Abstract:
A device for processing a signal of pressure measurement of an internal combustion engine combustion chamber. The device obtains, by a low-pass filter of the pressure measuring signal, a filtered signal, subtracts the filtered signal from the pressure measuring signal to obtain a difference signal, compares the difference signal to a predetermined threshold value, and determines a combustion starting time as a time at which the difference signal reaches the predetermined threshold value.

Description:
BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   The present invention relates to a device for processing a pressure-measurement signal of a combustion chamber of an internal combustion engine with a view to determining an instant of start of combustion. It also relates to a method employed by this device. 
   II. Description of Related Art 
   Such a device is the object of the American patent application published under the number U.S. 2003/0145829. In that document, the pressure-measurement signal is processed by means of a wavelet transformation. An analysis of the coefficients of the wavelet transformation of the pressure-measurement signal makes it possible to detect an instant of start of combustion. More precisely, a sudden increase of the absolute values of these coefficients constitutes an indicator of the instant of start of combustion. 
   Such processing of the pressure-measurement signal, which consumes considerable computing time, is costly and is difficult to install in a traditional engine control computer. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention seeks to remedy this disadvantage by providing a device capable of determining an instant of start of combustion by means of simple processing of the pressure-measurement signal. 
   The object of the invention is therefore a device for processing a pressure-measurement signal of a combustion chamber of an internal combustion engine with a view to determining an instant of start of combustion, characterized in that it comprises:
         a low-pass filter for obtaining a filtered signal from the pressure-measurement signal, and means for subtracting the filtered signal from the pressure-measurement signal to obtain a difference signal,   means for comparing the difference signal with a predetermined threshold value, and   means for determining an instant of start of combustion as the instant at which the difference signal attains the predetermined threshold value.       

   The processing applied to the pressure-measurement signal is effectively simple, since it involves low-pass filtering, and mere comparison of the difference between the filtered signal and the original signal with a predetermined threshold value makes it possible reliably to determine the instant of start of combustion. 
   In one possible embodiment of the invention, the low-pass filter is provided with digital means for first-order low-pass filtering, and the comparison means are provided with a digital comparator. 
   Digital implementation of the invention is possible, since the processing operations applied to the pressure-measurement signal are simple. The traditional engine control computers are capable of supporting such processing operations. In addition, the merit of digital implementation is that it allows reliable and predictable results to be obtained by simulation. Furthermore, adjustment of the device parameters is simple. 
   Optionally, the digital parameters of the low-pass filter and the comparison means are chosen in such a way that only operations between integral numbers are generated during processing of the pressure-measurement signal. 
   Also optionally, the digital parameters of the low-pass filter and the comparison means are chosen in such a way that any division of an integral number is replaced by displacement of its binary representation to the right during processing of the pressure-measurement signal. 
   In this way the digital operations are facilitated and processing thereof is correspondingly simplified. 
   In this embodiment also, the low-pass filter can be provided with a digital Euler integrator. Such a digital integrator is simple to use. 
   Advantageously, the device for processing a pressure-measurement signal is provided with a nonlinear processing element disposed in such a way as to broaden the pass band of the low-pass filter when the difference signal attains the predetermined threshold value. 
   In this way, it is possible to detect a possible second instant of start of combustion after detection of a first instant of start of combustion. 
   In another possible embodiment of the invention, the low-pass filter is provided with analog means for first-order low-pass filtering, and the nonlinear processing element is provided with a diode connected in parallel with the low-pass filter. 
   The advantage of such an analog embodiment is its cost. 
   In this embodiment, the device for processing a pressure-measurement signal can be provided with an analog circuit of the comparator type in order to deliver a step signal that changes from a zero value to a nonzero value when the difference signal exceeds the predetermined threshold value. 
   This step signal constitutes an alarm that can trigger subsequent processing steps. 
   Another object of the invention is a method for processing a pressure-measurement signal of a combustion chamber of an internal combustion engine with a view to determining an instant of start of combustion, characterized in that it comprises the following steps:
         low-pass filtering of the pressure-measurement signal in such a way as to obtain a filtered signal, and subtraction of the filtered signal from the pressure-measurement signal to obtain a difference signal,   comparison of the difference signal with a predetermined threshold value,   determination of the instant of start of combustion as an instant at which the difference signal attains the predetermined threshold value.       

