COMPRESSION APEX VARIABLE TYPE PISTON

PURPOSE: To improve combustibility at a low temperature by, in a piston comprising an inner piston part fit in an outer piston part, disposing a control valve for blocking an oil discharge bore at a low temperature and opening at a high temperature in the oil discharge bore that connects an upper control chamber between the two piston parts and a crankcase. CONSTITUTION: A piston 1 comprises an outer piston part 2 and an inner piston part 3 connected to a crankshaft through a piston pin 5 and a connection bar 6. An upper control chamber 10 is defined between the two piston parts 2 and 3. The control chamber 10 is connected to a lower control chamber 11 via a throttle 13 and a check valve 14. Further, the inner piston part 3 is provided with a first and a second oil discharge bores 16, 30 for connecting the upper control chamber 10 to an oil supply passageway and a crankcase. In this case, a control element 31 made of an expansive material is disposed in the second oil bore 30 for blocking the oil bore 30 at a low temperature of an engine and opening at a high temperature, thus preventing the lowering of a compression height at a low temperature.