How much does EGR affect fuel consumption though, really? It’s just replacing some of the air in the cylinder with exhaust, but most of that air you don’t need anyway when cruising…?

Speed12 – VW TDIs in the USA have a system for scrubbing NOx – could be similar to SCR.

Adding EGR lowers the AFR in the cylinder quite dramatically – even with a pretty standard EGRate of around 8-10% your AFR can drop down to 17:1 or so which means although diesels generally are “air rich” you have a lot lot less to play with to get all the fuel burnt. Add to that the combustion temperature lowering effect of adding EGR (which in case anyone was wondering, is the point of it – reduced temperature = reduced NOx) and you get much more unstable combustion which in turn means you need to throw in a wee bit more fuel to ensure you get enough torque. Depending on the combustion bowl design, swirl etc will depend on exactly how much extra you need to put in (i.e. how efficient the combustion system is anyway).

Yeah, the VW system would be SCR. US vehicles are starting to introduce it as the NOx requirements are a lot lower than in Europe. Once Euro 6 comes properly into force you will start to see more SCR on passenger cars in Europe. PSA (Peugeot Citroen) have said that all of their diesel fleet will be equipped with SCR in the coming years. You can also use an LNT (Lean NOx Trap) which is more like a gasoline catalyst, but aren’t quite as effective at getting rid of NOx.

Good stuff speed12, always good to talk to someone who knows their stuff 🙂 I read about the VW NOx scrubber, it seemed very complicated.. and people complain about EGR and DPFs!

How does boost pressure affect combustion and swirl? I’m thinking of the VAG 1.6 TDis that presumably use a higher boost pressure to compensate for smaller displacement.

Which reminds me of something.. a while back someone was telling us that their mate who worked for Volvo said that they sacrifice fuel economy a bit to reduce certain particulate emissions of some kind.. any ideas?

Haha, no worries, I’m a diesel calibration engineer by trade so it’s te sort of thing I could talk about for hours!

SCR systems are reasonably simple really – it’s just like a normal catalyst but needs urea sprayed on it to convert the NOx to NO2. It’s a good technology and will actually mean a lot less reliance on EGR valves as they are so effective – which in turn will mean better mpg (CO2) and lower PM. Downside is the control system is quite expensive at the moment but the SCR cat itself is quite cheap as it’s a non-precious metal cat (uses Iron or Copper rather than Platinum or Rhodium).

Boost won’t affect swirl very much as that is just a function of intake port/cylinder bowl geometry. Combustion wise though, a higher boost simply means more air in the cylinder for a given intake stroke which in a diesel is exactly what you need. It’s one of the key parameters we play with to decrease fuel consumption. It has to be calibrated carefully though as higher boost will draw through more EGR and so you could actually end up with higher CO2 and more PM by raising boost. But overall higher is better!

Not too sure about older Volvos, but having just finished the calibration work for the latest C30, S40, V50, V60, S60, S80 and V70 I can tell you they certainly don’t now! The cals on those cars are very low PM and with CO2 down to 99-119g/km (very low) and with good NOx as well. It took a bloody lot of work to get there mind!

I did notice that their driv-e or whatever it is eco range looks particularly good (geddit? See what I did there? Particle.. oh well 🙂 ).

Seems to me that most of the wastage in diesels is compressing air that you don’t need. So it would make sense to have a small displacement and bigger turbo as seems popular at the moment, so when cruising you can turn off boost and compress less air.

Is anyone working on small 6 or 8 cylinder diesel engines where you can turn off one side by leaving the valves open, like they do on big US petrol pickup engines? Cos I always thought that would be a good idea to reduce wasted compressed air…

I would say that keeping it clean and tidy inside by a periodic thrashing would help prevent that happening.. but perhaps not fix it.

Hmm. My wife has an Audi TDi and she drives like an old lady. She *never* floors it. When I drive that car it produces masses of black soot. Now I know this is normal for VAG engines but is there actually a sound basis behind ragging it every once in a while. I know knowitall blokes down the pub will say this but what about smart people?

Oh, and swirl flaps. I like it.

When I drive that car it produces masses of black soot

When you granny it, soot deposits build up in the engine. When you thrash it, the deposits get burned up and blown out of the exhaust, which is the smoke you see. It shouldn’t do it all the time. And VAG engines do not smoke any more than any other diesel! Take it on a long fast trip and you should be fine at the end. If it’s really bad you might have to clean some stuff like the EGR valve out manually, or some sensors might be clogged up so they might need a clean too – but I dunno.

Police officer – Can you tell me why you were thrashing the nuts off your land barge sir?

Diesel Car Driver – Italian Tune Up.

Police Officer – Oh thats fine, nice flaps BTW 😀

ok, ta.

And VAG engines do not smoke any more than any other diesel!

Nah, not all diesels produce smoke. Once my Honda is warm I can cane it with no obvious smoke at all.

My VAG doesn’t smoke either.

Sometimes you get a puff from certain cars, this I suspect is a bit of over-fuelling to spool the turbo up as fast as possible. Most produce a very fine blue haze when you floor it which you can’t see at all unless it’s night and someone’s behind you with foglights low down.

Hondas are supposed to be pretty clean tho.

I opened this thread with some trepidation, but find I am bored, not disgusted.

Swirl Flaps, have to like that – are they connected to the Flanges ??

speed12 – Member
Haha, no worries, I’m a diesel calibration engineer by trade so it’s te sort of thing I could talk about for hours!

SCR systems are reasonably simple really – it’s just like a normal catalyst but needs urea sprayed on it to convert the NOx to NO2.

How do you fill that SCR system up then? 😉

Interesting, you could wee in the tank if you find yourself without 🙂

the reason your diesels don’t smoke is because they have a dpf to reduce the smoke. If you whipped that out, you’d see a puff on accelleration like anything else.

My 180,000 mile mondeo is great for leaving a smokescreen. someone a bit too close behind on the motorway? 3rd gear, boot it. Car doesn’t actually go any faster (it had all of 88bhp when new. I doubt its got 50 now) but it annoys the guy on your bumper

My diesel doesn’t have a DPF, and I do sometimes (but not always) see a puff on acceleration at night. But of course I am not always looking and I also don’t have a good view of the exhaust from the driver’s seat 🙂 Certainly can’t see anything in the rear view in the daytime.

What make of car is VAG??? 🙄
Why not just say VW or Passat, as that’s what it is? Or does it make it better by association with an Audi?

VAG is easier to type than “VW/Audi/Skoda/Seat” and they all have the same engines which is what this thread is about 🙂

Yes, I know what it means, but YOU have a VW Passat, so why try and hide the fact.
Actually THIS thread is about BMW’s, NOT VAJ’s. 😉

Yes, I know what it means, but YOU have a VW Passat, so why try and hide the fact.

If I was trying to hide it I’d lie about it on here and you’d never know 🙂

Actually THIS thread is about BMW’s, NOT VAJ’s.

I can start a thread about Vaj, I’m away for the weekend so the 3 day ban is worth the laugh. 🙂

At least someone appreciated it!!!

What make of car is VAG???
Why not just say VW or Passat, as that’s what it is? Or does it make it better by association with an Audi?

Awesome, inverse snobbery 😀