Expander for a heat engine

A method of operating an expander for a heat engine, the expander being capable of converting a high pressure gaseous working fluid to useful work, the method including:

• reciprocating at least one piston in a cylinder between top-dead-centre (TDC) and bottom-dead-centre (BDC) with a long dwell time at TDC;
• opening an inlet valve to introduce working fluid from a high pressure working fluid supply means into an expansion chamber in the cylinder when the piston is at TDC;
• continuing movement of each piston from TDC to BDC under the influence of the working fluid in the expansion chamber, expansion of the working fluid in the expansion chamber thereby doing work on each piston;
• closing the inlet valve;
• and
• releasing expanded working fluid from the expansion chamber to a volume of low pressure working fluid via an exhaust valve;
• the work on the piston being delivered in a form of useful work output by power transfer means;
• wherein piston travel is small during transition of the inlet valve from open to closed and from closed to open.