I can't figure out if this is a "blog" or not so if this isn't the proper
 way to start a new "thread" I apologize.

 Mr. Lamar: Could you possibly explain (in relatively small words, if
 possible) what characteristic (rotor size, combustion volume, type of
 fuel,
 etc.) of rotary engines causes one engine to develop its maximum torque
 at,
 say, 9000 rpm and another to develop its maximum torque at 6500 rpm? Do
 you
 think a rotary engine could be developed that developed its maximum torque
 at, say, 3000 rpm (thereby possibly eliminating the need for reduction
 gearing for aviation applications)? Thank you.

 Bill Hobson.


 I can't figure it what it was either so I called it a "newsletter".

 All engines generate their maximum torque when the total torque generated
 by the engine
 minus the torque losses is a maximum. Internal engine friction is a torque
 loss.
 So too is the pumping energy required to fill the engine with combustion
 air.
 That is a torque loss. The internal friction loss torque goes up with the
 RPM
 While the total torque of the engine is a constant or straight line.
 Total torque is merely the pressure in the combustion chamber pushing down
 on the piston times the piston area (read displacement) times the
 stroke of the crankshaft or the e-shaft in this case.

 Now along comes pumping losses. If it were not for the organ pipe tunning
 effect
 pumping losses would go up with the square of RPM. Pumping losses manifest
 themselves in the form of a nagative pressure in the combustion chamber.
 Minimize the pumping pressure and you will maximize the net torque from
 the
 engine. This goes for the exhaust system as well.

 Intake manifold systems are just like a child on a swing. It takes very
 little
 shoves to keep that child swinging back and fourth. It is almost an
 energy amplifier. Intake pipes can resonate with just a little help
 from the engine so pumping torque losses are a minimum at some RPM which
 coincides the resonant frequency of the intake pipes. Hence maximum net
 torque
 out of the engine.

 What you want is the largest diameter organ pipe you can fit in there. The
 larger
 the pipe dia. the lower the skin friction drag with a given amount air
 flowing
 through it. For the rotary that works out to about 18 inches long and
 about
 1-7/8 of an inch in inside diameter and about 6000 RPM.

 You could build a turned intake system at 3000 RPM and you might see 200
 foot
 pounds with a p-port but the HP would be way down. HP is torque TIMES RPM.

 It is horse power that makes airplanes fly and not torque.
 The engine would be near useless for an airplane as the power
 to weight ratio would be very bad.  Better to use a much bigger
 piston engine like an O-360 designed to be light weight
 at 3000 RPM. The rotary is only 158 cubic inches.

 This is not the same as saying the O-360 is a better aircraft
 engine than a geared rotary engine. The rotary wins overall
 in the power to weight ratio category. A p-port rotary
 will be about 200 foot pounds at 6000 RPM while an O-360
 will be about 360 foot pounds at 3000 RPM. Now gear the rotary 2:1
 (or more) and you have 400 foot pounds at 3000 RPM.

 Paul Lamar


 Yes Bill a rotary engine will work just fine direct drive. That's the
 route I've taken after
 hundreds of hours of testing.

 It's basically what has held the rotary from becoming a aviation favorite.
 Running around the skies
 screaming the engine at high RPM's is totally unnecessary.

 Anyhow I've done it and it works(direct drive). I'm building two engines
 to do just that application.

 Skip Gormqn

 Have you had it on a dyno Skip?
 According to this RX8 power curve the engine has only 67 HP at
 3000 RPM and will weigh at least 260 pounds firewall forward.

 Paul Lamar in Aus


Thanks Skip. I'm still curious though as to why one single rotor rotary
engine develops its peak torque at a particular rpm and another similar
engine develops its peak torque at a significantly different rpm. (For
example compare the RotaMax 65 to the Aixro 50). There must be some
fundamental reason for the difference. When setting out to design a rotary
engine what would one do to to try to produce more power at lesser rpm?

Bill Hobson

Perhaps I did not explain intake tunning clearly.
Perhaps someone else will please  take a stab at explaining it?

