[DampRobot]

Originally Posted by Nate Laverdure

This definition leaves out steam engines, stirling engines, and perhaps others.

Maybe: an engine takes advantage of differences in pressure, specific volume, and/or temperature to do useful work on a system.

A steam or sterling engine would burn some material which would then heat water. On the other hand, I do like the idea of a pressure change being used to do work as part if the definition. That's probbably a more widely acceptable definition.