PURPOSE: To prevent the occurrence of impact noise by providing the primary oil chamber between the bottom of a piston body and the top surface of an inner piston to supply pressure oil, and also the secondary oil chamber between an inner ring and the crown part of the inner piston.
CONSTITUTION: When an engine is subjected to a light load, an electric auxiliary oil pump 12 functions to cause high pressure oil to flow through a check valve 9 into the primary oil chamber 5, and a piston body 1 is therefore pressed upward against combustion pressure acting on its top surface to increase the compression ratio of the engine. In this case, since the oil having flowed into the primary oil chamber 5 leaks partly through a gap between the inner periphery of the piston body 1 and the outer periphery of an inner piston 2 to flow into the secondary oil chamber 8, when upward inertia force acts on the piston body 1 for duration from an exhaust stroke to an intake one, the leaked oil is compressed to weaken impact due to the inertia force acting on the piston body 1 and also improve the durability of the engine. When the engine is operated under a heavy load, the function of the auxiliary oil pump 12 is stopped, and oil pressure in the primary oil chamber 5 therefore lowers to cause the compression ratio of the engine to decrease.
GOTO TAKAHARU
KATO MANABU
ARAI TAKAYUKI
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