Withstanding Peak Temperatures

OK. I have a basic question. Why is running an engine at peak
temperature (as in Rich-of-Peak operation) not good for the engine? In
the scheme of things 50-100 degress cooler than peak temperature (where
many folks recommend the engine be run) does not seem significantly
lower in temperature than the peak temperature. It would seem if the
engine parts can stand Peak-100 degrees, they would be able to stand
peak. What am I missing? (Oh, and I have read Deakin's articles on
AVWEB, and still have this question).

-Sami (N2057M, Piper Turbo Arrow III)

"O. Sami Saydjari" wrote in message
...
OK. I have a basic question. Why is running an engine at peak
temperature (as in Rich-of-Peak operation) not good for the engine? In
the scheme of things 50-100 degress cooler than peak temperature (where
many folks recommend the engine be run) does not seem significantly
lower in temperature than the peak temperature. It would seem if the
engine parts can stand Peak-100 degrees, they would be able to stand
peak. What am I missing? (Oh, and I have read Deakin's articles on
AVWEB, and still have this question).


As you yourself just said, the engine can "stand" it. It's all about wear
and tear. Why abuse the machine?

Do you drive your car at redline on the tach?

In article ,
"O. Sami Saydjari" wrote:

OK. I have a basic question. Why is running an engine at peak
temperature (as in Rich-of-Peak operation) not good for the engine?

There is nothing wrong with running an engine at peak EGT if it is happy
to run there. That's where I run mine, in accordance with my POH.


In
the scheme of things 50-100 degress cooler than peak temperature (where
many folks recommend the engine be run) does not seem significantly
lower in temperature than the peak temperature. It would seem if the
engine parts can stand Peak-100 degrees, they would be able to stand
peak. What am I missing? (Oh, and I have read Deakin's articles on
AVWEB, and still have this question).

When you read Deakin's articles, you are not too subtly encouraged to
take away the belief that running 50-100 ROP is actually worse for the
engine than running either much richer or much cooler. He provides
graphs and text to explain why.

The incorrect assumption explicit in your question suggests that you
have misunderstood something fairly fundamental about what you read.
You should go back and look at the graphs of EGT, CHT, HP, and peak
pressure, plotted against fuel mixture. Convince yourself of where CHT
and peak pressure are at a maximum, and see where that lines up against
EGT. It should be around 50-100 ROP. Note how as EGT continues to
increase, CHT and peak pressure (the real enemies) actually decrease.

Also look at your aircraft panel. Note that CHT has a redline. Note
that EGT (most probably) has no redline. Your engine is designed to
tolerate maximum EGT. It is not designed to tolerate arbitrarily high
CHT. You manage EGT to control CHT.

Because that additional 100 degrees of exhaust temperature translates back
to far more than an additional 100 degrees in combustion flame temperature,
and requires the heads, and the oil, and especially the exhaust valves, to
remove a larger total heat load than the additional 100 degrees on your egt
gauge looks like to you... Engineering is not always intuitive...
denny

"O. Sami Saydjari" wrote in message
...
OK. I have a basic question. Why is running an engine at peak
temperature (as in Rich-of-Peak operation) not good for the engine? In
the scheme of things 50-100 degress cooler than peak temperature (where
many folks recommend the engine be run) does not seem significantly
lower in temperature than the peak temperature. It would seem if the
engine parts can stand Peak-100 degrees, they would be able to stand
peak. What am I missing? (Oh, and I have read Deakin's articles on
AVWEB, and still have this question).

-Sami (N2057M, Piper Turbo Arrow III)

"O. Sami Saydjari" wrote in
:

OK. I have a basic question. Why is running an engine at peak
temperature (as in Rich-of-Peak operation) not good for the engine?
In the scheme of things 50-100 degress cooler than peak temperature
(where many folks recommend the engine be run) does not seem
significantly lower in temperature than the peak temperature.

There is nothing inherently wrong with running an engine at peak, or
ROP, or LOP. The issue is running the engine within the engineering
design constraints -- i.e. where it is not subjected to debilitating
conditions.

There are two temperatures to consider - the cylinder head temp, and the
exhaust gas temp, as well as cylinder pressure. If all three are well
within limits, then you are not hurting the engine. If they are getting
near (or beyond) limits... not good. Add in the fact that most
manufacturers tend to want to stretch the limits on temps in the POH,
and add in instrumentation error (especially with EGT), and a little
margin is a good thing.

In your turbo Arrow III (nice plane, fly one myself) you can run ROP, or
LOP (if it will run smoothly), or you can run at peak (see the POH -
this is recommended for best economy). But regardless, you must keep
the temperatures within limits. Running well rich (100 degrees or more)
is one way to help do this - it sure brings down the EGT and CHT. But
it also burns a LOT of excess gas, fouls the plugs, etc. Leaning it out
helps keep the engine clean, and cuts WAY down on gas (same airspeed LOP
in mine, vs similar temps and airspeed ROP is almost 4 gph difference).

One thing to be careful of on that engine -- at LOW power settings (25"
or so), the EGT's can get quite high due to the fixed mag timing.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------

O. Sami Saydjari wrote:
: OK. I have a basic question. Why is running an engine at peak
: temperature (as in Rich-of-Peak operation) not good for the engine? In
: the scheme of things 50-100 degress cooler than peak temperature (where
: many folks recommend the engine be run) does not seem significantly
: lower in temperature than the peak temperature. It would seem if the
: engine parts can stand Peak-100 degrees, they would be able to stand
: peak. What am I missing? (Oh, and I have read Deakin's articles on
: AVWEB, and still have this question).

Sami, I think that Deakin is using TEMPERATURE, which can be determined by
cockpit instrumentation to describe TIME, which cannot be seen on cockpit
instruments. When running 50-100 ROP, the temperatures are well controlled.
The difficulty as I understand it is because the power pulse from the piston
occurs very shortly after the piston reached Top Dead Center (TDC).

