Friday, July 06, 2007

"Hot fuel" and the 60 degree volume standard: Are we getting ripped off?

There's been a lot of talk in the news lately about thermal expansion of liquids. Usually that's not something that most people think about - and if they do, they're probably thinking about the way that water expands when it freezes (that is, when water goes from its liquid form to its solid form), which is pretty unusual (matter is usually more dense in its solid state than its liquid state) due to the peculiarities of its molecule, H20, and the way water molecules interact with each other.* But water in liquid form follows the same basic rules as other liquids: the hotter it is, the less dense it is, and the same number of liquid molecules occupy a greater volume; the cooler it is, the more dense it is, and the same number of liquid molecules occupy a smaller volume.

The liquid people are talking about these days is gasoline, and concerns about thermal expansion have to do with the fact that gas is priced per gallon at a temperature of 60 degrees Fahrenheit. At 60 degrees a gallon of gasoline contains a certain number of molecules of gasoline. At temperatures greater than 60 degrees, that same gallon contains fewer molecules, since the gasoline has expanded and the same number of molecules now fill a greater volume. Our internal combustion engines function by burning molecules of gasoline. If the gallon of gasoline you just put into your car contains fewer molecules, there are fewer molecules of gasoline to be burned in the combustion chamber.

So, don't you see? The gas companies are ripping us off! By pricing the gas at a cool temperature of 60 degrees, they get to sell us less gasoline in the Summer in each gallon but charge us an inflated price!

At least, that's how the argument goes.

I've heard folks on TV intone about this, and then grimly state that consumers can be seeing as much as X% less gasoline going into their cars in the Summer, resulting in a big loss of fuel economy.

Only that's not what my data shows.

1996 Toyota Tercel fuel economy
(miles per gallon)
10/1/2000 - 4/23/2006

For years I have kept track of some fairly simple data: What day I put gas in my car, how many miles I've driven since my last fill-up, how many gallons it takes to fill up my tank, where I'm getting the gas, and what the cost per gallon is. (I do this on a little notepad I keep in my car. Each pad lasts two or more years. Anybody can do this.) Lately my sampling rate has changed - instead of filling up three times every two weeks, it's not unusual for me to go two or more weeks without needing a refill. My driving habits have changed, too, since a 66-mile commute each day is no longer a given, and I'm consciously avoiding unnecessary trips. But none of that data is reflected in the chart above, which covers October 1, 2000 through April 23, 2006.

One critical piece of information allows you to immediately interpret the chart: the "year" label is centered on June 1 of each year, indicated by a blue line (visible by clicking on the chart for a bigger version.) The other lines on the chart, the ones between "year" labels, indicate December 1.

Now the chart may be interpreted by inspection alone: there is a sawtooth pattern - at first I thought it was sinusoidal, but it's much sharper than that - showing that the best fuel economy (as measured in miles per gallon**) occurs in the Summer, and the worst fuel economy occurs in the Winter. There are several proposed explanations for this, but the bottom line is that it is a real phenomenon. You get better gas mileage in the Summer than in the Winter. At its most extreme I'm seeing a 33% improvement - miles-per-gallon nudging into the low 40s in the Summer vs. the low 30s or below in the Winter.

Does the volume of gasoline expand above 60 degrees Fahrenheit? Yes. Does this mean that we're getting less fuel per measured gallon at higher temperatures? Yes. Does this have a noticeable effect on fuel economy? By my data, no.

And by rights, if we're going to complain about getting less dense gasoline in the Summer, we should also complain about getting more dense gasoline in the Winter. We should be paying more per gallon of gasoline in the Winter, when (according to my data) fuel economy plummets!

There are lots of things we can all do to improve fuel economy. Drive the speed limit - most car engines are designed for optimal performance at or around 55 miles per hour, so routinely driving faster or slower than that will affect your fuel economy. Make sure your tires are properly inflated. Remove excess weight from your vehicle. Cut out unnecessary trips, especially short hops. Dump your gas-guzzling "sheik's delight" and drive a car that gets 30 miles per gallon or better. Somehow, I don't think suing gas companies over the standard by which they measure volume will achieve anything much more than line the pockets of a few class-action lawyers.

UPDATE: This isn't really a "new" issue at all. Check out the dates on these links:
Land Line Magazine - March/April 2005
Hot fuel not a hot deal

The Blotter
Motorists: We're Being Cheated with "Hot Fuel"
December 15, 2006 3:15 PM

"Hot Fuel" Bilks Consumers, Lawsuit Charges
December 18, 2006

Petroleum Equipment Forum
September 6, 2006

Related stories:
Motorists steamed over 'hot fuel' losses sue oil titans, retailers - USA Today - Jul 4, 2007

"Hot Fuel" Costing Consumers Big Bucks? - CBS News, NY - Jul 4, 2007

Motorists Sue Over 'Hot' Fuel - Slashdot

A Wall Street Journal article that links to this post:
Why Your Car Has Lousy Gas Mileage

A KCET (a PBS station from Los Angeles) article that links to this post:

*The fact that water expands when it freezes explains why a glass bottle of milk left by the milkman in the little tin box on our porch one Winter some 35 years ago froze and cracked; why I had to go to the hospital for stitches one late Winter night back around 1997 after picking up a frozen jar of home-jarred grape juice while gathering the garbage from our back porch; and why there is life on Earth - if ice were more dense than water, it would have sunk to the bottoms of lakes, ponds, and other bodies of water and killed any overwintering life forms by smothering them.
**In some countries fuel economy is expressed in "liters needed to drive 100 kilometers". In these countries, the lower the number the better the fuel economy. But that's crazy talk.


