The octane number assigned to a motor fuel has very little to do with the actual chemical "octanes" in the fuel and everything to do with how well the fuel resists detonation (which is directly related to the amount of energy (heat) required to get the fuel burning in the first place).
Therefore, it is possible to assign octane VALUES to fuel which contain no octanes whatsoever.
The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, preignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases under controlled test conditions.
What is preignition?
What is detonation?
There are six things to consider when comparing hydrocarbon fuels:
The first three factors are often confused and interrelated when, in fact, they measure three completely separate things. There is no natural collelation between them.
Note that gasoline, partially, makes up for its (relatively) low energy-per gallon by the fact that a gallon of gasoline weighs less (by about 15%) than a gallon of jet fuel.
Octane rating is in no way correlated with engine power or efficiency. There is more potential energy in a gallon of diesel fuel than a gallon of gasoline, yet the diesel fuel has a much lower octane value (more on that below).
Ok, then, how is octane rating determined? First, you go out and get a suitable supply of the fuel which you wish to test. Then, you get yourself some heptane (made from pine sap) and some iso-octane (a petroleum derivative). Finally, you and your buddies arbitrarily, agree that iso-octane has an octane rating of 100 while heptane has an octane rating of 0.
Next, you call up Waukesha Motors and order yourself an ASTM-CFR test engine. Make sure you have about $250,000 available on your VISA before you order it. This single-cylinder wonder has a four bowl carburetor and a movable cylinder head that can vary the compression ratio between 4:1 to 18:1 while the engine is running.
You fill the ASTM-CFR full of your mystery fuel and, for automotive fuels, you run two test protocols using the ASTM. One protocol is called the motor protocol and the other the research protocol. You vary the compression ratio until the onset of knock and write down all kinds of various scientific parameters.
Next, you run your reference fuel, made up of various proportions of heptane and iso-octane through the ASTM-CFR. You keep varying the proportion of heptane to iso-octane until you get a fuel that behaves just like (knock-wise) your mystery fuel. Once you get that, you say to yourself "How much heptane did I have to add to the iso-octane to get the mixture to knock in the ASTM-CFR just like my mystery fuel?" If the answer is, say, 10% heptane to 90% iso-octane, your mystery fuel has an octane number of 90.
How do the motor and research protocol differ? Mostly in input parameters. In the motor protocol (ASTM D2700-92), the input air temp is maintained at 38C, the ignition timing varies with compression ratio between 14 and 26 degrees BTDC, and the motor is run at 900 RPM. In the research protocol (ASTM D2699-92) the input air temperature varies between 20C and 52C (depending on barometric pressure), timing is fixed at 13 degrees BTDC, and the motor is run at 600RPM.
The motor method, developed in the 1920s, was the first octane rating method devised. After its introduction, many more methods were introduced. During the 1940s through the 1960s one of those methods, the research method, was found to more closely correlate with the fuels and vehicles then available. However, in the early 1970s automobiles running on high-speed roads, such as the German Autobahn, started destroying themselves from high-speed knock. It was found that the difference in ratings between the research and motor method, known as the fuel's sensitivity was important as well. The greater the fuel's sensitivity, the worse it performed from a knock point of view in demanding, real-world, applications.
Remember, at the pumps, the results of the motor and research numbers are averaged together to get the value you see. The fuel's sensitivity is not published. Highly cracked fuels have high sensitivity while paraffinic fuels often show near zero difference between the two. While the fuel's sensitivity is not published at the pump it can be a valuable indicator as to the fuel's real world octane performance. Remember, the octane tests are conducted in a lab using a special test engine; the lower the fuel's sensitivity, the more likely it is that the fuel will, indeed, behave as expected. Generally, the closer the fuel's research rating to the published rating the more reliable the published rating. Because the motor and research methods primarily differ in terms of input parameters (the test engine is the same for both), the greater difference that a fuel exhibits between its motor and research test will be due to differences in input parameters (intake temp, timing, etc.). A fuel that has an octane rating that varies with intake parameters is said to be more "sensitive."
The octane of aviation fuel is not measured in exactly the same was as is automobile fuel.
