octane related question

[willyjp]

with several threads lately touching on "octane" rating of fuels, it's got me thinking:

on the commonly seen posted octane ratings (R+M/2), does anyone know if that is BEFORE or AFTER the addition of 10% EtOH???

the reason I ask this is, as is commonly stated, I know Ethanol increases resistance to knock/detonate (or effective octane),...

and I've read it stated that E85 has a "much higher octane than what they label it at" (so it must have been rated on the 15% gas before the 85% EtOH was added??)..or is that of the EtOH?...what IS the octane rating of 100% EtOH???...

and I've noticed that at my local pump (92 octane-highest available in this area), they put on the 10% Ethanol label during the fall/winter season but they don't change the octane rating label...

So: what's going on with Ethanol and Octane? Is the fuel we're buying potentially of higher octane than labelled, because of the added EtOH?, or do they "adjust" the gasoline before the EtOH is blended, so that it comes out the same in the end? (I have heard that ethanol blending is done after refining and often in regional fuel distribution points on the way to your local pump...suggesting possibly a potential lack of any "hi-tech" or precision in this step perhaps?)

it would be interesting to be able to get octane testing on fuel samples at points-of-sale to see if this octane thing isn't really a big bunch of smoke and mirrors and marketing B.S.!!

[desperado]

Perhaps with the VCDS, you could simply run the different fuels in your R8 and then see what the car is doing to the air-fuel and timing settings?

(PS, I think this is possible, but haven't actually done it so I can't give you the locations to look at in the ECU.)

[willyjp]

Perhaps with the VCDS, you could simply run the different fuels in your R8 and then see what the car is doing to the air-fuel and timing settings?

(PS, I think this is possible, but haven't actually done it so I can't give you the locations to look at in the ECU.)

would think that's a definite possibility...in theory; but you'd have to be careful to use a standardized drive profile, and to be sure ambient conditions were constant (temp, humidity, barometric pressure, etc.), because all of those things can impact all the adjustable variables the engine can use to compensate for octane and ambient conditions

seems to me I've read that the EPA mileage "numbers" are generated in actual, real sample cars run in a testing lab by a computer, where they can control all those variables (the infamous "EPA Test Cycle")---

The onboard systems probably have built in "prep" for that, just like they do for your state emissions testing (if you have it); in Oregon, they NO LONGER test your exhaust, just plug their computer into your car's data socket and ask your car if it's performing up to standard!! (guess the 5th Amendment to the Constitution doesn't apply to cars!!)

[marcuslf]

Octane is a pretty difficult topic. I think many people widely misunderstand how it applies to a car's performance. All that it really represents is the fuel's resistance to detonation. Higher octane is harder to ignite. It really is not necessarily related to the energy content of the fuel.

In general, all things equal, lower octane will provide more energy/power/economy, as long as the timing doesn't have to be pulled to prevent detonation. In modern cars, with all of the computerized detection and control,it's complicated as to which octane is best.

In a run of the mill car, running "premium" fuel will actually hurt power and economy. There's no benefit, because the ecu probably will not take advantage of the ability to increase ignition timing.

E85 has a very high octane rating, but around 20-30% less energy content than gas. So, people running it in flex fuel vehicles are getting maybe a 5% per gallon savings, but much lower mileage. There's no economic benefit, it actually costs more to run. If the subsidies go away, so will the minimal per-gallon discount.

In the real world, engines are generally designed with certain assumptions. The octane you need to run is at least as high as that, but no higher.

As to whether the gas companies octane ratings take into account ethanol additives or not, I don't know. My assumption is that a pump that normally gives 91 octane in the summer, probably has a higher octane rating in the winter - but will give less power and less mileage due to the decreased energy content the fuel now has.

[willyjp]

Octane is a pretty difficult topic. I think many people widely misunderstand how it applies to a car's performance. All that it really represents is the fuel's resistance to detonation. Higher octane is harder to ignite. It really is not necessarily related to the energy content of the fuel.

In general, all things equal, lower octane will provide more energy/power/economy, as long as the timing doesn't have to be pulled to prevent detonation. In modern cars, with all of the computerized detection and control,it's complicated as to which octane is best.

In a run of the mill car, running "premium" fuel will actually hurt power and economy. There's no benefit, because the ecu probably will not take advantage of the ability to increase ignition timing.

E85 has a very high octane rating, but around 20-30% less energy content than gas. So, people running it in flex fuel vehicles are getting maybe a 5% per gallon savings, but much lower mileage. There's no economic benefit, it actually costs more to run. If the subsidies go away, so will the minimal per-gallon discount.

In the real world, engines are generally designed with certain assumptions. The octane you need to run is at least as high as that, but no higher.

As to whether the gas companies octane ratings take into account ethanol additives or not, I don't know. My assumption is that a pump that normally gives 91 octane in the summer, probably has a higher octane rating in the winter - but will give less power and less mileage due to the decreased energy content the fuel now has.

