REFLECTOR: Exhaust systems

Tue, 15 Jul 2003 15:52:10 -0400

My experience with EGT's is that they are sensitive to the distance from
the valve.  I don't remember my sources, but I think ideal is about 3".  If
you are greater than the ideal distance, the temp falls.  As someone said
earlier, the air/fuel system has a theoretical efficient burn EGT.  I have
observed that peak EGT is about 1400 degrees on engines as diverse as A/C
and two-cycle motorcycles, so I will be surprised if the Franklin engine is
really 500 degrees cooler.

If you want to try an alternate analysis, you might try a CO2 monitor.
Easier still is do what is called a "hot cutoff" on the engine after a
power run.  If you can find the right situation, you just run the engine at
full power down the runway a short ways and cut the ignition.  Coast to a
safe location and tow the plane back and pull the plugs.  Do a plug
reading.  This is the way the best way to understand the real cylinder
mixture during max power.  This can also be used to calibrate your observed
EGT to the engine performance.

I always (with engines that have mixture adjustment) find peak (ignore
absolute value) and go from that point, not worrying about the absolute
value.  Has worked for many years on different engines.

-jerry

                      "Ronnie Brown"                                                                                                    
                      <romott@adelphia.        To:       <reflector@tvbf.org>                                                           
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                      Sent by:                 Subject:  Re: REFLECTOR: Exhaust systems                                                 
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                      07/15/03 01:38 PM                                                                                                 
                      Please respond to                                                                                                 
                      reflector                                                                                                         

Brian,

That makes sense.  That also explains the rough idle, sort of like a hot
cam
from a Corvette - lots of exhaust and intake valve overlap which allows
some
of the intake air to go out the exhaust valve at low rpms. And might
contribute to rough mag checks at lower rpms.

And that would lower the EGT's at lower rpms.  But shouldn't make much
difference at static or cruise rpms - right?

Ronnie

----- Original Message -----
From: "Brian Michalk" <michalk@awpi.com>
To: <reflector@tvbf.org>
Sent: Tuesday, July 15, 2003 12:58 PM
Subject: RE: REFLECTOR: Exhaust systems

| Franklin engines are 10.5:1 compression ratio with 350 cubic inches, and
are
| rated at 220 horsepower, normally aspirated.  Compare this to a Lycoming
| 0-360 rated at 180 horsepower.  More heat(energy) goes into the work.
| The second thing that could cause low EGT is the very high valve overlap.
| There is a lot of scavenging going on in a Franklin cylinder, causing
| volumetric efficiencies of 103% at the cruise RPMS.
|
| > -----Original Message-----
| > From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
| > Behalf Of Scott Derrick
| > Sent: Tuesday, July 15, 2003 11:09 AM
| > To: reflector@tvbf.org
| > Subject: Re: REFLECTOR: Exhaust systems
| >
| >
| > This sounds very odd.  I think av gas in a near optimal
| > mixture(Stociometric?) burns at 1500+ F. It doesn't matter whether its
| > in your Honda Civic, your lawn mower or aircraft engine.
| >
| > Maybe its the placement of the EGT probe.  The absolute temp is really
| > of not much concern because where you stick the probe in and how the
| > exhaust flows in the pipe will effect the probe temperature quite bit.
| > Its the trend that is of prime importance.
| >
| > Scott
| >
| > John Dibble wrote:
| > > I just spoke with an A/P that works on Franklin engines.  He
| > says I should not expect
| > > my EGT to exceed 1000 F.  Franklin engines run cooler.
| > >
| > > John
| > >
| > > steve korney wrote:
| > >
| > >
| > >>John...
| > >>
| > >>In that case your running way to rich...You should see temps of
| > about 1250
| > >>to 1300 (f) on take-off...
| > >>Maybe that's the reason it stumbles so much on run-up...Lean it
| > out...Keep
| > >>the temps above 1150 on the ground or you won't vaporize the lead in
the
| > >>fuel and it will foul your plugs...
| > >>
| > >>Best... Steve
| > >>
| > >>_________________________________________________________________
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