COMPRESSION QUESTION
09-25-2009 | 05:46 PM
#1
COMPRESSION QUESTION
HI can someone please explain why the compression is higher when turning a glo engine backwardsthen when the compression is felt in the direction the engine would normally run-this came up at the flying field today-love to have the answer for the boys -when i returnjust to surprise the hell out of them - REGARDS TONY
09-25-2009 | 09:00 PM
#5
RE: COMPRESSION QUESTION
hi the engine is not running when the compression is felt going clock wisethen counter clock wise in my comparisonclock wise rotation always has a higher compressionWHY ??
09-25-2009 | 09:17 PM
#6
My Feedback: (16)
RE: COMPRESSION QUESTION
The piston is pushed to one side of the cylinder when being pushed up by the crankshaft pin. When this happens, the skirt of the piston helps seal the compression.
In your clockwise direction the piston skirt seals the compression lost by the gap in the ring. In the counter clockwise direction , the opposite side piston skirt is help the non gap side of the ring seal the compression. This leaves the gap side of the ring with no sealing help and you notice less compression.
In your clockwise direction the piston skirt seals the compression lost by the gap in the ring. In the counter clockwise direction , the opposite side piston skirt is help the non gap side of the ring seal the compression. This leaves the gap side of the ring with no sealing help and you notice less compression.
09-26-2009 | 02:07 AM
#8
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From: Brisbane, AUSTRALIA
RE: COMPRESSION QUESTION
I believe there is more compression because the cam timing is different when rotated slowly in the opposite direction, have only noticed this in 4 strokes.
Mike.
Mike.
09-26-2009 | 02:27 AM
#9
RE: COMPRESSION QUESTION
ORIGINAL: Luna_Rendezvous
I believe there is more compression because the cam timing is different when rotated slowly in the opposite direction, have only noticed this in 4 strokes.
Mike.
I believe there is more compression because the cam timing is different when rotated slowly in the opposite direction, have only noticed this in 4 strokes.
Mike.
More compression in 4 stroke engine are due by the piston goes from TDC to BDC, therefore the engine get better cylinder filling.
The 2 stroke engine has poor cylinder filling than a 4 stroke engine due difficult to fill the cylinder by crankcase pressure..
09-26-2009 | 02:42 AM
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From: Brisbane, AUSTRALIA
RE: COMPRESSION QUESTION
ORIGINAL: Motorboy
Not whole correct answered..
More compression in 4 stroke engine are due by the piston goes from TDC to BDC, therefore the engine get better cylinder filling.
The 2 stroke engine has poor cylinder filling than a 4 stroke engine due difficult to fill the cylinder by crankcase pressure..
ORIGINAL: Luna_Rendezvous
I believe there is more compression because the cam timing is different when rotated slowly in the opposite direction, have only noticed this in 4 strokes.
Mike.
I believe there is more compression because the cam timing is different when rotated slowly in the opposite direction, have only noticed this in 4 strokes.
Mike.
More compression in 4 stroke engine are due by the piston goes from TDC to BDC, therefore the engine get better cylinder filling.
The 2 stroke engine has poor cylinder filling than a 4 stroke engine due difficult to fill the cylinder by crankcase pressure..
You have lost me??? We are talking about rotating the crank by hand, so if there is any pressure difference in a two stroke, then atmospheric pressure will balance it through the exhaust port.
In a normally running 2 stroke, then I agree with your 2nd statement.
09-26-2009 | 05:41 AM
#12
Senior Member
RE: COMPRESSION QUESTION
nor having a ring for that matter.
Two strokes without ring shouls have the same compression feeling, unless they have desaxe in the crankshaft.
Four strokes without ring have a totally different timing when turned the wrong way. i.e. the exhaust valve normally opens earlier than the inlet valve closes.
All this without ring.
If there is a ring, W8ye explained that.
Two strokes without ring shouls have the same compression feeling, unless they have desaxe in the crankshaft.
Four strokes without ring have a totally different timing when turned the wrong way. i.e. the exhaust valve normally opens earlier than the inlet valve closes.
All this without ring.
If there is a ring, W8ye explained that.
09-26-2009 | 09:05 AM
#13
RE: COMPRESSION QUESTION
hi this is my original posti was talking about two stroke enginessorry that was not clearthe ring theory does not apply as ABC ( non ringed engines ) have the same characteristic and they do not have a ringnow this is getting interesting ! TONY
09-26-2009 | 12:58 PM
#16
RE: COMPRESSION QUESTION
Also, as Pe noted, a lot of engines are built with desaxe which gives a small amount more more leverage when pushing the piston up going one way as opposed to the other. It's not difficult to understand if you look it up and see the pictures of what is happening.
09-26-2009 | 01:47 PM
#18
Senior Member
RE: COMPRESSION QUESTION
That`s my understanding too. My .91 FX acts just the same way, weak compression when turned in the running direction and strong compression when turned backwards.
09-26-2009 | 01:48 PM
#19
RE: COMPRESSION QUESTION
ORIGINAL: N429EM
OK, I gotta ask...
