Disclosed herein is an internal combustion engine (and a method of operation thereof) comprising a piston movable relative to a combustion chamber and to a crankcase to effect cyclical pressure variation in the crankcase in response to piston movement, a transfer passage for introducing fuel-air mixture into the combustion chamber from the crankcase in response to cyclical crankcase pressure variation, an auxiliary chamber separate from the transfer passage, and a port arrangement for providing the auxiliary chamber with pressurized air in response to cyclical crankcase pressure variation and for initiating communication of the auxiliary chamber with the combustion chamber in response to piston movement during the presence of pressurized air in the auxiliary chamber and at a time other than the time of initiation of introduction of fuel-air mixture into the combustion chamber.