Variable compression ratio apparatus of piston engine

The utility model relates to a crank connecting rod mechanism for engines, in particular to a variable compression device for reciprocal piston type engine crank connecting rod mechanisms, wherein a connecting rod (2) and a connecting rod axle neck (3) on the crank are connected via an eccentric upper bearing (9) and an eccentric lower bearing (10), a balance weight (7) on the crank is machined with a slide way and a step hole communicated with the slide way, an eccentric slider (6) is mounted in the chute and is connected with the eccentric lower bearing (10) via a draw rod (4), the small diameter part of the step hole is mounted with an adjusting bolt (11) mounted with a lock nut (12), while the large diameter part is mounted with a return spring (8) which two ends are respectively connected with the adjusting bolt (11) and the eccentric slider (5). The variable compression device has simple structure and small volume, and adopts the rotations of the eccentric upper bearing the eccentric lower bearing which are mounted on the crank connecting rod axle neck to change the stroke of a piston, thereby changing volume of the cylinder combustion chamber, to change the compression ratio.