Abstract:
A method is described for estimating the temperature of the exhaust gases upstream from a pre-catalyser disposed along an exhaust pipe of an internal-combustion engine, which is provided with a system for controlling the composition of the exhaust gases, comprising an oxygen sensor, which is disposed along the exhaust pipe, upstream from the pre-catalyser, a heater, which is associated with the oxygen sensor, and a control unit, which, inter alia, serves the purpose of piloting the heater. The method comprises the steps of: determining an operative quantity, which is correlated to an electrical power supplied to the heater, in order to keep the operative temperature of the oxygen sensor close to a target temperature; and determining the temperature of the exhaust gases upstream from the pre-catalyser, according to the said operative quantity.

Description:
The present invention relates to a method for estimating the temperature of the exhaust gases upstream from a pre-catalyser disposed along an exhaust pipe of an internal-combustion engine. 
     BACKGROUND OF THE INVENTION 
     Systems for controlling the composition of the exhaust gases of intern-combustion engines are known, which require acquisition and processing of a certain series of signals, which can either be measured directly by means or suitable sensors, or can be estimated from other values correlated to the signals, by means of use of predictive models. 
     For the sake of greater clarity, reference is made of FIG. 1, which illustrates a simplified block diagram of a known system for controlling the composition of the exhaust gases of an engine  20 , provided with a pre-catalyser  2 , which is disposed along an exhaust pipe  7 , in a position which is very close to the engine  20  itself, and a main catalyser  3 , which is disposed along the exhaust pipe, downstream from the pre-catalyser  2 , in a position further away rom the engine  20 . 
     The control system, which is indicated as  1  as a whole, comprises an oxygen sensor  5 , which is disposed upstream from the pre-catalyser  2 , and normally consists of a linear LAMBDA or UEGO sensor, and supplies a signal V OX  which indicates the quantity of oxygen present in the exhaust gases at the intake of the pre-catalyser  2 ; a temperature sensor  6 , which is disposed downstream from the pre-catalyser  2 , between the latter and the main catalyser  3 , and supplies a signal V T  which indicates the temperature T V  of the exhaust gases at the output of the pre-catalyser  2  itself, indicated hereafter in the description by the term “temperature downsream”; and a control unit  4  which is connected to the oxygen sensor  5  and to the temperature sensor  6 , receives the signals V OX  and V T , and, on the basis of these signals, serves the purpose of controlling the composition of the exhaust gases produced by the engine  20 . 
     In order to implement satisfactory control of the composition of the exhaust gases, in addition to the signals V OX  and T V , the control unit  4  also needs to have available additional values, which, if they are not in specific operating conditions, cannot be measured either directly or indirectly, and which must therefore be estimated on the basis of the operating conditions of the engine  20  (load, number of revolutions etc.), by means of use of predictive models. 
     In particular, it is necessary to use predictive models in order to estimate the temperature of the exhaust gases at the intake of the pre-catalyser  2 , since this temperature cannot be related directly to the signal supplied by the temperature  6  disposed downstream from the pre-catalyser  2 , except in specific operating conditions. In fact, the pre-catalyser  2  is normally the source of exothermal chemical reactions, and consequently the temperature of the exhaust gases increases during passage of the latter through the pre-catalyser  2 . 
     Only in cases when the engine  20  is functioning with an air/fuel (A/F) mixture which is significantly greater than the stoichiometric value (equivalent to 14.56) do the exothermal reactions stop, such that the ratio between the temperature of the exhaust gases at the intake and output of the pre-catalyser becomes known. 
     The predictive models which are used at present to estimate the temperature of the exhaust gases at the intake of the pre-catalyser  2  nevertheless have some disadvantages. 
     Firstly, the accuracy of the estimates which can be obtained by means of these predictive models is not always sufficient. In particular, during transient conditions between different operating conditions of the engines, the estimates which are supplied by the known predictive models cannot follow reliably and quickly the variations of the temperature values of the exhaust gases. 
     In addition, the predictive models which are used at present do not take into account differences from the nominal conditions, owing mainly to ageing of the components, and thus, the estimates which these models provide gradually become increasingly less reliable. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method for estimating the temperature of the exhaust gases, which is free from the disadvantages described, and which in transient can provide reliable estimates even in transient conditions, without requiring the addition of further sensors. 
     According to the present invention, a method is thus provided for estimating the temperature of the exhaust gases upstream from a pre-catalyser disposed along an exhaust pipe of an internal-combustion engine, which is provided with a system for controlling the composition of the exhaust gases, comprising oxygen sensor means which are disposed along the said exhaust pipe, upstream from the said pre-catalyser, and means for piloting the said heater means; the said method being characterised in that it comprises the steps of: 
