Roger,

Many piston engines are not well balanced, it is a matter of the design.
Horizontally opposed engines are inherently balanced. As long as the
components are close, the engine is smooth running. Inline six's and the
derivative V-12's are also well balanced.

Inline fours suffer from a secondary shaking force. When the engine is
at TDC
two pistons are at TDC and two are at BDC. The engine has a center of
gravity.
Now rotate the engine 90*. The center of gravity has changed. The
explanation
is that as the crank rotates, the conn rod and piston not only move down the
bore, but the rod also moves to the side. Pythagorean theorm states Asq'd +
Bsq'd = Csq'd. Now if the C is the rod length and B is 1/2 the stoke
the math
will tell you the effective rod length at 90*. The piston will be lower
in the
bore the 1/2 the stroke. Thus the center of gravity wants to move the engine
to keep the Cg in the same spot.

V6 engines are a nasty mess of vibrations with rocking couples in the mix.
They want to rock forward and back, and then side to side depending on where
in the rotation the rod throws are. Early Buick V6's had very soft engine
mounts to try to isolate the vibrations.

The rotor moving in the housing is sort of like an inline 6 crank. When two
throws are at TDC the other four are at 120* and 240*. The two low ones
have a
higher mass combined, but that is closer to the rotational center. Now the
rotor when at TDC has two apex seals that are vertically one over the other.
If you measure from a line between them and another horizontally to the
e-shaft center line and measure the distance it is less the the rotor
radius.
Thus you have a higher mass at a shorter radius. There is still some
imbalance
that the balance weights take care of. Many piston engines also have balance
weights. Sm block Fords, 400 Chev, BB Chevs to mention a few.

Dale Davies

Like an inline six? Hardly.

"When two throws are at TDC the other four are at 120* and 240*."

What do you mean by a "throw" ?

The two rotor has two eccentric 180 degrees out. The two rotors
exactly statically balance each other. Since there is an offset between
them they are dynamically out of balance. This is completely canceled by the
counter weights. As you add more rotors the counter weights get
smaller and smaller.

Piston engine balance is a complex subject.
The rotary is very simple by comparison.

I recommend For those of you with an engineering degree or equivalent
Taylor's
series of books is the best there is. The bible of engine design.

The Internal Combustion Engine in Theory and Practice.

Volume 2: Combustion, Fuels, Materials, Design. Revised. Charles
Fayette Taylor. The MIT Press Cambridge Massachusetts, and London
England.

Engine Balance Chapter 8 page 240.

When I was at the Mazda factory the vibration test was a glass of wine
sitting
on the top of the engine when running on the dyno. No disturbance.
All engines coming off the assemble line were run on dynos.

Perry Mick's rotary is bolted solidly in his Long Ez airframe for
the last 16 years or so. No rubber bushings. One can hardly
feel the engine running.


Paul Lamar

Dale,

You are right it is a messy affair. And it doesn't stop with the masses
moving around, we have dynamic forces emanating from each power stroke
as well.

If all opposed are well balanced...well don't know about that, four
cylinders and especially two cylinders have some less desireable short
comings.

Take one of the messiest engines to sort out..

The two cylinder opposed..

Each power pulse wants to push the piston down, but pull the engine up,
( action-reaction) ad to that that the counter cylinder doesn't cover
it's power pulse  until a full turn later...and is offset to the first
cylinder.

What a mess...I am surprised though how smooth the 1/2 VWs are running
after all, but they had to put a lot of work into it.

At 5 cylinders, you start to overlap the powerpulses, and that starts to
make a big difference in vibration, also efficiency as less and less
flywheel ( if that application is used) is needed to cover for the
pulses the more cylinders that you have that cover each others pulses.

Not that I know of any 5 cylinder engines that fly nowadays, maybe some
old radial.

Of the flying breed ( and still talking pistons) I would say that a 6
cylinder opposed have probably most of all the desirable features. There
seems to be a lot of happy Corvair engine guys out there, I keep reading
about the smoothness of their engines the whole time.

The forces we are talking about is pretty visible if you take a small
engine ( Briggs and Scrapiron), those with horisontal shaft, and a foot.
and start it up on the garage floor, not bolted down.
The engine wants to jump up the whole time, to counter the force of the
piston that wants to be pushed down the whole time. The garage dancing
Briggs can jump around so intensively that it could be hard to catch.

Any engine that has so few cylinders that it doesn't cover each others
pulses, 4 cylinder or less, will transmit  a full pulse each time a
cylinder fires., and as in an opposed engine they have to sit not
directly opposed to each other, but a bit ofset, there will be a scewing
pulse action. More so in a 2 cylinder than in a 4 cylinder engine
because the 4 cylinder engine has more mass.

It would translate to, like if you would go and grab the spinner and
rock it horisontally left and right.

V engines have their own idiosyncrasies, as their power pulses can not
be directed opposite of each other, but usually this is cast iron things
and especially in an 8 cylinder, even if the power pulses are not
directed against each other, there are so many of them that they really
start to cover each other genuinly now.

Yes inline 4's is a bit messy, add to the balance problem that the
firing order is 1342, so there will be two pulses wanting to lift up the
front part, then comes two pulses wanting to lift up the rear part,
that's an engine that really benefits from a flywheel or heavily counter
weighted crankshaft.

Then add to the problem that when you are accelerating an engine, the
torque wants to rip it of it's engine mounts and throw it in the
opposite rotating direction.

This is a phenomenon that has floored a few bikers, that wants to put
big V8's onto motorcycle frames.

Flywheels and harmonic dampeners are ways to dampen all those things,
but those gizmos are something that rarely, because of the heavy weight
penalty, can be used in airplane engines.

A propeller is hardly a flywheel, a flywheel has to conserve energy, but
a prop is designed to give away energy as effcient as possible, so a
propeller is more of a rotating brake.

You can take a fourstroke, and balanace it, and run it dry, not firing,
and you will find that it will be a fairly stable and not too wildly
shaking ting, it will actually be farily smooth.

Most of the shaking and vibrations, and different forces will come once
you start to put some real power into that puppy, once you flip the
ignition to ON.

Roger