Method and apparatus for monitoring deterioration of internal combustion engine exhaust gas purifier

An apparatus for monitoring the deterioration of an internal combustion engine having an exhaust gas purifying means provided within an exhaust pipe of said engine, said apparatus including:

• (a) sensor means disposed downstream of said purifying means;
• (b) further sensor means disposed upstream of said purifying means;
• (c) electronic control means for controlling an actual air-fuel ratio of the air-fuel mixture supplied to each cylinder of said engine to have a required mean air-fuel ratio;
• (d) an output of said downstream sensor means providing a signal indicative of said mean air-fuel ratio;
• (e) determining means for determining said engine to have a preferred air-fuel ratio with a value of 14.7 which is an air-fuel ratio midway between a rich condition and a lean condition;
• (f) measuring means for measuring said mean air-fuel ratio from an output of said downstream sensor means;
• (g) driving means controlled by said signal through a closed loop algorithm to change said mean air-fuel ratio from a first amount on one side of said preferred air-fuel ratio to another amount on another side of said preferred air-fuel ratio;
• (h) further measuring means for measuring the time taken from said change for said output of said downstream sensor means to reach a predetermined level, whereby said time is indicative of a deteriorative condition of said purifying means;
• and
• (i) judging means for judging whether the engine is in a steady state operative condition whereby only if such a condition is judged to exist is said driving means controlled by said signal through said closed loop control algorithm to change said mean air-fuel ratio from said first amount to said another amount.