Device and method for detecting engine combustion condition

A combustion condition detecting device for an internal combustion engine having a combustion chamber, the combustion condition detecting device comprising:

• a direct current source for supplying a direct current voltage;
• a pair of opposing electrodes disposed in the combustion chamber;
• a transformer having a primary side connected to the direct current source and a secondary side connected to the opposing electrodes, the transformer being turned on and off alternately to convert a direct current voltage of the battery to an alternating current voltage applied from the secondary side to the opposing electrodes to detect combustion ions existing between the opposing electrodes based on current signals corresponding to magnitude of current flowing between the opposing electrodes;
• and
• a low pass filter connected to the primary side of the transformer for eliminating current components having high order frequencies of the alternating current voltage so that a waveform of the alternating current voltage approximates a sine wave.