Combustion state-detecting system for internal combustion engines

A combustion state-detecting system for an internal combustion engine having at least one spark plug, and a crankshaft, comprising:

• crank angle signal-generating means for generating a crank angle signal with a predetermined period shorter than a firing period of said engine whenever said crankshaft rotates through a predetermined angle;
• engine rotational speed-detecting means for detecting a rotational speed of said engine, based on said crank angle signal whenever said crank angle signal is generated, and for generating a rotational speed signal indicative of the rotational speed detected by said engine rotational speed-detecting means;
• extraction means for extracting from said rotational speed signal a specific frequency component therefrom;
• variation rate-determining means for determining a rate of variation in said specific frequency component extracted by said extraction means every first predetermined time period;
• average value-determining means for determining an average value of said rate of variation every second predetermined time period which is longer than said first predetermined time period;
• difference cumulative value-determining means for determining a difference between said average value and said rate of variation, and for cumulating absolute values of said difference over said second predetermined time period to obtain a difference cumulative value;
• and
• combustion state-determining means for comparing said difference cumulative value with a predetermined value, and for determining that said engine is in a degraded combustion state when said difference cumulative value exceeds said predetermined value;
• wherein said combustion state-determining means includes combustion state kind-determining means for determining that said engine is in a continuous degraded combustion state when said difference cumulative value has continued to exceed said predetermined value, and for determining that said engine is in a sporadic degraded combustion state when said difference cumulative value once exceeds said predetermined value and then becomes smaller than said predetermined value, before said difference cumulative value again exceeds said predetermined value.