1.  Field of the Invention
The present invention relates to a device for determining an irregular combustion in an internal combustion engine. 
2.  Description of the Related Art
Japanese Unexamined Patent Publication No.  61-258955, discloses an irregular combustion determining device for an internal combustion engine having a plurality of cylinders.  In this device, an engine speed during a combustion process in each cylinder is sequentially detected, and variations in the engine speed at the successive stages of the combustion process in each cylinder are calculated.  Note the variation in engine speed is defined as the difference between two successive engine speeds at two successive stages of the combustion process.  The variation .DELTA.NE in the engine speed is calculated from the following equation.  EQU .DELTA.NE=NEB-NE
Where
NE: an engine speed in a present stage of the combustion process PA1 NEB: an engine speed in a stage of the combustion process immediately preceding the present stage of the combustion process.  PA1 When the variation in the engine speed is larger than a predetermined variation, it is determined that a misfire has occurred. 
In this device, however, the following problem arises It is assumed that a malfunction has occurred in one of the fuel injectors of the engine, and that the malfunctioning fuel injector has injected a large amount of fuel into the intake ports, whereby an air-fuel mixture fed into the cylinder corresponding to the malfunctioning fuel infector (hereinafter referred to as a failed cylinder) becomes extremely rich, and thus a misfire occurs in the failed cylinder.  In this case, the engine speed NE during the combustion process in the failed cylinder is lowered, and accordingly the variation .DELTA.NE in the engine speed becomes greater.  Therefore, it can be ascertained that a misfire has occurred when it is determined that the variation .DELTA.NE in the engine speed is greater than a predetermined variation.  In this case, an oxygen concentration detection sensor (hereinafter referred to as an O.sub.2 sensor) arranged in an exhaust passage of an engine continues to output a rich signal, and accordingly, when an air-fuel ratio feedback control operation is executed, the air-fuel mixture fed into normal-functioning cylinders supplied by normal-functioning injectors (hereinafter referred to as a normal cylinder) becomes extremely lean, and thus an misfire occurs in the normal cylinders.  When a misfire occurs in the normal cylinder immediately before the stage of the combustion process in the failed cylinder, the two successive engine speeds in the successive combustion processes in the normal and the failed cylinders are lowered In this case, the variation in the engine speed, which is represented by a difference between an engine speed during the combustion process of the normal cylinder and an engine speed during the combustion process of the failed cylinder, becomes smaller than the predetermined variation, and thus a misfire in the failed cylinder can not be detected. 
A similar problem arises, when the malfunctioning fuel injector injects a small amount of fuel into the intake port, i.e., a misfire in the failed cylinder can not be detected.