50 degrees rich of peak

Ok. Just want to see if I have this correct. When leaning using an EGT
gauge the 172R POH says to lean untill the EGT needle peaks then enrich to
50 degrees F rich of peak EGT.

So the EGT temperature will be 50 degrees less than the peak reading.
Right?

Right. Your understanding of what 50 degrees rich of peak means is
correct.

Whether that's a good way to operate the engine is highly debatable.

Michael

"Michael" wrote in message
ups.com...
Right. Your understanding of what 50 degrees rich of peak means is
correct.

Whether that's a good way to operate the engine is highly debatable.


The actual and extensive data shows it's about the worst place you can
operate. That's in terms of both CHT and cylinder pressures.

lardsoup wrote:

So the EGT temperature will be 50 degrees less than the peak reading.
Right?

Slowly roll out the mixture lever watching the EGT gauge. When the
temperature on the EGT gauge peaks and begins to fall, roll in the mixture
lever until the temperature peaks, then keep rolling it inward until the
temperature needle falls 50 degrees below (or less than) the peak
temperature you witnessed.

--
Peter


















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Depends on if you want best power or best range. 50 degrees is for best
range, and 75 for best power if you don't want to crisp your valves.
Think of the heat in the valve section....excess fuel cools the valves
and that is what the EGT is doing for you.

selway,

You might want to read Deakin's articles on engine ops in AVweb.

Crisping or burning the valves is an oft repeated, but untrue
statement. It is without any data to support it.

All the best,
Rick

excess fuel cools the valves
and that is what the EGT is doing for you.


Uhm, actually, no. Excess fuel is slowing the burn front, which in turn
cools down the burn process, which in turn reduces EGT. Now, lean of
peak, you have excess air cooling the exhaust gas.

--
Thomas Borchert (EDDH)

On Sat, 25 Jun 2005 13:35:35 +0200, Thomas Borchert
wrote:

Uhm, actually, no. Excess fuel is slowing the burn front, which in turn
cools down the burn process, which in turn reduces EGT. Now, lean of
peak, you have excess air cooling the exhaust gas.

Close, but not quite according to Deakin. The excess fuel causes the
mixture to burn more slowly, but what this does is delay the Peak
Pressure Point from occuring too close to the piston being at Top Dead
Center.

Either side of the precisely correct fuel/air mixture and the
combustion process occurs more slowly, but the actual combustion
process (the flame inside the combustion chamber) is not much less
hot.

The PPP has to occur with the piston at around 12 degree ATDC (After
Top Dead Center) in order for the maximum push on the piston with
minimum strain on the bearings and connecting rod. The closer to TDC
the piston is when the PPP occurs, the more pressure and strain and
more HEAT is produced.

The PPP doesn't happen instantly. The mixture has to be ignited a
certain number of degrees before TDC so that by the time the piston is
at the proper position, the combustion has reached it's maximum
pressure. For most fixed timing engines, that setting is around 26 to
28 degrees BTDC.

So the magnetos are set there. But this is a setting that requires
full rich in order for the PPP to occur at 12 degrees ATDC. If the
mixture is leaned for takeoff while the airplane is at or near sea
level, the combustion process speeds up. Because the combustion
process speeds up, the PPP begins to occur closer to TDC, which is a
bad thing.

So the additional fuel, or overly rich mixture, doesn't produce a
cooler flame front, it simply slows down the combustion so that it can
occur at the proper place. This produces acceptible cylinderhead
temperatures.

Leaning past the stoichiometrically correct mixture also slows down
the burning process (which keeps the engine cool), but this time,
you're using a lot less fuel while doing it.

And the above is just for takeoff. When cruising you usually slow the
engine down. Slowing the engine down brings the PPP very close to
TDC. So you have to either cruise rich or very lean in order for the
PPP to be where it needs to be to prevent long term damage to the
engine.

Corky Scott

Corky,

you got it right - I was quoting from memory. My key point was supposed
to be that there is no "vaporisation cooling" by excess fuel at rich
mixtures.

--
Thomas Borchert (EDDH)

Lardsoup,

go to www.avweb.com and search for columns by John Deakin on engine
management. Read them all. They will answer your question - and tell
you much more that's really good to know. Fun to read, too.

--
Thomas Borchert (EDDH)