Various embodiments include a method for identifying an inlet and an outlet valve stroke phase difference comprising: measuring pressure oscillations during operation; generating a corresponding signal; determining a corresponding crankshaft phase angle; applying a discrete Fourier transformation to the pressure signal to determine amplitudes of selected frequencies in relation to the crankshaft phase angle; determining lines of equal amplitudes of the frequencies based on the amplitudes depending on the phase differences using reference lines; determining an intersection of the lines by projection into a common plane; and determining the inlet valve stroke phase difference and the outlet valve stroke phase difference from the determined common intersection point of the lines of equal amplitudes of the selected signal frequencies.