RE: Engine Fires...
It depends on the model.
If it is an internal engine and you have a start fire and it hasn't gone out by the time you pick up the extinguisher I would give it a squirt. You shouldn't do any damage to the engine with a CO2 extinguisher.
If externally mounted I would give it a few seconds to see if it clears. If in any doubt then give it a squirt. It is generally a judgement based on experience but it won't take you long to figure out what is right and what is wrong.
p.s. if the engine doesn't enter cooldown then it won't catch fire. Don't use the extinguisher to cool it down in this case; just let it cool naturally. The only model I've seen crash and catch fire couldn't be put out with 4 full extinguishers[X(]