> Gordon,
> You lose the weight of the gearbox but gain the weight of the
> SUPERCHARGER and all it's plumbing and mounting hardware. I'd bet it
> would be close to a wash on weight, and the wear aspect would be MUCH
> higher. Paul hammers on this all the time but I'll take over this
time.
> The rotary is NOT a piston engine! There is no reciprocating
motion, and
> higher e-shaft RPMs are NOT a wear problem. As has been said many
times
> Mazda instrumented the engine and found that bearing load at 6000 RPM
> was NIL, ZERO, NADA, NOTHING. The compression and power loads
balanced
> out. I plan to cruise at or slightly below that figure using a 2.85:1
PSRU.
> Secondary item: Paul the figures on the screw Lysholm
supercharger are
> true. The efficiency improvement lead to the NHRA banning them at one
> time. I don't know it that is still true or not.
> Bill Jepson
>
Hi Bill,
Regarding the 'no bearing load at 6000 rpm' means that the inertia
force of
the
pistons equals the force generated by the combustion pressure. The
pressure
curve
at combustion has a very sharp, short duration peak and falls off quite
rapidly
at both sides of the curve. Also keep in mind that at the other
side, when
the
eccentric is at the 'bottom' meaning at the side of the ports, the full
inertia
load of the piston is loading the bearings, similarly to an RPE at
BDC or
rather
at TDC during open valves.
There are always bearing loads, no matter at what speed the engine
runs. Of
course, running near 6000 rpm reduces the bearing load near TDC, again,
similar
to an RPE. (Just don't want to get the wrong impression out there.)
Rolf
Rolf,
Thanks for that piece of technical insight. I always wondered about the
rotor's inertia with that elliptical orbit -- besides the circular
motion,
there is an up and down movement from one end of the oval to the
other that
no doubt generates sizable inertia with that big rotor.
As you correctly pointed out there are always bearing loads at any
rpm --
but it's clear that they go down to their minimum at just under
6,000 rpm,
and then go back up from there.
Still, looking at that graph, the difference between 4500 rpm and
6000 rpm
is not that great, about 25 percent less (160 kg/cm2 to 120 kg/cm2). The
graph doesn't show a data point at 3500 rpm but if we assume that it
continues in a straight line, we can interpolate about 180 kg/cm2 at
3500
rpm. So the bearing load at 6000 rpm is only 33 percent less than at
3500
rpm.
Hmm, maybe I was too quick to throw in the towel on the direct-drive
supercharged engine.
Still, as Paul pointed out, the thrust bearing might not be able to
take the
propeller loads.
Another thing is the side loads from the prop, especially if you need to
have a long prop hub. And even if you use a short prop hub, your
engine will
be farther forward, thus moving your CG forward as well.
So yeah, there are problems with this approach, like I said. Still maybe
this discussion will have been of benefit if there are some who might be
considering a direct-drive option. At least they will have some
points to
consider.
Regards,
Gordon.
Paul,
I know you will jump in to correct me, so I will correct myself. I
know the
rotor has a perfectly round orbit around the eccentric, not elliptic.
What I was referring to was the path of the outside of the rotor,
which does
describe an oval -- following the shape of the oval housing.
Now I may be wrong on this, but aside from the rotational momentum
of the
rotor, I think there is some linear momentum there as well, as the
mass of
the rotor accelerates from the bottom of the oval to the top, then back
down, etc.
The effect is like having an arm on the eccentric that translates the
rotational motion into reciprocating motion -- but instead of an
arm, it is
the shape of the rotor, triangular, with a thin pointy part and a
wide part,
that makes possible the up and down movement in the oval housing.
Regards,
Gordon.
It is not really an oval housing either. It is a trochoid
housings.
"I think there is some linear momentum there as well, as the mass of
the rotor accelerates from the bottom of the oval to the top, then back
down, etc."
No it does not. The center of gravity of the rotor travels
in a pure circle. There is absolutley no linear momentum.
BTW you are not alone. Some very very smart engineers still
don't understand the rotary. As I said in my article below
the engine is subtle.
