In this paper, a high gain observer is designed for a parameter varying polytopic model to estimate the enclosed mass in the combustion chamber of a spark ignited engine. The high gain strategy allows the design of an observer that handles the uncertain part of the system. The observer uses the cylinder pressure measurement during the compression stroke to estimate the enclosed mass. An engine compression model is used as a virtual engine to build the observer. The results of the observer are compared to the virtual engine model and a good agreement between the observed variables and the model was obtained.