Patent Publication Number: US-2011073078-A1

Title: High-pressure fuel pump

Description:
The invention relates to a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having a pump housing and having at least one pump element that includes a piston which is driven by a cam that cooperates with a roller which is defined on its ends by two lateral stop faces that in operation of the high-pressure fuel pump come into contact with opposite surfaces. 
     The opposite surfaces may be provided on a tappet body, for example, and they limit a motion of the roller in the axial direction. 
     DISCLOSURE OF THE INVENTION 
     It is the object of the invention to create a high-pressure fuel pump as defined by the preamble to claim  1  which can be produced economically and has a long service life. 
     In a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having a pump housing and having at least one pump element that includes a piston which is driven by a cam that cooperates with a roller which is defined on its ends by two lateral stop faces that in operation of the high-pressure fuel pump come into contact with opposite surfaces, this object is attained in that the lateral stop faces are embodied as circular-annular disk faces and are released in a central internal region from contact with the opposite surfaces. 
     A preferred exemplary embodiment of the high-pressure fuel pump is characterized in that the circular-annular disk faces are released in a coaxial outer region from contact with the opposite surfaces. As a result of the intentional releases, wear from friction caused in operation of the high-pressure fuel pump by a side run-up of the roller on the associated opposite surfaces can be reduced markedly. A face which is defined by two concentric or coaxial circles is called a circular-annular disk face. The circular-annular face can be embodied as either flat or curved. 
     A further preferred exemplary embodiment of the high-pressure fuel pump is characterized in that, viewed in longitudinal section through the roller, the circular-annular disk faces are each curved convexly. The radius of curvature in conventional rollers is also called the cap radius. In the roller of the invention, the conventional cap shape is varied as a result of the releases. The convexly curved circular-annular disk faces have the shape of spherical zones. The part of a spherical layer, belonging to the spherical surface, that is created when a sphere is intersected by two parallel planes is called a spherical zone. 
     A further preferred exemplary embodiment of the high-pressure fuel pump is characterized in that the central internal regions each include a flattened face, which is disposed perpendicular to the longitudinal axis of the roller. The flattened faces essentially have the shape of circular disks. 
     A further preferred exemplary embodiment of the high-pressure fuel pump is characterized in that the central internal regions each include an indentation. The indentations reliably prevent the central internal regions from coming into contact with the associated opposite surfaces. 
     A further preferred exemplary embodiment of the high-pressure fuel pump is characterized in that the circular-annular disk faces are raised relative to the associated coaxial outer regions. The raising or elevation reliably prevents the coaxial outer regions from coming into contact with the associated opposite surfaces. 
     A further preferred exemplary embodiment of the high-pressure fuel pump is characterized in that viewed in longitudinal section through the roller, the coaxial outer regions taper frustoconically. The cone angle is preferably selected such that the coaxial outer regions drop off more steeply toward the outside than the circular-annular disk faces do. 
     Further preferred exemplary embodiments of the high-pressure fuel pump are characterized in that the transitions between the central internal regions and the associated circular-annular disk faces, between the circular-annular disk faces and the associated coaxial outer regions, and between the coaxial outer regions and a cylindrical body of the roller are rounded. The rounded areas create gentle transitions. 
     Further advantages, characteristics and details of the invention will become apparent from the ensuing description, in which various exemplary embodiments are described in detail in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows a detail of a high-pressure fuel pump in an exemplary embodiment in longitudinal section through a pump element; 
         FIG. 2  shows an enlarged detail II from  FIG. 1  including a roller; 
         FIG. 3  shows the roller of  FIG. 2  by itself in plan view; 
         FIG. 4  shows an enlarged detail injection valve from  FIG. 3  in longitudinal section in a first exemplary embodiment; and 
         FIG. 5  shows the same detail as in  FIG. 4 , in a second exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     In  FIGS. 1 and 2 , a detail of a high-pressure fuel pump  1  with a pump housing  2  is shown in longitudinal section through a pump element  3 . The high-pressure fuel pump  1  is part of a fuel injection system of a motor vehicle and serves to subject fuel, which is pumped out of a fuel tank to the high-pressure fuel pump  1 , preferably with the aid of a prefeed pump, to high pressure. The fuel subjected to high pressure is then delivered to a central high-pressure fuel reservoir, also called a common rail. Fuel injection valves, which are also called injectors, are connected to the central high-pressure fuel reservoir, and by way of them, the fuel subjected to high pressure is injected into combustion chambers of an internal combustion engine. 
     Each pump element  3  includes an element bore  4 , into which an element body  6  that originates from a cylinder head (not shown) protrudes. In the element body  6 , a high-pressure piston  8  is guided movably back and forth. The high-pressure piston  8  essentially has the shape of a straight circular cylinder with a longitudinal axis  9 . A double arrow  11  indicates that the high-pressure piston  8  is guided movably back and forth along the longitudinal axis  9  in the element body  6 . 
     One end of the high-pressure piston  8  defines a pressure chamber in the cylinder head. Via a suction valve, the pressure chamber is in communication with the prefeed pump. The pressure chamber is also in communication with the central high-pressure fuel reservoir via a pressure valve. When the piston  8  moves out of the pressure chamber, fuel is aspirated into the pressure chamber. When the piston  8  moves into the pressure chamber, the fuel located in the pressure chamber is then subjected to high pressure. 
