Refining 101: Summer Gasoline

Just what is summer gasoline? Twice a year, in the fall and in the spring, you hear about the seasonal gasoline transition. However, most people probably don’t understand what this actually means. AAA recently provided a Top 10 list explaining the recent rise in gasoline prices, and summer gasoline checked in at #7:

7. The summer blend switchover. This transition from winter-blend to summer-blend fuel, a concoction that causes less smog, occurs every spring. It causes a dip in gasoline supplies as refineries in the U.S. shut down temporarily to retool their production facilities.

That’s only partially correct, and is probably the extent of most people’s understanding of this transition. But given that I am very keen that people should understand the energy industry, it is worth a review, and a layman’s explanation. I explained the details behind this transition in Here Comes Winter Gasoline. But let’s review some concepts.

There are two key (although not the only) specifications that refiners must meet for gasoline. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure (RVP). While the octane of a particular grade is constant throughout the year, the RVP spec changes with the seasons.

The RVP is based on a test that measures vapor pressure of the gasoline blend at 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 normal atmospheric pressure, that liquid boils. For example, when you heat a pan 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 vaporized 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. The particulars vary, but key considerations are the altitude and motor vehicle density of a specific location. The EIA summarizes the key points:

As gasoline evaporates, volatile organic compounds (VOC’s) enter the atmosphere and contribute to ozone formation. Gasoline’s propensity to evaporate is measured by Reid vapor pressure (RVP). In order to control VOC emissions, the Federal Clean Air Act Amendments of 1990 require that all gasoline be limited to an RVP maximum of 9.0 psi during the summer high ozone season, which the Environmental Protection Agency (EPA) established as running from June 1 to September 15. The Act also authorized the EPA to set more stringent standards for nonattainment areas. As a result, EPA limits areas designated as “high volatility non-attainment” to a maximum RVP of 7.8 psi during the high ozone season. Some States elected to require even more stringent restrictions to achieve local clean air goals, and require 7.2- and 7.0-psi gasolines.

Butane, which has an RVP of 52 psi, can be blended into gasoline in higher proportions in the winter because the vapor pressure allowance is higher. There are 2 advantages in doing this. First, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce. But butane is also abundant, so that means that gasoline supplies increase in the winter because more butane is thrown into the mix. Not only that, but this all takes place after summer driving season, when demand typically falls off. 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.

There are some misconceptions that I often seen repeated about this seasonal transition. One is that it is the reason that spring and fall maintenance are done. That is not the case. Most, if not all refineries can carry out this transition without shutting down or interrupting production. The reason that maintenance is done in the spring and fall is that it provides a combination of moderate weather (the inside of a vessel can be unbearable in the summer) and off-peak demand. Vessels must be inspected, new equipment must be installed, catalyst change-outs occur, etc. This is similar to tuning up your car to keep it in proper running condition. But the seasonal maintenance is unrelated to the gasoline transition. In fact, for reasons I won’t get into here, seasonal maintenance often complicates the transition.

Another misconception that some have is that they can save money by buying cheap gas in the winter and storing it for the summer. Remember that winter gasoline will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix. You will end up with less gasoline than you paid for, and you will be contributing to the air pollution problem that summer gasoline was designed to avoid. If, on the other hand, you were to buy summer gasoline and try to store it until winter, you might find yourself having problems getting the fuel to ignite, due to the lower vapor pressure. This would be like putting a little bit of diesel in your gasoline – not very good for your car. So buy and use gasoline in the correct season.

The Politics of Ethanol Blending

I should also mention a bit about ethanol blending. The blending of ethanol into the gasoline pool has been controversial because (among other things) it increases the vapor pressure of gasoline blends. This has resulted in the need for a 1 psi waiver for ethanol-containing fuels. From the previously linked EIA report:

As a part of the Clean Air Act Amendments, conventional gasoline containing 10 percent ethanol was allowed to exceed the Federal RVP maximums by 1 psi.

This of course means that ethanol will exacerbate smog at certain times of the year, and has resulted in a campaign by Senator Diane Feinstein to limit ethanol blending in California:

California contends its refineries can make clean-burning gasoline without oxygenates such as ethanol or MTBE. In fact, California’s Sen. Diane Feinstein contends ethanol’s volatility may be the cause for increasing smog levels in Southern California since the waiver was denied and more ethanol was added to the state’s gasoline supply.

Recently, Feinstein asked the EPA and the California Air Resources Board to investigate the impact of ethanol-blended gasoline on California’s air quality.