   Advantageously the pass band of the low-pass filtering is broadened when the difference signal attains the predetermined threshold value. 
   As indicated hereinabove, this makes it possible to detect a second instant of start of combustion, after detection of a first instant of start of combustion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by means of the description hereinafter, given solely by way of example, and referring to the attached drawings, wherein: 
       FIG. 1  schematically represents the general structure of a device according to the invention for processing a pressure-measurement signal; 
       FIGS. 2   a  and  2   b  are diagrams representing the time evolution of a pressure-measurement signal in a combustion chamber of an internal combustion engine; 
       FIG. 3  schematically represents the structure of the device of  FIG. 1  according to a possible analog embodiment; and 
       FIG. 4  schematically represents a combustion chamber equipped with a device according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Device  10  represented in  FIG. 1  is adapted to receive at the input a pressure-measurement signal Pr from a combustion chamber of an internal combustion engine. 
   This device  10  is provided with a subtracter  12  to obtain, from pressure-measurement signal Pr and a filtered signal Pf, a first difference signal ε between these two signals. 
   Optionally, it is additionally provided with an element  14  for nonlinear processing of the first difference signal ε, which will be described in detail hereinafter, to deliver a second difference signal. 
   This second difference signal, or in the absence of element  14  the first difference signal, is delivered to a device  16  for detection of an instant of start of combustion. This device  16  is provided with means for comparing the second difference signal with a predetermined threshold value σ, and with means for determining the instant of start of combustion as an instant at which the second difference signal attains the predetermined threshold value. 
   The filtered signal Pf is obtained from the pressure-measurement signal Pr by means of a low-pass filtering device provided with the following elements:
         an amplifier  18  of gain F=k f  applied to the pressure-measurement signal Pr, making it possible to regulate indirectly the cut-off pulsation that must be imposed on the pressure-measurement signal Pr by the low-pass filtering;   an adder  20 , to add the result of the processing applied by nonlinear processing element  14  to the signal Pr amplified by amplifier  18 , so as to amplify the signal Pr in such a way that the pass band of the low-pass filtering is broadened when the second difference signal attains the predetermined threshold σ; and   a low-pass filter  22 , whose parameters are predetermined, and which receives at the input the output signal of adder  20  and delivers at the output the filtered signal Pf delivered at the input of subtracter  12 .       

   In one possible embodiment of the invention, device  10  as described hereinabove is implemented by means of digital elements. 
   In particular, low-pass filter  22  can be provided with digital means for first-order low-pass filtering, such as a digital Euler integrator that performs the following operation: 
                     G   ⁡     (   z   )       =       T   e       z   -   1         ,           (   1   )               
where Te is the sampling frequency, z=e i2πfTe  is the transformation operator in z and f is the frequency.
 
   As another example, the comparison means are provided with a digital comparator. The digital parameters of the low-pass filter and of the comparison means can be chosen in such a way as to generate only operations between integral numbers and/or in such a way as to replace every division of an integral number by a displacement of its binary representation to the right during processing of the pressure-measurement signal. 
   Nonlinear processing element  14  executes an operation that is defined, for example, by the following system of equations:
 
 NL ( x )= m·x  if  x≧σ 
 
 NL ( x )=0 if  x&lt;σ   (2)
 
   Thus the pass band of digital low-pass filter  22  depends on the amplification coefficient k f  of amplifier  18  and on the output of nonlinear processing element  14 , in such a way that, when the output of nonlinear processing element  14  is zero, or in other words when the first difference signal ε is below the predetermined threshold σ, the cut-off pulsation of low-pass filter  22  at −3 dB ω c1  is given by the following equation: 
   
