3 rotor wrote:

perhaps someone already mentioned this?  wouldn't it make more sense to use the extra turbo
potential to drive simply the water pump and/or the alternator?  could a miniscule pump be run
directly to eliminate the need to gear it down?  isn't the additional power available about what
it takes to run the water pump?  the other idea would be to run the pump electrically and have the
turbo run an alternator.  would an alternator work at super high speeds?   kevin

The water pump is somewhere between one HP and three HP. That is mechanical
HP. We are still not sure the exact amount. If driven with an electric motor
that is 746 watts to 2,238 watts. That is 62 amps to 186 amps at 12 volts.
Assuming 100% efficiency for the electric motor to drive that pump.
Electric motors are only about 80% efficient so that is 
77 amps to 232 amps.

An alternator putting out 70 amps at 14 volts  requires 980 watts
or 1.3 HP to drive if you figure the alternator is 100% efficient.
They are only about 80% efficient so they would need about
1.6 mechanical HP to drive. Lets split the difference on the water
pump and say it requires 2 mechanical HP. Total accessory HP 
requirements are then about 3.6 HP. Hardly worth the trouble of 
compounding.  

Assume for the moment our turbo compound rotary is only cruising
at 150 HP and burning 8 gallon an hour. Say we are able to recover 
20% of that in the exhaust with the turbine. That will be 30 HP.
a far cry from the 3.6 HP  we need to drive the accessories.
That will up our available cruise HP to 180 HP. If we chose
to reduce that back to 150 HP our fuel burn should go down
to about 6.5 gallons an hour. 

Right now Tracy is cruising at about 6.5 gallons an hour 
at .47 BSFC or about 83 HP. Tracy is a poke or a putzer.
I run my Continental C182 flat out at altitude or about 170 HP,
150 MPH and about 11 to 12 gallons an hour. True! Tracy is going
almost as fast as I am but he is not flying a 182 and hauling 
88 gallons of fuel or as much stuff as I am :)

If we also decided to cruise our turbo compound rotary engine at 
83 HP then our fuel burn would go down to about 5.l gallons 
an hour. The basic engine would then be generating 66
HP and the turbine would still be generating about 16 HP.

All these calculations are very rough and the real results will
highly depend on the design point for the turbine.
One would start out by assuming some given cruise HP 
and optimize the exhaust turbine for that design point.

Paul Lamar
 
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