Engine Desing

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
cylinders perfect in line makes it so well balanced. These are all 2
cylinder engines, on a 4 cylinder engine with a spark every 180 degrees,
it seems like it would be even smoother. My question is why don't they
make any engines for real airplanes like that? The 180 - 540 degree
firing sequence probably isn't as big a deal for the high rpm of model
airplanes as it would be for the low rpm of typical GA planes, so it
would probably only be suitable for engines with a multiple of 4
cylinders. It seems like it would be easier to make too, simpler crank
shaft, perfectly symmetrical crank case. The only more complicated part
would be the connecting rods. Just curious.

--
Chris W

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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

I think the Rolls-Royce Merlin was built like that. It was a
V-12 and had forked rods as you describe. There are very few ideas that
would be new in the way of reciprocating engines.
Lycoming and Continental could do this too, but their engines
are already way too expensive. Don't give them ideas.

Dan

Experimental builders have no reason to complain about Lycoming
prices, because no law makes them use them.

I forgot to include a diagram in case it's not clear what I meant from
the text. Take a look at the piston connecting rods in the drawing.

http://www.thewishzone.com/cdw/index...e=Saito182.gif


--
Chris W

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com

Chris W wrote:
I forgot to include a diagram in case it's not clear what I meant
from
the text. Take a look at the piston connecting rods in the drawing.

http://www.thewishzone.com/cdw/index...e=Saito182.gif


That's two cylinder radial engine. I understand that Harley Davidson
makes one.

--

FF

wrote:
Chris W wrote:
I forgot to include a diagram in case it's not clear what I meant
from
the text. Take a look at the piston connecting rods in the
drawing.

http://www.thewishzone.com/cdw/index...e=Saito182.gif


That's two cylinder radial engine. I understand that Harley Davidson
makes one.


However the Harley Davidson is a Vee, not opposed like the example
above.

--

FF

The Harley is not made to be smooth. Both pistons connect to the same
crankpin and fire right after each other. This is part of the famous
Harley lope as the engine goes bang bang flup flup.

I don't believe so. Since it is a FOUR cycle engine, Each cylinder only
fires every other revolution. It is timed so that one cylinder fires each
revolution. They alternate but since they are staggered, so is the timing.


"Sport Pilot" wrote in message
oups.com...
The Harley is not made to be smooth. Both pistons connect to the same
crankpin and fire right after each other. This is part of the famous
Harley lope as the engine goes bang bang flup flup.

On Mon, 09 May 2005 01:54:33 -0500, 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
cylinders perfect in line makes it so well balanced. These are all 2
cylinder engines, on a 4 cylinder engine with a spark every 180 degrees,
it seems like it would be even smoother. My question is why don't they
make any engines for real airplanes like that? The 180 - 540 degree
firing sequence probably isn't as big a deal for the high rpm of model
airplanes as it would be for the low rpm of typical GA planes, so it
would probably only be suitable for engines with a multiple of 4
cylinders. It seems like it would be easier to make too, simpler crank
shaft, perfectly symmetrical crank case. The only more complicated part
would be the connecting rods. Just curious.

what you are trying to describe is called DESAXING after a french guy
with the surname desaxe.
the change in alignment is really not that much.
it does work.

I have heard of magic hot rod engine tuneups that just involved a
quiet shimming and refacing of the cylinder base to achieve a freer
and faster running engine by desaxing it.
Stealth Pilot