Super G said...

You should send this data off to a statistics department somewhere. It should be in a book for statistics students to analyze and ponder.

Hang in there.

D.B. Echo said...

I'm thinking of printing out this opost and handing it out at interviews.

Unknown said...

This post doesn't take into account that many states require ethanol to be added to gasoline that is sold during the winter. While ethanol may improve smog a bit in the winter, it most certainly does not help with mileage. The differential in your stats could very well be due to the reduced energy output from ethanol blended gasolines.

D.B. Echo said...

Brian, I think this was a suggestion someone raised in one of the earlier posts I've done on this subject - or if it wasn't, it should have been. But the problem is that the data doesn't show the sort of step-pattern you would expect if the cause were a binary one - ethanol/no ethanol. The data instead looks more sinusoidal or even sawtoothed, which suggests a cause or causes that produce best fuel economy around June 1, worst fuel economy around December 1, with a continuously ascending or descending pattern seen in between. I'm not sure what could bring that about, but I have a feeling it's not a single cause.

Anonymous said...

I'm certain it's not a single cause. There are 2 others that I can think of off the top of my head and there may be more.

1) Air is also less dense in the summer. Modern engines have MAF sensors (mass airflow) and they adjust the amount of gas injected based on the amount of air to keep the optimum mixture. Less air means less gas needed to keep the best mixture.

2) Many people (myself included) leave the defogger vent (not the rear defroster, the defogger vent in front) active all winter long. That causes the A/C compressor to turn on to dry the air so it can defog, and that draws power from the engine. Those without A/C don't have that problem of course. My 1981 Datsun 200sx without A/C didn't have much of a swing winter/summer (31-33mpg) but my 2007 Pontiac Vibe with A/C does (24-29mpg). Now that I'm aware of it, I'll try to not run the defogger as much next winter and see what that does to mileage. My '92 Nissan 240sx (with A/C, but the hoses leaked and I never filled it so it didn't 'work') had a swing of 25-28mpg, but it only got 28 in the hottest part of the summer for a couple months, whereas my Vibe swung up to 28-29mpg as soon as it stopped freezing overnight.


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D.B. Echo said...

Twice in two days I've gotten comments on this post from people identifying themselves as being in the Search Engine Optimization (SEO) business. Well, yay for that, and yay for trying to improve your overall link score by pointing to your site from my oh-so-popular blog. But if all you're trying to do is use my site to hawk some of your snake oil - well, I'm just gonna delete your comment.

If this keeps up, I'll have to close comments on this post.

Peter said...

Got here through google after getting an email from Costco about a class action lawsuit some shark, sorry, lawyer is starting, alleging that Costco, "misled consumers by marketing motor fuel at temperatures above 60 degrees Fahrenheit without adjusting for the fuel's temperature." Given that the volumetric coefficient of gasoline is 950 X 10^-6, the change in volume between, say, 60 and 90 degrees F is about 1.58% (0.000950*(305.22K - 288.56K)), which translates to less than a nickel ($0.0475) per gallon at $3/gallon. Filling a 16 gallon tank would cost an extra $0.76. If your car gets 25 miles per gallon, you're paying an extra cent every 5 miles or so. Highway robbery!

However, there are a couple of things to consider before getting all steamed up about this:
- as has been mentioned, the measuring apparatus may also change with the temperature.
- the tanks that store the gasoline at the gas station are quite deep underground, which means that the actual temperature of the gasoline being dispensed is not necessarily the same as the ambient air temperature. In fact, it's probably much more stable than the actual air temperature - probably hovers around 60 degrees most of the year.

Also, you suggested driving 55 because, "most car engines are designed for optimal performance at or around 55 miles per hour." While I think that's an excellent suggestion, the reasoning is, I think, flawed. It's not the engine that's designed for 55, it's the body of the car. Air resistance - drag - increases by the square of the velocity, and the power required to overcome that resistance increases by the _cube_ of the velocity. In other words, if you increase your speed from 55 to 65 - 10 mph - the drag goes up by a factor of 100, and the power requirement on your engine goes up by a factor of 1000. All that translates to much better fuel efficiency @ 55 than 65.

Bottom line: drive like a granny and don't worry so much about the crooks at the pump rooking you out of a few cents per gallon.