Abstract:
In a fuel injection pump with a housing, a metering pump and an injection adjuster for the metering pump, in which the injection adjuster has a piston that is acted upon by a restoring spring, the starting performance of the internal combustion engine supplied by the injection pump is to be improved by providing a starting spring which acts on the piston and acts counter to the restoring spring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 01/00032 filed on Jan. 8, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel injection pump, having a metering pump and an injection adjuster for the metering pump, in which the injection adjuster has a piston that is acted upon by a restoring spring. 
     2. Description of the Prior Art 
     One fuel injection pump is known from German Patent Disclosure DE 43 44 865 A1 is a so-called distributor injection pump, in which a single pump piston of the metering pump is used to inject the correct fuel quantity into the appropriate cylinder of an internal combustion engine. During the revolution of the drive shaft, the pump piston executes as many strokes as there are engine cylinders to be supplied; the drive shaft of the fuel injection pump rotates at half the rotary speed of the engine crankshaft. 
     Since the fuel need not always be injected into the applicable cylinders at the same instant, with reference to the angle of rotation of the crankshaft, an injection adjuster is provided, by means of which the instant of injection can be adapted to the prevailing operating conditions of the engine. In a distributor injection pump, this is done by adjusting a cam ring, which actuates the pump piston and is driven by the drive shaft, by approximately ±10° relative to the drive shaft. 
     For adjusting the cam ring, the piston is provided, which is pressed by the restoring spring against an end stop. This position in contact with the end stop is equivalent to an injection adjustment in the “late” direction. In order to bring about the injection adjustment during operation, the side of the piston opposite the restoring spring is subjected to a fluid that is under pressure and that is controlled by a control slide and is furnished by a prefeed pump of the injection pump. As a consequence, however, when the fuel injection pump is at a stop, the piston of the injection adjuster is in the “late” position, since at a stop no fluid flow can be furnished, and thus only the force that is furnished by the restoring spring acts on the piston. 
     A disadvantage of this is that the position of the injection adjuster piston, which is predetermined by the restoring spring, at a stop is not optimal for starting an internal combustion engine supplied by the fuel injection pump. 
     SUMMARY OF THE INVENTION 
     The fuel injection pump of the invention offers the advantage that the starting spring acting counter to the restoring spring keeps the piston of the injection adjuster in the optimal position for starting the engine when the fuel injection pump is at a stop and consequently no fluid flow with which the piston could be acted upon is available. This optimal position, which is selected especially with a view to starting performance at low temperatures, can preferably be determined by adaptation of the prestressing and the spring constants of the restoring spring and the starting spring. 
     In a preferred embodiment, it is provided that the starting spring is disposed in the interior of the piston. This produces an especially compact structural shape. 
     In one embodiment, it is provided that the starting spring is braced by one end on the housing of the injection pump. In this way, a simple, open construction at little additional cost is obtained. 
     In an alternative embodiment, it is provided that the starting spring is braced by one end on an auxiliary piston, which is supported displaceably in a receiving bore in the piston and is braced on the housing. In this encapsulated version, only a relatively short spring is needed, and such a spring can be designed well. 
     To prevent the auxiliary piston from coming loose from the injection adjuster piston, a securing ring that limits the displaceability of the auxiliary piston can be disposed in the receiving bore. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described below with specific reference to the drawings, in which: 
     FIG. 1 is a sectional view of a fuel injection pump [in a sectional view]; 
     FIG. 2 is a schematic view; partially in section of an injection adjuster of a fuel injection pump of the invention; 
     FIG. 3 is a sectional view of a detail of the piston of the injection adjuster in a first version; and 
     FIG. 4 is a view similar to FIG. 3 showing a second version of the piston of the injection adjuster. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fuel injection pump, shown in part in axial longitudinal section in FIG. 1, has a housing  10  in which a drive shaft  11  is rotatably supported with slide bearings  12 . The housing  10  surrounds a pump interior  13 , which is filled with fuel that is under pressure. The filling of the pump interior  13  is attained with the aid of a prefeed pump  14 , which is disposed in the pump interior  13  and is driven by the drive shaft  11 . 
     On the face end of the drive shaft  11  is a pair of claws  15 , which via a slaving piece  16  and suitable claws, not shown, drives an end cam plate  17  to rotate. A pump piston  18  is coupled in a manner fixed against relative rotation to the end cam plate  17  and is pressed against the end cam plate  17  by a spring  19  and presses a cam race  20 , disposed on the end cam plate  17 , against rollers  21 , which are supported in a radial orientation in a roller ring  22 . The roller ring is supported rotatably by its circular outer contour in a corresponding circular-cylindrical recess  23  in the pump interior  13  and is braced axially via an apron  24  on a disk  25  that covers the prefeed pump  14 . The coupling, formed by the pair of claws  15  and the slaving piece  16 , between the drive shaft  11  and the pump piston  18  protrudes through the roller ring  22 . 
