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