Premium Gas and MPG

The packages added at the station or to the tanker truck are the detergent packages. In that case, the better packages can mean better fuel economy by being more effective, requiring less, and thus less dilution of the gasoline.

Those 16oz bottles of octane boost only raise the octane in 20 gallons of gas one point, or 0.1 octane. So the octane is set at the refinery.

The fraction of straight gas in crude isn't enough to meet demand. It is also a lower octane than regular. So we have to crack the larger molecules in crude to get smaller, and then reform those and the straight gas into ones that will yield the right octane. Toluene and xylene are common products of the reforming process that are used for boosting the gas blend's octane. Being valuable products in their own right is part of the reason why premium costs more.

Premium fuel does tend to be lower in energy compared to regular, and it will make little difference burning it in a low compression engine in place of regular. A high compression engine though, has a longer power stroke. This allows more of the gasoline's chemical energy to be converted to useful mechanical energy. Which means a higher thermal efficiency in short.

So premium fuel can yield better fuel economy in engines designed to take advantage of the higher octane. The 1.4L turbo in my Sonic gets better fuel economy on higher octanes. It was designed with Europe, and it's more prevalent higher octane gas, in mind. The ECU has a premium octane fuel map, and just detunes the engine operation on the go to meet the regular fuel requirement Chevy says it has. The increase MPG is only worth it financially if the price difference is 25 cents or less for premium, but it is usually 40 cents here, and I've seen near 80 cents.

A couple of related points and thoughts.
Depending exactly how much range of valve adjustment an Atkinsonized engine has, it could make use of higher octane fuels for more power. They are already taking advantage of the long power stroke for efficiency though.
Flex fuel engines available in the US are based upon low compression ones. If they were based upon high compression ones, they could take more advantage of ethanol's higher octane, and not have their MPG drop as much when using E85.
The EPA test blend is 93 octane. 91 when the test is done for CARB.

I've heard the price difference is now larger because of supply and demand. Requiring octane higher than regular hurts car sales for all but the high performance models. Then modern engine controls with knock sensors of the past decade or longer mean many of those performance cars are only premium suggested instead of required.

So the demand for premium has dropped. With some of the premium gas blend's fractions being more valuable separate, the oil companies' have cut back on the amount of they make premium. much that they do, might go to bringing the straight gas fraction up regular octane.

 

I don't pay attention to the silly NEDC European regulations. I just read from prius owners in Europe, where there is no choice to buy lower than super unleaded 91 AKI/95 RON, get on average 40 mpg in real life.

 

If Europeans are driving significantly faster, that could easily overcome their small advantage of higher octane rating gasoline.

 

I ran a couple of tanks of Costco Premium, 92 octane, in our 2010 Prius and noticed no difference in mpg or performance. We also have a 2014 Volt and have been using Costco regular, 87 octane, and last trip over 80 miles just on gas was over 44 mpg. No noticeable difference over premium gas, even though owners manual requires premium in the 2014 Volt. Premium gas may have been useful and required before computer controlled engines, but not so today with modern computer technology in all new car engines. Also some premium gas is 40-75 cents per gallon more than regular, a rip off in my opinion...

 

There is no guarantee that Premium has more energy, so your observation could be totally correct. However, we do have some Oregon owners feel opposite, and that could be true also. Since Oregon is not a reformulated gasoline state, there could be more variations. Also, your Regular in Oregon could be juicier, as discussed below.

I am trying to recall how the EPA regulations work on gasoline recipes. I am thinking, to make cleaner burning reformulated gasoline, EPA mandated removal of the some of the higher energy (high octane aromatics) from gasoline in the areas that use RFG. Then if I recall correctly, EPA also said, large oil companies are not permitted to transfer the higher energy materials from their RFG pool to the non-RFG pool.

Smaller companies may not have the same restrictions. And Europe and Canada would not have the same rules, so they would have more freedom to use higher energy constituents in Premium ( and Regular for that matter).

I filled up twice in Oregon last week. But I had a rental Cruze.

 

GM called for Premium in the first Volt because it has a longer shelf life. They were worried someone might drive enough EV miles that old regular would have dropped in octane beyond what the engine can compensate for.

Going to a higher regular octane will save fuel. The higher compression ratio means a longer expansion stroke, and more energy extracted from the burning fuel for useful work. A high compression, high octane fueled engine has a higher thermal efficiency with all else being equal.

Mazda's SkyActiv gas engines are 13:1 compression in the US. In markets where the regular is higher octane, they are 14:1.

With the same content of alcohol, premium tends to be slightly lower in energy than regular. So it will likely net lower fuel economy in a low compression engine.

 

I don't really agree that Premium usually has less energy.
The general case is Premium would tend to have a little more energy, but in the USA, EPA forces removal of the higher energy constituents in RFG reformulated gasoline areas. So in RFG areas, I agree Premium has same or perhaps slight less energy.

 

In Australia, the Gen 3 & Prius v both specified "Fuel type octane no. 95" as well as "Minimum fuel octane number 95".

However, Gen 4 specifies Octane Number & Minimum Octane of 91.

Both state on their brochures that they are 13:1 compression ratio.

