Method for determining the compression of an internal combustion engine

1. A method for operating an internal combustion engine wherein the compression can be variably adjusted by means of an actuator depending on an actuating variable, the method comprising:

determining a value for a sensor signal indicative of a compression of the internal combustion engine;

checking the plausibility of the sensor signal by:

ascertaining a series of corresponding values of the sensor signal and values of the actuating variable;

determining a mathematical relationship between the sensor signal and the actuating variable based on the ascertained series of corresponding values of the sensor signal and of the actuating variable;

determining a deviation between the determined relationship and a predetermined characteristic relationship between the sensor signal and the actuating variable; and

determining that the sensor signal is plausible when the determined deviation is lower than a predetermined threshold value; and

if the determined deviation is higher than the predetermined threshold value, adjusting operation of the internal combustion engine.

2. The method as claimed in claim 1 , further comprising:

determining, based on the determined relationship between the sensor signal and the actuating variable, a value of the sensor signal at which the corresponding value of the actuating variable is at an extreme;

determining, based on the predetermined characteristic relationship between the sensor signal and the actuating variable, a reference value of the sensor signal at which the corresponding reference value of the actuating variable is at an extreme; and

determining the deviation as a deviation between the determined value of the sensor signal and the determined reference value of the sensor signal.

3. The method as claimed in claim 1 , wherein:

the internal combustion engine includes a multi-joint crank mechanism with an eccentric shaft;

the actuator rotates the eccentric shaft about its axis; and

the sensor detects the rotational angle of the eccentric shaft.

4. The method as claimed in claim 1 , wherein:

the actuator comprises an electromechanical actuator; and

the actuating variable comprises an electric current intensity.

5. The method as claimed in claim 1 , wherein:

the actuator comprises a hydraulic actuator; and

the actuating variable comprises a pressure.

6. The method as claimed in claim 1 , further comprising systematically ascertaining the series of corresponding values of the sensor signal and values of the actuating variable during a measuring process.

7. The method as claimed in claim 1 , further comprising ascertaining the series of corresponding values of the sensor signal and values of the actuating variable within a predetermined time window during operation.

8. The method as claimed in claim 1 , further comprising correcting the predetermined characteristic relationship between the sensor signal and the compression based on the determined deviation when the determined deviation lies between the predetermined threshold value and a further threshold value.

9. The method as claimed in claim 8 , further comprising generating a fault message when the determined deviation exceeds the further threshold value.