Single cylinder balancing question.
04-18-2003, 01:49 PM
#1
Member
Thread Starter
My Feedback: (1)
Join Date: Jan 2003
Location: Laurel,
DE
Posts: 80
Likes: 0
Received 0 Likes
on
0 Posts
Single cylinder balancing question.
I'm curious if anyone is balancing these engines? I am considering modifications to the piston on my 25cc poulan/featherlite engine. Since it has a single sided crank shaft, the con rod is easily removed, making balancing fairly simple.
What do you use for calculating the bob-weight? 50%?
Can it be done with some fair amount of accuracy by balancing the assembly like we do with a prop? I know automotive shops spin the cranks on big machines, but I would at least like to get this thing in the ballpark.
John
What do you use for calculating the bob-weight? 50%?
Can it be done with some fair amount of accuracy by balancing the assembly like we do with a prop? I know automotive shops spin the cranks on big machines, but I would at least like to get this thing in the ballpark.
John
04-18-2003, 03:01 PM
#2
Senior Member
Join Date: Mar 2002
Location: ramona, CA
Posts: 841
Likes: 0
Received 0 Likes
on
0 Posts
Single cylinder balancing question.
Articles I read say the piston/pin/rod assembly should balance with the counterweight. I think the article was in Model Aviation this past year on the helicpter page. They used a Dubro prop balancer, it's adjustable.
04-18-2003, 06:20 PM
#3
Single cylinder balancing question.
single cylider is a compromise
old model engine book of mine says 50% or 100% balance of reciprocating parts should yield the same results. rotating parts should be 100% balanced. bottom half of conrod is considered rotating, top half is reciprocating. hope this helps!
dave
old model engine book of mine says 50% or 100% balance of reciprocating parts should yield the same results. rotating parts should be 100% balanced. bottom half of conrod is considered rotating, top half is reciprocating. hope this helps!
dave
04-26-2003, 07:57 PM
#4
Senior Member
Join Date: Nov 2002
Location: cheshire, UNITED KINGDOM
Posts: 257
Likes: 0
Received 0 Likes
on
0 Posts
compromise indeed!
Even if you do balance the mass perfectly for the piston / crankpin / rod & crankshaft...
you will still have vibration - especially in the sideways plane from the rod / crankpin & counterbalance.
To get even near perfection, you need 4 cylinders to start, 6 is better.
But back to basics, if your engine is really bad for a typical glow, then you can bolt a small flywheel onto the crank.
It must be secure & marked so it cannot slip. Then drill a little hole (taking some mass away) when piston is at TDC in the flywheel at 180 degrees.
Is the vibration any different....? Its trial & error - but will give you an idea of weather the crank needs MORE mass balance - or LESS.
You can get vibration measurements using accelerometers and a 'Pico' device linked to your PC.
Depends again on how far you want to go...
have fun!
you will still have vibration - especially in the sideways plane from the rod / crankpin & counterbalance.
To get even near perfection, you need 4 cylinders to start, 6 is better.
But back to basics, if your engine is really bad for a typical glow, then you can bolt a small flywheel onto the crank.
It must be secure & marked so it cannot slip. Then drill a little hole (taking some mass away) when piston is at TDC in the flywheel at 180 degrees.
Is the vibration any different....? Its trial & error - but will give you an idea of weather the crank needs MORE mass balance - or LESS.
You can get vibration measurements using accelerometers and a 'Pico' device linked to your PC.
Depends again on how far you want to go...
have fun!
04-29-2003, 07:08 PM
#5
Senior Member
Join Date: Jan 2002
Location: richland, WA,
Posts: 114
Likes: 0
Received 0 Likes
on
0 Posts
Single cylinder balancing question.
One of the big reasons that a single cylinder engine cannot be balanced to no vibration is the variance in acceleration due to combustion and compression.
Imagine the typical two stroke engine cycle, starting with the combustion. At this point, the engine is at or near TDC. The combustion will cause a rapid acceleration of the piston downward due to the expansion of gasses. Those gasses are then vented out the exhaust port. This rapid acceleration downward also causes a rocking motion of the piston, due to the path taken by the connecting rod not being directly parallel to the cylinder bore's axis. As the piston returns back towards the top of the cylinder, the gasses now in the cylinder from the transfer port and the gasses pushed back into the cylinder by the reversion wave from the tuned pipe are compressed. That compression slows the piston down rapidly, or decelerates it. The difference is that the deceleration due to the compression, and the acceleration due to the combustion are quite different. That difference contributes to the vibration already occuring from piston rock and unbalanced reciprocating masses. If you could balance the reciprocating mass you would still have uneven accelerative forces causing vibration. One possible solution is to use more than one cylinder to try to balance the forces.
As a side note, an engine is not turning a "constant speed", but rather accelerating and decelerating rapidly enough for it to be hard to notice, and we give the average rotation a value relating to how many rotations the crank makes in a minute (RPM). The actual rotational speed (angular velocity) changes as a function of engine timing and where it is in the combustion process.
Imagine the typical two stroke engine cycle, starting with the combustion. At this point, the engine is at or near TDC. The combustion will cause a rapid acceleration of the piston downward due to the expansion of gasses. Those gasses are then vented out the exhaust port. This rapid acceleration downward also causes a rocking motion of the piston, due to the path taken by the connecting rod not being directly parallel to the cylinder bore's axis. As the piston returns back towards the top of the cylinder, the gasses now in the cylinder from the transfer port and the gasses pushed back into the cylinder by the reversion wave from the tuned pipe are compressed. That compression slows the piston down rapidly, or decelerates it. The difference is that the deceleration due to the compression, and the acceleration due to the combustion are quite different. That difference contributes to the vibration already occuring from piston rock and unbalanced reciprocating masses. If you could balance the reciprocating mass you would still have uneven accelerative forces causing vibration. One possible solution is to use more than one cylinder to try to balance the forces.
As a side note, an engine is not turning a "constant speed", but rather accelerating and decelerating rapidly enough for it to be hard to notice, and we give the average rotation a value relating to how many rotations the crank makes in a minute (RPM). The actual rotational speed (angular velocity) changes as a function of engine timing and where it is in the combustion process.
