Present paper analyses the flexibility of co-combustion power supply in a Spark Ignition (SI) engine fuelled with Natural Gas (NG) and biogas (BG) for cogenerative purposes. The biogas properties are strongly influenced by the source biomass and by the characteristics of the conversion process, thus the possibility of a double-ramp supply with NG taken from the national distribution network allows to compensate for any decay in engine performance linked to the worst quality of the fuel and, therefore, to ensure more stable operations over time. The effects deriving from the addition of NG are quantified through the development of a dedicated one dimensional (1D) numerical model of the engine in GT-Power environment. The combustion sub-model is properly customized according to the different fuel composition relying on a detailed kinetic model, where the laminar flame rate is evaluated for each specific fuel considered. As a result, co-combustion operation appears to be a feasible solution both on the technical and economic point. The developed model can be properly adapted for the executive design of energy systems powered by NG -biogas mixtures, helping in the optimization of the energy and environmental performance.