Internal combustion engines (ICEs) operated using gaseous fuels have shown significant potential in terms of the integration of renewable and traditional energy sources for an effective solution to clean energy production and storage challenges. In contrast, each mixture is characterized by different combustion properties that influence the overall ICE working conditions. The impact of the methane and biogas from anaerobic digestion under the emission levels and engine working conditions was investigated. The application of either a numerical or an experimental approach helps in adjusting the ICE operating conditions to fulfill strict emission regulations. The numerical model of an ICE provides the possibility to visualize the propagation of an emission front for different species, i.e., CO2, NO, and other parameters. The experimental studies included an in-cylinder pressure and exhaust-gas emission analysis for various excess oxygen ratios and spark timings. The numerical model accurately predicts the combustion process for variable excess oxygen ratios, fuel compositions, and the start of a spark. The results showed significantly lower emissions of NOx for biogas owing to a lower in-cylinder temperature. Moreover, the majority of NO and CO is located near the spark ignition and is not directly connected with the combustion in the flame front. (C) 2019 Elsevier Ltd. All rights reserved.