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Winter vs Summer Gasoline… yes there is a difference

November 28, 2011

It’s easy to notice some differences about gasoline.  If you pull up to the pump, there are often two or three nozzles per station covering the different octane ratings recommend for some cars.* There may also be separate pumps specifically for diesel** or E85.  But for the bulk of us who pull up, pump in a tank of regular unleaded, and roll out; how many are aware that the gas in July is different than the gas in January.

Sometimes there’s a brief mention of a shift from “summer blend” to “winter blend” or vice versa in the news.  Though equally or more often it’s a quiet shift that occurs June 1st and September 15th.  Just as under the radar as when it changes is what the point of the change is.  People hear “winter blend” gasoline and think it’s supposed to help make sure your car starts when it’s -10°F below zero and your coffee froze in it’s travel mug.  And in fact, some gas producers tend to spin it this way as the winter blend is more volatile, and thus ignites a bit easier in cold engines.  But this is just a side benefit.

First off, let’s define what we’re calling gasoline.  It’s toxic, readily flammable, and surprisingly translucent considering it usually comes from some variant of crude oil.  Chemically, it is a mix of molecules made up of hydrogen and carbon, with carbon being these molecules’ backbone.  These types of molecules are called hydrocarbons (hydrogen & carbon… get it?).  Here’s some examples:

There are plenty of rules and regulations regarding what can actually be in gasoline.  The molecules can vary but pretty much must have between 4 and 12 carbon molecules.  Less than this and you have molecules than are gases (think propane which has 3 carbon atoms or natural gas whose molecules are 1 or 2 carbon atoms long) and thus aren’t going to readily flow into your car’s tank or work with how your engine is designed.  More than 12 carbon molecules and you get something more sludgy or less volatile and thus hard to burn.  Diesel has molecules that are 8 to 21 carbons and as such requires a compression ignition engine instead of a gasoline vehicle’s spark ignition engine.  There’s also limits to types of molecules since crude oil and it’s variants aren’t just a mix of natural gas, gasoline, and pure American spirit. For example benzene has 6 carbon atoms and is a natural component of crude oil.  Exposure limits for benzene start at 1 part per million when inhaled, so making sure it’s not in the mix of gasoline is important.  No one wants to get bone marrow cancer just because they didn’t wear a respirator while filling up their tank.

So what’s the difference between summer and winter blends?  Well, it’s important to note that butane (4 carbons) is relatively cheap. Molecules with more carbon atoms are more valuable since it’s easier to break molecules down than build them up.  Which means that refineries looking to make the most money want gasoline blends that have the most shorter chain molecules while still having a mixture that is stable enough not to evaporate during the distribution process.  It’s also important to note that butane has less energy by volume than longer chain hydrocarbons.

In the summer, it’s warm out.  Which is nice for swimming, but bad for living in a world where we have tanks of hydrocarbons all over; zipping around us, stored in our garage, buried at gas stations, airports, and vehicle fleet facilities, etc.  When it’s warm, things evaporate easier, and shorter molecules evaporate easier than larger molecules (they are more “volatile”).  Reducing the volatility of gas cuts evaporative emissions, which contribute to ground level ozone and related environmental and health problems.  So regulations were put in place to protect us and dictate that summer blend gasoline have to effectively be heavier (less volatile, more longer chain molecules).

In winter, it’s cold, and so refineries are allowed to produce gasoline that evaporates more easily.  So they maximize the cheap, low energy butane in the mix.  Any benefit of your car starting more readily is really limited to old and/or poorly maintained vehicles.  Current vehicle technology is pretty hardy.

What’s the result?  Well, basically, winter blend gasoline has a larger percentage of butane in it.  And since butane is cheaper and has less energy, winter blend thus costs less and gives us a lower MPG when we burn it.  So it’s a wash, right?  Eh, maybe.  Your vehicle miles per gallon typically will drop 2-8% when you start filling up with winter blend.  Unfortunately, evidence suggests that the common price decline is 2-4%.  Of course this price change can easily be lost in the noise of a global commodity.  So you should probably just make sure you’re car is running optimally (oil change, tire pressure, reduce weight, clean off snow, etc. ) and cross your fingers on breaking even from a cost per unit of energy point of view.

*Unless your vehicle manual says differently, using a higher octane rating just wastes money as well as possibly damaging your car and/or reducing your MPG

**There’s actually a size difference in the nozzle for diesel that keeps the unaware customer from accidentally filling their gasoline vehicle with a fuel that needs and engine that utilizes completely different combustion mechanism


From → random musing

  1. Hedy Rubison permalink

    Definitely, what a great site and educative posts, I will bookmark your website.Best Regards!

  2. Lashaunda Teesdale permalink

    I regard something genuinely interesting about your website so I saved to favorites .

  3. pphenow permalink

    So, two comments so far, both spam. I hope Hedy and Lashaunda are still following the discussion. Seriously though, this was a very good article. I’ve always thought that my car just performed better in milder/warmer weather. I usually max out at about 28-30 mpg in the winter in my 2004 Sable. From late spring to early fall I can usually hit about 34-35 mpg. But I’ve been able to get it that high in the last week or so, so maybe that’s at least partially true that you get higher mpg in the warmer months? Or are refineries possibly so nimble in their production that they are able to suddenly switch to shipping summer blend in a particularly mild spring?

  4. Sue Hogge permalink

    I’ve had terrible gas mileage for the past 3 winters. It was so bad that the first time it dropped I thought someone was stealing my gas. A locking gas cap didn’t help. A tune-up and a thorough going-over by my mechanic didn’t help. Everyone kept telling me to check my tire pressure (I’d already been doing that routinely) and that it was probably just from letting my car warm up. Last winter was so mild there were only a few occasions when I let it warm up. Big Oil is selling us an inferior product for a superior price, and trying to dupe us into believing they’re doing us a favor. Give me the same gas all year round, and let me worry about whether or not my car starts!

    • Sue – You are probably correct in your assesment of “Big Oil”… but, whatcha gonna do? “Big Oil” (BO … yeah, something smells funny) only listen$ to money! Profit before logic / Congre$$ is paid for / rape the Earth. BO is deaf to public outcry … just don’t even look at the price on the pumps! It’s only money!!

      • Jeff permalink

        You could make your own ethanol. There was a study where a company burned Pure ethanol in a truck and regular gas in a different truck. The one running ethanol had less wear after 100000 miles. Both non flex fuel.

  5. Sue Hogge permalink

    True, Radford. We have to take it or walk.

  6. madeline permalink

    I need to to thank you for this very good read!! I definitely enjoyed every bit of
    it. I have you bookmarked to check out new things you post…

  7. rachael permalink

    It’s remarkable. thanks admin

  8. Leslie Brunell permalink

    I live in Utah where we have terrible winter air pollution due to inversions. Would using gas formulated for summer during the winter reduce air pollution?

  9. sam permalink

    where can i find the autor for this website for my reference.

  10. Breanna permalink

    This was a really wonderful article. Many thanks for providing this information.

  11. I have found that I get better mileage using 91 octane than 87 even though my ford manual says 87 is fine, I have also read that horsepower ratings for the vehicle I drive were generated using 93 octane Therefore I don’t understand the comment saying that higher octane gas is just a waste of money.

  12. rad skriba permalink

    It is a good article.

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