Abstract:
A device and a method for determining pressure fluctuations in a fuel supply system provide two signal filters to enable determining as much information about pressure fluctuations as possible with minimal sensor use. A sensor signal which is characteristic of a pressure in the area of a fuel injector is filtered using the two filters, which have filter characteristics that differ from one another.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a device and a method for determining pressure fluctuations in a fuel supply system including, e.g., a fuel injector. 
   BACKGROUND INFORMATION 
   Utilization of the sensor effect of the piezoelectric actuator for measuring the frequency of a pressure wave, which is generated by the opening and closing of the nozzles, is described in published German patent document DE 102 17 592, for example. The piezoelectric actuator is used to open and close the control valve of the fuel injector in order to control the injection operation. The fact that the piezoelectric actuator is able to convert electric voltage into force and electric charge into linear expansion is utilized for this purpose. The reversal of these effects is utilized to convert the mechanical force exerted on the piezoelectric actuator into an electrical voltage signal. This is known as the sensor effect. 
   SUMMARY OF THE INVENTION 
   The device and the method according to the present invention for determining pressure fluctuations in a fuel supply system provide a first filter and a second filter, to which filters a signal characterizing the pressure in the area of the first fuel injector is supplied, the first filter having a first filter characteristic and the second filter having a second filter characteristic which differs from the first filter characteristic. This arrangement makes it possible to filter the signal characterizing the pressure in the area of the first fuel injector in different ways, so that different information for processing may be obtained from the signal. The signal characterizing the pressure in the area of the first fuel injector is thus able to be analyzed in various ways. 
   It is particularly advantageous when a first limiting frequency of the first filter is selected in such a way that it is higher than first frequencies of low-frequency pressure fluctuations to be anticipated due to the fuel delivery by a fuel pump and/or low-frequency pressure fluctuations to be anticipated due to a pressure drop during at least one injection operation, a pass-band of the first filter below the first limiting frequency being selected in such a way that it includes the first frequencies. In this way, information about possible low-frequency pressure fluctuations due to the fuel delivery by the fuel pump, and/or due to a pressure drop during at least one injection operation, may be obtained in a targeted manner from the signal characterizing the pressure in the area of the first fuel injector, i.e., the information about possible low-frequency pressure fluctuations is differentiated or separated from other information in this signal. In addition, further processing of the filtered information of the signal characterizing the pressure in the area of the first fuel injector, obtained via the first filter, may be performed. 
   It is also advantageous when a limiting frequency of the second filter is selected in such a way that it is lower than a second frequency or second frequencies of high-frequency pressure fluctuations to be anticipated which occur during an injection operation of the first fuel injector, a pass-band of the second filter above the second limiting frequency being selected in such a way that it includes the second frequency or the second frequencies. In this way, information about high-frequency pressure fluctuations due to an injection operation of the first fuel injector may be determined from the signal characterizing the pressure in the area of the first fuel injector, and differentiated or separated from information of the signal characterizing the pressure in the area of the first fuel injector. The information of the signal characterizing the pressure in the area of the first fuel injector, obtained via the second filter, may then also be conveyed for suitable further processing in a targeted manner. 
   The two filters may be implemented in a simple manner if the first filter is designed as a low-pass or band-pass filter and the second filter is designed as high-pass or band-pass filter. 
   A further advantage arises if a control unit is provided to which a first output signal of the first filter is supplied and which controls the pressure in a fuel line of the fuel supply system as a function of the first output signal. In this way, the information of the signal characterizing the pressure in the area of the first fuel injector, obtained from the first filter, may be used for regulating the pressure in the fuel line of the fuel supply system. 
   A further advantage arises if a determination unit is provided to which a second output signal of the second filter is supplied and which determines a sound velocity of the fuel as a function of the second output signal. In this way, the information of the signal characterizing the pressure in the area of the first fuel injector, obtained from the second filter, may also be analyzed, e.g., in order to determine an error in the injected fuel quantity and to increase the metering accuracy of the fuel supply. 
