Abstract:
The invention relates to a fuel pump for a fuel system of an internal combustion engine, having a housing and a housing cap joined to the housing. In order to create a fuel pump which in its operation generates little airborne sound, structure-borne sound (vibration amplitudes) and pulsations in a low-pressure region of the fuel pump, it is proposed that the housing cap has at least one damping element, which is embodied as a sandwich construction having at least a first cover layer, a second cover layer, and a damping connection layer disposed between them. The damping connection layer has a markedly higher elasticity and/or higher material damping than the two cover layers, which may be constructed of sheet metal or the housing cap itself.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on German Patent Application No. 10 2007 038 984.3 filed on Aug. 17, 2007, upon which priority is claimed. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a fuel pump for a fuel system of an internal combustion engine, having a housing and a housing cap joined to the housing. 
   2. Description of the Prior Art 
   A fuel pump of this kind is known for instance from German Patent Disclosure DE 10 2005 033 634 A1. This fuel pump is a radial piston pump, that can be driven with the aid of an eccentric or cam portion and that can pump fuel from a low-pressure region into a high-pressure region of a fuel system of an internal combustion engine and subject it to high pressure. The fuel pump furthermore has a housing that is closed with a housing cap. In the operation of this radial piston pump, pulsations occur fundamentally in the low-pressure regions and they are damped using a pressure damper disposed in the low-pressure region. 
   Fuel pumps are also generally known that to vary a pumping rate have a quantity control valve which an be actuated to set an open or closed state. In these fuel pumps, as a result of mechanical contacts that occur in particular upon actuation of the quantity control valve between the parts present in the quantity control valve, structure-borne sound also occurs, which is transmitted to the housing of the fuel pumps. 
   OBJECT AND SUMMARY OF THE INVENTION 
   The object of the invention is to create a fuel pump which in its operation generates only slight vibration amplitudes and in particular emits little airborne sound. 
   According to the invention, it was recognized that the sound generated by a high-pressure pump can be reduced by damping vibration of a housing cap, occurring from pulsations or structure-borne sound in a low-pressure region, and caused for instance by a switching quantity control valve, and that a damping element embodied as a sandwich construction is especially suitable for this purpose. This is because such a damping element reduces the vibration of the housing cap above all in the following way: The damping element, upon deformation, absorbs mechanical energy, especially in the intermediate layer, and converts it into heat by a displacement of the individual layers of the sandwich construction. The reduction in the vibration amplitudes at the housing cap also reduces the emission of airborne sound. 
   A damping element of his kind is quite compact, so that the outside dimensions of the fuel pump increase only slightly once such a damping element is attached. For known fuel pumps, existing manufacturing and assembly concepts can thus continue to be used with only slight adaptations. Moreover, because of the reduced vibration, the material of the housing cap is less stressed dynamically and therefore has improved durability. 
   To obtain a robust, temperature-resistant damping element, it is preferred that the two cover layers each be formed by a respective metal sheet. 
   In order not only to reduce the noise generation but also to ensure that only slight hydro pulsations if any are imported into a low-pressure region of the fuel system, it can be provided that an inner side of the housing cap is subjected to a pressure that prevails in a low-pressure region. The damping element then cooperates directly with the low-pressure region and absorbs shock waves in the low-pressure region that are due to the pulsations. It preferably acts as a supplementary provision for pulsation damping, in addition to a pressure damper that is already present in known fuel pumps. The advantages of the supplementary pulsation damping are apparent especially when the contents of the pulsation spectrum are of high frequency. The supplementary pulsation damping moreover indirectly leads to a reduction in the tendency to vibrate as well and thus to a reduction in sound emission from further portions of the low-pressure region. These further portions as well, since they are coupled hydraulically to the fuel pump via the fuel located in the low-pressure region, can in fact be excited to vibration by the pulsations. 
   It can be provided that the damping element has a plurality of damping connection layers and corresponding cover layers. As a result, the damping action of the damping element is further improved. Nevertheless, the damping element remains relatively compact and can be made economically. 
