Engine knock sensor signals are amplified, band-pass filtered and rectified and then a sequence of integrated signal peak values are compared with reference signals relating to a typical engine knock signal. If the deviation of the integration values or peak values or their sum over a measurement period differs by less than a predetermined amount from the corresponding reference values, a knock indication is produced. The initial integration or peak value may be given less weight than those that follow. The knock sensor may be an engine block sound sensor, or a combustion chamber light, pressure or iron current sensor. Additional criteria, such as the interval between null transitions or signal maxima or the number of them in a measuring period may be used to inhibit a knock indication in case the main processing should occasionally provide an anomalous knock signal, but this precaution is not sufficiently needed to warrant its inclusion in simplified systems.