@William Pete Hodges - so pulling the black and blue knobs to lower CHTs, let’s say during a climb, worsens longevity due to peak cylinder pressure? How does the engine know the spam can is in a climb vs cruise, other than maybe less cooling air during climb, assuming no cowl flaps? The info you provided seems contradictory. I haven’t taken that course, so just asking.
Okay. At maximum power the WOT setting provides extra rich mixture which delays flame propagation and reduces stress, compared to backing off the throttle some which takes away the extra richness. As you climb the MP reduces with altitude so when you get to cruise altitude you can leave the throttle wide open and control power with RPM and Mixture. I cruise my Mooney WOT and 8 GPH and 140 kTAS. I usually cruise 8500-13500 depending on circumstances.
If you elected to level off at low altitude and stay WOT at max RPM and full rich the sustained high power would cause the cylinder heads to heat up, and get too hot. If you lean some the heads will heat up faster. If you lean to minimum propagation time they will heat up really fast. During all this ROP leaning the power produced will be about the same but the engine stress will go up dramatically. As you lean further to Peak ET and beyond you enter the LOP region as ET falls.
Not all engines will run well LOP, but assuming the engine continues to run smoothly, here's what happens next.
As you continue to lean engine stress is dropping because Flame Propagation is being delayed, but power is also going down as well further reducing engine stress. From Peak ET to about 100F LOP the power produced is about 14.9 or 15 HP per GPH. That means 10 GPH is about 150 HP.
Look at the chart. On my IO-360 9GPH at 50F LOP is 134 HP or 67% load. At 50F LOP this is safe for continuous operation, but at peak ET the cylinders will get too hot, as shown by running inside the Red Fin. The power is the same and the GPH is the same, but at Peak ET there is less air going through the engine, making the mixture less lean with shorter flame propagation time. At WOT increasing RPM will run more air through the engine and at the same 9GPH will make the mixture leaner, producing the same HP with less stress while keeping the cylinders cool. This illustrates how increasing RPM reduces stress on the engine.
Now at the full power setting it won't be possible to run continuously and prevent engine damage no matter how rich you go. Less stress buys you more time before backing off the power to keep the cylinders cool. If you are climbing with a naturally aspirated engine then power is reduced automatically as you climb. But IF you are not climbing and you can run lean enough the power will reduce to a point where the engine can run continuously. That is IF the engine can continue to run smoothly in LOP mode as lean as you need to run to reduce the power. In this regard the mixture control by itself has the capability to produce any power setting from zero to full power.
Clear as mud, right?