   To attain a wide-surface area and nonpositive-engagement connection of the damping element to the housing cap of the fuel pump, it can be provided that there is a glue layer between the damping element and the housing cap. A glue layer can also be produced quickly and with a small number of work steps and is thus economical. 
   To further simplify mounting the damping element on the housing cap, a self-adhesive glue layer can be provided, or a glue layer can be used of the kind whose adhesive action ensues only when the damping element and the housing cap are pressed against one another. 
   If a damping connection layer is disposed between the glue layer and the cover layer, then the damping action of the damping element can be improved still further while increasing the dimensions of the damping element only relatively slightly. 
   To reduce the outside dimensions of the fuel pump, the damping element can be integrated with the housing cap in such a way that at least a portion of the housing cap forms a layer of the damping element. The reduction in the outside dimensions is due to the fact that only past of the damping element is located on an outer side of the housing cap. 
   A further possible way of obtaining a compact fuel pump is for at least one region of the housing cap overall to form the damping element. If the entire housing cap is embodied as a damping element, then the result is on the one hand a low number of parts of the fuel pump and on the other a high damping action, since the individual layers of the sandwich construction embody the entire housing cap and thus have a relatively large amount of surface area. 
   It is especially preferred that the damping element is joined directly to the housing, in particular welded to it. It is advantageous for at least all the cover layers of the damping element to be joined to the housing, in particular by welding. Thus for given requirements in terms of stability of the housing cap, the housing cap can be produced using comparatively little material. 
   The requisite elasticity of the connection layer can be attained by providing that the connection layer is formed of an elastomer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a sectional side view of a fuel pump, in a first preferred embodiment of the present invention; 
       FIG. 2  is a sectional side view of a housing cap with a damping element, in a second preferred embodiment; 
       FIG. 3  is a view similar to  FIG. 2  of a third preferred embodiment; and 
       FIG. 4  is a sectional side view of a portion of a damping element in a fourth preferred embodiment, shown greatly enlarged. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the overall construction of a fuel pump  117  which has an overall cylindrical housing  13  and a housing cap  15  solidly joined to the housing on the top thereof. The fuel pump  11 , in its lower region, has a radially protruding securing flange  17  extending all the way around the housing  13 . A low-pressure connection  19  is disposed on the housing  137  protruding away radially. This connection communicates via a low-pressure line  21 , which is forced as a bore, with a filter  23  that is disposed in a pressure damper chamber  25  formed below the housing cap  15 . The pressure damper chamber  25  is bounded laterally and at the top by an inner side  26  of the housing cap  15  and at the bottom by the housing  13 . A pressure damper  273  which when viewed from above is overall circular in shape, is located in the pressure damper chamber  25 . Alternatively to the embodiment shown a housing  13  can also be provided that is not cylindrical in shape; for instance, it may be prism-shaped or angular and in particular block-shaped. 
   The pressure damper chamber  25  furthermore communicates, via a line not visible in the sectional view in  FIG. 1 , with a metering unit  29 , which has an electromagnetic actuator  31  connected to an engine control unit (not shown). By means of the electromagnetic actuator  31 , the degrees to which the metering unit  29  is opened can be set or adjusted. In an embodiment not shown, instead of the metering unit  29  and the inlet valve  33 , an inlet valve device typically known as a “quantity control valve” is provided, which has an electromagnetic actuator by means of which an open or closed state of the quantity control valve can be set or adjusted. All the parts and regions of the fuel pump  11  that communicate hydraulically directly with the low-pressure connection  19  form a low-pressure region  32 . This low-pressure region  32  includes in particular the pressure damper chamber  25 . The metering unit  29  is connected downstream to an inlet valve  33  embodied as a check valve, which leads to a work chamber  35  of the fuel pump  11 . Between the work chamber  35  and a high-pressure region is an outlet valve embodied as a check valve (neither shown). 
