Octane ratings: Where did they come from, what do they mean, and why are there so many?

Knocking in spark ignition engines has been around for a long time. I believe that the Wright brothers’ first flight was run almost exclusively on pre-ignition.

As the performance of engines increased, the need for better fuels became apparent. But the question then arose: How to test for anti-knocking characteristics?

First of all, what is knocking and why is it a problem?

Knocking is caused when the air/fuel mixture is ignited at an improper time in the engine’s cycle. It is named after the sound that is heard when the fuel mixture pre-ignites or detonates. The associated rapid combustion leads to pressure oscillations and an increase in heat flux that can cause overheating and the failure of components.

The American Society for Testing and Materials (ASTM), which was organized in 1898, arranged for the Cooperative Research Council (CRC) to develop a test to measure the anti-knocking characteristics of fuel. In 1928, the CRC developed a test using the Cooperative Fuels Research (CFR) engine.

This was a single cylinder engine on which different heads and cylinders could be fitted. The knock test engine was equipped with a separate head and cylinder that could be raised or lowered relative to the crankshaft center line. This would change the compression ratio on the fly. Knock was measured in the running engine with an accelerometer mounted on the head.

The engine had four float bowls on the carburetor and the test fuel was placed in one. After a run on that fuel, the head and cylinder would be lowered until a specified knocking level was reached. A chart would then indicate what rating that head height would relate to.

If the reading was about 90, they would then run a 90% isooctane, 10% normal heptane fuel to confirm the reading. By definition isooctane had a rating of 100 and n-heptane had a rating of 0. This would then be rated as a 90-octane fuel. This is where the term octane number comes from.

The first test procedure was called the Research Octane Number test (RON).

But the engineers could not leave well enough alone and kept trying to correlate the Research Octane Number to ratings in real cars and airplanes and found a poor correlation. So they modified the test procedure by raising the test RPM and intake temperature and this became the Motor Octane Number (MON), which also correlated to the aviation lean rating.

This related much better to real world engines in use, so they wanted to replace RON with MON.

But because the MON was usually eight to 10 numbers lower than the RON, the fuel sales people would not allow it.

The difference between the RON and the MON is called the fuels sensitivity. Most typical fuels have a sensitivity of around eight numbers. But occasionally there will be a fuel with a 10 to 12 sensitivity, which does not perform as well in the field.

After years of debate, they now list the R+M/2, which is the average of the RON and MON ratings.

The aviation world adopted the lean rating, which is close to the Motor method, so they developed a chart that converts the MON to a lean rating.

With the advent of supercharged aircraft engines, they found that the lean rating did not correlate well with the anti-knock performance needed in some applications. They then developed the rich rating, which uses the same base CFR engine, but they supercharge it with an outside air compressor.

Therefore, the spec for all aviation gasoline listed both the MON related rating and the rich rating like 100/130.

Historically, there have been a number of different octane aviation gasolines. In recent years there was 80/87, 100/130, and even 115/145.

Then in the 1970s, the volume of avgas dropped so low because of increased use of jet engines that oil companies decided to offer only one grade of fuel.

The fuel chosen was 100/130 with a two gram per gallon limit on lead. Although referred to as 100LL, the spec was still 100/130LL, but that was too long and confusing, so it was just called 100LL.

Back when refineries produced 100/130 high lead, they would take the aviation alkylate and add tetra-ethyl-lead (TEL) until the blend met the lean rating spec. They would then test for the rich rating and it would almost always be above the 130 mark.

When blending 100LL they would add two grams of lead and measure the lean and rich rating. The rich rating was usually below the 130 level so they would add toluene concentrate to bring it up to the 130 mark. They would then measure the MON and it would usually be above the 100 level.

So when the industry went to just one fuel, 100LL, the MON was almost always higher and the rich rating was usually lower than the 100/130 high lead fuel that it replaced.

The result was that the number of knock complaints from the field increased very sharply — not as sharply as the spark plug fouling, but that is another subject.

So what have we learned from all of this?

The first lesson is that the octane number of a fuel is not a physical property of that fuel but rather a rating in a particular engine under specified conditions — and that changing those conditions changes the octane rating of a fuel.

For example, changing the RPM of the RON engine test about 300 rpm and increasing the intake air temperature changed the octane rating of most fuels about eight numbers, RON vs MON.

And we are asking a new unleaded fuel to be all things to all engines under all conditions.

In addition, the rich or supercharged rating, which is usually not mentioned by some manufacturers of 100 octane unleaded fuels, is probably more critical than the MON in some engines and applications.

Another point that I should mention is a very well documented phenomena called the lead bonus. In the field, a leaded fuel will normally provide better anti-knock protection than an unleaded fuel with several numbers higher-octane rating.

Is this going to affect all general aviation aircraft? No, it should not have a negative effect on the vast majority of the GA fleet.

What it will probably affect is the large flat and round piston engines that are in commercial service.

When the industry went from 100/130 to 100/130LL most of the large engine carriers were forced to derate their aircraft, which meant less cargo. They may have to reduce the amount of boost used for takeoff even further when they switch to an unleaded fuel. Hopefully they do not have to reduce the loads to a point that the aircraft is not economical to operate.

The other part of this problem is that it is very difficult to hear knock in a large engine during takeoff. Many times they do not notice the knocking until after it has lead to pre-ignition and ventilated a piston or two, which gets rather expensive and dangerous.

The warbird people should be OK since reducing the takeoff boost will not hurt them too much.

The people who race at Reno are sort of up a creek. But since this is to be the last year of racing at Reno, this may be just one of many changes for them in the future.