A method of distinguishing piston slap from engine knock in a reciprocating device includes obtaining a fundamental frequency of a cylinder, the cylinder having a thrust face and an anti-thrust face, receiving a first signal from a first knock sensor mounted on the cylinder, and identifying piston slap by evaluating whether a first plurality of amplitudes of the first signal at the fundamental frequency and one or more harmonic frequencies of the fundamental frequency exceed a piston slap threshold value.

An engine control unit includes an extracting portion acquiring a sound or vibration of an engine from external and extracting vibration components of frequency bands each of which including a resonant frequency of a vibration mode, a waveform synthesis portion synthesizing the vibration components extracted by the extracting portion to generate a synthesis vibration waveform, a calculation portion calculating a time average value by averaging the synthesis vibration waveform in a predetermined time, a peak detection portion detecting a peak strength of the vibration component of the frequency band including a lowest resonant frequency, a storage portion previously storing a first threshold and a second threshold, and a determination portion determining an existence of a knocking and whether a knock level is a first knock level or a second knock level, by comparing the time average value with the first threshold and comparing the peak strength with the second threshold.