Octane Ratings

[Kickstart]

Hi

Common idea is that higher octane fuel will give more power, but on its own this isn't true.

To start off in the early years of internal combustion engines during WW1, some aircraft engine suffered major failures for unknown reasons. One problem was found to be different fuels propensity to ignite on their own (from just pressure) without waiting for the spark plug to fire. When this happens the fuel pretty much explodes violently (rather than a slow burn). Even if only a small pocket of fuel / air explodes like this the pressure in the cylinder will rapidly increase triggering the rest to explode anyway. This explosive force will rapidly damage parts of the engine. However at the time what allowed this to happen, and how it varied between different fuels was not understood.

To classify fuels on how easily they will detonate on their own the octane rating system was devised. The octane rating of a fuel is its equivalence to a test fuel made up of a mix of iso-octane and n-heptane. If the fuel detonates as readily as a mix of 80% iso-octane and 20% n-heptane then it is said to be 80 octane. Iso-octane is very resistant to detonating while n-heptane is very prone to detonation, but the mix isn't designed to be a useful fuel.

Fuels are compared using standard single cylinder test engine with a variable compression ratio. However there are 2 main ratings which are the RON rating (Research Octane Number) and MON rating (Motor Octane Number), an the difference between these is in the details of how the test is performed. For the RON rating the test engine is run at 60rpm with an inlet temperature of 52 degree centigrade, while for the MON rating the engine is run at 900rpm with an inlet temperature of 149 degree centigrade. The 2 ratings for the same fuel will be different. On a rule of thumb for normal road fuels the RON rating will be about 10 higher than the MON rating for the same fuel, but this is not consistant between all fuels (hydrogen has a very high RON rating and a very low MON rating). This difference is known as the fuels sensitivity.

There is yet another system used to rate fuels and that is the RdON (road octane number), determined using actual vehicles. This will be quite difficult to measure consistantly.

The USA uses the PON (Pump Octane Number). This is merely the average of the RON and MON ratings for the same fuel. This apparently reflects fairly closely with the RdON and so is probably more meaningful for a road vehicle.

What this does mean is that US PON ratings and European RON ratings are not directly comparable. The same fuel will probably be said to have a 5 point higher octane rating in Europe than the US.

Fuels that have an octane rating of over 100 are tested and their resistance compared with extrapolated results for the normal control fuels.

Note that none of these tests make any differentiation between the power of any engine when run on these fuels. Generally a higher octane fuel will need more energy to ignite, and will often release less energy when burned. You can easily have a fuel that has a very high octane rating but releases very little energy creating very little power (methanol is a bit like that, although it compensates by allowing a very rich mixture to be usefully used). Indeed a slow burning fuel is likely to have a high octane rating.

A normal petrol engine is not perfect. It relies on the fuel burning, but how much effect that fuel has will depend on the piston position at the moment it is burning. If the piston is at top dead centre then no matter how much force the burning mixture apples to the piston at that exact moment it can't push the piston down short of bending the con rod. Similarly when the piston is at bottom dead centre any last bits of mixture burning are wasted. When the connecting rod is making a certain angle with the crankshaft then the burning mixture can apply the maximum turning effect. As the mixture takes a certain time to burn the engine wants the time when the most mixture is burning to correspond with the moment when the crankshaft can take the most advantage of it. With normal fuels this means igniting the fuel before the piston reaches top dead centre, giving the flame front time to spread for maximum effect. If you advance the ignition timing you gain in this way, but you also have the mixture burning (increasing its volume) at the same time as the piston is still trying to compress it, and as the pressure increases the chances are that some of the unburnt mixture will have been compressed enough to spontaneously explode (resulting in detonation and probable engine damage). Thus having a fuel which is less prone to spontaneously explode (ie, a fuel with a higher octane rating) enables the ignition timing to be safely advanced and it is this change that gives a bit more power. Changing the fuel alone without changing the timing to take account of it means no benefit.

Similarly with compression ratios. The more the mixture is compressed the more energy that is released when it is burnt. However again if the mixture is compressed more then it is more likely to spontaneously explode. A higher octane fuel allows a higher compression ratio to be safely used.

As you can't really adjust the compression ratio while riding / driving you can't safely use a lower octane fuel once you have raised the compression ratio for more power. What you can do is retard the ignition timing (losing you power) to make it safe to use a low octane fuel once you have raised the compression ratio. It would be entirely possible to raise the compression ratio to gain power from using a higher octane fuel but then lose even more power by retarding the ignition timing to cope with the fuel you were previously using.

Some ignition / injection systems use knock sensors (although very few bike ones). These are basically a microphone which can pick up the noise of the exploding mixture with detonation compared to the normal noise of a burning mixture. The ECU can recognise this and then change things to remove the problem, probably by automatically retarding the ignition timing.

