retracted position which in the illustrated embodiment is the right hand cylinder 10 on the upstroke air is compressed and forced into air supply duct 16 suitable reed or poppet valves 13 are provided both at the intake and exhaust openings from cylinders 10 to permit the entrance of fresh air and the discharge of compressed air at the appropriate times in the cycle of movement of pistons 12 the compressed air is then passed through a valve 18 which has a sliding spool member 20 which permits the compressed air to be passed a through duct 22 into an accumulator tank 24 where compressed air may be stored b into a duct 26 which leads to an air preheater 28 and c permits a portion of the air to flow to accumulator 24 and the remainder to the air preheater in the preheater 28 the air is heated by indirect heat exchange with exhaust gases via heat exchange coil 30 heated air leaves the preheater through line 32 into the upper end of an external combustion chamber 34 in the illustrated embodiment a fuel pump 36 pumps a liquid or gaseous fuel through fuel inlet line 38 into the combustion chamber where the fuel is ignited by a conventional igniter 40 the resulting hot combustion gases which may be at a temperature in the order of 2 000 u00b0 f and are now.