Abstract:
A method, and corresponding device, for processing a pressure signal from a pressure sensor mounted in a combustion chamber to detect a combustion in an internal combustion engine. The method provides a pressure signal; filters the pressure signal with a filtration mechanism having a variable gain; generates an output signal representative of the filtered pressure signal; generates a signal representative of the error between the pressure signal and the output signal; and determines if the combustion has occurred during an expansion phase of the thermodynamic cycle.

Description:
BACKGROUND 
     The present invention relates to the field of combustion detection for an internal combustion engine. 
     More precisely, the invention relates, according to a first of its subjects, to a method for processing a pressure signal (p) originating from a pressure sensor mounted in a combustion chamber, for the purpose of detecting combustion in an internal combustion engine, comprising the steps consisting in:
         supplying a pressure signal (p),   filtering the pressure signal (p) by filtering means having a variable gain ( 7 ),   establishing an output signal (out) representative of the filtered pressure signal (p), and   establishing a signal (e) representative of the error between the pressure signal (p) and the output signal (out).       

     Such a method is known to those skilled in the art. 
     However, the various solutions proposed by the prior art in order to modify the bandwidth of the means for filtering the pressure signal lead to nonlinear filters. 
     In addition, these solution are based on the hypothesis that combustion takes place during the compression phase of the cylinder in the combustion chamber. 
     In certain cases, combustion may take place in the expansion phase of the thermodynamic cycle, in particular during combustions called exotic combustions. These combustions pose engine-control problems. 
     BRIEF SUMMARY 
     The object of the present invention is to remedy these disadvantages by proposing a method, and the device for applying it, the object of which is to determine the moment of beginning of combustion, with the value of the pressure signal (p) as the only initial item of information. 
     Although it is not limited thereto, the present invention is advantageously applied in an engine called an HCCI, for “Homogeneous Charge Compression Ignition” wherein the air-fuel mixture is mixed in the most homogeneous possible manner and compressed quite highly in order to reach the auto-ignition point. 
     In particular, the combustion in an HCCI engine begins in several locations at a time, the result of which is an almost simultaneous combustion of the whole of the air-fuel mixture. Engine combustion is then more difficult to control. 
     With this objective in view, the device according to the invention, moreover complying with the preamble cited above, is essentially characterized in that it also comprises a step consisting in:
         determining whether combustion has taken place during an expansion phase of the thermodynamic cycle.       

     In one embodiment, the method according to the invention also comprises the steps consisting in:
         establishing a signal (p 1 ) representative of the time differential filtered by a low-pass filter of the pressure signal (p), the establishment of the signal (p 1 ) and the filtering of the time differential being carried out by a block ( 1 ),   establishing a signal (p 2 ) representative of the comparison between the value of the signal (p 1 ) and a threshold (threshold  1 ),   establishing a signal (p 5 ) representative of the comparison between the value of the pressure signal (p) and a threshold (threshold  4 ),   establishing a signal (Q) representative of the beginning of the expansion phase in the thermodynamic cycle by combining the signals (p 2 ) and (p 5 ).       

     Advantageously, the time differential of the pressure signal (p) represents an image of the quality of combustion. 
     Preferably, the method according to the invention also comprises the steps consisting in:
         establishing a signal (e 4 ) representative of the comparison between the value of the error signal (e) and a threshold (threshold  2 ),   establishing an intermediate signal (X) representative of a great and rapid increase in the pressure in the combustion chamber in the expansion phase, by combining the signals (Q), (p 2 ) and (e 4 ).       

     Optionally, the method according to the invention also comprises a step consisting in:
         determining whether the combustion has taken place in a compression phase of the thermodynamic cycle.       

     In this case, the method according to the invention advantageously also includes a step consisting in:
         establishing a signal (e 6 ) representative of the comparison between the value of the error signal (e) and a threshold (threshold  3 ).       

     In one embodiment, the method according to the invention also comprises a step consisting in:
         generating a signal (z) for controlling the filtering means having a variable gain ( 7 ) in order to obtain a variable gain (K 0 , K 1 ) as a function in the value of the intermediate signal (X) or of the signal (e 6 ).       

