Improving the efficiency of Stirling cycle heat machines

A method of operating a heat machine with an external heat source and an external heat sink, the machine having:

• a first pair of displacers provided on a common first mount and working in opposed first bores formed in first cylinders;
• a first casing enclosing a volume between the first pair of displacers;
• a second pair of displacers provided on a common second mount and working in opposed second bores formed in second cylinders;
• a second casing enclosing a volume between the second pair of displacers;
• a mechanism interconnecting the first and second mounts and arranged to maintain a phase angle between the first and second pair of displacers;
• and
• working fluid chambers defined by spaces in the cylinders on the sides of the displacers remote from the mounts;
• in which method heat from the external heat source is supplied to the working fluid in the working fluid chambers adjacent the first pair of displacers, heat from the working fluid in the working fluid chambers adjacent the second pair of displacers is dumped to the external heat sink, the pressures in said casings and in said working fluid chambers are monitored by pressure monitoring means including pressure tappings and pressure transducers and compared by a comparison means, and the pressure of fluid in one or both of the casings and chambers is adjusted dependent upon the result of the comparison so that the pressure in the casings is maintained below the pressure in the working fluid chambers.