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Message
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:
BMEP = 150.8 x TORQUE ÷
DISPLACEMENT (in Cu.In.)
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!!
For further reading, the following URLs might be helpful:
This last URL is particularly helpful, as it
has some nice graphs which make the topic very clear, even to a
novice.
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
----- Original Message -----
Sent: Wednesday, February 02, 2005 2:55
AM
Subject: [FlyRotary] Re: more
flying
Al,
Where are these
graphs? Could you post them or email them?
They sound interesting
... Jim S.
It is one that PL
scanned and posted 4-5 years ago; so scanning the scan doesn’t give a very
good copy, even at 300dpi. It has curves for only two compression
ratios, so it takes a little interpolation/extrapolation to get an estimate
for other ratios.
Al
(I tried to send
the scanned file of 220KB, but it was rejected as being 301kb, over the
300kb limit. I’ll try something else. Al)
I pulled out a
graph out of my file showing HP, compression ratio and rpm for WOT
performance. It does show that the difference would small (about 2% at
6000rpm), and getting less as rpm increases. The curves only go to
6000. But the difference is significant in the, say, 3500 to 5500
range. At 5000 going from 9.7 to 9.0 loses about 4%, from 9.7 to 8.5
loses about 6.5%. I have no info on engine model or configuration used
for the measurement.
FWIW,
Al
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