WWDiesel wrote:
Surely the valve are designed to handle temperatures of exhaust gases.
This is hardly a foregone conclusion... And don't call me Shirley.
WWDiesel wrote:
Generally in a diesel engine, it it not recommended to run EGT above 1300 deg. F for any sustained periods to prevent piston damage.
As already said, to reduce EGT's, more boost, less fuel, and / or reducing exhaust backpressure can all lower EGT's.
Here's the thing about the pistons and the head though - there are cooling passages in the head that are RIGHT NEXT TO the combustion chamber, and the pistons have a continuous oil jet spraying up into the underside of the piston crown. So while there is an active explosion happening for 1/4 of every cycle and 500 times every minute (at 2000 rpm)... That heat is a very brief temperature rise against continually-cooled surfaces with rather large surface areas and immense thermal inertia to overcome.
I am going to change a term here for clarity - the bottom of a valve is called the "head" which is confusing when thinking about the entire assembly - Head contacts valve seat which is also contacting the head it is pressed into. So for clarity - the bottom of the valve shall be called the "coin" since it looks like a coin after they break.
Thinking about the valves, they only contact the head in two places: The coin contacting the valve seat, and the spring / keeper / rocker arm at the top. I do not know for certain if the valves are also oil-cooled, but I suspect they are. Every one that I have disassembled comes out coated in oil along the entire length of the stem. In other designs, the valves can ONLY cool themselves by contact with the valve seat - so there is a high potential for heat-soak and the valves actually being hotter than the surrounding metal of the head and piston which are continuously cooled.
Where the piston and the head surface also only have 1/4 of the cycle under explosive heat, the exhaust valves actually have 1/2 of the cycle - because the coin gets blasted by the explosion, then the stem gets blasted by the same heat during the exhaust stroke AND there is the potential for even more added heat with afterburn fuel injection events! So if the combustion hasn't finished before the valve opens... That isn't just the measured EGT (pre-turbo) that the alloy might be seeing... That is FULL ON combustion temps and the coin is also absorbing that heat without the benefit of being closed against the valve seat and water-cooled-head to draw off that heat.
Maybe there are after-burn events programmed in the tune, I don't know for certain. I do know that multiple injection events have been used to "quiet that diesel sound" and control the noise / vibration / handling as cars are being designed. Maybe there's something to this, I don't know.