What’s safer – a plastic fuel tank or a steel one?
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?”
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.
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.
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 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.
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.
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.
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.
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.