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06-22-2015, 07:07 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by Don at Airflow
There will be a change in the absolute EGT number for a given CR at the same F/A, but the delta change from peak to a given F/A will be the same regardless of CR. It’s a chemistry thing.
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Yep, absolute EGT decreases with increasing CR. From an operational standpoint, it's trivia. Non-turbo flyers can totally ignore the actual peak EGT value. As Don says, the difference from peak is all that matters.
Of course you do
It's an inside joke, folks. Don likes the chart because he includes it in the student materials package at the Airflow Performance "Fuel Injection 101" school, which is where I got it.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 06-22-2015 at 07:13 PM.
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06-22-2015, 10:43 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by DanH
Yep, absolute EGT decreases with increasing CR. From an operational standpoint, it's trivia. Non-turbo flyers can totally ignore the actual peak EGT value. As Don says, the difference from peak is all that matters.
Of course you do
It's an inside joke, folks. Don likes the chart because he includes it in the student materials package at the Airflow Performance "Fuel Injection 101" school, which is where I got it.
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Why the denotation of 7:1 CR in the variable description? Confusing since this is a variable, not a fixed condition in the chart.
Last edited by rv6ejguy : 06-22-2015 at 10:45 PM.
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06-23-2015, 07:08 AM
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Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
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Charts are interesting and I've forgotten much of what I once knew about working them.
There was a time in a previous life when flight planning was so complicated (working performance charts) it had to be done the day before the flight so someone could check the resulting numbers for accuracy, this was before computers. We were good at it or would have been without a job.
The engine was a P&W 4360; the flight engineers mecca, a pilots nightmare. (It was my experience one of the four failed every fourth mission. It had magnesium super chargers, if one caught fire it did not end until the airplane crashed and stopped burning about a day later. The entire airplane was a night mare waiting to happen from fires to fuel leaks, I nearly quit flying) (KC-97) Thankfully the super chargers rarely caught fire as it was hit the chute if one did.
Back to RV stuff....
An element missing in this discussion re A/F and EGT is rpm. The Don Rivera chart is turbo so CR does change. If variable rpm is introduced it has a huge impact on A/F, EGT and timing at WOT.
With Subby H6 I would loaf around at WOT, rpm 1900 and watch the numbers. Timing would advance to 32-35, A/F would settle around 15:1 and temps were cool. IAS was about 110 so it was great for local sight seeing at minimum cost. It had a knock sensor to prevent detonation.
I miss that feature with a FP prop.
WOT at low rpm does make for an efficient operation. I believe that technique was used by the P-38's that finally caught up with Admiral Yamamoto in 1943 and settled with him for the attack on Pearl Harbor in 1941.
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06-23-2015, 07:47 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rv6ejguy
Why the denotation of 7:1 CR in the variable description? Confusing since this is a variable, not a fixed condition in the chart.
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The data is from a Waukesha variable compression CFR (Cooperative Fuel Research) engine installed at Stanford U. The notation on this figure is misleading...CR was varied from 4 to 9 with fixed timing and mixture. 7:1 is the middle of the scale.
http://rescomp.stanford.edu/~efroeh/...erformance.pdf
Fun to read:
https://www.asme.org/getmedia/ffedc3...gine-1928.aspx
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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06-23-2015, 08:03 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by David-aviator
An element missing in this discussion re A/F and EGT is rpm. The Don Rivera chart is turbo so CR does change. If variable rpm is introduced it has a huge impact on A/F, EGT and timing at WOT.
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A chart like that is taken at a fixed RPM and MP.
A change in RPM and/or MP does not change the fundamental EGT relationship.
Again, the IO-540-K at Hughes, big change in RPM and MP. Note that best power is around 100 ROP regardless, and 11 AFR is still 200+ ROP. Note that the actual peak values change, but as previously stated, we don't care about those (well, ok, Big Boy Airplanes might have a Turbine Inlet Temperature limit). The only thing we're interesting in is the difference from peak:

__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 06-23-2015 at 08:08 AM.
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06-23-2015, 03:59 PM
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Join Date: Aug 2007
Location: Chico (KCIC) , CA
Posts: 264
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I'm posting graphs from Savvy of my last flight. The first is marked at peak EGT, the second is marked at full rich, which was 11.1 to 1 on the wide band AFR gauge and the third is just a graph of the flight with no marks. Please ignore the CHT temps -- there appears to be a conversion problem between the "J" type thermocouples on my CHTs, Savvy expects K type. Temps never got over 410.
Not sure if this helps with this discussion or not???
GEM

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06-23-2015, 04:07 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by GEM930
I'm posting graphs from Savvy of my last flight. The first is marked at peak EGT, the second is marked at full rich, which was 11.1 to 1 on the wide band AFR gauge and the third is just a graph of the flight with no marks. Please ignore the CHT temps -- there appears to be a conversion problem between the "J" type thermocouples on my CHTs, Savvy expects K type. Temps never got over 410.
Not sure if this helps with this discussion or not???
GEM

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Always interested in flight data. 
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06-23-2015, 04:09 PM
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Join Date: Aug 2007
Location: Chico (KCIC) , CA
Posts: 264
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ok ... so that did not work!!!
I have them on photobucket... whats wrong???
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06-23-2015, 04:22 PM
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Join Date: Aug 2007
Location: Chico (KCIC) , CA
Posts: 264
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[IMG]  [/IMG]
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06-23-2015, 07:45 PM
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Join Date: Aug 2007
Location: Chico (KCIC) , CA
Posts: 264
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