Patent Publication Number: US-7210639-B2

Title: Servo-valve-controlled fuel injector

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an improved fuel injector for internal combustion engines. 
   2. Description of the Prior Art 
   DE 101 23 913 A1 has disclosed a fuel injector for internal combustion engines, having a pressure boosting unit for boosting pressure and having a servo-valve for triggering the fuel injector in a pressure-controlled manner. The servo-valve, which is embodied in the form of a 3/2-way control valve, is triggered by and on/off valve embodied in the form of the solenoid valve that executes the pressure control of the fuel injector. The control valve is equipped with a control piston that can move longitudinally in a bore and has a control edge that disconnects a high-pressure system from a low-pressure system. The control piston of the control valve must be provided with various pressure chambers to permit the connection of control lines, the insides of which are subjected to the high pressure of the injection system. This exertion of pressure results in the dilatation of leakage points along high-pressure-tight guides, deformations and dilatations at the control edges of sliding seals, and high notch stresses at bore intersections. These effects due to the exertion of pressure impair the function of the control valve and consequently reduce its fatigue strength. 
   German patent application 103 37 574.0 has already proposed guiding the control piston in a bush that is externally subjected to system pressure. This significantly reduces dilatations of the high-pressure-tight guides and control edges, deformations of the valve, and high notch stresses at bore intersections. However, it is disadvantageous that the control piston must be ground to fit the bush at two different diameters, which entails high production costs. 
   OBJECT AND SUMMARY OF THE INVENTION 
   The fuel injector according to the present invention has the advantage that the pressure piston section and the control piston section of the control piston, which are embodied with different diameters, are guided in separate respective guide elements. It is consequently unnecessary to provide double guidance in a single guide element for the different diameters of these sections of the control piston, which reduces cost of producing the control valve. This also improves technical manufacturing-related controllability and reproducibility for serial production. At the same time, the forces of pressure acting in the control valve, against the control piston, and on the components connected with it, are compensated for, as a result of which the deformation forces exerted in the control valve are kept to a minimum. Therefore high notch stresses do not occur in the components, e.g. at bore intersections, so that the resulting stresses remain significantly lower than fatigue strength values. 
   It is particularly useful if the control cylinder encompasses the valve chamber and, in the pressure-balanced state when the sealing seat of the control piston is closed, a connecting conduit that cooperates with the control edge produces a hydraulic connection between the pressure chamber and the valve chamber. It is also useful if the control cylinder is provided with an additional sealing seat in the control chamber, which in the depressurized state, disconnects the pressure chamber from a valve chamber provided in the valve body. In this state, the connecting conduit between the pressure chamber and the valve chamber is simultaneously closed. The control piston is suitably embodied so that it has a closing pressure surface in a control chamber and an opening pressure surface that is acted on by the system pressure and exposed to the pressure chamber. In the inactive state of the fuel injector, the sealing seat of the control piston is closed and the sliding seal with the control edge on the control cylinder is open. In the active state of the fuel injector, the sealing seat of the control piston is open and the sliding seal with the control edge on the control cylinder is closed. 
   The movement of the control piston can be set to any desired speed by suitably matching a first throttle, which produces a connection between the control chamber and a control chamber in the vicinity the actuator, to a second throttle, which produces a connection between the control chamber and the high-pressure chamber. A constant, definite opening force acts on the control piston due to the system pressure continouously exerted against the opening pressure surface. This yields a precise valve movement and a causes the control piston to stably remain against the opening stop in the open state. As a result, it is possible to implement a slow opening motion of the control piston, thus permitting a stable partial opening, which makes it possible for the injection of an extremely small quantity to be reliably set. 
   The control edge between the control piston section of the control piston and the valve body can be embodied in a multitude of ways. The use of a flat seat for the sealing seat intended to seal the pressure chamber in relation to the low-pressure/return system is particularly suitable because this makes it possible to compensate for a potentially occurring axial offset of the components. In addition, the closing force of pressure by means of the pressure surface of the control piston provides enough closing force to assure a sufficiently high surface pressure against the flat seat to produce a good seal. It is also possible to assist the valve movement of the control piston by allowing additional spring forces to act on the control piston. 
   It is particularly suitable to use the control valve in conjunction with a pressure boosting unit that is connected between the high-pressure source and the injection valve; the pressure boosting unit has a differential pressure chamber that cooperates with a pressure booster piston and can be controlled by the control valve so that a pressure change in the pressure chamber causes a boosting of the pressure acting on the injection valve. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments, taken in conjunction with the drawings, in which the sole FIGURE shows a schematic, sectional view of a fuel injector according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The fuel injector shown in the sole FIGURE is connected to a high-pressure fuel source  5  via a fuel line  3 . High-pressure fuel source  5  has a number of elements that are not shown, including a fuel tank, a high-pressure pump, and a high-pressure line, for example an intrinsically known common rail system in which the pump delivers a high fuel pressure of up to 1600 bar via the high-pressure line. The fuel injector also has a fuel injection valve  10  whose injection openings  11  protrude into a combustion chamber of an internal combustion engine. 