     
       
         
           
             
               
                 
                   ω 
                   
                     c 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
                 = 
                 
                   
                     
                       1 
                       Te 
                     
                     · 
                     arc 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                     
                       ⌊ 
                       
                         
                           
                             k 
                             f 
                             2 
                           
                           + 
                           
                             2 
                             ⁢ 
                             
                               k 
                               f 
                             
                           
                           - 
                           2 
                         
                         
                           2 
                           ⁢ 
                           
                             ( 
                             
                               
                                 k 
                                 f 
                               
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       ⌋ 
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   On the other hand, when the output of nonlinear processing element  14  is nonzero, or in other words when it amplifies the difference signal by a factor m, the cut-off pulsation of low-pass filter  22  at −3 dB ω c2  is given by the following equation: 
   
     
       
         
           
             
               
                 
                   ω 
                   
                     c 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
                 = 
                 
                   
                     
                       1 
                       Te 
                     
                     · 
                     arc 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   cos 
                   ⁢ 
                   
                     
                       ⌊ 
                       
                         
                           
                             
                               ( 
                               
                                 m 
                                 + 
                                 
                                   k 
                                   f 
                                 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 m 
                                 + 
                                 
                                   k 
                                   f 
                                 
                               
                               ) 
                             
                           
                           - 
                           2 
                         
                         
                           2 
                           ⁢ 
                           
                             ( 
                             
                               m 
                               + 
                               
                                 k 
                                 f 
                               
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       ⌋ 
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   The value of the threshold σ is chosen such that it is exceeded by the first difference signal ε at the instant of start of combustion. 
   When this instant of start of combustion is detected, nonlinear processing element  14  causes broadening of the pass band of low-pass filter  22 , in such a way that the first difference signal ε resulting from subtraction of the pressure-measurement signal Pr and of the filtered signal Pf is greatly reduced. In this way it is possible to detect a second instant of start of combustion as an instant at which the first difference signal ε once again attains the predetermined threshold value σ. 
   In  FIG. 2   a , curve  22  represents the time evolution of pressure-measurement signal Pr in the combustion chamber of the internal combustion engine. It is observed that the instant of start of combustion corresponds to the first instant at which the pressure-measurement signal intensifies abruptly, at point  24 . 
     FIG. 2   b  represents a detail of curve  22  around point  24 . Curve  26  corresponds to the time evolution of the first difference signal ε, which exceeds the predetermined threshold value σ for the first time at point  24 . Finally, curve  28  represents a step signal that is delivered at the output of the processing device and that assumes a nonzero value when the instant of start of combustion is detected. 
   In another possible embodiment of the invention represented in  FIG. 3 , processing device  10  is constructed by means of analog elements. 
   The assembly represented in this figure is provided with a first input stage  30 , in which the voltage V A  at a point A is related to the input voltage V in  of the pressure-measurement signal by the following equation: 
                       V   A       V   in       =       1   +       R   1       R   2         =     A   v         ,           (   5   )               
where R 1  and R 2  are two values of resistors.
 
   Between point A and a point B, first input stage  30  is provided with a circuit comprising a diode D disposed in parallel with a first-order low-pass filter (provided with a resistor R 3  associated with a capacitor C). This circuit constitutes a nonlinear processing element. In effect, when diode D is nonconducting, the voltage V B  at point B is given by the following equation 
                     V   B     =         1     1   +     sCR   3         ⁢     V   A       =         A   v     ⁢     V   in     ⁢     1     1   +     sCR   3           =       A   v     ⁢     V   filtered             ,           (   6   )               
where s is the Laplace operator and V filtered  is the voltage of the filtered signal Pf.
 
