This article investigates the possibility of utilizing the standard knock sensor for an in situ injector calibration. The goal is to estimate the actual injection duration by means of the structure-borne sound emission during the injection process. Since the sound signals are highly nonstationary and contain various transients, a time–frequency analysis is applied. Based on the findings of the signal analysis, a method is presented for detecting the beginning and the end of injection by applying the theory of the change-point problem.