No disagreement with MOST of that (not sure how higher octane rating than you need can actually "hurt" your power), but in the R8 forum, we have a group of people who are forced (in the US) to run fuel that, at "best available" is at the lower margin of the "certain assumptions" made when the car was designed.

In California (and a LOT of Oregon, because we share their petro dist. system to some extent), 91 octane (R+M/2) is the HIGHEST available, and under some ambient conditions (or if you're running aftermarket s/c) that is NOT going to be "all that's required." Also, as has been amply stated in other current threads, the high altitude West only gets premium fuel rated 89....which may be OK in Denver, but by the time you get to Kansas City it may not!

I'm aware that in Europe, octane ratings are given using "R" numbers, where our's are R+M/2 and therefore (usually) somewhat lower on the same fuel; but I don't believe most of the US premium would have an R number equal to the 97 commonly available in Europe....and, Thanks Be, Audi did NOT choose to "detune" the R8 for the US. I've no inside knowledge, but I have "heard" it said in forums that the ECM programming is not substantially different and that European "chips" work in the US (maybe our chip vendors could enlighten us on that).

So, in light of all that, I think it would be VERY interesting to know just exactly what effect Ethanol is having on the octane rating of our fuels, and whether the labelled ratings are accurate, conservative or over stated. With careful looking, it IS possible to still get EtOH free premium, even in winter. But maybe we should be GLAD to be getting it. I just don't know.

Given that this is a forum for owners of officially labelled (and taxed) "gas guzzlers", the fuel economy issue was the furthest thing from my mind in raising this issue. Not that it isn't important, but if you're driving an R8, it's probably WAY down your list of concerns when it comes to fuel choice!

[desperado]

There is a nice discussion of octane in chapter six and seven of What is Gasoline?.

From the same source:
Dynamics of Normal Combustion
Detonation and Preignition Theory

[Rit2]

Perhaps with the VCDS, you could simply run the different fuels in your R8 and then see what the car is doing to the air-fuel and timing settings?

(PS, I think this is possible, but haven't actually done it so I can't give you the locations to look at in the ECU.)

Desperado is right. You can view the ECU performance for the various fuel qualities using VCDS by going to:
ENGINE, MEASURING BLOCKS, then, enter 020 for first GROUP, 004 for the second GROUP.
You will see the Time Stamp, Timing Retardation, Engine Speed, Battery Voltage, Temperatures being displayed. ECU performance can be data-logged by recording the ECU performance during runs with gears at 3rd and 4th, and engine speeds between 2000-6000 RPM. It's good to do a Throttle-body Adaptation for each fuel quality.

[willyjp]

But don't you agree that examining the ECU response to various fuel qualities could only be interpreted meaningfully IF you have compared them with the SAME operation profile (ambient conditions, load, throttle opening, etc.)...in other words if you gave each fuel sample the IDENTICAL DRIVE...which is just about impossible in actual practice, unless you're talking about a chassis dyno run.

Sure, you could get some hints about it from just "driving around", but you'd have to wonder if you weren't fooling yourself, I would think.

[Rit2]

@willyjp,
The R8's ECU is very fuel sensitive. Fuel octane affects the timing, which affects performance. In a generally stable environment, if you are testing 92 vs 93 those differences may not be as appreciable compared to you testing 91 vs 93. Regardless, with the data from the VAG-COM, you need to know what you are looking for/at. I appreciate your concerns To avoid getting very technical on fuel-blending (where more questions will just pop-up), just adhere to "highest fuel octane from a top-tier fuel company. This will avoid timing retardation and enables the ECU to run higher timing values, thus achieving the best possible performance for you car for that condition of the day". So Ethanol? Toluene? %? ... and after all this, it all eventually comes down to that driving skill

[willyjp]

@willyjp,
The R8's ECU is very fuel sensitive. Fuel octane affects the timing, which affects performance. In a generally stable environment, if you are testing 92 vs 93 those differences may not be as appreciable compared to you testing 91 vs 93. Regardless, with the data from the VAG-COM, you need to know what you are looking for/at. I appreciate your concerns To avoid getting very technical on fuel-blending (where more questions will just pop-up), just adhere to "highest fuel octane from a top-tier fuel company. This will avoid timing retardation and enables the ECU to run higher timing values, thus achieving the best possible performance for you car for that condition of the day". So Ethanol? Toluene? %? ... and after all this, it all eventually comes down to that driving skill

I accept all that as a "given." Bottom line (and reason I posted this thread) is:

locally, I can get Chevron premium labelled 92 octane, from the same distributor (comes via tanker from So. Calif.) WITH or WITHOUT 10% Ethanol. So, if you didn't have a VAG-COM or necessarily the desire to prowl around in there, which fuel would YOU choose?? Maybe I will have to resort to "reading my ECU's entrails" to answer this, I'd just be afraid that comparing 2 spells of "just driving around" wouldn't give me data I could rely on.