The Switch to Winter Gasoline and a Primer on Gasoline Blends
Posted by Robert Rapier on October 10, 2009 - 11:16am
Every year in late summer, you will start hearing references in the media about the conversion to winter gasoline, such as the following (originally in the Bradenton Herald, but the link is long dead):
Motorists can thank a mild hurricane season in the Atlantic for the lower gas prices, according to the American Automobile Association.
Other factors include the end of the summer driving season and a cheaper winter fuel mix.
Gas stations sell a special, more expensive fuel blend during the summer to cut down on smog during hot months. Stations nationwide will start selling a less-expensive winter fuel blend Friday, which could lead to even lower prices, analysts said.
So what does this mean, and why does it make winter gasoline less expensive?
A Primer on Gasoline Blending
Gasoline is composed of many different hydrocarbons. Crude oil enters a refinery, and is processed through various units before being blended into gasoline. A refinery may have a fluid catalytic cracker (FCC), an alkylate unit, and a reformer, each of which produces gasoline blending components. Alkylate gasoline, for example, is valuable because it has a very high octane, and can be used to produce high-octane (and higher value) blends. Light straight run gasoline is the least processed stream. It is cheap to produce, but it has a low octane. The person specifying the gasoline blends has to mix all of the components together to meet the product specifications.
There are two very important (although not the only) specifications that need to be met for each gasoline blend. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure, or RVP. While the octane of a particular grade is constant throughout the year, the RVP spec changes as cooler weather sets in.
The RVP is the vapor pressure of the gasoline blend when the temperature is 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than local atmospheric pressure, that liquid boils. For example, when you heat a pot of water, the vapor pressure increases until it reaches atmospheric pressure. At that point, the water begins to boil.
In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is boiled off ends up in the atmosphere, and contributes to air pollution. Therefore, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others.
A typical summer gasoline blend might consist of 40% FCC gas, 25% straight run gas, 15% alkylate, 18% reformate, and 2% butane. The RVP of the gasoline blend depends on how much of each component is in the blend, and what the RVP is of each component. Butane is a relatively inexpensive ingredient in gasoline, but it has the highest vapor pressure at around 52 psi.
In a gasoline blend, each component contributes a fraction to the overall RVP. In the case of butane, if there is 10% butane in the blend, it will contribute around 5.2 psi (10% of 52 psi) to the overall blend. (In reality, it is slightly more complicated than this, because some components interact with each other which can affect the expected RVP). This means that in the summer, the butane fraction must be very low in the gasoline, or the overall RVP of the blend will be too high. That is the primary difference between winter and summer gasoline blends.
Why Prices Fall in the Fall
Winter gasoline blends are phased in as the weather gets cooler. September 15th is the date of the first increase in RVP, and in some areas the allowed RVP eventually increases to 15 psi. This has two implications for gasoline prices every fall. First, as noted, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce. But the increased ability to add butane also means that gasoline supplies effectively increase as the RVP requirement increases. Not only that, but this all takes place after summer driving season, when demand typically falls off. On the other hand, refiners usually draw down inventories of summer gasoline leading up to September 15th to make room for the changeover, and this can lead to vulnerabilities should hurricanes come into play (as they did this year).
These factors normally combine each year to reduce gasoline prices in the fall (even in non-election years). The RVP is stepped back down to summer levels starting in the spring, and this usually causes prices to increase. But lest you think of buying cheap winter gasoline and storing it until spring or summer, remember that it will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix.
And that's why gasoline prices generally fall back in the fall, and spring forward in the spring.
This is a reposting of an article written in 2006 explaining the transition to winter fuel that takes place each fall.
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I my younger years I became very familiar with RVP but not because of anything you mention in the article.
I used to be a huge gear head and built and restored among others 2 1968 Corvettes. When I lived in California I had no trouble but when I moved to Texas in the early 1990s I had unsolvable problems in the summer with vapor lock and percolation due to the high ambient temperatures but even more due to the extremely high under hood temps the engine would generate. Of course this was a non-pressurized Holley Carburetor.
