Doctor Diesel

New wheels for Bill

Dear Doc, I regret that this may be my last communication in relation to my Vengaís appetite for winter diesel. (We shall all miss your updates! Doc.) I will shortly be swapping it for a Mazda 2 diesel, first registered March 2016, and with 7,000 odd miles on the clock. Despite your reservations about the SkyActiv-D technology, I’m curious to see how it will perform in the real world. I am a fan of the Venga, and would dearly have liked to buy an updated version, but the current Venga is virtually identical to the car I bought nearly five years ago, when it was not a new design then. Its stablemate, the Soul, which used to share a platform with the Venga, has seen multiple updates in that time, including a switch to the more modern cee’d platform, with its more compliant suspension. The problem is that the Venga is a supremely rational design in a world where rationality doesnít attract the same development budget as the ìcoolî quotient in vehicle design. (How perceptive of you! Doc.)  

I’m currently burning Esso diesel, which I previously found to be the least satisfactory of all the branded fuels, and that is now showing only 51.8mpg, though that is after only 100 miles or so, and one DPF regeneration episode, so I am hopeful that this figure represents the nadir, and will improve with more miles untroubled by DPF regeneration.

 Speaking of DPF regeneration, I think your sensible advice to your correspondent James Tickle, concerned that the UK limit of 70mph is not fast enough for his X3 engine to reach the rpm level required for effective DPF regeneration, that he should simply drop a gear to bring the rpm within range, should be reiterated widely ñ in particular to fellow Diesel Car contributor Andy Goodwin, who is still repeating the commonly-held misconception of ìthe necessity for prolonged driving at higher speedsî (An Insight into AdBlue). This is a myth. DPF regeneration is perfectly feasible at urban speeds, provided that normal engine temperature is maintained, and adequate rpm levels sustained. Road speed is irrelevant: only engine temperature and engine rpm are the parameters that count. In my own case, the manufacturerís handbook suggests that the car be driven at 1,500 to 2,500rpm for about 20 to 25 minutes. I maintain third gear up to 30mph at around 1,800rpm (fourth gear drops below 1,500rpm at 30mph), and DPF regeneration occurs happily in town driving. I have a routine journey of six miles ñ three miles out and three back ñ mainly within 30mph limits; if I start with a cold engine and DPF regeneration is needed, the process will initiate halfway into the return leg, presumably when the engine reaches full working temperature (no temperature gauge on the Venga), and at 30mph in third gear. The only concession I make is to extend my journey, if possible, once regeneration starts, until the process is totally complete.

 So there we have it: my Venga will be gone before the current tankful of Esso is consumed, so we wonít know the end of that story. But I look forward to finding out if the Mazda is as sensitive to the switch over between summer and winter diesel as the Venga, and I may well update you when I find out!

Finally, a word of appreciation for your colleague Victor Harman, whose ìExtra Mileî column this month has really hit the nail on the head. It baffles me that people are willing to pay extra to drive around in overweight, bulky, blunt and inefficient SUVs at a time when we are all meant to be reducing CO2, and cleaning up on other emissions in order to save the world (and ourselves). Peace and goodwill to the Doc Diesel household. 

Bill Lawrie

Hello Bill, a few thoughts on the car change. Fine choice, by the way, based on what I have read about the car, although not driven it. But surely the cabin is possibly a bit tighter for space all-round than the Venga?

Firstly, my off-the-cuff forecast for your long-term average with the Mazda2 is 62 to 64mpg, and we’ll see whether I turn out to be anywhere near right! Yes, I do agree with you, the Venga has been sorely neglected by Kia, and that Soul thing looks awful, drives awfully, and primarily seems to appeal to non-motorists and those with no sense of style. But your Venga was a fine choice at the time, and has apparently served you well. I hope that it ends up at a good home, deserving of the care that you have given the car.

Now I need to swot up a bit on the 1.5 SkyActiv-D engine, to see in what ways it differs from the bigger 2.2-litre engines in the 6 and CX-5. But I have to say that your assertions regarding DPF regeneration relate primarily to your experiences over some years with the Venga. Not all engines perform in this agreeable way, and there are various designs of DPF systems, some of which respond fairly well, and give little trouble, and others which can be more problematic. The latest designs feature close-coupled oxidation catalysers and DPF filters placed very close to the exhaust manifold, which helps generate the DPF temperatures that facilitate trouble-free regeneration. Much does, however, and I think we would agree on this, come down to how sensibly, and sympathetically, DPF equipped cars are driven.


The Doc

P.S. The first report from Bill on the Mazda 2 1.5 Diesel (which came to him with 7,000 miles plus on the clock) is a first brim-to-brim tankful fuel economy figure of well over 70mpg, over a period of what he describes as normal mixed, fairly typical motoring.

They are very impressive early figures indeed!

The Doc

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