Quote:
Originally Posted by dcb
...dont know what engine you are thinking of either...
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Every diesel engine with a VGT and EGR has high exhaust manifold pressure.
A VGT (Variable Geometry Turbocharger) is simple: when the ECM wants more boost, it closes down the volute, much like when you squeeze the end of garden hose. The pressure in the hose increases and the water sprays much farther. When more boost is required, the volute closes, creating a) backpressure, and b) high velocity exhaust that spins the turbine much faster, raising boost levels. VNTs (Variable Nozzle Turbocharger) are slightly different, changing the pitch of the volute fins, but the principle is the same: more restriction = more backpressure + more boost.
Also, backpressure in a EGR diesel isn't a nasty side effect of the VGT, but necessary for EGR to occur. Unlike a gas auto engine, diesel engines require EGR flow at full power where NOX production is extremely high. In order to jam recirculated exhaust into an intake manifold with high boost levels, the exhaust manifold pressure has to be higher than the boost pressure. If is wasn't, EGR flow would be backwards. In the 12-13L and 15L HD diesels, I typically see 40 psi of boost and around 50-55 psi of exhaust manifold pressure at max HP. And lots of EGR flow. This pressure difference between the "hot" and "cold" sides of the engine, combined with an EGR valve for control, is what causes EGR flow in a diesel engine.
Diesels require much more EGR flow than a gas engine to have the same effect, because diesel exhaust contains a higher % of oxygen (which aids combustion, not what you want), and a lower % of inert gas (which cools combustion, what you do want) in comparison. EGR levels in 2007 + diesels (without SCR) are as high as 30% of the intake charge!
I would say that the your calculations for a 7L engine are nifty on paper, but in a real working diesel engine with many things that affect many other things - the kind of things that only engineers know the real calculations for - they are pretty much meaningless. But, so is this entire thread unless someone (AKA myself) actually tries the theory in question.