[embee]

Rather a lot of "nearly right" comments being made. Just to try to clarify a few ……………

"Octane number" is indeed the measure of the tendency of a particular fuel to result in "knock" (aka detonation, pinking, pinging etc). NOTE that knock is different to "pre-ignition". As the name suggests, pre-ignition is where the burn starts before the spark, usually due to excessively hot items like a wrong grade spark plug or hot deposits. "Knock" occurs at the end of the burn process, normal combustion is a progressive burn, taking maybe 40 or so crank degrees from initiation, it is not an explosion. Knock has to be severe and extended for any real damage to occur (I've run engines in heavy knock to determine the effects, and one took 10hrs at full power knocking heavily until a ring land broke on a piston). Pre-ignition will destroy an engine in seconds due to massive heat input into pistons.

Fuels are assessed in specific test engines run at specific conditions, the tendency for the fuel to knock is then compared to a theoretical mixture of two specific hydrocarbons, iso-octane and n-heptane.

Iso-octane (usually 2-2-4 trimethyl pentane) is branched molecule and reasonably stable, defined as 100 "Octane". N-heptane is a straight molecule and breaks easily, and is defined as "zero octane". The test fuel is given a number which describes how it behaves compared to a mixture of the 2 reference fuels, so 95 octane bejhaves like 95% iso-octane and 5% n-heptane. IT IS NOT MADE OF THESE, but behaves like it. It has no DIRECT relation to volatility, that's a different characteristic. It is also not directly related to alcohol content, any commercial pump fuel is allowed to contain "up to" 5% alcohol without having to declare it, 10% or more must be labelled. Alcohol fuels have different octane ratings and content will affect how the fuel will behave, but it is the overall mix of constituents which determines the behaviour.

Different test conditions (speed/load/temperature) give rise to different numbers, one is "research octane number" RON, another is "motor octane number" MON.
MON is usually around 7 or 8 numbers lower than RON for most commercial petrol. In the USA they usually quote the average of the 2 ((RON+MON)/2), in Europe we quote RON. Thus USA "pump" numbers are usually about 4 lower than RON.


Knock is when a fuel/air mix is exposed to high pressure and temperature long enough for the fuel to break down into chemicals which will spontaneously react without the need for an ionisation source (flame/spark). It usually happens in the gas away from the plug where the mixture gets heated and compressed by the advancing flame front, and significant amounts of the charge can spontaneously react simultaneously resulting in a very rapid pressure rise, giving the metallic "knock" sound.

Commercial higher RON fuel tends to have slightly higher density and slightly higher calorific value than regular, but the difference is relatively small, of the order of 2-3%. This is down to the different constituents of the mix of hydrocarbons it contains.
95RON might be around 42.7MJ/kg and 0.74 kg/l, 98RON might be typically 43.5MJ/kg and 0.75kg/l. Since we buy fuel by the litre we usually get slightly more energy per litre for higher RON, but the effect is small.
The ignitability characteristics can be different, and this is often one aspect which you can notice. Combustion stability is measured as statistical variation in IMEP or the first 5 or 10% mass burned, and tests can demonstrate the effects of different fuels.

A higher octane number means a fuel can tolerate higher temps and pressures for longer, allowing the compression ratio to be increased (in simple terms). This improves the thermal efficiency (power and fuel economy). CR is selected according to what the commercially available fuel is in a specific market. In reality many manufacturers use a common spec for the engine for different markets, so the lowest common denominator applies.
At low speed and high load most engines are "knock limited", i.e. the ignition cannot be advanced to the point where the optimum output is achieved before knock is encountered. This is down to the choice of CR being to suit mid/high speeds where it can be optimised (often termed "best ignition" or "maximum brake torque" MBT ignition etc) in the interest of output and economy. Knock will happen at low engine speeds because the burn rates are low and cycle times long so the mix can be exposed to the high temps and pressures for longer and will reach the critical condition.

Running a higher RON fuel than the minimum recommended is not detrimental. Different speeds and loads will have different octane requirements in any given engine so the majority of the time the fuel rating is higher than actually needed.

It is a perennial topic, and I usually end up saying just try it. If you find it makes your engine run smoother or start easier etc then fine, if you can't tell the difference then don't bother. Some engines/operators are more sensitive that others to fuel characteristics. If your vehicle uses systems which optimise it for the use of higher octane fuels it will usually be worthwhile, but ultimately it's up to you.