Cooling Fuel: Safety or Performance Issue?

Temperatures at the Dover race were unseasonably high. Kurt Busch’s Stewart-Haas 41 team was told by NASCAR officials to remove “heat shields” from their fuel cans. The cans (shown at right) have an 11-gallon capacity. Not shown in the pictures is a tube that connects the nozzle at the top with the vertical part coming straight up from the can. This attachment recovers overflow fuel – remember when we used to have a ‘catch can man’?

Apparently, Busch’s team was using some type of heat shield on the cans to keep them cool. All of the things I’ve read about NASCAR’s response seem to mention safety. This is an important consideration, especially given the incident we had at Richmond where three people were burned seriously enough by a fuel fire to have to go to the hospital.

What hasn’t been mentioned is whether this is actually a performance issue.

Density

As you probably know from middle school, “dense” means “thick”. But we’re going to use it in its precise scientific meaning.

 

Density has units like grams per liter or pounds per cubic foot.

Simplifying Assumptions

OK – let’s make some simplifications for the purposes of discussion.

1.  Gasoline is made up of a mix of molecules, so there’s really no such things as “a gasoline molecule”. In reality, gasoline contains a bunch of hydrocarbons with four to twelve carbons atoms per molecule.  For you specialists, it’s a mix of alkanes, cycloalkanes and alkenes. For the sake of discussion, I’m going to talking about “a gasoline molecule”.

2. Molecules are absurdly small. and talking about their mass becomes unwieldy.  Octane (one of the hydrocarbons in gasoline) has a molecular weight of 114.22852, which means that if you put Avogadro’s number of octane molecules (which would be 6.023×10²³ molecules) on a scale, the scale would read 114.22852 grams.

This means that a single octane molecule weighs 1.897 x 10^-22 g. That is  0.00000000000000000000001897 g.  You get the point: they’re very small. So we’re going to talk about density in terms of number of molecules more than their mass. The two are related, of course (mass = number of molecules x mass of one molecule), but I think it’s easier to visualize with number.

3. Finally, there ought to be a couple billion billion billion molecules in the drawings, but I just don’t have the patience to draw them. So we’re using simpler numbers like “10” and “20”.

Density of Gasoline and Temperature

The density of typical gasoline is 6.073 lb/gal at 60°F. Whenever you list a density, you must list the temperature at which the density if measured, because density changes with temperature. If you blow up a balloon, then put it in a freezer, the volume of the balloon shrinks -that’s because molecules slow down when it gets cold (like most of us do).

Most things in an automobile that deal with gasses or liquids work on volume. A fuel injector, for example, is set to let a particular volume of gasoline into the combustion chamber. So let’s think about what the change in density with temperature means in terms of a constant volume.

Most liquids become less dense at the temperature gets warmer. So if you get a gallon of gasoline at a higher temperature, the molecules are spaced out more, which means you get fewer molecules when it’s warm than you do when it’s cold.

How the density of gasoline changes with temperature is pretty well known and shown below. Let’s check the axes here to see the magnitude we’re talking about.  I’ve plotted a 126 degree change in temperature, over which the density changes by about 8 percent. If you’re looking at a ten degree change, say from 60°F to 70°F, you’re talking about a little more than a half a percent change in density.

Combustion works on the basis of a precise chemical equation. Each fuel molecule needs a particular number of oxygen molecules to combust. If there are too few oxygen molecules, then some of the gasoline molecules do not combust. If there are too many oxygen molecules, then some of the oxygen molecules just hang around. Either way, you’re limited by whichever component of the combustion process is smaller.

At high altitudes, or high moisture in the air, you get less power from the engine because there are fewer oxygen molecules in the air coming into the engine.

This is the idea behind turbochargers. Turbochargers compress the air going into the engine, so in a fixed volume of air, you get more oxygen molecules. More oxygen molecules means you can inject a larger volume of gasoline and make more power with each combustion.

The same idea can be used on the fuel side. There are systems on the market you can buy that use compressed gasses to cool the fuel – essentially they’re an air conditional, but for the gasoline. That lets you pack as many fuel molecules as possible into each charge that goes into the cylinder. You can let in more air, and – voila – more power. Of course, you reach a point of diminishing returns. The fuel has to be heated to combust and if the fuel is too cold, it won’t heat fast enough and some of those molecules won’t combust and won’t produce any power.

Is It a Performance Advantage?

Did having heat shields on the fuel cans help Kurt Busch? If we’re just talking mechanical heat shields – metal that reflects heat and keeps it from being absorbed by the can – I don’t see how they could’ve gotten more than a five to (maybe) ten degree decrease in temperature. That’s less than one percent change in density, which is pretty small. But also remember that over a 400-mile race, a typical NASCAR race car will use 100 gallons of gasoline, so you’re getting a 1% advantage over the entire course of the race. And races are determined on very small margins, so it’s not impossible that it’s a performance advantage – but it’s not a huge one.

Is This a Safety Issue?

No. The auto-ignition temperature of gasoline (the temperature at which gasoline will spontaneously ignite) is around 500°F. Cooling the gasoline on pit road will have pretty much zero effect on safety aspects.

What about my car? Do I get cheated when I fill up when it’s hot?

There’s an urban legend that you should always fill up your car in the morning instead of in the evening because you get fewer gas molecules for the price when you’re dispensing warmer gasoline. Maybe on those rare days when you have a 40°F temperature change, but on most days… it’s not going to make a heck of a lot of difference. Consumer Reports actually did the experiment.

But the winter/summer change and the sheer amount of gasoline we use does have an effect. The Today Show had a report a couple years ago (2012) on this very phenomenon. If gas pumps are calibrated in cool weather, then you’re actually getting less gasoline for the dollar when you fill the tank in hot weather.  They cite a 2007 Congressional report that says Americans paid an estimated $1.5 billion extra for gas that summer. That sounds like a really big number, but remember there are 300 million people in the U.S. and we use a lot of gas. If each person in the country gets $1 (edit  – I should never do math in my head…)  less gasoline in the summer, there’s 300 million dollars right there.

A group called for a mandate for gas stations to use equipment that measures the temperature and takes that into account when calculating gas prices. After all, they do it in Canada and have been doing it for a couple decades now.  The problem is that most cost-effectiveness studies show that if the government mandates temperature compensating pumps, the cost for installing and maintaining them gets passed along to the consumer. In the short term, no one would save any money.

Plus, there are a lot more important things to be worrying about in the world, don’t you think?