Nox control for an internal combustion engine

Abstract

An improved NOX control for an internal combustion engine having an exhaust system with a NOX storage catalyst ( 105 ) includes a gas composition sensor ( 110 ) upstream of the NOX catalyst. Whether the NOX catalyst is storing NOX onto the catalyst or purging NOX from the catalyst is estimated from the signal output from the gas composition sensor. The composition of exhaust gasses input to the catalyst is also estimated using variables like engine load, engine speed, engine temperature and space velocity of the exhaust gases. This enables the NOX flow rate input to the catalyst and the aggregate quantity of NOX stored on the catalyst to be estimated.

1 . A method of operating an internal combustion engine having a first catalyst with NOx storage capability; said method involving determination of the composition of exhaust gas products input to the catalyst to thereby estimate an aggregate quantity of nitrogen oxide compounds (NOx) stored on said catalyst, said method including the steps of 
 estimating a quantity of NOx that has recently either been stored onto said catalyst or reduced from said catalyst, and    summing said quantity of NOx estimated to have recently been stored or reduced with an aggregate quantity of NOx estimated to have previously been stored on said catalyst;    said determination of the composition of gases input to said catalyst also being used during certain engine operating conditions to control the engine air fuel ratio so as to control the composition of exhaust gas products produced by the engine and input to the catalyst.    
     
     
         2 . A method as claimed in  claim 1  wherein said estimate of said aggregate quantity of NOx stored on said catalyst is performed at intermittent time intervals.  
     
     
         3 . A method as claimed in  claim 2  wherein said intermittent time intervals further comprise a background loop of an electronic control unit of said engine.  
     
     
         4 . A method as claimed in  claim 2  further comprising the step of estimating the quantity of NOx that has either been stored onto said catalyst or reduced from said catalyst over a recent one of said intermittent time intervals, and summing said quantity of NOx estimated to have been stored or reduced over said recent one of said intermittent time intervals with an aggregate quantity of NOx estimated to have been stored on said catalyst over a previous one of said intermittent time intervals.  
     
     
         5 . A method as claimed in  claim 1  wherein said step of estimating the quantity of NOx stored on said catalyst further comprises the step of estimating NOx flow to said catalyst.  
     
     
         6 . A method as claimed in  claim 5  wherein said NOx flow is derived from at least engine speed and engine load.  
     
     
         7 . A method as claimed in  claim 5  wherein said step of estimating the quantity of NOx stored on said catalyst further comprises the step of estimating engine temperature.  
     
     
         8 . A method as claimed in  claim 5  wherein said NOx flow and said intermittent time interval is used to determine a total quantity of NOx input to said catalyst during a recent one of said intermittent time intervals.  
     
     
         9 . A method as claimed in  claim 1  wherein said step of estimating the quantity of NOx stored on said catalyst further comprises the step of estimating the space velocity of said gases input to said catalyst.  
     
     
         10 . A method as claimed in  claim 9  wherein space velocity and said total quantity of NOx input to said catalyst are used to determine said quantity of NOx stored on said catalyst.  
     
     
         11 . A method as claimed in  claim 1  wherein said step of estimating NOx reduced from said catalyst further comprises the step of estimating a NOx purge time period based on said aggregate quantity of NOx stored on said catalyst.  
     
     
         12 . A method as claimed in  claim 11  wherein said NOx purge time comprises the time required to purge a pre-determined quantity of NOx from said catalyst under predetermined engine operating conditions.  
     
     
         13 . A method as claimed in  claim 11  wherein said step of estimating NOx reduced from said catalyst comprises the further step of estimating a NOx reduction flow to said catalyst.  
     
     
         14 . A method as claimed in  claim 13  wherein said NOx reduction flow is estimated from the space velocity of gases input to said catalyst and from the composition of said gases.  
     
     
         15 . A method as claimed in  claim 13  wherein said step of estimating the quantity of NOx reduced from said catalyst is derived from said NOx reduction flow and said NOx purge time.  
     
     
         16 . A method as claimed in  claim 1  wherein determination of the composition of gases input to said catalyst further comprises determination of the molar weight ratio of carbon monoxide (CO) and NOx in said gases.  
     
     
         17 . A method as claimed in  claim 16  further comprising the steps of estimating that NOx is stored on said catalyst when said molar weight ratio is less than 3.0 and estimating that NOx is reduced from said catalyst when said molar weight ratio exceeds 3.0.  
     
     
         18 . A method as claimed in  claim 1  wherein said engine further comprises a first gas composition sensor located upstream of the first catalyst and wherein said determination of the composition of gases input to said catalyst further comprises monitoring an output signal of said first gas composition sensor.  
     
