Amongst popular bio fuels is producer gas, or wood gas. It is a combustible gas derived from the thermo chemical conversion of woody biomass and generally composed of a mole fraction of 18-20% H-2, 18-20% CO, 2-3% CH4, 12% CO2 and 48-50% N-2.The composition of producer gas can vary depending on the inherent production technology. Five different compositions of producer gas were chosen from literature in order to evaluate their knocking characteristics. These compositions were simulated using a gas blending system and the methane number was evaluated on a CFR F-2 engine. The methane number of the chosen producer gas compositions ranged from 54 to 131. Further, the effects of the specific concentrations of CO2 and H-2 were quantitatively evaluated. It was determined that addition of CO2 increases the critical compression ratio while H-2 decreases it. Overall, the effect of CO2 on changing the critical compression ratio was found to be over twice that of H-2. (C) 2011 Elsevier Ltd. All rights reserved.