Rwy20 wrote:
What I meant was this, where the TIT (1620°) is constantly higher than any of the EGTs (1425° to 1505°) in cruise
I guess it is because the TIT probe is heated by the power stroke of each and every cylinder, i.e. more often than the EGT gauges which get heated once in two revolutions, so the gauge does not have to cool down as much in between two events.
Yes, I think that is the explanation. It goes with this graph that Mike Bush explains in the video (from 08:20 onwards) that boscomantico linked to:
https://d3hbcrfcqdtpdz.cloudfront.net/static/images/article-figures/01-figure-3.45235df2cb02.png
The EGT probe only sees the hot gases for 1/4 of the time (cf. the bottom part of the graph, “exhaust valve open”), whereas the TIT probe will be constantly heated, even though the absolute temperature passing by the TIT probe will be lower than that which touches the EGT probes!
The fact that the gases don’t get heated on the way from the EGT probe to the TIT probe, but the latter shows a higher temperature, underscores why these absolute EGT values are just arbitrary numbers.
That can’t be it. If the exhaust pipes were perfect insulators, the entire exhaust stack would be the same EGT throughout the cycle. When the valve closes, the gas temperature behind the valve can only lower because of two reasons – a change in pressure, and heat lost through the manifold / exhaust pipes.
I reckon the gases cool substantially after the exhaust cycle, by expansion within the tube. They have to if the peak temp is way above 1500F, to get a 1500F indication on the probe.
OTOH the probe must have some thermal inertia…
A piston TIT is not constantly heated, but sees a lot more power pulses than a single EGT gauge.
A turbine engine would deliver a constant flow of exhaust temperature.
The fact that the gases don’t get heated on the way from the EGT probe to the TIT probe, but the latter shows a higher temperature, underscores why these absolute EGT values are just arbitrary numbers.
I agree that they’re arbitrary numbers, but there must be conditions where combustion is still occurring in the exhaust manifold and the heat from this late combustion exceeds any heat lost by expansion until the exhaust gas has been through the turbine.
As for the OP’s question, if cruise EGTs were relevant to valve life, the engine manufacturer would state a limitation.
Not in a conforming engine set up.
Combustion finishes in the cylinder.
If combustion continues in the exhaust one knows about very quickly
Jacko wrote:
As for the OP’s question, if cruise EGTs were relevant to valve life, the engine manufacturer would state a limitation.
Although that’s probably true, I got a few problems with that:
So rather than just accepting what the manufacturer tells me, I prefer understanding what’s behind the recommendation.
I disagree. Combustion in the manifold is not limited to automotive anti-lag systems. Valve overlap means that unburnt fuel/air mixture enters the exhaust manifold and can be a contributory factor to the (admittedly inconsequential) EGT/TIT difference – cooling the first and warming the second.
Yes, I trust the manufacturers. The normal operating range for the Lyc O-360 in my Maule manual is 200-435 F. For my CAP 10 it is 150-400 F. I don’t think those are outrageous figures, give or take 10%.
So I’m inclined to regard the 500 F limit a bit like Vne – i.e. I should activate Plan B before we get there.