     a) determining a first operative quantity, which is correlated to the exchange of heat between the said oxygen sensor means and the exhaust gases; and 
     b) determining a temperature of the exhaust gases upstream from the said pre-catalyser, according to the said first operative quantity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to assist understanding of the present invention, a preferred embodiment is described hereinafter, purely by way of non-limiting example, and with reference to the attached drawings, in which: 
     FIG. 1 is a simplified block diagram of a system of a known type for controlling the exhaust gases; 
     FIG. 2 is a more detailed block diagram of a system for controlling the exhaust gases, which implements the method for estimating the temperature according to the present invention; and 
     FIGS. 3 and 4 are flow diagrams relative to the method for estimation according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The system for controlling the exhaust gases, which implements the method for estimating the temperature according to the present invention, has a general circuit structure which is similar to that previously described with reference to FIG. 1, and thus hereinafter in the description, parts which are identical to those in FIG. 1 will be indicated by the same reference numbers. 
     FIG. 2 shows a more detailed block diagram of the control unit  4  and of the oxygen sensor  5 . 
     In particular, the oxygen sensor  5  comprises an oxygen sensor  10 , which in use is immersed in the exhaust gases, and supplies as output a voltage V S  which is correlated to the internal resistance R S  of the oxygen sensor  10  itself, which is supplied at the intake of the control unit  4 ; and a heater  11 , which is controlled by the control unit  4 , and serves the purpose of keeping the temperature of the oxygen sensor  10  within a pre-determined operative interval of values, in which the information supplied by the oxygen sensor  10  is reliable. 
     The control unit  4  comprises a calculation block  12 , which receives as input the voltage V S , and supplies as output operative temperature values T S  or the oxygen sensor  10 . In detail, Inside the calculation block  12 , the voltage V S  is sampled with a period of sampling ô, and is converted into a digital signal, on the basis of which the calculation block  2  itself determines initially, at each sampling interval and in a manner which is known and is therefore not described in detail, a value of internal resistance R S  of the oxygen sensor  10 , and on the basis of this, and of the known ratio which associates the internal resistance R S  and the operative temperature T S  of the oxygen sensor  10 , the block then calculates an operative temperature value T S  of the oxygen sensor  10  itself, which is stored in a work memory, which is of a known type and is not shown. 
     The control unit  4  additionally comprises a subtracter block  13 , which receives as input the operative temperature T S  and a target temperature T°, and supplies as output an error signal T E , which is provided by the difference between the operative temperature T S  and the target temperature T°; a controller block  15 , which is preferably a controller of the PI (proportional-integral) type, which receives as input the error signal T E  and supplies as output a control voltage V C , which is correlated to the amplitude of the error signal T E  itself; and a block  16  for piloting the heater  11 , which receives as input the control voltage V C , and supplies as output a piloting voltage V P , which is supplied to the heater  11 , and has an effective value V PEFF  such as to supply to the heater  11  itself the electrical power W E  necessary to take the temperature of the oxygen sensor  10  to a value which is close to the value of the target temperature T°, for example 770° C. 
     The control unit  4  additionally comprises an estimation block  17 , which receives as input the control voltage V C , the operative temperature T S , and a value of flow rate M G  of the exhaust gases, and supplies as output a temperature T G  of the exhaust gases at the intake of the pre-catalyser  2 , which is indicated hereinafter in the description by the term “temperature upstream”, estimated by using an estimation algorithm described in detail hereinafter; and a correction block  18 , which receives as input the temperature upstream T G , by implementing an adaptation procedure described in detail hereinafter, and supplies as output a correct temperature T C . 
     In particular, the method for estimating the temperature upstream T G  of the exhaust gases implemented by the estimation block  17  is based on the fact that the amplitude of the control voltage V C  is correlated to the difference which exists between the real temperature of the exhaust gases and the operative temperature T S  of the oxygen sensor  10 . In fact, the control voltage V C  is used to control the effective value V PEFF  of the piloting voltage V P , and, consequently, the electrical power W E  which needs to be supplied to the heater  11 , in order to compensate for the variations in the temperature of the sensor  10 , caused by heat exchange with the surrounding environment, constituted by the exhaust gases which flow in the exhaust pipe  7 . 
     In detail, the estimation block  17  calculates the temperature upstream T G  from the operative temperature T S  of the oxygen sensor  10  and from the control voltage V C , in the manner described hereinafter. 
     Since no mechanical work is carried out on the oxygen sensor  10 , the energy balance, with reference to a sampling period ô between two successive moments of sampling n and n+1, is represented by the equation: 
     