     The roller ring  22  is supported rotatably to a limited extent in the circumferential direction and is actuated by an injection adjuster  26  in a known way. To that end, the roller ring  22  is coupled to a piston  28  of the injection adjuster via a bolt  27  extending radially to the drive shaft  11 . The coupling is effected via a sliding block  29 . 
     The injection adjuster with the piston  28  can be seen in FIG.  2 . The piston  28  is displaceable in a bore  30 , forming two work chambers  32 ,  34  separated from one another by the piston  28 ; each of the work chambers can be supplied with fluid from a respective supply bore  36 ,  38 . For controlling the applicable fluid flow, a control slide  40  and a proportional magnet  42  are provided. 
     The piston  28  is provided on its two face ends with a respective receiving bore  44  and  46 , which serves to receive a spring. A restoring spring  48  is disposed in the receiving bore  44  and seeks to urge the piston  28  downward, in terms of FIG. 2, into a position in which the work chamber  34  has a minimal volume. A starting spring  50  (not shown in FIG. 2) is disposed in the receiving bore  46  and acts counter to the restoring spring  48  and thus urges the piston in the direction of a reduction in size of the work chamber  32 . 
     In FIG. 3, the end of the piston  28  provided with the starting spring  50  is shown in a first version. An auxiliary piston  52  is disposed displaceably in the receiving bore  46  in the piston  28  of the injection adjuster; the starting spring  50  is embodied as a compression spring and is braced between the bottom of the receiving bore  46  and the head of the auxiliary piston  52 . Thus the auxiliary piston  52  is urged into contact with the housing  10 , and a securing ring  54  is disposed in the receiving bore  46  and prevents the auxiliary piston  52  from being forced out of the receiving bore  46  by the starting spring  50 . The auxiliary piston  52  is provided with a vent opening  56 , so that the chamber in which the starting spring  50  is disposed is at the same pressure as the work chamber  34 . 
     A second version of the piston  28  of the injection adjuster is shown in FIG.  4 . Unlike the version shown in FIG. 3, the starting spring  50  disposed in the receiving bore  46  is braced, with its end opposite the bottom of the receiving bore, directly on the housing  10 , and a spring plate  58  is provided which is disposed on a centering lug  60  on the housing. 
     The mode of operation of the injection adjuster piston  28 , provided with the restoring spring  48  and the starting spring  50 , is as follows: When the fuel injection pump is at a stop, or in other words no fluid is furnished by the prefeed pump  14  via the supply bores  36 ,  38  to subject the work chambers  32 ,  34  to pressure, the piston  28  is approximately in the position shown in FIG. 2, in which the starting spring  50  assures that the piston  28  is not resting on the bottom of the work chamber  34  as would be the case in an injection pump of the prior art. The optimal early starting position of the fuel injection pump for optimal engine starting conditions is obtained in this position of the piston  28 . Once the engine has been started, there is initially an rpm that is equivalent to the lower idling rpm level. First the work chamber  32  is subjected to fluid by the control slide  40 , so that the piston  28  of the injection adjuster adjusts downward to the “late” position in terms of FIG. 2, and in this position it is pressed against the bottom of the work chamber  34  into the terminal position which it would assume (0° camshaft angle) if the starting spring  50  were not present. If the rpm is increased further, the pressure chamber  34  is supplied with fluid by the control slide  40 , so that the piston  28  is moved back in the direction of “early”. At maximum rpm, it is then at the end stop for the “early” position, which is equivalent to approximately 2° camshaft angle. 
     In the version shown in FIG. 3, because of the presence of the auxiliary piston  50  and the securing ring  54 , the starting spring  50  is effective over an adjusting range of only about 5° camshaft angle; at a greater adjustment of the piston  28 , the auxiliary piston  52  lifts away from the housing  10 . The particular advantage of this version is the good designability of the spring; however, a disadvantage is that the resultant of the spring forces acting on the piston  28  experiences a kink in the range in which it lifts the auxiliary piston  52  from the housing  10 . 
     In the version shown in FIG. 4, the starting spring  50  remains effective over the entire adjusting range of the piston  28 . What is critical in this version, however, is the fatigue strength of the spring. 
     In the version shown in FIG. 3, the position of the piston  28  in the bore  30  is due to the disposition of the securing ring  54  in the receiving bore  56 ; on the assumption of a sufficiently strong dimensioning of the starting spring  50 , the piston  28  is pressed by the auxiliary piston  50  so far in the direction of “early” that the auxiliary piston  52  rests on the securing ring  54 . In the version shown in FIG. 4, the corresponding position of the piston  28  is due to a position of equilibrium between the forces of the restoring spring  48  and the starting spring  50 . For precise calibration of this position, adjusting screws  62  can be used, which are disposed between the bottom of the receiving bore  46  and the starting spring  50 . 
     The injection adjuster arrangement described above can also be employed for radial piston pumps. In that case, the roller ring  22  is replaced by a cam ring, which has a knuckle pin with an articulation ball. The coupling is effected by engagement of the articulation ball of the knuckle pin and a slaving bush, which in turn is pressed firmly into a transverse bore of the injection adjuster piston. 
     The foregoing relates to a preferred exemplary embodiment 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.