open and close such valves at the appropriate times for smooth operation of the engine these valves and timing devices form no part of the invention because they are well known and commercially available accordingly they are not shown in the attached drawings ports 25 and 26 are entrances and exits from intermediate cylinders 16 and 17 respectively these cylinders and the cooperating pistions 11 and 12 are preferably employed to precompress air to be used in the combustion of fuel in chambers 18 and 19 one port 25 or 26 is an entrance passageway for air and the other port 25 or 26 in an exit passageway for compressed air these ports all require suitable valves which are not shown because they too are conventional commercial devices ports 27 and 28 communicate with the two ends of central cylinder 15 one of each pair of ports 27 and 28 is an exit it is of course necessary to have an entrance and an exit from each portion of cylinder 15 the portions being separated by central piston 10 the function of central piston 10 central cylinder 15 and ports 27 and 28 is to pressurize and pump a suitable fluid e g a gas or a liquid since central piston 10 is double acting there will be a pumping stroke for each movement of central piston 10 toward one end or the other end of central cylinder 15 if a gas is pumped by piston.