Valin -
We had a Halon fire extinguisher in each engine compartment of the Javelin (FJ-33 jet engine). The tanks were spherical and surprisingly small, larger than a grapefruit but smaller than a volleyball. I think they had 3 pounds of extinguishing agent (Halon) in each. It was understood that this was way more than we needed, and I recall that we were looking for about one pound per engine for the production version. These are not precise recollections but is is enough to tell me that your proposed system is more than adequate as far as total quantity of Halon is concerned.
The ball had one exhaust port which was triggered by electrically firing a pyrotechnic burst disk. There was a manifold attached to the exhaust port which directed the extinguishing agent to two places. One was aimed at the engine accessories (fuel pump, oil pump, etc) which were near the bottom center of the engine, and the other was aimed near the top front of the compartment. I do not recall a fire extinguisher nozzle which actually squirted the stuff into the engine air flow.
This was set up by someone way more knowledgable than myself.
The take home message for me, which I am passing on to the group, is that 3 pounds is way more than enough to put the fire out. Also, that you probably want to concentrate the Halon in the areas where fire is likely to be fed. I think that if you put one nozzle above the engine, as in between the cooling air inlets and the cooling fins, then you can put out anything that is burning between the engine and the top of the cowling. Of course, there is not much to burn there (there is one fuel line there on my engine). I would direct a second nozzle to the accessory area behind/under the engine. I don't know about the 540 engine but the 320/360 has a lot of tubes with flammable liquids between the engine and the firewall, from bottom to top of the engine compartment. This is stuff like fuel pumps, oil coolers, oil filters, etc. I think that area is the most likely area for a fire to break out.
I think it would be hard for Halon to get from the area above the engine to the area behind and above the engine, and vice versa. That's why two nozzles are desirable. We did not have a nozzle per se -- it was simply the open end of a tube.
The other guys are right. Once you put the fire out, you need to make sure it won't start up again. On the Javelin the fire extinguisher handle would shut off fuel lines and engine bleed air lines before firing the extinguisher. (We just used the other engine to come home...) We would pull the handle to shut off the fuel lines, then rotate it to fire the extinguisher if the fire did not go out just by turning off the fuel.
On our airplanes, all we can do is shut off the fuel heading into the engine compartment and hope for the best.
Along these lines, we have pretty good designs for keeping engine fumes frm entering the cockpit, except for the cabin heat. It would be bad to put out the fire and then asphixiate (sp?) by inhaling Halon which entered the cockpit from a cabin heat SCAT tube that just burned up in the engine compartment. I don't know about you, but if there is an engine fire I probably won't remember to turn off the cabin heat. No recommendation here -- just food for thought.
One final note -- the shop floor foreman had built many airplanes at the Lockheed Skunk Works. He looked at the pyrotechnic actuation system and bet me that there was no way we would finish the project without accidentally popping off one of the extinguishers. In fact, we did not (he still owes me a seven-adjective Starbucks drink) but it tells me that odds are, if you have a pyrotechnic system you are likely to accidentally actuate it. As a NASA rocket scientist you probably are more aware of how to keep this from happening than the average Lancair builder but I caution you to be careful with your electric actuation system. It shouldn't hurt you if it goes off as long as you can keep the Halon fumes from getting you, but it would be an exciting day, A cable actuation system would not be a bad idea here.
Best of luck. The engineering of your stuff is always top-notch and I look forward to seeing what you end up with.
- Rob Wolf