50 degrees rich of peak

I suggest that you go to Cessna Owners Org & search lean of peak in the
archives. Walter Atkinson has posted a lot of excellent material on this
topic.

HTH

Tony

--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE

In article ,
"lardsoup" wrote:

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?

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)

Peter,

The OP cannot be faulted for leaning as per the POH.


And he isn't. Nobody faulted him, but many tried to educate him ;-)

--
Thomas Borchert (EDDH)

Thanks to every one for responding.

"Peter R." wrote in message
...
wrote:

As stated above, read John Deakin's columns in AVweb about what's going
on inside the engine. Either run it about 100 degrees rich of peak or
slightly lean of peak, although, thankfully on the little GA engines so
long as you are at 65% power or less you can't hurt them, the temps and
pressures don't get high enough.

The OP cannot be faulted for leaning as per the POH. The late model C172
R and S model POH (page 4-26) recommends 50 degrees lean of peak. I would
suspect the OP is simply following the POH recommendation.

And before you flame me for simply pointing out this fact, keep in mind
that I am a 60-80 degree lean-of-peak leaner. :-)

And you have something the 172's, etc., don't have: GAMIjectors.

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)