eightinavee, although I'm not disputing that the egr valve serves a purpose, it's the real world value of it that I'm questioning, and the motive the manufacturers have to use it, and then the benefit they try to apportion to it in sales literature and in the appearance of being 'green'.
At tickover, where emissions tests are carried out it will be open, at which point the drop in NOx will be reduced (beneficial as vehicles spend an awfull lot of time there), but the increase in fuel needed to sustain the same engine speed with it open and running, and running at less than perfect Stoichiometric value is neither green nor beneficial, just helpfull in passing a standardised test taking very few variables into account. HC and Particulate emissions generally increase under these conditions.
Under light acceleration requires that you apply more throttle to gain the desired acceleration giving the same results.
On overrun, the engine would normally be running on next to nothing but fresh air, so the gain is negligible - back to the percentage values quoted last post about re-burn.
Not being familiar with the 300TDi engine this thread was originally about, but does it also open at the throttle value equivilent to 100km per hour in top gear used in mpg testing and drive by noise tests (and also some emissions tests) as it does in other vehicles, accounting for the often massive flat spot seen on torque output graphs, mapped by the manufacturers for no reason other than to pass tests imposed on them, allowing them to produce un-green engines that appear to be greener than they really are?
I'm not a member of the green lobby, having owned numerous v6 and performance vehicles, but as I said previously I get fed up with manufacturers colluding with governments and 'independant' testing organisations to put up a smoke screen of economy and 'greenness' where the owner ends up being the loser, having to drive vehicles that are deliberately de-tuned at the point where the performance would be most useful and then sold on the idea that they are in fact benefitting from all of this.
Rant over.....for now.....