Engine simulation has traditionally been an instrument for the early phase of engine design in order to choose the optimal compromise between the different requirements, such as cost, packaging, performance and fuel consumption.However, the problem has somewhat changed from an engine design to an engine calibration and function development task, as new technologies increase the degree of freedom to such an extent that it is almost impossible to find the optimal setting in the test bench.The purpose of the model is to be used as a Model-In-the-Loop model for offline simulation and debugging during the algorithm development phase of the engine controller. It can also be used during the rapid prototyping calibration phase as the amount of measurements needed for the model is relatively small. From the measurements, simulation can rapidly be executed to find a calibration setting which is in the correct ballpark. A final fine-tuning is then required in the test cell and the time saved can be substantial.The model presented in this paper is a thermodynamic model for rapid simulations. It uses all the geometric properties of the engine and it includes the effect of heat transfer and crevice losses with a model of the combustion deficiency. The specific heat ratio is calculated based on fuel, temperature and lambda, using chemical equilibrium package for accurate calculation of specific heat ratio in the presence of dissociation.It has been verified in a single cylinder test cell running two different fuels, Indolene and E85, with 3 different compression ratios running loads from 2-18 bar IMEP. The model is implemented in Matlab and it takes approximately 0.5 seconds to execute one cycle. The standard deviation of the model, with respect to the indicated mean effective pressure error is less than 2%.