Once again, you start with your trusty ASTM-CFR engine. First you set up the ASTM-CFR for the motor method and use that method to determine the motor rating of your fuel. You then correct that rating to the "Aviation Lean" rating using a conversion table. Below about 110 motor octane (a performance number of 110), the aviation lean and motor octane numbers will differ by only about 1 or 2 points. Above 110 motor octane the differences can be significant. Next you pull out another version of the ASTM-CFR engine. This one has a fixed compression ratio but allows you to supercharge the intake manifold. You pressurize the intake to higher and higher values until the onset of knock. Other than that, the parameters are the same as for the motor method used for automobiles. The supercharge method is then used to assign the Aviation Rich value of the fuel. Supposedly the pressurization method (as opposed to changing compression ratios) is a throwback to the 1950s and 60s when supercharging was common in aircraft engines. The engineers were particularly concerned with the fuel's behavior under boost.
Because of the different ways in which automotive and aviation gasoline octane is measured one must be very careful when comparing absolute numbers. 100 octane avgas is not equal to 100 octane autogas (but it's close!). Note that the lean number rating of an aviation engine will correspond very closely to its autogas (mogas) motor rating requirement (see the above paragraph). Thus when shopping for autogas for your 91/96 O-360, you should look for a filling station at which the motor octane rating of the fuel is at least 91.
You should also be careful leaning the engine as this may cause its octane requirements to go above what the autofuel can provide. Look for an autofuel with an octane number as far above the lower aviation octane number as you can. If you can get one which is at or above the rich octane requirement (the higher number) then you should be a-OK.
Thus an engine rated for 80/87 aviation should have no trouble whatsoever running on 89 octane (or higher) unleaded. Engines rated for 91/96 should run on at least 91 (motor) octane unleaded but note that this is lower than the rich limit requirements (96) of the engine. Therefore it is especially critical to limit leaning with such an engine/fuel combo when running at high power settings.
Often it's done by pure extrapolation. A more reliable method, however, is through the use of so-called performance numbers. Briefly, these are arrived at by determining the instantaneous mean effective cylinder pressure (IMEP), using the fuel under test, at the highest boost that does not cause knocking. This number is then multiplied by 100 and the resultant is divided by the IMEP at the highest boost that does not cause knocking on the 100 octane equivalent fuel.
Note that, technically, there is no such thing as an octane number above 100. If you're at a party, avoid saying things like "110 octane gasoline" because people will get up and walk away from you. You should say, instead, "a gasoline with a performance number of 110." That will bring the help scurrying over with more champagne.
Tetraethyl lead raises the octane rating of a fuel not because it adds more "octanes" to the fuel but because it makes the fuel knock at a higher compression ratio in the ASTM-CFR. According to the latest research, octane ratings go down with fuels comprised of long, straight, hydrocarbon chains (paraffinic fuels). Fuels with branching hydrocarbon fuels, and aromatic fuels, have a higher octane ratings.
Oxygenates and alkyl lead affect the pre-flame reaction pathways by retarding branching sequences. Lead was previously believed (by many, including myself) to work by slowing the flame front, thus leading to a slower pressure rise in the cylinder. While general flame-front propagation speed does affect octane ratings, lead does not significantly affect it.
Combustion chamber design, localized hot spots, piston speed, and a host of other factors can all contribute to a engine's propensity to ping.
Additionally, in the aviation world, altitude extremes and super/turbo charging affect octane requirements. Increased induction pressures (such as would be encountered in a turbo/supercharged engine) cause more rapid flame-front propagation. Likewise, decreased exhaust pressure (as would occur at altitude) also tends to increase flame-front propagation speed. Both of these effects can combine to raise octane requirements - especially at altitude.
Note that the latter effect also affects the proper fuel/air ratio for BEST ECONOMY operation. The reason is left as an exercise for the reader.
Diesel and Jet fuel (along with kerosene) have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.
Diesel fuel is rated by its cetane number which is determined, like octane, by running the fuel in a test engine. Instead of heptane and iso-octane they use napthalene (cetane rating = 0) and n-cetane (cetane rating = 100). In total opposite to octane ratings, the higher the cetane rating the higher the fuel's propensity to knock!
Just as using a fuel with an octane number higher than necessary in a gas engine will gain you noting, using a fuel with a cetane number higher than necessary in a diesel engine gets you nothing. On the other hand, where using a fuel with too low an octane number in a gas engine will result in a damaged engine, using a fuel with too low a cetane number of a diesel engine will just result in a rough-running (or not running at all) engine with no damage.
