When we think about saving fuel, and driving to save fuel, one of the significant factors involved is too often glossed over, and that’s the origins, the quality, and specification of the fuel that we are using. How much difference can there be between different fuels from different sources? Well, we have it on good authority that, if you are a regular buyer of a nationally available fuel grade, let’s call it Delta Diesel Plus, it’s most unlikely to be the same stuff every time that you buy it, as you travel around the country. This is particularly relevant to “standard” diesel fuels, rather than any premium price fuels, and it only needs an hour or two sitting outside a regional fuel distribution depot and watching the familiar brand names as the tankers come and go. Are they all delivering the same stuff, one might ask? These depots generally don’t keep individual bulk tanks for the various brands they supply, and basically the difference between Delta Diesel Plus and Omega Super at this depot is probably just the additive package added on despatch to the “standard” diesel fuel which, without supplements, is what haulage companies buy, and to which they often add their own individual additive package.

There are five or six operational fuel refineries producing diesel fuel in Britain, and around 50 distribution terminals, and the refineries that supply them draw crude oil supplies from vastly different geographical sources. This accounts for some of the chemical variation of the diesel that we buy in various parts of Britain. North Sea Brent Oil and Saudi Light Oil, two of the “lighter” crude oils, make the best diesel, whilst Venezuela Heavy Oils, some from Canada, and from Kuwait, are high in aromatics, tars, and sulphur. Restructured chemically by “catalytic cracking”, these can produce lighter petrol and diesel fractions, but this is costly, and it’s not possible to fully replicate the high quality diesel produced from light crude oils. In Europe, the refineries are primarily geared to producing diesel fuel, and high diesel demand has hitherto created a relative scarcity of diesel and an excess of petrol. So European refineries generally ship petrol to the USA, whilst the USA’s surplus of diesel comes to Europe, and its quality is not always brilliant, although good enough to meet the EU specification for diesel, EN 590, which is set at a pretty low level. There’s consequently a potentially large variation in the diesel fuel quality based on geographical location and time of year, and there’s also the matter of “summer diesel” and the gelling-resistant “winter diesel” that has a lower energy content and often affects winter fuel economy. 

Nevertheless, not all diesel engines respond in the same way to variations of the fuel, but, with increasingly high power outputs of 100bhp per litre and more, these more advanced engines will naturally perform better on higher quality fuels. The cetane value of diesel fuel is one such quality, and it’s critical to have good values to avoid “diesel knock”. This can be avoided with poorer fuels by automatic ECU adjustments to the injection timing, but often at the expense of power output and fuel economy. Cetane values can also be uprated with inexpensive additives containing cetane improvers, and other combustion enhancers, and these additives can often give your car a fuel economy boost that costs you less than buying premium price fuels. 

The point about all these observations is that only by buying your fuel from the same location, week after week, month after month, can you realistically hope to compare mpg figures for each tankful. But let’s conclude, though, by saying that if you can be bothered to experiment with various fuel brands, closely monitor your fuel economy, and find one fuel that consistently delivers better fuel economy in your current car, then it’s probably wise to stick with it. The same goes for any combination of fuel and fuel additive that gives you the same beneficial results – but just remember that your next car, or even your partner’s car, might just have a different favourite tipple!

Victor Harman