Patent attributes
The air/fuel ratio of exhaust flowing into a catalytic converter is forcibly modulated, between a lean air/fuel ratio leaner than a target average air/fuel ratio and a rich air/fuel ratio richer than the target average air/fuel ratio, with a specific period, a specific amplitude, a specific modulation ratio and a specific waveform. During the forcible modulation (S10, S12), the ratio of a time for which the output of an oxygen sensor is greater than a standard value Sb for the output set between the maximum and minimum values of the output (“rich” output time), or of a time for which it is smaller than the standard value Sb (“lean” output time), in a predetermined period of time, or a value correlating with this ratio is obtained (S14), and the air/fuel ratio of the exhaust is controlled on the basis of this ratio or the value correlating with this ratio (S16, S18).