 

Avgas is higher octane than automotive fuel. It might even still have lead. But it has a lower energy density.

Premium could have higher energy than regular, but the difference either way is tiny, and ethanol was sometimes being used to raise the octane before it was required in RFG.

My main point is that the difference between regular and premium gasoline with the same amount of ethanol is too small to account for any improvements or not in fuel economy. Any gains with premium is because the engine can run more aggressively on the ignition timing, and has a longer expansion stroke to capture more of the fuel's energy.

Australia uses RON for reporting octane, correct? The US uses AKI, an average of RON and MON. It would be better if we adopted RON only.

The Prius engine is Atkinsonized. It simulates the short compression stroke of the Atkinson cycle by leaving the injection valves open at the beginning of the compression stroke. So some of the air:fuel mix is pushed back into the air intake to be sucked back in next cycle, and the effective compression ratio is around 9:1 to 10:1. The longer expansion stroke means more fuel energy converted to work, but the Atkinson cycle gains efficiency at the expense of power.

If the valve timing could be adjusted so that the compression stroke was fully 13:1, the Prius engine would produce more power. I can't say if it will gain in fuel efficiency, as it takes more energy to compress the fuel charge now.

Valve timing is moving to the point where an engine can switch between full Otto cycle and Atkinson cycle to get the best of power when needed and efficiency when at low load.

 

When the Gen3 was introduced in early 2009, the brochure very clearly labeled this as 'expansion' ratio, not 'compression' ratio. Sometime in the next couple years, the brochure terminology changed, but the Atkinson-style engine did not.

For fuel grade and knock- or ping-control purposes, the Prius engine's 'compression' ratio is no where near 13:1, it isn't even in double digits.

 

Thanks

Any idea why Gen III needed 95 octane ("Premium 95") but Gen IV is happy with 91 Octane (our Standard/Regular)?

95 is typically 10-11c/litre dearer, ie 10-11% dearer here. With Gen IV using about 14% less fuel, added to the saving 10% in fuel price, that adds up.

I'm in the throes of deciding on a superceded (but NEW) Gen III and a new Gen IV.

 

I don't think Gen 3 required Premium:
What is the minimum octane rating fuel that I should use in my vehicle?

 

Regarding octane index, prius engine can use low grade octane fuel, as it's a low compression engine. I think it's 8:1 compression.

What limits the type of fuel you can use in this case is not the octane index, but the sulphur concentration. Where I live premium gas is required for the Prius and all Toyota vehicles, because that type has a concentration of 15ppm (parts per million). Regular fuel here has up to 300ppm.

The catalitic converter can't handle properly that concentration, so you have to go with premium instead to guarantee that the catalitic converter is working properly and that you don't exceed emissions.

On the other hand, I don't know if this requirement is because using regular fuel will damage the catalitic convert or it's just to keep emissions according to regulations.

SM-N900W8 ?

 

Why car companies design gas engines today to run on premium gas is beyond me. In my opinion any gasoline car engine designed to run on premium gas is a fail.

When Honda with their 2016 Honda Civic, not exactly a Ferrari, can build a gasoline 1.5 engine, turbo charged, that can obtain 174 HP on regular gas thats a win in my book

Premium gas only benefits, for 90 % of all the gasoline engines out their, the oil companies. Many people use premium in their regular 87 octane gas car because they believe its better for the engine, when its only better for the oil companies.

 

Because it is cheaper to design an engine to run on Japanese or European regular, and let its knock sensors and ECU handle running American regular, than to make modifications to an engine designed for American regular in order for it to run on the other markets' regular.

American regular is probably the lowest octane gasoline avialable for private cars. If we want to improve fuel economy, we'd bump the octane rating of our regular up, and switch to RON from AKI for it. RON pertains more to fuel injected engines.

On what test blend? The gas for EPA testing is premium octane.
I don't want higher octane fuel for power, I want it for fuel economy.

Someone will always profit off of another's ignorance.

 

I thought higher octane is only good for improved pre-ignition resistance, does nothing for fuel economy. No?

 

In the USA market, Prius has always required lowest octane (regular). We saw no change G1/G2/G3/G4/PiP all the same. Years ago, in the USA, when Premium was marketed as a better fuel, more detergent additives were often added to Premium. In the USA, basically the EPA/Congress has taken control of gasoline formulation, and we would have no detergent or other benefit for Premium for Prius, excepting an outside chance it could have more energy in certain regions less impacted by EPA formulation requirements.

 

I suspect burning that high-sulphur regular would result in stinky exhaust, which we endured in the US in converter-equipped cars, back when the sulphur content was higher.

 

there will always be some idiot trying to sell you on premium, or saying Chevron is the best and don't fill up elsewhere. don't listen to them, they failed high school chemistry. unless your engine runs a high enough compression ratio to require premium, there is zero benefit to burning it. anyone claiming otherwise is experiencing the observer expectancy effect.

 

Ditto
In the US, I suggest you only use regular gas on any vehicle that is not designed to run on higher octane fuel. Lots of reasons for this. Believe the manual may address this issue. As many have stated:
carbon build up is the biggy. Save your money and use regular. Once you have carbon build up it is not easy to remove without a rebuild.