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 spark plug whenever said crankshaft rotates through a predetermined angle;
• engine speed-detecting means for detecting a value of rotational speed of said engine whenever said crank angle signal is generated;
• engine rotational speed averaging means for continuously averaging values of the rotational speed of said engine detected by said engine speed-detecting means, over a period of one rotation of said crankshaft, to obtain an average rotational speed value;
• firing period averaging means for averaging said average rotational speed values of the engine rotational speed obtained by said engine rotational speed averaging means, over said firing period of said spark plug, to obtain an average firing period rotational speed value;
• variation rate-determining means for determining a value of a rate of variation in said average firing period rotational speed value of the rotational speed of said engine obtained by said firing period averaging means, over said firing period of said spark plug;
• variation rate average valve-determining means for averaging values of said rate of variation obtained by said variation rate-determining means, over a predetermined number of ignition cycles of said engine, to obtain an average rate of variation value;
• difference cumulative value-determining means for determining a difference between said average rate of variation value obtained by said variation rate average value-determining means and said value of said rate of variation obtained by said variation rate-determining means, and for determining a cumulative value of an absolute value of said difference over said predetermined number of ignition cycles;
• and
• combustion state-determining means for comparing said cumulative value with a predetermined value, and for determining that said engine is in a degraded state of combustion, when said cumulative value exceeds said predetermined value.