The application of current CFD techniques to the crankcase of a conventional two-stroke engine is described. The model was based on a single-cylinder research engine and included features to simulate the motion of the reciprocating piston, the rotating flywheel, the reed valve and the opening and closing of the transfer ports. The model did not take account of the piston geometry and connecting rod, as their inclusion would have added undue complexity to this initial treatment of the problem. The resultant flow predictions were largely dominated by the flywheel induced recirculation. However, during the induction phase a strong ‘s’ shaped flow developed to carry the inlet charge directly through the crankcase and into the under-piston area.