This work has conducted a numerical study using AVL‐Boost engine simulation utilizing biogas (CH4 and CO2)–air mixtures in a SI engine model. The effect of varying the air manifold temperature from 25 to 50°C on the combustion characteristics as well as engine performance under full load condition in a SI engine fueled by gasoline and biogas mixtures has been investigated. The performed tests involved using variable concentrations of CO2 from 10% to 40% by volume. The current model has been validated with experimental results and it was found that the error in predicting the P‐θ behavior of the engine is about 4.89%. The results showed that, as the air inlet temperature increases, the volumetric efficiency decreases and the brake specific fuel consumption (BSFC) increases for gasoline and all biogas compositions. However, the peak pressure, brake output power, maximum pressure rise rate, and maximum heat release rate were decreased slightly with the increase in air inlet temperature. The effect was only observed for gasoline and the 10%‐CO2 biogas, whereas the effect on the maximum heat release rate was detected on gasoline and biogas compositions up to 20%‐CO2 only. On the other hand, the existence and growth of CO2 content in the biogas reduced significantly the peak pressure, brake output power, maximum pressure rise rate, volumetric efficiency, and both net and maximum heat release rates, though its effect in reducing the volumetric efficiency stopped at 20%‐CO2 biogas. Furthermore, BSFC rises noticeably with the increase in CO2 ratio in biogas. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.