About Octane Numbers, Engine Performance, And More...

Some time ago I had a discussion with some friends about the virtues of premium, high-octane gasoline as opposed to regular, low-octane gasoline. Basically the majority opinion was that a car basically runs better on premium gas than on regular gas, since premium boosts engine performance. So, I decided to investigate this to see if it was true or not. Boy, did that open up a can of worms...

The following describes what I found, as well as some conclusions I drew based on common sense. It will turn out that no matter how much you search on the Internet, some questions still remain.

Note that I am by no means an expert on these matters. I'm not a mechanical or chemical engineer, nor am I particularly handy with cars. The following is based on information found on the Internet, supplemented with just a little bit of basic logic reasoning. Draw your own conclusions, and feel free to disagree...

Whassup With Those Octane Numbers?

Or, what does the octane rating of gasoline mean?

Many years ago I heard someone claim that the octane number of gas simple meant the octane percentage. That is simply not true.

The octane number of a fuel indicates its resistance against self-ignition when put under pressure. Two hydrocarbons are used as a reference: iso-octane (or more precisely: 2,2,4 trimethyl pentane), and heptane, also called n-heptane. The chemical formulas are C8H18 and C7H16, respectively. Note that in the case of octane, we are talking about the branched isomer, and not the linear molecule.

The exact chemical structure of these hydrocarbons is not all that important for this discussion. What matters is the fact that the two have rather different properties when it comes to self-ignition. If you heat up the vapor of these hydrocarbons enough, and put it under sufficient pressure (as in an internal combustion engine), it will eventually ignite (or burn) by itself. Octane has a high resistance to self-ignition, heptane has a low resitance to self-ignition.

We have to discuss this self-ignition stuff at this point. When gasoline is burnt inside an internal combustion engine, the air-fuel mixture should ignite when the spark plug produces a spark, and not any sooner. If the air-fuel mixture ignites too soon, very bad things happen. Temperatures inside the cilinder spike, and the piston and valves will be damaged severely. This phenomenon is called knocking, pinging (after the sound that it produces), pinking, or pre-ignition.

Apparently there is a difference between knocking and pre-ignition. As far as I can determine, knocking occurs when combustion of the air-fuel mixture starts at more than one point in the combustion chamber. Pre-ignition seems to describe the situation when the air-fuel mixture ignites even before the spark plug produces a spark. Pre-ignition is even worse than knocking, apparently. For the sake of this discussion, we will not differentiate between the two phenomena, since the causes and remedies are basically the same. From this point on, we will only talk about knocking.

Back to the octane number. As we mentioned, octane has a high resistance agains knocking, and heptane has a low resistance agains knocking. A 50/50 mix of octane and heptane would have a moderate resistance against knocking. This is where the octane number comes into the picture. To determine the octane number of a fuel, it is burnt in a special (1-cilinder) reference engine, of which the operational parameters can be varied (most importantly, the compression ratio). The compression ratio of the engine is increased until the engine starts knocking. Then, it is determined what mixture of octane and heptane will behave the same with regard to knocking as our fuel under test. To give an example: say our fuel has the same knocking behavior as a mixture of 87% octane and 13% heptane. Then our fuel has an octane number of 87. Simple, huh?

Yeah, right. If only things were that simple. First of all, if this method works under all circumstances, then how can you have fuel that has an octane rating higher than 100? Fuels with these octane ratings of 100+ are actually used in racing and in the world of aviation. Obviously, the octane number of these fuels cannot have been determined with our reference engine. The short answer is that these ratings are determined by extrapolation. See this link for a simple description.

The next confusing thing is that octane ratings in the USA and Europe differ significantly. In the USA, you can typically buy regular (87 octane), medium (89 octane), and premium (91-94 octane). The octane ratings differ slightly in high altitude states like Colorado. In Europe, you can typically buy 95 octane fuel and 98 octane fuel. So why do European cars need this high-octane fuel?

Well, they don't.

The story about octane measurement was not quite complete. There are two different sets of conditions under which the octane number is determined. The first set of conditions simulates low load conditions, sort of corresponding to everyday, regular driving conditions. The number that results is called the Research Octane Number, or RON. The second set of conditions simulates a higher load on the engine. The number that results is the Motor Octane Number, or MON.

The choice of these names is terribly unfortunate, in my humble opinion. Research Octane Number suggests a lab test, whereas Motor Octane Number makes you think that they did some road test. Some websites actually state that this is the case. As far as I can determine, both are laboratory tests.

It turns out that in Europe, the octane number posted is determined by the RON method (if you live in Europe, check the small print on the gas pump next time you fill 'er up. Wait, filling it up requires taking out a loan nowadays...). In the USA, the number posted is according to the "(R+M)/2" method. Turns out that this is simply the average of the RON number and the MON number. So, would the MON number typically be lower than the RON number? The answer is....yes! Apparently, for modern fuels, the difference between RON and MON (also called sensitivity) is about 10 (RON is higher than MON). Knowing this, we can now calculate the following:

87 octane in the USA = 92 octane in Europe
89 octane in the USA= 94 octane in Europe
92 octane in the USA = 97 octane in Europe

Go figure, huh? It seems these regular and premium fuels have roughly the same octane rating after all.

So, why is the (R+M)/2 method used in the USA? Well, in addition to RON and MON, there is also something called RdON, or Road Octane Number. This number is apparently determined with actual test vehicles (also known as "cars"). I won't make this any more confusing than necessary, but apparently (R+M)/2 is a good approximation for the RdON rating. The (R+M)/2 number is also known as the Anti-Knock Index, or AKI.

Something I haven't quite figured out yet: the fact that the MON is lower than the RON seems quite obvious, given the fact that the test engine is run under a higher load. So, higher compression ratio and temperature would logically result in a higher sensitivity to knocking. This all seems perfectly logical, until you start thinking about it. The knocking sensitivity is compared to a given octane/heptane mixture! So, the fact that the MON is lower than the RON implies that an octane/heptane mixture has a higher resistance to knocking under high load conditions than a typical gasoline. Hmmm...I wonder why that would be...
I can't help but wonder if I'm missing something here.

The main point of all this is that the octane number of a fuel is a measure of its resistance to knocking. It does not necessarily indicate anything about the performance of the engine. Or does it? We will explore this in excruciating detail in the next part.

Next: Octane Number and Engine Performance

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