   It is also advantageous if at least one sensor is provided which generates a signal as a function of an existing pressure, the at least one sensor being situated in the area of the first fuel injector. In this way, the pressure may be determined at a point of the fuel supply system at which the pressure includes a representative part of the low-frequency pressure characteristic in a common fuel supply due to the fuel supply by the fuel pump and/or due to the pressure drop during at least one injection operation of the first fuel injector, as well as a representative part of the high-frequency pressure characteristic in a fuel line between the common fuel supply and the first fuel injector, this high-frequency pressure characteristic being a function of the injection operation of the first fuel injector. The low-frequency part and the high-frequency part of the signal characterizing the pressure in the area of the first fuel injector, determined by the sensor, may be separated from one another using the two filters and may be conveyed for suitable further processing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic illustration of a fuel supply system. 
       FIG. 2  shows a block diagram for illustrating an example embodiment of the device, as well as the corresponding method, according to the present invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , reference numeral  5  indicates a fuel supply system, e.g., of a motor vehicle. Fuel supply system  5  supplies, for example, a combustion chamber of an engine with fuel, diesel fuel in the present example, via at least one injection valve, which is also referred to as a fuel injector. According to the example in  FIG. 1 , four fuel injectors  10 ,  15 ,  20 ,  25  are provided which directly inject the fuel into assigned cylinders of the engine (not shown in  FIG. 1  for the sake of clarity). A high pressure pump  40 , having an upstream fuel metering unit (not shown in  FIG. 1  for the sake of clarity), supplies fuel from a fuel tank (also not shown in  FIG. 1 ) via a first fuel line  95 , a pressure regulation valve  60 , and a second common fuel line  100  to what is known as a rail  85  which represents a third common fuel line in the form of a fuel pressure container and distributes the supplied fuel to individual fuel injectors  10 ,  15 ,  20 ,  25  via fuel lines  65 ,  70 ,  75 ,  80 , respectively. Pressure regulation valve  60  could alternatively also be situated on rail  85  or on high pressure pump  40 . Individual fuel lines  65 ,  70 ,  75 ,  80  are high pressure lines. Fuel is supplied from rail  85  to a first fuel injector  10  via a first fuel line  65 ; to a second fuel injector  15  via a second fuel line  70 ; to a third fuel injector  20  via a third fuel line  75 ; and to a fourth fuel injector  25  via a fourth fuel line  80 . First fuel injector  10  includes a nozzle  105  via which fuel is directly injected into a first cylinder. Second fuel injector  15  includes a second nozzle  110  via which fuel is directly injected into a second cylinder. Third fuel injector  20  includes a third nozzle  115  via which fuel is directly injected into a third cylinder. Fourth fuel injector  25  includes a fourth nozzle  120  via which fuel is directly injected into a fourth cylinder. As described in  FIG. 1 , the four cylinders are not shown for the sake of clarity. Fuel could alternatively be injected into a cylinder via multiple fuel injectors. Intake manifold fuel injection may alternatively be considered for direct injection, in a gasoline engine in particular. 
   Continuing with  FIG. 1 , a controller  90  is provided, which controls pressure regulation valve  60  for setting an intended fuel pressure in common fuel lines  95 ,  100 ,  85 . Moreover, controller  90  controls four fuel injectors  10 ,  15 ,  20 ,  25  with respect to a predefined opening time and a predefined open duration, in order to inject an intended fuel quantity into the cylinders in an intended time window. This takes place in a suitable manner for setting a torque intended by the driver, predefined via an acceleration pedal of the vehicle, or for setting a predefined air/fuel mixture ratio. A pressure sensor  55  is situated in at least one of high pressure lines  65 ,  70 ,  75 ,  80 , which sensor measures the fuel pressure in this high pressure line and conveys the measuring result to controller  90 . Pressure sensor  55  is situated in the area of the assigned fuel injector. As described in published German patent document DE 102 17 592, for example, the pressure sensor may be identical with a piezoelectric actuator which may be provided in an example as a control element for opening and closing the nozzle of the respective fuel injector. In the example in  FIG. 1 , pressure sensor  55  is situated in first high pressure line  65  in the area of first fuel injector  10 . The time signal of the pressure characteristic, detected by the pressure sensor, is conveyed to controller  90 . In a similar manner, one or several of high pressure lines  70 ,  75 ,  80  may each be equipped with a pressure sensor and a signal line to controller  90 . 
