I've been meaning to post this revelation ever since I got back from last month's European assignment. Another one of life's mysteries unraveled.

You hear it all the time. It's often repeated on this very site. Somebody in Europe (or someone who has spent a lot of time there) will remind us "...sure, our gas costs $4.50 per gallon, but our LOWEST grade is 93 octane, and the premium stuff is 98...". Everybody knows that Europe practically has rocket fuel pouring out of every pump, and us poor Americans have to suffer through the usual 87-93 octane juice. Well, not exactly.

If you look at the fine print beneath the square yellow octane sticker (or somewhere close by) on practically any gas pump in the US, you will be informed that the octane rating is calculated by the "R+M/2" method. This simply means that the advertised octane in all parts of the US is the arithmetic average of the "research" and "motor" octane, which are two different, commonly-used test methods.

Our (distant) cousins "across the pond", of course, have to be contrary. I first noticed this while fueling up at an Elf station in France, and subsequently observed the same phenomenon across the whole of France and Germany. I will assume, for sake of discussion, the the same holds true throughout the rest of "the continent".

The Europeans sell their fuel (benzin, petrol, carburant, or whatever) based on just one of the measurements (either research or motor octane, I can't recall which). The relevant part of this is that it's the higher of the two measurements. In the small print under the big octane declaration, they actually do list both the research and motor octane. For example, the 93-octane "low-grade" stuff tested 93/83 by the two methods, equal to our 88-octane economy juice by the R+M/2 method. Their 98-octane super-premium sported 98/88 numbers, equal to 93-octane premium in the US.

So indeed, their gas is no better, can't be found in higher octane ratings, and still costs more than twice as much. Didn't those claims sound just a little bit "fishy" all along?

Now, if only people actually understood what "octane rating" really means. That's a topic for another post altogether.

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Silver Y2K Lightning
Bone Stock w/ Duraliner, Ford Hitch Cover

Silver (matching) Y2K ML320 Benz
Burl (ML430) Shift ****

Beater 1992 Grand Prix
Commuter Mule

 

I spoke to a family member in the gas-refining business and this is what he had to say:

"Essentially, the octane rating is the fuels resistance to pre-detonantion under compression. The greater the compression, the more heat is generated during the compression stroke, which in turn raises the temperature of the fuel-air mixture. Depending on the characteristics of the fuel, it may reach the auto-ignition temperature before the piston reaches full compression, and before the spark plug fires. This is the infamous knock (if severe) or ping (if just slightly premature).

The reference fuel is iso-octane, a highly branched, eight carbon molecule. It is has been designated as 100 octane (research number, which is the higher of the two). I'll refresh my brain cells tomorrow and give you a short discourse on how research and motor octane are determined. (They are done on a stationary engine - a "knock"- engine, at specific speeds. The timing is varied during the run until knock is detected.) I need to check my facts since I can't recall what the speeds and load conditions for RON and MON are, nor whether the actual measured octane is a ratio of the degrees off TDC for iso-octane and the test fuel."

[This message has been edited by Nathan (edited 09-25-2000).]

I also saw this link referenced in another thread. Its a good read. http://www.dynopower.freeserve.co.uk...reignition.htm

[This message has been edited by Nathan (edited 09-25-2000).]

 

Nathan:

Not sure what this has to do with the original post, but I suppose ANY discussion about fuel octane ratings is a good one, as this is a VERY misunderstood subject, on the balance.

Your definition of octane rating (resistance to detination/pre-ignition/knock/ping) is correct. This is the ONLY thing that is measured/delivered by the octane rating. If you're not knocking on your current fuel, you have nothing to gain by raising octane. Let me repeat that - if you're not knocking on your current fuel, you have nothing to gain by raising octane.

In fact, if you dig out some organic chemistry books and do a little research, you will find a general trend of DECREASING energy content (on both a mass and volume basis) for common organics as their octane rating (resistance to detonation) increases. Think about that one for a moment.

I got into this topic really heavily in my motorcycle dragging days. I had the delightful combination of a very fast, modded Suzuki GS1100E, a fresh master's degree in chemical engineering, and access to almost any organic solvent you can name in large quantities. I was dragging the bike very hard in hot conditions, and experiencing a lot of knocking. I eventually ended up running it on almost straight reagent-grade toluene, which despite its poor vaporization (forget cold starting without a can of ether) and lower energy content (can't get around that one), delivered a magnificent 131 octane (stack on the timing and compression!) and made the exhaust smell sweeter than Chanel Number 5.

Indeed research and motor octane are measured on standard test engines. The particulars of the methods reside in the hazy recesses of my mind, but I almost remember that the knocking was induced in test engines not by increasing ignition timing, but by the much more precise method of increasing compression (a threaded, adjustable cylinder head-to-piston clearance on a flathead engine). I could be wrong on that one.

The method involves running the fuel to be tested and setting up the test engine to to point where knocking "just" occurs. The fuel is then switched over to a variable mixture of iso-octane and some other (forgotten, but very poor knock-resistance) fuel. When the mixture is adjusted to the point where knocking "just" occurs (matching the "test" fuel), the octane rating is reported as the percent iso-octane in the mixture. For ratings above 100 (more knock-resistant than pure iso-octane), there is another fuel used (forgotten as well, but very high knock-resistance) for blending with the iso-octane.

Interesting that the research and motor octane results differ with the same fuel, despite (per my hazy recollection) using the same iso-octane-and-whatever blend for testing. This implies that sensitivity to knocking is a combination of fuel and engine - switching to a differnt fuel with the same octane rating may cure a knocking problem with a given engine, despite COMPARABLE knock resistance on standard test engines. I think that the experiences of folks on this site support that.

And the claims in the original post still stand - the Europeans use a different accounting system for reporting the octane rating of their fuels. Expressed by the R+M/2 method (as we do in the US/Canada), the European unleaded fuels are of comparable octane as the US (87-93 octane at most stations). They don't get "better" (more knock-resistant) fuel.

By the way, Nathan, we can post information like this all day, and there will still be a very loud, vocal majority that just can't accept that knock resistance is the ONLY benefit delivered by higher-octane fuels (although some refiners also add higher detergency to their "premium" fuels, also). They will swear to all sorts of incorrect, unsubstantiated things like "a hotter burn", "higher energy content", blah, blah, blah. I've preached the gospel in the past, and it's an impossible uphill fight.

If your Lightning doesn't knock on 87-octane econo-juice (fat chance), fill it up and go faster. Chew on that one. Why do you think that F-15's run on 50-octane kerosene? Knocking isn't a concern in a turbojet and lower-octane fuels, in general, have a higher energy content. You can believe that if it were the other way around, "Maverick" and his buddies would be filling up on Turbo Blue. It's not quite this simple (safety, vaporization, cost), but you get the point.