Plastic versus steel fuel tank: Which is safer?

Coming up: What’s safer - a plastic fuel tank or a steel one? Plus - the idiot’s guide to destroying a nuclear submarine. (That sounds like fun.)

Free audio.jpg

Here's a question from Craig.

 
"Recent crashes on Australian roads over Christmas ended in explosions and fireballs. Are plastic fuel tanks a likely contributor to these events? A fire at Sydney Olympic Park also led to the destruction of vehicles, which exploded. Are plastic fuel tanks a likely culprit?" - Craig
 

We can probably all agree that it’s unacceptable to die in a fire if you survive the crash. This nightmarish scenario is quite the challenge for engineers, because cars are essentially a pyromaniac’s wet dream.

Petrol versus diesel >>

Carfire1.jpg

LIGHT ME UP

Cars are so inherently flammable that it amazes me they all don’t spontaneously conflagrate every time you drive one. Petrol - gasoline in ‘Merica - is especially willing. The pyromaniacal girl who can’t say ‘no’.

The flashpoint of petrol is minus 43 degrees C (minus 45 F). So if it’s warmer than that, petrol evaporates and forms a highly flammable mixture. All it needs is a spark - and you are (literally) toast.

Not to fearmonger, but petrol is the most dangerous chemical most people ever go near. Society is generally completely unaware how potentially dangerous it is.

There’s so much energy - about 35 million joules in every litre. So 50 litres spilled all over the road can rapidly convert to 1.75 billion joules of heat energy. You could lift a Toyota Landcruiser about 65 kilometres into the air with that. If you could find a big enough crane.

And there’s electricity everywhere in a car - there’s even an electric fuel pump submerged in the fuel tank… What could possibly go wrong?

The only protection in the pyromaniac’s paradise we call the automobile is good engineering design.

PLASTIC VS STEEL

Plastic tank.jpg

Plastic fuel tanks are made from high density polyethylene, or HDPE. Steel is cheaper, but there’s more design freedom with plastic. Plastic tanks are about 30 per cent lighter, and they generally don’t have seams, which makes them less likely to rupture in a crash. That’s a big plus.

Plastics also don’t generate sparks, and they’re a bit more flexible than steel - in the sense of springing back into shape after impact.

When a steel tank absorbs impact energy, it crumples and often bursts at the seams. The volume decreases and fuel is ejected under pressure - which is exactly what you don’t want. Because it evaporates rapidly and becomes the girl who can’t say no.

Plastic fuel tanks are very tough. (It’s hardly as if a plastic tank is a glorified Coke bottle.) It’s even a better insulator if there’s a fire beneath the vehicle. So the fuel inside the tank takes longer to heat up … but this is a ‘good news/bad news’ story.

The good news is: in an under-car fire the fuel in a plastic tank stays cooler longer. It’s is less likely to become rapidly pressurised and vent flammable vapour. The bad news: The fire will ultimately melt the tank, and then you’ll be sitting on a two-gigajoule bunsen burner, which is pretty far from ideal.

BURNING VS EXPLODING (AND HOW TO BLOW UP A SUB)

Mercedes-Benz-C-63-AMG-fire-at-GG2H.jpg

Craig mentioned fuel exploding in a fire, but liquid petrol does not really explode. It burns. If you want to be a complete pedant, the best petrol and vapour can do is to deflagrate rather than actually detonate.

Hand grenades and blocks of C4 detonate. If you see a fireball, like in Mad Max or Transformers, it’s a deflagration. They’re different things.

Blowing stuff up is always pleasant. So let us digress and blow up a full-on nuclear submarine. That could be. The modern military torpedo is essentially a hydro-dynamic kerosene-powered rocket that punches a hole in an aircraft carrier with a high-explosive warhead. Because people just cannot get along.

There’s energy in the kerosene, for propulsion; and power in the warhead, for destructive mayhem. Oddly enough, there’s more energy in the fuel than the warhead.