   Fuel supply system  5  shown in  FIG. 1  represents what is known as a common rail injection system. As described, rail  85  represents a high pressure fuel storage. Using pressure regulation valve  60 , the fuel in rail  85  is set to a predefined pressure. The predefined pressure may suitably be calibrated on a test bench, for example. Each injection of fuel into the combustion chamber of the engine via fuel injectors  10 ,  15 ,  20 ,  25  causes a slight pressure drop in rail  85 . In order to maintain the predefined pressure in rail  85 , an appropriate fuel quantity is re-supplied to rail  85  by high pressure pump  40 . The pressure in rail  85 , necessary for this purpose, is regulated optionally via pressure regulation valve  60  or via an adjustable throttle point (not shown in  FIG. 1 ) of, for example, the fuel metering unit at a fuel inlet of high pressure pump  40  from the fuel tank (not shown in  FIG. 1 ). In conventional fuel supply systems, the pressure to be adjusted is measured by a rail pressure sensor which is situated directly on rail  85 . 
   Since rail  85  has a relatively large volume in comparison with the connected high pressure lines  65 ,  70 ,  75 ,  80  and the high pressure bores (not shown in  FIG. 1 ) within the individual fuel injectors  10 ,  15 ,  20 ,  25 , the rail inner diameter is much greater than the line inner diameter of high pressure lines  65 ,  70 ,  75 ,  80  and the high pressure bores, and high-frequency pressure oscillations which occur in high pressure lines  65 ,  70 ,  75 ,  80  and in fuel injectors  10 ,  15 ,  20 ,  25  during injection of the fuel are dampened by the rail volume. These high-frequency oscillations, whose frequencies lie approximately between 1 kHz and 3 kHz, for example, thus may not be detected by the rail pressure sensor. Only the pressure increases caused by the delivery strokes of high pressure pump  40  and the pressure drops due to the removal of fuel during the injection of fuel into the cylinders via fuel injectors  10 ,  15 ,  20 ,  25  may be detected by the rail pressure sensor. 
   The present invention thus provides for the pressure sensor to be relocated to a position in which the low-frequency pressure fluctuations due to the fuel supply by high pressure pump  40  and the fuel removal due to the injection, necessary for the regulation of the fuel pressure in rail  85 , as well as the previously undetectable high-frequency pressure oscillations between the nozzle of the respective fuel injector and the end of the associated high pressure line facing rail  85 , are measurable, the high-frequency pressure oscillations being caused by the injection operation itself. Suitable signal processing of the measured pressure signal makes it possible to separate the high-frequency and low-frequency components, so that a single sensor may be used for the rail pressure regulation and the measurement of the high-frequency pressure oscillation in the appropriate high pressure line. This results in substantial cost savings in comparison to a system having two separate pressure sensors which are specialized, e.g., with regard to their position in fuel supply system  5 , one in the rail pressure regulation and the other in the measurement of the high-frequency pressure oscillation of the associated high pressure line. 
   According to the present invention, pressure sensor  55  is situated in the area of first fuel injector  10 . As shown in  FIG. 1 , pressure sensor  55  may be situated at one end of first high pressure line  65  facing first fuel injector  10 . As described in published German patent document DE 102 17 592, pressure sensor  55  may also correspond to a piezoelectric actuator as a control element of first fuel injector  10  and may utilize the piezoelectric actuator&#39;s sensor effect as described in published German patent document DE 102 17 592. For detecting the high-frequency pressure fluctuations in second high pressure line  70 , in third high pressure line  75 , and in fourth high pressure line  80 , a pressure sensor may also be situated in a corresponding manner in the area of the associated fuel injector, the pressure signal of the pressure sensor being conveyed to controller  90  in an appropriate manner and analyzed there. However, this procedure is described in the following as an example for pressure sensor  55  and first high pressure line  65 . 
   The relocation of pressure sensor  55  from rail  85  to a position near the injector on one of the available high pressure lines  65 ,  70 ,  75 ,  80  results in the detection of the high-frequency pressure oscillation in the high pressure line, on which pressure sensor  55  is situated, in addition to the low-frequency pressure fluctuations due to the pump supply of high pressure pump  40  and the fuel removal due to the injection of one or several of fuel injectors  10 ,  15 ,  20 ,  25 , the high-frequency pressure oscillation being caused by the injection operation of the associated fuel injector. In the present example, the high-frequency pressure oscillation in first high pressure line  65 , which is caused by the injection operation of first fuel injector  10 , is detected by pressure sensor  55  situated on first high pressure line  65 . 