   The fuel injection valve  10  has a closing piston  12 , which is embodied in the form of a valve needle and has a pressure shoulder  13  encompassed by a pressure chamber  14 . At an end oriented away from the combustion chamber, the closing piston  12  is guided in a guide region  15  that adjoins a closing pressure chamber  16 . A closing spring  17  prestresses the closing piston  12  in the closing direction. 
   To boost the pressure, the fuel injector also has a pressure boosting unit  20 . The pressure boosting unit  20  has a booster piston  21 , which is supported in a sprung fashion by a return spring  18  and has a first partial piston  22  and a smaller-diameter second partial piston  23 . The partial pistons  22 ,  23  are associated with a corresponding diametrically stepped cylinder  24  so that the smaller-diameter partial piston  23  separates a high-pressure chamber  25  in the cylinder  24  from a differential pressure chamber  26  in a fluid-tight fashion. The larger-diameter first partial piston  22 , which is guided in the larger-diameter cylinder section of the cylinder  24 , also separates the differential pressure chamber  26  from a pressure boosting chamber  27  in a fluid-tight fashion. The pressure boosting chamber  27  contains the return spring  18  that is prestressed between a spring retainer  28  and a ring element  29  in order to produce an appropriate return movement for the booster piston  21 . 
   The fuel injector also has a servo-valve that includes a hydraulic control valve  30  and an electrically actuatable on/off valve  50  that is actuated by an electromagnetic or piezoelectric actuator  51 . Connected to the actuator  51 , the control valve  50  has an actuator piston  52  that is guided in an actuator bore  53 . In cooperation with an actuator sealing seat  54 , the actuator piston  52  shuts off a low-pressure chamber  55  of the actuator from a control chamber  56  of the actuator in a fluid-tight fashion. 
   The control valve  30  has a valve body  31  with a receptacle  32 . The receptacle  32  contains a bush  33  in which a pressure piston section  35  of a control piston  34  is guided. The control piston  34  also has a control piston section  36  that has a smaller diameter than the pressure piston section  35 . The control piston section  36  has a guide region with a control edge  45  that functions as a sealing edge. The guide region of the control piston section  36  is guided in a piston guide  43  of a control cylinder  41 ; the control cylinder  41  is likewise contained in the receptacle  32  and functions as a guide element for control piston  34 , which guide element is separate from the bush  33 . On the pressure piston section  35 , the control piston  34  has a pressure surface  38  facing into a control chamber  37 . Between the pressure piston section  35  and the control piston section  36 , there is an annular surface that constitutes an opening pressure surface  39  that will be explained in greater detail below. 
   The receptacle  32  constitutes a pressure chamber  40  in which system pressure is externally exerted on the bush  33  and the control cylinder  41 . The end surface of the control piston section  36  is provided with a sealing seat  46  that cooperates with the bottom surface of the pressure chamber  40  and separates a valve chamber  47  inside the control cylinder  41  from a connecting chamber  48  connected to a low-pressure/return system. The control cylinder  41  also has an end surface with a sealing surface or sealing edge that constitutes an additional sealing seat  42  at the bottom of the receptacle  32 , separating the pressure chamber  40  from the valve chamber  47 . A compression spring  49  acts on the control cylinder  41  and presses the sealing seat  42  against the bottom surface of the pressure chamber  40 , particularly in the depressurized state. In the guide region of the control piston section  36 , a connecting conduit  44  is provided, which cooperates with the control edge  45  and, when the sealing seat  46  is closed, produces a hydraulic connection between the pressure chamber  40  and the valve chamber  47 . 
