[J.Jensen]

The AFFF (if you missed an F) film/foam liquid extinguishing media is HIGHLY corrosive - don't let anyone tell you anything else.

The positive side is - foam can be quickly washed away, unlike powder media which gets in everywhere and eats up your whole wiring...

For gas based systems you can choose between CO2 and Ingergen. Halon is off the market unless you know someone "on the inside" in the aircraft industry that will give you an old bottle.
The gas required to extinguish isn´t that high if the gas can be contained, but in an engine bay a lot will bleed out. Which means you need an obnoxious amount of gas. That rules out CO2 - because of the low pressure in the bottle (1000psi vs Inergens 4500). But we´re talking a lot of space and a lot of weight.

In an aircraft - the nozzles are spread out in the entire engine compartment and the pipe diameter is quite large to let as much gas into the compartment in as short time as possible.

The problem with gas systems is they may very well put out the fire - but it does so by removing the oxygen. Its still hot enough to burn, so it will relight when the O2 levels increase again. An AFFF foam system will cool as well as take away the O2 - plus it doesn't bleed out and become ineffective. Once the foam is in the engine bay - nothing will catch fire again.
I haven´t seen anyone make a system for installation with nozzles fixed under the hood though. There are plenty of hand held AFFF systems out there - but if the thing catches fire a dry powder ABC handheld, preferably 55A classified, will be a HELL of a lot more effective than the AFFF option.

JJ (Aircraft Maintenance tech and workplace firefighter).