The Transition to Winter Gasoline--Revisited

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?

NB: This is a reposting of an article written by Robert two years ago on how winter gasoline differs from summer gasoline, and why this tends to make winter gasoline less expensive than summer gasoline. We also now have a lot of reports of gasoline outages due to short supply following Hurricanes Gustav and Ike. Feel free to discuss those in this thread or scroll down to the refinery/pipeline/gas shortages thread.

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.

ED: This post originally ran September 15, 2006.

Gas Shortage Hits Western NC

The question is how long it will take suppliers to catch up -- a tall order considering that many stations have emptied their tanks or fallen to lower-than-normal levels during the past two weeks.
"I think we'll see three to four weeks of improving conditions," said Pete Sodini, president and CEO of The Pantry, the Sanford chain that owns more than 1,600 convenience stores throughout the Southeast. "In four weeks, we'll be back to normal."

Until then, stations will continue to be pinched, especially for premium gas.

(Emphasis mine - It looks like it will be several weeks before things get back to normal, with premium and mid-grad worse than regular.)

Is there any word on this spreading, or should it remain restricted primarily to the currently affected areas?

I would expect it to spread as it gets fixed in the Atlanta area. The amount of gasoline going in doesn't seems to be adequate. It is the old "whack-a-mole" problem. If there is not enough to go around, and you get the problem sort of fixed for one participant, someone else will be left without.

Just a note - talked to the local fuel distributor in this part of Germany a couple of years ago, and he had never heard of different summer/winter blends of gasoline. This is someone who picks up his fuel directly from the refinery, and not merely someone who has it delivered.

Of course, the extremes of temperatures are much less in Germany than a 'mild' state like Virginia, but I suspect that this is pretty much a distinctly North American practice, especially the stockpiling aspect. Possibly, German refineries simply adjust their output over time, without worrying excessively about the issue.

Even the northern states in the US have some hot days, so the summer blend becomes more of an issue.

Adding ethanol to the gasoline increases the problem of summer vaporization (wonderful!), so this may be another reason we have to be especially watchful. If we weren't putting ethanol in our gasoline, summer/winter blend would be less of an issue. I don't think Germany uses much ethanol to stretch its gasoline.

I believe the coming German ethanol mandate in gasoline is 10% from next March, and seems to be currently set at 5%.

However, there seems to be some 'flexibility' in this - the reasons for ethanol were related to climate change, and as further research shows the destructive habitat and social costs of bio-fuels, the EU has been reconsidering its general approach.

Mainly because ethanol is imported into the EU, and not manufactured within its borders, with fat subsidies for taking fossil fuels and converting them to ethanol. However, at least bio-diesel is working pretty well - Raps(rapeseed) grows pretty well in this area, and a good number of trucks are set up to use bio-diesel, and have been for several years at this point.

i am fairly sure there are differences between summer and winter fuel in Germany (and the UK) since I remember reading reports some yars ago about the addition of ethanol and tests of RVP. I guess that since Germany is a much smaller land area than the USA there is one national vlaue that is used rather tahn regional variations.

another guess is that there are EU regulatuions and standards.

Here in Yurop, Reid Vapour Pressure is expressed in kPa

For any of you who are preparing prudently (not hoarding) both the RVP and octane rating will fall over time.

You would think so, from a technical perspective at least, but this concept was completely unknown to the fuel distributor, including the idea of making sure to buy the correct blend of gasoline to store. Germany is not really a place noted for wide ranging curiosity, so it is possible that his not knowing about it merely reflects the fact that only people that really need to be concerned are those at the refinery.

I don't think you have the whole picture.

Generally gasoline in cars needs a RVP between 5 and 15; 15 for easy vaporization (smoother running, acceleration)and 5 for slower vaporization(worse). OTH, if the RVP is too high you get vapor lock as the fuel vaporizes on warm days and the liquid doesn't get to the engine.

Refinery gasoline has a RVP of ~5, so to raise the blending 'average' RVP to 8 or 10 you have to add a small amount butane (RVP of 52!) or ethanol(RVP of 18) or MTBE(RVP of 8). In winter the tendency to evaporate is less because the ambient temperature is much lower.

You also HAVE to raise the (anti-knock)octane rating of gasoline with oxygenates (ethanol, MTBE(ether)) which replaced tetraethyl lead(poison). The octane number should be around 90. MTBE(ether) has an octane number(average) of 120, ethanol is 110, while refinery gasoline might have an octane rating of 85.

So producing a fuel that does two jobs which is always a balancing act.

To reduce ground level ozone(poison) producing smog from evaporating hydrocarbons, the EPA mandates that the RVP in summer be around 8(it varies around the country). Ethanol with an RVP of 18 does a better job keeping the RVP low than butane with an RVP of 52.

