Wiktionary
simple engine
n. 1 An engine that has only a single stage, contrasting to a compound engine, compound locomotive, or triple expansion engine, which use multiple stages to improve efficiency. 2 A steam engine that does not use a separate condenser to create suction and thereby improve efficiency.
Usage examples of "simple engine".
Finally, I just drew up a simple engine lathe, even though I didn't see how we could possibly build one.
A ferrous-alloy barrel, basically, the aft section holding a reaction-mass tank and a simple engine.