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The quote came from Fred Moreno's post of 5 Feb (pasted in below).
John
On 2/10/09 10:32 AM, "John Schroeder" <jschroeder@perigee.net> wrote:
> John -
>
> I probably missed a message or two in this thread. But where did this
> quote come from? Max CHT is really not defineable in any useful or
> practical sense at all. My CHT's are alarmed at 400 and from everything I
> have read your do irreparable damage to the cylinder at around 425 degrees
> in an air-cooled aircraft engine. "Max CHT" may equate to "Max Power" in
> some theroetical sense, but the concept is useless as an engine management
> tool.
>
> Regards,
>
> John Schroeder
> LNCE
Go to John Deakin¡¯s ¡°Pelican¡¯s Perch¡± series on line
(http://www.avweb.com/news/pelican/182544-1.html ) and you will find a
series of interesting articles on leaning derived from GAMI test results.?
For the IO-550 he refers to a ¡°red box¡± and elsewhere talks about a ¡°red
wedge¡± which shows the forbidden territory.? Above about 60-65% you can set
mixture with impunity and do no damage according to his reports.? He does
show you can lean at 80% power with this engine, but as I recall it must be
75-80F lean of peak.? I copied his tables and the picture of the red wedge
and have them on my clip board in the airplane.?
Here is one version for maximum power (speed) at the higher altitudes (rich
of peak).? For best economy you stay at 50F lean of peak at these higher
altitude settings.? His columns explain his reasoning about what constitutes
¡°optimum.¡±? Briefly, if power is low enough to permit operation over a wide
range of mixture settings (generally below 65%), operating a bit rich of
peak burns a bit more fuel, but provides significantly more HP and speed
compared to the pure economy case (50F lean of peak) so for best combination
of speed and economy, run between the green lines.? (Being cheap, I run at
best economy all the time, but you may be in a hurry.)
and here is a version showing his recommended mixture management plotted on
the chart.??
Clarification on mixture settings:
Best power is usually 75-150F rich of peak depending on the engine.? The
curve in this region is relatively flat.? You can find it by climbing to
cruise altitude, setting cruise power, and then starting rich, slowly lean
until you get best speed.? Use the altitude hold on your autopilot to
carefully control speed and make your changes slowly.? I recommend doing
this at 8500 or above so that power will be at a nice cruise power setting.
Highly boosted engines run a lot richer than best power for take off for
additional cooling and to prevent detonation when the cylinders are hot.?
Factory recommended setting (based on a lot of tradition and history before
we had good instrumentation) is 50F rich of peak for many engines.? This
turns out to be the worst place to operate for many engines, particularly at
high power settings. ?But will the factory change its recommendation??
Nope.? It opens them to criticism that past advice may be wrong.
Best fuel economy (lowest brake specific fuel consumption) is about 50F lean
of peak.?
I have taken the liberty of copying some of Deakin¡¯s material below since it
is directly relevant to this discussion. ?I highlighted some key points.?
Read carefully, or even better read his series of articles. ?It may take a
couple of evenings to fully digest, but the education is worth it.
Fred Moreno
From: http://www.avweb.com/news/pelican/182084-1.html
Now, please stop right here. DO NOT just skip over that chart, it's
important. It looks a little complicated, because there are four parameters
on one chart, and each needs to be understood. If you are not prepared to
understand that chart, I don't think you can understand how to operate your
engine properly, much less understand the rest of this column. Please take
the time, right now, to follow me through on this chart!
The entire chart represents data while running at 25.0 inches of manifold
pressure and 2,500 RPM. Those are held constant ¡ª only the mixture changes.
Please read the preceding paragraph again, it's VERY important.
The only variable is the fuel flow, plotted across the bottom of the chart,
and all four parameters are plotted against that fuel flow from 75 to 120
pounds per hour (PPH). This gives a lovely picture of the relationships.
Note that "Richer" is again to the right on this chart (I could have swapped
some of these charts left to right to make them all the same, but preferred
to stick to "authentic.")
The right side of the chart is not necessarily "Full Rich". For whatever
reasons, TCM chose to show just the range of fuel flows from 75 to 120 PPH
on this IO-550, while "full rich" at sea level is about 162 PPH
The top curve shows EGT, aligned with the numbers on the right side, from a
low of 1,350¡Æ at 75 PPH (lean), peaking at about 1,520¡Æ F at 95 PPH, and
dropping again to 1,380¡Æ F at 120 PPH (rich). This is a classic EGT curve,
and many of you will be familiar with it. There, now that's not so bad, is
it? And you thought this chart was too complicated!
Below that is the CHT curve, with the scale at the left. About 310¡Æ F at 75
PPH (lean), peaking at about 425¡Æ F at 105 PPH, dropping again to about 405¡Æ
F at 120 PPH (rich). Note the CHT peaks at a substantially richer mixture
than EGT. (In fact, you'll see that the hottest CHT occurs at the point
where EGT is about 50¡Æ F rich of peak (ROP). Hmmm, where have we heard that
before?)
The third trace from the top is "brake horsepower" (BHP), which we can just
call "Power" as delivered to the shaft. The numbers for this trace are on
the right, and our trace shows 180 HP at 75 PPH (lean), peak at about 250 HP
for fairly broad range of fuel flows, and drops only slightly to about 245
HP at 120 PPH (rich). Note that going all the way to "Full Rich" would
produce a good deal more fuel flow and a good deal less power.
Finally, the bottom curve depicts "Brake Specific Fuel Consumption" (BSFC).
This fancy-sounding engineering term is nothing more than the fuel required
to produce one horsepower (HP) for one hour.
