The subject matter of this specification can be embodied in, among other things, a method that includes determining a gas supplement ratio (GSR) of a mixture of natural gas and diesel, identifying a start event of a stroke of a piston of a combustion cylinder of a dual-fuel engine, identifying a first observation period that is offset from the start event based on the GSR, identifying a second observation period based on the GSR, receiving signals representative of vibration of the combustion cylinder during the stroke, identifying an abnormal combustion event based on the signals, determining that abnormal combustion occurred during at least one of the first observation period and the second observation period, and causing a change in operation of the engine based determining that abnormal combustion occurred during at least one of the first observation period and the second observation period.