How petrol pumps work

Peek inside the fuel-pumping petrol dispenser

Next time you’re lost in boredom at the petrol pump, think about this. Several metres below you are three massive storage tanks. Two of them contain regular unleaded petrol – one with the highest octane grade of fuel, the other with the lowest – and the third holds diesel.

When you select your fuel grade and pull the trigger, suction pumps inside the petrol dispenser draw up fuel from both the high- and low-octane tanks and blend them to the precise octane level. A spinning ‘fuel-ometer’ inside the pump records how much petrol flows past, keeping track of your purchase. Since petrol expands and contracts with hot and cold weather, a temperature probe compensates for fluctuating volume, ensuring you get what you pay for.

In the old days, caustic petrol vapours seeped out of the tank during a fill-up. Modern petrol stations are equipped with vacuum pumps that siphon out the offending fumes and store them safely below ground in the fuel tanks.

If you were to look inside the fuel nozzle, you’d see that the dispenser line is held open by air pressure from within the tank. When the fuel level reaches the tip of the nozzle, the air pressure is choked off and the dispenser switches off automatically.