     The use of a single integrator (1/S) downstream of the filtering means having a variable gain, as described below, is advantageously simple and of limited cost. 
     According to another of its objects, the present invention also relates to a device capable of applying the method according to the invention. 
     Accordingly, the device according to the invention is a device for acquiring and for processing a pressure signal (p) originating from a pressure sensor mounted in a combustion chamber of an internal combustion engine, for the purpose of detecting a combustion therein, the device being capable of applying the method as claimed in any one of the preceding claims, and comprising:
         means for acquiring the pressure signal (p),   means for processing said pressure signal (p),   means for detecting the evolution of the pressure signal (p),   filtering means having a variable gain ( 7 ) in order to filter the pressure signal (p),   means (1/S) for establishing an output signal (out) representative of the filtered pressure signal (p), and   means for establishing a signal (e) representative of an error that can exist between the pressure signal (p) and the output signal (out).       

     According to the invention, the device is essentially characterized in that the means for detecting the evolution of the pressure signal (p):
         are configured to at least detect a pressure drop from the pressure signal (p), so as to determine whether the thermodynamic cycle is in the expansion phase, and   comprise means for detecting an increase in pressure during said expansion phase of the thermodynamic cycle, so as to determine whether the combustion took place during said expansion phase of the thermodynamic cycle.       

     The device according to the invention also comprises:
         first comparison means ( 2 ) for establishing a signal (p 2 ) representative of the comparison between the value of the signal (p  1 ) and a threshold (threshold  1 ),   second comparison means ( 5 ) for establishing a signal (p 5 ) representative of the comparison between the value of the pressure signal (p) and a threshold (threshold  4 ),   third comparison means ( 4 ) for establishing a signal (e 4 ) representative of the comparison between the value of the error signal (e) and a threshold (threshold  2 ),   means ( 3 ) for establishing a signal (Q) representative of the beginning of the expansion phase in the thermodynamic cycle by combining the signals (p 2 ) and (p 5 ).       

     Preferably, the means ( 1 ) for establishing a signal (p 1 ) representative of the time differential of the pressure signal (p) filtered by a low-pass filter comprise a linear filter the transfer function of which is as follows:
 
 F ( s )= s/ (1+ τs )
 
     wherein s is the Laplace variable and τ is the time constant chosen as a function of the frequency band of the noise of the pressure signal (p). 
     Also preferably, the means ( 3 ) for establishing a signal (Q) representative of the beginning of the expansion phase in the thermodynamic cycle comprise an RS flip-flop the input (S) of which is connected to the output of a logic AND port the inputs of which are connected to the signals (p 2 ) and (p 5 ), and the input (R) of which is connected to the inverted signal (p 5 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will appear more clearly on reading the following description given as an illustrative and nonlimiting example and made with reference to the appended figures in which: 
         FIG. 1  represents the evolution over time of a pressure signal when combustion takes place in the compression phase, 
         FIG. 2  represents the evolution over time of a pressure signal when combustion takes place in the expansion phase, 
         FIG. 3  represents an embodiment of the device according to the invention, and 
         FIG. 4  represents an embodiment of the block  3  of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In the present description, no distinction is made between a signal or the communication line through which it passes. 
       FIG. 1  illustrates the speed of a pressure signal, that is to say originating from a pressure sensor mounted in a combustion chamber, when the engine has a normal behavior, that is to say combustion takes place in the compression phase. 
     In the compression phase, the pressure increases in the combustion chamber. 
     When combustion takes place, the pressure increases in the combustion chamber. 
     In this example, combustion takes place between the moments t 0  and t 1 . The two effects are combined, and the signal p passes through a maximum, in this instance corresponding to the time t 1 . 
     However it happens that combustion takes place during an expansion phase. 
     In the expansion phase, the pressure reduces in the combustion chamber. 
     In this case, illustrated in  FIG. 2 , the signal p passes through a first maximum corresponding to the top dead center, and a second maximum, in this instance corresponding to the time t 3 , combustion taking place between the moments t 2  and t 3 . 
     Such combustions risk damaging the control of the engine, and even the engine itself. Also, the object of the present invention is to detect the moments of combustion, in particular when the latter take place in the expansion phase, and thanks only to items of information originating from a pressure sensor mounted in a combustion chamber. 
     Accordingly, a pressure sensor delivers a pressure signal p representative of the pressure in the combustion chamber in which it is mounted. 
     A pressure signal p is conventionally noised. 
     One of the subjects of the present invention relates to a device for processing a pressure signal p, illustrated in  FIG. 3 . 
     The device according to the invention comprises filtering means  1  (block  1 ), configured to filter the pressure signal p. 
     The filtering means produce a linear filter the transfer function of which is as follows:
 
