Abstract:
A valve assembly for use in a fuel pump comprises a body member with a valve aperture, a valve member movable within the body member and adapted to close the valve aperture, and a biasing arrangement having a first part fixed with respect to one of the body member and the valve member and a second part fixed with respect to the other of the body member and the valve member. The biasing arrangement is adapted to bias the valve member to close the valve aperture. The biasing arrangement comprises a helical spring with a first diameter at the first part and a second diameter at the second part. The first diameter of the helical spring is different from the second diameter of the spring. The helical spring is at its first part retained by an interference fit against the member with respect to which it is fixed.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a valve assembly suitable for use in a fuel pump. Embodiments of the valve assembly described are particularly suitable for use in a fuel pump for use in a common rail fuel injection system for supplying high pressure fuel to a compression ignition internal combustion engine. 
       BACKGROUND TO THE INVENTION 
       [0002]    Fuel pumps are employed in a variety of engine systems. Common rail fuel injection systems for compression ignition (diesel) internal combustion engines provide excellent control of all aspects of engine operation and require a pump to act as a source of high pressure fuel. One known common rail fuel pump is of radial pump design and includes three pumping plungers arranged at equi-angularly spaced locations around an engine driven cam. Each plunger is mounted within a plunger bore provided in a pump head mounted to a main pump housing. As the cam is driven in use, the plungers are caused to reciprocate within their bores in a phased, cyclical manner. As the plungers reciprocate, each causes pressurisation of fuel within a pump chamber defined at one end of the associated plunger bore in the pump head. Fuel that is pressurised within the pump chambers is delivered to a common high pressure supply line and, from there, is supplied to a common rail or other accumulator volume, for delivery to the downstream injectors of the common rail fuel system. 
         [0003]    Such a fuel pump has an inlet valve for admitting fuel under low pressure and an outlet valve for letting out the pressurised fuel. Both inlet and outlet valves are non-return valves (also known as check valves)—each have a valve member which is a moving element biased by a spring to close a valve aperture. 
         [0004]    For the inlet valve, the valve member forms a plunger. One end of the plunger is biased to close the valve aperture. The biasing spring is fixed to the other end of the plunger, the spring extending around the plunger shaft to a seat in the pump body. A spring seat is formed at the second end of the plunger to retain the biasing spring in compression between the two seats. A variety of approaches have been used to fix this spring seat: clipping the spring seat around the plunger shaft; press fitting a spring seat on to the plunger shaft; and welding or screwing the spring seat to the plunger shaft. 
         [0005]    For the outlet valve, the valve member is a ball biased to close the valve aperture by the biasing spring. The ball is located in one end of the biasing spring. A spring seat fixed to a body of the valve retains the other end of the biasing spring. The same variety of approaches are used to fix this spring seat as for the inlet valve: clipping the spring seat inside the bore of the valve body; press fitting a spring seat into the valve body; and welding or screwing the spring seat to the valve body. 
         [0006]    An example of such an arrangement is shown in WO 2006/125690 A1. This document describes a high pressure pump with an outlet valve in which a spring retainer is inserted into the outlet bore of the pump body and fixed into it by press fitting. 
         [0007]    It is an object of the present invention to provide a valve assembly suitable for use in a fuel pump and which avoids or overcomes the limitations of the aforementioned types of valve assembly. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the present invention, there is provided a valve assembly for use in a fuel pump comprising: a body member with a valve aperture; a valve member movable within the body member and adapted to close the valve aperture; and biasing means having a first part fixed with respect to one of the body member and the valve member and a second part fixed with respect to the other of the body member and the valve member, the biasing means being adapted to bias the valve member to close the valve aperture; wherein the biasing means comprises a helical spring with a first diameter at the first part and a second diameter at the second part, wherein the first diameter and the second diameter are different from each other, and wherein the helical spring at either the first part or the second part is retained by an interference fit against the member with respect to which it is fixed. 
