Abstract: Controlled variation of piston .displacement and adjustment of compression ratio are achieved by an adjustment mechanism located between the vehicle crank shaft and the piston which changes the effective length of the piston rod. The piston rod is divided into upper and lower portions and the adjustment mechanism connected to both portions at the point of division. The adjustment mechanism allows controlled lateral displacement of the two parts of the piston rod from each other at the point of division while still transmitting energy of motion from the piston to the crank shaft.