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In the
article on piston kinematics
we investigated the piston position, speed and acceleration
as it moves up and down the cylinder once every crank revolution. The effect of rod-ratio on piston
acceleration was of particular interest, as this acceleration at TDC and BDC leads to large
forces on the bearings (rod and main) as well as on the wristpin.
In this article we concentrate on the forces caused by combustion, as opposed to forces caused by piston motion. We know that the pressure from combustion produces a force on the piston which is transferred to the crank via the rod. But due to the fact that the rod is free to rotate at both ends, there is also a "resultant" force produced, which pushes the piston into the cylinder wall. This force is the side-wall load and leads to power robbing friction as well as faster component wear. Also, if the cylinder walls are thin (due to boring the block close to its limits) then the side-wall load can cause significant block distortion on the expansion stroke. This distortion adds even more friction, but it can also lead to excessive blow-by of combustion gases due to improper ring sealing, leading to a loss of power. | |||
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Also of interest is the actual percentage of the piston force that causes the crank to rotate. As with side-load, part of the force exerted by the rod onto the crank is perpendicular (normal) to the crank arm. And only this component of the rod force actually causes the crank to rotate, which leads to torque production. The remaining component of the rod force exerted onto the crank arm does nothing more than push on the bearings (rod main and wrist-pin). This component produces no work and therefore no torque. How does this normal force on the crank vary through the expansion cycle? And what effect does rod ratio have on this normal force? These are interesting questions. | |||
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The equations for both the side-wall load and the normal force on the crank are derived as shown in the following link: | |||
| Click here for a derivation side-wall load and crank normal force... | |||
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The equations resulting from the derivation can easily be plotted in a spreadsheet so that any peculiar trends can be observed. | |||
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