Method and apparatus for preventing locking of engine-operating-characteristic changing device

A method of controlling an engine-operating-characteristic changing device operable between a first and a second operating state and including a locking device for locking the changing device in the first operating state in which a compression ratio of an intake air of an internal combustion engine (e) is maintained at a value higher than that in the second operating state, the method including steps of determining whether there is a possibility that the engine-operating-characteristic changing device is locked in the first operating state by the locking device, where the changing device should not be placed in the first operating state, and if it is determined that there is the possibility, implementing a preventive operation to prevent the locking unit from locking the changing device in the first operating state.