A numerical model for a prechamber autoignition gas engine under development has been established based on an experimental test case. With a simplified finiterate/ 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. The predefined empiric constant of the eddydissipation 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 a simulation. 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 cylindrical geometry tested did not show any improvement in the combustion homogeneity inside the prechamber and its volume (limited by the real engine geometry) is not sufficient to properly ignite the main chamber.