Cylinder combustion monitoring apparatus

An apparatus for monitoring cylinder combustion in an internal combustion engine having at least one cylinder to control said engine, comprising;

• sensor means sensitive to combustion pressure in said cylinder for providing a sensor signal indicative of a sensed cylinder combustion pressure level;
• means coupled through a signal line to said sensor means for extracting, from said sensor signal, a knock signal in a predetermined high frequency range and a cylinder combustion pressure signal in a predetermined low frequency range;
• a circuit responsive to said high frequency knock signal and said low frequency cylinder combustion pressure signal for providing a failure indication of said sensor signal being unreliable for use in controlling said engine, said circuit including first decision means responsive to an abnormal condition of said high frequency knock signal for providing a first indication, second decision means responsive to an abnormal condition of said low frequency cylinder combustion pressure signal for providing a second indication, and means for providing said failure indication based upon said first and second indications;
• means to responsive to said failure indication for controlling said engine independently of said sensor signal;
• and
• said first decision means includes means for sampling a predetermined number of values of said high frequency knock signal at predetermined intervals, means for calculating a variance of said sampled values each time a new value is sampled, means for comparing said calculated variance with a reference value, means for providing a caution indication in response to said calculated variance smaller than said reference value, and means for providing said first indication in response to said caution indication held for a period greater than a predetermined value.