Patent ID: 7543443

Claim:
A method of determining an NO x reduction ratio in an exhaust emission control device in which a reducing agent is added upstream of an NO x reduction catalyst positioned in an exhaust passage of an engine comprising: measuring a detected upstream NO x value at a location upstream of a position at which the reducing agent is added; correcting said detected upstream NO x value based upon at least one of exhaust temperature and exhaust flow rate to provide a corrected upstream NO x value; measuring a detected downstream NO x value at a location downstream of the catalyst; correcting said detected downstream NO x value based upon at least one of exhaust temperature and exhaust flow rate to provide a corrected downstream NO x value; determining the NO x reduction ratio based on the corrected upstream NO x value and the corrected downstream NO x value; sensing exhaust temperature at a location upstream of said catalyst to provide an upstream exhaust temperature and using said upstream exhaust temperature to correct the detected upstream NO x value to provide the corrected upstream NO x value; and sensing exhaust temperature at a location downstream of said catalyst to provide a downstream exhaust temperature and using said downstream exhaust temperature to correct the detected downstream NO x value to provide the corrected downstream NO x value.