Misfire detection apparatus for internal combustion engine

A misfire detection apparatus for a spark ignition type of multi-cylinder internal combustion engine, comprising:

• measuring means for measuring an amount of fluctuation in rotation of a crankshaft of said multi-cylinder internal combustion engine in a measurement period indicative of a predetermined range of rotation angle for a plurality of engine cylinders successive in order of ignition;
• misfire judging means for judging whether or not a misfire occurred in any one of said plurality of engine cylinders in accordance with a result obtained by comparing said amount of fluctuation measured by said measuring means with a predetermined judging threshold value;
• operating condition judging means for judging whether or not said multi-cylinder internal combustion engine is in operation under a predetermined low load operating condition;
• and
• ignition angle judging means for judging whether or not an ignition angle is larger than a predetermined ignition delay angle, wherein
• said misfire judging means is provided with a first judging threshold value map representing said judging threshold value as a first judging threshold value which is equal to one specific amount of fluctuation specified by a function of rotational speed of said multi-cylinder internal combustion engine and a second judging threshold value map representing said judging threshold value as a second judging threshold value which is equal to another specific amount of fluctuation larger than said one specific amount of fluctuation corresponding to said first threshold value as a function of rotational speed of said multi-cylinder internal combustion engine, said misfire judging means judges whether or not said misfire occurred by comparing said amount of fluctuation with said first judging threshold value and said second judging threshold value when said operating condition judging means judges that said multi-cylinder internal combustion engine is in operation under said predetermined low load operating condition and said ignition angle judging means judges that said ignition angle is larger than said predetermined delay angle, and only with said first judging threshold value when said operating condition judging means judges that said multi-cylinder internal combustion engine is not in operation under said predetermined low load operating condition or said ignition angle judging means judges that said ignition angle is not larger than said predetermined ignition delay angle, and wherein
• said misfire judging means is operative to judge whether or not said misfire occurred in any one of said plurality of engine cylinders from comparison results based on values of said second judging threshold value map in said measurement period subsequent to said measurement period when said misfire judging means judges that the misfire occurred in one of the plurality of engine cylinders from comparison results based on values of said first judging threshold value map while said operating condition judging means judges that said multi-cylinder internal combustion engine is in operation under said predetermined low load operating condition and said ignition angle judging means judges that said ignition angle is larger than said predetermined ignition delay angle.