   Since the above-described effects occur in different frequency spectra, separation of the low-frequency pressure fluctuations from the high-frequency pressure fluctuations, which are contained in the signal of pressure sensor  55 , is possible using suitable filtering. A corresponding device according to the present invention for determining different pressure fluctuations in the signal of pressure sensor  55  is indicated in  FIG. 2  by reference numeral  1  and may be implemented in controller  90  in the form of software and/or hardware. Device  1  includes a first filter  30  and a second filter  35 , to which the signal of pressure sensor  55  is conveyed. First filter  30  has a first filter characteristic and second filter  35  has a second filter characteristic. The first filter characteristic is different from the second filter characteristic. In the present example, the two filter characteristics are formed by different, in particular, but not necessarily, non-overlapping pass-bands. A first limiting frequency of first filter  30  is selected in such a way that it is higher than the first frequencies of low-frequency pressure fluctuations to be anticipated caused by the fuel supply by high pressure pump  40  and/or low-frequency pressure fluctuations to be anticipated due to the fuel removal during at least one injection operation of one of fuel injectors  10 ,  15 ,  25 . A pass-band of first filter  30  below the first limiting frequency is selected in such a way that it includes the first frequencies. First filter  30  may be designed as a band-pass filter, for example; a third limiting frequency for the pass-band of first filter  30  must then also be defined in such a way that it lies below the above-mentioned first frequencies. It is even simpler to design first filter  30  as a low-pass filter, so that the third limiting frequency no longer has to be defined. A signal is applied to the output of first filter  30  which includes only the pressure fluctuations having the first frequencies and from which the high-frequency pressure fluctuations due to the injection operation of first fuel injector  10  have been filtered out and are thus no longer present. As shown in  FIG. 2 , for example, this output signal of first filter  30  may then be conveyed to a processing unit which is characterized in the example of  FIG. 2  as a control unit  45 . Control unit  45  is used for regulating the pressure in rail  85  to a predefined pressure value P v , which is conveyed to control unit  45  in addition to the output signal of first filter  30 . Control unit  45  subsequently forms the difference between predefined pressure value P v  and the output signal of first filter  30  as the actual value of the rail pressure. Control unit  45  then generates a control signal for the pressure regulating valve  60  in such a way that this difference is minimized and the low-frequency pressure fluctuations due to the fuel supply by high pressure pump  40  and/or due to the pressure drop during removal of fuel by one or several of fuel injectors  10 ,  15 ,  20 ,  25  are largely compensated. 
   A limiting frequency of second filter  35  is selected in such a way that it is lower than a second frequency or second frequencies of the high-frequency pressure fluctuations to be anticipated which occur during an injection operation of first fuel injector  10 . A pass-band of second filter  35  above the second limiting frequency is selected in such a way that it includes the second frequency or the second frequencies. Second filter  35  may also be designed as a band-pass filter which closes the pass-band of second filter  35  upward by a fourth limiting frequency which is higher than the second frequency or the second frequencies. The second limiting frequency, for example, may be selected to be slightly lower than or equal to 1 kHz, e.g., 900 Hz, and the fourth limiting frequency, for example, may be selected to be slightly over 3 kHz, e.g., 3.1 kHz. Second filter  35  may be implemented even more simply as a high-pass filter; in this case, the fourth limiting frequency no longer has to be defined. Since the first frequencies are lower than the second frequency or second frequencies, the first limiting frequency and the second limiting frequency should lie between the first frequencies and the second frequency or second frequencies, in order to be able to cleanly separate the first frequencies from the second frequency or second frequencies. The first limiting frequency may be selected to be equal to the second limiting frequency. In order to reliably separate the different frequency spectra it is also advantageous to select the second limiting frequency to be higher than the first limiting frequency. However, the second limiting frequency may also be selected to be lower than the first limiting frequency, in which case the pass-bands of the two filters  30 ,  35  overlap. In the present example, the first and the second limiting frequencies may also be selected to be 1 kHz each. Thus, the signal at the output of second filter  35  is cleared of the low-frequency pressure fluctuations of the output signal of pressure sensor  55  and only includes the high-frequency pressure fluctuations due to the injection operation of first fuel injector  10 . The output signal of second filter  35  may then be conveyed for suitable further processing. This may be characterized, as shown in  FIG. 2  as an example, by a determination unit  50  which determines the frequency of the high-frequency pressure oscillation from the output signal of second filter  35 , by way of a Fourier analysis, for example. The frequency of the high-frequency pressure oscillation in first high pressure line  65  is directly proportional to the sound velocity of the fuel, so that, after determining the proportionality constant on a test bench, for example, and its storage in a memory assigned to determination unit  50 , the sound velocity of the fuel in first high pressure line  65  may be calculated with the aid of this proportionality constant and the determined frequency of the high-frequency pressure oscillation. The determined sound velocity may then in turn be conveyed to further processing by determination unit  50 , it being possible that this further processing takes place in controller  90  or in a different control unit. 
