It is not widely known that the operation of internal combustion engines requires little trolls to push the pistons.

It is important not to put contaminated fuel in your engine because this may cause the trolls to become angry and refuse to do any more work. If this happens nothing short of divine intervention is likely to get the engine running again.

Keiichi does not seem to like the engine trolls very much, or at least he seemed very distressed when he learned of their existence. (Ch 30: Mystical Engine.)

From MangaWiki, a Wikia wiki.