Primary and secondary piston movements


  • Primary and secondary piston movements

Primary and secondary piston movements:
The up and down (translation) movement of the piston is converted into a rotary movement by the crank-connecting rod mechanism. The piston moves up and down in a straight line. This is called the primary piston movement. However, the connecting rod moves not only up and down, but also sideways. Due to the lateral movement of the connecting rod, the piston will travel a slightly greater distance. At this point, the piston has the fastest speed of movement. This extra distance is called the secondary piston movement.

The picture shows the piston movement. The top blue piston shows where the TDC (Top Dead Center) is. The blue piston in the center right indicates the distance of the primary piston movement (ie where the connecting rod has not become oblique). The lower red-colored piston indicates the extra distance created by the rotation of the crankshaft and the oblique position of the connecting rod; this is the secondary piston movement.

When the crankshaft is rotated 90 degrees, the speed of movement of the piston is highest. The secondary piston movement provides a greater distance covered. By adding the distance of the secondary movement to that of the primary movement, the total distance traveled by the piston can be determined.

The graph shows the primary piston movement (blue) and the secondary piston movement (red). During the primary piston movement, the piston is at TDC at 0° of rotation. At half a revolution of the crankshaft (180°), the piston is in ODP.

At 90° crankshaft rotation, the piston is exactly halfway up the cylinder and the secondary piston speed is at its highest. At a rotation of 180°, the piston is back in TDC and the secondary movement is 0. Here the direction of movement of the piston is reversed.

The secondary piston force is partly responsible for engine vibrations. To reduce vibrations, motors are often used with balance shafts executed.

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