A useful trick to help you understand this chart is to lay a clear plastic
ruler vertically on the chart. Keeping it vertically oriented, move it back
and forth horizontally. As one end of the ruler moves over the fuel flow
scale at the bottom of the page, the four curves will move up and down along
the ruler's straightedge, just as those parameters move up and down in the
real world.
Remember earlier, where I said we could lean a lot and only lose a little
power? Well, this curve gives us a visual idea of how that works. At 75 PPH
(lean), it takes about 0.425 pounds to produce one HP for one hour. In the
range 85 to 95 PPH, less fuel is required: about 0.385 pounds of fuel.
Enrich further to 120 PPH, and you'll be burning about 0.480 pounds of fuel
to produce one HP for one hour. In this case, "less is better" if we're
looking for economy. BSFC is a very useful term, as it is a direct
indication of an engine's efficiency. Modern automobile engines, contrary to
popular belief, are not very efficient, having BSFCs above 0.42 or so. They
are designed to run very clean, however, and sacrifice some efficiency as a
result. TCM engines can achieve BSFCs as low as 0.385 from the factory. With
modifications, we're going to do better than that. Not too shabby for "World
War II technology," as so many call it!
Now, here's the crucial concept behind this chart: All four curves are
carefully plotted with reference to the fuel flow scale at the bottom, and
this gives us a lovely opportunity to look at the relationships between
them! If you'd like to know what is happening to CHT as you fiddle with EGT,
the answer is here.
For example, let's say we've leveled off in cruise, allowed things to settle
down and it's time to lean, at whatever MP and RPM you choose. Starting from
full rich (perhaps well off the right side of this chart and due to the
enrichment feature at full throttle), we start leaning. According to the
chart, the EGT and CHT will rise, and we know this to be true from
experience. The power rises only slightly. If you're going for absolute
maximum altitude or speed, that very flat "peak" in the BHP curve that
occurs around 105 to 110 PPH fuel flow will be helpful, and if you're making
a high-altitude takeoff, it will be very helpful. The BSFC is dropping as
you lean, of course, you're getting "more efficient."
So as the mixture is leaned, power peaks first, with CHT peaking at very
close to the same point. In practical terms, if we lean to max CHT, we'll
have max power for that MP/RPM setting. Doesn't that make sense,
intuitively? Max power, max CHT? It's not precisely true, but it's close
enough.
Ok, so power peaks first, stays pretty flat, and then CHT peaks shortly
thereafter. With continued leaning, power and CHT drop together ¡ª very
gradually at first, then progressively more steeply ¡ª while BSFC continues
to improve and EGT continues to rise.
Continue leaning, and EGT peaks and begins to fall, while BSFC continues to
improve. Sure, we're losing power, but fuel consumption is declining even
faster, so our "economy" (as measured by BSFC) is still getting better.
Finally, BSFC bottoms out (at "best economy mixture"), and stays pretty flat
between 85 and 90 PPH. If we disregard the small difference between 0.385
and 0.400, we could even stretch the point a little, and say the BFSC curve
is "kinda flat" between 85 and 100 PPH. Look immediately above at the HP
curve, and note that for a small loss of fuel efficiency, we can pick up 25
to 30 HP? Isn't that interesting? We'll see later how that affects airplane
performance and efficiency, a very different subject. But to make a long
story short, if the power setting that results from optimizing engine
efficiency causes your airplane drop below the airspeed range that provides
optimum aerodynamic efficiency, you're not gaining anything! That's a whole
'nother can o' worms. (I feel another column coming on.)
But What If We Change MP or RPM?
Let's say we have carefully set up a real 65% of rated power, using the
required MP/RPM, with the mixture leaned to Best Power.
What would happen to this if we increased the MP and/or RPM, keeping the
mixture constant at Best Power? More power to the shaft, right? Think about
what this might do to that chart. Since the power moves up, all the curves
must move up. Since the peak EGT will occur at a higher fuel flow, it must
move to the right, and all the other traces will move right, too, to
preserve the interrelationships.
>
> On Tue, 10 Feb 2009 00:50:05 -0500, John Hafen <j.hafen@comcast.net> wrote:
>
>> Here©ös the quote:
>>
>> ©øSo as the mixture is leaned, power peaks first, with CHT peaking at very
>> close to the same point. In practical terms, if we lean to max CHT, we'll
>> have max power for that MP/RPM setting. Doesn't that make sense,
>> intuitively? Max power, max CHT? It's not precisely true, but it's close
>> enough.©÷
>>
>> John
>>
>>
>> On 2/8/09 11:23 AM, "Colyn Case at earthlink" <colyncase@earthlink.net>
>> wrote:
>>
>>> John,
>>> Did they really print that about CHT?
>>> I don't remember that max CHT equates to anything useful although
>>> it may
>>> correlate very well to max internal cylinder pressure (ICP), which is a
>>> bad
>>> thing. ...but it's also affected by cylinder cooling, while EGT is
>>> (mostly)
>>> not. Max EGT would be stoichiometric which is most efficient if your
>>> engine
>>> can take it. Usually slightly rich of that (e.g. 50 dF ROP) is better
>>> power
>>> but also involves more pressure before top dead center and is close to
>>> max
>>> ICP.
>>> There's a really scary picture you get early in the APS course
>>> which shows
>>> how the internal cylinder pressure varies in relation to crank position
>>> when
>>> you are running at "best power". A measurable amount of the combustion
>>> expansion is actually pushing backwards on the crank until it comes
>>> over the
>>> top. Great for torsional stress.
>>>
>>> Colyn
>>>
>>>
>>
>
>
>
>
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