On 09/24/06 12:07, Wade Hasbrouck wrote:
"Thomas Borchert" wrote in message
...
Wade,

however running an
engine too lean can, because if you lean too much the cylinder head
temperatures will be higher than normal and can cause detonation.

..

No... Higher EGT indicates leaner mixture. Excess fuel in the cylinder
has
a cooling effect.

Sorry, but you're wrong. Running not lean enough can cause detonation. Or,
more exactly: Putting the mixture in the wrong place can. Cylinder
pressures
are highest at about 50 to 75 rich of peak. Higher EGT does indicate
closeness
to peak EGT, but not a leaner mixture. The leaner the mixture, the cooler
the
EGT - once you're beyond peak.

The misconceptions behind your statements lead to people not liking to
contemplate LOP operations, which is why I point them out.


--
Thomas Borchert (EDDH)


What is "LOP operations"?

Let me give you a pointer to a great deal of engine operational information.
John Deakin has written a series of articles for AVWeb. The series is
called Pelican's Perch. You can see the index of his engine-related articles
he

http://www.avweb.com/news/pelican/182544-1.html

They make a very good read. You need to be a member, but the membership
is free. If you're not familiar with AVWeb, there are lots of great
resources there, including lots of other regular columns like John's.



I realize I got the leaning procedure for EGT wrong... As yes, you lean
until it peaks and then goes down by 25 - 50 (or what ever the POH says).
Plane I currently fly does not have an EGT and was going by what I have
read... Was taught "Lean to max RPM then richen slightly..."

I am confused now... Are you saying "running full rich" can cause
detonation? As this caused me to go look in my copy of Rod Machado's
Private Pilot Handbook to see what the causes of detonation are and he lists
the following as "several know causes of detonation"

1. Using a lower grade of fule than what is recommended by the
manufacturer. Higher octoane fuels have additives which help prevent
detonation.

2. Using time-expired fuel.

3. Over-leaning. Excessive leaning can raise cylinder head temperatures
increasing the likelihood of fuel exploding insteadd of burning within the
cylinders.

4. Overheating and engine by climbing too steeply on hotdays, as waell as
using excessive engine power (some engines have restriction on the time
maximum power can be used).

5. An abrupt opening of the throttle.

In the section "Too Rich and Too Lean" he talks about what happens if it is
too rich or too lean. The following are quotes from that section.

"Aside from inefficient fuel combustion, mixtures that are too rich or too
lean cause difficulties with engine operation. A mixture that is too rich
causes engine roughness. Spark plugs are easily fouled when unburned fuel
residue builds up between the plug gap..... "

"It's interesting to note that a fouled spark plug in flight can be detected
by an increase in EGT. Why? With only one flame source operating with the
cylinder, it takes longer to reach the maximum combustion temperature of the
cylinder's fuel-charge (it simply burns slower). As a result, the mixture
is hotter as it exits the exhaust valve, resulting in a higher EGT."

"Aside from a rough running engine, an excessively rich mixture contributes
to high fuel consumption...."

"A mixture that 's too lean produces other problems. First, too lean a
mixture means liss power is produced...."

"The biggest danger with an excessively lean mixture is that it burns hot.
It does so because it burns slower. This exposes the cylinder, postion and
valves to higher temperatures and high temperature is the worst enemy of
metal, causes reduced cylinder life and other problems..."

"High cylinder tempreatures also lead to something know as detonation.
Instead of a smoth even expansion of the fuel-air charge within the
cylinder, the mixture detonates (explodes), causing damage to the
engine...."

And there is even a question on the private pilot knowledge exam that asks
what to do if you have an overheating engine or detonation... and I believe
the the answer is "richen the mixture."




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA