Tag Archives: petrol

You Should Buy Your Petrol in the Morning


Petrol expands as it gets warmer, and it expands a lot: 950 parts per million per kelvin rise in temperature. This may not seem like a lot, but you might be surprised.

Energy is released when petrol is burnt because the energy required to break the bonds between the carbon and hydrogen atoms in the petrol is less than the energy released when new bonds are formed between the carbon and hydrogen and the oxygen atoms from the air. See, for example, the combustion of octane (one of the main components of petrol) shown below.

2\,\textrm{C}_{8}\textrm{H}_{18} + 25\,\textrm{O}_{2} \rightarrow 16\,\textrm{CO}_{2} + 18\,\textrm{H}_{2}\textrm{O} *

This means that the amount of energy released by burning petrol depends on the number of atoms in the petrol, and therefore on the petrol’s mass, but you pay for petrol by the litre. You use mass but pay for volume.

Imagine buying petrol on a day when there is a fifteen degree Celsius difference in temperature between morning and afternoon; the petrol will expand by 14250 parts per million. One litre of petrol bought in the morning will expand to 1.014 litres in the afternoon. This means that the same amount of energy will cost you 1.43% more if you buy it in the afternoon rather than in the morning.


This is all a bit of a moot point, as petrol is stored in large underground tanks, and the temperature underground is a lot more stable than the temperature above ground. Perhaps you’d be better stockpiling petrol in the winter when ground temperatures are lower.

* Breaking eighteen carbon-hydrogen bonds, seven carbon-carbon bonds and twenty-five oxygen-oxygen double bonds requires (18×413)+(7×348)+(25×498) = 22320 kJ. Forming thirty-two carbon-oxygen double bonds and thirty-six hydrogen-oxygen bonds releases (32×360)+(36×459) = 28044 kJ.

Things From Movies That Cannot Exist Number 3: The Nervous Petrol Tank

If you were an alien who had learnt everything you know about Earth from watching TV and movies you would probably think that the sole purpose of a car’s petrol tank is to explode.

It doesn’t take much to cause a movie petrol tank to explode: a serious crash, a well-placed gunshot or even a lightning strike1 will do it; and driving the car off a cliff makes it an absolute certainty.

Reality is far more boring. Petrol tanks are specifically designed with safety in mind: most are produced from tough plastic (HDPE to be exact) and have built-in features that ensure that the mixture of liquid fuel, fuel vapour and air never reaches the flammability limit (petrol must be between 1.4% and 7.6% (by volume) in air in order for it to catch fire). Petrol does not usually detonate as explosives do, but rather it deflagrates, burning subsonically (below the speed of sound) and therefore not creating a shock wave.

In certain circumstances petrol can explode, especially if trapped in a sealed container (petrol tanks have relief valves to prevent this) which can create a BLEVE (pronounced “blevvy”) which stands for Boiling Liquid Expanding Vapour Explosion2 in which a sudden change in pressure inside the tank causes the liquid fuel to turn into vapour, causing the container to explode and releasing a cloud of vapour that can then ignite, causing a (usually larger) secondary explosion.

1 This is despite the fact that inside a car is one of the safest places to be in a lightning storm.

2 Also known to some as “Big Loud Explosion, Very Exciting!”.