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