1. Jan 17, 2010

### Chantry09

Hi,

I have a question about one of the engines im looking at for a project. I am linking two cylinders together. One is a cylinder with a pivoting shaft (piston A), the other piston has a cylinder with a fixed axle so it can only move in and out (piston B). To allow piston be to move with the cam, it will have to be attached to a pivot.

However, what I am concerned with is, these cylinders have to be "in phase" so they are moving the same speed and are at the same point during the cycle. Im thinking that the difference between the way piston a and piston b are pivoted will mean they will not always be in perfect sync.

To achieve this, do I need to fix Piston A so that it has the same straight length of shaft and the same pivoted shaft length? As shown in the diagram below:

Please let me know if I have explained it well enough. I basically want the relationship between the cylinder head moving and the cam moving to be exactly the same on Piston A and Piston B, and I think the increased pivoting length will alter this relationship.

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2. Jan 17, 2010

### Q_Goest

Every point on the piston must move together. So every point on the "straight length" must also move along with the piston. There is no difference in motion between the straight length and piston.

There are two rotational joints, one on the 'cam' (generally called a crankshaft) where it connects to the 'pivoted length' (generally called a connecting rod) and one between the connecting rod and piston. As long as the center of the crankshaft is in line with the piston, the two pistons will operate 180 degrees out of phase with each other, regardless of the length of the straight part.

3. Jan 18, 2010

### Chantry09

Hi,

Thank you for the reply. That makes sense, but when I consider the length of the shaft that it pivoted, I still am I bit confused. After thinking about this for a little bit, I think the best way I can explain it is the following:

In my first image, will the travel of the cylinder head created by the rotation of the crankshaft be the same for piston 1 and piston 2 at all stages of rotation?

I think they will be different, using Pythagoras' theorem, since the length of the shaft that is allowed to pivot is larger on Piston A. Can someone confirm this? If so, I need it to be the same. Can this only be achieved by having the same length of pivoted shaft on both piston a and piston b?

4. Jan 18, 2010

### Q_Goest

If the length of the connecting rods is the same, then yes, the motion of the two will be perfectly identical.

Consider what happens if the straight bit is removed such that the two pistons are mirror images of each other. Would you agree their motion is identical in this case? You could put some equations down to prove it to yourself if you doubted that. ... Next, consider what happens when the piston is lengthened slighty. The pivot at the piston hasn't moved, just the length of the piston. Consider the extreme case of a piston that is very long. The pivot at the piston still moves the same way. The length of the piston has no affect on its linear motion.

5. Jan 18, 2010

### Chantry09

Ok, I understand all of that. However what happens when the length of the pivoting connecting rod is different between piston A and piston B? Say piston A has a length of 10 cm and piston B has a length of 5cm?

6. Jan 18, 2010

### Q_Goest

The longer the connecting rod, the closer the piston movement comes to producing a perfect sine curve. An infinately long piston rod will essentially give you a scotch yoke, which gives you the perfect sine curve.

http://grove.ufl.edu/~wgsawyer/Laboratory/Wear/scotchYoke.gif [Broken]

As the connecting rod gets shorter, the sine curve becomes distorted. So having two different length connecting rods will result in a slightly different motion for the piston. They will be similar to a sine curve but they won't be exact.

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