Saturday, August 07, 2010
I must start by stating that this write-up on gasoline octane ratings isn't my work, it’s something I found on the Internet in a news group. I have edited it to correct spelling and grammar, and improve readability however the informational content remains unchanged. I am hoping this information will be useful to others as it was to me. Despite its being perhaps 20 years old the theory hasn't changed.
The octane number of a gasoline IS NOT a measure of its hotness or coolness in the burning process, nor is it a measure of how powerful it is—it is simply a measure of how good the gasoline is at resisting detonation (knocking/pinging).
The internal combustion engine is, in the most simple of terms, a self-powered air/fuel pump.
It drives itself by creating pressure to push on a piston connected to a crankshaft via a connecting rod. The pressure is created by heating a cylinder full of air and fuel; and then igniting it with a spark causing the mixture to release energy and expand. The higher the pressure inside the cylinder, the more push there will be on the pistons and the higher the engine’s power output will be.
One goal of internal combustion engine engineers is to get the highest possible pressure without creating uncontrolled burning or even an explosion of the fuel mixture.
Detonation 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; and should not be confused it with pre-ignition, which occurs when the fuel is ignited before the spark occurs. The reason detonation occurs relates to the nature of gasoline, which is a mixture of different hydrocarbon molecules; some of these molecules decompose (burn) more easily than others when heated under pressure.
As the fuel/air mixture is ignited by the spark plug, the flame front begins moving across the cylinder from the point of ignition. When detonation occurs the increasing the temperature and pressure of the remaining air/fuel mixture and causing it to decompose before the flame front reaches it. If this decomposition produces auto-ignition compounds (compounds that can and will start burning without a spark) the result is an uncontrolled overly-rapid burning of the remaining fuel. This uncontrolled burning generates an opposing pressure wave (relative to the intentionally triggered wave) in the cylinder and the opposing pressure wave and results in the piston getting a hammer blow instead of a steady push. The clicking and pinging (pinking to those on the other side of the pond) noises are the sounds of detonation.
These hammer blows of detonation can quickly destroy an engine.
There are three main sources of heat inside the cylinder which contribute to the decomposition of the fuel:
- Residual heat in the heads, cylinders and pistons;
- 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);
- Heat of compression before the spark. (compression of a gas raises its temperature, which we want to happen. However if the sum of residual heat, heat from combustion, and heat from compression exceeds the fuel’s decomposition limits then things go BOOM....
Using Higher Octane Fuels:
The benefit of higher octane fuels is that they are better at controlling their 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. Octane numbers came about as a result of research carried out in the 1920s and 30s by Sir Harry Ricardo (The Internal Combustion Engine; 1925/35) and Kettering (of Kettering ignition system fame).
When Ricardo was asked to develop an engine for a British WW1 tank in 1916, he used a previously developed and quite ingenious variable compression test engine in his research.
- (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.)
Ricardo discovered that iso-octane (2,2,4 trimethylpentane) had a very high knock resistance, but that n-heptane (dipropyl methane) had a very poor knock resistance, and because these two compounds are very similar in other respects, they made a useful comparison point for gasoline. Ricardo assigned iso-octane to represent an anti-knock value of 100 and heptane to a value of 0.
So, the octane rating of any given fuel is based upon its anti-knock properties as compared to a mixture of iso-octane and heptane with the same anti-knock characteristics. I.e. a 91 Octane rating is equivalent to a mixture containing 91% iso-octane and 9% heptane.
In the late 1920s it was discovered that certain compounds of lead enhanced the anti-detonation characteristics of internal combustion engine fuels—gasolines could be doped with tetra-ethyl or tetra-methyl lead to enhance their octane numbers. This revolution in fuel design led to engines that could operate at higher compressions for better efficiency and increased power.
