Abstract: An expander for a heat engine, the expander being capable of converting a high pressure gaseous working fluid to useful work, the expander including: •high pressure working fluid supply means; •at least one reciprocating piston reciprocating in a cylinder between top-dead-centre (TDC) and bottom-dead-centre (BDC) with a long dwell time at TDC; •a working fluid inlet valve that opens and closes to introduce, while open, high pressure working fluid from the working fluid supply means into an expansion chamber in the cylinder at or near TDC; •power transfer means that transfers work done on a piston by the working fluid to a form of useful work output; and •an exhaust valve to release expanded working fluid from the expansion chamber to a volume of low pressure working fluid; wherein piston travel is small during transition of the inlet valve from open to closed and from closed to open.