         [0009]    Such an arrangement provides a valve assembly with fewer parts than in conventional prior art valve assemblies. Such an arrangement may also be beneficial in reducing tolerance requirements for the mounting of the biasing means. The resilience of the spring allows for a significant tolerance in the spring diameter with regard to the diameter of the component that forms the other part of the interference fit. Prior art arrangements, such as the use of spring seats without such resilient properties, will not allow such a significant design tolerance. 
         [0010]    Preferably, the first part of the helical spring has a helical pitch throughout. 
         [0011]    Advantageously, the first part of the helical spring comprises at least two close wound turns. 
         [0012]    It is also preferred that the first part of the helical spring contains a closed loop at an end of the helical spring. This closed loop may be ground to form a flat end surface to the helical spring. 
         [0013]    Advantageously, the helical spring comprises a variable diameter section, comprising the first part of the helical spring and in which the diameter of the helical spring varies, and a constant diameter section, comprising the second part of the helical spring and in which the diameter of the helical spring is substantially constant. Substantially the whole of the variable diameter section may be close wound. 
         [0014]    In a first arrangement, the first diameter is larger than the second diameter, and wherein the first part of the helical spring forms an interference fit against an inner wall of the body member. Furthermore, in this arrangement the second part of the helical spring may include a free end that abuts the valve member. 
         [0015]    In a second arrangement, the first diameter is smaller than the second diameter, and wherein the first part of the helical spring forms an interference fit against an outer surface of the valve member. In this arrangement, the helical spring is received over the valve member and the second part of the helical spring forms an interference it with an annular groove on the body member. 
         [0016]    In one aspect of the invention, a fuel pump comprises an inlet valve assembly and an outlet valve assembly, wherein one or both of the inlet valve assembly and the outlet valve assembly is a valve assembly as described above. 
         [0017]    In one form of fuel pump according to this aspect of the invention, the inlet valve assembly is a valve assembly as claimed in the second arrangement of valve assembly, and wherein the outlet valve assembly is a valve assembly as claimed in the first arrangement of valve assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will now be described, by way of example only, by reference to the following drawings in which: 
           [0019]      FIG. 1  is a cut-away view of the valve assembly of a first embodiment of the present invention; 
           [0020]      FIG. 2  is a cut-away view of the valve assembly of a first embodiment of the present invention; and 
           [0021]      FIG. 3  is a cut-away view of the fuel pump assembly comprising the valve assemblies of the first and second embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Referring to  FIG. 3 , a fuel pump assembly  10  comprises an inlet valve  20  to allow low pressure fuel into the fuel pump assembly  10  and an outlet valve  30  to allow high pressure fuel to leave the fuel pump assembly  10 . The fuel is pressurised in a fuel chamber  50  by a pumping plunger  40  reciprocating in a bore  42  provided in a pump body  43 . This plunger may, for example, be driven by a cam (not shown) and is used to pressurise the fuel. 
         [0023]    The inlet valve  20  comprises a valve member  22  in the form of a plunger. This valve member  22  reciprocates in an inlet bore  21  of the pump body  43 . The inlet bore  21  joins the fuel chamber  50  at a valve aperture  26 . A valve closure end  24  of the valve member  22  is biased to close the valve aperture  26  by a first biasing arrangement in the form of a spring  28 . The biasing spring  28  works in compression, one end located in an annular groove  27  on the pump body  43  and the other end fixed to a part of the valve member  22  remote from the valve aperture  26 . The biasing spring  28  varies in diameter along its length—it has a first, smaller, diameter at the end remote from the valve aperture  26 , and at this end forms an interference fit around the valve member  22 . It should be noted that the term ‘interference fit’ is used here to mean a fit between two mating parts whose tolerances are such that one part will have a slightly larger dimension than that of the part into which it will be inserted such that the fastening between the two parts which is achieved by friction after the parts are pushed together. 
         [0024]    The outlet valve  30  comprises a valve member  32  in the form of a ball. This ball  32  is located in an outlet bore  31  of the pump body  43 , the outlet bore  31  joining the fuel chamber  50  at a valve aperture  36 . The ball  32  is biased to close the valve aperture  36  by a second biasing arrangement, also in the form of a spring  38 . The biasing spring  38  works in compression, one end located around the ball  32  and the other end fixed to a part of the outlet bore  31  remote from the valve aperture  36 . The biasing spring  38  varies in diameter along its length—it has a first, larger, diameter at the end remote from the valve aperture  36 , and at this end forms an interference fit with the inner surface of the outlet bore  31 . 
