This is a direct lift of "Wikepedia" publishing re. the above.

In diesel engines

In modern diesel engines, the EGR gas is cooled with a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, when there is otherwise a large excess of air) in controlling NOx emissions.[citation needed] Exhaust recirculated back into the cylinder can increase engine wear as carbon particulate wash past the rings and into the oil.[6]
Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines. Exhaust gas—largely carbon dioxide and water vapor—has a higher specific heat than air, so it still serves to lower peak combustion temperatures. However, adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR.[7] [8]
Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter (PM) call for further emission controls to be introduced to compensate for the PM emissions introduced by EGR. The most common is a diesel particulate filter in the exhaust system which cleans the exhaust but reduces fuel efficiency. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen, these filters need to function properly to burn off soot. Automakers inject fuel and air directly into the exhaust system to keep these PM filters from becoming blocked up.


This link provides the full text, and in my opinion a very good read.
https://en.wikipedia.org/wiki/Exhaus...diesel_engines


The EGR effects on fuel consumption are very different in petrol (SI) engines from diesel engines (CI).



Leedsman
p.s. There is much controversy over EGR: See this part of Wikepedia for full text.

https://en.wikipedia.org/wiki/Talk%3..._recirculation

 

An interesting read my mate. An unscientific 20 minute scan read of all the material listed above suggests that a guesstimate of 80% for the argument that EGR's reduce miles per £ ............ and almost a 0% for the argument that EGR's increase miles per £.


This suggests to me that while the minority 20% of the above who defend EGR's see the reduction as a price worth paying for reduced NOx. For myself I'd prefer my £ to be converted into MPG - I mean it really is that simple. I have choices, there's no VOSA requirement in the UK for an EGR and until it becomes a mandatory requirement in law then I make the choice to defend my right if I so choose to eliminate the EGR from my engine and decouple my wallet from Europe's bad legislation, I've been banging on about the right to choose EGR or no-EGR for many years, and continue so to do. They have made some weak [visual only inspection] attempt close the door on DPF's but the EGR remains the same as it was many years ago.


I already have the efficiency of my £ per gallon reduced by a significant 20% by Methanol which GOV had legislated out of my control, a further 20% with the combined EGR & DPF is just the usual politicians way of making Joe public spend his wonka supporting something we choose not to. This might explain why with much more efficient modern engines our CRD common rail vehicles do less miles per £ gallon in 2015 than they did when they returned more MPG per £ in the year 2K than they do now.

 

Looks like this result was as I thought expected. The exhaust recirculated back into the cylinder via the EGR as said here again and again blocks the EGR and its pipework in the form of carbon.
So why of why should we want this carbon going into the cylinders, as we already have an air filter to clean the intake air. Is there any point of using an air filter come to think of it.

 

An aspect (not a good one either) of this is that crankcase fumes are also fed ino the inlet manifold along with this EGR proportion making a pernicious mixture of oil products that is bound to come out of the tailpipe into the atmosphere. It's no wonder a DPF had to be fitted to clean up this horrible mixture.

1) Crankcase fumes should be vented to a SEPARATE container. Only low-tech is needed here.
2) EGR should be abandoned and the engine designed to burn ALL the fuel leaving practically no particulates, as current technology can.
3) There ARE other ways of reducing NOX, and these should be investigated.

Leedsman.

 

Our old XK jags used to have straight crank breather tanks for purging air and particularly the water that was the actual 'carrier' for the impurities.