   The work clamber  35  has a cylindrical bush  37 , in which a pump piston  39  is supported axially displaceably. Below the cylindrical bush  37  is a sealing element  41 , which is retained by a seal holder  43 . Somewhat above a lower end of the pump piston  39  is a spring holder  45  of circular-annular cross section that is solidly joined to the pump piston. A spring  47  is tensed between the spring holder  45  and the seal holder  43 . Above the sealing element  41  is a hollow chamber  49 , which is defined by the seal holder  43 , the cylindrical bush  37  and the housing  13 , and which communicates with the low-pressure connection  19  through a return line  51  formed by a bore. 
   A damping element  53  embodied as a sandwich construction is disposed on the housing cap  15 . This damping element  53  has three layers; a middle layer is a connection layer  55  formed of polymer, and an upper layer is a cover layer  57  of sheet metal. A lower layer  59  is formed by the housing cap  15  itself. 
   In operation of the fuel pump  11 , the pump piston  39  is pressed upward at regular intervals, for instance by a cam or eccentric portion, so that the work chamber  35  decreases in size. At the times when the pump piston  39  is not being pressed upward, the spring  47  assures that the pump piston  39  moves downward and thus increases the size of the work chamber  35 . 
   Fuel which is at a relatively low pressure is delivered to the low-pressure connection  19 . From the low-pressure connection  19 , the filet passes via the low-pressure line  21  to reach the pressure damper chamber  25 , and therefore the inner side  26  of the housing cap is subjected to a pressure prevailing in the low-pressure region  32 . Upon an enlargement of the work chamber  35  because of a downward motion of the pump piston  39  (intake stroke), fuel from the pressure damper chamber  25  reaches the work chamber  35  via the open metering unit  29  and the also-open inlet valve  33 . Upon a reduction in size of the work chamber  35  following the intake stroke, because of an upward motion of the pump piston  39  (supply stroke), the fuel located in the work chamber  35  is subjected to a pressure and pumped into the high-pressure region via the outlet valve of the fuel pump  11 . By means of a suitable setting of a degree of opening of the metering unit  29  with the aid of the electromagnetic actuator  31 , a pumping rate of the fuel pump  11  is set. In the embodiment not shown that has the quantity control valve, this quantity control valve is actuated at suitable times to set a defined pumping rate of the fuel pump  11 . In this process, for setting a reduced pumping rate compared to a maximum pumping quantity, a portion of the fuel located in the work chamber  35  is not pumped into the high-pressure region but instead is returned to the low-pressure region  32 . The engine control unit executes a control or regulating method accordingly. In operation of the fuel pump  11 , a slight fuel quantity reaches a region between the pump piston  39  and the cylindrical bush  37  and accumulates in the hollow chamber  49 . This leak fuel quantity is returned to the low-pressure region  32  with the aid of the return line  51 . 
   Because of the constant alternation between intake stroke and pumping stroke and because of abrupt interruption in the volumetric flows in a quantity control valve—if present—an uneven flow of fuel into the low-pressure region  32  results. This causes pulselike pressure fluctuations pulsations) in the low-pressure region  32 , which if they were not damped could impair the operation of the fuel pump  11 , or of a fuel system to which the fuel pump  11  belongs. A fundamental frequency of the pulsations, depending on the operating state of the fuel pump  11 , is typically on the order of magnitude of approximately 15 Hz to 200 Hz. Because of the nonharmonic, uneven pumping, the pulsations include higher-frequency harmonics and broadband spectral contents at higher frequencies. 
   Because of the pressure fluctuations, caused by the pulsations, inside the low-pressure region  32  and thus inside the pressure damper chamber  25  as well, the housing cap  15  is deformed outward and inward in alternation. The damping element  53  is deformed accordingly as well. The connection layer  55  and the cover layers  57  and  59  of the damping element  53  shift relative to one another. In the process, the cover layers  57  and  59  become curved, and the connection layer  55  experiences shear stress. In this deformation, the damping element  53  absorbs mechanical energy and converts it into heat. In this way, the pulsations in the low-pressure region  32  are damped, and sound generation in the housing cap  15  caused by these deformation motions is reduced as well. 