MTBE(ether) was found to be poisoning groundwater which is why it has been banned so you're left with basically ethanol to simultaneously lower RVP and raise octane.

If you have to blame somebody blame the EPA for requiring an average summer RVP of ~8. Then let the RVP rise with butane and be prepared for more ozone days.

Bottom line, ethanol is GOOD.

Ethanol with an RVP of 18 does a better job keeping the RVP low than butane with an RVP of 52.

But because ethanol is slightly polar, and gasoline isn't, the effect of adding ethanol has a greater impact on increasing the RVP. In other words, 10% ethanol will increase the RVP by more than 10% of ethanol's RVP. This is why the EPA had to grant RVP waivers for ethanol blends:

You can also read here of Dianne Feinstein's efforts to get a waiver from having to blend ethanol:

The California Air Resource Board (CARB) researched this issue at length and found that ethanol-blended gasoline does not help California meet the goals of the Clean Air Act as it relates to reducing ozone formation, particularly during the summertime, and, in fact, ethanol actually increases the emission of pollutants that cause ozone during the summer months.

Bottom line, ethanol is GOOD.

It's hardly that simplistic. It depends on how you make it and how you use it.

This is great topic! (note:...I just crashed, so if this shows up twice, forgive me)

I have a couple of questions...
1) Why is the RVP of ethanol 18? When I look up the vapor pressure ( wiki_ethanol ) it looks like at 100 F the vapor pressure is just over 100 mmHg, or 2-2.5 psi. Am I missing something, or is this a result of the significant deviation that leads to ethanol/benzene azeotropes? Everything I've looked up supports ethanol raising the RVP...except the numbers.

2) All this vapor pressure stuff is dependant on mole fractions, so a very small amount of butane by mass or volume can have a huge effect. Thus a 10% butane mix (mass) will be much greater than 10% by mole fraction, thus much more than 5.2 there really much butane in any but the coldest gasolines?

3) More of a thermo question. How does decreasing energy content relate to mileage? For example, let's say a hydrocarbon fuel has 100 j/mL, and ethanol has 60 j/mL (made up numbers for easy math). If the unadulterated fuel gives you 20 mpg, what would a 50/50 fuel give you? The energy content would be about 80 J/mL, and a simple calculation would suggest that that translates to 16 mpg (20% reduction).
-my question is whether it is this simple. I suggest that it should be worse, as work that goes into friction and heat will largely stay constant, meaning that the fraction of energy that goes into moving the car takes most of the loss. If half the fuel's energy moves the car then the original fuel used 50 j/mL for friction and heat. If we switched to the mix, 50 J taken from the 80 leaves only 30 for proplusion, for a 40% loss (50 to 30), or a drop in mileage to 12 mpg. Maybe there are advantages (less wasted heat?) that mitigate this?


btw...great to hear you live at ASPO, Mr. Rapier

Thanks for that first link....If I had waited a few minutes before my previous posting I mighta saved a little time!

Why is the RVP of ethanol 18?

It isn't, but my guess is that what he was reporting above was something like an 'effective RVP' when blending. Based on RVP only, ethanol should lower the RVP when added to gasoline. However it raises RVP as noted.

is there really much butane in any but the coldest gasolines?

It is at about 2% in the winter, and can get up to 12% or so in the winter depending on what else is in the blend.

Maybe there are advantages (less wasted heat?) that mitigate this?

Compression ratio can be made higher if octane is higher. Ethanol increases octane. In theory, you could build a car that gets much better gas mileage than you would expect from a straight BTU calculation by raising compression ratio. Think about diesel. It has only about 10-15% more BTUs than gasoline, but diesel engines get 30% or so better fuel efficiency. Why? Higher compression ratios.

btw...great to hear you live at ASPO, Mr. Rapier

After hearing me, Debbie Cook said that she would always read my essays now envisioning a southern accent. But what I didn't tell her is that I actually write with a British accent. :-)

The RVP of denatured ethanol is 3psi , but the RVP of ethanol above a gasohol mix is 18( vapor pressure varies per Henry's Law). The reference below says 17 but I've seen 18 psi in other places. So for purposes of what I am talking about the RVP of ethanol to be added to gasoline mix is 18psi(17psi) and I standby it.

The RVP of gasoline product prior to blending can very greatly depending on where it comes from in the refinery--allkylate or reformate gasoline has a different RVP than FCC cracked gasoline and the amount of these 'gasolines' depends on the kind of oil supplied to the refinery. Reformate or FCC gasoline typically has RVPs of less than 5, so blending in butane, ether or ethanol is necessary.
The reason you can't reduce the RVP below 5 psi is that you'd have starting problems from low rates of fuel evaporation.

