35 MPG RX8

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

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

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,
Let me ask why two extra rotors are needed?  Two rotors wouldn't be
enough grunt for a full-size sedan, but it seems that this would be
adequate for the typical econo cars of today.  So you would have a two
rotor, only they wouldn't be directly coupled, and one rotor would be
down-sized.

Mark S.


Well you still have 240 HP out of the main 2 rotor RX8 engine. That has not 
changed. Total HP is now 290 HP including the small one rotor.

OK I see what you are driving at. Something like this for a Mazda 3  or 
MX5 (Miata) say. 170 HP total. 50 HP in the front and 120 HP in the back. 

Paul Lamar
 
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