The paper presents a few original elements about the dynamics and kinematics of piston the mechanism, used like motor mechanism from OTTO engines. One presents an original method to determine the efficiency of the piston mechanism, used like motor mechanism. This method consists of eliminating the friction modulus. One determines the efficiency of the piston mechanism in two ways: 1. When the piston mechanism works like a motor; 2. When the piston mechanism works like a steamroller. Finally one determines the total motor efficiency, for the four-cycle engine and for a two-cycle engine. With the relation of motor efficiency, one optimizes the Otto mechanism, which is the principal mechanism from the internal-combustion engines. This is the way to diminish the acceleration of the piston and to maximize the efficiency of motor mechanism. One optimizes the constructive parameters: e, r, l, taking into account the rotation speed of drive shaft, n.