Engine Desing

On 16 May 2005 00:28:47 GMT, Bertie the Bunyip
wrote:

Stealth Pilot
m:

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.

Umm, don't thnk that's what he's talking about. A few early radials used
desaxe cyliners, but that's a cylinder that's been offset form the
centerline of the crank to take advantage of a straighter conrod during the
power stroke. prolly worked OK, bu ti think they only use it in two strokes
nowadays if at all.
What th ewirter seems to be describing is an arrangement where two opposing
pistons share a crankpin. I've seen twins like this and they vibrate like
hell. A boxer (whihc most all flat opposed engines are) is a much smoother
arrangement.


Bertie

you know I think you are right bertie.
I misread the post. I agree totally with your analysis as well.

shapers use a scotch link as well. they can get quite a vibration up
even at low rpm.
Stealth Pilot

Stealth Pilot
:

On 16 May 2005 00:28:47 GMT, Bertie the Bunyip
wrote:

Stealth Pilot
om:

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.

Umm, don't thnk that's what he's talking about. A few early radials
used desaxe cyliners, but that's a cylinder that's been offset form
the centerline of the crank to take advantage of a straighter conrod
during the power stroke. prolly worked OK, bu ti think they only use
it in two strokes nowadays if at all.
What th ewirter seems to be describing is an arrangement where two
opposing pistons share a crankpin. I've seen twins like this and they
vibrate like hell. A boxer (whihc most all flat opposed engines are)
is a much smoother arrangement.


Bertie

you know I think you are right bertie.
I misread the post. I agree totally with your analysis as well.

shapers use a scotch link as well. they can get quite a vibration up
even at low rpm.
Stealth Pilot

Scotch link? Is that where one big end is forked to go round the other rod?
On that subject, I seem to remember one of the Flying and Glider manuals
from the thirites has insturctions on how to turn one of those diabolical
contraptions into a boxer engine. It meant a new crank and bending the con
rods!
And I thought Moslers were crap...


Bertie