Factoid - Octane Ratings

First off, I must start by saying that this write-up on gasoline octane ratings isn’t my work – its something I found on the Internet in a news group some time ago – when I first started building hot rods. I am hoping this information will be useful to others - it was to me. The theory hasn’t changed.

The octane number of a gasoline is NOT a measure of it's hotness’ or coolness in the burning process, and it is NOT a measure of how 'powerful' it is. The octane number is simply a measure of how good the gasoline is at resisting detonation (knocking/pinging).

The internal combustion engine is - in simple terms - a gas pump. The higher the gas pressure inside the cylinder, the more 'push' there is on the pistons, and this means the higher the power output will be.

We create this pressure by heating a cylinder full of air; and we do this by adding a little gasoline to the air and igniting it with a spark.

The engineers aim to get the highest possible pressure without creating uncontrolled burning of the gasoline.

Detonation (pinging/knocking) occurs after the fuel is ignited by the spark plug, but before the flame front has finished moving across the cylinder to burn all the fuel/air mixture (don't confuse it with pre-ignition, which occurs when the fuel is ignited before the spark occurs).

The reason why detonation occurs relates to the nature of gasoline. Gasoline is a mixture of different hydrocarbon molecules, and some of these molecules decompose more easily than others when heated under pressure.

We ignite the fuel/air mixture with a spark, and the flame front starts moving across the cylinder. This increases the temperature and pressure of the remaining fuel/air mixture, which starts to decompose before the flame front reaches it. If this decomposition produces 'auto-ignition' compounds (those which will start burning without a spark), you end up with an uncontrolled over-rapid burning of the remaining fuel, which sets up an opposing pressure wave in the cylinder. This uncontrolled burning and the opposing pressure wave produces the characteristic clicking/pinging sound of detonation, and results in the piston getting a 'hammer blow' instead of a steady push.

These hammer blows can quickly destroy the engine.

Higher octane fuels are better at controlling the decomposition into auto-ignition compounds than lower octane fuels. They do this in several ways - by interfering with and reducing the actual decomposition, or by chemically reacting with the decomposing gasoline so less auto-ignition compounds are formed.

There are three main sources of heat inside the cylinder which contribute to the decomposition of the fuel:

1. The residual heat in the heads, cylinders and pistons.

2. The heat produced by the ignition of the fuel itself. This depends on the nature of the fuel, and on the fuel/air mixture - rich mixtures burn a little cooler, lean mixtures burn hotter.

3. The heat of compression before the spark. Compression of a gas raises the temperature of the gas. We want this to happen, because the higher the compression, the higher the pressure rise after the fuel is burned - giving us more power. The heat of compression (compression ratio) is easy to adjust in the design of an engine, so this is the one used to match an engine with the fuel it will be using. It's all a balancing act, and because the air-cooled engine runs hotter (more residual heat), you need to limit the amount of additional heat produced in the cylinders prior to ignition (lower compression ratio).

The octane number came about as a result of research carried out in the 1920s and 30s by Sir Harry Ricardo ("The Internal Combustion Engine" 1925 and 35 and other books) and Kettering (of Kettering ignition system fame). Ricardo had previously developed an ingenious variable compression test engine when he was asked to develop an engine for the British WW1 tank in 1916, and this test engine was used in his subsequent research. The British War Ministry used to order fuel by Specific Gravity and the fuel they gave him to use in the tank he assessed (years later) as having an octane rating of about 45. His tank engine was limited to a compression ratio of about 3.5:1 to cope with this poor fuel!

(Incidentally, this engine was extremely innovative for it's day, and was utterly reliable - so it also got used as a stationary (generator) engine by the British army for their field stations all over France, and by the British Navy for it's patrol boats, as well as about 12,000 tanks. The Army and Navy loved it because it would run on just about any liquid fuel - it would even run on a kerosene/gasoline mix if that was all they had! It was just as happy (but gave no extra power) on high grade aviation gasoline.

It was discovered that Iso-Octane had a very high knock resistance, but Heptane had a very poor knock resistance. Because these two compounds are very similar in other respects, they made a useful comparison point for gasoline. So the octane number is a comparison with a mixture of Iso-Octane and Heptane. 91 Octane is equivalent to mixing 91% Iso-Octane with 9% Heptane.

The discovery in the late 1920s that certain lead products enhanced the anti-detonating characteristics was a revolution in fuel design, as engines could be designed to operate at higher compressions for better efficiency. So gasoline’s became 'doped' with tetra-ethyl or tetra-methyl lead to enhance their octane numbers.

Another useful feature of lead in gasoline is that the burned lead products coated the hot exhaust valve seating area, and prevented a problem called Valve Seat Recession (VSR) which results in the exhaust valve 'eating' it's way into the head. With the less advanced 'soft' cast iron heads of the day, this was a real bonus.

VSR is not a problem with newer engines with aluminum heads, as they typically have hardened valve seat inserts.

Gasoline which is high in Aromatics has a high 'natural' octane rating and so needs less additives to increase the octane rating. Unfortunately, the aromatic compounds are also those most responsible for atmospheric pollution, so these compounds are being reduced in gasoline in many countries. This creates another dilemma - how to increase the octane rating without lead additives, and with reduced aromatic compounds in the fuel.

A number of other chemical compounds called Oxygenates have been developed to enhance the natural octane number of gasoline’s. The most common one used is Methyl Tertiary Butyl Ether (MTBE). Other compounds include TAME, ETBE, Methyl Alcohol and Ethyl Alcohol (Gasohol). But MTBE and the other oxygenates contains 'used' oxygen, so cars using oxygenates fuels burn MORE fuel (because there is less 'fuel' in the fuel) and this increases pollution anyway (Source - "Cleaner Burning Gasoline" California EPA).

And there is a second effect here too - carburetor cars cannot adjust the fuel/air mixture 'on the run' like computer equipped fuel injected cars can, so they run lean when run on oxygenated fuels. This is because carburetors meter out a volume of fuel into the intake air; they can't measure the amount of 'fuel' in the fuel. Lean burning creates more heat in the cylinders, and this 'excess' heat raises the octane number needed.

It's a vicious circle, so If you can avoid using oxygenated fuels - do so. If you have to use oxygenated fuels, you may improve the car's performance by using a slightly larger main jet in the carburetor. Doing this brings the mixture back to the correct setting, which helps reduce the extra unwanted heat in the engine, and reduces the likelihood you'll need a higher than normal octane gasoline to compensate. And if your engine is due for a rebuild, and you have to use oxygenated fuels, consider using a slightly lower compression ratio.

Sir Harry Ricardo used the 'research' method of measuring the octane number using a constant speed (1500 rpm) engine in laboratory conditions. This is the RON - Research Octane Number. The other method is the MON - Motor Octane Number, which uses a harsher test regime more closely related to road conditions. So the MON is usually lower than the RON.

Often you may see the octane rating quoted as (R+M)/2. This means an average of the two methods is used to give the fuel a number. This number method is often called 'pump octane' in the US.

Using a higher octane gasoline in an engine designed for low octane WILL NOT increase it's performance - the octane number is a MINIMUM needed to eliminate detonation, and that's all it is.

In conclusion, the octane rating is a measure of the fuel's ability to CONTROL the burning process (to prevent detonation); it is not a function of burning 'hotter' or 'colder'. And the higher the compression ratio (in the same engine), the higher the octane number needed.

Hope this helps.