Method and device for controlling an scr catalytic converter of a vehicle

Abstract

The present invention provides for a method and apparatus for controlling an SCR catalytic converter ( 2 ). The invention describes a model-based control method including a physical model of the SCR catalyst ( 2 ) with more than one NH3 storage cell and a physical model of a NOx sensor ( 4 ). The observer feedback gain forces the estimated sensor outputs from the model to converge to measured ones, therefore, there is not ambiguity in the determination of the operating point.

1 . Method for controlling an SCR catalytic converter of a vehicle, comprising the step of using as reference value an output of an estimated sensor of nitrogenated gases by forcing the estimated sensor output to converge to a measured value. 
     
     
         2 . Method according to  claim 1 , wherein, in an observer, the convergence is forced by means of adjusting the amount of stored ammonia or other state variables using variable feedback gains, which are dependent on the operating conditions such as temperature, space velocity, stored ammonia or others. 
     
     
         3 . Method according to  claim 2 , wherein the convergence is forced by means of additional adjusting of constant to slowly drifting parameters such as catalyst storage capacity or offsets of urea injection or sensors, or urea quality. 
     
     
         4 . Method according to  claim 2 , wherein the quantity or sign of the feedback gains in the observer are used to detect divergence and to initiate a recovery step from said divergence, said divergence being due to a wrong interpretation of the NOx sensor signal, caused by the NH3/NOx ambiguity of the sensor characteristic. 
     
     
         5 . Method according to  claim 4 , wherein said recovery step provides for adjusting the amount of stored ammonia or other quantities in the observer, such as to converge to the correct side of the NOx sensor characteristic. 
     
     
         6 . Method according to  claim 1 , wherein said nitrogenated gases are NOx and/or NH3, and/or NO2, and/or NO, and/or N2O. 
     
     
         7 . Method according to  claim 1 , further comprising the step of considering the SCR catalytic converter as divided in a succession of two or more storage cells, and the step of controlling the NH3 stored only in one of said cells, preferably the first one. 
     
     
         8 . Method according  claim 1 , further comprising the step of controlling an injected urea quantity or NH3 quantity or any other reduction agent which is converted into NH3, by means of a calculation of a catalyst conversion efficiency. 
     
     
         9 . Method according to  claim 1 , further comprising the step of controlling an injected urea quantity or NH3 quantity or any other reduction agent which is converted into NH3, a urea injection, by means of a calculation of a NH3 slip level in the SCR catalytic converter. 
     
     
         10 . Method according to  claim 8 , wherein the injected urea quantity, or NH3 quantity or any other reduction agent which is converted into NH3, is controlled by means of both set points calculated by imposing as target an NH3 storage level and a catalyst conversion efficiency. 
     
     
         11 . Method according to  claim 9 , wherein the injected urea quantity, or NH3 quantity or any other reduction agent which is converted into NH3, is controlled by means of both set points calculated by imposing as target an NH3 storage level and a catalyst conversion efficiency. 
     
     
         12 . Method according to  claim 10 , wherein said set points are calculated separately and wherein the actual control set point is obtained from a minimum selection of said set points. 
     
     
         13 . Method according to  claim 1 , wherein, if no sensor is available or the NOx or NH3 sensor output is not or partly reliable, the corresponding feedback gain is adjusted or switched to zero. 
     
     
         14 . Device for controlling an SCR catalytic converter of a vehicle, comprising means for implementing the method of any of the preceding claims. 
     
     
         15 . Computer program comprising computer program code means adapted to perform all the steps of  claim 1 , when said program is run on a computer. 
     
     
         16 . A computer readable medium having a program recorded thereon, said computer readable medium comprising computer program code means adapted to perform all the steps of  claim 1 , when said program is run on a computer.