In a previous job I was involved in testing fire extinguishers which were intended for use in cars, I have let off hundreds, and without doubt halon is best.
The problem you can get with powder is that the fire can continue in the less accessible parts of the engine bay, whereas with halon as long as you aim it in the rough direction it worked ever time.
Accessibility is a real issue because you need to get to the base of the fire and if that is covered by an engine cowl you will have to find some way of getting the powder in so it can to work. On my engine cover there is a port hole so you see if there is a fire and beside that a capped hole which allows you to squirt the fire extinguisher in. The only problem I see with this design is that the cap is made of steel so if there is a fire it will be red hot
and not much fun to open (must remember oven gloves next time I go out)