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It is apparently a form of Stirling engine: <a href="https://en.wikipedia.org/wiki/Malone_engine" rel="nofollow">https://en.wikipedia.org/wiki/Malone_engine</a>
Thanks. So the liquid part is the working fluid. I had been wondering if the pistons were liquid; some engines like that have been built, but not very efficient ones as far as I know.
I find it a bit funny. Robert Stirling invented the Stirling engine because steam engines were dangerous (at that time) and could explode.<p>Malone(and others apparently) took Stirling engines and filled it with compressed water as a working fluid with some decent efficiency!<p>The advantage, based on what I could gather from limited info available, was that these liquid engines could be run at lower temp differential making them great for low grade heat recovery.
Wanting to read more about this, I came across this useful page: <a href="http://www.douglas-self.com/MUSEUM/POWER/maloneliquid/maloneliquid.htm" rel="nofollow">http://www.douglas-self.com/MUSEUM/POWER/maloneliquid/malone...</a>