For a car engine:
* Vroom: This is the classic, general sound of an engine starting or accelerating.
* Zoom: This emphasizes speed and movement.
* Brum: This is a quieter, more rumbling sound.
* Chug-chug: This is a more old-fashioned, chugging sound.
* Rattle: This suggests a noisy, older engine.
For other types of engines:
* Whirr: This could be used for a quieter, electric engine.
* Clank: This could represent the sound of a steam engine.
* Roar: This could describe the powerful sound of a large, heavy-duty engine.
Ultimately, the best onomatopoeia for an engine will depend on the specific context and what you want to convey.