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Date: Fri, 18 May 2012 12:55:41 -0400
From: Ernest Christley <echristley@att.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: BSFC and EGT
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I think this is going to be useful information, and concurs to what I was taught as a trucker...to save fuel, run slow
and gear high.

  The first step for me is going to have to be mapping the EGT to AFR.  Has anyone done this, BTW?  Is there a direct
and consistent correlation?  Once I have that, tuning then becomes a matter of setting the target AFR in a table of RPM
vs [TPS | MAP] and then running the engine through several power cycles across the timespan of a couple minutes.  The
computer will then adjust the fuel injector open times as needed to obtain the target AFRs.

One question I have about the data presented is, "What was done with ignition timing?"  If the timing was held constant
throughout the test, that explains a lot about the numbers.  As the EGT drops off from the AFR increases, the mixture is
taking longer to burn.  You could possibly be getting peak pressure well after the 50* ATDC that Lynn has called out.
The same would inform the peak heat transfer to coolant numbers.  The heat absorbed by the coolant is going to be as
dependent on how long the fully burned mixture stays in the chamber as it is on the AFR.

I can't speak directly to this experiment, because it doesn't mention timing.  An interesting enhancement to the data
generated would be to vary the mixture as before, but to tune the timing for max power at each data point.  If that was
done, I'd be interested to see what the graph you shared yesterday would look like with the timing numbers added.


Steven W. Boese wrote:
> Doug Dempsey wrote the following response to the BSFC and EGT plot that
> I posted recently:
>  
> As we discussed at Paducah, the operating condition around * 50 deg F*
> *LOP* was not used as a test point and while we indicated we were
> planning to do so, it has not yet been performed. Steve, I believe you
> have confirmed this approximate value in many hours of flying with your
> RV-6?
> From a study of gas dynamics and combustion, it is expected that the
> "minimum peak" BSFC (that which produces the most power for the least
> fuel consumption.....and most MPG at that specific power condition and
> resultant velocity) will be in the *-30 to -70 deg F lean of Peak EGT*.
> This depends very slightly on engine design, but is fundamentally
> related to the physics of gas combustion and mixture ratios of fuel and air.
> Note also that the operating condition of the graph was at 22.7" Hg MAP
> and WOT due to our testing density altitude. It is possible that at some
> other test condition, ie...-50 deg Lean of Peak, the BSFC could be lower
> (better ie.. less fuel per hr per HP produced-lb/HP-hr.). Data from WWII
> engines proves that on those large radials, slower and heavier loads
> produced better BSFC and resulted in greater range. The story of
> Lindbergh's instructions to new pilots on this condition is legend for
> most pilots (unfortunately those conditions require constant speed
> props)! My data on some of those engines indicates a best BSFC of 0.58
> in cruise condition! Lots better than the .75-.80 at takeoff power!!
> While it is equally possible that our HP readings may be in error in
> magnitude, we suspect that the myth of Huge HP expectations is more the
> case. Our plots of HP vs. MAP show minimal hysteresis, that is... the
> data retraces the curve both up and down the MAP excursion within +/-3%,
> which should indicate reasonable error (<+/- 3%). Corrections for
> Density Altitude on numerous testing occasions have indicated that the
> engine would produce 165 +/- 5 HP @6,500 RPM at sea level, which we
> consider probable.
> If one is propeller limited to 5,200 rpm instead of 6,500 rpm.....165 HP
> cannot be generated. If one is N.A. and at 10,000' Density Altitude, 165
> cannot be generated! No magic....just God's rules as we know them.
> The book "*Internal Combustion Engine Fundamentals*" by John B. Heywood,
> ISBN 0-07-028637-X, McGraw-Hill, 1988 is a wonderful and readable book
> with in-detail discussions of this area of interest. Heywood lists the
> minimum BSFC of the Wankel rotary as 0.51. We believe this is possible
> under the most advantageous conditions and have experienced 0.52-0.54
> BSFC during several test conditions.
> *Note of caution*: the highest rate of *heat transfer from gas to
> combustion chamber to coolant* (NOT peak power) is -25 to -75 deg F RICH
> of peak......even though PEAK POWER is nearby at -90 to -160 RICH of
> PEAK, the transfer to coolant goes down, not up. If one wants to go
> fast...a "little rich" is NOT an advised operating condition. Got to
> -100 to -125 deg F RICH of peak....more power, less heat to coolant.
> *Bottom-line*: Leaning more lean than -70 deg F of Peak EGT does NOTHING
> but lower power faster than fuel consumption. If you want to fly slow
> set the MAP, lean to find PEAK and then lean to -50 deg F from that
> point. Will it run super lean?? Perhaps, but who wants an even greater
> loss of power??
> Each installation is slightly different and the absolute value will
> vary. Procedure: lean to find your Peak EGT, then ....either richen to
> -120 deg F Rich of Peak for maximum power....or lean to -50 deg F for
> maximum fuel efficiency (minimum BSFC/maximum MPG) at that MAP. Do not
> then increase MAP to recover the lost velocity.......you just upset your
> tuning for max MPG apple cart!
> Regards,
> Doug
>  
> Steve Boese
> RV6A, 1986 13B NA, RD1A, EC2