I don't understand your comment about polarity. MTBE ether is also polar. Do you recommend putting lead into gasoline?
At any rate Feinstein wanted to waive the whole oxygenate requirement. Oxygenates improve performance and reduce the production of carbon monoxide, isn't this important?

As far as the 10% allowance for ethanol goes, the EIA example assumes 9psi RVP gasoline prior to blending. I suppose a refinery could make one pre-blend for ethanol and a different RVP for straight gas to be used if the EPA requirements were too tight.

I don't understand your comment about polarity. MTBE ether is also polar.

Minor compared to ethanol. Ethanol is much less like a hydrocarbon, thus it doesn't mix as well.

Do you recommend putting lead into gasoline?

You throw out a lot of straw men? I have blended millions of barrels of gasoline. I have blended very high octane gasoline. I never used ethanol, MTBE, or lead.

Good stuff, thanks.

BAck to my mpg question. Assuming my '99 Tacoma is not re-tuned when I put gasoline with ethanol in it, what happens to my mileage? I assume the car is smart enough to adjust the air mix and the timing, but then what? 10% ethanol drops the energy content by 3% relative to the straight hydrocarbon fuel. In practice does anyone know (via careful tests) what that does to mileage? This seems an important part of the arguments for or against using ethanol as motor fuel. I recognize that adding ehtanol likely means removing other high octane components from the original fuel so exact comparisons are of limited value, but...?

Is there some rough equation that relates energy content to mileage?

(mileage) = EC * a where EC is energy content and "a" is a proportionality constant
(mileage) = (EC - b)*a where b is energy lost to friction/heat


In practice does anyone know (via careful tests) what that does to mileage?

There have been a number of independent tests done, and they show the expected decline in mpg. Most of these are on E85 though:

I have around here somewhere a University of Wisconsin-Milwaukee study from the 90's that looked at 5% ethanol blends, and concluded that they dropped fuel efficiency by around 2%. The American Coalition of Ethanol commissioned a study recently and came out with the conclusion that fuel efficiency generally drops, but by less than the BTUs would warrant. The ethanol guys like that study, but I don't suppose they would like a study funded by the American Petroleum institute that concluded just the opposite.

Some insights from a Christian Science Monitor article:

Pre-Ike Gas Shortage May Take Weeks to End

But Mr. Baen, the UNT industry expert, says tough stances on price gouging may have exacerbated the shortage as jobbers focused on getting gas to areas that were easier – and cheaper – to reach. Without financial incentive to get to outlying areas, those pumps simply remained dry.

That's what seems to have happened in the mountain city of Asheville, N.C. With many independent dealers and jobbers facing long distances over steep grades, distributors with limited supplies chose their routes based on what was expedient – and profitable.

As a result, the city of Asheville cancelled several events. Landscapers, florists, and plumbers all declined work for lack of gas.

The article also notes that the Atlanta situation is the result of having a special boutique blend requirement. This requirement was lifted on Wednesday, so the Atlanta situation may start getting better in the next few days.

I live in the mountian city of Asheville, I would like to expand on landscapers, florists, and plublers declining work for lack of gas. I teach engineering at the local community college(~7,00 students), which is shut down indefinately until our students, staff and faculty have sufficient fuel to make the commute. Ditto local government offices and the hosiptals are making arrangements for staff to stay either in the hospital or within walking distance.

Do I believe in Peak Oil, you bettcha. In fact, Asheville may be the first city in the US to actually feel the impact; one expects a reprieve but the fact is we right now have no fuel. Our situation would make an excellent case study of what to expect (fantasy, not my problem, and 2 weeks of "fuel will be here tomorrow") from local/state/federal government officials when this situation begins to impact other areas and could be expanded to a primer of what should be done to mitigate the impact. Interestingly, the local authorities did not even realize that they had the authority to declare a state of emergency and invoke odd/even tag 10 gal maximum, which would have helped significantly as much fuel is in containers. Evidently, the emergency planners have not yet tumbled to physical Peak Oil at all, to say nothing of the social dynamics

If anyone has a doctoral candidate who needs a dissertation topic, please suggest this.

Irre, please keep notes on what happens and also ask your students, it will be very useful to know. Being engineers I would ask them to work out solutions assuming that the cavalry don't come riding up the hill with their ten thousand gallon hats full of gas. It might help focus their minds. Most problably have internet so ask them to read up on PO.

Not a doctoral candidate, but perhaps Orlov might like to use this for an early chapter in his next book Collapse: the USSA example :-)

One thing you will want to look at is whether diesel is available. I know here in Atlanta, that is the case. This makes the situation at least a little better than the worst case.

You are in an extraordinary position to help a lot of people with future shortages.

1)You have a technical background.
2)Access to a large sample of local population (your students)
3)Peak oil aware.

