    Patent Document 1: Pamphlet of International Publication No. WO 92/02722    Patent Document 2: JP-A-04-347352    Patent Document 3: JP-A-05-26106    Patent Document 4: Japanese Patent No. 2988010
Techniques for reducing the friction loss between the piston and the cylinder on the thrust side due to thrust force acting on the piston include, among others, a pamphlet of International Publication No. WO 92/02722, JP-A-04-347352, JP-A-05-26106, and Japanese Patent No. 2988010 (Refer to Patent Documents 1 to 4). In the techniques described therein, a gas chamber is formed between piston rings for compression provided on a piston upper body, i.e., on a second land portion, and high pressure gas above the piston is introduced into this gas chamber in an initial stage of the expansion stroke of the engine operation, such that the piston is supported by this introduced gas pressure in opposition to the thrust force generated due to the tilting of a connecting rod, to thereby attain reduction of the friction loss between the piston and an inner surface of the cylinder.
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