Here is a scenario I've been thinking about that might explain the piston burning, the larger ratio of "Combustion Temps / EGT's".
Imagine first, for reasons unknown, that we have a good spray pattern, but due to weak injectors, or the IP being too advanced. So, our nice fuel spray occurs too far advanced, so, the initial compression induced burning begins at multiple sites at the periphery of the cloud (which, I believe it does), then, as is normal, radiative heat from the initial compression burn sites ignites the rest of the fuel. So, the pressures and temps are much higher for a longer duration before the piston reaches TDC. We see in the image of the piston that the melting seems predominant at the periphery of the piston, which is a very normal thing as edges have more heat to move and it is a slower process. As background, I don't know how many compression rings this piston has, for arguments sake let's say two compression rings and one oil scraper. The compression rings revolve around the piston not in a uni-directional fashion at from 3 to 6 rpm, but at a rate of 3 to 6 rpm, but as the ring gaps begin to near each other pressure is built up by the first gasses passing the end gap of the ring, over the first land making it's way to the gap of the 2nd compression ring pushing both in the opposite direction relative to each other, back & forth across these 180 degrees. Now, this initial problem of way higher than designed pressures & temps during the compression stroke begins a partial melt of the edge of the piston, eventually some of this melt making it's way to the end gap of the 1st compression ring and filling it. Continued partial melt enters the end gap gradually pushing the first ring tighter & tighter against the cylinder wall. Now we have that increased heat from friction, plus we've blocked the nominal amount of gas that normally get's around both both compression rings. Eventually the 2nd compression ring make's it's way so that it's end gap lines up with the now non-existent gap of the first ring. Heat is greater there because the initial fit interference at the frozen gap prevents any up & down movement, so the 2nd compression ring now locks up, with the land between the two rings expanded by the heat locking it up. So, now we have both rings locked exerting greater pressure on the cylinder wall and concomitant heat build up. The initial problem of an early injection, prior to the sealing, whether partial or complete, overheats the oil scrape ring resulting in either partial lockup, but whatever, it now doesn't scrape the hot oil well, which removal helped to keep cylinder wall & piston temps down. At this point we have a troublesome environment in progress, with more and more of the piston at the initial melt spot having nubs and good piston top/skirt edge burning. Maybe some to the same fate of the initial burn spot and the rest being ejected to the exhaust port. As material is lost from piston this lowers compression by increasing the volume normally seen at TDC, perhaps resulting in the increased smoking & the lowered EGT, while maintaining the higher peak combustion temps because the hot piston is still igniting the cloud too early due to hot spots, even though overall minimum cylinder volume has increased. Also, if the early or later piston disintegration that I hypothesized above is true, the oil scraper is not removing cylinder wall oil as it should resulting in higher temperatures. So, yes, we do get some of the excess temp gone from wall to coolant jacket, but we also have the locked rings on the piston, deficient oil scraping, and I surmise that the piston itself transfers much heat from piston top and skirt, with poor heat transfer from skirt to oil that is normally making it's way back to the oil pan. In my guess here I am saying that we're seeing combustion temps way above normal, but much of the energy has been transferred too early contributing to higher pre-TDC temps & pressures than is normal. Yet, if the EGT's are correct, where has all the heat gone from the abnormally high peak combustion temperature? Perhaps our piston/cylinder interface can transfer a lot of this heat (it doe's have more time to do so). I'm guessing, and here I just can't visualize or intuit the physics, the way advanced peak pressure and temperature is losing it's energy partly in a normal fashion via the cylinder wall and what the head absorbs, but I would think the majority of the heat is lost through friction not only on the compression stroke, but also on the power and exhaust strokes due to locked up rings, whether complete or partial lockup, so the EGT's are normal or relatively okay as this is just a gas temp we're looking at & a good deal of the gas partial pressure energy dissipated through the piston and cyl wall much before it should have relative to TDC. I realize I can't have it both ways as regards an increase in exhaust smoke, though I'm not sure if we're talking about fuel or oil being the main component. By both ways I mean I can't say the compression rings are locked tighter to the cyl walls and less oil is making it's way past the oil scraper, and we're seeing increased exhaust smoke from either oil or fuel. though perhaps there is some balance of these processes that allows my argument to have it's cake and eat it too. My argument says we're seeing rather complete burning of fuel, more so than normal, unless normal means complete burning; so, I stick with the scenario of combustion heat, per se, exclusive of all the frictional heat, having the additional time to cool by heat transfer by the time we're on the exhaust stroke at that point where maximal EGT are normally noted. My previous post of a poor, streamlike spray pattern on a single spot of the piston most certainly wouldn't be at the interface of piston top and skirt. So, I guess on the relatively low EGT's, with all the variables I've thrown into this, I would have to say that early combustion from beginning to complete is what dominates & EGT's are low because combustion is complete way too early, much of the pressure, hence heat, is lost as described previously. The fuel:air mix has a defined maximum pressure/heat it can create. I've said by a larger percentage of that pressure/heat working against the piston as it travels towards TDC is why we see relatively low EGT's, while burning pistons with relatively high combustion temps. I don't know. I wish I did. This just seems such a problem that reveals much, if understood, of both normal & abnormal diesel function that I wish I knew if my guesses are even partially on track.