Supercharged engines (either mechanical superchargers or exhaust driven turbo chargers) have extra things to play with. A supercharger will push more mixture into the engine and this will give more problems with detonation. However the boost pressure can be fairly easily changed, so if a high octane fuel is being used the ECU will up the boost to generate more power until knock is detected, and then back off a touch. With supercharged engines quite a lot more power can be generated from using more boost, hence a higher octane fuel can give a substantial (or at least quite noticeable) increase in power if the ECU can adjust the boost to take advantage of it.

I hope this has explained things a bit, and hopefully shown why a higher octane fuel on its own won't give any more power unless things are adjusted to take advantage of it.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Frost]

Haha, welcome back, been taking post writing lessons off Mike?
As a general rule if you need more power out of your slow or little engine, higher octane fuel isn't the way to go.
If you are prepared to spend the money you can bike bikes to run on Aviation fuel, which does give a bit more power

[Kickstart]

[weasley]

Another small difference between the RON and MON engine test is that the RON test is carried out at fixed ignition timing, whereas MON uses variable ignition timing, as the compression ratio increases. Because of this and the other differences in testing, MON will always be lower than RON for a given fuel sample.

Also, the way octane ratings are adjusted in fuel is not by blending more or less octane, but by the use of octane improvers - chemicals which adjust the ignition properties of the fuel. One of the best and most widely used was tetra ethyl lead, or TEL. This us what made fuel "leaded". When it burnt it deposited lead compounds on the valve seats which helped cool and lubricate them. When TEL was banned it was necessary for valve seats to be harder as the replacement chemicals did not have the depositing feature.

Great post though.
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[ZRX61]

[Kickstart]

[Teflon-Mike]

Tetra-ethyl-lead
It's not the metal, lead, but a compound containing lead, which was commonly added to mineral oil distilate 'Petrol' to increase its knock resistance, and RON number.
It had a side effect in that metalic lead in ionic form would leach out of the fuel, and attach itself to ferric metal, iron, in the engine, where it provided some useful lubricating properties, mainly to the valve stems and the valve seats, which don't see much by way of positive lubrication from engine oil.

Still used in some blends of Aviation Fuel, it's the 'stuff' they took out of pump road fuel to make the 'Unleaded'* pump fuel we have today.

Modern 'unleaded' is one or two RON points lower in value than old 'leaded' fuel.

In the UK thy started phasing in unleaded fuel in the late 1980's and it was entirely phased out at the pumps in the mid to late 90's.

When Unleaded was introduced, it was given a lower rate of duty to encourage people to use it, and many specialists offered 'engine conversions', which in some cases were quite extensive, and included new 'self lubricating' valve stems and harder valves and valve seats, and cylinder head re- profiling to lower the compression ratio, and better optimise the chamber shape for the different burn speed. Most though simply retarded the ignition and ritchened the mixture to avoid detonation!

Most Japanse motorcycles however were already designed to run on unleaded fuel, having been designed significantly for the North American market, where Unleaded had been available since the late 1960's**

Two stroke motorcycles, tend not to have such high 'actual' compression ratio's as four strokes, due to the 'trapping efficiency' of having holes in the cylinder wall, which substitute for valves, so they don't need such a high octane rating, nor the valve lubricating properties.

Japanse four stoke motorcycles were standardising on high speed four cylinder engines, which could tolerate lower grad fuel with less power loss.

So significantly only owners of older and European machinery were notably effected.

So, if you have a Japanese bike, or a two stroke, you don't rally need to worry about tetra ethyl lead, only a concern if you have a European 'Classic', where chances are, if its been in use for the last 20 odd years its already had an unleaded conversion, or if not, retarding the ignition and ritchening the mixture will let you run it on unleaded until thee valve guides and seats wear out, as they would naturally anyway, just a little quicker, and you can convert to unleaded compatible valves guides & seats when it needs reconditioning anyway!

*{Political Asside:
When regulations were changed to 'promote; unleaded petrol, it was for concerns over 'heavy metal particulates' other compounds of lead, basically, in exhaust emissions that could cause heath issues. Significantly brain development in infants
Which was why they insisted on taking the lad out of paint, as well.
Scientific study, reckoned that something like 1 in 2000 children suffered 'brain development problems', of which probably less than half, could be considered significantly retarded, and of all of them, HMP probably only contributed to about a fifth.
SO, in short, they took the lead out of petrol, to protect, one child in 20,000, from being 'more' retarded than they would otherwise.
Having removed the Tetra-ethyl-lead from petrol, though, 'something' was needed to help bring its octane rating back up, and something ls to provide some lubricating properties for valves and valve seats.
High graphite iron for the valve guides and valve seats provided some self lubricating properties for valve gear, but to bring octane ratings back up to an 'accptable' level... modern 'Regular' unleaded is 92 RON, where old 'Two Star' was around 94/95... not quite its former potency The Fuel manufacturers had to add a substance called Benzene... a volatile liquid and carsonogen.
Which is why petrol pumps have them great rubber rings around the nozles to try and avoid you breathing the lighter benzene containing vapors....
So, the 'retards' in Whithall, made the country safer for a few hundred retarded children, who, thanks to all this are not quite as retarded as they might have been, but exposed EVERY ONE to a significantly increased risk of CANCER!}
**The USA's conversion to Unleaded was nothing to do with retarded children; it was due to Federal EPA 'Smog' legislation, to reduce Carbon Monoxide 'smoke' emissions, lingering in skyscraper laden cities, not being naturally blown away. And demanded fitment of catalytic converters in exhaust systems.
Unfortunately these contain rare metals, and the ionic lead and HMP produced from burning Tetra-ethyl-lead in the fuel, resulted in the ionic lead and particulates attaching themselves to the 'activation sites' in the catalyst and 'clogging them up' making them useless; hence they demanded Unleaded, NOT to protect children, but to save their exhaust systems!)
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Kickstart]

