Study finds wider use of premium gas could save fuel, money

October 28, 2014
Credit: iStock

If the majority of light-duty vehicles in the United States ran on higher-octane gasoline, the automotive industry as a whole would reduce its carbon dioxide emissions by 35 million tons per year, saving up to $6 billion in fuel costs, according to a new analysis by MIT researchers.

In a study published in the journal Environmental Science & Technology, the team considered a scenario in which is manufactured under a redefined octane rating—the measure of a gasoline's ability to resist engine knocking during combustion.

Currently in the United States, a car's octane rating is based on the antiknock index (AKI)—a specification for fuel composition that is determined by a standard research octane number (RON) and a motor octane number (MON). The resulting octane ratings for today's car engines typically range from 87 (regular fuel) to 93 (premium, or high-octane, fuel)—numbers that are commonly displayed at the pump. The higher the octane rating, the more resistant the fuel is to knocking.

However, the MIT researchers deemed AKI—and more specifically, MON—to be an outdated measure of engine performance, originally designed to apply to older, carbureted engines rather than modern, fuel-injected engines. To bring the octane up to date, the team considered doing away with MON, and basing engine performance solely on RON.

The revised octane rating system would boost the fuel grade of regular gasoline to 93, and premium to 98. The researchers reasoned that the higher fuel grades, while still appropriate for use in today's engines, could also give oil refiners the opportunity to produce higher-octane fuel, which in turn could spur manufacturers to design vehicles to run on higher-octane—an innovation that could lead to more efficient vehicles.

"The efficiency of gasoline engines depends on the octane number, but that's not something that's changed in quite a while," says Raymond Speth, a research scientist in MIT's Department of Aeronautics and Astronautics. "If [manufacturers] know the gasoline is higher-octane, they can design engines to have a higher compression ratio, which would make the engine smaller and more efficient, both of which are a benefit."

A high-octane lifecycle

To explore the economic and environmental consequences of a higher-octane fleet, Speth and his colleagues first modeled a vehicle fleet transition from regular to higher-octane gasoline. The team factored in a policy-decision period of about three years to put in place a revised octane rating system, and an additional three to five years for manufacturers to redesign engines to meet the new standards. Based on these constraints, the researchers estimate that by 2040, 80 percent of light-duty vehicles in the U.S. would transition to higher-octane fuel.

The researchers estimated the environmental and economic cost of such a scenario by running simulations of the oil-refining process and vehicle performance, and performing a life-cycle analysis of the resulting .

The team found that vehicles running on higher-octane fuel would be more efficient, consuming 3 to 4.5 percent less gasoline, for a projected savings of up to $6.4 billion per year by 2040.

Based on its oil-refinery modeling, the group found that producing higher-octane fuel would increase an oil refinery's emissions by 6 percent—an increase that is minor when compared with the balance of emissions from fuel production. When assessing the emissions produced by everything from extracting crude oil to transporting it to refineries and burning it in car engines, the team found that a higher-octane vehicle fleet would reduce overall emissions by 35 million tons per year—a decrease that stems mostly from more-efficient engines.

"Overall, you're decreasing carbon by 3 to 4 percent, at negative cost," says Steven Barrett, an associate professor of aeronautics and astronautics, and co-author of the paper. "It's one of the few things you can do to decrease carbon dioxide, while at the same time saving money."

In sum, the researchers found that redefining the octane rating system in the U.S. to encourage higher-octane consumption would have both economic and environmental benefits. To change the system, however, will require consensus from multiple parties.

"It's an issue that has a lot of stakeholders that will have to agree," Speth says. "When you have this evidence that there's this real economic benefit, hopefully you can say, 'Look, there are pieces of the pie that can be divvied up to provide a benefit to almost everyone involved: the refineries, the vehicle manufacturers, and the consumers.'"

Explore further: New filtration material could make petroleum refining cheaper, more efficient

More information: "Economic and Environmental Benefits of Higher-Octane Gasoline" Environ. Sci. Technol., 2014, 48 (12), pp 6561–6568 DOI: 10.1021/es405557p

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12 comments

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antialias_physorg
5 / 5 (2) Oct 28, 2014
I've been trying this out for quite some time as my car is rated for 98 octane premium fuel (but may also run on the 95 octane fuel). At least two large chains of gas stations also offer 100 octane fuel. While I did not notice an increase in power I did notice a slight decrease in consumption. And during checkups the garage says the spark plugs are cleaner than expected (though that doesn't change the service interval - so no savings there). The added cost and the savings in consumption pretty much balance out.
So I'm sticking to the high octane stuff simply for the extra range and the less exhaust.
MR166
5 / 5 (2) Oct 28, 2014
First of all the article seems to "raise" octane levels by changing the way they are computed. I doubt that engines, even the newer ones, can read the sign on the pump.

Also the highest octane gas here in the US costs about 10% more than regular so a 4% savings in consumption still adds 6% on to your gas bill.