Your chance to be a trail blazer is unique. Please consider collecting information and to start posting it (blog?).

Questions it would be helpful to have answers to would be: the reaction of the local population, government, economic impact, food and medicine availability, essential services, peoples evolving attitudes.....

Most of your students probably have email access at home and would like to share their experience.

Yes ! Bitteroldcoot hit the nail on the head.

A blog would be ideal.

Is there any other way of working it elsewhere into the curriculum at the college ?

I've sent your post to a few of my colleagues (I, too, work at a Community College). And while a shockingly low percentage have any comment, those that do ask why we don't see this in our news. ( The news does havee the time and resources to have 5 helicopters video a bear that wandered into someone's backyard, ,however.)

I just looked up your local guvm'nt pages: Drought and fuel shortages!

Thanks for the post.

I only paid attention to gasoline prices in this election season and 2006 election season. I wrongly attributed the falling gas prices to a conspiracy of the oil companies trying to help the Republicans win with lower prices. I do think there is manipulation of prices but are of the kind that would be hard to prove in court.

As I was filling up this morning in Dallas, a guy in a delivery van drove up to the spot next to me and said that he had to go to five gas stations before he found one that still had fuel.

All Things Considered, September 25, 2008 · Gas stations across the Southeast have been running out of fuel, a lingering effect of hurricanes Gustav and Ike, which struck Texas and the Gulf Coast. Refineries had to shut down temporarily, halting the supply of gas into some areas.

The shortage has been felt strongly in places like Nashville, Tenn.; Tallahassee, Fla.; and North and South Carolina. In Atlanta, many stations are still out of gas, and others have long lines.

On Thursday, there was a strange sight at one of the few stations in the northern Atlanta suburbs that had gas: Dozens of cars waited in a line that snaked out of the station and right onto a main thoroughfare.

Strange sight? What's so strange about having a line at "one of the few...that had gas"? And if the station is on a thoroughfare, where else would a person expect the line to spill out to? I guess these young whippersnappers neither read or view any history, nor do they talk to anyone not almost exactly their own age... they just "report"... completely failing to connect up even words in the very same sentence...

Arrgh... about those utterly worthless failed schools with the jobsworth "teachers" who hand out free A's for the sake of "self-esteem"... oh, never mind, it's useless...

Does Deisel also have these seasonal variations or is this just an attribute of petrol (gas)?

Robert can answer this better than I can. I think the issue with diesel is preventing it from "gelling up" when it gets too cold. There is a different blend when temperatures are low for this reason.

Gail is correct. There isn't a summer/winter variety for diesel, but in really cold weather the blend will be toward more jet fuel which gels at very low temperature.

Gail, is the real issue on the rate of evaporation? Or is it the amount we have initially to be evaporated in the first place? If we don’t have enough we may see some stretching of the gasoline combining additional cheaper ingredients to make the winter as affordable as possible. Even the slightest reduction would result in cost savings (obviously) This has been a dilemma for many of the smaller and locally owned gas stations in the US. They have been stretching their gas my combining cheaper fuels. And less face it we are ALL feeling the pressure. I have to agree with majorian that ethanol IS good, however we saw the impact it had on farmers and how it resulted in devastation for many. All in all we are still on a quest for the best alternative fuel, and right now ethanol seems attractive to ensure a warm winter.

There is definitely as issue regarding the amount to be evaporated as well.

The shorter chain hydrocarbons evaporate at lower temperatures than the longer ones. Some of these shorter chain hydrocarbons evaporate in summer temperatures (80+), but not in winter. It is my understanding that some of these are taken of summer gasoline, and stored for use in winter gasoline.

When the use of ethanol was first discussed, EIA memorandums were concerned that adding ethanol instead of MTBE would reduce the summer fuel supply. One concern was that adding ethanol would require even more short-chain hydrocarbons to be removed than adding MTBE. I am not sure that Robert agrees that this is really an issue. Theoretically, the saved fuel can be stored for winter use.

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.

Thanks for this bit of info. I had always thought high octane gasoline cost the same to produce as regular gasoline.

That said, I still have a problem with the oil companies calling high octane gasoline super or premium since the octane is simply a requirement based on engine design and has nothing to do with the implied performance enhancement the name gives it. Too many people still believe they get a benefit from premium gasoline when their vehicle is not designed to use it. I'm betting most TOD readers know this already. Still, it is good info to pass on to others you know. Gas costs enough already without paying another .18¢/gallon for premium when you don't need to.


Circa 1895 a car called the Stanley Steamer did over 100mph.

Emission-wise would a wood burning steam car be polluting less than a petrol car. CO2 NOX etc.

Peak oil may put us all back to sailing ships and steam cars.