I knew of particular aircraft that were damaged by running too lean.
And I am trying to get across the point that they were
NOT damaged by running
to lean...they were damaged because they were
not run lean enough....or in some circumstances rich enough.
This is important...the biggest hurdle to understanding and practicing modern engine management (assuming aircraft capability-which the Cirrus has in spades) is precise language...and not just from relatively innexperienced pilots...veteran engineers/pilots are often the worst offenders.
The graph above is one of the ones used in the pilot seminars referenced earlier in the thread.
It is a portion of the mixture spectrum...left side is rich (but not full rich) and the right is lean (but not idle cutoff). It is important to realise that MP/RPM
are held constant in this graph...25/25 from memory...the
ONLY thing being varied is mixture.
From top to bottom the lines show EGT, CHT, ICP (internal combustion pressure in the cylinder), HP and 'mpg' for want of a better term...economy if you like. It is actually Brake Specific Fuel Consumption (BSFC= how much fuel it takes to make 1 hp for 1 hr) inverted so it peaks with the rest rather than curving the other way as less fuel is required to make 1 hp.
Think of this being for just a one cylinder engine...the reality is that this mixture sweep represents an average of 6 cylinders with balanced injectors...but for the sake of clarity think of it as just 1 cylinder.
As you lean from
richer towards
leaner the EGT, CHT, ICP and HP all rise and the engine becomes more efficient...HP, CHT and ICP
peak first...so the most heat and pressure experienced by that 1 cylinder happens
before peak EGT...about 50F before (ROP) peak EGT.
That is why 50F ROP is deadly for engines
above 65% power...because there is enough oxygen to mix with all that fuel so that those peak CHTs/ICPs are high enough to weaken the cylinder metal or cause imperfections like a busted helicoil tang to get hot enough to set off the next intake fuel/air charge prematurely (pre ignition)...which can lead to detonation and engine failure.
Now we lean a bit further...at peak EGT our CHT, ICP, HP are all falling away- a cooler, less stressed engine developing a little less HP...but economy is still improving.
A fraction leaner is where most engines start to run rough...and where we have been taught historically to stop leaning and enrichen "2-3 graduations" on the old EGT gauge...typically each graduation on an GT gauge = 25F...that takes you straight back to peak heat and pressure at about 30-80F ROP EGT
Now a single cylinder engine doesn't run rough..only a bunch of cylinders developing different HP run rough. This is because standard injectors can have up to +/- 4% variation from nominal flow rates. Look at the above graph and imagine that the 1 cylinder of your 6 cylinder engine that has an EGT & CHT probe shows you have just hit peak EGT...freeze the action...now think about all the other cylinders that have different flow rates from the other individual injectors...those flowing at a higher rate will be producing a richer mixture in their respective cylinder and those with a lower flow rate will be producing a leaner mixture in their respective cylinder....6 cylinders all developing different HP, CHT and ICP...and economy for that matter...all bolted together causing the engine to shake like a dog passing a watermelon.
As we have 'frozen the action' at peak EGT which cylinders are better off?
Tell me again about 'too lean'?