Combustion state detecting apparatus for an internal-combustion engine

A combustion state detecting apparatus for an internal-combustion engine, comprising:

• a crank angle sensor which issues, in synchronization with the revolution of the internal-combustion engine, a crank angle signal having a pulse edge which corresponds to a reference crank angle position;
• a plurality of cylinders which are subjected to ignition control in synchronization with the revolution of the internal-combustion engine;
• an ignition coil for applying high voltage for ignition to spark plugs of respective cylinders;
• an ionic current detecting circuit which detects an ionic current flowing via a spark plug, which has just been subjected to ignition control, by applying bias voltage to at least one spark plug;
• a comparison device which compares a detected ionic current signal with a first threshold value and outputs a first ionic current pulse signal;
• and
• an electronic control unit (ECU) which drives the ignition coil according to the crank angle signal and determines a combustion state of the internal-combustion engine according to the first ionic current pulse signal;
• wherein the ECU changes the first threshold value by a predetermined value for each ignition control and determines the combustion state according to each change in the first threshold value and according to whether the first ionic current pulse signal is generated for each change in the first threshold value.