 F ( s )= s/ (1+τ s )   (1)
 
     wherein s is the Laplace variable. 
     The filtering means  1  deliver at the output a signal p 1 . The signal p 1  is a signal equivalent to the differential of the raw pressure signal p received by the filtering means  1  at the input, this differential being filtered by a low-pass filter in order to limit the noise. 
     According to the invention, the time constant T of the equation (1) is chosen as a function of the frequency band of the noise of the pressure signal p. 
     The signal p 1  is brought to the input of a block  2  the output of which delivers a signal p 2 . 
     The block  2  is configured to perform a thresholding function relative to a negative or zero threshold threshold 1  (first threshold), such that the binary signal p 2  is equal to 1 if the value of the signal pl is less than or equal to the threshold threshold 1  , and equal to 0 otherwise. 
     This therefore gives:
 
p2=1 if p1&lt;threshold1
 
p2=0 if p1&gt;threshold1
 
     where threshold 1 ≦0 
     The signal p 2  takes the value 1 when the differential (filtered) p 1  of the pressure signal p is negative and less than the value of the threshold threshold 1 , that is to say when the pressure falls in the combustion chamber, and this fall is significant, the threshold threshold 1  being equivalent to a certain gradient of the pressure signal p. 
     In a similar manner, the blocks  4  and  6  produce a thresholding step. 
     The block  4  is configured to produce a thresholding step relative to a positive threshold threshold 2  (third threshold), such that the binary signal e 4  is equal to 1 if the value of the error signal e is greater than or equal to the threshold threshold 2 , and equal to 0 otherwise. 
     The error signal e corresponds to the output signal of the device OUT subtracted from the pressure signal p. 
     This then gives:
 
e4=1 if e≧threshold2
 
e4=0 if e&lt;threshold2
 
     where threshold 2 ≧0. 
     The signal e 4  takes the value 1 when the error signal e is positive and greater than the threshold threshold 2 . 
     The block  4  is configured to detect an increase in the pressure in the expansion phase (for example: combustion). 
     The block  6 , which is optional, is configured to produce a thresholding step relative to a positive threshold threshold 3  (fourth threshold), such that the binary signal e 6  (fourth signal) is equal to 1 if the value of the error signal e is greater than or equal to the threshold threshold 3 , and equal to 0 otherwise. 
     This then gives:
 
e6=1 if e≧threshold3
 
e6=0 if e&lt;threshold3
 
     where threshold 3 ≧0 and, preferably, threshold 3 &gt;threshold  2 . 
     The signal e 6  takes the value 1 when the error signal e is positive and greater than the threshold threshold 3 . 
     The block  6  is configured to detect an increase in pressure in the compression phase (for example: combustion). 
     The block  5  is also configured to perform a thresholding step relative to a positive threshold threshold 4  (second threshold), such that the binary signal e 6  is equal to 1 if the value of the pressure signal p is greater than or equal to the threshold threshold 4 , and equal to 0 otherwise. 
     This then gives:
 
P5=1 if p≧threshold 4
 
p5=0 if p&lt;threshold 4
 
     where threshold 4 ≧0. 
     The signal p 5  takes the value 1 when the pressure signal p is positive and greater than the threshold threshold 4 . 
     The block  3  receives at the input the signals p 2 , p 5 , e 4  and e 6 , and delivers at the output a signal z transmitted to the input of the block  7 . 
     The block  3  is illustrated in greater detail in  FIG. 4 . It comprises an RS flip-flop comprising an input R and an input S, and delivering an output signal Q. 
     The signals p 2  and p 5  are connected to a respective input of an AND port the output of which is connected to the input S of the flip-flop. 
     The input R of the flip-flop is connected to the inverted signal p 5 . 
     This then gives:
 
S=1 if p2=1 and p5=1
 
S=0 otherwise
 
R=1 if p5=0
 
R=0 otherwise.
 