   The input signal can therefore be considered as having been amplified then filtered by a first-order low-pass filter, whose cut-off pulsation at −3 dB ω c1  is given by the following equation: 
   
     
       
         
           
             
               
                 
                   ω 
                   
                     c 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
                 = 
                 
                   
                     1 
                     
                       CR 
                       3 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   If σ denotes the threshold voltage of diode D, the latter remains nonconducting as long as V A −V B  is below σ. 
   Now, V A −V B =A v (V in −V filtered )=A v ε, represents the error between the input signal V in  and the low-pass filtered signal V filtered . 
   Diode D is then nonconducting as long as the value of ε is below a predetermined threshold and it becomes conducting when the error ε exceeds this predetermined value. 
   When the diode becomes conducting, the pass band of the low-pass filter is broadened by the presence of a resistor R 4  at the diode input and in parallel with resistor R 3  (disregarding the voltage drop at the diode terminals). 
   The diode therefore behaves as a nonlinear processing element whose function is to broaden the pass band of the low-pass filtering when the diode is conducting. 
   The threshold voltage σ of diode D is chosen in such a way that the instant at which the voltage at the diode terminals exceeds this threshold voltage for the first time corresponds to the instant of start of combustion (instant at which the large-amplitude high-frequency rise of the pressure-measurement signal occurs). 
   By means of the gain A v  given by equation (5), it is possible to modify the value of the threshold σ. That makes it possible easily to regulate the processing device and consequently to make it very flexible and usable for every type of combustion. 
   If I D  denotes the current passing through diode D, the voltage at the terminals of R 4  is proportional to this current:
 
 V   R4   =R   4   I   D .  (8)
 
   In view of the considerations stated hereinabove, the instant of start of combustion also corresponds to the instant at which the voltage V R4  exceeds a certain threshold value s v . The voltage V R4  at the terminals of R 4  is therefore measured and amplified by a differential amplifier in a second stage  32  of the assembly of  FIG. 3 . 
   If G denotes the gain of this differential amplifier, the output voltage of second stage  32  is then given by the following equation:
 
 V   det   =G·V   R4 .  (9)
 
   This voltage Vdet is then compared with the threshold value S v =G·s v  by an analog comparator in a third stage  34  of the assembly of  FIG. 3 . 
   Finally, the signal delivered at the output of third stage  34  constitutes the “clock” input of an analog flip-flop  36 , whose input D is constantly set to logical value “1”. This flip-flop must be reset to zero at the end of each thermodynamic cycle of the engine. 
   The output signal of analog flip-flop  36  is a step signal whose rising front corresponds to the instant of start of combustion in the thermodynamic cycle: it corresponds to signal  28  represented in  FIG. 2   b.    
   Finally,  FIG. 4  represents a combustion chamber  40  of an internal combustion engine. This combustion chamber is associated with a processing device  10  such as described hereinabove, receiving at the input a pressure-measurement signal Pr delivered by a pressure sensor  42 . The data delivered by processing device  10  and those delivered by a crank-angle sensor  44  disposed in combustion chamber  40  are transmitted to an electronic box  46  for subsequent processing operations. 
   As indicated hereinabove, device  10  for processing the pressure-measurement signal may be of analog or digital type. 
   It is clear that a device for processing a pressure-measurement signal such as described hereinabove makes it possible in simple manner to detect one or more instants of start of combustion of a combustion chamber of an internal combustion engine. 
   It will be noted, however, that the invention is not limited to the embodiments described hereinabove. 
   In fact, in particular, as a variant, low-pass filter  22 , which in the digital embodiment executes a Euler integration, can be replaced by a low-pass filter that executes, for example, a trapezoidal integration based on the following transfer function: 
   
     
       
         
           
             
               
                 
                   G 
                   ⁡ 
                   
                     ( 
                     z 
                     ) 
                   
                 
                 = 
                 
                   
                     Te 
                     2 
                   
                   · 
                   
                     
                       
                         ( 
                         
                           z 
                           + 
                           1 
                         
                         ) 
                       
                       
                         ( 
                         
                           z 
                           - 
                           1 
                         
                         ) 
                       
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 10 
                 )