Abstract:
A method for controlling the titer of the air-fuel mixture in an internal combustion engine provided with at least two cylinders, in which the exhaust gas present in a common exhaust manifold is analyzed in order to measure at least two successive values of the overall air-fuel ratio of the cylinders; a value of the air-fuel ratio of a final combusted cylinder being estimated by carrying out a linear composition of the two successive values of the overall air-fuel ratio of the cylinders and the value of the air-fuel ratio of the final combusted cylinder being attributed to a first of the cylinders and being used to correct a titer of the air-fuel mixture introduced into the first cylinder.

Description:
[0001]    The present invention relates to a method for controlling the titre of the air-fuel mixture in an internal combustion engine, in particular an internal combustion engine for driving vehicles.  
         BACKGROUND OF THE INVENTION  
         [0002]    The regulations relating to road vehicles are requiring an increasingly thorough reduction of the pollutant emissions emitted by internal combustion engines. These pollutant emissions can be reduced substantially in two ways: by optimising the combustion process in the cylinders of the engine or by treating the exhaust gases before they are emitted into the atmosphere (typically using exhausts of a catalytic type). In order to optimise the combustion process in the cylinders it is necessary to maintain the titre of the air-fuel mixture as close as possible to the stoichiometric value in each cylinder.  
           [0003]    The internal combustion engines that are currently in use are provided with a plurality of cylinders (generally four), each of which has a respective exhaust duct communicating with a common exhaust manifold disposed upstream of an exhaust provided with a device for reducing pollutant agents. In order to contain costs, only the overall stoichiometric ratio of all the cylinders is measured by means of a linear oxygen sensor disposed in the common exhaust manifold.  
           [0004]    By means of appropriate reconstruction methods and starting from the measurements of the overall stoichiometric ratio, the stoichiometric ratios of the individual cylinders are estimated and these stoichiometric ratios are used to control the intake of fuel into the individual cylinders, in order to cause each individual cylinder to work as close as possible to the stoichiometric value.  
           [0005]    These known reconstruction methods for estimating the stoichiometric ratios of the individual cylinders from the measurements of the overall stoichiometric ratio are, however, relatively imprecise and very complex.  
         SUMMARY OF THE INVENTION  
         [0006]    The object of the present invention is to provide a method for controlling the titre of the air-fuel mixture in an internal combustion engine, which is free from the above-described drawbacks and which is, moreover, simple and economic to implement.  
           [0007]    In accordance with the present invention, a method for controlling the titre of the air-fuel mixture in an internal combustion engine according to claim  1  is provided. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will now be described with reference to the accompanying drawings, which show a non-limiting embodiment thereof, in which:  
         [0009]    [0009]FIG. 1 is a diagrammatic view of an internal combustion engine using the control method of the present invention; and  
         [0010]    [0010]FIG. 2 is a diagrammatic view of a control unit of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    In FIG. 1, a device for controlling the titre of the air-fuel mixture in an internal combustion engine  2  provided with four cylinders  3  (shown diagrammatically) disposed in line is shown overall by  1 . Each cylinder  3  receives the fuel from a respective injector  4  of known type and is provided with a respective exhaust duct  5  which communicates with an exhaust manifold  6  common to all the cylinders  3 .  
         [0012]    The exhaust manifold  6  communicates with an exhaust device  7  of known type and comprises a linear oxygen probe  8  (commonly known to persons skilled in the art by the name “UEGO probe”), which is adapted to measure the percentage of oxygen present in the manifold  6 ; as is known, the percentage of oxygen in the exhaust gases of the cylinders  3  is in a bi-univocal relationship with the overall air-fuel ratio of the cylinders  3  and a measurement of this oxygen percentage therefore corresponds substantially to a measurement of the overall air-fuel ratio of the cylinders  3 .  
         [0013]    The control device  1  comprises a control unit  9 , which is connected to the probe  8  in order to receive the measurements of the overall air-fuel ratio of the cylinders  3 , and is connected to the injectors  4  in order to provide each injector  4  with a correction value of the quantity of fuel injected into the respective cylinder  3 . Each injector  4  is in particular controlled in a known manner by an injection control unit (not shown) in order to inject a predetermined quantity of fuel into the respective cylinder  3  (or into an intake duct of this cylinder  3 ); each injector  4  also receives a signal for the correction of the quantity of fuel to be injected from the control unit  9  in order to try to cause the respective cylinder  3  to work as close as possible to the stoichiometric value.  
