Abstract:
An injection system for internal combustion engines with a pressure booster is proposed, in which between injections, a compensation of the hydraulic forces engaging the stepped piston is performed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 00/03594 filed on Oct. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates pressure boosters and particularly to a pressure booster for a fuel injection system for internal combustion engines. 
     2. Description of the Prior Art 
     As exhaust gas standards become increasingly stringent, ever-higher injection pressures are required to improve mixture formation and combustion. The result is greater mechanical and thermal stresses on the fuel injection system. Furthermore, the demand for driving power is increasing disproportionately, since with the pressure, the losses in the fuel injection system rise as well. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The object of the invention is to furnish a fuel injection system in which the hydraulic forces acting on the pressure booster are reduced, and in which the pressure booster is simple to control. In addition, higher injection pressures should be made possible and at the same time the stress and the demand for driving power on the injection pump should be reduced. 
     This object is attained according to the invention by a pressure booster for a fuel injection system for internal combustion engines, having an injection nozzle and having an injection pump that has a high-pressure part, the high-pressure part of the injection pump being in operative communication with the injection nozzle via a control line communicating with the low-pressure side of the pressure booster and via a high-pressure path communicating with the high-pressure side of the pressure booster, characterized in that means to compensate for the hydraulic force acting on the low-pressure side of the pressure booster between injections are present. 
     This pressure booster has the advantage that between injections, the pressure forces acting on the stepped piston are reduced, so that after the end of the injection, the stepped piston can be returned with less force to its outset position toward the pump, and the pressure booster can be controlled in a simple way. The leakage and throttling losses of the fuel injection system are reduced as well, which leads to a reduction in the demand for driving power and improves the hydraulic efficiency of the fuel injection system. In addition, the high injection pressures allow smaller injection port diameters for the injection nozzle, which improves the mixture formation at all operating points. 
     In a feature of the pressure booster of the invention, it is provided that the pressure booster has a stepped piston, which is displaceable in a bore and whose end faces each define one pressure chamber; that a first, larger end face of the stepped piston defines a first pressure chamber, communicating with the control line; that a second, opposed, smaller end face of the stepped piston defines a second pressure chamber, communicating with the high-pressure path; that an annular face opposite the first end face defines a control chamber; and that between injections, the pressure is the same in both the first pressure chamber and the control chamber. This version has the advantage that despite a simple design, it has the advantages of the invention. 
     In a further feature of the invention, it is provided that the first end face and the annular face are essentially the same size, so that the hydraulic forces on the stepped piston are compensated for entirely between injections. 
     Another embodiment provides that the control chamber communicates hydraulically with the first pressure chamber via an inlet throttle, so that during the injection, pressure equalization between the first pressure chamber and the control chamber is prevented. 
     In one feature of the invention, the control chamber can be pressure-relieved via a control valve, in particular a 2/21-way control valve, so that the onset of supply by the pressure booster is controllable by opening the control valve. 
     A further variant provides that a restoring spring is fastened in the first pressure chamber and is braced on a support and a shoulder of the stepped piston and as a function of the pressure in the first pressure chamber and the pressure in the control chamber presses the stepped piston, between injections, against its stop toward the pump, so that after the end of injection, the stepped piston is put into its outset position regardless of the pressures that prevail. Furthermore, the restoring spring requires only little installation space. 
     In a feature of the invention, the shoulder is separably connected to the stepped piston, thus simplifying production and assembly. 
     In a further feature of the invention, the fuel flows out of the first pressure chamber into a leaking oil return via an outlet throttle, so that the pressure level in the leaking oil return is created indirectly by the injection pump. 
     A further variant of the invention provides that the second pressure chamber is filled from the leaking oil return, and that a check valve, which blocks the return flow of fuel from the second pressure chamber to the leaking oil return, is disposed between the second pressure chamber and the leaking oil return, so that the filling is simple, and only a low pressure prevails in the second pressure chamber between injections. 
     Further in the invention, it is provided that the pressure in the leaking oil return is less than the opening pressure of the injection nozzle, so that the injection nozzle closes securely between injections. 
     In one feature of the invention, to simplify production and assembly, the stepped piston is embodied in two parts. 
     One version of the invention provides that the control quantity of the control valve is carried away into the leaking oil return, so that a simple hydraulic circuit is attained. 
