Hey Tinman, I've been mulling things over too in my head about the effects of timing/EGT/fueling/etc. Forgive me if I'm being elementary and stating things you already know. I don't know if I've got any answers, but I'll just try to expound a little on what's been floating around in my head. :-laf
First, I do work at a place where we have fully instrumented diesel engines set up on dynomometers... this typically includes individual cylinder temps, every other temp imaginable, pressures on everything, in-cylinder pressure, crankshaft encoder (for measuring velocity, acceleration, etc of the crankshaft at 1/5 of a crank angle degree resolution), high speed data acquisition, real-time heat release, full emission benches, air/fuel ratio UEGO sensors, and on and on.
While I can't speak particularly concerning engine makes and models, I might be able to provide some insight on general trends for turbocharged, inline, 6-cylinder diesels.
As a general rule of thumb... given a fixed fueling duration and rate, EGTs will decrease as engine rpm increases. This is one reason for keeping rpm above 2100 or so when pulling heavy up hills with our trucks. The engine is "pumping" more air due to the higher rpm. Sure, the truck will pull the load up the same hill at 1600 rpm, but at a much higher temperature. It takes a certain amount of power to get the load to the top of the hill at speed... and thus, a certain amount of fuel. When this quantity of fuel is mixed with an abundance of air (higher rpm), things will stay cooler than when burning the same amount of fuel with less air. I guess another way of thinking about this would be that there has to be more fuel per stroke injected at lower rpm, and so higher temperatures. If the same amount of fuel is injected on an engine running much higher rpm, the heat loading per combustion event is much less.
At low load conditions, diesel engines run very lean... sometimes 40:1, 50:1, and even 60:1 (stoichiometric ratio being around 14. 7:1). Contrary to how many people think, running lean generally means low EGTs. However, most people try to rely upon knowledge of naturally-aspirated gasoline engines, and say that running lean will produce high EGT. Again, this is not true for turbocharged diesels.
Now, things change when adding chips, programmers, large turbos, etc. Most fueling boxes or programmers increase the duration of the fuel across the board, but particularly at lower rpm. This is done in part to "overfuel" on the bottom end to light a big, laggy turbo quicker with all the extra thermal expansion and heat available. This can produce very hot EGTs. Much more fuel is being injected than can be burned cleanly. For those with a manual transmission and modified trucks, you know you can shut down an interstate by shifting to low rpm and going full throttle. The engine goes extremely rich, and particularly with a large turbo, you are so far out of its efficiency range, that it can take a very long time to build any boost. But, when it does "light", the smoke clears up quickly... because there is now enough air being forced into the engine to bring the fuel/air ratio back to slightly lean levels.
So, high EGT on modified trucks is a complicated issue, that I'm not sure I fully grasp. However, heavy overfueling at low rpm can definitely tend toward higher EGT.
Another consideration is the issue of advanced timing. Diesel fuel (and all other fuel) has a certain ignition delay. On our data acquisition systems, you can see the fuel injection profile, and overlay the heat-release and pressure trace... if you look closely, you can see the "delay" period between when fuel is injected, and when it begins to "burn". People typically discuss the "burn" of the fuel as 10% burn time, 50% burn time, and 90% burn time.
Obviously, the engine is only making power when the "burn" occurs on the downward "power" stroke of the piston. In order to compensate for ignition delay and other factors, the fuel is injected several degrees before TDC. This allows the heavy flame propagation to occur very shortly after the piston passes TDC and begins its power stroke.
When fueling is increased with programmers, chips, etc, the timing is usually advanced as well. However, it is not really the timing directly that causes higher EGTs. Rather, because more fuel is burning over a longer duration of time in the cylinder (hence, more power), the added residence time of this high combustion temperature transfers much more heat to the piston.
At least how I understand it in my mind, is it's more like a time-weighted average of high in cylinder temperature that is harmful. For example, assume short spurts of 1600° F is allowable without damage at OEM fueling and timing. Clearly the melting point of aluminum is well below 1600° F... but the piston doesn't melt, because the 1600° is only "in the cylinder" for a short amount of time, and the oil provides cooling to the piston as well.
