A control device for an internal combustion engine including a variable compression ratio mechanism for changing a compression ratio by using an electric actuator prohibits control of changing the compression ratio to a higher compression ratio when a failure occurs in the variable compression ratio mechanism. Thereby, upon failure occurrence, the compression ratio of the variable compression ratio mechanism at the time of the failure occurrence is maintained or reduced to less than this compression ratio so as to protect the electric actuator and prevent abrupt changes in torque.

In a method for operating an internal combustion engine a compression ratio is set to a setpoint compression ratio by means of an adjustment device, wherein in a normal operating mode of the internal combustion engine the setpoint compression ratio is determined as a function of an, operating variable of the internal combustion engine. The internal combustion engine is at least temporarily operated in a predictive operating mode in which the setpoint compression ratio is determined on the basis of an anticipated estimated operating variable which is estimated on the basis of the instantaneous gradient of the operating variable over time.

A control device for a variable compression ratio mechanism for changing a compression ratio of an internal combustion engine by rotating a control shaft connected to an output shaft of an actuator, wherein with respect to a reference position, the control device determines the rotation angle of the control shaft by sequentially integrating the output values of an angle sensor that detects the rotation angle of the output shaft of the actuator in a range of 0 to 360 such that the rotation angle approaches a target angle. When the internal combustion engine is stopped, the control device rotates the control shaft to a high-compression-ratio side and a low-compression-ratio side, respectively, toward a stopper mechanism that defines the rotatable range and compares the determined rotation angle and a reference operation angle to diagnose whether a failure has occurred in the variable compression ratio mechanism.