Why can diesel engines tolerate a low octane fuel? In all gasoline engines, (including injected gasoline engines!) the fuel/air mixture is present in the cylinder the entire time the piston is travelling upward on its compression stroke. This means it could be ignited at any time whereas we only want it to be ignited when the spark plug fires, some time just before the very top of the stroke. Furthermore, we want a nice, even, steady, pressure rise in the cylinder as a result of ignition. This means that we want the flame-front to travel linearly from the source of ignition (the sparkplug) to the other side. We do not want combustion to occur randomly within the mixure as that may cause a too-rapid pressure rise which will throw off all our calculations about where the piston should be and when.
First, In a diesel engine there is NO fuel in the combustion chamber as the piston starts up on its compression stroke. Instead, fuel is injected at high pressure (up to 3000PSI!) into the combustion chamber at the exact moment when ignition is desired. In a diesel engine with a compression ratio of around 20:1 (compared to 7:1 for many modern gas engines), the heat of compression will have raised the combustion chamber temperature to arond 1000-1500F. The injection time takes about .002-.004 seconds during which the fuel spontanously ignites from the heat of compression at just the right time. Even so, a diesel fuel with too low a cetane rating may not ignite, or may ignite poorly - especially on cold days starting a cold engine.
The second critical difference is that Diesels are set up to burn the fuel in a slightly different way.
In a gas engine, you typically set it up so that the mixture is ignited before the piston hits the top of the stroke. What you're aiming for is for the mixture to be fully burned around the top of the stroke - thus combustion pressures are maximized at the top of the stroke and gradually fall off as the piston moves downward on the power stroke (and increases the volume in the cylinder). Diesels, on the other hand, are set up to inject fuel very close to the top of the compression stroke. The fuel spontenously ignites (auto-ignition) and, actually, knocks just like it does in does in a gasoline engine (hence the classic diesel "knocking"). The combustion pressures in the diesel increase evenly as the piston goes down. The net result is that the diesel piston "feels" a constant pressure on it as the piston travels from top dead center to bottom dead center whereas a normally operating gasoline engine piston "feels" a constantly decreasing pressure as it travels to the bottom of the stroke. The net result is that the diesel feels a lot lower PEAK pressure while the pressure is maintained over a longer period. The gasoline engine feels a much higher peak pressure which starts to fall off immediately as the piston travels downward. The implication, for the latter, is that it periodically operates very close to the capabilities of the base metals. Anything, such as knocking, which increases those peak pressures even more is apt to push beyond the capabilities of the base metals and result in engine damage.
Knock in a gasoline engine tends to occur at the end of combustion, when pressures inside the cylinder have reached, as a result of spark ignition, very high values - values high enough to auto-ignite the fuel.
Knock in a diesel engine happens at the beginning of combustion as a direct result of piston compression only. It is what allows further combustion as the piston moves downward. This continued combustion keeps the cylinder pressure constant as the piston moves towards BDC.
In the absolute worst case, if the fuel is too low octane, it may spontaneously ignite before the spark plug fires due to thermal rises from the heat of compression or from hot spots in the cylinder itself. This kind of ignition is called pre-ignition (as opposed to knocking) and is a pathological case which will just turn an engine to scrap. Diesel fuel is low enough octane that mixing it with gasoline can cause pre-ignition!
What usually happens, and what we usually call knocking or pinging is that the fuel/air mixture does not ignite before the spark plug fires but does ignite spontaneously after that. The sparkplug fires and this causes an immediate, rapid, rise in combustion chamber pressure. This causes fuel on the other side of the flame-front to ignite before the flame-front reaches it. In turn, this causes combustion chamber pressure to rise even more rapidly. The result is an explosion inside the combustion chamber as opposed to the desired rapid burning.
A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel (such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the engine (more below). The higher the compression ratio, the higher the octane of the fuel that's needed.
Again, if the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence. To reiterate, what we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug).
Now, back to aircraft. We want to make aircraft engines with the following characteristics:
The easiest way to do this, without involving lots of complex machinery that might fail and add weight, is to raise the compression ratio of the engine. An engine's efficiency is in direct proportion to its compression ratio. Unfortunately, raising the compression ratio means we need to protect against knock/detonation. How do we do this? We use high (100 octane) fuel!