The performance of a spark ignition internal combustion engine (SI ICE) fueled with biomass gas (woodgas) is evaluated using an analytical mathematical model. For the evaluation, the model was based on the fuel-air thermodynamic cycle for spark ignition engines, taking into account the composition of woodgas used as fuel, the thermodynamic properties of the fuel supplied, the cylinder geometric characteristics, the engine operational conditions, the effects of heat losses in the cycle through the walls of the cylinders and due to the loss as gas “blow-by”, the influence of dissociation processes during the combustion and the residual gases remaining in the cylinders at the beginning of the compression stroke. The model can predict of the internal temperatures profiles, heat flow, as well as the work and pressure in relation to crank angle. It was used also to evaluate the influence of the rotation speed, the air ratio and the ignition timing on the engine indicated power. It was found that when feeding the engine with woodgas, a power output between 59 and 65% can be obtained, in comparison it's powered by gasoline. Additionally, the analysis shown the engine performance when feeding it with pure methane (100%), to give an idea about what would happen if the same engine is fueled with natural gas under the same conditions (the main component of natural gas is methane and the presence of this in composition generally varies between 70 and 95%).