Due to the increasing gasoline prices I heard the other day in the news that truck drivers fill the tank at the morning when fuel is cold so as to save some money. I really dont understand what is the theory behind that. Notwithstanding, I made the following reasoning:
When the fuel is cold it is denser than normal. The pump measures litters (not kilos) of gasoline, then, for a fixed amount of litters paid after filling the tank, it will be more kilos of gasoline when it is cold (in the morning).
I think the reasoning is right, but, I think the saving will be ridicolous, as I suppose that the thermal expansion factor of gasline must be pretty small, as many liquid's are. That is different on what happen on gases, that have a much greater expansion factor than liquids.
What do you think?
Well, the coefficient of expansion for petrol is 0.00095 C^-1
Using the simple formula b=DV/(V.DT) you get
DV=b.V.DT
(where V is volume, DT is change in temp, b is expansion coefficient and DV is change in volume).
If we work that for a hypothetical case - say 50litres of petrol with a 20 degree change, we get:
DV=0.00095*50*20 = 0.95 litres
^So, for a normal car, it isn't that much savings, but considering that a normal semi might carry around (I'm guessing at the conversion from gallons) 2000 liters... With that much, the effect becomes significant.
Well, over here a litre is about £1.20, so saving a litre on each fill, whilst not a huge amount, is enough to buy a packet of crisps, a twix and a soft drink. Better than nothing 
So, a trucker could save around £48.00 per fill up!
Not bad at all.
| ocalhoun wrote: |
So, a trucker could save around £48.00 per fill up!
Not bad at all. |
Yep - it is certainly not chicken-feed.
(Although, given the current price of cereals and grains, it might well be )Bikerman: thank you for the estimation, I checked it with an expansion coeff table and is Ok. I had never imagined that the expansion was so big (1 litre in 50 l is much more than I imagined). I will take this into account next time I fill the tank.
I suppose pump designers should take this into account and measure kilos instead of litres. It amuse me why this haven't been made yet. Would it be technologically difficult to measure weight instead of volume? I know it is more difficult, but the savings of the petrol company would be huge if they implement that, justifing the investment needed to change all pumps.
Do you understand why this haven't been made yet?
My assumption is that currently the fuel/volume issue works out in the fuel stations favor or comes out about evenly as most people don't pay attention to when they buy fuel, and buy during the day when it is warmer. Who knows though as more people realize this and prices increase, changes may be made. Or maybe the fuel stations will automatically calculate the difference and make an increase in the price.
Either way I don't think 75% of people would hear about this or even understand it anyway so, I don't think it will matter.
Most gas stations (around here at least) use underground tanks, where the temperature is relatively constant.
Well, the temperature of the petrol will certainly change when it is pumped from the storage tank to the car petrol tank. I have no empirical data to suggest what that temperature would be (ie how close to the ambient temperature the petrol is when it settles in the tank) but clearly there will be a difference.
Hummer010 is right: the storage tanks are underground and so are at a relatively constant temperature. The fuel is transferred so rapidly from the storage tank to the fuel tank that I can't imagine that its temperature increases much, if at all.
| Voodoocat wrote: |
| Hummer010 is right: the storage tanks are underground and so are at a relatively constant temperature. The fuel is transferred so rapidly from the storage tank to the fuel tank that I can't imagine that its temperature increases much, if at all. |
Well I'm afraid I must disagree with you. I have recently watched the refurbishment of a local garage - they have installed new petrol and diesel storage tanks as part of the job. The tanks are about 1 metre below normal ground level and, at that depth, they are certainly subject to ambient temperature variation. The rate of transfer to the car is not so 'rapid' as you might think and there is plenty of time for the fuel to warm-up - particularly since the very act of pumping the fuel will inevitably increase the temperature. You also have to figure that the car's fuel tank is at ambient temperature and will, according to basic thermodynamics, warm the fuel as it enters (particularly since there is likely to be a reasonable volume of existing fuel at ambient temperature). | Bikerman wrote: |
| You also have to figure that the car's fuel tank is at ambient temperature and will, according to basic thermodynamics, warm the fuel as it enters |
This gives me a very interesting idea... I wonder how much effect it would have on money spent on gas if the exhaust pipe was located far away from the gas tank, as opposed to right next to it (as it is in my truck, but not all vehicles)... It must heat the gas tank up considerably, depending on how close it is and how much airflow there is across surfaces in between them.
That would be an interesting experiment to set up, but unfortunately it is well beyond my practical means... Perhaps a car manufacturer would be interested in doing it (or already have done it).
Edit:
Doh! No effect at all on money spent! By the time is is heated up in the tank, you've already paid for it.
The volume of gasoline pumped in the tank is mesured at th outset from the underground tank, wo it really doesn't matter the temperature of the tank neither the temperature outside, except for the heat transference to the gasoline 1 meter underground that mus be negligible. So I think now that it is not necessary to go to the pump on the morning and get a cold, because what is mesured is what came out inmediatly from the underground reservoir, not the volume in the car tank.
