engine vs heat engine

engine and heat engine both are nouns.

Word	Noun	Adjective	Verb	Adverb
engine	Yes	No	No	No
heat engine	Yes	No	No	No

As nouns, heat engine is a hyponym of engine; that is, heat engine is a word with a more specific, narrower meaning than engine:

engine (noun)	heat engine (noun)
motor that converts thermal energy to mechanical work	any engine that makes use of heat to do work
a wheeled vehicle consisting of a self-propelled engine that is used to draw trains along railway tracks
an instrument or machine that is used in warfare, such as a battering ram, catapult, artillery piece, etc.
something used to achieve a purpose