Ion current sensing device for an internal combustion engine with spurious voltage preventing filter

An ion current sensing device for an internal combustion engine having a spark plug with electrodes present in a combustion chamber in a cylinder, said device comprising:

• a) a power source (14) for supplying a bias voltage to the electrodes of said spark plug;
• b) ion current sensing means (15, 16) connected between the electrodes of said spark plug and said power source for sensing as a voltage an ion current which is generated between the electrodes of said spark plug each time a mixture in the combustion chamber is combusted;
• c) signal processing means (10, 11) for comparing the voltage corresponding to the ion current as sensed by said ion current sensing means with a reference voltage and generating an output signal when the voltage corresponding to the ion current is higher than the reference voltage;
• and
• d) means for preventing a spurious voltage from being applied to the signal processing means due to a leakage current flow through the power source and the ion current sensing means resulting from spark plug contamination, said preventing means comprising:
• e) a filter (21) connected between said ion current sensing means and said signal processing means for passing only AC components of the voltage corresponding to the ion current to said signal processing means while blocking a DC component of the voltage as sensed by said ion current sensing means.