Disclosed is a reciprocating piston engine, comprising a combined structure with an optimized double-crankshaft and variable compression ratio pistons, characterized in that the variable compression ratio piston is a piston serving as a double-acting hydraulic cylinder, a control valve bush of a slide-valve type directional control valve is fixed in a central mounting hole of the inner piston, and a control valve core is mounted in a rotatory sliding or nut-ball screw manner in a central mounting hole in the inner surface of the piston top; and the double-crankshaft engine is formed by two reverse rotating crankshafts which are coupled by gears to be in synchronous reverse rotation motion together, each piston being connected to a connecting rod shaft of two crankshafts, and a piston control valve driving mechanism being mounted between the two crankshafts.

A variable compression ratio apparatus may include a plunger configured to move up and down in response to rotation of a crank shaft, a piston having a chamber formed thereinside, into which the plunger is inserted, and configured to move up and down with the plunger, the chamber including an upper chamber formed above the plunger and a lower chamber formed below the plunger, a spool valve configured to selectively supply oil to the upper chamber or to the lower chamber, and a controller configured to control the spool valve so that the piston moves up and down with respect to the plunger.

A connecting rod (10) has a big end bearing eye (11) for connection to a crankshaft, a small end bearing eye (12) for connection to a piston of a cylinder, and an eccentric adjustment device (13) for adjusting an effective connecting rod length. The eccentric adjustment device (13) has an eccentric (36) that interacts with an eccentric lever (14), and eccentric rods (15, 16) that the eccentric lever (14). The eccentric lever (14) has segments (38, 39) positioned axially in front of and behind the small end bearing eye (12). Each eccentric lever segment (38, 39) is connected to the eccentric (36) for conjoint rotation. Each eccentric lever segment (38, 39), end faces (43, 44) of the small end bearing eye (10) adjacent the respective eccentric lever segment (38, 39), and adjacent end faces (45, 46) of the eccentric (36) have a beveled contours.

This invention provides a piston in which the moving piston unit can be prevented from rotating with respect the base piston unit, while suppressing the increase of the sliding resistance the piston has with respect to the cylinder. The circumferential wall of the moving piston unit has a pair of skirt parts opposing each other across a piston boss and a pair of side-wall parts coupling the skirt parts and each having an outer circumferential surface concaved toward the central axis X1 of the piston. The side-wall parts have a penetration part each, which penetrates the side-wall part and extends in the directions the moving piston unit reciprocates with respect to the base piston unit. The base piston unit has rotation preventing projections that project perpendicular to the central axis of the piston and are inserted into the penetration parts.