The piston is continually moving inside the cylinder. The piston of the cylinder is not constant throughout the cycle. The piston speed is zero at TDC and BDC where it reverses direction.

The speed is zero when the crank angle is 0 and 180 degrees. The piston speed is maximum when the crank angle is 90 and 270 degrees.

The piston traverses a distance equal to the stroke length in half the rotation of the crank. In one complete rotation of the crank, the piston would have covered a distance equal to twice the stroke length, 2L

Therefore if N is the speed of the engine, the mean speed of the piston will be