Doctor Diesel

Kindred spirits, and more

Hello, Doc! 


Yes, me again – I had to write for a couple of reasons. Firstly, the July edition of Diesel Car cheered me up more than usual – not just because you generously squandered a number of your column inches on some of my mutterings, but because I learned that the bees that buzz in my head don’t only live under my bonnet. I found myself nodding along with your correspondents. The Delivery Man DP’s sceptical view of petrol-electric hybrids, which exactly mirrors my own, and in Chrissy from Leeds I believe I detect a kindred spirit – I applaud her stout defence of diesel in the face of criticism from her ill-informed petrol-driving friends. Perhaps you could have recommended that she invite these unbelievers to stick a finger up their tailpipes (in the nicest possible way) and discover for themselves just how mucky the exhaust output of their ‘clean’ petrol engines really is?


The good news is that I can tell you with confidence that, since I started keeping details until just recently, I have covered 13,944 miles and used 884.84 litres of fuel in the Mazda2, at an average of 71.61mpg, with brim-to-brim figures ranging from a best of 79.1mpg to a worst of 63.33mpg. The variation seems to be linked to the seasons, but I have also noticed a correlation between DPF regeneration frequency and fuel consumption – more frequent regenerations when fuel consumption is higher, less frequent when fuel consumption is lower. I can only speculate as to the cause – is it chicken or is it the egg? Do extra regenerations affect fuel consumption, or does winter diesel cause more regenerations? My prejudice is to think the latter, but perhaps the lower ambient temperature in winter might simply be the trigger, I don’t know.


Since doing the above calculation, I’ve refilled the tank again after 681 miles, for an average of 74.9mpg. I also ran a check, which I haven’t previously done, and found the results intriguing. My dashboard usefully allows me to see simultaneously the number of miles completed since filling up and the estimated number of miles left in the tank – so add the two figures together and you get the anticipated range for the whole tankful. When my last regeneration was initiated, this combined figure stood at some 693, and at the end of the cycle it had shrunk to 681 miles. It was interesting to watch the projected figure for miles remaining shrink more rapidly than the elapsed mileage grew. It’s something I’ll look at more closely in the future. 


Best wishes

Bill Lawrie


Hello Bill, 


I am glad you found a common wavelength with two other readers – I think there has to be some sort of a common thread with diesel owners who pay their own running costs and read Diesel Car! That there are quite a few thousand of us should be something of a consolation, when we read so much nonsense about the motoring world today and have fears that we are nothing but a shrinking minority. If enough people read my nonsense every month, and some care enough about what I write and then bother to write back to me, it all adds to the warm feelings this end that match yours at your end.


I’ve been thinking about the “chicken and egg” situation with the regeneration frequency. The particular regeneration that you followed “only” cost you 12 miles worth of fuel, in warmer weather, and with summer diesel in your tank. You have written in the past about how the unfavourable characteristics of your last tankful of winter diesel can run right into spring, and I veer towards that explanation, of it being the fuel to blame for more frequent regenerations, not the weather. It fits in better for me than any other argument. Your last calculated tankful that you wrote about of 74.9mpg is indeed impressive. I know you were really hoping to crack 80mpg one of these days. I think if you could use a full tankful on a suitably long journey, your 80mpg would be well attainable. 


That’s all for now, and thanks for taking the trouble to write in. 


Best regards

Doc D

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