As far as what extinguisher and type - this is a good clip from a firefighter/Jeeper.
"Basic fire chemistry says that you need four things in order to have a fire:
(The Fire Tetrahedron)
1. Fuel
2. Heat
3. Oxygen
4. An uncontrolled chemical chain-reaction.
In order to extinguish the fire, you have to remove one or more of the 4 elements.
1. Fuel. Self-explanatory. Remove the fuel, fire goes out.
2. Heat. I'll get back to that one. Please bear with me.
3. Oxygen. Halon extinguishers work by displacing the ambient oxygen from the fire, thereby extinguishing it.
Halon units only work effectively in confined areas, such as electrical and computer rooms. In the outdoors, they are virtually worthless. Also expensive. Recent environmental regulations (the same ones that outlawed R-12 refrigerant) make replacement Halon very expensive, since it can no longer be manufactured in the US.
4. An uncontrolled chemical chain-reaction. This is what a fire really is. A fire can be extinguished by chemically altering its state. Infusing a working fire with certain chemicals will inhibit this chain-reaction, and the fire will go out. This is how a dry chemical extinguisher works. A chemical agent in a dry powder form is sprayed into a working fire and this in turn inhibits the chemical chain reaction and suppresses the fire.
OK, so back to the heat question. Yes, removing the heat of a fire will put the fire out. BUT, this is much easier said than done with a running (or recently running) motor vehicle. If the heat source is an electrical issue, the heat will remain until the power is cut. Re ignition is a very real issue. If the problem is a fuel or oil leak onto a hot engine, headers, etc, then cooling the immediate area with a tiny CO2 extinguisher will work only for a short time. Eventually, the fuel will find another ignition source (such as the other header) and reignite. In fact, most of the engine surfaces on a Jeep will ignite fuel under normal operating conditions. Keep in mind that in industry, CO2 extinguishers are used to extinguish CHARGED electrical equipment until the power can be removed, nothing more. Everywhere else in the plant, you will find dry chemical units. True, the big draw toward CO2 extinguishers is the fact that they leave no messy residue behind, and they won't ruin electrical equipment or materials. The downside is their very limited effectiveness.
Even the big 20# CO2 extinguishers we carry on the Rescue Pumpers at my Fire Department would be hard pressed to extinguish a fuel fire on a motor vehicle and keep it extinguished. My weapon of choice is a charged 1-3/4" hoseline with Class-B foam concentrate flowing at about 125 gallons per minute in this case! That hoseline will cool just about anything on a motor vehicle to the point where it cannot reignite even raw gasoline. As a quick-knockdown effort, or if there are other extenuating circumstances, a 20# dry-chem extinguisher would be a perfectly acceptable alternative to that big hoseline full of water and foam.
But seeing how very few of us have that kind of suppression equipment at hand, I simply wanted to make the point that the MOST effective extinguishing system you can carry in most cases is a class BC dry chemical extinguisher, and the bigger the better. Most folks carry a 5# unit, which is OK, but when it comes to fire extinguishers, size matters. 10# or even 20# is better, although these units are generally considered too big to carry in a Jeep. In any case, the extinguisher needs to be carried where the driver can get to it in a hurry, such as next to his seat, or under the dash. Roll bars are a convenient mounting point, usually behind the driver's seat. Try to get one of these extinguishers out and put it into service from OUTSIDE the Jeep though.....
When I go out to teach extinguisher training at the nursing homes, we use a big galvanized washtub with about a gallon of Kerosene floating on about 5 gallons of water. We can put out about 4 fires like this with a 20# dry chem extinguisher. However, there is no heat source to provide re ignition in this scenario. Were this a Jeep with a fuel leak, we would be able to put this fire out 4 times, and then it would be left to burn after it reignited until it ran out of fuel. AND THIS IS WITH A 20-POUND EXTINGUISHER, WHICH IS 4 TIMES THE SIZE OF THE ONES MOST OF US CARRY IN OUR JEEPS! With a 5# unit, you have about one shot to hit that fire and knock it out.
The moral of the story here folks, is that the ONLY extinguisher that should be acceptable to use in a Jeep is a dry-chemical unit. Yes, they make a huge powdery mess when used, but they are the ONLY effective unit to use on a motor vehicle fire. CO2 units may put out a small vehicle fire, but they won't keep the fire from reigniting, unless they have some seriously huge tanks onboard.
Oh, and once you have that dry-chem extinguisher mounted in your jeep, take it out once a month a turn it upside-down and whack it with your palm or a rubber mallet to uncake the powder inside. By all rights, these units should be replaced or discharged/recharged every year, but few of us do this. every three years would be reasonable. It is usually cheaper to simply replace a 5# unit than to have it tested and refilled. Just discharge it somewhere safely and toss it in the recycling can with your used brewskies after removing the valve from the cylinder.
Carry a Dry-chemical extinguisher in your rig, and the bigger the better. Put it where you can get to it in a hurry, whether you are in the Jeep or standing next to it. Leave the CO2 extinguishers in the computer rooms, and the electrical vaults. And when all else fails, call 9-1-1. Trust me, we answer a TON of calls every day that are more stupid than a Jeep on fire.
This message brought to you by your friendly local firefighter.
P.S. In case you were wondering:
* Class-A = Ordinary combustible material. Wood, paper, textiles and the like.
* Class-B = Flammable liquids. Gasoline, Kerosene, Oils, etc.
* Class-C = Energized Electrical Equipment
Here is a link to a quick primer on fire extinguishers: http://www.hanford.gov/fire/safety/extingrs.htm#fetypes"
I may be adding a 5lb BC for the Jeep (to supplement the cheap 2lb ABC). Looks more effective for off road type fires (BC is geared more towards fuel and electrical fires. The "A" in ABC is geared towards fires that produce ash (hence the "A") paper, wood, etc...