         [0014]    The control device  1  further comprises a sensor  10  of known type (typically an angular encoder) which is connected to the control unit  9  and is adapted to read the angular position of a drive shaft  11  (shown diagrammatically).  
         [0015]    As shown in FIG. 2, the control unit  9  comprises a device  12  for filtering the measurement signal from the linear oxygen probe  8 .  
         [0016]    The filtering device  12  comprises a filter having a transfer function of a “high pass” type in order to filter the measurement signal of the overall air-fuel ratio of the cylinders  3  from the linear oxygen probe  8 . The filter of the filtering device  12  has a transfer function in the Laplace domain comprising a zero and two poles which are disposed at frequencies higher than zero. The filtering device  12  further comprises a limitation of the filtered signal within a predetermined acceptability range in order to eliminate any noise pulse components.  
         [0017]    The measurement signal from the liner oxygen probe  8  needs to be filtered to recover some dynamics weakened as a result of the response characteristics of the linear oxygen probe  8 , particularly as a result of the capacitance effect due to a protective hood (known and not shown) of this probe  8 . In order to obviate this critical factor, the filtering device amplifies the frequencies characteristic of the combustion phenomenon and at the same time reduces the high frequencies in order not to amplify noise.  
         [0018]    The signal filtered by the filtering device  12  is strongly under-sampled by a sampling device  13 , which stores four measurement values AFR COMPL  of the overall air-fuel ratio of the cylinders  3  for each complete revolution of the engine shaft  11 . The measurement values AFR COMPL  are in particular stored at the exhaust phase of each cylinder  3  such that each measurement value AFR COMPL  is as indicative as possible of the state of combustion of a respective cylinder  3 . According to a preferred embodiment, the measurement values AFR COMPL  are stored at each top dead centre of each cylinder  3 .  
         [0019]    As output from the sampling device  13 , each measurement AFR COMPL  is transmitted to a reconstruction device  14  which is adapted to estimate the values AFR CIL  of the air-fuel ratio of each cylinder  3  by processing the measured values AFR COMPL  of the overall air-fuel ratio.  
         [0020]    After many experimental tests, it has been decided to use a model with two coefficients to represent the relationship existing between the measured values AFR COMPL  of the overall air-fuel ratio and the estimated values AFR CIL  of the air-fuel ratio of each cylinder  3 . This model is summarised by the following equation:  
           AFR   comp ( k )= B   RICOSTR   *AFR   CIL ( k )+ A   RICOSTR   *AFR   COMP ( k− 1)  
         [0021]    where AFR COMP  (k) represents the k th  measured value of the overall air-fuel ratio (i.e. the value measured at the moment k), AFR COMP (k-1) represents the (k−1) th  measured value of the overall air-fuel ratio (i.e. the value measured at the moment k−1), and AFR CIL (k) represents the k th  estimated value of the air-fuel ratio of the last cylinder  3  combusted (i.e. the estimated value of the air-fuel ratio of the cylinder  3  combusted at the moment k). A RICOSTR  and B RICOSTR  are two identified coefficients which are characteristic of the engine  3  and are obtained experimentally.  
         [0022]    Resolving the above equation with respect to AFR CIL (k) provides:  
           AFR   CIL ( k )=1 /B   RICOSTR *( AFR   COMP ( k )− A   RICOSTR   *AFR   COMP ( k −1)  
         [0023]    which can be rewritten as:  
           AFR   CIL ( k )= C 1 *AFR   COMP ( k )− C 2 *AFR   COMP ( k −1)  
           C 1=1 /B   RICOSTR    
           C 2 =A   RICOSTR   /B   RICOSTR    
         [0024]    It has been observed that the coefficients C1 and C2 are not constant but depend on the operating point of the engine  3 , and in particular on the number of revolutions and the torque transmitted (or the quantity of air introduced) by the engine  3 . It is preferable, therefore, to implement a table which supplies the values of C1 and C2 corrected for the current operating point of the engine  3  in a known manner within the reconstruction device  14 .  
