pony engine
Noun
Meaning
pony engine (plural pony engines)
A small engine used for shunting wagons, etc.
Related terms
Examples
In the past, Caterpillar and John Deere used a small petrol pony engine in their tractors to start the primary diesel engine.
The pony engine heated the diesel to aid in ignition and used a small clutch and transmission to spin up the diesel engine.