Message-ID: <52548863F8A5D411B530005004759A93012FE5@QBERT>
From: George Braly <gwbraly@gami.com>
To: "'lancair.list@olsusa.com'" <lancair.list@olsusa.com>
Cc: "Timothy C. Roehl" <troehl@gami.com>
Subject: RE: BMEP
Date: Mon, 31 Dec 2001 11:56:10 -0600
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Reply-To: lancair.list@olsusa.com

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Walter,

The radial engine torque gage was driven off of the ring gear arrangement in
the gear reduction unit.

Depending on the specific installation, the display of that information in
the cockpit would be presented as "B.M.E.P." or as Torque, or as Horsepower.


I have an instrument sitting on my shelf that has a dial wheel on the face.
You set in the RPM, and then the outer scale reads horsepower, directly.
The inner scale reads torque or BMEP.

BMEP is calculated "bass ackwards".   You do this by asking "how much would
the mean pressure above the pistons have to be, for an engine with this
displacement and making  xxx amount of torque?"   One can then do a
straightforward calculation of how much the "mean" pressure would be to
generate the measured amount of torque at the end of the prop flange.
Because the calculation is done in this manner,  it is truly a purely
calculated number and does not represent a real world physical event.

Those old instruments were remarkably accurate, and when properly set up and
calibrated, probably close to as good as one can do on an engine test stand
at measuring torque.

And there is nothing "misleading" about the BMEP used as an indicator of the
overall level of torque and horsepower (if RPM is known).  

A lot of BMEP in a small displacement engine with a large amount of torque
are "normal".  A small amount of stated BMEP with a large amount of stated
torque, implies a very large displacement for the engine.   If the engines
all have the same displacement, then different BMEP values for engines of
identical displacements, tells you torque on each engine.   If you know RPM
then you know horsepower.  

 BMEP is "useful" as a general indicator that an engine is being run "hard"
or "easy", but as pointed out below, some BMEPs are a bit more like your
grandmother and some are more like your teenage son.   They both went to the
store for milk.  But in one case, the durability of the engine is challenged
more than in the other! <g>

The problem is that having stated the "BMEP" one is at the beginning of the
inquiry, not at the end.  Frequently, I see "discussions" about engines all
over the WWW in which  various  "BMEP" values are put forward in support of
some issue involving engine durability, without further qualification.  THAT
use of BMEP is seriously misleading, for the reasons I stated in my response
to Jack.

If one says to me,  the BMEP is 175 PSI, and the peak cylinder pressure is
950 PSI and that 950 PSI maximum pressure was observed and measured to have
occurred at 14 degrees After Top Dead Center,  THEN,  that combination of
data tells me nearly the complete picture of what is going on inside the
engine and I can make apples to apples comparison from one engine to
another.
   

The formatting from my last message was screwed up on the critical data.
Let me try to restate that, again:


Example (these numbers are approximations, from memory, from recent
experience on the test stand):

		Peak 				Peak Torsional
		Combustion			Crankshaft Stress 
    BMEP	Pressure	Theta(p-p)	Reversals

A)  	175	1100 psi	 5-8deg	~5  x mean 

B)	175	 850 psi	17-19deg	~3.5 x mean 

Where "Theta(p-p)" is the rotational angle between TDC and the peak of the
combustion pressure event.


Regards,  George 

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