Originally Posted by
andrewr
Is there evidence to support the idea that the same mixture always gives the same EGT? (Actually we know it doesn't - one ignition inoperative will raise EGT, detonation will lower EGT, but lets ignore malfunctions.)
I would be slightly surprised if the EGT was always the same - my guess would be that lower RPM, lower manifold pressure and lower air density would each lower EGT to some extent. That is just a guess though, I would be very interested in actual data.
Well obviously you’ll get different EGTs for the same mixture if you change MP or RPM (or timing or turn off a mag).
The data comes from the engine monitor and comparative climb performance.
If you take off in an aircraft at sea level with a normally aspirated aero engine at mixture full rich, pitch full fine and throttle wide open, and touch nothing during the climb, the mixture will become more and more rich as you climb, because the manifold pressure becomes lower and lower. The further rich you go the less power you are producing (on this part of the power curve). There is no doubt or controversy about this.
If you instead take off and touch nothing, other than the mixture so as to lean during the climb to get around the same EGT as you got at sea level on take off, you’ll get more power out of the engine than you would if you’d stayed full rich. The CHTs will remain nice and cool. The CHTs on the engine monitor and the rate of climb compared with full rich shows this.
I see it every time I take off and climb to a ‘decent’ height. SOP is I don’t touch the throttle or RPM (unless it’s early in the morning and near a populated area, in which case I’ll pull the RPM back a bit until I’m clear) and lean every minute or so during the climb to get back to the ‘target’ EGT. If I go full rich during the climb, the rate of climb goes down compared with the rate of climb when the mixture is set to achieve the ‘target EGT’.
PS: One correction to the above. Some engines have an altitude compensating fuel pump which automatically leans to match the density altitude. It does, automatically, what I’m trying to do by leaning manually to the ‘target’ EGT.