>I have to confess that I need someone to give me a short, one paragraph
  >lesson on the mechanical factors involved in turbo-compounding.  I'm not
  >an engine guy but I can learn quickly.  Here's what I understand so far.
  >
  >If the turbo is always turning at, say, 75k rpm cruise power, then it
  >would be easy enough to direct drive thru gear reduction to assist the
  >final drive at the prop hub.  OK.  Reduce 75k rpm down to 2500 rpm
  >(30:1).  But, and this is the part that I don't understand completely,
  >things are not always turning at these speeds.  As the prop slows down
  >for cruise flight (say down to 2,000 rpm) that would also drag the
  >governor down to 60k.  Not good because that's not its efficient speed
  >and will build up backpressure.  It wants to keep turning at 75k.  Also,
  >if the engine is throttled back to idle and the aircraft dives, the prop
  >may be back up to 2,500 rpm but the turbo only wants to turn at half
  >that speed.  I think I understand that much.  What's needed is a
  >variable connection between the 2 with high efficiency (i.e. direct
  >gears, not hydraulic or electric motors).  Also, I believe that some
  >system needs to identify what the correct ratio should be and
  >automatically set the gearing accordinly.  From here, I'm lost.  What
  >are we trying to accomplish?  Are we trying to allow the turbo to turn
  >at whatever speed it naturally wants, but also connect it somehow to a
  >prop shaft that can vary in rpm from 500 to 2500?  Or am I missing
  >another aspect completely.
  >
  >Anyone got a "turbo-compound for Dummies" book they can loan me??
  >
  >Greg
  >
  >Since the aircraft engine spends 95% of the time at constant cruise RPM
  >it is easy to optimizes the gearing for that cruise RPM. Also back
pressure has
  >just a little effect on the engine power.
  >
  >Here are some numbers.
  >
  >Paul Lamar
  >
  >Greq did you watch these youtube videos?
  >
  >http://www.youtube.com/watch?v=iy_Jd5kewbM  I want my MPG!
  >
  >http://www.youtube.com/watch?v=luoUHrX6DeQ  I want my MPG Diesel
  >
  >http://www.youtube.com/watch?v=emLKhfm6MWE  I want my MPG Q and A
  >
  >Any questions? Where did I go wrong?
  >
  >This chart pretty well sums it up. 30 inches of Hg exhaust pressure is
  >sea level pressure. The big difference between a piston engine
  >and a rotary is a piston engine has only half the kinetic energy available
  >for blow down turbines due to the exhaust valve. If it is this good
for a piston
  >engine think what it would be for a rotary.
  >
  >Paul Lamar
  >

Paul,

   I wonder if this chart data is actual,  predicted or both?    The first
two lines of data seem believable the second questionable.
gear efficiency at 95%.

Doug in Japan

I assume you got the entire NACA-822 pdf.

Here is a planetary drive where the gear eff. is claimed to be 99%.
Granted it is not 10 to one or 60 years ago.

Paul Lamar

Greg,

Here is the other piece of the puzzle. Kinetic energy loss across the
exhaust
valve. This is the fundamental reason the rotary is twice as good as the
poppet
valve piston engine such as this CW R3350 TC.

Paul Lamar

Paul:

I'm still not sure what would happen to the turbo if it's output were
geared down and direct connected thru an RGB to the prop shaft.  I know
the rpm range of the engine, from idle all the way up to max 2,500 rpm.
What I'm missing is the relationship with the prop speeds to the turbo.
Yes, the turbo varies its speed depending on the throttle settings, but
is it just too simple to say that an idle setting (i.e. 800 rpm) would
produce a low turbo speed (low exhaust gas velocity) and a high throttle
setting (i.e. 2400 rpm) would correspondingly speed up the turbo (high
exhaust gas velocity)??  Yes, there would be mismatched gear ratios
along the way from idle up to full and back to idle, but 95% of the life
of the engine would be at 75-80% max cruise power.  Why could the turbo
not be geared for this flight condition and forget about the
inefficiencies during throttle up and down?  Why did Doug in Japan say
that we all agree there would be some broken gear teeth if we did this?
If the turbo is trying to turn faster than the engine, it will slow
down.  And if it's trying to turn slower, it's because the aircraft is
in a descent with the prop turning faster than the engine wants to push
it (or spin the turbo).  I still don't understand why a torque converter
or fluid coupling is needed other than more efficient at all speeds.

Greg

Yes that is it exactly.  However what I have implemented is an over running
clutch. If the e-shaft is running faster than the turbine no torque is
transmitted to the the turbine. Only when the turbine is running faster than the
engine does torque transfer from the turbine to the e-shaft.

See the attached jpgs.

The torque coupling is designed to isolate the engine torgue pulses from
the turbine gear box. In my design I don't need it as I am using a friction
belt drive from the e-shaft to the gear box that can slip when it sees a torque
pulse.

Paul Lamar

Greg,

Here is the type of clutch I used.

Paul Lamar