Stirling engine fuels and efficiency

[richard.duffy]

I'm curious. Just what kinds of fuels can be utilitzed in a Stirling
engine? I heard that these units are not very efficient. What would
be the range of efficiency be?

[romanjd]

The Stirling engine is an external combustion engine, so any material that burns can theoretically be used (ie: gas, solid fuel).

I'm not sure about the efficiencies of currently produced engines, but in theory the Stirling cycle (which is ideally approximated by the Carnot cycle) comes near the upper limit for efficiency in heat engines