   Injection quantity errors may occur due to the high-frequency pressure fluctuations in first high pressure line  65  and first fuel injector ( 10 ), since injection via nozzle  105  of first fuel injector  10  takes place at a time at which the pressure wave of a previous injection of first fuel injector  10  has not yet decayed. However, if this pressure wave, which corresponds to the described high-frequency pressure fluctuation between nozzle  105  of first fuel injector  10  and the rail-side end of first high pressure line  65 , is known, i.e., in the form of the output signal of second filter  35 , a suitable injection quantity correction may be carried out as a function of the output signal of second filter  35  which takes the pressure wave of the previous injection of first fuel injector  10  into account. However, the exact implementation of such further processing of the output signal of second filter  35  is not critical to the present invention. Such an injection quantity correction makes it possible to increase the metering accuracy of the fuel supply system. 
   The described high-frequency pressure oscillation in first high pressure line  65  and first fuel injector  10  is a hydraulic oscillation which has its maximum pressure amplitude at the closed nozzle  105  of first fuel injector  10 ; its pressure amplitude at the rail-side open end of first high pressure line  65 , however, is very low. Therefore, this high-frequency oscillation cannot be detected by a conventional pressure sensor within rail  85 . This is achieved in the described manner by placement of pressure sensor  55  in first high pressure line  65  near the injector. Although pressure sensor  55  is no longer situated in the area of rail  85 , it is nevertheless possible to reconstruct the pressure characteristic in rail  85  from the measured pressure of pressure sensor  55  in first high pressure line  65  with great accuracy. The level of the pressure peaks of the low-frequency pressure signal, in particular, which are used for regulating the rail pressure, differ only marginally from the level of the pressure peaks of the pressure signal which was measured directly in rail  85  for test purposes and was filtered with the aid of filter  30 . Regulation of the rail pressure is thus possible without any accuracy losses by using the filtered pressure signal determined by pressure sensor  55 , situated near the injector in first high pressure line  65 . The method and the device according to the present invention have been described based on the pressure signal provided by pressure sensor  55 . The pressure fluctuations may generally be determined by appropriately analyzing a signal, which is characteristic for the pressure in the area of first fuel injector  10 , this signal being formed by a sensor or it may be modeled from performance quantities of the fuel supply system and/or the internal combustion engine which is supplied with fuel by fuel supply system  5 . The pressure signal of pressure sensor  55  has been analyzed in the present example as the signal characteristic for the pressure in the area of first fuel injector  10 . However, a signal which is proportional to pressure, e.g., the oscillation amplitude of the diaphragm of a pressure sensor, could also be used. 
   According to  FIG. 2 , device  1  according to the present invention includes first filter  30 , second filter  35 , control unit  45 , and determination unit  50 . In addition, device  1  may alternatively also include pressure sensor  55  and/or pressure regulation valve  60 . However, device  1  should essentially include at least the first filter  30  and second filter  35  so that, in a further alternative, device  1  may include only first filter  30  and second filter  35 . Predefined pressure PV may be provided from a memory (not shown in  FIG. 2 ); this memory may be associated with controller  90  and may be situated inside or outside of device  1 . It may be assumed in the present example that this memory is situated outside of device  1 .