     When the input S is set to 1, the output Q of the flip-flop goes to 1. 
     When the input R is set to 1, the output Q of the flip-flop goes to zero. 
     The transition to 1 of the output Q indicates that the value of the pressure signal p is diminishing (because p 2 =1) but that this value is above the threshold threshold 4  (because p 5 =1). 
     Therefore, the transition to 1 of the output Q makes it possible to determine the beginning of the expansion phase in the thermodynamic cycle of combustion. 
     However, according to the invention, it is not desired to increase the bandwidth of the filtering means for the whole of the expansion phase but only when a combustion takes place. For this reason, the combustion is detected by means of the signals e 4  and e 6  as described below. 
     The output signal from the flip-flop Q is connected as an input of an AND port to three inputs the second input of which is connected to the inverted signal p 2 , and the third input is connected to the signal e 4 . 
     The AND port with three inputs delivers an intermediate signal X at the output. 
     This then gives:
 
X=1 if e4=1 and Q=1 and p2=0
 
X=0 otherwise.
 
     The intermediate signal X makes it possible to detect a great and rapid increase in the pressure in the combustion chamber, that is to say a steep gradient of the pressure signal p in the expansion phase. 
     Consequently, the intermediate signal X makes it possible to control the command of the increase in the bandwidth of the filtering means. 
     The condition e 4 =1 in the construction of the intermediate signal X makes it possible to increase the bandwidth of the filtering only during the rising phases of the pressure signal p. 
     The optional step consisting in determining whether combustion has taken place during a compression phase of the thermodynamic cycle is carried out by the logic OR port an input of which is connected to the signal X, and the other to the signal e 6 . 
     Therefore, the output signal z from the OR port is the output signal z from the block  3 . 
     This then gives:
 
z=1 is X=1 or if e6=1
 
z=0 otherwise.
 
     The condition e 6 =1 in the construction of the signal z makes it possible to detect a rapid and great amplitude increase (as a function of the value of the threshold threshold 3 ) of the value of the pressure signal p, in the compression phase. 
     The output signal z of the block  3  is connected as an input of the block  7  ( FIG. 3 ). 
     The block  7  is configured to perform a step of variable gain and deliver at the output a signal Der. 
     The variable gain generated by the block  7  is a function of the value of the output signal z of the block  3 . 
     Let K 0  be a first gain that can be generated by the block  7 , and K 1  a second gain that can be generated by the block  7 , thus giving: 
     If z=0, the block  7  generates the gain K 0   
     If z=1, it is considered that a combustion has taken place, and the block  7  generates the gain K 1 . 
     The choice of the value of the gains K 0  and K 1  makes it possible to define the bandwidth of the filtering means. 
     Preferably, K 1 &gt;K 0 , which makes it possible to filter the pressure signal p with a large bandwidth, corresponding to the gain K 1 , when the signal z is equal to 1, that is to say:
         either when the pressure in the combustion chamber increases rapidly, and this increase is of great amplitude, and the cylinder of the combustion chamber is in the compression phase,   or when the pressure increases rapidly and the cylinder of the combustion chamber is in the expansion phase.       

     In the other cases, that is to say when the signal z is equal to zero, the device according to the invention makes it possible to filter the pressure signal p with a small bandwidth, corresponding to the gain K 0 , in order to filter the high-frequency and small-amplitude components (noise). 
     The output signal Der of the block  7 , corresponding to the differential of the filtered pressure signal p, is connected to the input of an integrator (1/S) the output of which delivers an output signal OUT, corresponding to the filtered pressure signal p, connected to the aforementioned subtractor for the construction of the error signal e. 
     The assembly comprising the subtractor, the block  7  and the integrator forms a first-order filter when the gain of the block  7  is fixed. In this case, the gain K 0  (or K 1 ) fixes the bandwidth of the filter. 
     By virtue of the invention, the pressure signal p can be filtered even during exotic combustion, that is to say in the expansion phase, when combustion takes place after top dead center. 
     The output signal OUT can be connected to control means, in particular to the electronic control unit (ECU) of the engine.