[Richard G]

those of us with diesel might like to look at this article

http://en.wikipedia.org/wiki/Diesel_engine

I especially note the comment about black smoke if the injection timing is too late. It started me thinking about the effect of EGR on the flame speed and a whole lot of other technical factors

 

[Richard G]

Yes EGR does affect the flame speed
The reason for all this is that the flame has to travel a finite distance through the cylinder in order to provide the "bang" that drives the piston down. The problem is that as the engine speed increases this fixed time takes more degrees of crank rotation to achieve so the injection has to be advanced more (like ignition timing in petrol) If the EGR valve allows excess exhaust through (stuck open or similar) then the injection is late and as per above helps to cause the black smoke

These Bosch publications may be of interest
http://www.boschautoparts.co.uk/tpDEMM1.asp?c=2&d=2
The only dealer I've found at present is in the states though I'm investigating (unless anyone knows of a UK source)