Most engine fires happen when one of several things occur:
1. You're away from the Jeep and all the blow-by and valve cover leakage finally makes it's way down to the hot manifold and smolders. After you shut off the Jeep and walk away the smoldering continues untill there's enough fuel and O2 for the fire to errupt. The fuel can be gas, more oil, vacuum lines, wiring, almost anything that burns.
This type of fire has done most of it's damage by the time you've gotten back to the Jeep... and may even have burned itself out.
2. When you're running rich and backfire through the carb. This can be dangerous and very, very scary because you're in/or around the Jeep. The engines running and pouring fuel into the Carb. This is not a pretty sight. If this type of fire occurs, <font color=red>SHUT OFF THE ENGIE</font color=red> so more fuel ain't poured on the fire.
This fire can be VERY hot, and infact by the time you've jumped out an are at the hood latch you're probably feeling the heat. This ought to tell you something... as the other posters have said you can guess what's going to happen to your face, hands, shirt, hair, eyebrows, etc. if you lift the hood.
With this type of fire, you've got to act quickly... and lifting the hood is about the only choice. But the question is how far to lift it. IMHO just far enough to get the nozzel of the extinquisher into where it can do it's work... an then Lette 'er rip. You can clean up the mess later.... just be very, very vigilent.
3. Electrical fires occur when you've got some bear wiring that carry some hefty current hitting up against ground. The wire heats, the insulation melts and sometimes flames up. It's like watching the crew of the Enterprise, you'd swear the Jeep's wired with maganese wire.
With this type of fire the source of the igniton is usally removed by shutting off the ignition or a blown fuse or the wire becomes it's own fuseable link. Then you're left with the residual fire. Though it can be hot, it's usally more confined than a gas related fire.
4. In any event... don't become this:
Most engine fires are preverntable by keeping the engine clean of oil and leaking fluids, keeping the timing in specificaiton and the wiring harnesses in good conditon.