A heat engine, such as the Stirling engine, typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. They operate in a cyclic manner and are usually illustrated by a PV diagram.
This site provides a concept map that explains heat engines.
A heat pump can be considered to be a "heat engine in reverse", in which energy is applied to transfer heat into or out of an area as desired.
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User: kory_kusick 11:12, 5 October 2007 (PDT)