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The standard light petroleum distillate for vehicle engines, "regular gasoline," is (or is equivalent to) some mix of heptane (C7) and octane (C8). Higher proportions of C8 are more knock-resistant, which allow for higher compression ratios and thus more efficient energy utilization in internal combustion engines.

Don't modern refineries produce pretty much any hydrocarbon mix they want through combinations of distillation, cracking, and alkylation?

If so, then why is "regular" gasoline produced in such quantities, and why is a premium charged for more knock-resistant ("higher octane") blends? For example, if a refinery had to produce only one light fuel distillate, couldn't it just as easily and cheaply produce a "100 octane" blend as the current "87 octane"?

Or is it actually cheaper to produce "lower octane" petrol?

Note that "100 octane" doesn't mean 100% octane, as the "octane" number of a hydrocarbon depends on its isomers, with more highly-branched isomers having more knock resistance. So 100 octane fuel can be produced with many blends of C7, C8, and even include lighter and heavier hydrocarbons.

A characterization of the isomeric constituents of crude oil might answer this question. For example, if crude feedstock tends to have more linear isomers, then energy would have to be put into isomerization to produce more knock-resistant distillates.

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  • $\begingroup$ "regular" is 95 Octane, at least in the UK. If it's lower where you are, that must be a different market with different demands. What region/country are you referring to? $\endgroup$ – Brian Drummond Nov 21 '16 at 19:50
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    $\begingroup$ @Brian: In most of the US, "regular" gasoline is 87 octane, as measured by the (R+M)/2 method. I was surprised to see 85 octane sold as "regular" in Colorado last summer, although I didn't notice any adverse effects or reduction in milage over several tanks of it. $\endgroup$ – Olin Lathrop Nov 21 '16 at 20:44
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    $\begingroup$ @BrianDrummond - United States, where we quote the (R+M)/2 rating, which is known to be 4-6 points lower than the RON number you must use in U.K.. $\endgroup$ – feetwet Nov 21 '16 at 20:46
  • $\begingroup$ @OlinLathrop - Higher altitude and air temperatures reduce compression and hence knock susceptibility. Likewise, you'll notice "regular" octanes are increased when many regions change to "winter" blends. $\endgroup$ – feetwet Nov 21 '16 at 20:49
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    $\begingroup$ @agentp - True. But would also be economically advantageous for refiners to engage in price discrimination. So, for example, even if they only made 93 octane, they might mark their base output as 87 and charge a premium for the stuff they certify as 93. $\endgroup$ – feetwet Nov 22 '16 at 14:05
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Well first and foremost, refiners are a business so they must make the decisions that make them the most money. Considering this, lets think about cracking and isomerization via catalysis. The refiner is going to require additional reactors, piping and pumping, most likely additional distillation columns, catalyst loading etc. The energy alone to do distillation is generally very burdensome and if you're dealing with isomerization, you could likely be dealing with similar chemicals that don't separate easily (meaning huge energy and thus monetary operating costs). Catalysts can often be cheap, but usually are made from zeolite bases doped with rare or heavy metals, and can cost anywhere from $5 to thousands of dollars per kilogram, and they don't last forever (sintering, poisoning, etc).

So even without numbers, you can see where it could become very involved for an entity to produce the higher octane materials. But if they can get a premium for it and make money they'll do it right (some do)? Here's another factor: Biofuels.

Biofuels are currently mandated through the Renewable Fuel Standard for blending into the US fuel supply. This includes many different renewable and biofuels, but the most prominent is fuel ethanol (from starch currently, cellulosic later). The link below provides an overview of the RFS mandate. It is important to note that the parties obligated to blend these biofuels are Refiners and importers of crude or finished petroleum fuel products.

https://www.epa.gov/renewable-fuel-standard-program/final-renewable-fuel-standards-2017-and-biomass-based-diesel-volume

Because these entities are required by law (effectively under the clean air act) to blend these fuels, it makes the most sense to utilize them in the best way possible. Ethanol provides a very specific advantage to refiners, as it can drastically impact the final RON, MON, or combined Octane Number of lower quality gasoline fuels. enter image description here

So, from a business perspective, it makes the most economic sense to invest little to no capital or operating expense in octane enhancing process equipment, meet the mandated obligation and at the same time blend the ethanol with lower quality cuts of fuel but still make the same octane grade to the rack for the distributor / consumer.

