Internal combustion engine

A two-stroke engine is for a portable handheld work apparatus such as a motor-driven chain saw, cutoff machine, brushcutter or the like. A combustion chamber ( 3 ) is configured in the cylinder ( 2 ) of the engine ( 1 ) and is delimited by a reciprocating piston ( 5 ). The piston ( 5 ) drives a crankshaft ( 7 ) via a connecting rod ( 6 ). The crankshaft ( 7 ) is rotatably journalled in the crankcase ( 4 ). The two-stroke engine includes a discharge opening ( 10 ) for conducting exhaust gases out of the combustion chamber ( 3 ) and an inlet ( 11 ) via which an air/fuel mixture is conducted to the crankcase ( 4 ). The air/fuel mixture passes via at least one transfer channel ( 12, 15 ) from the crankcase ( 4 ) into the combustion chamber ( 3 ). The transfer channels ( 12, 15 ) open at one end with a transfer window ( 13, 16 ) into the combustion chamber ( 3 ) and are connected at the other end to the crankcase ( 4 ). A bypass channel ( 20 ) conducts essentially fuel-free gas and opens via a membrane valve ( 19 ) into the transfer channel ( 15 ). The sum of all areas of the inlet windows ( 13, 16 ) referred to the piston displacement of the engine is approximately 1.4 mm2/cm3 to approximately 5.90 mm2/cm3 whereby good exhaust-gas values are obtained at low fuel consumption.