Combustion state-determining system for internal combustion engines

A combustion state-determining system for an internal combustion engine having an exhaust system, and at least one exhaust system component part arranged in said exhaust system, comprising:

• misfire-detecting means for detecting misfires occurring in said engine;
• a first counter for counting a number of times of occurrence of misfires detected by said misfire-detecting means every predetermined number of firing cycles;
• a second counter for counting a number of times of occurrence of misfires detected by said misfire-detecting means every predetermined repetition period which is a predetermined integer times as long as said predetermined number of firing cycles, said second counter having a count value thereof corrected every said predetermined number of firing cycles;
• comparing means for comparing a count value of said first counter with a first predetermined reference value and a second predetermined reference value which is smaller than said first predetermined reference value, every said predetermined number of firing cycles;
• first combustion state-determining means for determining that said engine is in a combustion state which adversely affects said at least one exhaust system component part arranged in said engine, when the count value of said first counter is larger than said first predetermined reference value;
• count value-correcting means for correcting the count value of said second counter, by an updated value of the count value of said first counter, when the updated value of the count value of said first counter is smaller than said first predetermined reference value and at the same time larger than said second predetermined reference value;
• resetting means for resetting the count value of said second counter when the count value of said first counter is smaller than said second predetermined reference value;
• and
• second combustion state-determining means for comparing the count value of said second counter with a third predetermined reference value, and determining that said engine is in a combustion state which degrades exhaust emission characteristics of said engine, when the count value of said second counter is larger than said third predetermined reference value.