   In particular, vibrations in the form of natural vibration, in particular bending vibrations of the housing cap  15 , are at least partially eliminated. The term “natural vibration form” is understood to mean a vibrational motion caused by the nature of the housing cap  15  and characterized among other factors by a resonant frequency. Its elimination is accomplished in that certain natural vibration forms are damped and/or resonant frequencies of certain natural vibration forms are altered in such a way that in the operating states intended for the fuel pump  11 , these natural vibration forms occur at most with only a slight amplitude. The nature of the housing cap  15  is thus defined by the damping element  53  in such a way that the pulsations cannot, or can to only a limited extent, engender independent vibrations of the housing cap  15 , especially at a frequency that is within the range of audible sound. 
   Since the housing cap  15  is exposed directly to the pressure prevailing in the low-pressure region  32 , interactions occur between the low-pressure region  32  and the housing cap. As a result, the housing cap  15 , damped with the aid of the damping element  53 , also brings about pulsation damping of the fuel in the low-pressure region  32 . This pulsation damping occurs in addition to the pulsation damping effected by the pressure damper  27 . 
   Which natural vibration forms of the housing cap  15  have to be damped and to what extent depends in particular on the precise construction of the fuel pump  11  and on the planned operating states of the fuel pump  11 . It is therefore necessary that the nature of the damping element  53 —in particular, the properties of the connection layer  55  and the thickness of the individual layers  55 ,  57  and  59 —be adapted to an intended use for the fuel pump  11 . 
   Such an adaptation can thus lead for instance to the embodiment shown in  FIG. 2 , in which the damping element  53  has a total of three layers once again, and there is an adhesive or glue layer  61  between the damping element  53  and the housing cap  15 . This glue layer  61  is applied to the damping element  53  in the manufacture of the damping element, and in the manufacture of the fuel pump  11 , the damping element  53  together with the glue layer  61  is pressed onto the housing cap  15 . The glue layer  61  is self-adhesive. In an embodiment not shown, however, the glue layer  61  is pressure-activated; that is, it does not develop its adhesive action until the damping element  53  and the housing cap  15  are pressed against one another. 
   As shown in  FIG. 3 , the housing cap  15  can itself be embodied as a damping element  53  also. The damping element  53  again has the connection layer  55 , which is sandwiched by two cover layers  57  and  59 . The two cover layers  57  and  59  are formed by metal sheets and are welded at their edges  62  to the housing  13 . In an embodiment not shown, only one cover layer  57  is welded to the housing  13 . 
   In an embodiment not shown, the entire housing cap  15  is not embodied as the damping element  53 ; instead, only a portion of the housing cap  15  forms the damping element  53 . In a further embodiment, not shown, cover layers and connection layers are disposed in alternation not only above the housing cap  15 , or in other words outside the pressure damper chamber  25 , but also below the housing cap  15 , or in words inside the pressure damper chamber  25 . The portion of the housing cap  15  that is directly contacting the layers of the damping element  53  thus itself acts as a layer of the damping element  53 . 
   A further possible way of realizing a damping element that can be glued to the housing cap  15  is shown in  FIG. 4 . This damping element  25  has two cover layers  57  and  59 , each made from sheet metal, below each of which is a respective connection layer  55 , which is formed from an elastomer. The glue layer  61  is applied to the lowermost connection layer  55  in  FIG. 4 . Also in this embodiment, the number and thickness of the individual layers  55 ,  57 ,  59  and  61  can be varied in order to meet special requirements made of a certain fuel pump  11  or for the sake of planned operating states of the fuel pump  11  (such as a planned range of a stroke frequency of the pump piston  39 ). In the other embodiments, the connection layer may likewise be formed of an elastomer. 
   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.