Fuel Saving Tips – Commentary II

There’s this email being circulated touting the best way to fill up your car. You know the one, giving advice such as:

ONE OF THE MOST IMPORTANT TIPS IS TO FILL UP WHEN YOUR TANK IS HALF FULL. The reason for this is, the more fuel you have in your tank, the less air occupying its empty space. Petrol evaporates faster than you can imagine. Petroleum storage tanks have an internal floating roof. This roof serves as zero clearance between the petrol and the atmosphere, so it minimizes the evaporation. Unlike service stations, here where I work, every truck that we load is temperature compensated, so that every litre is actually the exact amount.

What kind of tripe is this? “… occupying its empty space. Petrol evaporates…” if I did this kind of logical leap in an exam, I would be shot.

Consider this. All things being equal (temperature, driving style, etc), two half tanks would take you the same distance as one full tank. So, for the same distance driven, when you refill your tank, you would displace either:

  • 2 x 1/2 tank volume, or
  • 1 x 1 tank volume.

Aren’t these the same?

The article further confuses the issue by introducing internal floating roofs to eliminate vapour space and minimise creation of fuel vapours, therefore have more fuel to burn. Problem is that floating roofs have guides to maintain the roof position and levelness, and these don’t take kindly to the jostling a typical car ride will provide. A jammed roof would limit your tank capacity.

2 Responses to Fuel Saving Tips – Commentary II

  1. zzeed says:

    the fact that the system itself sprays the fuel (and become vapor like!) to a mixture of air afterwards… kind of invalidate the argument that you lose it when it turns to vapor bla bla bla.

  2. Wata says:

    Zzead: same thing, liquid that has been metered becomes vapour. The spraying increases the rate of vapourization, but it can’t be more than the liq vapour pressure.

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