Doctor Diesel

Gargantuan Gas Guzzler

Web_Letter three - 3I last wrote to you over two years ago concerning diesel engine emissions control equipment – and we had an interesting online debate about DPFs and SCR etc. which I much enjoyed. Since then, at my wife’s instigation, I replaced our E 320 CDI with a 2008 Mercedes-Benz GL 420 CDI in December 2011, my wife having applied emotional blackmail. So I gave her the chance to choose, and this is what we got. Anyway, it’s a really nice car, very fast, very big, spacious, comfortable, and with a luxurious ride and unstoppable feel, but economy in the low 20s mpg, and expensive road tax. The car does a mixture of short runs, with the occasional longer run plus the odd high speed motorway trip, and the current mileage is 44,000. Basically it’s a much better handling and more practical German Range Rover. It costs about £140 to fill the tank, but we don’t do a high annual mileage now, so that isn’t a big deal.

I still have my old Citroën Xsara, which I use for short trips to work and the shops etc. It’s too good a car to change, and it just continues to run very well, with proper maintenance and Millers fuel additive. Anyway, the GL is equipped with twin Diesel Particulate Filters, which I was concerned about before buying the car – for the reasons given in my original correspondence. The dealer assured me, however, that DPFs on Mercedes Benzes don’t go wrong, provided the correct oil is used, the car gets an occasional longer run, and it’s correctly serviced. I wasn’t totally convinced, but I took the decision to buy it anyway. I’ll admit now that my concerns over DPFs were apparently misplaced and I haven’t had any problems to date. The engine, being large and powerful, doesn’t get as much “right foot” as a lesser car, so presumably the exhaust temperatures won’t be particularly high, which isn’t ideal for DPF regeneration. I only notice that the car is going through regeneration when the engine is automatically held in a lower gear than normal for a few minutes, say every 300 to 400 miles, and when driving at over 40mph or so, with the engine oil and coolant up to temperature. This regeneration strategy seems to happen for a few minutes only. I always use a double-dose of Millers diesel fuel additive and the engine runs sweetly, with loads of power and torque. I wonder, however, how many miles per gallon these filters cost me – but if it stops soot entering the air, then I suppose I have to accept that for the common good. I had the Mercedes-Benz dealer check the ash levels in the DPFs at the last service, using the Mercedes “STAR” diagnostic system, and was told that they only contain two per cent ash, which seems to be reassuring.

The next technology I’ll have to worry about though, I suppose, is SCR (Selective Catalytic Reduction that has proven extremely reliable in North American trucks, although these are early days in Europe for use in cars. Do Mercedes-Benz still use EGR in their latest cars fitted with the BlueTec SCR system, can you tell me? I see that the power of the E 350 BlueTec is lower than the previous model, even if fuel consumption seems to be noticeably better. I presume then that the lower power output is as a consequence of the SCR system back-pressure, plus internal engine changes such as a lower compression ratio to reduce NOx formation within the cylinders. I wonder if it will be possible to eventually have a SCR equipped car remapped as per non-SCR equipped diesels, or is it likely that the SCR system could not cope with a higher power output and (presumably) higher NOx  emissions? Before I finish, I’ve attached an article from the “Car Mechanics” Magazine for your information. The author, a car dealer, isn’t overly impressed by diesel cars and concerned by the more technically complex Euro 6 cars about to arrive! Is he talking nonsense, Dear Doctor? I’d love to have your comments on this.
Danny Gillis

Many might disagree with your choice of car Danny and suggest that the GL is a greedy fuel guzzler that should be taxed off the road. I’m not so sure about your dealer’s assertion that GLs don’t suffer DPF problems. Plenty of GL owners, the majority I guess, just won’t want to know about such things. If a warning light comes up, they probably drive to the dealer who sorts the problem and presents a nasty bill that probably has little effect on most owners’ bank balances! I think you need to try and find a non-franchise garage that knows its way around GLs, or certainly the engine, suspension, electrics and fuel system (which according to Warranty Direct are the problem areas in GLs of your vintage), and be ready to take refuge there instead of your Mercedes-Benz dealer, if/when problems arise. Average repair costs for four to five year old GLs (mostly 350 CDI, I would guess) are quoted at around £650 a year, so maybe you are having a good run of luck, and long may it continue! However, evidently your driving habits do give the engine a good enough work out at frequent enough intervals to keep the DPF clear and with suitable engine conditions to ensure fairly regular regenerations.

As for the elimination of EGR systems with SCR, it appears that this will not be the case, and Volkswagen have recently introduced a new twin circuit EGR system on the Euro 6-equipped Golf GTD, one circuit cooled, and one un-cooled mainly for use during warm-up, alongside an additive-free SCR system. Regarding your inference of reduced power on the latest GL 350 CDIs, I’m looking at data showing the last old model GL 350 CDIs having 221bhp/376lb ft, with the new 2012 model having 256bhp/457lb ft, so rather than any loss of power with SCR, there is a significant gain.

The Car Mechanics article is very much black and white. Some of what is written is true, but it doesn’t alter the case for diesel’s greater efficiency and far superior driving characteristics. SCR is close to changing the whole emissions game, I feel, and a new generation of diesel engines will offer strong resistance against the petrol lovers! As somebody pointed out, when you are filling up and you spill a bit of highly volatile petrol, you are putting as much hydrocarbon pollution in the air as you would in a thousand miles of motoring. Nobody ever attempts to regulate this kind of petrol engine car pollution. I hope that I’ve made some relevant and intelligible comments, but this is really something to chatter about over a long pint, or maybe three! Don’t fret that if I’ve been a little hard on your GL, but I suspect that many owners don’t really give a damn about running costs and pollution and all that. So don’t get talking to any fellow GL owners in the hopes of an interesting technical discussion – they might be able to offer you one on 4-4-2, and transfer fees, but they won’t have a clue what you’re on about!
All the best,

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