To avoid a situation such that a compression ratio of an engine may be carelessly fixed to a high compression ratio.
An adjustable compression ratio engine 20 modifies the compression ratio variously according to an operating state of the engine, and judges whether the compression ratio after modification (the real compression ratio i) may be proper or not. When some troubles occur under such a condition that the compression ratio is a high compression ratio considered to be appropriate for low load operation, power transmission between a compression ratio modification mechanism 30 and a servo motor 57 is intercepted by a clutch 70. In the compression ratio modification mechanism 30, a piston 28 receives the combustion pressure of fuel in a combustion chamber, and a connecting rod 27 bends naturally so that the bending degrees of a first connecting rod 31 and a second connecting rod 32 may become large. As a result, since the position of a piston top dead center is away from a ceiling wall of the combustion chamber, the compression ratio of the engine becomes low.
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