Article well worth a read, regarding advantages of higher octane fuels, from the Independent Newspaper in the UK.
I started posting about compression and got lost in my own severely limited knowledge and techno-babble - this sums it up better, as one might expect, from a science and motoring journo...
"Super fuel: Is it myth or magic?
Paying a little more for your petrol can be good for your car, your wallet and the environment. But few understand why says John Simister
24 August 2004
Why, In the era of high petrol prices, would you ever buy premium priced super unleaded petrol? It costs around 5p a litre more than regular unleaded, and most cars made since 1989 can run on the regular brew anyway. So, does super fuel - Shell Optimax, BP Ultimate and the like - make your car go better and run more efficiently?
Marketing campaigns stressing the engine-cleaning benefits of these fuels, pushing the environmental angle and also hinting at extra power, have some basis in fact - but not for the reasons you might expect. It is a confused and misunderstood subject area for the consumer, so I shall try to clear the fog.
Let us go back in time to the late 1980s, when unleaded fuel first appeared. The four-star petrol used by most cars then had an octane rating of 97, a measure of its resistance to detonation. Lower-octane fuels are more likely to burn uncontrollably quickly, so require engines with lower compression ratios to stop things getting too hot, too soon and setting off the destructive "knocking" that signifies detonation (uncontrolled burning) or pre-ignition (the mixture igniting before it should). Higher octane fuels allow higher compression ratios, which release more power while burning less fuel.
Confusion arose because standard unleaded petrol has an octane rating of 95, a figure arrived at by pan-European consensus. But many cars around at the time were designed to run on 97-octane fuel, so needed to have their ignition timing retarded to prevent pre-ignition. Many older cars also needed the lead of four-star to protect their engines' soft valve seats; the tetra-ethyl lead, originally intended as an anti-knock ingredient, had the side effect when combusted of lubricating the very hot valves where they contacted the cylinder head.
These two issues became lumped together in the public's mind as the need to "convert" an engine to unleaded. But a little while later, super unleaded with an octane rating of 98 came on the UK market (it had been available much earlier in mainland Europe). It was the perfect fuel for the many cars that needed the octanes but did not need the lead. Such cars had no need of any "conversion" if they used this fuel. Unfortunately, super unleaded was considerably more expensive than the ordinary version (and still is) and, besides, the moment for educating the public had passed.
In mainland Europe, however, 98-octane fuel was, and is, only a little more expensive than 95-octane, so it was widely bought. But in the UK it was sold in quantities far smaller than it could have been if every car that ran best on it used it, and that low demand kept prices high and kept the vicious circle rotating. That super unleaded is more expensive to produce here than in mainland Europe, (owing to the different oils used), has also played a part.
And then we in the UK were dealt another blow. Leaded four-star met its end, so for the relatively few cars that still needed an additive to protect valve seats the oil companies came up with LRP, or lead-replacement petrol. This used a potassium additive, remained at four-star's 97 octane - and often cost less than super unleaded even though, from that point, super unleaded dropped also to 97 octane. This was iniquitous: they were the same fuels apart from the additive. It was almost as if the oil companies wanted to sabotage super unleaded, but blame could not be placed at the Treasury's doors.
This, then, could be the end of the story. Most modern cars can run on 95-octane fuel, and super unleaded is surely doomed as its sales drop from a low starting point. But "can run" is not the same as running optimally. Many cars today have engine management systems able to adapt to different fuel grades (Saab was one of the first to do this over a decade ago), using "knock sensors", which retard the ignition if required. Their engines' power and economy figures may well be quoted on the basis of 98-octane fuel (cheaply available in mainland Europe, remember); running on 95-octane, the engines will feel fine but use more fuel for less power. Typically, the reduced fuel consumption when running on higher-octane fuel more than makes up for the fuel's slightly higher cost.
That equation is less valid in the UK, given the smaller octane difference and greater price disparity, but BP claims that its Ultimate super unleaded has extra benefits: keeping engines clean and free of carbon deposits. There is another dimension, though, which is the advent of sulphur-free or very low sulphur fuels. Already petrol has a much lower sulphur content than it had a couple of years ago, but some of the latest generation of direct-injection engines require a sulphur-free or near sulphur free diet to stop the tiny holes in their injectors from becoming clogged.
Direct-injection engines also run at a very high compression ratio, and are happiest on 98-octane fuel. So for the motor industry to produce cars as clean and fuel-efficient as possible, it needs sulphur-free, 98-octane petrol. The only such fuel sold in the UK like this is Shell Optimax (an extremely low sulphur fuel) and it is this lack of truly universal availability that has so far kept Mercedes-Benz's CGI direct-injection engines out of UK price lists.
In a few years' time, there will be more cars in need of Optimax-like fuels and the oil companies will need to provide them here. Maybe the Government could help; if it wants to encourage sales of ultra-clean cars, it should reduce duty on ultra-clean fuels. Existing cars that run more efficiently on higher-octane fuel could benefit, too. Fuel duty coming down? What a sea-change that would be."