OK. How good is the Si4 vs the SD4

Thanks for clarifying. I realise that diesels arent as efficient on short runs but from experience (and I may be wrong), a diesel is still more efficient than an equivelant petrol on the same run. I understand the higher cost per litre but looking at other suv's, the resale value on diesels are so much better.

(21-10-2011 07:02pm)DDX Wrote:  Does anyone mind explaining why the DPF is particularly bad for short journeys? I have a SD4 on order but my wife will only do around 5000 miles a year, all of which are short stop/start journeys

Yeah just to add to what yts said, a DPF (Diesel Particulate Filter), in case you didn't know, is designed to do exactly what it sounds like - filter out particles from a diesel's exhaust. Every now and then though it needs to clear itself out and it does this by "regenerating". This can be done in a few different ways depending on the specific DPF design, but often (and in the case of the Evoque's I believe) it is done by the engine / exhaust management system changing the mix of fuel and doing a "burn" of all the crap that the filter has collected by running the exhaust hotter than normal for a period of about 20 minutes or so which burns off all the gubbins leaving the filter clear to efficiently filter the particulates from the exhaust again. This "burn" needs to be done at a moderate vehicle speed in order for the exhaust to run hot enough for the process to properly clean out the filter.

If you are only ever doing short journeys then the filter will be gradually clogging up but never getting the chance to properly regenerate and clean itself out. It is important therefore to perform a longer journey than you normally would (i.e. 20mins+ at a decent speed) to allow the DPF regeneration cycle to complete, otherwise the DPF will go faulty and it becomes a trip to a workshop to sort out. I believe the dashboard will indicate when the car is performing (or wanting to perform) a regeneration so you know when to do the longer journey, but it could still be an inconvenience for some.

At least that is my understanding of it - I'm happy to be corrected! Last diesel I had didn't have a DPF so haven't experienced it myself first hand.

Thansk for the further explanation. If the car does provide a message when its regenerating the dpf then a short trip up an 'a' road shouldnt be a problem.

Thanks again and apologies for hijacking the thread.

I am missing the point completely of the DPF, It's meant to cut down emissions but surely whenever it "Regenerates" it coughs out all of the emissions that it's trapped. Net reduction in emissions = zero. Or is there a more scientific explanation?

Take a look at this it explains it in non-scientific terms http://www.theaa.com/motoring_advice/fue...lters.html
and also from the AA there is this training thread on their website http://www.aa-academy.com/Training/Learn...0(DPF).asp

(21-10-2011 08:23pm)DDX Wrote:  Thanks for clarifying. I realise that diesels arent as efficient on short runs but from experience (and I may be wrong), a diesel is still more efficient than an equivelant petrol on the same run. I understand the higher cost per litre but looking at other suv's, the resale value on diesels are so much better.

Given the Dynamic is more sporty orientated & highly desireable I don't think resuduals will suffer too much.

With this stupid DPF thing a diesel even if I wanted it would be a no no given I do mostly short journeys in my Freelander 2 & the fuel consumption on that is poor.
To add wasting time & fuel regenerating the DPF that is a pain or worse getting the dealer to recharge it at an exorbident cost no doubt!

(21-10-2011 08:33pm)DDX Wrote:  Thansk for the further explanation. If the car does provide a message when its regenerating the dpf then a short trip up an 'a' road shouldnt be a problem.

Thanks again and apologies for hijacking the thread.

Im sorry my explanation was rather short but I was rushing out for dinner. I agree that in general the diesel will be more economical and because it has lots of torque it performs well. The smart money is on it holding its value better too. I'm pleased with mine and yesit does tell you when its working but more than that you probably smell it, especially if you have to stop as the first time it happened to us we thought the car in front had a problem. Finally, as understand it the reason for the higher than average temperature is so that the dpf can actually burn the soot efficently and harmlessly. cheers YT