         [0025]    It has further been observed that the coefficients A RICOSTR  and B RICOSTR , and therefore the coefficients C1 and C2, are not independent from one another, but are connected by the equation:  
           A   RICOSTR =1− B   RICOSTR    
         [0026]    and therefore:  
           C 2 =C 1−1  
         [0027]    It is therefore possible to reduce the mathematical model to a single coefficient.  
         [0028]    It will be appreciated from the above description that it is possible to estimate the value AFR CIL (k) of the air-fuel ratio of the final cylinder  3  combusted by means of a linear composition of the last measured value AFR COMP (k) and the penultimate measured value AFR COMP (k−1) of the overall air-fuel ratio.  
         [0029]    On each complete revolution of the engine shaft  11 , the sampling device  14  carries out an estimate of the values AFR CIL  of the last four cylinders combusted applying the formulae:  
           AFR   CIL ( k )= C 1 *AFR   COMP ( k )− C 2 *AFR   COMP ( k −1)  
         [0030]    Once the values AFR CIL  of the last four cylinders combusted have been estimated, the reconstruction device  14  supplies the four values AFR CIL  to a synchroniser device  15  which associates each value AFR CIL  with a respective cylinder  3  by means of a predetermined criterion of association stored in a memory of this synchroniser device  15 .  
         [0031]    According to a preferred embodiment, the above-mentioned association criterion is formed by a bi-univocal law of association, which associates each AFR CIL  with a respective cylinder; for instance AFR CIL (k) is associated with the cylinder  3 -I and will subsequently be indicated by the symbol λ CIL1 , AFR CIL (k−1) is associated with the cylinder  3 -III and will subsequently be indicated by the symbol λ CIL3 , AFR CIL (k−2) is associated with the cylinder  3 -II and will subsequently be indicated by the symbol λ CIL2  and AFR CIL (k−3) is associated with the cylinder  3 -IV and will subsequently be indicated by the symbol λ CIL4 .  
         [0032]    The association law is initially determined in a theoretical manner by associating each estimated value AFR CIL  of the air-fuel ratio with the cylinder  3  which, on the basis of the angular position of the engine shaft  11 , is combusted at the moment closest to the moment of measurement of the value AFR COMP  of the overall air-fuel ratio used in the estimate. This association criterion is not always valid, as it does not take account of the output velocity of the exhaust gases from the cylinders  3 , which velocity is substantially different depending on the speed of rotation of the engine  2 .  
         [0033]    The above-mentioned association law is not constant but may be modified during the operation of the engine  2  in order to adapt to the changed operating conditions of this engine  2 . The synchroniser device  15  in particular implements an algorithm which verifies the overall stability of the system in order to verify the accuracy of the current association law. It is also the case that if the association law is not correct the system becomes unstable, i.e. the difference between the estimated values λ CIL  of the air-fuel ratios of the cylinders  3  and a reference value λ TARGET  of the air-fuel ratio over time tends to increase and not to decrease (i.e. tends to diverge and not to converge towards zero).  
         [0034]    If the synchroniser device  15  discovers an instability in the system, this synchroniser device  15  modifies the association law, typically by modifying the bi-univocal association functions by one step; for instance:  
       Initial Association Law  
       [0035]    AFR CIL  (k)→Cylinder  3 -I (λ CIL1 )  
         [0036]    AFR CIL    
         [0037]    (k−1)→Cylinder  3 -III (λ CIL3 )  
         [0038]    AFR CIL  (k−2)→Cylinder  3 -II (λ CIL2 )  
         [0039]    AFR CIL  (k−3)→Cylinder  3 -IV (λ CIL4 )  
       Modified Association Law  
       [0040]    AFR CIL  (k)→Cylinder  3 -III (λ CIL3 )  
         [0041]    AFR CIL  (k−1)→Cylinder  3 -II (λ CIL2 )  
         [0042]    AFR CIL  (k−2)→Cylinder  3 -IV (λ CIL4 )  
         [0043]    AFR CIL  (k−3)→Cylinder  3 -I (λ CIL1 )  
         [0044]    In order to verify the stability of the system, the synchroniser device  15  calculates a value D of divergence of the estimated values λ CIL  of the air-fuel ratio. This divergence value D is calculated using either the value of the derivative over time of the estimated values λ CIL  of the air-fuel ratio of each cylinder  3  or by using the absolute value of the differences between the reference value λ TARGET  and the estimated values λ CIL  of the air-fuel ratio of each cylinder  3 .  