     
         19 . A method as claimed in  claim 1  further comprising the steps of monitoring said aggregate quantity of NOx, determining if said aggregate quantity exceeds a threshold value and operating said engine so as to reduce NOx stored on said catalyst when said aggregate quantity exceeds said threshold value.  
     
     
         20 . A method as claimed in  claim 19  wherein said engine is operated with a rich air fuel ratio providing exhaust gases having molar weight ratios for CO and NOx of 3.0 or greater in order to reduce NOx stored on said catalyst.  
     
     
         21 . A method as claimed in  claim 20  wherein said engine is operated with a closed loop air fuel ratio to thereby provide said rich air fuel ratio for reducing NOx stored on said catalyst.  
     
     
         22 . A method as claimed in  claim 1  wherein said engine further comprises a second catalyst upstream of said first, said second catalyst being a three way catalyst and said first gas composition sensor located intermediate said first and second catalysts.  
     
     
         23 . An internal combustion engine and exhaust system, the engine comprising an exhaust manifold with outlet for egress of raw combustion gases and an electronic control unit (ECU) for controlling operation of the engine and the exhaust system comprising a three way catalyst (TWC) located separately from a lean NOx catalyst (LNC), the TWC having an inlet port located so as to receive the raw combustion gases from the outlet of the exhaust manifold and an outlet port for egress of partially treated combustion gases, the LNC having an inlet port to receive the partially treated combustion gases and an outlet port for egress of exhaust gases, and an O 2  sensor located intermediate the outlet of the TWC and the inlet to the LNC wherein during at least pre-determined TWC operating conditions said ECU monitors the output of said O 2  sensor to thereby control the engine air fuel ratio so as to control the composition of treated exhaust gas products input to the LNC.  
     
     
         24 . An internal combustion engine and exhaust system as claimed in  claim 23  wherein said ECU monitors the output of said O 2  sensor to thereby determine whether said LNC is storing NOx or being purged of NOx.  
     
     
         25 . An internal combustion engine and exhaust system as claimed in  claim 23  or  claim 24  wherein said ECU monitors the output of a second O 2  sensor operatively associated with the inlet to the TWC to thereby provide closed loop air fuel ratio control.  
     
     
         26 . An internal combustion engine and exhaust system as claimed in  claim 23  wherein the closed loop air fuel ratio control is λ≈1 control.  
     
     
         27 . An internal combustion engine and exhaust system as claimed in  claim 23  wherein the closed loop air fuel ratio control perturbates the air fuel ratio and wherein this perturbation is between lean and rich air fuel ratios to thereby provide λ≈1 control.  
     
     
         28 . An internal combustion engine and exhaust system as claimed in  claim 27  wherein said rich perturbations are controlled in response to the output of said O 2  sensor associated with the output of the TWC so as to control whether said LNC is purging or storing NOx.  
     
     
         29 . A NOx model for estimating the aggregate quantity of NOx stored on a lean NOx catalyst for use with a lean burn engine and an exhaust system having a first catalyst for treating engine out emissions and a second catalyst wherein said second catalyst is a lean NOx catalyst and an oxygen sensor located intermediate said first catalyst and said second catalyst wherein said model receives input data at least derived from said oxygen sensor wherein said data indicates the composition of exhaust gas input to said lean NOx catalyst whereby said model estimates whether said lean NOx catalyst is storing NOx or reducing NOx.  
     
     
         30 . A NOx model as claimed in  claim 29  wherein the input data derived from said oxygen sensor is also used during certain engine operating conditions to control the engine air fuel ratio so as to control the composition of the exhaust gas products produced by the engine and input to the lean NOx catalyst.  
     
     
         31 . An electronic control unit for controlling operation of an engine having an exhaust after treatment system with a NOx storage catalyst, said ECU determining the composition of gases input to the catalyst to thereby determine an aggregate quantity of nitrogen oxide compounds (NOx) stored on said catalyst.  
     
     
         32 . An electronic control unit as claimed in  claim 31  wherein determining the composition of gases input to the catalyst further comprises determining the molar weight ratio of carbon monoxide (CO) and NOx in said gases.  
     
     
         33 . An electronic control unit as claimed in  claim 32  wherein said ECU estimates that NOx is stored on said catalyst when said ratio is less than 3.0 and estimates that NOx is reduced from said catalyst when said ratio exceeds 3.0.  
     
     
         34 . An electronic control unit as claimed in  claim 33  wherein said exhaust after treatment system further comprises a gas composition sensor upstream of said catalyst, said ECU receiving signals output by said gas composition sensor which said signals indicate said molar weight ratio.  
     
     
         35 . An electronic control unit as claimed in  claim 34  wherein the signals output by said gas composition sensor are also used during certain engine operating conditions to control the engine air fuel ratio so as to control the composition of the exhaust gases produced by the engine and input to the NOx storage catalyst.