Paul,
When I was at the Seven Stock Show, I noticed that all of the rotors were
cast iron.
Has anyone made aluminum rotors? Is there too much thermal expansion to
use aluminum?
I was also thinking that the rotors could be made of a ceramic--low
expansion, lighter
weight, and possibly lower cost.
Steve Brazil
I think it is a heat issue. The rotors can run 500 F or more. Titanium
seems to work
but it is expensive.
Paul Lamar ...No rotor no motor.
Here is a photo of a lightened Mazda 13b rotor. It can be found on the
website
http://www.der-wankelmotor.de/Motoren/Motoren_Mazda/Mazda_13B/mazda_13b.html
Does anyone know anything about this? It seems to me that not only can we
make the
rotors out of a lighter, more durable substance, we can alter the
construction.
The rotor does not need to be a solid. It can have just the material it
needs to
have. It will be lighter and can still be reliable.
I do not read German. I do not know if the German written beside the photo
has
anything to do with the photo of the lightened rotor. I have included it,
just in case.
http://s12.photobucket.com/albums/a234/JacobPotts/Atom/Rotary%20Engine/?acti
on=view¤t=LightenedRotary13BEngine001.jpg
Les Nordman
This photo only shows the cross section of a stock RX-8 rotor cut in half.
As you
can see, there is not a huge amount of excess material in the rotors. The
face
of the rotor must be able to hold the maximum combustion pressures with
minimum
flexing in order to survive.
On the subject of titanium, I have built many race engine parts out of ti.
It
is it's own animal when it comes to designing with it. Any surface that will
see
any friction has to be coated with DLC (diamond like carbon) or a similar
coating
to prevent galling. The apex seal area would have to have a steel insert.
The other
issue would be the rotor running much hotter, as ti does not transfer heat
well and
you could run into the case of the seals overheating. With enough money for
building
the investment casting die, we could test this... but the question remains
as to why
we need it so much lighter if we> don't plan on spinning the engine above
7500 rpm.
Larry
1. About the photo of the seemingly skeletal Wankel rotor, Larry wrote:
"This photo only shows the cross section of a stock RX-8 rotor cut in half."
Ah, so stock Mazda rotors have a solid surface on the side, but, internally,
they are hollow, with "spokes" of metal radiating out from the center to the
outer edges, just enough material in it to take the stresses. I though the
stock rotors were solid iron all the way through. Thanks, Larry, for making
this clear! :-)
2. On the general topic of lighter rotors, Larry says, "but the question
remains
as to why we need it so much lighter if we don't plan on spinning the engine
above 7500 rpm."
Okay, it may very well be that this noob does not understand, but that is
what this
forum is for, right? :-)
The noob responds, "Larry, I don't know about other contributors to this
forum, but I
was thinking that lighter rotor construction, as well as lighter rotor
materials,
would be part of an overall effort to reduce the weight of a rotary engine.
If the
engine is lighter, we don't have to lug as much weight around up in the air.
We will
go faster, turn better, get better fuel mileage, and go farther.
And yes, Larry, I am still interested in also applying this to The Dark Side
of
rotary engines: a lightweight rotary engine for lightweight cars such as the
Ariel
Atom and the Caterham."
(Cue Star Wars Imperial Death March)
Les :-)
Les,
I agree with you in principle... all parts made lighter would be a good
thing! Rotor and eccentric shaft bearing life would be better with lighter
rotors, but at the expense of rotor life. I think the technology exists to
build lighter rotors out of something other than cast iron, we just can't
afford it.
If I were building rotaries for the Reno Air Races, different story as the
engine has a short fuse. Make max power for a limited amount of hours,
knowing is will die quickly. For general aviation with my family on
board.... different story altogether. For that I will live with a heavier
but more robust rotor.
But keep the thinking going! That's what makes this forum so useful to all.
Larry
I think the easiest way to get a very light, strong rotor would be to
investment cast 400 series stainless steel. This is the same stuff Ruger
uses to make firearms. Investment casting does not use dies in the sense
that die casting lower melting point metals does. It uses a consumable form
called a "burnout" made of wax or some clean burning plastic.
The best way to make burnouts for small numbers or prototypes of intricate
parts is to use stereo lithography. The burnout is coated with a plaster
slurry and the coating is built up to the point it can be put in a can to
encapsulate it in a manageable form. When the plaster mold is heated the
burnout goes away exposing the cavity. Because of the intricacy of the
forms in the rotor, I think centrifugal casting would be best.
Steve
The current RX8 rotors are centrifugal castings.
This outfit can make sand cores and molds directly with stero lithography.
I saw some parts at the SAE expo. No need for core boxes or patterns.
The sand is bonded together with a laser. They can do 2 mm walls. .079 thin
walls. Amazing! We were lucky to get .2 inch walls with the bell housings.
www.rapidcastings.com
Francois, they are also in Germany
www.actech.de
Paul Lamar ...No rotor no motor.
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