rv6ejguy said:
1) I stated that EGT (heating) and CHT (quenching or cooling against the seat and guide) are the primary factors in exhaust valve temperature.
2) Since intake valve temps are several hundred degrees lower, it is obvious that EGT has a higher impact on valve temps than combustion temps when both valves are closed. If combustion temps were the primary factor in valve temps as you say, intakes and exhausts would run at the same temps.
3) To say that EGT does not have a major effect on valve temp is simply incorrect seeing as it is immersed in high mass flow 1600F exhaust gases when open.
4) Pressure has no effect on temperature at a fixed volume.
1) You still have not explained how the exhaust valve gets cooler as EGT is going up. Until that fact is addressed, any theory that says that EGT heats the exhaust is not viable. The EGT is NOT what's heating the valve in the above case, it's actually COOLING it. If the exhaust valve is cooling as the hot gas is getting hotter and flowing past it, it must be cooling it! <VBG> That problematic fact must be reconciled.
2) The intake and exhaust valves are both exposed to the same 3800 degree heat of combustion. The intake valve is exposed to a HUGE flow of cold air as it is open. The metal around the intake valve seat is cooler than that around the exhaust valve. No wonder it runs cooler. The metal around the exhaust seat is hotter. No wonder the exhaust valve is not cooled as well--it's sitting on a hotter seat.
3) OK, as the gas is getting hotter, why is the valve getting cooler as the mixture is moved from 25dF ROP toward peak EGT? If you're right it oughta be getting hotter, but it's getting cooler. Please explain that.
4) Internal cylinder pressure does have an effect on the BTUs transfered into the cylinder, head, valves, and piston. High pressure means more heat transfer across the thermnal boundary layer. THIS effects valve temperature (along with CHT) more than anything else.
Think about this:
Diesels run very high head and valve temps and must shed much more heat than gasoline engines but have very, very low EGTs. Why the higher valve temps with lower EGTs?
The answer is in Boyle's Law and the expansion of the combusting gases. Based on one mixture, EGTs are what they are based on the compression ratio and the timing. The earlier the thetaPP, the lower the EGT, but the higher the exhaust valve temperature!
The heat of the exhaust valve is related to the temperature of combustion, the internal cylinder pressure and the cooling effect of the valve seat/head contact. Yes, it cools less with hot gas going by it than the intake valve that has cold gas going by it.
Many are surprised to find that the exhaust valve is often hotter than the exhaust gas and is actually being cooled by the exhaust gas going by it!
Walter