19

I mean, should I refuel my vehicle in the morning?

The reasoning is simple, in the morning the temperature is low and the density of the fuel would be higher and I get more energy per litre(or whatever metric system you use). But in afternoon, or by evening, the temperature increases and the density decreases, thus reducing the energy per litre.

(I have tried Google, but couldn't find any satisfactory answers, or could only find conflicting answers.)

Improve this question
3
  • What sources did you find when you searched?
    – Freiheit
    Commented Oct 11, 2016 at 13:29
  • 20
    You should refuel your vehicle when your vehicle is low on fuel.
    – cory
    Commented Oct 11, 2016 at 15:37
  • 1
    Great question but a bit confused on the energy per liter as that less relates to refueling but varies with the cars tank temperature... The answer does relate to issues of volume at temperature when pumping... With that if your passing the station in AM and in PM then maybe a saving occurs. But if you drive just to fuel in the AM it a loss.
    – spicetraders
    Commented Oct 11, 2016 at 15:45

4 Answers 4

Reset to default
31

According to wikpedia, the volumetric constant of thermal expansion for gasoline is 

av=950*10^-6/K

For example, if the temperature changes by 20K (20°C; 36°F), the volume changes by a factor of 

950*10^-6/K * 20K = 0.0192

The warmer fuel has a volume increased by about 2%, and since energy content depends on the mass, the energy per volume decreases by about 2%.

But this doesn't mean you get more for your money in the morning. Most fuel stations have their tanks under ground, where the temperature is quite constant over the day.

Temperature in the tanks of the gas station can change significantly when they are refilled from a tank truck, but since you usually don't know when the truck comes, nor how much new fuel is dumped into which tank, you can't save money here.

At least in Europe, more and more gas pumps are temperature compensated, i.e. they don't display the actual volume, but the volume the fuel would have at 15°C (59°F). In this case, there is no way to get more for the money.

Improve this answer
5
  • 1
    Good answer. LPG (propane) tanks are always above ground because any leaked gas is heavier than air and will pool. So filling your BBQ bottle or LPG vehicle in the colder morning is a good idea.
    – Criggie
    Commented Oct 11, 2016 at 9:59
  • 2
    @Criggie Probably for that reason, volume compensation to 15°C is optional in Canada for gasoline and diesel (though most pumps seem to do it) but mandatory for propane sold for automotive fuel. See this
    – Spehro 'speff' Pefhany
    Commented Oct 11, 2016 at 11:12
  • Why do you think that there will be a change in temperature after the tank at the station is refilled?
    – RogUE
    Commented Oct 11, 2016 at 13:32
  • 1. When there are 1000l of fuel at 15°C and the truck adds 9000l and 30°C in the summer, there will be 10000l with 28.5°C in the tank. But within hours/days, the temperature will sink to the usual 15°C. This means, when the tank truck was there, you get warmer fuel in this case.
    – sweber
    Commented Oct 11, 2016 at 17:53
  • 2
    2. Yes, K is Kelvin. It's the common unit in engineering, though it differs from Celsius by an offset only.
    – sweber
    Commented Oct 11, 2016 at 17:55
7

Many countries (but notably not US) actually have regulations regarding temperature compensation of fuel volume. For example, UK has Standard temperature accounting for fuel dispensers, which allows the volumetric measurements to be corrected for the standard temperature of 15°C. Needless to say, it makes no difference when to tank in this case, as you essentially pay for the mass of the fuel, even if it is measured in litres.

Even in the US, it won't make any practical difference between mornings and afternoons (but it will make some difference for summer vs winter). From Wikipedia:

Modern tanks are non-metallic and sealed to stop leaks. Some have double walls or other structures that provide inadvertent thermal insulation while pursuing the main goal of keeping gasoline out of the soil around the tank. The net result is that while the air temperature can easily vary between 30 and 85 °F (−1 and 29 °C), the gasoline in the insulated tank changes temperature much more slowly

Improve this answer
2
  • I wonder if they account for barometric variations.
    – spicetraders
    Commented Oct 11, 2016 at 16:07
  • @spicetraders liquids are virtually non-compressible, so no.
    – Dmitry Grigoryev
    Commented Oct 11, 2016 at 16:11
4

As @sweber said, the tanks are usually underground, which keeps their temperature constant. However, if you are filling a smaller bottle with gasoline (ex: for a small engine), the first few gallons (litres) that come out of the nozzle will be a slightly different temperature.

This is because the pumps are above ground, and if they are sitting in the sun, they will be even more effected by morning-evening temperatures.

However, if you are filling totally up, professionals have determined that it makes next to no difference to the average consumer.

Improve this answer
1

It doesn't make ANY difference to the cost

If you read the small print on the pump, you'll find the cost per volume advertised is actually based on a particular temperature and density. If the density is different for any reason this is automatically accounted for in the price you pay.

So the actual price you pay for a particular amount of fuel will be the same regardless of the volume that fuel actually occupies.

Improve this answer

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .