Abstract:
The invention relates to a fuel injector ( 1 ) having a high pressure region ( 32 ) which contains fuel that is under high pressure during operation, and a low pressure region ( 38 ) in which a lower pressure is present during operation than in the high pressure region ( 32 ). A sensor ( 12 ) is located in the low pressure region ( 38 ) and a transmission means ( 9 ) is arranged such that a force which is equal to the pressure of the fuel in the high pressure region ( 32 ) is exerted on the sensor ( 12 ) at least some of the time.

Description:
BACKGROUND OF THE INVENTION 
     Pressure sensors in fuel injection systems for measuring the fuel pressure are known in the state of the art. Such known pressure sensors are generally arranged in a central pressure accumulator of the injection system where they are subjected to the high pressure of the fuel in the injection system. In modern common rail systems the fuel pressure may be a few thousand bar. The pressure sensors therefore have to be sealed off from the high pressure of the fuel in the injection system and thus constitute an elaborate and cost-intensive component. Furthermore, finding a suitable location at which to fit the pressure sensor is something of a problem in systems having no central pressure accumulator. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a simple and inexpensive device for measuring the fuel pressure in a fuel injection system. 
     This object is achieved by a fuel injector as claimed in the independent claim  1 . Advantageous developments are set forth in the dependent claims. 
     A fuel injector according to the invention has a high-pressure area, which in operation for at least some of the time contains fuel under high injection pressure. A fuel injector according to the invention also has a low-pressure area, which in operation contains no fuel and/or is connected to an outlet, so that a high fuel pressure does not build up in the low-pressure area and a lower pressure prevails than in the high-pressure area. A sensor is located in the low-pressure area and a transmission means is arranged so that for at least some of the time it exerts a force, which corresponds to the pressure of the fuel in the high-pressure area, on the sensor. 
     The invention also encompasses a fuel injection system having a fuel pump, at least one fuel injector according to the invention and a regulating valve. By using a fuel injector according to the invention having an integral pressure sensor, it is possible to dispense with a central pressure accumulator, in which the pressure sensor is fitted. Such an injection system can be designed with few components and is therefore inexpensive. 
     Since the pressure sensor is located in the fuel injector itself, the pressure can easily be measured, even in systems that do not have a central pressure accumulator. Since the pressure sensor is located in the low-pressure area of the fuel injector, the sensor does not need to have a special high-pressure seal. It is therefore possible to use simple and inexpensive sensors. 
     In one embodiment the force exerted on the sensor by the transmission means is proportional to the pressure in the high-pressure area. This particularly facilitates evaluation of the values measured by the sensor for determining the fuel pressure prevailing in the system. 
     In one embodiment the fuel injector comprises a control valve and the sensor is arranged in the low-pressure area of the control valve. Fitting the sensor in the low-pressure area of such a control valve is particularly advantageous. 
     In one embodiment the fuel injector comprises a pressure-balanced control valve. A fuel injector having a pressure-balanced control valve can be opened and closed by small forces and thus allows particularly short operating times. Such a control valve can be actuated by a small and inexpensive actuator. 
     In one embodiment the control valve comprises a sleeve-shaped valve needle and the transmission means is embodied as a moveable pressure pin inside the valve needle. Such a valve having a pressure pin arranged inside a sleeve-shaped valve needle allows an especially easy transmission of the pressure from the high-pressure area to a sensor located in the low-pressure area and is easy and inexpensive to produce. 
     In one embodiment the control valve can be actuated by a solenoid actuator. Solenoid actuators have proven successful when used in fuel injectors and are inexpensive to produce. 
     In one embodiment the control valve can be actuated by a piezoelectric actuator. Piezoelectric actuators allow particularly short operating times. 
     In one embodiment a compensating element is arranged between the pressure pin and the sensor. Such a compensating element makes it possible to compensate for angular tolerances between the sensor and the pressure pin and thereby to improve the accuracy of the measurement. 
     In one embodiment the fuel injection system comprises at least two fuel injectors, the fuel pump, the fuel injectors and the regulating valve being connected in series in such a way that in each case the outlet of one fuel injector is connected to the inlet of a succeeding fuel injector. Such a series arrangement makes it possible to minimize the overall length of the pressure lines of the fuel injection system. Such an injection system is therefore particularly inexpensive to produce. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below with reference to the figures attached, of which: 
         FIG. 1  shows a sectional representation of a fuel injector according to the invention, and 
         FIG. 2  shows a schematic representation of a fuel injection system having at least one fuel injector according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a sectional representation of a fuel injector  1  according to the invention. The fuel injector  1  comprises a cylindrical nozzle body  2  shown in the lower area of  FIG. 1 , and a cylindrical union nut  4 , which is arranged above the nozzle body  2  and is tightly screwed to the nozzle body  2 . At its end shown at the top in  FIG. 1 , the injector  1  is closed by a closure plate  41 , which is screwed hydraulically tight to the union nut  4 . 
     A high-pressure chamber  32 , which by way of a fuel inlet  30  can be filled by an external fuel pump  28  with fuel under high pressure, is formed inside the nozzle body  2 . 
     Injection ports  3 , through which fuel can flow from the high-pressure chamber  32  into a combustion chamber (not shown), which encloses the lower end of the nozzle body  2 , are formed at the end of the nozzle body  2  shown at the bottom of  FIG. 1 . 
     The upper end of the high-pressure chamber  32  situated opposite the injection ports  3  is defined by a valve plate  24 , which is tightly clamped to the nozzle body  2  by the union nut  4  and hydraulically seals the high-pressure chamber  32  tight. 
     A cylindrical projection, which encloses a control chamber  18 , is formed on the side of the valve plate  24  facing the high-pressure chamber  32 . A nozzle needle orifice is formed on the side  17  of the projection remote from the valve plate  24 . 
     A nozzle needle  6  having an upper end  6   a  facing the valve plate  24  and a lower end  6   b  facing the injection ports  3  is arranged along the longitudinal axis of the high-pressure chamber  32 . The nozzle needle  6  may be composed of one piece or it may be built up from multiple parts which are operatively connected together. 
     The upper end  6   a  of the nozzle needle  6  is introduced through the nozzle needle orifice, which is formed in the side  17  of the control chamber  18  remote from the valve plate  24 , into the control chamber  18 , in such a way that the volume of the control chamber  18  can be varied by movement of the nozzle needle  6  parallel to the longitudinal axis. 
     Below the control chamber  18  a step  14  is formed on the circumference of the nozzle needle  6 . A nozzle needle spring  16  is arranged between the step  14  and the side  17  of the control chamber  18  remote from the valve plate  24 , so that it elastically braces the nozzle needle  6  against the projection of the valve plate  24 . In so doing, the nozzle needle spring  16  presses the lower end  6   b  of the nozzle needle  6  against the injection ports  3  in the lower area of the nozzle body  2 , in such a way that the lower end  6   b  of the nozzle needle  6  closes the injection ports  3  and no fuel flows out of the high-pressure chamber  32  through the injection ports  3  into the combustion chamber. 
     A pressure shoulder  7  is formed above the lower area  6   b  of the nozzle needle  6 . In operation the high-pressure chamber  32  is filled with fuel under high pressure and the fuel exerts an upwardly directed force on the nozzle needle  6  by way of the pressure shoulder  7 . 
     An inlet restriction  20  formed in a side wall of the control chamber  18  connects the control chamber  18  hydraulically to the high-pressure chamber  32 , so that in hydraulic equilibrium the same pressure prevails in the control chamber  18  as in the high-pressure chamber  32 . 
     The side of the control chamber  18  remote from the nozzle needle  6  is defined by the valve plate  24 . An outlet bore  21 , which hydraulically connects the control chamber  18  to a cylindrically formed valve chamber  19 , which is formed above the side of the valve plate  24  remote from the control chamber  18  and the high-pressure chamber  32 , is formed in the valve plate  24  in the area of the control chamber  18 . 
     An outlet restriction  22  is formed in the outlet bore  21 . The dimensioning of the outlet restriction  22  allows the flow through the outlet bore to be regulated. 
     Two outlet bores  36 , which hydraulically connect the valve chamber  19  to a low-pressure chamber  36  formed above the control plate  24  in the union nut  4 , are formed in a wall  35 , which encloses the valve chamber  19 . The low-pressure chamber  36  is hydraulically connected to a fuel outlet  40 , through which fuel runs out of the injector  1 , so that a high fuel pressure does not build up in the low-pressure chamber  36 . 
     A seal seat  34  is formed on the control plate  24 , at the end of the outlet bore  21  which faces the valve chamber  19 . A valve needle  8 , which is moveable in the longitudinal direction of the injector  1  between a lower, closed position and an upper, opened position, is located in the valve chamber  19 . Here, when it is in the lower, closed position, the valve needle  8  rests on the valve plate  24  and closes the seal seat  34 . When it is situated in an upper, opened position, the valve needle  8  is lifted off from the valve plate  24  and exposes the seal seat  34 . 
     When the valve needle  8  is in an upper, opened position, and exposes the seal seat  34 , the control chamber  18  is hydraulically connected to the valve chamber  19  via the outlet bore  21  and the outlet restriction  22 . When the valve needle  8  is in the lower, closed position and closes the seal seat  34 , the connection between the control chamber  18  and the valve chamber  19  is interrupted. 
     The valve needle  6  extends through an aperture formed in an upper boundary  27  of the valve chamber  19  remote from the valve plate  24  into the low-pressure chamber  38  and at its upper end in the low-pressure chamber  38  remote from the valve plate  24  comprises an armature plate  25 , which extends at right-angles to the longitudinal direction of the injector  1  in the low-pressure chamber  38 . 
     An armature spring  36  is arranged between the armature plate  25  and the closure plate  41 , which closes the injector  1  at its upper end. The armature spring  36  braces the armature plate  25  elastically against the closure plate  41 , so that the valve needle  8  is pressed by the force of the armature spring  26  into the lower, closed position against the seal seat  34  formed on the valve plate  24  and hydraulically seals the seal seat  34  tight. 
     Between the armature plate  25  and the closure plate  41 , inside the union nut  4 , is a solenoid  10 , which is designed in such a way that the armature plate  25 , through activation of the solenoid  10 , is moved against the force of the armature spring  26  upwards towards the closure plate  24  into an upper, opened position, and in so doing lifts the valve needle  8  out of the seal seat  34 . The seal seat  34  is therefore opened by activation of the solenoid  10 . Fuel flows out of the control chamber  18  through the outlet bore  21  and the outlet restriction  22  into the valve chamber  19  and out of the valve chamber  19  through the outlet ports  36  on into the low-pressure chamber  38  and into the fuel outlet  40 . 
     Due to the discharge of fuel from the control chamber  18  as described, the fuel pressure in the control chamber  18  is reduced and is no longer sufficient to hold the nozzle needle  6  in the lower, closed position against the force which the fuel under high pressure in the high-pressure chamber  32  exerts on the pressure shoulder  7  formed at the lower end  6   b  of the nozzle needle  6 . The nozzle needle  6  moves upwards towards the valve plate  24  and exposes the injection ports  3 . Fuel flows out of the high-pressure chamber  32  through the injection ports  3  into the combustion chamber (not shown), which encloses the lower end of the nozzle body  2 . 
     To terminate the injection sequence, the solenoid  10  is deactivated. The armature spring  26  presses the armature  2  into the lower, closed position, in which the valve needle  8  closes the seal seat  34 . With the seal seat  34  closed, fuel, which flows out of the high-pressure chamber  32  through the inlet restriction  20  into the control chamber  18 , cannot run out of the control chamber  18  into the valve chamber  19  and the pressure in the control chamber  18  increases. The increased pressure in the control chamber  18  exerts a downwardly directed force on the nozzle needle  6 , which together with the force of the nozzle needle spring  16  presses the nozzle needle  6  into the lower, closed position. The lower end  6   b  of the nozzle needle  6  closes the injection ports  3  and no further fuel flows out of the high-pressure chamber into the combustion chamber through the injection ports  3 . 
     A central valve needle bore  23  is formed in the valve needle  8  along the longitudinal axis of the injector  1 . A pressure pin  9 , which is moveable along the longitudinal axis of the injector  1 , parallel to the direction of movement of the nozzle needle  6  and the valve needle  8  inside the valve needle bore  23 , is fitted into the valve needle bore  23  so that it is high-pressure tight. The pressure pin  9  extends above the valve needle  8  centrally through a bore formed in the armature plate  25 , the armature spring  26  and the solenoid  10 , and above the solenoid  10  is operatively connected to a sensor  12  arranged on the closure plate, in such a way that a force acting on the lower end face of the pressure pin  9  facing the outlet bore  32  in the valve plate  24  is transmitted to the sensor  12 . 
     In the exemplary embodiment shown in  FIG. 1  a compensating element  11  is provided between the upper end face of the pressure pin  9  facing the sensor  12  and the face of the sensor  12  facing the pressure pin  9 . The compensating element  11  makes it possible to compensate for angular tolerances between the sensor  12  and the pressure pin  9  and to increase the accuracy of the measurements made by the sensor  12 . 
     In the lower, closed position of the valve needle  8 , that is to say when the valve needle  8  is resting on the seal seat  34  formed on the valve plate  24  and closes the connection between the control chamber  18  and the valve chamber  19 , the high fuel pressure prevailing in the control chamber  19  acts through the outlet bore  21  on the end face of the pressure pin  9  facing the outlet bore  21 . The pressure pin  9  transmits a force, which is proportional to the fuel pressure in the control chamber  18 , to the sensor  12 . 
     Since the control chamber  18  is hydraulically connected via the inlet restriction  20  to the high-pressure chamber  32 , in hydraulic equilibrium the fuel pressure in the control chamber  18  is equal to the fuel pressure in the high-pressure chamber  32 . The force exerted on the sensor  12  by the pressure pin  9  is therefore proportional to the fuel pressure in the high-pressure chamber  32 . The fuel pressure in the high-pressure chamber  32  can easily be determined from the value measured by the sensor  12 . 
     Since the sensor  12  is located in the low-pressure area of the injector  1 , it is not necessary for the sensor  12  to be embodied as a high pressure-resistant sensor. Instead a sensor  12  of simple construction that is inexpensive to produce may be used. 
     During the injection sequence, that is to say when the valve needle  8  has been moved into an upper, opened position by activation of the solenoid  10 , so that the seal seat  34  is opened, the outlet bore  21  is hydraulically connected to the low-pressure chamber  38  via the valve chamber  19  and the ports  36 . In this state there is no high pressure bearing on the lower end face of the pressure pin  9 , so that in this state the high pressure of the fuel system cannot be measured by the sensor  12 . The period in which the seal seat  34  is closed between the injection sequences, and the lower end face of the pressure pin  9  is exposed to the high fuel pressure of the system, is sufficient to undertake a reliable pressure measurement. 
       FIG. 2  shows a fuel injection system having four fuel injectors  1 ,  1   a , a fuel pump  28  and a pressure regulating valve  42 . 
     The outlet of the fuel pump  28  is connected to the inlet  30  of a first injector  1  via a fuel inlet line  46  and an inlet restriction  47 . The fuel outlet  40  of the first injector  1  is connected to the inlet  30  of a second injector  1  via a connecting line  44 . The fuel outlet of the second injector  1   a  is connected to the fuel inlet of a third injector  1  via a further connecting line  44 . The fuel outlet  40  of the third injector  1  is connected to the fuel inlet  30  of a fourth injector  1  via a third connecting line  44 . The fuel outlet  40  of the fourth injector  1  is connected to a pressure regulating valve  42  via an outlet line  38 . The second fuel injector  1   a  is embodied as a fuel injector according to the invention having a pressure sensor  12  incorporated into the low-pressure area. 
     The fuel injection system shown in  FIG. 2  does not have a central pressure accumulator. Nevertheless, the fuel pressure in the fuel injection system can be reliably measured, since at least one of the fuel injectors  1  is embodied as a fuel injector  1   a  according to the invention having an integral pressure sensor  12 . One or more of the fuel injectors  1  may be embodied, as required, as a fuel injector  1   a  according to the invention having a pressure sensor  12  or as conventional fuel injectors  1  having no pressure sensor  12 . The pressure regulating valve  42  serves for adjusting the fuel pressure in the system. Since the fuel injectors  1 , la are connected to one another in series, a single pressure regulating valve  42  is sufficient to regulate the fuel pressure in the injection system. This eliminates the need both for a central pressure accumulator and for additional connecting lines. Such a fuel system is easy and inexpensive to produce.