Please reread this revised text of the article on the ACRE web site and let me know
what you think before I upload it back to the web site.

Paul Lamar
________________________________________________________________________________

The Rotor Verses the Piston.  By Paul Lamar

Lets analyze the motion of an aircraft piston engine rotating at 2500 RPM.
The bore is five inches and the stroke is five inches. At the three o'clock
position of the crank the piston is moving down the bore at maximum speed.
At the nine o'clock position the piston is moving up the bore at maximum
speed. At noon and six o'clock the piston is stopped at top and bottom of
the bore respectively.

The circumference of the crank is pi times five inches or 15.7 inches. That
is 1.31 feet.

Since the RPM is 2500 the RPS is forty two revolutions per second.  So the
speed of the crank-pin is forty two RPS times 1.31 feet or fifty five feet
per second moving in a circle. The piston speed is therefore the same as the
crank pin speed at the three o'clock and nine o'clock positions.

Bear with me here as we are steadily getting to the point. It takes the
crank one second divided by forty two or .024 seconds to rotate once. To
rotate from noon to three o'clock it took one quarter of that or .006
seconds. Therefore the piston accelerated from a dead stop to fifty five
feet per second in only .006 seconds! The average acceleration is then 9200
feet per second in one second. Well now lets see. One G is only thirty two
feet per second in one second so 9200 divided by thirty two is about 290
G's!!!!!

Let's assume for the moment the connecting rod, the piston pin and the
piston with the rings weighs seven pounds. They all move the same way the
piston does. Mr. Newton said, and he seems to be right, force equals weight
times G's. The three pound piston alone looks like 870 pounds to the rod in
the form of inertial load.  Mr. Connecting rod has to push and pull that 870
pound piston back and fourth once every .024 seconds. As far as Mr. 
Crank-pin is concerned that seven pound collection of parts looks like it
weighs about 2000 pounds! 290 G's times seven pounds is about 2000 pounds of
inertial load. Mr.  Crankshaft says "OUCH!".

This is a simplification of course. The compression and combustion pressure
loads sometimes adds to and sometimes subtracts from these inertial loads. I
cannot imagine a piston engine made without light weight pistons. The piston
engine, as we know it today, would not be possible without aluminum pistons.

Metal fatigue happens when the force and therefore so too the stress
repeatedly reverses on a part. The cylinder spacing, rod length, rod journal
diameter, main journal diameter, cylinder bore have a large effect on the
fatigue life of a piston engine crankshaft.

According to Charles Fayett Taylor "The Internal Combustion Engine in
Theory and Practice" Volume 2 page 493 the major problem with piston engine
crank shafts is the fillet radius between the rod and main journals and the
crank cheeks. The fatigue limit stress can be as little as 5% of the
breaking stress of the crankshaft if these fillet radii are too small. The
fatigue limit is defined as that stress level where the part will live
indefinitely. Usually if the crank does not crack in two million revolutions
or 13 hours at 2500 RPM max power it will live forever. If you run an
aircraft engine at mostly 75% power it will probably live about 2000 hours.

The automotive engine cylinder spacing is too close to allow adequate fillet
radius in the crank cheeks. Engine size and length is everything to an
automotive design engineer. This was a major factor in the downfall and
bankruptcy of the Chev big block based Orenda V8 aircraft engine. If you run
an automotive piston engine at 6000 RPM full power you will exceed two
million cycles in about six hours. There is some manifold pressure where the
engine will live indefinitely however.  What that is is unknown but most
auto engines are designed to run indefinitely while generating only about 50
HP or less. That include the 400 HP Chev V8. What you wind up with is an
engine that has a rather poor power to weight ratio or limited life for use
in an aircraft. 

This can be circumvented to a certain extent and the life
increased by substituting better materials such as 4340 for the cast iron
typically found in automotive crankshafts and rods. That of course increases
the cost of the engine almost to the point of a real aircraft engine while
the TBO is unlikely to exceed 1200 hours. If you reduced the bore and
consequently the displacement to limit the peak HP that too would help. If
you shortened the stroke to reduce the displacement that would help even
more as the rod journal to main journal overlap would be increased.

An over the road tractor trailer diesel engine on the other hand is designed
to run at peak power almost indefinitely. Such a 300 HP piston engine would
weigh in the neighbor hood of 2000 pounds. BTW I have always maintained such
an engine installed much further aft in a crop duster airframe would make a
great banner or glider tow airplane. Unfortunately it would be illegal in
the eyes of the FAA.

If you are going to design a V8 or V12 aircraft piston engine you need to
start with a clean sheet of paper. V engines (and all other in line piston
engines) are trying to shove their crankshafts through the bottom of the
blocks. A horizontal opposed engine will weigh less due to the more
efficient structure. Lycoming built some horizontal opposed air cooled 8
cylinder engines and they too had problems with the crank despite the wider
cylinder spacing.

It is no wonder your aircraft engine crank shaft and other parts are a mass
of cracks at over haul time.  Its a miracle that piston engines last as long
as they do. It has taken over 100 years of minute detail development and
billions of piston engines to make them viable.

The beauty of the Wankel on the other hand is the shaft is straight through
yielding no concentrated stress problems up through four rotors and maybe
more.

All other things being equal, like intake breathing, the HP of an ICE engine
is directly proportional to the RPM it can be run. The wankel engine breaths
extremely well at high RPM's due to to the lack of poppet valves getting in
the way of the intake and exhaust ports. Unlike any and all piston engines
the wankel rotary is in complete and precise balance. Just like a turbine or
electric motor. Despite commonly held beliefs it is impossible to precisely
balance any piston engine as there are always reciprocating parts that
oscillate back and fourth.

You might surmise from this the wankel rotary is able to revolve at
unlimited RPM's because there are no reciprocating parts. You would be
wrong. The wankel rotary also has its RPM limits.  Centrifugal force rears
its ugly head. The weight of the rotor spinning around the center of the
e-shaft creates a terrific centrifugal force on the rotor bearing at speeds
of 10,000 RPM or more. By the way the turbine engine is also subject to the
same centrifugal force limits on its compressor and turbine blades.

Never the less the wankel rotary can rotate far faster than an equivalent HP
piston engine adding to its intrinsic power to weight ratio advantage. Also
this centrifugal load due to the weight of the rotor never changes direction
on the e-shaft at high speeds so a cracked or broken e-shaft on a wankel
rotary is unheard of. This is why the wankel rotary engine designer can
afford to make the rotor out of low cost and extremely durable but heavy
cast iron.

Further development of the rotor is possible of course. Better casting
techniques are used by Mazda to reduce the weight of the rotor in the RX8
engine. The RX8 engine is rated at 250 HP at 8500 RPM. Not bad for a 200
pound engine twelve inches wide, fourteen inches high and seventeen inches
long. This is an exceptional power to weight ratio and power to size ratio
that exceeds any piston aircraft engine despite the use of low cost cast
iron rotors and end housings.

Rotors made from other material with a better strength to weight ratio than
cast iron may eventually be used. A more costly aluminum rotor is possible
with steel inserts in the high wear areas of the apex seal slots. A titanium
rotor is also possible. So too would be a sheet steel rotor perhaps laser
welded together.

I hope I have adequately got the point across of why the wankel rotary is a
better engine than the piston engine.


 
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