Originally Posted by
Walter Atkinson
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The "temperature" does not create the pressure. The expansion of the burning gas that creates the pressure, the increasing pressure affects the heating. It's a small nit, but it's real.
That's not truly accurate either... the total gas is not expanding until top centre, sure the the volume of gas that has ignited is expanding.
Originally Posted by
Walter Atkinson
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The NACA report 754 alludes to this in the section on the effect of timing on valve temperature:
"The rise in valve temperature with greatly retarded spark is probably caused by the higher exhaust-gas temperature resulting from a decreased expansion after combustion"
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They got this "probably" wrong. This is a perfect example of a correct observation leading to an incorrect assumption of causality. The later thetaPP of a retarded spark has the resultant EGT higher at a much reduced pressure. The rising valve temp in these conditions are caused by the still burning 3800dF combustion gasses going past the open valve. There was no time for the gas to expand after burning and lower the EGT. We can see this effect in very well-balanced F:A engines that are leaned beyond optimal. This is not a "normal" condition.
Correct.
I would agree with that, now that your considering the valves exposure to the flame, I would submit that this is also why the valve temperature curve is reasonably flat from 5 to 25 degrees, even though EGT increases, the valve face exposure to the advancing flame is reducing as the volume of the gas expands and moves away from the face.