Paul Lamar
--------------------------------------------------------------
-How the Wankel Works Rev. 1 copyright by Paul Lamar
The Wankel rotary consists of a three sided rotor moving in a
modified oval
chamber or rotor housing. There are at least two holes in the walls
of one
side of the rotor housing. In the other side of the rotor housing
there is
at least one spark plug. One hole is used as an intake port and the
other as
an exhaust port. Just like a one cylinder two cycle piston engine.
The rotor
rotates around the center of the eccentric on the output shaft while
at the
same time rotating around its own axis. This is very similar to the
way the
earth rotates around the sun while rotating around its own axis. The
engine
is sometimes called a planetary engine for this reason. The rotor
rotates
one third as fast about its own axis as it does around the center of the
output shaft. Actually what is happening is the center or CG of the
rotor is
moving in a circle around the center of the output shaft while the
output
shaft rotates. Very very clever on the part of Felix Wankel to
invent and
engine that uses this motion.
Many engineers and mechanics that should know better still think the
rotor
translates, reciprocates or wobbles in the rotor housing. Nothing
could be
further from the truth. The key is to think only of the center of
the rotor.
That is the center of gravity of the rotor and what that does is all
that
counts. It rotates around the center of the output shaft pure and
simple.
The wankel is a subtle engine. This is motion very important when it
comes
time to balance the engine. Since there are no reciprocation parts
what so
ever it is possible to completely balance the Wankel engine just like a
turbine or electric motor and unlike any piston engine despite some
claims
to the contrary. The rotor is balanced around it own axis. The front
rotor
balances the rear rotor as they are 180 degrees apart on the output
shaft.
That leaves a small couple which is finally completely balanced by the
counter weights.
As this motion is taking place the tips of the triangular shaped
rotor are
passing the ports in the housing. As one tip, call it tip 1, passes the
intake port in the rotor housing the face behind tip 2 starts to
draw in the
fuel air mixture. Call it face or chamber A. When the next tip call
it tip
2 passes the intake port the mixture is captured and starts to
compress. As
the chamber continues around the housing it encounters the spark
plug and
the mixture inside it is fully compressed. The spark plug fires and the
chamber starts to expand imparting the pressure energy in the burning
mixture to the output shaft. As the rotor continues on around tip 1
passes
the exhaust port and the burnt mixture starts to leave chamber A. This
completes the four cycles for chamber A. In the mean time chamber B and
chamber C are completing various cycles of this same sequence
simultaneously. Since the rotor is rotating one third as fast as the
output
shaft only one firing event and one chamber's volume worth of air is
mixed
with fuel and processed per revolution of the output shaft. This is
the same
as a one cylinder two cycle piston engine of the same displacement
or a two
cylinder four cycle piston engine of twice the displacement.
This three chamber simultaneous action is the reason the Wankel
engine is
so light and compact for the power it is able to generate.
Furthermore the
forces in each chamber due to the pressure in each chamber are
completely
contained in the one piece cast iron rotor. In a four cycle piston
engine
this is not the case. The compression force must pass down a
connecting rod,
into the crankshaft and up another connecting rod into another
chamber that
is expanding and extracting power from the burning mixture. This makes
piston engine force paths and consequent stresses far far more
complicated
and problematic than the wankel engine. In fact the Mazda Wankel
engine is
so robust, structurally speaking, it is possible to get as much as
400 HP
per rotor. This HP per rotor number is still increasing while auto
racers
around the world are still developing the engine.
Although the wankel has similarities with a two cycle engine,
particularly
in regard with the way the porting mechanism works, it is in no way
a two
cycle engine. All four cycles are continuously taking place
simultaneously
and those are intake, compression expansion and exhaust.
I think about it like this- everything in the wankel spins- the e-shaft
spins, and the rotor spins on the e-shaft eccentric lobe. The rotor
spins exactly 1/3 the speed of the e-shaft, and it is this that
describes the shape of the trochoid/hypotrochoid shape. Nothing goes
back and forth, just around in circles, the trochoid shape just follows
the path that the apexes of the rotor follow on their revolutions. Felix
Wankel was a genius. The concept is simple, but not obvious. The steam
engine gave rise to the internal combustion engine-
pistons, cylinders valves, reciprocating motion. These things were
around way before steam engines. Da Vinci designed pumps with these
items. So I can see where the idea of the internal combustion piston
engine came from.
But the Wankel- where did Felix Wankel pull that idea from?
Brian Trubee
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