Abstract: An internal combustion engine has at least one pair of first and second cylinders (12, 14) with the first (12) cylinder having a larger swept volume than the second cylinder (14) and respective first and second pistons (16, 18) reciprocable in the cylinders. The second piston (18) has a drive stem (234) and divides the second cylinder into a first volume (15a) containing the drive stem of the second piston and a second volume (15b) between the two pistons. The first cylinder (12) has an air inlet (25) and an exhaust outlet (12). A common combustion space (20) is formed between the pistons when the pistons are substantially at their inner dead center positions, the combustion space comprising the second volume (15b). The engine also has transfer means (38) enabling gas flow between the first and second volumes (15a, 15b) towards the end of the compression stroke of the second piston (18).