Patent ID: 7266942
Filing Date: 2007-09-11
Classification: F02D,Y02T

Abstract:
1. A diagnostic device of an engine exhaust purifying system including a plurality of cylinder groups, a pre-catalyst made of a three-way catalyst and disposed in each of a plurality of upstream-side exhaust passage portions connected to the respective cylinder groups, a downstream air-fuel ratio sensor disposed downstream of each said pre-catalyst, and a main catalyst made of a three-way catalyst and disposed in a downstream-side exhaust passage portion downstream of a merging area where said upstream-side exhaust passage portions are merged together, said diagnostic device comprising: exhaust air-fuel ratio control means for performing control to keep an air-fuel ratio of exhaust gas flowing into said main catalyst near a stoichiometric ratio in accordance with outputs of said downstream air-fuel ratio sensors; rich/lean switching means for switching over the air-fuel ratio in each said cylinder group from rich to lean in accordance with information obtained from said exhaust air-fuel ratio control means, thereby diagnosing said downstream air-fuel ratio sensor; response detecting means for detecting a response of said downstream air-fuel ratio sensor when the rich/lean switching of the air-fuel ratio in each cylinder group is performed; and downstream sensor deterioration determining means for determining a deterioration of said downstream air-fuel ratio sensor based on the detected response, wherein said diagnostic device is configured such that, upon starting of diagnosis, the cylinder groups are temporarily changed to one of rich or lean and thereafter by way of the rich/lean switching means, one of the cylinder groups is switched from one of rich to lean or lean to rich and then another of the cylinder groups is likewise switched from one of rich to lean or lean to rich, such alternating switching routine occurring repeatedly with a sum of fuel modification amounts being kept at zero and under-floor exhaust gas is maintained nearly at the stoichiometric ratio.