operated by hydraulic pressure or pneumatic pressure the piston 15 is firmly engaged with a side of the main bearing 10 an operation of the actuator 14 is controlled by an electronic control unit 16 in accordance with the operational state of the engine the electronic control unit may comprise a processor and other associated hardware and software as may be selected and programmed by a person of ordinary skill in the art based on the teachings contained herein when the engine is operated at high speed and high load conditions such that a high compression ratio is required the piston 15 of the actuator 14 withdraws and pulls the main bearing 10 so that a center of the crankshaft 4 supported by the main bearing 10 becomes in line with a reciprocating center of the piston 2 therefore as shown in fig1 a and 1 b tdc and bdc of the piston 2 become as high as possible according to the design and accordingly the compression ratio of the engine becomes a maximum as measured by equation 1 when the engine is operated at low speed and low or partial load conditions so that a low compression ratio is required the actuator 14 pushes the main bearing 10 a predetermined distance d such that the center of the crankshaft 4 become offset from the reciprocating center of the piston 2 as shown in fig2 a and 2 b therefore tdc and the bdc of the piston 2 are lowered correspondingly to the moved distance d of the crankshaft 4 and accordingly the compression ratio of the engine becomes reduced.