Another useful feature of lead in gasoline was that the burned lead products coated the exhaust valve seating area, greatly reducing a wear problem called valve seat recession (VSR) which results from the exhaust valve “eating” it's way into the head. With the less advanced softer cast iron heads of the day, this was a real bonus. VSR is not a significant problem with newer engines as they typically have hardened valve seat inserts.
Gasoline which is high in aromatics (sweet smelling hydrocarbon compounds) has a high natural octane rating, and so needs fewer additives to increase the octane rating—unfortunately however the aromatic compounds are also those most responsible for atmospheric pollution, so these compounds have been 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 (compounds infused with oxygen) have been developed to enhance the natural octane number of gasoline. The most common one used is Methyl Tertiary Butyl Ether (MTBE), some other compounds include TAME, ETBE, Methyl Alcohol and Ethyl Alcohol (Gasohol). But since MTBE and the other oxygenates contain oxygen, cars using oxygenates fuels burn more fuel because there is less “fuel” in the fuel and this increases pollution anyway (Cleaner Burning Gasoline California EPA). California and other states have banned MBTE as an environmental hazard; ethanol is often used as a replacement.
Ricardo used the research method of measuring the octane number using a constant speed (1500 rpm) engine in laboratory conditions—this is the RON or Research Octane Number. The other method is called MON (the Motor Octane Number), which is measured using a harsher test regimen, more closely related to road conditions. MON is usually lower than RON. Often you may see the octane rating quoted as (R+M)/2. This means an average of the two methods was used to rate the fuel’s anti-knock characteristics—this method is often called pump octane in the US.
The bottom line is that using a higher octane gasoline than that for which an engine was designed and tuned WILL NOT increase its performance—all that is necessary is the octane rating needed to eliminate detonation, and you will find that the manufacturer’s recommended grade of fuel has been selected to exceed that need. There are of course exceptions to those recommendations, generally the result of one or more of the following conditions:
- Using higher octane fuel and retuning the engine to take advantage of same can increase power output, but only up to a point. Spark timing can be advanced on most modern engines that were originally tuned for 87 octane fuel by about 2° per unit increase in the octane—however when spark advance reaches the point that the peak cylinder pressure occurs to close after TDC power output will decline.
- At that point using a yet higher octane fuel will accomplish nothing.
- Extremely high operating temperatures resulting from constant high output demands and/or extreme ambient temperature;
- Increased compression ratio resulting from carbon build-up in the combustion chamber or piston crowns;
- Higher compression pressures brought about by forced induction (turbo or super charging) modifications. If the air/fuel mixture is introduced into the engine at higher than atmospheric pressure (which can never happen with a “normally aspirated” engine) then more gets in, at higher initial pressure
In conclusion, a fuel’s octane rating is ONLY a measure of that fuel's ability to CONTROL the burning process and to prevent detonation—it is not a result of its burning “hotter” or “colder”, or of its ability to produce more power, and is nearly entirely determined by the compression ratio of the engine in question.
Higher compression engines must run high octane fuel, and lower compression designs are able to run lower octane fuel. (that’s a “period”, as in the end of the story…)
 Higher compression produces higher final pressures after the fuel is burned, which produces more power. The heat generated by compression is easy to adjust in the design of an engine, as it is directly related to and engine’s compression ratio. Therefore, compression ratio directly links an engine design to the grade of fuel it will require.
 The British War Ministry had traditionally purchased fuel specified by specific gravity, Ricardo estimated some years later that it had 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!
 Carbureted engines do not adjust the air/fuel ratio on-the-run as do modern electronically controlled fuel-injected engines. They will run lean on oxygenated and ethanol fuels and their carburetors must be re-jetted to recalibrate the air/fuel ratio.
 Ethanol produces a whole new set of issues however, pure ethanol contains some 34% less energy than conventional gasoline (about 30% less than oxygenated gasolines) which means that an engine must burn more of it for equal power output—which means fuel economy is reduced. At mixtures of up to 10% ethanol this reduction is only 3% or so, however with E85 (85% ethanol) fuel the reduction is 27%.