If you've gone to the gas station, you've undoubtedly seen that most pumps offer three grades of fuel, "regular", "mid-grade" and "Premium," with octane ratings of 81, 86 and 90 octane or thereabouts. You may also have heard people claim that 'premium" fuel "has more power." That's not exactly the case, so don't be fooled into buying the "high priced spread" unnecessarily.
The exact chemistry of octane numbers gets confusing, but what they boil down to is a rating of how likely a particular blend of gasoline is to "knocking." This phenomenon, also sometimes called "ping" is most correctly called "pre-ignition." It's what happens when the gasoline-air mixture in the engine ignites before the spark plug fires.
Here's what happens: When the piston comes to the top of the compression stroke, the act of compressing the mixture to a fraction of its original volume (say, 1/9th, in a typical engine) generates a good deal of heat. Under normal circumstances, the mixture will wait until the spark plug fires to ignite, then the expanding gases push the piston down for the power stroke. Sometimes, though, if the compression is too high or the engine is running too hot, something else starts the fire -- it may be the spark plug itself over heating, some other projection in the cylinder head glowing, or maybe even a carbon build-up on the piston itself. This effect is also called "dieseling" for good reason; diesel engines rely exclusively on compression ignition to fire their fuel-air mixture.
Whatever causes the pre-ignition, the effect is the same. The flame starts in one place, then the spark plug fires, causing the mixture to ignite in a second place. When the two flame fronts (the leading edge of the expanding fireball) meet, they interfere with each other, causing an audible "ping" or "knock" sound. Light pinging is annoying. Strong pinging, or a pre-ignition that is allowed to continue for some time can damage the engine.
What is happening here is the battle of the immovable object -- the rising piston, which is being driven by the other pistons -- versus the irresistible force; namely the expanding gases of the burning fuel-air mixture. When this happens, something has to give. Most of the time, the piston wins out through sheer force. Sometimes, the piston loses when the burning mixture is so strong it breaks a hole through the top of the piston.
So what's an "octane rating?" It is a number grading a fuel blend's resistance to compression ignition or "ping" compared with Isooctane (2,2,4-trimethylpentane), which has a rating of 100, and Heptane, another petroleum distillate that burns so readily, it has an octane rating of almost zero. Most gasoline blends are somewhere in between, with some high performance fuels rating slightly higher.
These ratings are found using a rather unusual test engine. It runs relatively slowly -- tests are conducted at 600 rpm and 900 rpm. What makes it so odd is that it has a variable compression ratio; its stroke can be lengthened while the engine is running. The test starts with a short stroke, and thus a low compression ratio. Then the stroke is slowly increased until the compression ratio starts pre-igniting the fuel. The result is then compared with the known characteristics of Isooctane and the rating is given as a percentage of the isooctane mixture. Thus, and this is only an example, if isooctane pre-ignites at a 10 to 1 compression ratio and the sample being tested starts pinging at an 8.5 to 1 ratio, it is given an "85 octane" rating.
To be fair, these tests are performed several times, and at two different rpm speeds. The lower number, at a 600 rpm "idle speed" is called the "research octane number," or "RON." Then the engine is sped up to 900 rpm, giving a "Motor Octane Number," or "MON." The octane rating you see on the pump is an average of the two. The higher this average, the more resistant the fuel is to pre-ignition. Click here for an article on About.Com that explains in greater detail about octane ratings and how they are determined.
Now to dispel the "higher octane is more powerful fuel" myth. Gasoline is gasoline; it produces a set amount of power, no matter how high the octane rating is. So why the high-grade stuff? Because not all engines have the same compression ratio.
A high performance engine has a high compression ratio -- some racing engines go as high as 12 to 15 to one. Even some street engines touch on 10-to-1 or more. The advantage of that is, the higher the compression ratio, the more space the burning fuel-air mixture has to expand and thus the more power that can be used before the piston hits the bottom of its stroke. For reasons of longevity, most car engines have a compression ratio of between 8-to-1 and 9-to-one. That is a compromise between power (from high compression) and longevity, since low compression ratios don't put as much strain on the engine.
Complicating the picture even more is that some cars have superchargers or turbochargers. These pumps force more air into the cylinder and thus give it more power. In the process, they also effectively raise the compression ratio, even though the cylinder head and piston clearances remain the same. In the days when Indy cars used turbochargers, the basic compression ratio was low -- sometimes as low as 6-to-1. But as the turbocharger spun faster and pumped more air into the cylinders, the effective compression ration could go as high as 16-to-1, which is a huge change. The worst part is, at low boost pressure (when little extra mixture is pushed into the cylinder) the engine behaves like a low compression engine, which it is. But as the boost pressure increases, the extra air makes it more likely the fuel-air mixture will light off on its own. To prevent that, you need to feed your turbocharged engine higher octane fuel.
Here's another thing to consider. Years ago, when engines had carburettors and the spark timing was set by a mechanic, "ping" was a problem. An engine might work fine in some conditions, then knock at other times. To combat this, auto makers developed electronic engine controls. These computers have sensors that read exhaust gas chemistry, fuel use, the amount of air going into the engine, etc. They also have "knock sensors" that can detect the earliest hint of "ping" before a human ear can hear it.
When the computer detects "ping" starting, it will adjust fuel mixture and spark timing to compensate. This is what the mechanic used to do when he tuned-up your car and adjusted the settings, but now the computer does it automatically, several times a second. So even if your car is running a lower octane gasoline than the owner's manual suggests for "optimum performance," the computer will save your engine; albeit with some loss of power.
So there you have it. A simple explanation of what octane is, why it's important and how you can probably save a few cents a gallon at your next fill-up. One caution, though: Read your owner's manual and buy the grade of gasoline recommended. This is especially important if your car has a supercharger, turbocharger or high compression engine. The engine management computer can help prevent "ping," but it can't do miracles. So be careful, keep your engine in good condition and it will live awhile longer.