A numerical simulation of a prechamber autoignition gas engine has been performed based on an experimental test case. With a simplified finite-rate/eddy-dissipation model for the combustion of natural gas, it was possible to properly reproduce the experiment considering the combustion duration, ignition timing and overall energy balance. However the predefined empiric constant of the eddy-dissipation model had to be increased by a factor of 10. A modification of the original cylindrical-conical prechamber geometry to a simpler cylindrical one was tested with the simulation model. The influence of burnt gases inside the prechamber was assessed simulating the mixture formation inside the prechamber. The simulations showed little effect of taking into account the non-homogeneities in the gas phase on the combustion duration. The simulation showed that the new and cylindrical geometry envisaged did not show any improvement in the combustion homogeneity inside the prechamber and its volume (limited by the real engine geometry) is in fact not sufficient to properly ignite the main chamber. The model can be used to further guide design modifications of the prechamber engine to improve performance.