         [0045]    In particular, if the value of the derivative of an estimated value λ CIL  is positive and the estimated value λ CIL  itself is greater than the reference value λ TARGET , there is a potential situation of instability.  
         [0046]    If the divergence value D is higher than a predetermined threshold, the synchroniser device  15  then modifies the association law.  
         [0047]    Once the association has been carried out, the synchroniser device  15  communicates the four values λ CIL  (λ CIL1 , λ CIL2 , λ CIL3 , λ CIL4 ), each of which indicates for a respective cylinder  3  an estimate of the air-fuel ratio with which this cylinder  3  is working, to a calculation device  16 .  
         [0048]    Once the four values λ CIL  have been received, the calculation device  16  calculates a mean value λ mean  of the air-fuel ratio of the four cylinders  3 , and calculates for each cylinder  3  a respective dispersion value Δ CIL  indicating the difference between the corresponding value λ CIL  of the cylinder  3  and the value λ mean.    
         λ mean =(λ CIL1 +λ CIL2 +λ CIL3 +λ CIL4 )/4  
         Δ CIL1 =λ CIL1 +λ mean    
         Δ CIL2 =λ CIL2 +λ mean    
         Δ CIL3 =λ CIL3 +λ mean    
         Δ CIL4 =λ Cil4 +λ mean    
         [0049]    The calculation device  16  communicates the value λ mean  and the values Δ CIL  to a regulator  17  which is adapted to supply, to each injector  4 , the above-mentioned correction signal for the quantity of fuel to be injected into the respective cylinder  3 .  
         [0050]    The regulator  17  receives the reference value λ TARGET  of the air-fuel ratio from a memory  18  and attempts to cause each cylinder  3  to work with an air-fuel ratio which is as close as possible to the reference value λ TARGET . The regulator  17  comprises two control loops  19  and  20 , which are closed (i.e. work in feedback), are separate from one another and are disposed one within the other.  
         [0051]    The control loop  19  corrects the dispersion values Δ CIL  by attempting to bring them to a zero value; in particular, the inner loop  19  has the task of recovering the imbalances of the air-fuel ratio of the various cylinders  3  by making corrections bearing a zero mean value.  
         [0052]    The outer loop  20  carries out an overall control (i.e. without distinction between the various cylinders  3 ), attempting to adapt the mean value λ mean  of the air-fuel ratio of the four cylinders  3  to the reference value λ TARGET .  
         [0053]    The outer loop  20  has a comparator  21 , which compares, in negative feedback, the reference value λ TARGET  with the mean value λ mean  of the air-fuel ratio of the four cylinders  3 ; the error resulting from this comparison is supplied to a control device  22 , which is typically a control device of PID type and is able to generate, as a function of the error signal received as input, a control signal for the injectors  4 .  
         [0054]    The inner loop  19  comprises four control devices  23 , each of which receives as input a respective dispersion value Δ CIL  from the calculation device  16 , is typically a control device of PID type and is able to generate, as a function of the signal received as input, a control signal for a respective injector  4 . The inner loop  19  is for all purposes a closed feedback loop, wherein each dispersion value Δ CIL  is already an error signal to be cancelled out.  
         [0055]    According to a preferred embodiment showed in FIG. 2, a filter  24 , which has a transfer function of a “low pass” type and is adapted to cleanse the values Δ CIL  of high frequency noise, is disposed between the calculation device  16  and the control device  23 .  
         [0056]    The signal from each control device  23  is combined with a signal from the control device  22  by means of a respective adding device  25  and is supplied to a respective injector  4  to correct the quantity of fuel injected into the respective cylinder  3 . In this way, the value of the air-fuel ratio of each cylinder  3  is corrected by combining a first correction signal, which is determined on the basis of a mean value ο mean  of the air-fuel ratio of all the cylinders  3 , with a second correction signal, which is determined on the basis of the estimated value λ CIL  of the air-fuel ratio of the cylinder  3 .  
         [0057]    According to a preferred embodiment, the outer control loop  20  has lower time constants than the inner control loop  19 ; in other words, the outer control loop  20  is slower to respond than the inner control loop  19 . This ensures a greater overall stability of the process of correction of the quantity of fuel injected by the injectors  4 .