Right now the RX8 engine is running at about 3250 RPM at 65
MPH
and
getting 25 MPG. My guess it is using about 30 HP to go 65
MPH.
That would be a fuel burn of around 2.6 gallons an hour or
15.6
pounds per hour or a BSFC of around .52.
BSFC is defined as the number of pounds of fuel burned for
every HP generated in one hour.
An RX8 engine running at the best BSFC point should be
around
.47
according to the measurements some have made in aircraft
service.
This is a problem in common with all cars. The best BSFC
point for the engine is rarely at highway speeds or highway
RPMs.
The main reason smaller engines get better MPG on the
highway is
the small engines are running in a higher efficiency levels.
Namely wider throttle openings. Putting smaller engines in
cars
with higher axle ratios (lower engine RPM) to improve the
MPG is
called down sizing and down speeding. So a smaller rotary
engine
would get better MPG in an RX8 car at steady speeds of
around 65
MPH.
A turbo compound rotary could achieve a BSFC of about .38.
Turbo compound engines use small turbines extracting HP from
the
50% waste energy in a gallon of gas and feeding it back into
the
output shaft. A well known technology from the 1950's used
in
airliner piston engines. One of the problems with these A/C
engines
was the failure of an exhaust valve would take out the
turbine.
Needless to say the rotary has no exhaust valves. The 8 HP
turbine
would be geared down by at least ten to one.
Working backwards a BSFC of .38 would be a fuel burn for 30
HP
of
11.4 pounds per hour or 1.9 gallons per hour or a MPG
of 34.2 MPG at 65 MPH.
The trouble with turbo compounding a 240 peak HP engine
while
running at 30 HP is it does not appear to work. What we need
is a small rotary running at 80% of peak power or more to
achieve a BSFC of .38 with a turbo compound configuration.
One of the unique features of a wankel engine is the
possibility
of drilling a straight hole through the output shaft from
end to
end.
If we design a small single rotor turbo compound engine of
around
50 HP we can achieve 35 MPG on the highway at a steady speed
of
65
MPH. This
engine could feed its power through the large two rotor
engine
shaft while the
two rotor engine is shut down. The large engine is only
needed
for
acceleration
and climbing hills. All accessories such as water pump and
alternator would
be driven by the small engine.
Here is a 3D of such a configuration.
Comments are solicited. This is going on the web site soon.
Paul Lamar
Really good idea!
I was wondering..... Could one use a 3 rotor, the third rotor
modified
to act as the turbo compound? The compression side of the #3
rotor
housing plumbed to the exhaust of rotors 1 & 2, timed so the
exhaust
pulse powers the 3rd rotor.
Maybe the timing of the rotors is not conducive to this and
you
would
need a custom e-shaft. Weight wise it's probably not as
appealing
either.
Just thinking....always thinking...
Chris Cosman
What you want to do is stop the not needed rotors as they
consume
friction HP.
Paul Lamar
Mark R Steitle wrote:
Paul,
Couldn't you accomplish this by "selective decoupling" of the
third
rotor? Maybe a spraig clutch could be made to work. The 20B
e-shaft
is
already split.
Mark S.
That could be tricky as there is little room in the engine for
that.
Also a stock size one rotor would still be too large for road
load.
What you would need is something about half the size of the stock
one
rotor.
Paul Lamar
Paul,
Let me try again and see if I can explain my concept a little
clearer.
The three rotor e-shaft connects rotors 2 & 3 pretty much like a
13b
does, except for the "snout" sticking out the front end that
couples
the
#1 rotor to the rear two rotors. The two piece shaft uses a
keyway
and
friction to lock them together. If you left out the keyway, added
a
plain bearing or two, drilled the main e-shaft to accept a
through-shaft
(as you described) you could then put a clutching mechanism on the
front
side out near the oil pump and drive back through the engine and
into
the transmission.
The #1 rotor could be a smaller displacement as it would have to
be
individually balanced anyway.
Mark S.
(going back to lurking)
How about a sketch? :) I think I understand what you are saying.
Paul Lamar
Paul,
Sorry, but I don't have CAD capabilities on my work computer. The
best
I can do for now is this exploded view of the 20b e-shaft. The
small #1
rotor e-shaft would need bearings to allow it to spin freely on the
main
e-shaft while operating in economy mode (#1 rotor only). It already
has
a front bearing, so one additional bearing is all that would be
needed.
As in your concept, it would need a way to couple the #1 rotor
e-shaft
to the transmission by boring through the main e-shaft and using an
inner shaft, similar to how it was done with the Corvair. A small
coupling/counter-weight would connect the inner shaft with the #1
e-shaft. Power from #2 & 3 rotors would be through the rear as
before.
If you handled the coupling/de-coupling in the transmission, you
wouldn't need a spraig clutch on the engine.
If you kept all rotor dimensions the same and only reduced the rotor
width, you could still use 20b/13b side housings. It would require
a
special side housing (or adapter plate) if you're using a
scaled-down
rotor.
This would make a great engine for a luxury euro-coupe.
Mark S.
I think my way is cheaper and simpler as the power from the front
rotor
does
not have to go forward first and then back. The clutch is always
connected
to the front (small) rotor. This beats the current Daimler/Chrysler
4-6-8
engine as the
deactivated pistons don't have to go along for a free ride. Less
overall
engine friction.
BTW I also think the current 13B and RX8 rotors are not wide enough
yet.
I think one would get a better combustion chamber shape with wider
rotors
like the Diamond/Midwest rotary engine has. A square based combustion
chamber
has a better surface to volume ratio than a rectangular based shape.
Paul Lamar
Paul The primary surface area here is the planfom area, so I doubt that
you
would get a measurable improvement in the ratio. What you would probably
see is an increase in the oil heating from the wide rotor.
Vance
I don't follow you. If the rotor is wider the displacement is higher.
What is better is the max distance from the center of the fire to the far
reaches of the combustion chamber is less for for a square than a
rectangle
the displacement being equal. Just like the ratio of the surface area of
a sphere to its volume is less than the ratio of the surface area of a
cube
to its volume both containing the same
volume. In theory then there will be less quenched area.
Paul Lamar
Paul - Forget about spheres and cubes - If you start with the same R value
(rotor size etc) and widen
the rotor the swept volume (displacement) will incease as a linear funtion
of the width. for the same
compression ratio the TDC volume will incrase as the displacement (linear
with width) the projected
area also has the same length - increase in area being linear with width -
ie the volume ratio to area stays
the same. only the average distance spark to wall increases.
Vance
Displacement should be adjsted to stay the same.
Flame travel distance in unit time is the bottom line. Flame front speed
is constant.
Paul Lamar
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