     On its end remote from the pressure chamber, the piston  8  has a piston base  12 , which essentially has the shape of a circular disk and is connected in one piece to the piston  8 . The face end, remote from the piston  8 , of the piston base  12  rests on a roller shoe  14 , in which a roller  15  is guided rotatably about its longitudinal axis  16 . The longitudinal axis  16  of the roller  15  extends transversely to the longitudinal axis  9  of the piston  8 . The roller  15  cooperates with a cam  18  of a drive shaft  20 , by which shaft the piston  8  is driven. The drive shaft  20  is rotatable about an axis of rotation  21 . 
     A tappet  22  is received in the element bore  4 , movably back and forth in the direction of the longitudinal axis  9  of the piston  8 . The tappet  22  essentially has the shape of a hollow circular cylinder, and radially inward it has a land  24  with which the tappet  22  is braced on the roller shoe  14 . A spring plate  25 , which has a central through hole through which the piston  8  extends, rests on the upper side, remote from the roller shoe  14 , of the land  24 . The base  12  of the piston  8  is disposed axially between the spring plate  25  and the roller shoe  18 . 
     A restoring spring  26  for the piston  8  is fastened between the spring plate  25  and the cylinder head. The prestressing force of the restoring spring  26  keeps the piston base  12  in contact with the roller shoe  14 , and keeps the roller shoe  14  in contact with the roller  15 , or in other words keeps the roller  15  in contact with the cam  18  of the drive shaft  20 . An arrow  28  indicates that the drive shaft  20 , with the cam  18 , rotates about the axis of rotation  21  in operation of the high-pressure fuel pump  1 . 
     In  FIG. 3 , the roller  15  is shown by itself in plan view. The roller  15  essentially has the shape of a straight circular cylinder with caplike protuberances on its ends  31 ,  32 . In the high-pressure fuel pump, the roller  15  serves as a transmission member, for converting a rotary motion of the drive shaft, in particular a camshaft, into an oscillating motion. 
     In operation of the high-pressure fuel pump, forces in the radial and axial directions that can affect the function of the roller  15  occur at the roller  15 . To make it possible to control or intercept these forces, the roller is guided in the radial and axial directions. 
     In the exemplary embodiment shown in  FIGS. 1 and 2 , the roller  15  is guided in the radial direction in the roller shoe  14 . During the rolling process, the roller shoe keeps the roller  15  in position on the opposite running face of the drive shaft and absorbs the radial motions. Axial forces are transmitted to the tappet  22  via the caps embodied on the ends  31 ,  32  of the roller  15 . 
     As a result of the lateral limitation of the axial motion of the roller  15 , a stop state occurs, in which the roller  15  stops at the tappet  22  that is upright relative to it. In this run-up process, at one or more contact points, friction wear occurs, which can lead to failure of the components. In an essential aspect of the invention, the ends  31 ,  32  of the roller  15  are optimized geometrically, to reduce wear and lengthen the service life of the components. 
     By means of a geometrically adapted cap geometry of the ends  31 ,  32 , the contact region between the roller  15  and the tappet  22  is optimized in terms of the friction wear that occurs in operation, this being done by releasing the center and the peripheral region of the cap at the ends  31 ,  32 . It is thereby attained that the center of the caps, at the ends  31 ,  32 , where as a rule the greatest wear occurs, no longer intervenes in load-bearing fashion, and the incident frictional forces are distributed over a larger region that is better capable of absorbing them. As a result, the lateral stop behavior is improved markedly. 
     In  FIG. 4 , a detail injection valve of  FIG. 3  is shown on a larger scale, in longitudinal section. In an essential aspect of the invention, the lateral stop face of the roller  15  is embodied as a circular-annular disk face  40 . The circular-annular disk face  40  has the shape of a spherical zone and is convexly curved. The circular-annular disk face  40  can also have the shape of a frustoconical portion. The circular-annular disk face  40  extends about the longitudinal axis  16  of the roller  15 . 
     The circular-annular disk face  40  has an inner diameter  41  radially inward and an outer diameter  42  radially outward. The region inside the inner diameter  41  is embodied flat, as a flattened face  44 . A coaxial outer region  46 , which has the shape of a frustoconical portion, extends radially outside the outer diameter  42 . 
     In an essential aspect of the invention, the roller cap is purposefully released inside the inner diameter  41  and outside the outer diameter  42 , in order to limit the contact of the side stop of the roller  15  to the circular-annular disk face  40  between the inner diameter  41  and the outer diameter  42 . This releasing, as seen in  FIG. 4 , can be achieved by means of a change in the angle at the transition between the circular-annular disk face  40  and the coaxial outer region  46  and/or by means of the flattened face  41 . 
     In  FIG. 5 , the end  31  is shown with a circular-annular disk face  50 , which extends between an inner diameter  51  and an outer diameter  52 . The circular-annular disk face  50  is embodied essentially identically to the circular-annular disk face  40  shown in  FIG. 4 . In a distinction from the preceding exemplary embodiment, an indentation  54  is provided in  FIG. 5 , inside the inner diameter  51 . The depth or height of the indentation  51  is indicated by reference numeral  55 . 
     Furthermore, a coaxial outer region  56 , which extends radially outside the outer diameter  42 , is offset from the circular-annular disk face  40  in such a way that the circular-annular disk face  40  is raised or elevated relative to the coaxial outer region. The height of the elevation is also indicated by reference numeral  55 . The dimensioning of the dimensions  41 ;  51 ,  42 ;  52  and  55  is done as a function of the peripheral operating conditions.