       
         Δ Q   S   =ΔQ   SG   +ΔQ   SR   (1) 
       
     
     in which ΔQ S  is the heat stored by the oxygen sensor  10 , whereas ΔQ SG  and ΔQ SR  represent the heat exchanged respectively by the oxygen sensor  10  with the exhaust gases for convection, and with the heater  11  for conduction. 
     The quantities ΔQ S , ΔQ SG  and ΔQ SR  are calculated on the basis of the following equations: 
     
       
         Δ Q   S   =C[T   S ( n= 1)− T   S ( n )]  (2) 
       
     
     
       
         Δ Q   SG   =H[T   S ( n )− T   G ( n )]  (3) 
       
     
     
       
         Δ Q   SR   =KW   E   (4) 
       
     
     in which C is the thermal capacity of the oxygen sensor  10 , H is the coefficient of convective heat exchange between the oxygen sensor  10  and the exhaust gas, which is dependent on the flow rate of the exhaust gases M G , according to a known ratio, and K is the coefficient of conductive heat exchange between the oxygen sensor  10  and the heater  11 . 
     In addition, the value of the thermal power W E  is provided by the expression: 
     
       
           W   E   −V   2   PEFF   /R   H   (5) 
       
     
     in which R H  is the resistance of the heater  11 . 
     As previously stated, the effective value V PEFF  of the piloting voltage V P  depends in a known manner on the control voltage V C  which supplied as input to the estimation block  17 . 
     When the equations (2), (3), (4) and (5) are substituted in (1), the following ratio is obtained:                  T   S          (     n   +   1     )       =         (     1   -     H   C       )            T   S          (   n   )         +       H   C            T   G          (   n   )         +       K   C            V   PEFF   2       R   H                   (   6   )                                
     in which the only unknown term is the temperature upstream T G (n). 
     Since the variations in the temperature of the exhaust gases are slow compared with the variations of the electrical values and of the times required for processing of the signals, it is always possible to select an appropriate value for the sampling period ô, such that successive samples of the temperature upstream T G  can be considered approximately equal, i.e.: 
     
       
           T   G ( n+ 1)≅ T   G ( n )  (7) 
       
     
     By replacing (7) in (6), the required value of the temperature upstream T G  is obtained, according to the equation: 
     
       
         
           
             
               
                 
                   
                     
                       T 
                       G 
                     
                      
                     
                       ( 
                       
                         n 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       C 
                       H 
                     
                      
                     
                       [ 
                       
                         
                           
                             T 
                             S 
                           
                            
                           
                             ( 
                             
                               n 
                               + 
                               1 
                             
                             ) 
                           
                         
                         - 
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 H 
                                 C 
                               
                             
                             ) 
                           
                            
                           
                             
                               T 
                               S 
                             
                              
                             
                               ( 
                               n 
                               ) 
                             
                           
                         
                         - 
                         
                           
                             K 
                             H 
                           
                            
                           