Now, with heavy fueling and really advanced timing, it may be that an indicated 1400° exhaust temperature can melt a piston, whereas at 1600° with stock fueling and timing things were okay. Why is this? Well, one reason is because the 1400° is acting on the piston for a lot longer time than the 1600° OEM point... and thus, there is comparatively, alot longer time that the piston is absorbing heat. When running with advanced timing and heavy fueling, you can see oil temperature begin to creep up, as more heat is being transferred into the piston, and subsequently, the oil.
Another reason that advanced timing may be more harmful, is that the EGT read by the gauge is not exactly comparing apples to apples. With the stock timing and fueling maps, the exhaust temperature read by the gauge is the temperature of the exhaust being picked up by the thermocouple probe around 180° - 360° degrees after SOC (start of combustion). However, if you're running a lot of timing advance, the same probe is measuring EGT 205° - 385° after SOC. Thus, the exhaust has had longer to cool down before being read by the gauge.
The exhaust gas temperature indicated by an in-cab gauge is NOT the peak combustion temperature of the gas in the cylinder. It is the temperature of the gas after it has burnt and expanded through the power stroke and after being "pushed" out of the cylinder by the exhaust stroke, and entered into the exhaust manifold. Thus, when advancing timing 20° - 25°+ before TDC, the exhaust has been allowed to cool more by this longer amount of time.
Another thing that is sometime overlooked, is when a fueling modification increases rail pressure, timing is also being "advanced". This is because a larger amount of fuel is able to be pushed through the same duration of the injector. The same thing happens when running larger injectors... more fuel is being injected in the same duration... which is effectively increasing the timing. This is why on the Smarty programs, they advise you to run less timing with large injectors than with the OEM sticks.
Do all these things really matter? Well, actually it appears that they do. Many people with modified trucks have reported melting pistons, dropping valves seats, etc. at temperatures that a stock engine would have no problem with.
Clearly, things are much more complicated that what I've described here, but I believe this is something not everyone understands. When modifying these engines, you can't always go by "safe" limits defined on a stock engine, and consider these to be "safe" limits on a highly modified engine.
I think one of the most useful "tools" on these trucks would be a timing gauge. When stacking boxes, programmers, higher flow CP3s, injectors, cams, etc. , it's unclear what is really going on inside the cylinder.
At work, we can use a current probe around one of the injector wires, and the crank shaft encoder as a TDC reference, to "look" at timing. I would imagine that the same thing could be done on our trucks using an inexpensive probe around one of the wires, and synchronizing it with a reference signal from the cam shaft position sensor or crank shaft position sensor. This could then be fed into a stepper motor type gauge, and give a real-time indication of commanded injection timing. This wouldn't account for inherent electric/hydraulic delay of the injector, larger injectors, or other mechanical-related mods, but it would allow you to see when fuel is being commanded... and based upon a combination of timing and EGT, much safer operating conditions could be defined.
As for cylinder pressure, and "knocking" of the engine... when timing is advanced, the pressure in the cylinder can climb much faster. This is often termed "peak pressure rise", and is measure in bar/degree. Typically, for engine longevity, you would want the peak pressure rise to stay below 10 bar/degree. On an older, singe injection event diesel, you can often hear when this gets too high, as knocking is often present. However, on common rail diesels with multiple injection events (some up to 13 per cycle!), damage may be lingering before it's audible.
As far as EGR and low load vs high load... well, kindof running out of time, but here's some thoughts. At low load, the exhaust is relatively cool, and very high EGR rates may be achieved (we've experimented 60 - 70% egr). However, at high load, it's hard to "drive" egr, simply because back pressure pushing the egr has to be higher than the boost pressure for things to go in that direction. Higher egr rates at load can generally be achieved with variable geometry turbochargers, because you can change the vane angle to alter backpressure and drive egr to higher levels. However, with a fixed-vane turbo, you're kindof stuck. This is one of the biggest reasons that Detroit, Cummins, etc have implemented VGT turbos at the same time they introduced external EGR.
Well, lunch break is over...
--Eric
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Attention: TDR Forum Junkies 
This is where an EGR equipped diesel used more exhaust gases to cool combustion temps for decreased NO. EGR gases become less the closer the engine gets to WOT b/o a "richer" fuel air environment. Does this mean cylinder temps are cooler(est) at WOT??
