The method of claim 24 wherein said cool down filter in discrete form is represented by the difference equation TCool Filt1 , 2 = TAmb ??? ( n ) - TCool Filt1 , 2 ??? ( n - 1 ) ?? Cool1 , 2 + TCool Filt1 , 2 ??? ( n - 1 ) where TAmb is said ambient temperature; TCoolFilt1,2 is said cool down catalyst temperature while cooling down for both filters, and ??Cool1=??Cool2 and is a time constant which