What is desaxe?
EJ
OK, I gotta ask...
What is desaxe?
EJ
See the pic..
09-26-2009 | 03:02 PM
#20
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From: henry, IL
RE: COMPRESSION QUESTION
When the engine is broke in, the wear to near perfect fit happens on the power stroke. When turning the engine backwards, you are compressing with the loaded side of near perfect fit. Normal rotation compression stroke doesn't cause near the load and wear as the power stroke.
Some engines are designed with the piston pin off center to mimick the above illistration.
Rod angles and leverages come into effect with offset cylinders and wrist pins. May be why some engines run better and longer than others.
Some engines are designed with the piston pin off center to mimick the above illistration.
Rod angles and leverages come into effect with offset cylinders and wrist pins. May be why some engines run better and longer than others.
09-26-2009 | 03:26 PM
#21
RE: COMPRESSION QUESTION
ORIGINAL: Kimhoff
If w8 is correct then there is a wrong and right place to put the gap when assembing a 4 stroke engine.
If w8 is correct then there is a wrong and right place to put the gap when assembing a 4 stroke engine.
i dont think it matters in a 4 stroke as there is no porting
09-26-2009 | 05:00 PM
#22
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From: ROCKWELL,
NC
RE: COMPRESSION QUESTION
Hey Tony, I hope this crude drawing helps you understand the question you had. All the answers so far are also correct. W8ye had the reason I think you were looking for. I think the greatest "feel" of the compression difference is from the pistons running clearance & its changing sides from the engine being rotated by the crank. (or prop) While the engine is running however, it is the piston that moves the crank in the power stroke. In the right hand drawing, the force of the piston tranfered to the rod will cause the piston to the intake port side of the wall & will help to seal the ring gap better while running. As far as a right or wrong way to install on a 4-stroke, It might help to put the gap on that side of the piston. But I have seen some rings rotate within the cylinder depending how tight of a fit they are. So it really wont matter in those case's. The gap is so small & the speed of the piston is so great, that while running, there is not much time for much leakage to take place. In a perfect fit, (no gap) there would not be any room for heat expansion & the trouble that would cause would be bad. Also, in a 4-stroke, it is through this gap that the engine will get its lubricating oil. They tend to be a little larger in gap than a 2-stoke engines ring. Hope all this helps, Bill
09-26-2009 | 05:39 PM
#23
RE: COMPRESSION QUESTION
ORIGINAL: MILDBILL-RCU
Hey Tony, I hope this crude drawing helps you understand the question you had. All the answers so far are also correct. W8ye had the reason I think you were looking for. I think the greatest ''feel'' of the compression difference is from the pistons running clearance & its changing sides from the engine being rotated by the crank. (or prop) While the engine is running however, it is the piston that moves the crank in the power stroke. In the right hand drawing, the force of the piston tranfered to the rod will cause the piston to the intake port side of the wall & will help to seal the ring gap better while running. As far as a right or wrong way to install on a 4-stroke, It might help to put the gap on that side of the piston. But I have seen some rings rotate within the cylinder depending how tight of a fit they are. So it really wont matter in those case's. The gap is so small & the speed of the piston is so great, that while running, there is not much time for much leakage to take place. In a perfect fit, (no gap) there would not be any room for heat expansion & the trouble that would cause would be bad. Also, in a 4-stroke, it is through this gap that the engine will get its lubricating oil. They tend to be a little larger in gap than a 2-stoke engines ring. Hope all this helps, Bill
Hey Tony, I hope this crude drawing helps you understand the question you had. All the answers so far are also correct. W8ye had the reason I think you were looking for. I think the greatest ''feel'' of the compression difference is from the pistons running clearance & its changing sides from the engine being rotated by the crank. (or prop) While the engine is running however, it is the piston that moves the crank in the power stroke. In the right hand drawing, the force of the piston tranfered to the rod will cause the piston to the intake port side of the wall & will help to seal the ring gap better while running. As far as a right or wrong way to install on a 4-stroke, It might help to put the gap on that side of the piston. But I have seen some rings rotate within the cylinder depending how tight of a fit they are. So it really wont matter in those case's. The gap is so small & the speed of the piston is so great, that while running, there is not much time for much leakage to take place. In a perfect fit, (no gap) there would not be any room for heat expansion & the trouble that would cause would be bad. Also, in a 4-stroke, it is through this gap that the engine will get its lubricating oil. They tend to be a little larger in gap than a 2-stoke engines ring. Hope all this helps, Bill
I have some model engine with 2 rings and loss of leakage is so small that it keeps the compression pressure longer than others model engines have only one ring.
See the pic of my engine here: http://www.rcuniverse.com/forum/fb.asp?m=4985295
09-26-2009 | 07:06 PM
#25
RE: COMPRESSION QUESTION
HI all the talk about ringed engines is still not the answerfor as i said ABC engines do the same thing -more compression when turned backwardsthere is no ring in them
