Quote:
Originally Posted by serialk11r
1. An infinitely long rod would make the piston follow sinusoidal motion. A finite length rod will behave differently near the top and bottom of the stroke. The linear acceleration of the piston is higher in the top half of the stroke with a short rod, and slower in the bottom half of the stroke, while a longer rod has lower acceleration in the top half and greater in the lower half (roughly speaking).
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[Point 1] What are you talking about in bold sentence? The piston should be in linear motion. What does that have to do with infinite long rod? Even if you have the infinite rod, the piston should be just moving linearly back and forth because it is limited by the engine block and the o-rings.
However, if you chart the crank angle vs piston position, you see a sinusoidal graph. Is there any correlation with rod length? It seems to me that by increasing rod length the curve is moved upward on this diagram (re. chart shown in the wiki link). The amplitude is changed when you change the crank diameter (stroke).
Quote:
Originally Posted by serialk11r
Perhaps the more obvious difference though is the difference in the angle between the piston motion and the rod. At a greater angle, the force on the side of the piston is greater. Since the piston skirts and rings are responsible for half the total friction/fluid pumping loss in the engine, there is an appreciable decrease in friction with a longer rod, given the same masses everywhere. A longer rod probably weighs a little bit more though...
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How would you correlate the rotating angle between rod/piston with the force on the side of the piston? Given the same piston head, the surface area between the head and the block should be the same. The friction on that surface area should remain the same. With longer rod, the rotating angle at the piston side is decreased. So one can argue that the total *work* required to rotate the rod at the piston side is reduced with longer rod, but let's not confuse that with friction. The friction is based on the surface area and the content between the surfaces.
So without considering the increase weight of longer rod, with the same piston head and stroke, the engine with longer rod will have less rotating angle, i.e. less work required to rotate the rod. Thus the total work required to move the piston to rotate the engine is less compared with the one with shorter rod.
However, with increase weight of longer rod, the total *work* required to move the piston would increase due to the higher weight. The question becomes:
would the effect of reduced rotating angle at the piston side, thus less work to rotate rod, *greater* than the penalty of increase weight of the rod? 
On the other hand, based on the formula shown in the WIKI page, I did an quick Excel spreadsheet to show the difference with different rod lengths.
With a 6" rod and 2" radius (half stroke), the rotating angle = 38.94 degree, and the absolute maximum values of velocity shown is 2.11 at 75 degree (I was using 15 degree increment). The theoretical maximum velocity should happen at 73.17615 degree. The maximum angular acceleration happens at 0/360 degree with -2.67.
If I change the rod length to 7 inch with the same 2" radius, the rotating angle becomes 33.2031 degree, and the absolute maximum velocity is 2.08 at 75 degree with maximum angular acceleration at 2.57.
So with these values, one can argue that
by increasing the rod length, the maximum angular velocity and acceleration is reduced so that less stress to the rod "if the mass of the rod does not change". However since longer rod = more weight with the same material, one should be careful not to make quick statement simply based on one or two factors.