Engine catalyst monitor

A method for determining a condition of a catalyst disposed in an exhaust of an engine, comprising:

• providing oxygen sensors in the exhaust upstream and downstream of the catalyst;
• estimating the mean value of the upstream and downstream sensor signals, mf and mr respectively, in accordance with:
• m f = 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v f ⁡ ( t ) ⁢ ⁢ and ⁢ ⁢ m r = 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v r ⁡ ( t ) , ⁢ respectively
• where vf(t) and vr(t) are the voltages of the front and rear oxygen sensor respectively at time t, and N is the number of samples over the period of time;
• estimating the variances and correlation coefficient, denoted by sf2, sr2, and r respectively, in accordance with:
• s f 2 = 1 N ⁢ ∑ t = 1 N ⁢ ⁢ ( v f ⁡ ( t ) 2 ) - ( 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v f ⁡ ( t ) ) 2 s r 2 = 1 N ⁢ ∑ t = 1 N ⁢ ⁢ ( v r ⁡ ( t ) 2 ) - ( 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v r ⁡ ( t ) ) 2 ⁢ ⁢ or rs f ⁢ s r = 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v f ⁡ ( t - T ) ⁢ v r ⁡ ( t ) - ( 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v f ⁡ ( t - T ) ) ⁢ ⁢ ( 1 N ⁢ ∑ t = 1 N ⁢ ⁢ v r ⁡ ( t ) )
• where T is a positive constant;
• and
• determining the condition of the catalyst by comparing the differences between the estimated means of the upstream and downstream sensor signals, the difference in the estimated variances of the upstream and downstream sensor signals, and the estimated correlation coefficient.