From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Thursday, February 03, 2005 10:20 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: For Al & Paul - BMEP Torque BHP & Hi Comp rotors was Re: [FlyRotary] Re: more flying

Hey Al,

I just had a look at your BMEP graph.  Sorry to rain on your parade Al,  but I just have to disagree with your conclusions,  because they fly in the face of both theory,  and practical results.  I find it distressing that Paul Connor (and others) are again being fed incorrect information.

Firstly,  I assume you know that BMEP is a THEORETICAL,  TOTALLY MYTHICAL number calculated from actual Torque output from a dyno?  ('Cause it's a MEAN figure - an average - therefore NOT measured directly).  WE all know that torque varies with rotation of the E-Shaft,  and is therefore not constant (which is why we run a damper between the engine & the PSRU).  BMEP is usually calculated thus:

I also assume that you understand that Power is related to Torque via RPM.  It has been said that:

"High BMEP and a low rpm, or a low BMEP and a high rpm, can equal the same power".

A better formula then is:

BMEP = ( BHP * 13000 ) / ( L * RPM )

L = Displacement in Liters

Or 

BHP  = (BMEP * L * RPM) / 13000

BMEP doesn't relate directly to BHP.  You must ALSO take into account the RPM in the equation!!!!  BHP is RPM dependant,  BMEP is not!!!!   But we can play theoretical mathematics all day and prove nothing ...  Been there,  done that with our "Running the numbers" mate at that other place.  (Remember the EWP fracas - nah,  it can't work,  just proved it mathematically - even though they DO work in practice!!).

Even a casual glance at a typical dyno sheet will show that the torque peak (and therefore BMEP peak) for a rotary mild port is somewhere around say 4,500 RPM.  This is the point of MAXIMUM Volumetric Efficiency,  after which,  Vol Eff drops off.  However,  the power plot continues to climb to a peak somewhere around say 7,000 RPM (I'm generalising here - so no NITPICKING please).  So even though the Vol Eff/Torque/BMEP is dropping off,  the power is continuing to climb.  So even small increases in Vol Eff/Torque/BMEP at high RPM can mean a considerable increase in BHP!!

Now additionally,  I'm wondering as to how the BMEP figures on your graph were arrived at.  This graph looks like it was lifted by Lamar out of Kenichi Yamamoto's excellent book,  "The Rotary Engine".  However,  while most of the stuff in the book is still valid,  it is some 20-25-30 years out of date in other respects.   Since then,  sealing grid technologies have progressed,  and, for any GIVEN compression ratio,  the calculated BMEP (based on observed torque) has risen due to better sealing.

So due to the above reasons,  I question your calculated increase in BHP with increased compression ratio based on your BMEP chart.  More to the point,  it also flies in the face of hard dyno data that I have accumulated over the past 20 years or so.  Which is why I have been beating the drum for the use of high comp rotors for aero use.  As an aside,  in Improved Production Racing over here,  if you DON'T  have 9.7:1 rotors in your "Chook Cooker" or "Rice Burner",  you are relegated to 5th or 6th row of the grid.  Must be a lesson there somewhere ... ??

However,  I can only happily agree with your figures for your own 20B (done on a dyno),  and your pro-rata calcs for a 13B on the same basis,  although I must confess that I could never understand why you (or anybody else for that matter) would run an NA motor with turbo rotors in it ...  But then again,  Lamar has always reckoned that I'm an idiot!!

Now after much BS on ACRE,  I initially told Paul Connor that he would be lucky to get 140 BHP @ 5,300 RPM with his 8.5:1 turbo rotors.   You're pro-rata estimate was 142 @ 5,200 RPM for a 13B,  based on your own dyno figures.  So my ballpark prognostication of 140 was PRETTY CLOSE!!  I will also happily agree with your figure of 173 BHP @ 6,000 RPM (I can't even pretend to dispute it,  after all,  it's based on practical experiment on a dyno - and it agrees within a few BHP of what I'd expect to get down here in the Antipodes anyway). 

However,  I MUST disagree with you about the BHP increase when using 9.7:1 rotors. After all,  going from 8.5:1 to 9.7:1 compression is a considerable step!!  Seems like no body over your neck of the woods has bothered to do any dyno runs with these rotors. Well, ...my PRACTICAL experience tells me that I can expect about 82 - 85 BHP per rotor @ around 5,000 RPM,  slightly in excess of 100 BHP per rotor @ 6,000 RPM,  and around 112 - 113 BHP per rotor @ 7,000 RPM,  using the same carefully mild ported REW /  Cosmo engine ports,  (obviously assuming good inlet and exhaust systems).. 

Incidentally,  for those that are interested,  the only difference between the REW and Cosmo ports is that the Cosmo inlet runner is bigger at the manifold flange,  and tapers down to the port.  The actual port on the side face is the same size.  See attached pic.  The pic shows from left to right:

Cosmo rear,  REW rear,  Renesis 4 port rear,  Renesis 4 port front, Cosmo front, REW front.

So based on past experience,  I'd be expecting about  175 BHP @ 5,200 RPM,  200 BHP @ 6,000 RPM,  and 225 BHP @ 7,000 RPM out of a 13B with 9.7:1 rotors running on nicely ported REW or Cosmo end plates.  (Again,  that's assuming you have a decent inlet and exhaust system - no cast iron boxes  etc).

So summarising,  to compare compression ratio changes with changes in BHP,  you have to compare compression ratio changes with changes in BHP,  NOT BMEP (which is just a theoretical mythical calculation dreamed up by armchair engineers,  and mathematically derived from torque readings from a dyno,  and doesn't take into account the RPM factor in BHP calcs!! )

You must also take into account the Volumetric Efficiency of the engine,  which,  among other things, has to do with the sealing grid,  as well as the compression ratio.  Higher comp rotors SUCK harder as well as squeeze harder .. but only if the sealing grid is up to scratch!!.  A properly ported and manifolded rotary will make 125% - 130%  Volumetric Efficiency,  assuming the sealing grid has been cut and clearanced properly ( another black art about which very few people have the slightest clue).  This is why good rotaries on the race track run rings around most piston engines of a similar capacity, which rarely exceed 110% Vol Eff.

Cheers,

Leon