She said air quality in the South Coast Air Quality Management Zone has gotten worse this year compared to last and “the switch to ethanol-blended gasoline is considered one of the main culprits in increased ozone.”

“Since ethanol’s volatility increases smog, particularly in the summer, I believe we need to look carefully at its impact on air quality,” said the senator.

However, it looks like she is losing this battle for political reasons:

In the face-off between California and Corn Belt states over ethanol, California lost again this month. Federal officials concede that the corn-based fuel additive can increase smog and soot pollution from vehicles. But in a ruling shocking in its disregard for public health, the U.S. Environmental Protection Agency refused for a second time to scrap the rule requiring California to blend ethanol in its gasoline.

The EPA conceded that California air quality officials are right about ethanol’s polluting effect in summer. Nonetheless, in its tortured ruling, the federal agency said California had not “clearly demonstrated” that the ethanol requirement would delay or interfere with the state’s ability to meet federal clean air standards. Incredibly, the ruling said that even if California had demonstrated that the ethanol rule prevented the state from meeting clean air standards, the EPA “would deny the waiver.” Why? “This reduction in the use of ethanol would undermine the potential benefits vis a vis energy security and support for rural and agricultural economy that Congress expected” from its ethanol rule.

The EPA ruling’s effect is to increase payouts to one special interest, Midwest corn producers. For that California endures higher gasoline prices and dirtier air.

California cannot afford to let this assault on public health, fairness and common sense stand. U.S. Sen. Dianne Feinstein, D-Calif., has persuaded the Senate Energy Committee to add a clause to a pending energy bill that would exempt California from the ethanol rule during summer months. All the state’s elected officials should join her in that fight.

All energy issues seem to be completely entangled with politics, and the sad fact is that the politics often trump the science. Ethanol blending is a perfect example where we are willing to exempt certain pollution issues “for the greater good.”


Hopefully that was an easy-to-understand explanation of the seasonal gasoline transition in the U.S. The purpose of the transition is to curb pollution, but as the last section demonstrates the politics often interfere with the original intent. Now the next time you hear “season gasoline transition”, you will know exactly what they are talking about and what the expected impact on supply and price will be.

14 thoughts on “Refining 101: Summer Gasoline”

  1. For balance, you might note that ethanol does rather help in meeting the octane requirement, and therefore means there’s less of a need for some other troubling compounds, like say benzene (or MTBE or lead).

  2. Actually there isn’t a need for any of those. That was what Senator Feinstein was arguing – that the specs can be met without those additives. Well, benzene isn’t an additive anyway; it comes from the crude oil. There are new specs to greatly lower the benzene limit, and I know that studies are being conducted to see the best way of doing that.

    Cheers, RR

  3. Thanks Robert for an interesting post.

    I wounder, though, it gasoline left in an open container at high temperature would really boil. In contrast to the melting point, which is well defined for a pure substance except at very high pressures, BP is more complicated– or at least the act of boiling is. It is not as simple as the temperature at which the VP equals the ambient pressure.

    The most observed phenomenon which illustrates this is heating a cup of water in a microwave oven. The water can get much hotter than 100C without showing evidence of boiling.

    Boiling is defined by the visible bubbles which form and rise to the surface, releasing vapor to the atmosphere. For a bubble to form, it has to first nucleate and then grow to a sufficient size to be thermodynamically stable. A pan bottom is heated from below so is hotter than the bulk liquid, and there are imperfections on the surface which act as nucleation sites. If the pan is deep, the bubbles have to grow against not just atmospheric pressure, but also the weight of the liquid above. And without the heating element below, the bottom surface might not be hot enough to support bubble formation.

    Here is an interesting video of boiling in zero gravity.

  4. One way of upgrading bio-oil is the use of zeolites, which unfortunately gives a lot of aromatics.

    A few weeks ago, I decided to look into that a bit more, because I wanted to know how bad aromatics are and what refineries do to deal with the matter.

    I am not an expert on that at all, I’ve only looked at it for a few hours to get a bit of background that might be helpful for my actual area of work (fast pyrolysis).

    A bit of googling gave me this

    And a bit more for the layperson:

    “As a petrol additive, benzene increases the octane rating and reduces knocking. Consequently, petrol often contained several percent benzene before the 1950s, when tetraethyl lead replaced it as the most widely-used antiknock additive. With the global phaseout of leaded petrol, benzene has made a comeback as a gasoline additive in some nations. In the United States, concern over its negative health effects and the possibility of benzene entering the groundwater have led to stringent regulation of petrol’s benzene content, with limits typically around 1%. European petrol specifications now contain the same 1% limit on benzene content.”

