1ASTRACTWe have developed an in–vehicle detection system that can directly monitor the combustion conditions of individual cylinders by detecting the ions produced near to the combustion flame surface as an electric current and applying the derived data to engine management. This system consists of existing ignition plugs used as ion probes, ignition coils with additional electronic parts for the detection, and a detection module which processes the ionic current signals, and yields various information to inform the engine control system of combustion conditions. This detection system can discriminate between misfires and normal combustion over a wide operation range of an engine, and detect the knocking level by cylinder. Knocking control is achieved by abstracting the specific frequency components identifying any knocking from the ionic current signals detected by cylinder and by combustion cycle, converting them into digital pulses, and determining the knocking level using the number of pulse counts. Then, the retard spark angle at the ignition timing will be controlled, based on this knocking level determination.This paper reports the features of this system, based on the data obtained from the experiments on actual engines: detecting the knocking level using the number of pulse counts, and controlling the retard spark angle at the ignition timing.