   The individual components of the injection valve  10 , pressure boosting unit  20 , control valve  30 , and on/off valve  40  are connected by pressure lines that are, for example, integrated into the fuel injector. A first pressure line  61  connects the pressure chamber  14  of the injection valve  10  to the high-pressure chamber  25  of the pressure boosting unit  20 . From the closing chamber  16  of the injection valve  10 , a second pressure line  62  leads to the differential pressure chamber  26  of the pressure boosting unit  20 . There is also a connecting line  63  with a throttle between the closing pressure chamber  16  and a high-pressure chamber  25 . The hydraulic pressure of the high-pressure fuel source  5  travels through the high-pressure line  3  into the pressure chamber  40  and from there, via a pressure chamber line  64 , into the pressure boosting chamber  27  of the pressure boosting unit  20 . The pressure boosting chamber  27  is thus connected to the pressure chamber  40  of the control valve  30 . A differential pressure chamber line  65  connects the differential pressure chamber  26  of the pressure boosting unit  20  to the valve chamber  47  of the control valve  30 . A first return line  71  leads from the connecting chamber  48 , through the low-pressure/return system, and back into a fuel tank that is not shown. A control line  66  provided with an outlet throttle  67  connects the control chamber  37  of the control valve  30  to the actuator control chamber  56  of the on/off valve  50 . A second return line  72  leads from the actuator low-pressure chamber  55  of the on/off valve  50  into the low-pressure/return system. The return lines  71 ,  72  can also be embodied in the form of a combined return system. Finally, a connecting bore  68  leads from the control chamber  37 , via an inlet throttle  69 , and into the pressure chamber  40  of the control valve  30 . 
   The fuel injector functions as follows: at the beginning of the injection process, as a result of the constant pressure in the high-pressure chamber  5 , the pressure prevailing in the pressure boosting chamber  27  is also present in the differential pressure chamber  26  via the differential pressure chamber line  65  and is also present in the high-pressure chamber  25  via the second pressure line  62  and the connecting line  63  and from there, is also present in the pressure chamber  14  of the injection valve  10  via the first pressure line  61 . The actuator  51  of the on/off valve  50 , which in the present exemplary embodiment is a solenoid valve, is supplied with current so that the actuator piston  52  disconnects the control line  66 , which communicates with the control chamber  37  of the control valve  30 , from the actuator low-pressure chamber  55  that communicates with the second return line  72 . As a result, the system pressure or rail pressure prevailing in the pressure chamber  40  travels into the control chamber  37  via the connecting bore  68 . The high-pressure prevailing in the control chamber  37  acts on the pressure surface  38  and presses the sealing seat  46  of the control piston  41  against the bottom surface of the pressure chamber  40  so that the sealing seat  46  shuts off the connecting chamber  48  that communicates with the return line  71 . In this position of the control piston  41 , the control edge  45  is positioned outside the piston guide  43  of the valve body  31  so that a hydraulic connection is produced between the pressure chamber  40  and the valve chamber  47  via the connecting conduit  44 . The first return line  71  is consequently decoupled from the high pressure or system pressure and the injection valve  10  is closed. 
   The opening stroke motion of the closing piston  12  of the injection valve  10  is initiated by correspondingly supplying current to the actuator  51  to lift the actuator piston  52  away from the actuator sealing seat  54  so that the control chamber  37  is connected to the actuator control chamber  56  and the actuator low-pressure chamber  55 . The flow resistances of the inlet throttle  69  and the outlet throttle  67  are dimensioned so that the pressure in the control chamber  37  drops and the end surface of the control piston section  36  of the control piston  34  lifts away from the sealing seat  46  and at the same time, the control edge  45  on the piston guide  43  closes the connecting conduit  44 . This disconnects the valve chamber  47  from the rail or system pressure prevailing in the pressure chamber  40  and at the same time, the differential pressure chamber line  65  that leads into the valve chamber  47  is connected via the connecting chamber  48  to the return line  71  and therefore to the low-pressure system. Consequently, the pressure prevailing in the differential pressure chamber  26  of the pressure boosting unit  20  is relieved via the return line  71  and the pressure drops in the differential pressure chamber  26 . As a result, the pressure boosting unit  20  is activated and the second partial piston  23  with the smaller effective area compresses the fuel in the high-pressure chamber  25  so that in the pressure chamber  14  connected to the high-pressure chamber  25 , the force of pressure acting on the pressure shoulder  13  in the opening direction increases and the closing piston  12  lifts away from the injection openings. The pressure boosting unit  20  remains activated and compresses the fuel in the high-pressure chamber  25  for as long as the differential pressure chamber  26  remains depressurized. 
   In order to terminate the injection process, the on/off valve  50  is switched back into its starting position. As a result, the placement of the control piston  34  against the sealing seat  46  disconnects the differential pressure chamber  26  of the pressure boosting unit  20  from the return line  71  and causes it to be acted on with system pressure again via the valve chamber  47 , the connecting conduit  44 , and the pressure chamber  40 . The system pressure also travels into the differential pressure chamber  26  via the return line  65 , thus moving the pressure booster piston  21  back into its starting position assisted by the return spring  18 . As a result, the pressure in the high-pressure chamber  25  falls to the system pressure, which means that system pressure once again prevails in the pressure chamber  14  and the high-pressure chamber  25  is filled from the fuel source  5  via the connecting line  63 . The system pressure also exerted via the second pressure line  62  returns the closing piston  12  to its starting position, assisted by the closing spring  17  contained in the closing pressure chamber  16 . 
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.