Winter vs Summer Gasoline… yes there is a difference

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