A few years ago some SCCA rotary road racers had discovered the lower
the oil temp the more the power. It did not dawn on me
why that was necessarily true until I read the SAE paper I just up
loaded.
Here is the reason. The volumetric eff. of the engine is being
affected by the temperature of the face of the rotor which in
turn is being cooled by the oil.
Keep the oil hot during cruise for best BSFC and cool for max power.
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A 6% improvement in the BSFC would drop the BSFC from the current
lean burn .47 to about .44 which is about the same as a Lycoming.
The typical cruise fuel burn would go from about 10 GPH to about 9.4 GPH.
Easy to do. Just put a cowl flap on the oil cooler air outlet.
Not only will that warm the oil during cruise it will cut the
aerodynamic drag. This has pertinence in endurance road racing as
well as aircraft.
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Come to think of it this is also why the rotary BSFC is so much
better at higher power levels compared to what it is in a car piston
engine.
Mazda needs to find a way to turn off the rotor oil cooling squirter's
under
typical low power car cruise. In other words they need to find a way
to get more
heat in the rotors at 4,000 RPM at the 30 and 40 HP power levels
typical of 80 MPH
cruise. I would not be a bit surprised if this did not increase the
EPA highway
numbers by one or two MPG.
--
Paul Lamar ...No rotor no motor.
This is about all low heat rejection engines. Not just turbo
compounding.
Good list of references on other work.
Paul Lamar ...No rotor no motor.
Paul
So it looks like we need an rotor that is light weight, strong and has
insulating properties. Titanium. The recessed area within the rotor
faces
could be steel to absorb and retain heat.
Can't imagine how expensive this rotor would be:)
Doug in Japan
I am thinking stainless Steel. Easy to cast and I think it does
not conduct heat as easily as cast iron. I am still looking into it.
You can still cool the bearing without cooling the combustion surface.
----------------------------------------------------
http://hypertextbook.com/physics/thermal/conduction/
Iron ......80 .... Current
SS ........ 14 ... Cheap
Plain steel 45 - 65
Titanium... 22 .... Expensive
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Paul Lamar ...No rotor no motor.
People have been ceramic coating rotors (and other bits) to enhance heat
rejection for years - I don't know the details or effectiveness.
If you are thinking of casting custom rotors, go whole hog and do them
in some ceramic/MMC ;)
3-4# rotors would probably raise the rev limit to formula 1 levels and
racers would line up to buy them...
Is David Mathes still on? Any progress on the engine lightening?
Welcome back Doug - don't ask how much for Al-SiC rotors.
Cheers
Cary
I would not coat rotors with ceramic. I have seen what happens when
it comes off. Your rotor housings will look and feel like sand paper.
It is also too thin to act as an insulator.
However what is the heat conduction coef. of solid silicon nitride? The same
stuff they make apex seals out of.
It would be very very expensive. RPM would not go up much
as the cent. force is mass x V^2. Real short stroke Formula 1 engines
are revving to 19,000 RPM. You won't see that in a Mazda based rotary.
The only way to see that in a rotary is make a half scale version of
the Mazda with pressure fed bearings and wider rotors. Half the V
at a given RPM. Then go to four rotors to get the power back.
The fundamental problem is the weight/bearing-area of the rotors.
The smaller the rotor the better.
Here is a diagram. The 16B is going off in the other direction.
--
Paul Lamar ...No rotor no motor.
Paul, Cary
Here is an European outfit that specializes in induction heating and spin
casting of titanium down to thicknesses of 0.5mm.
http://www.azom.com/details.asp?ArticleID=286
Yes some stainless has a slower heat transfer characteristics compared to
cast gray iron, but some grades have a larger specific gravity.
I suspect if you are thinking stainless you would want a good corrosion
resistance alloy like SUS 316L.
Doug in Japan
For aircraft use I would gladly trade a little max RPM for lower fuel burn.
We don't run the engines over 7500 RPM anyway. The red line on the RX8
engine is 9000 RPM.
Paul Lamar ...No rotor no motor.
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