The kerosene is burned using hydrogen peroxide for an oxidiser because there’s no free oxygen for burning kero under water. You should make a mental note that kerosene has about the same destructive potential as petrol, per kilo.

An ex-Soviet 65-class torpedo is nine metres long, and five tonnes in weight. So you’d probably never get it on board in your carry-on luggage. Which would be a bad idea anyway because they’ve got 500 kilos of kerosene and a thousand kilos of peroxide on board.

Kursk Foto.jpg

And those two liquids are all it takes to destroy a nuclear submarine. You can buy kerosene anywhere, and you’ve probably got hydrogen peroxide in the medicine cabinet or the laundry. Kerosene costs about $3.00 per litre and peroxide is about $4.00. You won’t need the warhead. We know this because that’s exactly what happened on the 12th of August 2000.

A single 65-class torpedo on the Russian Kursk submarine malfunctioned. (The Soviets were profoundly shit at maintenance at the end.) One torpedo was in the tube; the outer doors were closed. The kerosene and the peroxide mixed together, and deflagrated violently in an enclosed space. Nothing known to science can stop that reaction once it starts.

The torpedo’s explosive warhead did not detonate. 500 kilos of hydrocarbons (that’s about 10 automotive fuel tanks worth) sank a 16,000 tonne submarine. It dropped to the floor of the Barrents Sea, killing all but 23 of the 118 sailors on board very quickly.

Next time you’re in a traffic jam, perhaps in a tunnel, take a look around at how many cars you see. Every 10 to 12 cars is one of those torpedoes worth of destructive potential. There’s a pleasant thought...

Upon hearing this news, Russian President, and Donald Trump organ grinder, Vladimir Putin, chose to remain at his luxurious presidential resort on the Black Sea for the five days that the 23 surviving remaining crew members were trapped on the floor of the ocean.

Big Vlad refused all international rescue support offers. Ultimately a flash fire in a potassium superoxide air scrubber consumed the remaining oxygen and the last 23 crew members aboard the Kursk perished.

A pretty shitty way to check out, if you ask me. Which just goes to show that politicians everywhere are insufferable arseholes who cannot be trusted with anything important.

HIGH EXPLOSIVES VS VAPOUR

When a fuel tank goes up, it’s visually compelling, but it’s not a detonation. Detonations are violent explosions that deliver supersonic, high-pressure shock waves. Petrol generally cannot do that.

A big petrol deflagration can easily kill you with radiant heat, or suffocate you, or maybe knock you off your feet. But a high explosive detonation in the wrong place will turn you into a red mist. Which is what an Apache helicopter does, routinely. There’s generally not much left for the autopsy.

When a fuel tank is breached in a car crash, liquid fuel runs onto the road. It starts to evaporate. If there’s a source of ignition, like a hot turbo or catalytic converter, the vapour deflagrates and the liquid burns aggressively. There’s usually lots of black smoke - which is the signature of an oxygen-limited fire.

maxresdefault (1).jpg

CONCLUSION

The thing for engineers to concentrate on is: doing everything they can to avoid breaching the tank in a crash. Because it’s never a good day when you let the girl who can’t say no out of the bottle. For this reason alone, I give plastic the win for fuel tanks - yes!

However, please don’t run out and poke your head under the car, and decide you’re in a death trap if it’s a steel tank. Detail design really matters - and both steel and plastic tanks can be designed to be quite safe.

I blame engineers for this. It’s a miracle that every car on earth is not on fire, right now. You’ve got the girl who can’t say no, plus 100 different ways to generate a spark, in bed with extreme temperatures and ridiculously high airflow. And you’re sitting on top of it. Awesome!

Arthur C Clarke summed it up nicely. If you think we live in a world without magic - I put it to you that your car is unlikely to catch fire today - even if it’s a Ford. And if that’s not magic, I don’t know what is.

MORE REPORTS