    The first link above is very interesting and very detailed.


    I think ethanol production has increased food prices a little recently. I am not so sure whether this is a net negative. It’ll stimulate more grain production and help farmers, both in the US and in developing countries.

    In case of real shortages, it’s good to have a bit of a cushion, and both animal feed and ethanol provide that.


    As for California, ethanol and pollution,

    I don’t think the article you cite gives the whole story, it appears decidedly biased to me.


    That article is of course biased as well. It’s from an ethanol lobby group after all.

    But, it’s a nice summary of the counterargument.

    They say that the 1 psi waiver is ozone “neutral” (or close to it), because reduced CO makes up for increased VOC, and most other emissions are improved, especially if aromatics and benzene in particular is displaced by ethanol.

  6. Here is what Senator Feinstein’s office reported:

    “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.

    The Secretary of the California Environmental Protection Agency quantified the impact of ethanol on air quality in a letter to me dated August 1, 2003:

    “…our current best estimate is that the increase in the use of ethanol-blended gasoline has likely resulted in about a one percent increase in emissions of volatile organic gases (VOC) in the SCAQMD [South Coast Air Quality Management District] in the summer of 2003. Given the very poor air quality in the region and the great difficulty of reaching the current federal ozone standard by the required attainment date of 2010, an increase of this magnitude is of great concern. Clearly, these emission increases have resulted in higher ozone levels this year that what would have otherwise occurred, and are responsible for at least some of the rise in ozone levels that have been observed.”

    In September 2004, CARB sponsored a study by the Coordinating Research Council (CRC). The CRC issued a report entitled Fuel Permeation From Automotive Systems. The study was designed to determine the magnitude of the permeation differences between three fuels, containing MTBE, ethanol, or no oxygenate, in the selected test fleet. The study found that emissions increased on all 10 vehicle fuel systems studied when ethanol replaced the MTBE. In fact, the ethanol blended gasoline caused emissions to increase by 65% when compared with MTBE blended gasoline, and by 45% when compared with non-oxygenated gasoline.

    In a November 2004 report, CARB staff issued a preliminary analysis of increased emissions due to ethanol blended gasoline. The staff reported that “on-road vehicles hydrocarbon emissions increase[d] by 40-50 tons/day, statewide, [in] 2004.” CARB staff is currently working on a final analysis of the impact of ethanol blended gasoline on emissions.

    Over 90% of Californians breathe unhealthy air. The purpose of the federal reformulated gas program is to protect public health by reducing harmful vehicle emissions of smog-forming compounds and air toxics. I am concerned that the ethanol mandate in California limits CARB’s ability to issue the most effective regulations to best reduce air pollution from vehicles and to protect the health of all Californians.”

    Source: Senator Feinstein Renews Call for Federal Oxygenate Waiver for California

    I am not sure why the California Air Resources Board would be biased, and they actually conducted lab tests to come up with their conclusions.

  7. I wounder, though, it gasoline left in an open container at high temperature would really boil.

    It all depends. I can assure you that I can make a blend with 20% butane, and it will boil – at least up until the point that the butane is concentration is reduced and the vapor pressure gets back down toward atmospheric.

    But you are right that these aren’t absolutes. If I make a blend with a 14.696 psi RVP and heat it up to 100 degrees F, it might not boil. But if I did that 100 times, you would see that on average the initial boiling point is plus or minus a degree or so of 100 F.

    Cheers, Robert


    Surprise, surprise, the ethanol lobby quotes CARB to say that their actual measurements show reduced ozone after the introduction of the ethanol blend, and that their modelling is wrong, their study faulty, and

    for example a Michigan government funded study is much better.

    Methinks that there is definitely bias in the article you cited, and in Feinstein’s press release,

    and quite possibly bias in what California and Michigan government funded studies have to say on the subject.

    CARB may be right, at least in the narrow sense that the CO effect is negligible for new cars and that VOC’s do make it (at least a little) harder to reach the ozone targets with ethanol blended petrol.

    On the other hand, the Michigan Department of Environmental Quality may be right that ethanol blended gasoline reduces ozone levels slightly.

  9. How would you recommend responding to these people? Or is it pointless?

    I just jumped over to the Digg thread where this article is being discussed. Look at that, and fear for what we are up against. A sampling of comments:

    this is complete propaganda. if you believe this crap your dumb. Why is Exxon making records profits. and where were these summer/winter blends in 1998 or earlier?when gas was 1.09 a gallon. why have gas prices just skyrocketed since the war?? dont just stop at this little report about it, continue to research oil and gas and im sure you’ll stumble upon some hard to take truth.

    well despite the reasons behind the price increases the best way to drive the prices down is to purchase the cheapest gas around, so by forcing higher priced stations to compete with lower competitors will drive the cost down.

