I wonder if I might engage you on the topic of winter diesel? I used to take it as fact that anti-waxing additives were introduced into diesel fuel during the winter months, with an adverse effect on fuel consumption. However, in recent years I’ve been led to believe that this is a bit of a myth, or that, if it were ever true, it no longer applies. Lately though, I’ve begun to question this latter view, for a number of nagging reasons. I run a Kia Venga 1.6 CRDi, and in your used car review of its sister car, the Hyundai ix20, you reported moderately disappointing fuel consumption of 50 to 55mpg. I should, therefore, be fairly satisfied with my recorded average consumption over 20,000 miles since new of nearly 57mpg. However, this average conceals wild variations between winter and summer months. During the summer, each tankful of fuel returns between 57 and 60mpg, and I often do journeys of better than 60mpg, but in winter it seems impossible to eke out more than 52mpg, however hard I try. This was brought home to me during this last month – my last tankful returned just shy of 58mpg but, having filled up again in mid-November, I find that my current tankful, now nearly consumed, is showing an average of 51.8mpg. This mirrors my experience of last winter, and leads me to believe that I am now using my first tankful of winter diesel. What do I blame the fuel for this discrepancy? Well, the weather isn’t to blame: the last month here on the balmy Moray Firth coast has been mild (11 degrees Celsius on 6th December); my driving routine doesn’t vary much, and the car is fed pretty much the same fuel – usually Esso, sometimes Shell ordinary diesel, with the occasional shot of Miller’s EcoMax. And, of course, I’ve eliminated suspects such as sticky brakes and under-inflated tyres etc. Tellingly, last winter I had access to a lightly-used Renault Modus 1.5 dCi which ran through the winter on the remnants of the summer diesel in its tank, and its fuel consumption showed no winter deterioration. I am reminded also of a conversation you had early this year with a correspondent who swapped his old Golf for a new one and was disappointed by its fuel consumption. It occurred to me then that, depending on when the exchange was, he might have been comparing his old Golf’s summer mpg with that of his new Golf on winter diesel. Nevertheless, I would value your definitive opinion on the subject. Am I right to blame the fuel for the poor winter mpg figures? If not the fuel, then what? I suppose there might be some quirk in the design of the Venga’s engine or fuel system which makes it gobble fuel whenever the temperature drops below 12 degrees Celsius, but I can’t imagine it. I hope you can shed some light!
Bill Lawrie

You may indeed! A very thoughtful and interesting letter Bill. The difference between winter and summer grade diesel is essentially the CFPP, or Cold Filter Plugging Point, which in the UK are required to be minus 15 and 19 degrees Celsius to meet EN 590 regulations, with the changeover dates in November and March, although when in those months is not specified. In colder parts of Europe, an arctic grade also exists, with an even lower CFPP, and there are actually subdivisions within both these cold weather grades, which may be specified within individual countries. Addition of anti-waxing additives is not necessarily required, particularly in warmer countries like the UK and, these additives being expensive, fuel suppliers will often meet the CFPP requirements without using additives. What they do is to supply a thinner diesel blend, as produced by the refineries, which will usually be less viscous, and thus less prone to waxing problems.

Now, these winter grades will differ from summer grade in two areas: the chemical make-up will have a slightly lower calorific content, but also a lower Cetane Value, both of which will tend to reduce performance, although the effects will differ from one engine to another. So there’s a bit less energy to be had from every gallon (although cold diesel is more dense than warm diesel!) and the ignition properties of winter diesel, as measured by Cetane Value tests, may be inferior. The lower energy content will affect all engines, but the Cetane Value reduction not necessarily all. In my motoring history I have been unable to pin down which of these, apart from the significantly lengthened warm-up period, (which might be the major effect, depending on your motoring habits and climate) and I haven’t actually owned a car that showed winter to summer economy variations as extreme as those that you are; so it may be that the Hyundai and Kia engine is more sensitive to Cetane Value than other engines. I would urge you to switch to using Millers Diesel Power Ecomax additive and, rather than adding it occasionally as you do now, and probably with injector and fuel system detergency more in mind than fuel economy. Standard addition rates of Millers raise the Cetane Value by up to four units, which is quite significant. So why don’t you run another tankful with no additive, run the tank down as low as possible, and then run at least two tank fulls with Millers, or maybe a tankful with a double dose, and the next with standard dose rate? If your winters are so mild that you noticed no deterioration in economy with the Modus running on summer fuel through the winter, then you should really notice a difference if my Cetane Value theories are correct. I shall now dig a little deeper and see if I can obtain more precise figures on the difference between calorific values of summer and winter diesel, and also, hopefully, find out the precise effects of temperature on the density of the fuel. If your figures are brim-to-brim checks, this is all fine; if they are computer figures though, who knows what subtle effects might be in play. Perhaps you could let me know? I hope this is helpful as a start though, although I think we may yet have more to say to each other on this interesting subject! Best regards,
Doc Diesel