Peter wrote:
Jeff wrote:
These are a different beast - because they are "external combustion"
(like a house furnace) they are very efficient and can be set to run
unattended, so they claim.

If 'external combustion' engines were more efficient than internal
ones, we'd be using them more widely. This is probably some variant on
a Stirling cycle engine.

Yes, it is a Stirling. They are enjoying a bit a of renaissance as
new technologies have brought the price down. They are quite
efficient, but have limitations that prevent their use in some
applications, like cars. There are a number of work boats that use
them for propulsion.