Lastly engines that were designed to run on regular gas ( lower compression ratios) will not see any advantage at all when fueled with high test gas. Only high compression or turbocharged engines will benefit from the higher octane.
Zenmaster
5 / 5 (1) Oct 28, 2014
Almost all auto manufacturers using gasoline engines have been moving to higher compression combustion with tech such as direct injection, super/turbo charging which does require higher octane. Smaller engines with higher compression are lighter, yield more power, and produce less CO emissions per volume.
Changing the pump octane method might spur some consumer consciousness about combustion efficiency, but the industry is already phasing out old engine tech.
Eikka
5 / 5 (2) Oct 28, 2014
First of all the article seems to "raise" octane levels by changing the way they are computed. I doubt that engines, even the newer ones, can read the sign on the pump.


I think the point is that using RON instead of AKI deals away with the use of the "standard" engine in testing the fuel, which forces the producers to make their fuels differently rather than just optimizing them for the particular testing setup used to come up with the MON rating.

Also the highest octane gas here in the US costs about 10% more than regular so a 4% savings in consumption still adds 6% on to your gas bill.


Your math is incorrect. (1 - 0.04) x 1.1 is 1.018 or 1.8% more, not 6% more.

Lastly engines that were designed to run on regular gas ( lower compression ratios) will not see any advantage at all when fueled with high test gas.


Most cars made in the last 20 years have anti-knock sensors that retard ignition and change fuel maps in response to low octane fuel.
MR166
not rated yet Oct 28, 2014
Ford makes a turbocharged 2.3L 4 that produces 270hp and runs on regular. That is pretty amazing.
Eikka
5 / 5 (1) Oct 28, 2014
To be more specific:

RON is measured by comparison to a mixture of Heptane and Octane in an engine with a variable compression ratio. A fuel that starts to knock at the same compression ratio as say 20/80 blend is equal to 80 RON because the comparison mixture contains 80% octane. Higher than 100 octane ratings are extrapolated.

MON is measured in an similiar engine, at higher speed and with pre-heating the fuel, with different ignition timing to stress it and induce knocking earlier.

The MON for pure heptane/octane is the same as the RON but many fuel blends behave worse with MON, so dropping the MON will, with some blends, actually lead to significantly lower quality fuel being sold at the same octane number.

But since the numbers are changing it's easier to convince people to buy 95 RON instead of 93 RON (87 AKI) for "regular" to mask the difference. The car manufacturers then simply have to deal with the fact that the new "regular" may vary in quality.

Eikka
5 / 5 (1) Oct 28, 2014
While I did not notice an increase in power I did notice a slight decrease in consumption.


The European E10 95 RON fuel contains twice as much ethanol as the E5 98 RON, which means it actually contains less energy per weight. Ethanol is an octane booster, so it also means the E10 uses lower grade fuel for the actual gasoline, which also results in lower energy density. The ethanol is also hygroscopic, so it draws in water from the surrounding air and actually dilutes slightly while in storage.

The combined effect is that the 95 RON fuel in Europe actually contains about 3-5% less energy per liter than the 98 or 100 RON fuel, and since the price difference is practically non-existent, you're just better off using 98 RON.

If it was just a difference in the octane rating, and not the energy contents, you wouldn't notice a difference under normal driving conditions with light engine loads.
MR166
not rated yet Oct 28, 2014
Eikka if high octane gas needs to be reformulated to meet the needs of modern vs old engines there is no doubt it should be done.

As far as the math goes you have made and error.
Lets say you can drive 25 miles on a gallon of gas that costs 2.50. That costs you 10 cents a mile. Use high test and get 25mi x 1.04= 26 miles at a cost of 2.50 x 1.1= 2.75. 2.75/26= 10.58 cents/ mile. You have increased your costs by .58 cents.mile or 5.8 %

As for your last point, a knock sensor works by delaying the ignition spark in order to make up for gas that is under the designed for octane levels. Thus increasing octane over the designed for levels advances the spark and increases gas mileage a little but the gain is not really that significant. Whereas increasing the compression ratio of an engine can result in more significant gains.
antigoracle
1 / 5 (1) Oct 28, 2014
Your math is incorrect. (1 - 0.04) x 1.1 is 1.018 or 1.8% more, not 6% more.

Your calculator is broken. Mine gives 1.056 or ~ 6%
Charlie_G
5 / 5 (1) Oct 28, 2014
Not just theory & math.
I noticed years ago that although 98 RON fuel costs approximately 10% more than 91 RON, range is actually increased on average by a little more than 10%. So premium fuel is cheaper (and supposedly cleaner) in vehicles designed to run on standard fuel.
Try it for yourself, can't do any harm.
Eikka
5 / 5 (1) Oct 29, 2014
As far as the math goes you have made and error.


It seems so.

Thus increasing octane over the designed for levels advances the spark and increases gas mileage a little but the gain is not really that significant.


Advancing the ignition beyond the optimal point does not increase power or efficiency.
Most modern engines are designed for higher octane than you can get from the pump in order to get better results in the fuel economy and emissions tests.

The trick is that the EPA tests are outdated and don't stress modern engines sufficiently, so they can run more agressive timings in the test than they'd use on the road.

If a car has never been driven, the anti-knock sensor has never adjusted to the fuel and will start with the most optimal timings. Putting such a car on the EPA dynamometer will see better efficiency and emissions than the same car after one tank of regular.
MR166
not rated yet Oct 29, 2014
I just did a quick check at my local station here in the US. Gas comes in 3 grades, regular, mid level and premium. There is a 14% difference in price between regular and premium. That is a big difference in price to make up with better efficiency.

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