Paul,

 Just playing around with some ideas on TC..This concept I am sending for
 critique, understanding that this was put together in just a few
 minutes...

 The Idea here is to bolt the 2 exhaust housings up close and personal,
 with some kind of adapter so they align with a sandwiched gear box that
 resides where the compressor used to be, reducing rpm to the upper
 shafts..

 One shaft translates outbound to the single compressor in a housing
 where it bumps the rpm back up to compressor speeds needed..the other
 translate in the opposite direction that reduces rpm for the compound..

 You can see built as oune complete body but don't have to tell you this
 is all very modular with gear box housing and shaft housings as
 separate..

 All gear ratios and dimensions will adjust obliviously according to
 design load and clearances..Certainly supported on the ends for rigidity
 as well...

 Jim talked about getting the turbo close to the exhaust port and also
 keeping the compound shaft in perfect alignment outboard, this may be a
 solution....


 Be nice, I am very sensitive as you know...

 jeff


 No jpeg attached??????

 jeff

 Sorry Jeff. I think you are gearing it up and not
 down.

 Paul Lamar

 Jeff, Although I haven't done the CAD work on this I did research to find
 out what type of system to use. There are manifolds allowing substitution
of
 one large turbocharger on a 13B which  is thought to extract more power
than
 the two stock turbines which are limited in size due to the tight spacing.

 Although you could connect two hot side turbines with an inconel coupling
 that is a really harsh environment and temperature expansion has to be
 considered.  The major problem with turbo compounding is the 12 to 1 speed
 reduction needed to connect back to the engine shaft. This is done with a
 traction drive by rotrax or more commonly with multistage spurr gears by
 procharger. To put these off the shelf items together the shaft of the
 turbo supercharger would have to be replaced with one slightly longer and
 providing a coupling for the procharger gearbox. Also  needed is a bearing
 housing fabricated to fit the compressor air horn inlet with the opposite
 end adapted to the procharger gearbox. As with your model above the engine
 drives the compressor at low speed and the exhaust  turbine drives
 everything at high speed. A waste gate is still needed in case the gearbox
 becomes disconnected from the engine shaft.

        I have seen an advertisement on the Internet for an assembled unit
 incorporating these design features as a kit for isuzu engines which is
 available only in Japan. The compounded supercharger is built by
 rotrax and manufactured  in Holland.  I couldn't get any information from
 rotrax or a price. There is apparently only one size and I don't know the
 displacement of the isuzu engine it is used on or the amount of boost.

      The shaft seems like a great excuse to use your CNC lathe. There are
 steps, precision journals, tapers, and threads. It would also seem that
 splines for a coupe ling might be necessary since it runs at 100K RPM.

     The torque of the high speed shaft should peak under 30 ft lbs at
 start up and  fall to 7 ft lbs at full speed. I like the one large
 turbocharger because it can  have a larger diameter shaft which would be
 better to adapt a coupling to.

       One question about your design above. Are you gearing down to the
 drive shaft then gearing back up to get a different speed  on the
compressor
 than the exhaust turbine rather than having them run 1 to 1 ?

 george grimes

 Doug Fir has a rotrex George.

 Here is some info. I am using the same friction gear box in my design.


 Paul Lamar



Paul,

My recollection is that turbine efficiency is somewhat tied to RPM
because the speed of the gas flow must be matched to the speed of the
turbine blade..  Why not go to a larger turbine which would allow a
lower RPM for a given blade speed?  I realize a larger turbine might
be heavier overall, but if it were limited to a lower RPM, savings
might be possible.

RPM (Speed) is directly related to circumference, which has a linear
relationship to diameter..  Double the diameter of the exhaust, and
that should halve the RPM for a given blade speed.  We'd want a very
large exhaust wheel with very tight flows compared to what's probably
already on the market.

I wonder if it would be cheaper to have a custom turbine wheel made
for an existing large housing than to use a design that requires such
a large and high speed reduction ratio.  It would certainly be
simpler.

I know we have talked about a direct drive to the e-shaft system in
the past, but I think an intermediate step that only required a 3:1
reduction might be worth considering.


Regards,

Matt-

The key is blade velocity as a ratio of gas velocity out of the nozzle.
The A/R relates to the nozzle.

Torque is a maximum when the blade is stalled and zero when
blade velocity equals gas velocity.

Here is the HP out.

Paul Lamar


That doesn't contradict anything I said about using a larger wheel, does it?

Matt-

Not directly. There is a practical limit and that has to due
with leakage around the circumference. Total blade area will be
limited by gas flow consideration. As you make it bigger the blades
get shorter for the same area and the leakage off the ends of
the blades becomes a higher percentage. I don't have the empirical
data to know what the optimum size is yet :)  The trend in the turbo fan
industry is to go smaller, up the RPM and gear down the turbine. You can
see this trend in turbo chargers as well.