     In another variant, it is provided that the control piston is guided in a sleeve, so that the guidance of the stepped piston is improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention can be learned from the ensuing description, taken with the drawings, in which: 
     FIG.  1 : a schematic illustration of a fuel injection system of the invention; and 
     FIG.  2 : an illustration of a pressure booster of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows a fuel injection system with an injection nozzle  1  and an injection pump  3 , which has a high-pressure part  5 . The high-pressure part  5  communicates operatively with the injection nozzle  1  via a control line  9  and a high-pressure path  10 . A pressure booster is disposed between the control line  9  and the high-pressure path  10 . 
     In FIG. 2, a pressure booster  11  of the invention is shown. In a housing  12 , the pressure booster  11  has a first pressure chamber  13 , a second pressure chamber  15 , a one-part or multi-part stepped piston  17  that is guided in a bore  18 , and a control chamber  19 . 
     The first pressure chamber  13  and the end face of the stepped piston  17  that protrudes into the first pressure chamber  13 , which end face has the diameter d 1 , form the low-pressure side of the pressure booster  11 . The second pressure chamber  15  and the end face, having the diameter d 3 , of the stepped piston  17  protruding into the second pressure chamber  15  form the high-pressure side of the pressure booster  11 . 
     The control chamber  19  is defined in the longitudinal direction by an annular face  20  of the stepped piston  17  and a shoulder in the housing  12  of the pressure booster  11 . 
     Between injections, a 2/2-way control valve  21  communicating with the control chamber  19  is closed. The first pressure chamber  13  and the control chamber  19  communicate through a connecting line  23 , which has an inlet throttle  25 , so that when the control valve  21  is closed, the same pressure prevails in both chambers  13  and  19 . Via the end face, protruding into the first pressure chamber  13 , of the stepped piston  17  and the annular face  20 , a compensation of the hydraulic forces takes place. The compensation is complete when the condition            d   1   2     4     =         d   2   2     -     d   3   2       4                            
     is met. A slight overcompensation of the hydraulic force acting on the end face of the stepped piston  17  can be advantageous. 
     The invention is tripped by opening the control valve  21 . The fuel located in the control chamber  19  flows through the control valve  21  into a leaking oil return  27 . The inlet throttle  25  causes the pressure in the control chamber  19 , when the control valve  21  is opened, to drop below the pressure in the first pressure chamber  13 . As a consequence, there is no longer a force compensation between the end face of the stepped piston  17  protruding into the first pressure chamber  13  and the annular face  20 . The stepped piston  17  begins to pump. 
     By means of the ratio between the end faces protruding into the first pressure chamber  13  and into the second pressure chamber  15 , the ratio of the pressures in the first and second pressure chambers  13  and  15  is specified. As soon as the pressure in the second pressure chamber  15  or in the high-pressure path  10  exceeds the opening pressure of the injection nozzle  1 , the injection nozzle  1  opens and the injection begins. 
     As soon as the control valve  21  is closed again, a pressure equalization and force compensation takes place between the first pressure chamber  13  and the control chamber  19 , so that the stepped piston  17  is moved into the position shown in FIG. 2, in the direction of the first pressure chamber  13 , by a restoring spring  29 . As soon as the stepped piston  17  moves in the direction of the first pressure chamber  13 , the pressure in the second pressure chamber  15  collapses, and the injection nozzle  1  closes. 
     The second pressure chamber  15  is filled via a supply line  31  subjected to leaking oil pressure. A check valve  33  is disposed in this supply line  31 . The check valve  33  can be spring-loaded, as seen in FIG. 2, or can be embodied without a spring, and it prevents the return flow of fuel during injection. The leaking oil pressure is below the opening pressure of the injection nozzle  1 . 
     The stepped piston  17  is guided with its diameter d 1  by a sleeve  35 . The sleeve  35  is radially fixed in the housing  12 . In the axial direction, a shoulder  37  of the housing  12  and a cup spring  39  assure the fixation of the sleeve  35 . The housing  12  can be split along the shoulder  37 , to make production and assembly of the pressure booster  11  of the invention easier. The cup spring  39  prevents the stepped piston  17  from a “hard” landing on its stop toward the pump. 
     The restoring spring  29  is fastened between the sleeve  35  and a shoulder  41  of the stepped piston  17 . FIG. 2 shows an embodiment in which the shoulder  41  is screwed into the stepped piston  17 . However, still other embodiments are also conceivable. 
     Via an outlet throttle  43 , fuel flows out of the first pressure chamber  13  into the leaking oil return. Via the outlet throttle  43 , the pressure level in the leaking oil return can be varied. 
     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.