If you have an engin room with a fresh air feed, you also need a way to block that fresh air from feeding the fire in the same way as lifting the engine cover will cause a flash over
If you don't have a way to block the fresh air feed, you may be a fire extinguisher that has a volume in excess of the engine room volume.
If the engine i running the engine would consume of the fire extinguisher gas so again reducing the effective volume