Paul Lamar
--
Hi Bill, Try this explanation:  1. Torque is a direct result of the
combustion
chamber's volumetric efficiency (rotary or piston engine). When this
efficiency peaks; torque peaks.

2. The V.E. is mainly governed by the intake system. In the intake pipe
the rush of air resonates, that is each time the combustion chamber
intake closes (whether rotary, 2cycle piston or 4cycle piston) the
air rush stops (sometimes even reverses).

3. This very brief stop sends an energy pulse back up the pipe toward the
open inlet.

4. # 2,and 3 change with RPM.

5. A perfect length intake pipe for a GIVEN RPM will contain that energy and
revert it to push the next charge TOWARD the combustion chamber.
This is "resonating."

6. This is important: Look at #5 again. The pipe length that works
perfectly at XXX RPM is not the correct length for YYY rpm. So, what to do?

Adjustable length intake pipes have been used on racing engines but
are too cumbersome for aircraft use. So the next best choice is to
pick an RPM that the engine likes and tune the intake to operate
there and accept less V.E. at RPM's above and below that engine
speed.

The aircraft piston engine (big bore/big stroke = lots of
mass moving back and forth large distances) is happiest at about
2500 RPM and the intake is designed around that.

NASCAR engines
(big bore/ short stroke = lightweight mass moving back and forth
 short distances) are happiest around 8000 RPM and their intake is
designed around that (put a NASCAR intake manifold on your family
 ride and you will be terribly disappointed)

The rotary engine
(lots of mass but NO DIRECTIONAL CHANGE) is happiest around
6000 RPM (rotor speed is around 2000 RPM, bearing loads are
lightest, V.E. is pretty good, vibration is quite low, power
output vs. engine wear is very good).

So with an engine that
wants to run forever at 6000 RPM that's where aircraft conversions
are run and the intakes are designed for that speed. To get back
to a proper propeller speed requires a +/- 40% reduction box.
Seems like a hell of a penalty until you look at the whole picture.
Even with the added weight, the power to weight ratio for a P-Ported
rotary easily surpasses today's standard aircraft engine.

If you've
 ever shock cooled your Lycoming, your repair bill exceeded the cost
of TWO COMPLETE rotaries. Can't happen with liquid cooling. Have you
priced a Continental exhaust valve lately? Rotaries have no exhaust
valves. This is only some of why we're here. Stay with the newsletter,
there are nuggets of valueable info here. Hope this affords some more
 understanding.
Dave M



I agree with most everything you said Dave - except for the bit about
aircraft engine intakes being tuned for 2500rpm.  I don't think
they're tuned at all except for easy packaging.  Regardless of the
engine type, tuning for 2500rpm will require _long_ runners.  This
site is interesting:

http://www.wallaceracing.com/runnertorquecalc.php

This page has a model for calculating runner specifics.  2700 rpm and
360 cu-in in four cylinders (a Lycoming) gives runners 31 inches long.
A similar length pipe would be required for the exhaust.  Most
aircraft don't have room for such long runners.  The F1 racers are
probably the closest - with headers-collectors on some of the O-200's.
They're quite long (24" - a guess), but the engines turn ~4000 rpm.
Headers optimized to work at the stock 2750 rpm red line would be
significantly longer.  The F1 rules don't allow diddling with the
intake configuration - otherwise you'd see long intake spiders too.

The model doesn't really appear to be set up for a Wankel rotary.  A
160 cu-in two rotor (13b) at 6000 rpm calculates to a 14" runner,
which experience has shown is too short.  There's probably something
about the multiple rotor faces sharing the same runner, and that the
rotor turns at 1/3 of the e-shaft speed that comes into play - I don't
have that all figured out...  It seems like the calculated runner
length is about 1/2 what it should be.

I would add to #2 above that the reason the intake is so important to
VE is that on a naturally aspirated engine, only atmospheric pressure
is available for cylinder filling.  I think things look different if
you have forced induction.


Regards,

Matt-