Chris W wrote:
As most of you probably know the opposing cylinders on most engines
are
slightly offset to allow for the piston connecting rods to attach to
the
crank at different spots. I know of a few model airplane engines,
where
instead of having the cylinders offset and having the connecting rods
on
different sections of the crank, the connecting rods interlock and
are
on the same section of the crank. I have been told that the engines
that are built like this run very smooth with almost no vibration.
Obviously they have an odd firing pattern, but I guess having the 2
... My question is why don't they
make any engines for real airplanes like that?

Radial engines are made that way.

http://travel.howstuffworks.com/radial-engine.htm

One or more of the newer paramotoring engines uses opposed
cylinders joined with what is called a 'Scottish Yoke'. If you
imagine the pistons as being horizontally opposed the yoke is
a plate with a vertical slot in it. The crankshaft passes
though a bearing that rides up and down in that slot. This
allows the pistons to be rigidly attached to the yoke, and
therefor to each other--the rods joining the pistons to the
yoke does not swivel or pivot as in the more common engine
designs. This reduces the number of moving parts in the
engine and the associated wear and energy losses.

Whereas in the radial engine many pistons are attached to a
central bearing on the crankshaft, only two can be attached to
the Scottish Yoke and they have to be opposed.

Also the motion of the yoke about the crankshaft is different
than an engine with a pitman. One consequence of that is the
piston moves slower at TDC and BDC. This means more complete
combustion and in a two-stroke, better evacuation of the
exhaust gasses. Combining the scottish yoke crankcase with
fuel injection would seem to be the way to go to maximise
the fuel economy for a two-stroke engine.

There is an engine called a Bourke engine, I _think_ the essential
feature of Bourkes is that they scottish yokes.

I do not know what the downside is of using a Scottish yoke,
perhaps the sliding motion in the yoke causes excessive wear
or maybe there is a timing problem for _exactly_ opposed
cylinders. Or maybe the only real problem is inertia in
management of engine manufacturers.

--

FF