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With a modern petrol engine which is able to adjust ignition timing in response to detonation detection, it is to be expected that a higher octane fuel will produce a higher mean cylinder pressure and hence more useful work.
This does not apply to a diesel. Putting it crudely, diesel engines knock (but don't detonate) all the time and no engine to my knowledge can adjust injection timing according to the output of any kind of "knock" transducer.
Leaving aside the hot topic of lubricity, one of the many advantages of the diesel is its tolerance to fuel variations. As long as a minimum cetane value is achieved, detonation will not occur and mean cylinder pressure will not increase with an increase in cetane value.
Craig, thanks for your input but Mother Nature has been out to get you. Ambient conditions of air temperature, humidity and barometric pressure amongst others will cause noticeable performance changes - and your test must necessarily have spanned a significant seasonal change in weather conditions, affecting all of the above. The other foreigner in the wood pile will be that for some of the time, you would (or should) be using winter diesel.
Winter fuel, although meeting the cetane requirement of EN590 has a lower calorific value per unit volume, this being caused by the addition of shorter chain hydrocarbons which are needed to depress the temperature at which waxing will occur. Most vehicles will give a poorer fuel economy in winter due to this factor alone.
Having tested both fuels (we are lucky in the UK) and many engines, my own conclusion is that the best value is obtained by buying basic fuel to EN590 from a high volume supplier (cleanest tanks) at the lowest available price. I will not go out of my way - but frequently buy fuel at the supermarket.
659.
Edited by 659FBE on 21/02/2014 at 23:28
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