Hi

Quite a few 2 strokes do have fairly high compression ratios, but confusingly they are often quoted as effective ratios. Eg, early Aprilia AF1 was quoted as a normal (non effective) ratio of 15:1 (from memory), although the effective ratio was probably about 8:1 or 9:1. Run these on low octane fuel and you will burn a hole in the piston (bit of a bugger as they were designed to run on 98 octane fuel).

It wasn't until the 1970s that US leaded fuel levels went down. First cats were in 1973 but US emission standards while stricter than Europe were still lax enough that a decent non cat car could get get through them. Alfa for example started to use fuel injection in the late 1960s on US market cars and got away without resorting to cats until about 1980.

Quite a few Japanese bikes did need the lead. Hondas post 1976 were meant to be unleaded safe. Often said that the CB400/4 was meant to have leaded fuel as they were all built before Hondas switch to unleaded even if not actually sold until a few years later.

Normal unleaded is 95 RON. While lower is available in some countries it isn't sold in the UK. 2 star was 91 RON I think (4 star was 97 RON).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Kickstart]

Hi

Slightly wrong on the star ratings. 2 star was 92 octane, 3 star 95 octane 4 star 98 octane and 5 star 101.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[weasley]

Actually, under the influence of high octane red wine, I got the ignition timing thing arse about face. The more severe way of testing is with a fixed ignition timing, which is how MON is tested. With the RON test, the ignition timing is retarded as the compression increases to delay the onset if knock (and hence give a less severe test).

More trivia: the test engine is (was) made by Waukesha. I may remember more with more wine.
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[unitynotsocri...]

More trivia....that saved us from the Nazi jackboot.....but not Brussels!
the germans had superior fighter aeroplanes in ww2 but we just about held our own because we had high octane petrol and they did'nt.
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Teflon-Mike]

[Kickstart]

[Ariel Badger]

If you want more power you need more compression and a fuel that will not detonate such as Methanol we run M very rich (My 500 single runs a 600 main jet)to stop overheating. On the down side I only get 5 miles to the gallon.

For ultimate power you need Nitro but you do not get many miles to the engine on top fuel
____________________
Bikers make great organ donors, get 115 on your licence today.

[unitynotsocri...]

Thanks Kickstart,
it appears I've been the victim of dastardly Nazi propoganda,once,along time ago we ruled the world yunno.
Well at least we stole their 2 stroke technology to create uber 2 strokes in the fifties.
____________________
nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Kickstart]

[ZRX61]

[CaNsA]

Why do i get an extra 20miles from a tank on the posh stuff?

[Kickstart]

[CaNsA]

So the fact i get about 120miles from a tank on the standard fuel compared to the 140 miles from a tank on the posh stuff has nothing to do with the higher octane?


I cant see that being the case considering the only difference is the type of fuel.

[Kickstart]

Hi

Octane rating is just one factor of the fuel, and if sufficient for the engine any extra is just a waste.

As an example, methanol is a fuel that a 4 stroke engine can run on. It has a higher octane rating than petrol (about 108, or higher according to some sources) but produces a pathetic amount of power for the same volume used compared to petrol (but with methanol you can burn substantially more at once).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Noxious89123]

The fact you get higher MPG means that the "posh stuff" you are using might contain more enegry per gallon than "normal stuff". Doesn't mean the resulting higher MPG is anything to do with the high octane!
____________________
'06 Honda CBR125RW-6 ~ '00 Honda CBR600F-Y ~ '07 Honda CBR600RR-7 ~ Bikeless
'53 Ford Ka 1.3 ~ '03 Vauxhall Astra SRi 1.8 ~ '52 Vauxhall Astra SRi 2.2 ~ '53 Vauxhall Astra GSi

[Teflon-Mike]

[unitynotsocri...]

I 've heard that with lpg the engine oil lasts a lot longer and the engine internals are cleaner,and cold startings easier,is that right Teflon-Mike?
____________________
nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Kickstart]