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EGR has little to do with the EGT directly. Putting less fuel into the cylinders reduces EGT on a diesel. On a gasser, the result is the exact opposite - Run too little fuel (lean) and the engine will run VERY hot in the cylinders and risk burning a piston. Towing / working hard on a diesel? Reduce fuel to cool the engine / EGT. Towing / working a gasser hard? Increase fueling to cool the engine. When heavily loaded however, EGR is never used. Only at part-throttle.
As I understand the double-speak... The theory behind EGR operation is that by reducing the available oxygen, or sometimes referred to as "cooling the cylinders" with piping hot non-burnable exhaust gas... The minor production of Nitrous Oxides in the exhaust will be reduced and acid rain won't fall.
The problem is: Diesels are already a lean-burn engine when not at WOT. EGR is only used when not at WOT, so without EGR getting in the way, the engine should be completely combusting the available fuel just fine, thanks. The stupidity of the EGR argument is that engines in some vehicles (such as hybrids, off the top of my head) are FORCED by the computer to run the gas engine for as long as 10 minutes to ensure that the cylinders are HOT, so as to (supposedly) do exactly the same thing, and to waste fuel heating up the Catalytic Converter, which (apparently) cannot perform a chemical air-contact reaction unless at significant temperature.
So to recap... Engines have to be hot and completely combust to reduce nitrous oxide formation, but EGR is used to "cool" the cylinders and reduce the available oxygen TO combust.
The end result of this is reduced efficiency, and attempts at after-treatment when a more effective solution would be to remove the more volatile elements from the fuel before they are ever burned. This idea would be similar to removing the bullets from a gun, rather than trying to plug the holes in the target CAUSED by such bullets. The exhaust of my CRD doesn't smell like acrid burning sulfur because I have an efficiency-choking catalytic on there... It doesn't smell b/c there isn't any sulfur IN the fuel I put into the tank. The combustion doesn't need after-treatment for what has finally been removed from the fuel after years of the oil companies crying that cleaning their fuel wasn't possible. (Biodiesel is still MUCH cleaner, and the exhaust shows it)
If you read through the various statements and releases about exhaust treatment from the EPA and CARB, you will very quickly see that a lot of what they say is contradictory, especially when illuminated by simple logic.
In a diesel, soot (particulates) are a particular complaint... Because that soot is VISIBLE. ALL combustion, from a campfire to the plume of a NASA launch has combustion results. Everything has particulate matter, the difference is the size. Gasoline engines aren't any more "clean burning" than a diesel, they make particulates just as much, but the SIZE of the particles is much smaller. Through EGR, you are carrying these particles BACK into your very-tight-tolerance cylinders... And scouring the cylinder walls with a NASTY abrasive. Do you want to do this with your engine? I don't. Not for a dubious benefit, such as making my engine use MORE fuel and create MORE emissions b/c of some idiot device on there.
The kind of "logic" and the contradictions in justifications they use is the reason for things like the DPF on new diesels. It "reduces emissions" by injecting RAW FUEL into the EXHAUST LINE, (After the engine, just to clarify) where 100% of that fuel will be burned in the exhaust line, creating ZERO POWER for the engine... And 100% emissions, as a means of "reducing emissions" from the tailpipe.
What have they been smoking? Cuz that must be some GOOOOOD chit mon.
As for a diesel and EGT, you can't have too low of an EGT. Too high, and things melt as you might imagine. Low EGT = Proper amount of fuel being completely consumed in the cylinders, for maximum efficiency. That is why boost is lower with lower EGT - Less unburnt fuel leaving the cylinders still burning (and expanding) in the manifold to create the pressure to spin the turbo. But with low fuel use, there must be low demand on the engine, so lower boost is OK.
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