>The chemical cylinder would need to be big enough to
>completely flood the engine compartment sufficiently so that even with
>the chemicals that would escape out the bottom under the engine, there
>would be enough going into the area to suffocate any flames.

The thing we were told on our extinguisher course was that you must
not only out the fire out, you must cool things down so that there is
no reignition. This is one thing where dry-powder extinguishers are
superior. We were told that for fires above floor/ground-level,
carbon dioxide flows down and this often allows the fire to reignite.