everytime i fill up my car, the pump always shuts off at just the right time to fill my tank. every tank is a different size, so how does the machine know?
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Your fuel pump doesn’t know when to shut off the refueling.
Inside your tank there is a Fuel Level Vent Valve (FLVV). This valve works to allow the air to escape the tank to the carbon canister and EVAP lines during fueling. Once the fuel reaches a certain point it lifts the flat inside the FLVV cutting air off to your canister external EVAP lines. This sudden drop in pressure causes the nozzle to click off letting the user know the tank is full.
This valve is placed in specific area to ensure the tank design meets the required fueling point for advertised volume. Depending on the make or model it could be in Liters or US Gallons.
Older vehicles have an externally welded valve and newer ones have them completely internal for reduced emissions. Newer vehicles most likely have the lines coming out the top of the fuel pump giving the impression that it is the pump doing the work.
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