    Show me any other year where the seasonal fluctuation is over 90 cents per gallon. Until gasoline refining is nationalized we will always face these kinds of price gougings.

    Don’t the refineries own calendars? Last time I checked, summer comes at the same time every year. You’d think they’d be able to figure that out by now.

    Summer blend my ass!

    I didn’t rtfa, but I am pissed off that these scumbag thieves (big oil) can take gas from 2.20 to 2.80 in Three ****ing weeks.

    While this article clearly states the variations in winter and summer gasoline, I’ll clairify it very simpler for you in a way that the oil companies never wish you to hear, but know about anyways. First of all, Menthol Hydrate is used in winter gasoline and since many people don’t drive pleasureably in winter, the only way to do it is to reduce the price in order to get people on the go.

    Is it any wonder that educating people on energy is such an uphill struggle? This the mindset of the majority of Americans, and our politicians are trying to pander to this sort of nonsense. That’s why our energy policy is so screwed up. The one comment just typified the average person: I didn’t read the article, but I am going to share my uninformed opinion anyway.

  10. Robert

    Nowhere is the role of politics and energy so apparent as in the case of global warming.

    The actual news on global warming is potentially much worse than we are being told. There is little confidence about predicting what the world will be like with 500ppm CO2, for example. My conversations with scientists and policymakers have been chilling in this regard.

    It seems as if there is a conspiracy, tacit and unofficial, not to tell us of the worst effects of global warming because:

    – there is a fear that a hopelessness will sink in, ie that nothing will be done

    – no one can quite believe what the evidence is telling them. There is an insistence that, somehow, our models are wrong in what they predict, or our reading of the latest evidence and the historic record is somehow wrong, or that ‘technology will come along and save us’

    – real change might require real sacrifice, and if that becomes clear, it might be politically impossible to impose

    I sense the same issue with Peak Oil. Policymakers are choosing mental frames of the world that don’t involve having to deal with Peak Oil.

    An example of the real costs that might have to be incurred might be a massive scaling up of nuclear power, or of unsightly wind farms. Or distinct rationing on our ability to fly and drive when we wish. Each of these measures would offend huge political constituencies.

    Ethanol to my mind is part of this. It is being sold as a ‘costless’ solution, when it is no solution at all (except for a handful of countries, like Brasil, that have massive sugarcane surpluses).

    Ethanol is all about the first US presidential caucus being held every 4 years in Iowa, and the key role of the midwestern states in the electoral college (given that the Republicans have a lock on the South, and the Democrats on New England and usually the West Coast).

    It’s not about reducing CO2 emissions or making America more energy independent. Selling more diesel cars would have a greater impact on that.

    As an example of that, friends of mine traded in their ancient SAAB (25mpg highway, Imperial Gallon) for a Toyota Avensis diesel (60mpg, Imperial Gallon). Now that is a giant leap towards energy efficiency!

    That latter car isn’t even *sold* in America.


  11. Heiko,

    Biofuels are an economic, ecologic, and social disaster. They exist only because diligent lobbying by corn farmers and ethanol producers led to government ethanol mandates.

    Browse over to the link to learn out just how much of an ecologic disaster biofuels are.

    I had to break the link to keep from blowing out the margins on the comment page.

    Copy and paste into your browser window and remove the space between maindish and /2006 and it will work.


  12. Robert,

    Where is the thread on digg. I could not find it.

    Some thoughts on responding.

    1. What is the lunatic to thinking person ratio on the thread? If they are all frothing at the mouth it is probably not worth your time.

    2. If there are people who seem to be open to discussion it would be more worth your time.

    3. Many more people read comment threads then comment. The frothers can actually provide you with a teachable moment for these people.

    If someone is making reasoned, sensible arguments, and providing additional information the contrast between the reasonable person and the ignorant frothers helps make the reasonable persons points.

    4. You will probably never persuade one of the frothers they have their beliefs and don’t want to be bothered by the facts.



  13. Where is the thread on digg. I could not find it.

    Sorry, the Digg submission was done from The Oil Drum. I posted this essay there as well, and they submit them all to Digg. Here is the thread:

    Refining 101: Summer Gasoline (and Why Gas Prices Go Up)

    As of today, 626 Diggs. I think that’s the most any post has ever gotten at TOD. Not bad for a post I wrote almost as an afterthought, and knocked out in under 2 hours.

    Cheers, Robert

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