Want to save some money? It does matter what time of day you should be purchasing your gasoline from a petrol station.
WHEN?
Optimum Time…
Cool of the day such as early morning; and never during or right after the gasoline tank truck has loaded the petrol station’s underground storage tanks. Once you purchase the gasoline it is stored in your car’s tank and as the temperature changes, the liquid gasoline volume will expand or contract in that tank, but the mass or weight of the gasoline you purchased, remains the same and stays in your tank).
WHY?
First a discussion about gasoline’s density…
- Density is a measure of how much weight or mass is in a certain volume, such as pounds of gasoline in a gallon of gasoline… to think how ‘heavy’ or ‘dense’ something is – a gallon of gasoline is much more ‘heavier’ (‘denser’) – a lot more of it – than say a gallon of sunshine – measured in mass per unit volume, more commonly so many pounds in a gallon;
- which can change (by shrinking or expanding);
- by approximately 0.125% of its volume for every 1 degree Centigrade change in temperature (equivalent to about 0.058% of its volume for every 1 degree Fahrenheit change). For example, the volume of 100 gallons of gasoline will expand 0.125 gallons when its temperature is increased by 1 degree Centigrade temperature. (To put in further perspective, often times you can run hot water over a tight metal screwed on bottle cap and the cap will easily come off — because the hot water causes the metal cap to expand and become less tight – while the ‘size’ of the cap has expanded, its weight or mass has remained the same).
The energy (how much heat can be generated) contained in a denser (shrunken) or heavier gallon of ‘cold’ gasoline purchased by a motorist in Nome, Alaska in January could, theoretically, be as much as 8-10% greater than the same but hotter (less dense, not as heavy, expanded) gallon of gasoline purchased by a motorist in Phoenix, Arizona in a hot July.
Gasoline is purchased at the pump by the gallon, a volume measurement, and not by the pound, a weight or mass measurement. When buying gasoline, the temperature at which the gasoline is purchased can influence its density (pounds per gallon where a cold gallon of gasoline, which is denser, has more energy content – more weight or mass – than a hot gallon of gasoline). If hot lighter, less dense, gasoline is purchased, you are actually purchasing less gasoline by the pound (you are buying less energy) than you would if you purchased the same colder heavier, more dense, gallon of gasoline. Because at the hotter temperature the gasoline’s density is lower – lighter – there are fewer pounds per gallon (or less energy per gallon) actually being purchased. Since your car engine is interested in burning pounds (energy content such as BTU per pound) of gasoline and not gallons, it is important to optimize your purchase of pounds and not gallons. More bang for your buck if you please!
Tanker gasoline transport trucks can have their contents heated up by the sun as they travel, and when unloaded into petrol station storage tank, could result in hotter, less dense gasoline being discharged into the petrol station storage tank. The heat of the day can cause the petrol station petrol pumping unit to heat up and could also cause heat to be transferred to the underground gasoline storage tanks, which can cause gasoline density to expand and be less dense.
[For completeness, in colder climates, gasoline composition is adjusted by adding more expensive volatile compounds to maintain the octane rating of the gasoline, so if you calculate the value difference between hot and cold gallon of gasoline, the gasoline composition and higher valued components needs to be taken into account].