Hello Doctor. 

I couldn’t help but think of you when I read a letter in the ìHonest Johnî letters column in last Saturday’s Daily Telegraph. Someone wrote in asking about the benefits of using premium petrol in his BMW 535i, and was told that premium petrol would give him up to 10 per cent more power, and more torque at lower engine speeds with Shell V-Power 99 octane. He also wrote that it contains beneficial detergents and lubricants, and that the owner should notice the difference. Be that as it may, for petrol cars, but I think that you have generally always in the past slammed premium priced diesel fuels, like Shell V-Power and BP Ultimate. Is that still the case, and why, or have things perhaps changed? 

Tony Haddon

I would be somewhat less likely to argue against premium price high octane petrol than premium price diesel. In high performance (and high compression ratio) petrol engines, high octane petrol can offer benefits, although suggesting 10 per cent more power is laying it on a bit. Engines like those in the 535i adjust automatically to suit octane levels, by using ìknock sensorsî that detect problems with low octane fuels and retard the ignition suitable to prevent ìknockî, or detonation, and in the process of this the maximum power output is inevitably reduced. With diesels, the key factor is the ìcetaneî level of the fuel, where a low cetane number can make the fuel somewhat slow to ignite when injected into the hot compressed air in the combustion chamber. That can lead to lower performance, and/or excessive ìdiesel knockî (uncontrolled and ragged combustion) which the diesel engine’s knock sensor (or sensors ñ some engines have one for each individual cylinder) will detect, and adjust the fuel injection timing and/or sequence (in multi-pulse injection programmes) accordingly to eliminate it.

It is worth pointing out here that the arrival of multi-pulse fuel injection programming initiated by Fiat, along with the development of ultra-fast piezo-electric injectors, enabled much more precise diesel combustion, and significantly reduced emissions. As with petrol engines though, any adjustments for lower quality fuel may well be at the expense of power output, and engine efficiency. But most UK diesels are alright regarding cetane number (much higher than USA diesel fuel) and the benefits of using premium priced diesel fuels lie more in the additives that can prevent carbon build-up and fuel injector deposits. This comes at a significant cost though, with most oil companies adding around 15 per cent (10p plus VAT) to the pre-excise duty cost of standard diesel fuel on their premium grades. 

Standard fuels usually do contain such detergents, but possibly at a lower level than premium fuels, as Esso points out in their product information. Shell says that their V-Power diesel ìHelps to clean intake valves and/or fuel injectors, and key fuel system components?î and BP says that their Ultimate Diesel ìStarts working from the first fill, helping to remove harmful dirt in your engine and to stop it coming backî and ìwith ongoing use, BP Ultimate Diesel with active technology helps keep your engine running smoothly and efficiently and performing at its best.î It all sounds to me a bit too much like washing powder advertising of the 1960s to me! 

With a cost of around 12p or more a litre for these benefits, owners keen to get the best out of their engine would be far better off buying a quality diesel fuel additive, such as from Wynns or Millers, which often contain a cetane booster and fuel system lubricants, as well as the detergent additives, and using them regularly for similar benefits at a much lower effective cost of as little as 4p a litre. 

I hope this answers your question Tony, although I have neatly avoided getting into the politics of petrol prices and grades! 

Regards,
The Doc