Doc, itís a year since I last wrote, with respect to monitoring my Kia Vengaís fuel consumption on winter diesel. Fear not ñ Iím not going to dull your brain with a blow-by-blow account of my subsequent record-keeping ñ other than to say I have good and bad news to report.

The good news is that I did improve my fuel economy significantly over the last winter, simply by using only those brands that had demonstrated good performance in earlier observations, but I also marginally improved my overall recorded average since new 37,000 miles ago. This normally deteriorates in winter and recovers over the summer. The bad news is what I think of as ìthe incredible shrinking summerî ñ seemingly the availability of summer grade diesel creeps later each spring, and that of winter grade diesel creeps ever earlier in the autumn. This year, my tankful of Shell bought on April 29th proved to be (still) winter diesel, returning an average of 54.1mpg over 518 miles, whereas the tankful of Gulf bought a week later (May 7th) returned 60.0mpg over 500 miles ñ my first summer diesel of the year. My last summer diesel of this year was a tankful of Gulf on September 27th, which delivered 59.6mpg over 556 miles, and was followed by my first tankful of winter diesel (Esso) on October 13th, which returned only 55.7mpg over 474 miles. My current tankful looks like itís heading the same way, with an interim reading under 55mpg ñ clearly the winter stuff! It seems absurd that weíre forced to buy winter diesel until the end of April and from the middle of October. I can see the logic from the fuel retailersí point of view ñ after all, if I have to buy 10 per cent more winter diesel than I do of summer diesel for the same mileage, why not if they can get away with it? But itís beginning to feel like a rip-off.

On a quite a different topic, I recall your comments in the past on how NO_x is formed in diesel exhausts, partly as a result of combustion taking place under high pressure in the typically high-compression-ratio diesel engine. Iíve been reading some material about Mazdaís SkyactivñD engine, which claims to be ìthe worldís lowest diesel-engine compression ratioî at 14:1, and this delivering ì20 per cent better fuel efficiencyî and compliance with ìstrict exhaust gas regulations globally without costly NO_x after-treatment.î I wonder what you think about this. Can a simple reduction in compression ratio deliver such benefits? If so, why are other manufacturers not rushing to copy the technology? I did read somewhere that its introduction in the USA was postponed at one point, but Iím not sure how current that information is. Perhaps Mazda thought twice about it, after the Volkswagen debacle, given that their troubles stemmed from attempting to gain approval for engines that didnít apply NO_x aftertreatment? 

I would appreciate your thoughts.

Bill Lawrie

Hello Bill.

You are, without doubt, the most observant reader that we have, in terms of spotting the winter diesel effect, although I’m not sure still precisely why you seem to notice the differences so clearly. Looking at my own records, I can pick out two periods covering motoring on six tankfuls in each period, from end April to end September 2015, and from end October 2015 to end April 2016, and the figures for those two periods are 53.7mpg and 54.7mpg! What do you make of that? It seems that your Venga must be more sensitive to the seasonal change, although precisely why I cannot suggest. I shall continue to dwell on this, but I’ll have to leave it there for now.

Regarding the Mazda Skyactiv-D, as I have pointed out previously, such figures refer to the strict geometrical compression ratio ñ that is, the volume ratio between the swept volume of the cylinder and the combustion chamber volume. In turbocharged engines, the intake air is not at atmospheric pressure (well, rarely), and it therefore makes something of a nonsense of comparing the classic compression ratio figures. You can imagine a 14:1 compression ratio engine running on high turbo boost and a 20:1 compression ratio engine running on low boost, and at any given time they are effectively running under similar conditions in terms of the weight of air in the combustion chamber available to burn the fuel. Anyway, the latest report that I can find says that, despite previous reports that put Mazdaís Skyactiv-D diesel powerplant on indefinite hold in the USA, the companyís CEO says that thereís an internal timeline for bringing the engine to the USA.

I do take on board some of the theoretical benefits of the low-compression ratio Mazda engines though, where Mazda claims that ignition at top dead centre, the optimum timing, rather than later as in many higher compression diesel engines, can achieve higher thermodynamic efficiency. But there is little information available to confirm whether their postulated later ignition in conventional higher compression diesels is significantly later, and over what range of engine conditions this might be so. There’s no argument that the engine friction is probably lowered, and that lighter construction is possible, but Mazda are certainly not the only manufacturers using aluminium cylinder blocks.

I also took a look at some real-life mpg figures declared by CX-5 owners and, unfortunately, the Skyactiv-D engine’s theoretical economy gains don’t appear to materialise into real life results. The range of powerplants, paired variously with 2WD, 4WD, and both manual and automatic transmission, produce average fuel economy of around 36 to 44mpg. This compares with, for example, a very similar 36 to 44mpg range for the 2008 to 2016 Volkswagen Tiguan 2.0 TDI. It also only represents 70 to 80 per cent of the EC combined figure, against the Tiguan’s real life figures of 80 to 90 per cent of their own EC combined test figures. So, NO_x control issues aside, maybe that’s another reason why others are not rushing to duplicate the technology?

Best regards, The Doc

I must add that since this exchange of letters, Mazda has announced the availability of the Skyactiv-D engine in the USA, with supporting NO_x emissions control using selective catalyst reduction. Maybe this will, in time, be offered in European cars. Doc D