1. ABSTRACTThe idea to monitor the combustion in an internal combustion engine and using the obtained data to control combustion in the engine has been around for some time now. There are two well-known methods, although in the capacity of lab experiments, which had been developed under this principle. One features the analysis of combustion pressure and the other features the analysis of ionic currents detected in the combustion gas. Although highly precise analysis can be achieved by the former, there are problems in the installation of sensors for detecting combustion pressure, also in the durability and cost of such sensors. As for the latter, there are also problems in installing sensors for detecting the ionic currents and the reliability of obtained data from such sensors is still questionable. For these reasons, no internal combustion engine control systems have been practicalized that use these methods, i.e. for continuously monitoring all cylinders in an internal combustion engine of a car.The authors carried out research on a method which uses spark plug voltage analysis for monitoring the combustion in an internal combustion engine. By this method, the spark plug voltage becomes measured by voltage sensors, installed (non-contact) on the high voltage zone, in the vicinity of the spark plugs. The monitoring can be easily done in real time by processing the waveform data and measuring the density of ions. This method can be effectively used for continuous monitoring of all cylinders in the internal combustion engine. Such monitoring can also achieve controlling the combustion to be constantly maintained under optimal preset conditions, even when the combustion fluctuates due to effects by the air/fuel ratio, EGR, ignition timing, etc., be it for lean-burn engines or other conventional engines. By being able to control the driving condition to be within the lean limit, the EGR limit, and the retard limit, it has become possible to satisfy the three factors, fuel consumption, exhaust gas emission, and driveability, a feat considered difficult to achieve, in high dimensional terms.