                             
                               V 
                               HEFF 
                               2 
                             
                             
                               R 
                               H 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     The value supplied by the equation (8) represents the output of the estimation block  17 , and is also valid in transient conditions. 
     FIG. 3 shows a flow chart relating to the operations implemented by the estimation block  17 , in order to calculate the value of the temperature upstream T G . 
     As illustrated in this figure, initially acquisition takes place of the value of the operative temperature T S  of the oxygen sensor  10  which is stored at the moment n, as well as of the flow rate of the exhaust gases M G  (block  100 ). 
     On the basis of the control voltage V C , there is then calculation of the effective value V PEFF  of the piloting voltage V P  (block  110 ), whereas the flow rate of the exhaust gases M G  is used in order to determine the value of the coefficient of convective heat exchange H (block  120 ). 
     Finally, the estimation of the temperature upstream of the exhaust gases at the moment n+1 is calculated on the basis of the equation (8) (block  130 ), and the algorithm is concluded (block  140 ). 
     FIG. 4 shows a flow chart relating to the method for adaptation implemented by the correction block  18 . 
     The method for adaptation is based on the fact that, as previously stated, the exothermal reactions within the pre-catalyser  2  stop in specific conditions of operation of the engine  20 , and consequently, the temperature gap T GAP  of the exhaust gases between the intake and the output of the pre-catalyser  2  itself is constant and known, since a nominal value can be determined experimentally, or calculated in a manner which is well known to persons skilled in the art. Thus, it is also possible to calculate the temperature of the exhaust gases at the intake of the pre-catalyser  2 , on the basis of the temperature downstream T V  measured by the temperature sensor  6 , and to compare it with the temperature estimated on the basis of the equation (8). Any divergence T OFF  is represented by the error which is committed by estimating the temperature upstream T G  in accordance with the equation (8), and is added to the temperature upstream T G  itself, in order to obtain the correct temperature T C , which provides a more accurate estimate. 
     In detail, the method for adaptation begins with a test to check whether the engine  20  is being started up for the first time (block  200 ). If this is the case (YES output from the block  200 ), the divergence T OFF  is set to zero (block  210 ), whereas otherwise (NO output from the block  200 ), a value of the divergence T OFF  stored in a previous operating cycle of the engine  20  is loaded (block  220 ). 
     Subsequently, a further test is carried out in order to check whether the conditions exist for carrying out an update of the divergence T OFF  (block  230 ). In particular, it is checked whether the air/fuel ratio (A/F) of the mixture supplied to the engine  20  is kept without interruption above a threshold ratio (A/F) S , which is greater than the stoichiometric value, for a time interval which is greater than a minimum time ô M . If this condition exists (YES output from block  230 ), the value of the divergence T OFF  is updated on the basis of the equation (block  240 ): 
     
       
           T   OFF   =T   V   +T   GAP   −T   G   (9) 
       
     
     If on the other hand the updating condition has not seen found (NO output from block  230 ), the correct temperature T C  is calculated directly on the basis of the following ratio (block  250 ): 
       T   C   =T   G   +T   OFF   (10) 
     A further test is then carried out, in which it is checked whether switching off of the engine  20  has been ordered (block  260 ). If the result of the test is negative (NO output from block  260 ), the updating method is ended (block  280 ); otherwise (YES output from block  260 ), before abandoning the method, the present value of the divergence T OFF  is scored in a permanent memory, which is of a known type and is not shown, which can retain the value stored even in the absence of a power supply (block  270 ). 
     The method for estimation described has the following advantages. 
     Firstly, the estimation of the temperature upstream T G  is based on processing of the data supplied by the oxygen sensor  5 , and not simply on predictive models. Consequently, the temperature value calculated by the estimation block  17 , in accordance with the equation (8), represents a more accurate estimate than those supplied by the conventional methods. In particular, the method makes it possible to calculate accurately the temperature upstream T G  even in transient conditions. 
     Secondly, the method can adapt the calculation of the temperature upstream T G , and supply a correct temperature T C , which takes into account any differences from the nominal operative conditions. By this means, for example, it s possible to compensate for the variations caused by ageing of the components, thus preventing deterioration of the performance of the system. 
     In addition, the present method for estimation advantageously makes it possible to obtain the results illustrated by using only the sensors which are already present in the systems currently available, and therefore without needing to use a larger number of sensors. 
     Finally, it is apparent that modifications and variants can be made to the method for estimation described, which do not depart from the protective context of the present invention. 
     In particular, the regulation function implemented by controller block  15  can be of the proportional-derivative (PD) type, proportional-integral-derivative (PID) type, or of another type.