Today with fuel injection being the norm I suspect there are zero problems and I also expect that the RVP is lower and only given attention to minimize evaporation.
I got rid of my last muscle car years ago and have turned my attention to efficiency rather than fun......better late than never.
There are still problems. This spring we had a very warm day early on. My sister took my mother and another sister both of whom are good size and wheel chair bound to town shopping in her 2001 Ford Focus Wagon with air conditioning. After shopping and on her way home the car stalled and could hardly move.
Fortunately she was able to limp home. The next day she told me of the problem and I took the car out on the road. It ran fine. I told her of reading RR's article and that this can happen. It appears that she had filled up at a station that still had winter gas.
The car has a small four cylinder engine and automatic all crammed into a small space under the hood. It appears the weight of 3 people, wheel chairs and shopping stuff was too much. The winter gas vaporized and stalled the car on this very warm early spring day. It was around 80 degrees.
She was relieved and has not had the problem since.
Interesting diagnosis.
I would have just said, "it's a Ford."
:)
Porge: Small world-I had a 1976 Vette with dual 4 barrels and side pipes. Loved that car.
I have a problem with a fuel injected vehicle on summer gas. After running for about 30 minutes it idles at about 2000 rpm and if I shut it off it doesn't start back up and must cool for about 10 minutes. When the gas switches in the fall the problem goes away. It has 478K miles on it so that may have something to do with it.
i am going to store some gas. a procedure i have read is 55 gal. in a sealed tight metal drum- 2in. air space- PRIG stabilizer. Needs far away from buildings/ people, etc. Shaded too.
aviation fuel was recommended. winter/summer in that even?
i want longest term storage.
I think you will find that there is no good way to store gasoline for long terms of time. Storage over a years time is probably the longest that I would recommend.
If you want to go to long term fuel storage, you should switch over what ever engine you will be using it in over to propane and then you can buy 250, 500 or 1,000 gallon tanks and store the propane for very long periods with no serious degradation of the fuel.
What about diesel ?
Can diesel fuel be stored for 5+ years ?
yep. if gets black it has a 'biogrowth'; but still my neighbor uses it in his tractor. probably plugs his filter sooner on the tractor; & he has a filter on his tank.
is under consideration. thanks.
First, 55 gallon drums are not UL approved to store gasoline anywhere. Second, if gasoline is stored for more than a few weeks it cannot be an ethanol blend. The octane rating of ethanol is 115. Lower octane gasoline is on the horizon for blending with ethanol to bring the octane rating of the blend back up to 87, 89, or 93. The ethanol will separate over a few weeks and you risk filling with pure ethanol or lower octane gasoline.
You could run into some real problems with your insurance if you have a fire associated with stored gasoline at your home or business, for sure.
But most farmers and small tradesmen that need it store some anyway.
I haven't heard any one complain about problems with stored gasoline because it seperates-perhaps the minor mixing associated with removing some from the drum keeps it from seperating.
Nearly everybody around here who farms and quite a few homeowners keep some gasoline on hand-but it usually gets used up over a period of a few months max.
Not that I actually know anything about this, but about 50 years ago when I was taking upper division Chemistry courses, they taught us that chemical reactions proceed about ten times faster for each ten degrees centigrade (18 degrees F) of temperature increase. Assuming that the deterioration of gasoline follows that rule, what you should probably be looking at is storing the gasoline (safely) someplace cool.
If you happen to live near Anchorage or Winnipeg and have an outbuilding with a root cellar, you might -- and I emphasize might -- be able to stash gasoline for quite some time. OTOH, if you are storing the stuff in a tin shed in the Southwest in Summer, it may well deteriorate a hundred or thousand times faster ... maybe.
Perhaps someone who actually know something about the subject will offer a comment.
good to know so temp. dependent. i wonder if the sealing helps as pressure would increase w/ temp.; [a safety consideration too]? thanks.
Actually it's a 2X increase in the speed of most chemical reactions for every 10C increase in temperature.
A lot of farmers and country folk keep a 55 gallon drum of gas on hand for convenience and as insurance aginst getting caught without in the event of a winterstorm or hurricane or something closing local roads.
