I have little doubt that your statistics are right, markr. Lithium-ion fires get reported for two reasons: (1.) Prior to the lithium-ion "boom," battery experts predicted that lithium-ion battery packs would have overheating problems if they were not engineered properly. Luckily, they have been engineered properly (with rare exceptions), which is why your point is on the mark. (2.) The devil you don't know is always more frightening than the devil you do know.
I realize it's unrealistic to design for every possible scenario and trying to prevent the fire before it happens is the goal but why not focus on minimizing and controlling the fire once it has started. Fire suppression has grown leaps and bounds over the past decade and deserves more consideratioin in automotive design. Stat-X is one of the companies that I have worked with first hand and has an incredible line of fire suppresion products that are very inexpensive and non-hazardous to the occupants of the vehicle. The US military currently have systems in place in the engine compartments of many of their vehicles and NASA's "crawler" also utilizes Stat-X fire suppression devices. I believe if a fire suppresion device similar to what I mentioned above were installed on the tesla, this incident would have been avoided all together. http://www.statx.com Here is the website in case you would like more information on the fire suppresion capabilities.
Charles, The formula is quite real, derived from our friend F=MA, which while a sort of abstract formula is easily turned into a hard xample. And the incident of the car with the dragging muffler happened on Woodward avenue in front of the ice cream shop where we had stopped. It was quite a startling thing to see. The dragging muffler made enough noise that we all tured to look, which was just an instant before the impact.
I hope that when a very detailed analysis and studt is made of the wrecked Tesla that they can see just how far the battery pack was impacted, and possibly reconstruct just exactly what happened. Not that the media would ever be able to understand, but perhaps the event could be reconstructed by some crew like "mythbusters", or shown in animation. Then people would possibly have a clue as to what a rare fluke the incident was .
This is a freak incident that caused the destruction of an expensive and rare car. But as the posts have brought up; what is really more dangerous gas or batteries? I would submit that it is not as intuitive as one may think.
To ignite gasoline, you must have the correct combination of heat, oxygen, and vaporized fuel. Gasoline only provides one part of the fire triangle. The combustion engine and the exhaust system provide a lot of heat, while the air provides the oxygen.
For a battery to ignite you need pretty much the same ingredients: heat, oxygen, and fuel. However, a battery is a system. When damaged, such as happened in this case, the battery is capable of generating intense heat. The battery case (and possibly electrolyte) becomes fuel and oxygen is readily available from the air.
My first car was a '74 Plymouth Scamp that had a 318 V-8 with a Holley 600 four barrel carburetor. The carb secondaries were cracked and would occasionally leak fuel onto the hot intake manifold. There were times I opened the hood and found fuel standing on the engine after hard running. Moral of the story is that gasoline is not as volatile as it seems.
Gasoline has an ignition temperature of 500°F, so aside from the exhaust manifold of a car that gets very hot or an open flame it is not that dangerous. To ignite gasoline with a spark requires the right concentration of fuel vapor to be present, which is not always reliable.
It should be quite obvious that the armor plate is not present in the picture that shows tha battery and drive assembly. And the curved piece of metal sounds like a piece of a leaf spring, which is some mighty strong material. And if the car was going 55 MPH, and the forward end of that chunk of metal caught a joint in the roadway, then it would be the equivalent of hitting a cold chisle with a hammer the weight of the vehicle, which would punch quite a hole in almost anything. I have seen what happens when a car with the front end of the muffler draging hit a joint in the road at about 30 MPH. It jammed the rear axel back a foot, ripping the spring mount out of the frame, and threw the car a few feet into the air.
I believe that the force of an impact can be described by the expression: Mass times Dv/Dt, and when there is a catch against a joint in the roadway that Dv/Dt approaches infinity. So the 75 Tons is probably a conservative estimate of the penetrating force.
It probably both ripped open the battery and caused multiple short circuits.
And the one gasoline spill fire that I was involved with had all of the skilled gas burning within 3 seconds. So I would say that this car was MUCH safer.
That said, I don't assume the bottom of the chassis is a simple flat plate, like the picture suggests. Flat plates in metal have very poor stiffness, and I seriously doubt the battery itself is part of the structural system. That plate either has soms sort of support beans attached, or more likely, is corrugated in a way to improve stiffness. The depth of the corrugations provides some protection, as well as the strength of the corrugations. If the battery were mounted so that a space of say, an inch existed between the chassis pan and the battery, you would mitigate a lot of deformation issues, especially from foreign objects on the road. Add the kevlar or carbon somewhere in there, and you have a strong, light weight protection system.
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