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Depending on where you are, "regular" automotive fuel is anything between 85 and 95. However a number of, say, 95, does not mean it's a mixture of 95% octane and 5% heptane. It merely means that the fuel has knocking resistance properties similar to this mixture. The actual composition can be any mixture of isomeric hydrocarbons.

As the number that denotes the knocking resistance goes up, the mixture becomes progressively more expensive to manufacture.

Also note that the ideal fuel composition for a given internal combustion engine is subject to variables. Air pressure, for example, is a factor. Ideal fuel/air mixtures vary for different fuel compositions. At, say, 6000ft altitude (which is where I live) various fuel mixtures perform differently than they do at sea level. So local conditions may also be a consideration when it comes to formulating fuel mixtures.

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  • $\begingroup$ You beg the question: Why is it more expensive to produce distillates that are more knock-resistant? For example, is it the case that natural crude oil is dominated by linear isomers, and so it is always necessary to crack and alkylate to produce the branched isomers that increase knock-resistance? $\endgroup$ – feetwet Dec 22 '16 at 13:40
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As has been noted; It costs money to make "octane". And because there are many processes to make these many octane components , there are many different costs and "stories" . One story; xlyenes are very high octane and key components in polyester raw material , so very high competing value. Second story; reforming produces many high octane components, but because of the high hydrogen pressure and high total pressure and platinum group catalyst make this an expensive process. Maybe get a book on how refineries work.

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The simple answer is profits. They can charge more for the higher numbers even if the cost is not relative to the increase. I can't tell you how much more it costs to produce high octane, but am confident there is additional expense.

The winter blends as mentioned above are more for evaporation rates. If you consider a large city has many hundreds of petrol stations the total gasoline evaporated from all of those tanks is significant. Since the evaporation rate of fuel directly correlates to the temperature they can use a more evaporative fuel in the winter than they can in the summer.

They also have zones where they have to use the heavier gasoline even in the winter due to the concentration of petrol stations there.

I learned all this while working for a pipeline company that stored and transferred fuel for all the major oil companies from their refineries to the terminals where they were loaded on trucks. Interesting trivia, the gasoline in my pipelines had an octane rating in the 60's when we tested it. The octane rating you see at the pump is from the ethanol they add at the terminal while loading the trucks. This is mostly due to the fact that you don't want alcohol in bulk storage tanks because it would soak up a tremendous quantity of water from the tanks.

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Another reason that hasn't been mentioned yet is the 'network effect'. One US state or European country can't sensibly increase its octane requirements. The reason you want higher octane ratings is because it allows for higher compression ratios in engines, which increases the fuel efficiency. In most engines the compression ratio is fixed, and is optimised for the fuel that is available in the area where the car is sold. (Compression ratios sometimes differ for EU and US variants of the same car, due to the higher octane ratings of fuel in the EU.)

If octane requirements were increased somewhere, that would only allow new cars sold there to utilise the higher octane fuel by having a higher compression ratio. But if only one US state or EU country had this higher octane requirements, cars using a corresponding higher compression ratio would not work well when driven to another state/country using the lower octane rating, so car manufacturers would not start selling cars with a higher compression ratio unless a large area (e.g. the entire US or EU) would change its fuel octane. And changes that influence a continent are very slow with lots of stakeholders, if they happen at all.

So once octane ratings are set, they are difficult to change. And the focus on fuel efficiency is probably relatively recent (though I don't know when these octane ratings were standardised).

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  • $\begingroup$ Incorrect. Many states and counties ( in the US anyway) make up their own gasoline standards. The idiots in the CA legislature in particular seem to think they are all chemical engineers. These "laws" frequently change so it is not worthwhile trying to list them . Cook Co. , IL ( Chicago) has so many special requirements that gasoline costs more because it is a boutique blend that many refiners can not or will not make. $\endgroup$ – blacksmith37 Apr 6 '18 at 20:24
  • $\begingroup$ I wasn't aware of that. But you seem to agree that one state or county can't sensibly set their own gasoline requirements <wink> $\endgroup$ – JanKanis Apr 6 '18 at 20:48

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