Myth-Busting: Should you fill your tank with fuel in the morning?
Another one where your mate will tell you it’s better to fill your car with fuel in the morning when it’s cooler, because you’ll get more fuel into your tank…
THIS IS ONE OF THOSE myths that has an element of truth in it but one that’s been spread backwards. Let me explain.
‘They’ will often tell you to only fill your car with fuel in the morning when it’s cool as you’ll get more fuel for your buck. And there’s an element of truth in that, because when fuel, a fluid, is cold it is denser; when it heats up it’ll increase in volume but not density. But…
Fuel, you need to remember, is stored in giant tanks beneath a thick layer of concrete which acts like an insulator (as does the ground around it) and, so, keeps the fuel at a constant temperature. Meaning, even if it’s a hot afternoon when you fill up your car, the fuel coming from the bowser will be the same temperature as if it was a cold day. So, whether you fill up in the morning or afternoon the fuel coming from the bowser will be, more or less, the same temperature.
Sure, the bowser itself might heat up, but that’s why there are big shade-providing roofs over the top of them, not just to keep the rain off you when you’re pumping fuel. That said, yes, on a hot day, just like your garden hose heats up in the sun, the first couple of litres of fuel being pumped will be warmer than those deep in the tank underground, but it won’t take long for the cooler fuel to flow through the nozzle and equalise that initial flow of warmer (expanded) fuel.
And don’t go and try and argue that if it’s a hot day when the tanker arrives to fill the tanks at the petrol station then the fuel pumped into the ground will be hot, etc etc. Because the tankers are insulated and designed to maintain the fuel at a set temperature. And then, once it’s sat in the ground it’ll cool down too.
But, what can have an effect is pumping fuel into a hot car. Meaning, if your car is on, say, a quarter of a tank and has been parked in the sun all day then the petrol in the tank, despite its insulation will have heated up and expanded, creating vapour. This vapour is generally captured to try and reduce the pressure in the fuel tank. And this pressure in the tank escaping is usually what you hear when you turn the cap on your tank (the hissing sound). Ideally, you shouldn’t hear the hiss as the vapour from the expanding fuel should be captured by the evaporative fuel recovery system (EFRS) which is usually just a brick of charcoal and a closed line that allows this vapour to be directed back to and burnt off by the engine.
On a particularly hot day this expansion of the fuel in your tank might prevent you from filling up your tank to the same level as on a cold day. But while I haven’t studied this myself, I’d expect the ‘fuel’ loss would be minimal.
What you shouldn’t do, in this situation, is to keep ‘topping off’ your fuel tank after you hear the first click on the bowser. What that does is overwhelm the EFRS and eventually lead to it failing, which means fuel vapours are able to escape into the atmosphere. And that’s very bad. You’ll usually see an engine warning light or be able to smell fuel near your car as evidence that something’s not quite right, you should have it checked out immediately.
With the heat wave that’s hit much of Australia recently a friend asked me if fuel could explode in a petrol tank in the heat. The answer is, no. See, without a spark to ignite the fuel, petrol won’t self-combust until the temperature reaches 400-degrees C.