Paul Lamar



George,
Rotrex is manufactured in Copenhagen, Denmark,   I've been to
the factory and met the two engineers who developed it.  They
confirmed it can be used 'backwards' and they have had some
requests from wind and hydro power manufactures to do this.
HOWEVER, the housing needs modification.  The oiling direction
and vaned  oil pump design need to be changed. In addition a new
power shaft and power transfer plate needs to constructed.
Basically Rotrex makes 3 series of one stage superchargers plus
a complex dual speed unit.  Ratios  vary with design and with
the amount of power one wants to transfer.  The unit I have now
(30 series number 75) can transfer a maximum of 40 hp.  at a
ratio of near 12;1.   I would rather move up to  the  35 series
and  have a more robust unit that could conceivable transfer
80hp.  at a ratio of 7.5:1. and depend on the rubber multi-V
belt ratio to arrive at  the final targeted rpm.


I've come a full circle with my thoughts on design and presently
consider a  turbocompound system as a nice addition. IE,  if it
works fine, but I realize that it is a time eater project  (200
hours to date) and a 'plus alpha' device.  That is Japalish for
nice to have, but not absolutely necessary for flights I am
attempting.

Here is what is necessary for long distance flight at over 150
knots TAS.
1.  A turbo normalized engine  at  18,000 ft.
2. An exhaust 'bypass valve' that can disable the turbo-charger
if necessary.  No need to cook the engine oil or start a fire if
a seal starts to leak.


Now with the obvious advantages of speed at 18K or preferably
the  flight levels above comes the high delta Ps that  gives
turbocompounding a chance to shine.

And here comes the hard part.  Balancing the system.  Turbo
compounding implies that the engine needs to boosted first of
all and secondly, the power turbine exhaust gas need to be
evacuated very well.  First this  means a supercharger or
turbocharger has to be functioning to make the exhaust gas flow
robust enough that recovering energy is indeed possible.
Running a power turbine without up front boost is going to
restrict gas flows/ and or recover little.  Secondly at high
altitude the difference in the boost pressure and the ambient
(delta P) combined with a  short well-designed exhaust pipe,
creates a suction after the power turbine.  This comes with a
slight drag penalty.


T.C. is complex, no doubt about it, and complex experimental
items need to be designed so that their failures can be
isolated. Just like a bypass valve can isolate a turbocharger,
the power turbine and drive box have to be isolated with a
bypass valve before and sprague clutch after the traction drive.

A power recovery turbine should be downstream or parallel with
the turbocharger.  Notice the plurality  turbines here.  I am
not a fan of one-big-turbine-does-all with assumption energy
goes where it is needed.  It could perhaps but at the expense
of having a quick reacting variable ratio drive in between it
and e-shaft.

The other way to do the T.C. system is have boost provided by a
direct drive supercharger.  This puts a design challenge on the
T.C. system in that it the drive mechanism needs to be robust
enough to transfer all the gas flow and potentially double the
torque. In other words the S.C is borrowing HP and the TC needs
to recover that plus some to be of merit. I gave up on that
approach after running the numbers.

With a plurity of turbines, the T.C. design  quite rightly
becomes an add-on device.  Gases could be shared between the
turbines (parallel) if the gas flows  are robust enough.
Unlikely and back to directing  energy flows problem.    A more
likely scenario that existing art has shown works, is an  axial
power turbine receiving the downgraded energy left over from the
turbo-charger.  The power turbine  (axial) is downstream of the
radial turbocharger. The inlet is conical and the outlet
tangental. The exhaust gases should be able to be diverted from
the downstream inlet housing if it fails.  This allows a
functioning turbocharger to remain on line.  It also allows for
the T.C. system to be brought on-line gradually.  A plus in the
R&D phase.  Again an good suction on the exhaust side is
probably necessary to help evacuate the engine.

A custom axial wheel and casing is needed of course which is not
as painful cost wise as you would imagine.  Lot has changed in
the last six years in custom turbine wheel casting. Jet Bettle
Inc. inTaiwan has a nice selection and will design and custom
cast a wheel if needed.   120mm diameter and running around 80K
rpm is ball parking it.

This approach has some advantages:
For instance if the  Rotrex traction box is oriented to provide
the power into the accessory side of the engine, it  could be
used 'as is' when a 'clock wise'  rotating  axial turbine is
chosen.   Other points.  The  turbocharger's exhaust outlet
needs to be aimed at the axial power turbine central inlet.  The
turbocharger then needs to  be flipped.   For  tractor oriented
engines this is an advantage for the straight in approach of
rammed air to the compressor. Of course the turbine and the
compressor can be clocked to obtain the best orientation.  The
down side, is the accessory side needs another bracket and
bearing to support the overhanging loads a turbo-compound device
will apply to it.  This implies the choice of an earlier 13B
front housing with the casted in motor mount area.

Running the power into the other side of the engine can also be
done but that means a custom traction drive, as well a pulley on
the starter gear, slots in the bell housing etc.  Either design
approach requires a custom exhaust manifold.

Wisdom dictates the exhaust gas flows and temps be measured at
simulated or actual operating altitude before a T.C  is
designed.


Doug in Japan