Patent ID: 6040698
Filing Date: 2000-03-21
Classification: F02P

Abstract:
A combustion state detecting apparatus for an internal-combustion engine, comprising:an ignition coil composed of a transformer which has a primary winding and a secondary winding, and which generates a high voltage for ignition at a high voltage end of the secondary winding when supply of current to the primary winding is cut off;a spark plug, comprising opposed electrodes, connected to the high voltage end of the secondary winding and which discharges under application of a high voltage for ignition to ignite a fuel-air mixture in a cylinder of the internal-combustion engine;an ionic current detecting circuit which includes biasing means connected to a low voltage end of the secondary winding and which detects ionic current flowing from the biasing means via the spark plug after the combustion of the fuel-air mixture;rectifying means, inserted between the biasing means and the low voltage end of the secondary winding, for controlling flow of the ionic current in a forward direction;voltage clamping means, inserted between the low voltage end of the secondary winding and ground; andan ECU which detects a combustion state at the spark plug according to the ionic current;wherein the biasing means applies a bias voltage of a polarity opposite to the high voltage for ignition to the spark plug via the rectifying means and the secondary winding; andthe voltage clamping means limits the voltage at the low voltage end of the secondary winding to a predetermined value when the high voltage for ignition appears;an absolute value of the predetermined value being set to the absolute value or more of the bias voltage of the biasing means,and wherein:current limiting means is installed between the junction of the rectifying means and the voltage clamping means and the low voltage end of the secondary winding; andthe current limiting means controls the current flowing from the biasing means to the spark plug via the secondary winding so as to control the voltage at the high voltage end of the secondary winding when current begins to be supplied to the primary winding,and wherein the current limiting means comprises a resistor and a diode connected in parallel to each other; andthe diode sets the direction of the secondary current flowing through the secondary winding at the time of applying the high voltage for ignition to the forward direction so as to suppress a potential difference across the resistor during ignition control.