A free-piston internal combustion engine comprising: a first compression chamber (29a, 29b) a first compression piston (23, 23a, 23b) reciprocable in the first compression chamber (29a, 29b); a first power chamber (18a, 18b) adapted for internal combustion therein; a first power piston (11a, 11b) reciprocable in the first power chamber (18a, 18b); conduit means for conducting fluid from the first compression chamber (29a, 29b) to the first power chamber (18a, 18b); and valve means (13a, 13b, 16a, 16b, 14a, 14b, 17a, 17b, 26a, 26b, 27a, 27b) for controlling the flow of fluid into and out of the first compression chamber (29a, 29b) and the first power chamber (18a, 18b); wherein the first compression piston (23, 23a, 23b) and the first power piston (11a, 11b) are rigidly coupled for reciprocation in unison; characterised in that the first compression chamber (29a, 29b), first power chamber (18a, 18b) and valve means (13a, 13b, 16a, 16b, 14a, 14b, 17a, 17b, 26a, 26b, 27a, 27b) are configured such that during each reciprocation cycle of said first compression and power pistons (23, 23a, 23b, 11a, 11b): the first compression piston (23, 23a, 23b) performs one intake stroke and one compression stroke of a four-stroke engine cycle, and the first power piston (18a, 18b) performs one power stroke and one exhaust stroke of the four-stroke engine cycle, wherein fluid is discharged from said first power chamber by the first power piston, and wherein said valve means is configured such that compressed fluid discharged from said first compression chamber is delivered from said conduit means to said first power chamber between an exhaust stroke and a following power stroke of the first power piston. a first compression chamber (29a, 29b) a first compression piston (23, 23a, 23b) reciprocable in the first compression chamber (29a, 29b); a first power chamber (18a, 18b) adapted for internal combustion therein; a first power piston (11a, 11b) reciprocable in the first power chamber (18a, 18b); conduit means for conducting fluid from the first compression chamber (29a, 29b) to the first power chamber (18a, 18b); and valve means (13a, 13b, 16a, 16b, 14a, 14b, 17a, 17b, 26a, 26b, 27a, 27b) for controlling the flow of fluid into and out of the first compression chamber (29a, 29b) and the first power chamber (18a, 18b); wherein the first compression piston (23, 23a, 23b) and the first power piston (11a, 11b) are rigidly coupled for reciprocation in unison; characterised in that the first compression chamber (29a, 29b), first power chamber (18a, 18b) and valve means (13a, 13b, 16a, 16b, 14a, 14b, 17a, 17b, 26a, 26b, 27a, 27b) are configured such that during each reciprocation cycle of said first compression and power pistons (23, 23a, 23b, 11a, 11b): the first compression piston (23, 23a, 23b) performs one intake stroke and one compression stroke of a four-stroke engine cycle, and the first power piston (18a, 18b) performs one power stroke and one exhaust stroke of the four-stroke engine cycle, wherein fluid is discharged from said first power chamber by the first power piston, and wherein said valve means is configured such that compressed fluid discharged from said first compression chamber is delivered from said conduit means to said first power chamber between an exhaust stroke and a following power stroke of the first power piston. the first compression piston (23, 23a, 23b) performs one intake stroke and one compression stroke of a four-stroke engine cycle, and the first power piston (18a, 18b) performs one power stroke and one exhaust stroke of the four-stroke engine cycle, wherein fluid is discharged from said first power chamber by the first power piston, and wherein said valve means is configured such that compressed fluid discharged from said first compression chamber is delivered from said conduit means to said first power chamber between an exhaust stroke and a following power stroke of the first power piston.