18 on the downstroke of piston 14 then compressed and thereby heated on the upstroke of the piston and injected into the inlet end of combustion chamber 13 in the combustion chamber the hot compressed air mixes with fuel introduced into the chamber through a fuel inlet 19 to form a mixture which burns throughout the chamber and produces a volumetric increase in the gas the hot expanding gas is delivered to the expansion chamber where it drives expander piston 16 in a downward direction spent gases are expelled through an exhaust port 21 during the upstroke of the expander piston communication through the inlet and outlet ports and between the chambers is controlled by valves 22 25 the sizing of the compression and expansion chambers the movement of the pistons in them and the timing of the valves can be made such that the temperature or pressure within the combustion chamber remains substantially constant throughout the operating cycle of the engine as illustrated in fig2 the valve assembly 24 that controls gas flow between the combustion chamber and the expander includes a valve body 31 having an inlet port 32 and an outlet port 33 on opposite sides of a chamber 34 the ports.