Air/fuel ratio controller and control method

1. An air/fuel ratio control method for an internal combustion engine ( 1 ) equipped with a three-way-catalyst ( 8 ) and with an oxygen sensor ( 9 ) upstream the three-way-catalyst ( 8 ) and a NOx sensor ( 10 ) downstream the three-way-catalyst ( 8 ), whereas the output (λ up ) of the upstream oxygen sensor ( 9 ) is used in an upstream control loop that controls the air/fuel ratio by maintaining a certain optimum upstream air/fuel ratio set-point (λ SP ), the method comprising the steps of:

adding incremental offsets (Δλ) to the upstream air/fuel ratio set-point (λ SP ) to get a current air/fuel ratio set-point (λ SPC ) while the NOx sensor ( 10 ) output is monitored,

repeatedly adding incremental offsets (Δλ) until a first turning point (SP 1 ) in the NOX sensor ( 10 ) output is reached and storing the current air/fuel ratio set-point (λ SPC ) at the first turning point (SP 1 ) as first air/fuel ratio set-point boundary value (λ SP1 ),

adding incremental offsets (Δλ) to the current upstream air/fuel ratio set-point (λ SPC ) in the opposite direction while the NOx sensor ( 10 ) output is monitored,

repeatedly adding incremental offsets Δλ in the opposite direction until a second turning point (SP 2 ) in the NOx sensor ( 10 ) output is reached again and storing the current air/fuel ratio set-point (λ SPC ) at the second turning point (SP 2 ) as second air/fuel ratio set-point boundary value (λ SP2 ), and

calculating a new optimum air/fuel ratio set-point (λ SP ) for the upstream control loop as mean value of the first and second air/fuel ratio set-point boundary values (λ SP1 , λ SP2 ).

2. The method of claim 1 , wherein the output of a second oxygen sensor ( 11 ) downstream of the three-way-catalyst ( 8 ) is interpreted as rich or lean and the first air/fuel ratio offset (Δλ) is added in the rich direction if the output of the second oxygen sensor ( 11 ) is interpreted as lean and vice versa.

3. The method of claim 1 , wherein the first air/fuel ratio offset (Δλ) is added in a predefined direction and the adding of the air/fuel ratio offset (Δλ) continues in the same direction if the NOx sensor ( 10 ) output decreases, or the adding of the air/fuel ratio offset (Δλ) starts in the opposite direction if the NOx sensor ( 10 ) output increases.

4. The method according to claim 1 , wherein the output of the NOx sensor ( 10 ) is allowed to stabilize for a certain time period before the next air/fuel ratio offset (Δλ) is added.

5. The method according to claim 1 , wherein the determination of the optimum air/fuel ratio (λ SP ) is repeated for a given number of times (i) and the new optimum air/fuel ratio (λ SP ) is calculated as mean value of the number of times (i) optimum air/fuel ratios (λ SP (i)).

6. An air/fuel ratio controller for an internal combustion engine ( 1 ) with a three-way-catalyst ( 8 ) arranged in an exhaust line ( 7 ) of the engine ( 1 ) and with an oxygen sensor ( 9 ) upstream the three-way-catalyst ( 8 ) and a NOx sensor ( 10 ) downstream the three-way-catalyst ( 8 ), whereas the controller ( 6 ) uses the output (λ up ) of the upstream oxygen sensor ( 9 ) in an upstream control loop to maintain a certain optimum air/fuel ratio set-point (λ SP ), whereas

incremental offsets (Δλ) are added to the upstream air/fuel ratio set-point (λ SP ) to get a current air/fuel ratio set-point (λ SPC ) while the NOx sensor ( 10 ) output is monitored,

the incremental offsets (Δλ) are repeatedly added until a first turning point (SP 1 ) in the NOX sensor ( 10 ) output is detected and the current air/fuel ratio set-point (λ SPC ) at the first turning point (SP 1 ) is stored as first air/fuel ratio set-point boundary value (λ SP1 ),

incremental offsets (Δλ) to the current upstream air/fuel ratio set-point (λ SPC ) are added in the opposite direction while the NOx sensor ( 10 ) output is monitored,

incremental offsets (Δλ) are repeatedly added in the opposite direction until a second turning point (SP 2 ) in the NOx sensor ( 10 ) output is reached again and the current air/fuel ratio set-point (λ SPC ) at the second turning point (SP 2 ) is stored as second air/fuel ratio set-point boundary value (λ SP2 ), and

a new optimum air/fuel ratio set-point (λ SP ) for the upstream control loop is calculated in the controller ( 6 ) as mean value of the first and second air/fuel ratio set-point boundary values (λ SP1 , λ SP2 ).

7. The air/fuel ratio controller of claim 6 , wherein the output of a second oxygen sensor ( 11 ) arranged downstream of the three-way-catalyst ( 8 ) is interpreted by the controller ( 6 ) as rich or lean and the first air/fuel ratio offset (Δλ) is added in the rich direction if the output of the second oxygen sensor ( 11 ) is interpreted as lean and vice versa.

8. The air/fuel ratio controller of claim 6 , wherein the first air/fuel ratio offset (Δλ) is added in a predefined direction and the adding of the air/fuel ratio offset (Δλ) continues in the same direction if the NOx sensor ( 10 ) output decreases, or the adding of the air/fuel ratio offset (Δλ) continues in the opposite direction if the NOx sensor ( 10 ) output increases.

9. The air/fuel ratio controller of claim 6 , wherein the output of the NOx sensor ( 10 ) is allowed to stabilize for a certain time period before the next air/fuel ratio offset (Δλ) is added.

10. The air/fuel ratio controller of claim 6 , wherein the controller ( 6 ) determines the optimum air/fuel ratio set-point (λ SP ) a given number of times (i) and the new optimum air/fuel ratio set-point (λ SP ) is calculated in the controller ( 6 ) as mean value of the number of times (i) optimum air/fuel ratio set-points (λ SP (i)).