What this means is that the force of the piston is being transferred to
the crankshaft at a very small angle - this can be demonstrated by holding
your arm out, elbow straight, and pushing on the wall: you can't make
your elbow bend. If you bend your elbow a little bit, and then push the
wall, it is quite easy to bend your elbow more. (In my example, your elbow
represents the connecting rod/crankshaft, and your hand the piston).

Running at LOP values, or very rich values, slows down the buring of the
gases above the piston, resulting in the push force being delivered when the
crankshaft has turned a couple more degrees (Deakin claims that 16 degrees
after TDC is optimum). This is easier for the engine to take, the bearings
are exposed to lower forces, etc. The use of TEMPERATURE to determine
TIME (crankshaft angle) is because there is no instrument to determine
crankshaft angle vs. combustion event timing.

Please note that this is a significant difference from auto engines! Car
engines have adjustable spark timing to overcome these difficulties, while
airplane magneto engines almost always have fixed timing (there is an
adjustable timing system for airplane engines).

I am not sure that I explained this as well as it is possible to do so,
but the basic point that I am trying to make is that the TEMPERATURE is not
in itself the critical issue, so long as you don't melt the cylinder heads.
Rather, TEMPERATURE is used to indirectly indicate another, immesurable,
parameter, crankshaft angle vs. combustion event timing.
--
Aaron Coolidge (N9376J)

"Aaron Coolidge" wrote in message
...
O. Sami Saydjari wrote:
: OK. I have a basic question. Why is running an engine at peak
: temperature (as in Rich-of-Peak operation) not good for the engine? In
: the scheme of things 50-100 degress cooler than peak temperature (where
: many folks recommend the engine be run) does not seem significantly
: lower in temperature than the peak temperature. It would seem if the
: engine parts can stand Peak-100 degrees, they would be able to stand
: peak. What am I missing? (Oh, and I have read Deakin's articles on
: AVWEB, and still have this question).

Sami, I think that Deakin is using TEMPERATURE, which can be determined by
cockpit instrumentation to describe TIME, which cannot be seen on cockpit
instruments. When running 50-100 ROP, the temperatures are well
controlled.
The difficulty as I understand it is because the power pulse from the
piston
occurs very shortly after the piston reached Top Dead Center (TDC).

What this means is that the force of the piston is being transferred to
the crankshaft at a very small angle - this can be demonstrated by holding
your arm out, elbow straight, and pushing on the wall: you can't make
your elbow bend. If you bend your elbow a little bit, and then push the
wall, it is quite easy to bend your elbow more. (In my example, your elbow
represents the connecting rod/crankshaft, and your hand the piston).

Running at LOP values, or very rich values, slows down the buring of the
gases above the piston, resulting in the push force being delivered when
the
crankshaft has turned a couple more degrees (Deakin claims that 16 degrees
after TDC is optimum). This is easier for the engine to take, the bearings
are exposed to lower forces, etc. The use of TEMPERATURE to determine
TIME (crankshaft angle) is because there is no instrument to determine
crankshaft angle vs. combustion event timing.

Please note that this is a significant difference from auto engines! Car
engines have adjustable spark timing to overcome these difficulties, while
airplane magneto engines almost always have fixed timing (there is an
adjustable timing system for airplane engines).

I am not sure that I explained this as well as it is possible to do so,
but the basic point that I am trying to make is that the TEMPERATURE is
not
in itself the critical issue, so long as you don't melt the cylinder
heads.
Rather, TEMPERATURE is used to indirectly indicate another, immesurable,
parameter, crankshaft angle vs. combustion event timing.
--

http://www.avweb.com/news/columns/182084-1.html (Review the charts and
graphs)

"James M. Knox" wrote in message
...
There are two temperatures to consider - the cylinder head temp, and the
exhaust gas temp, as well as cylinder pressure. If all three are well
within limits, then you are not hurting the engine. If they are getting
near (or beyond) limits... not good.

Is there a limit on EGT? I've never seen an explicit one, but I've only
operated normally aspirated engines. Are there TIT limits?

Julian Scarfe

On Tue, 2 Mar 2004 08:32:34 -0000, "Julian Scarfe"
wrote:

Is there a limit on EGT? I've never seen an explicit one, but I've only
operated normally aspirated engines. Are there TIT limits?

There are TIT limits -- IIRC around 1650°F I don't know what the limit is
on the Lycoming valves.

In my TN Lyc IO360, I've never seen the TIT above about 1500°F. But in
fully turbocharged engines, the TIT limit can be exceeded.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Ron Rosenfeld wrote in
:

Is there a limit on EGT? I've never seen an explicit one, but I've
only operated normally aspirated engines. Are there TIT limits?

There are TIT limits -- IIRC around 1650°F I don't know what the
limit is on the Lycoming valves.

In my TN Lyc IO360, I've never seen the TIT above about 1500°F. But
in fully turbocharged engines, the TIT limit can be exceeded.

There are limits, depending upon the material used to construct the
exhaust system. It's usually in the 1550 F to 1650 F range. [I want to
say that the Iconel system is TCM and good for 1650, but please don't
take my word for it.]

The problem is that EGT probes are VERY subject to error, and it is hard
to truly measure the temperatures accurately. The probes themselves are
very accurate, but the tiniest changes in placement, or even
positioning, can make a significant difference in reading. That's why
we tend not to look TOO CLOSELY at absolute EGT readings, and to want
some "margin" between what is allowed and what is read.

For instance, my TSIO-360-FB allows temps up to 1650. I try to keep
them below 1550 in cruise, however, and only VERY seldom let them go
anywhere near 1600 even in hot/heavy climb.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------