International Octane Ratings

Well, I finally got around to learning about how octane ratings are measured. Found the following post in the Thailand Expat Forum at http://www.thaivisa.com/forum/index....0&#entry467488 . Credit must be given to the writer, Crossy. So far this is the "simplest" and easiest to understand write-up I found. I never really knew about RON and MON and PON and AKI etc....


Are octane ratings different country to country?

The short answer is YES, US octane rating is lower than the international octaine value, the long answer is here:-


The octane rating is the most important characteristic of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It is a measure of how resistant the fuel is to premature detonation (knocking). It is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane. A complementary standard for diesel fuel is the cetane number.

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.

There is another type of octane rating, called Motor Octane Number (MON), which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.

In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "normal" gasoline in the US and Canada, would be 91 in Europe.

It is possible for a fuel to have a RON greater than 100, because isooctane is not the most knock-resistant substance available. Racing fuels, straight ethanol, Avgas and LPG typically have octane ratings of 110 or significantly higher. Typical "octane booster" additives include tetra-ethyl lead and toluene.

Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction- since higher octane fuels have higher activation energies, it is less likely that a given compression ratio will cause knocking.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane lets an engine be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane gasoline. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption; this is false—engines perform best when using fuel with the octane rating they were designed for.

The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of the Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.


So now I know

[Earthworm]

You're right...that's a good explanation. I like the part about the higher octane not equating to more power/efficiency.

[Beav]

BTUs for a given quantity is a better rating of the 'power' available from a specific fuel. Now if someone would come up with a new rating that reflects BTUs and knock resistance....

[svxistentialist]

That all is highly informative.

Does anybody know if the Japanese octane system is equivalent to the European one?

I use octane booster in my petrol which typically brings it up 2 "points". So far as I know, we are mostly 95 here, and high test petrol is non-existant since lead became a four-letter word. So I expect I am running with 97, maybe 98, most of the time, and the car is tuned for 100 octane in Japan.

But is 100 in Japan the same as 100 in Europe? Or in Australia?

Anybody know anything about this?

Joe

[Beav]

I do know one thing for certain - I've been watching for changes in mpg on my car, 87 vs. 93. No mpg changes whatsoever, in town or interstate, with exception to the one tank I purchased in Gainesville mentioned elsewhere. I'm not going to say it's safe or not, but I have not experienced ping, knock or detonation (that should cover all the bases with the sticklers.) Besides, BIG FRIGGIN" WHOOP! Six whole points! Now if I was towing a trailer up a 7% grade in 110° heat...

[svxistentialist]

Originally Posted by Beav

I do know one thing for certain - I've been watching for changes in mpg on my car, 87 vs. 93. No mpg changes whatsoever, in town or interstate, with exception to the one tank I purchased in Gainesville mentioned elsewhere. I'm not going to say it's safe or not, but I have not experienced ping, knock or detonation (that should cover all the bases with the sticklers.) Besides, BIG FRIGGIN" WHOOP! Six whole points! Now if I was towing a trailer up a 7% grade in 110° heat...

Well, same as that, Beav. I used to buy super unleaded for my SVX until Arthur in England told me it was a waste of money. With the SVX, there is a detector on each bank of cylinders, the vast majority of cars make do with one. So I changed to using ordinary unleaded, 95 octane here. I check each tankful, brim to brim, and I watched the mpg for months afterwards.

If the car is any less efficient for using lower octane fuel, it is hard to see in the mpg figures. Worst case I could see is a possible 1 mpg change, and like Beav says, absolutely no evidence of ping, zing or ding.

However, there is a slight increase in power available, it is a good bit less sluggish with the good petrol. With ignition retardation, I imagine it could be up to 10% less powerful in certain situations. I've never measured it on a dyno or anything, just a seat of the pants feeling about how well the car is pulling.

However, with the twin turbo Legacy, the octane booster makes a radical difference. When the ECU pulls the timing back on that baby, it's very noticable. Fundamental difference between NA and forced induction, I suppose.

Joe