If you have a really good cool shady spot gasoline will keep ok for a year in a capped steel drum, at least around here, where it rarely exceeds ninety in good deep shade.Almost any modern cars(in the nineties and up ) left sitting for a couple of years will fire right up on the gas in the tank with no problems at all.Cars left sitting for even longer will start and run fine on the gasoline in the tank most of the time -I have started one that has been sitting for five years by using a whiff of starting fluid and once started up it ran fine for the trip to a service station for a fillup with fresh gas.
But all cars less than about twenty or so years old have tightly sealed fuel systems that keep air and moisture out.The fuel system also maintains a slight positive pressure that helps retard evaporation.
We keep a drum of gasoline in deep shade in the edge of the woods over fifty yards from anything valuable-other than the trees that shade it.Better safe than sorry.
Gasoline vapors are heavy and lay on the ground and spread just like water-traveling along the lowest available path and puddling up in spots-a spark might set off a fire a considerable distance from a leaking container.
There are some products on the market that can be added to gasoline to increase it's storage life.I can say from experience that these products do work to some extent if you use them in lawn mowers, tillers,generators and other machines that are left unused for extended periods.
Personally I run my small engines dry of fuel before storing them for extended periods.
Robert,
Does what is known as LPG constitute the blended ingredients used in reformulated gasoline?
Secondly, I would like to know what LPG is used for besides winter fuel blends. If I understand things properly I think the fuel blend component can be seen pretty clearly in the seasonal cycle part of this plot from the JODI Databrowser:
But what kinds of uses account for the base demand of LPG? And what could account for that demand falling off in recent years? I mean what specifically? I know the economy is down.
More questions than answers.
-- Jon
"Natural Gas Liquids and LRGs" are 5% of US exports, 3.2% of LPG production is exported too, exports are 71% of imports, that should register on your graph, perhaps JODI data differs greatly from that from the EIA, or the scale is too coarse to show this.
LPG seasonality greatly increased in the early 90s - in the 80s it was pretty staid. Must be all those char grills. Production actually started into an uptick the last few years, post-Katrina.
Rereading the JODI data manual I see that their definition of LPG is
The JODI databrowser is currently only showing net exports/imports. Perhaps, for a more technical audience, I should show both imports and exports rather than combining them for a net value. And the graph has other problems with the production numbers consistently different from the consumption numbers. Makes me worry seriously about the JODI dataset, or at least my presentation of it.
Your graph with the longer history is fascinating with the seasonal variability becoming the dominant feature only after 1988 or so.
Has the difference between summer and winter blends been happening all along or did it not start until the late '80's.
Wat are there other main uses of propane and butane besides gasoline blending, backyard grills and cigarette lighters?
Thanks for any answers.
-- Jon
Here is perhaps the answer to the disparity between consumption and production in the JODI data:
It looks like my consumption line includes LPG from gas plants whereas my production curve only includes LPG from refineries.
That complicates the presentation of things.
The major use for LPG here in Australia as well as other countries such as New Zealand and South Korea is for cars. Around 10% of the Australian car fleet runs on LPG. I recently had my Mitsubishi Lancer converted to a vapour injection LPG system. Power output is slightly less (3%), fuel consumption increased 15% over using straight petrol, but the fuel usually sells for less than half the price of petrol. Australia is self sufficient is LPG while we import 40% of our daily oil use. Getting my car off imported oil and saving money (within 12 months when the conversion pays for itself) were my primary motivations for using LPG.
I should mention I'm using EIA data: U.S. Refinery and Blender Net Production. They define Liquefied Refinery Gases (LRG) as:
There are some serious discrepancies between various sources of data, I've been trawling over EIA and BP figures on consumption, sometimes the percentage diffs for a given year are a bit different.
Here in Santa Cruz County in California we use LPG, (Propane), for hot water, and home heating. It used to be much cheaper than electric heat, but is now comparable. If you are not in a city or suburbs it does not pay to have long buried gas lines so they are not installed, hence no natural gas service. Propane is used as backup heat for some industrial plants. Butane is not used here because its low vapor pressure. On cold days the pressure is too low to hold a flame in a heater.