         [0025]    Both the inlet valve  20  and the outlet valve  30  are non-return valves, sometimes alternatively referred to in the art as check valves. Each valve is biased so that it will only open at a distinct opening pressure. The opening pressure for the inlet valve  20  is lower than the opening pressure for the outlet valve  30 . The fuel pump  10  works in the following way. When the plunger  40  moves down, it expands the size of the fuel chamber  50  and lowers the pressure in it. When the pressure is sufficiently low, the difference in pressure between the fuel inlet pressure and the fuel chamber pressure becomes sufficient for the inlet valve  20  to open and for fuel to be admitted into the fuel chamber  50 . When the fuel chamber  50  fills and the plunger  40  starts to move upwards, the pressure in the fuel chamber  50  increases. When the inlet fuel pressure no longer exceeds the fuel chamber pressure sufficiently to hold the inlet valve  20  open, the inlet valve  20  closes. Throughout these stages, the outlet valve  30  has been closed as there has not been sufficient fuel chamber pressure to open it. As the plunger  40  continues to move upwards in the bore  42 , the pressure in the fuel chamber  50  rises to the point where it is sufficient to open the outlet valve  30 . When the outlet valve  30  opens, pressurised fuel passes out through the outlet until the fuel chamber pressure drops to the point when the outlet valve  30  closes again. The cycle described above then starts again and repeats. 
         [0026]    The inlet valve  20  will now be described in more detail with reference to  FIG. 1 , which provides a first embodiment of a valve assembly according to the invention. The biasing spring  28  is a helical spring that varies in diameter along its length. The main section  206  of the spring  28  is in the form of a conventional cylindrical helical spring. The free end of this main section  206  is located in the groove  27  in the pump body  43  which surrounds the valve member  22 . At the other end of this main section  206 , the free diameter of the spring  28  reduces to less than the diameter of the valve member  22 . The spring  28  is put in place by pressing it over the narrow end of the valve member  22  (the valve member  22  ends in a tapered section  208 , making it easier to carry out this process effectively) and is forced to a position sufficient to provide the necessary biasing force on the valve member  24 . The end of spring  28  is closed and ground to form a flat end surface  212 —this allows force to be applied evenly in positioning of the spring  28 , enabling the spring to be accurately positioned. 
         [0027]    At the reduced diameter end  202  of the spring  28 , the inner surface  204  of the spring  28  forms an interference fit with the outer surface of the valve member  22 . At the very end, the spring  28  forms a closed loop. To provide an effective interference fit, at least two turns of the spring  28  are in contact with the outer surface of the valve member  22 . This provides rigidity to the reduced diameter end  202  of the spring  28 , and the number of turns may be further increased if greater rigidity is required, for example for a significantly heavier duty valve. 
         [0028]    As shown in  FIG. 1 , the reduced diameter end  202  of the spring and a transitional zone  210  of the spring (between the reduced diameter end  202  and the main section  206 ) are both close wound, with one turn of the spring adjacent to and touching the next turn. For the transitional zone  210 , this close winding is desirable to minimize stress in the transitional zone (this may be particularly significant for heavier duty valves). For the reduced diameter end  202 , the close winding further increases the rigidity of the spring and hence improves the interference fit with the valve member  22 . Where both the reduced diameter end  202  and the transitional zone  210  are close wound, the only working turns of the spring  28  in compression are those in the main section  206  of the spring. This has the benefit that the properties of the spring  28  in compression will be similar to those of a conventional cylindrical spring of the length of the main section  206 —in particular, there will not be the significant non-linearity that would be found in a spring of variable diameter that was not close wound—so the performance of the spring will be easier to predict and model. 
         [0029]    The outlet valve  30  will now be described in more detail with reference to  FIG. 2 , which provides a second embodiment of a valve assembly according to the invention. The biasing spring  38  is a helical spring that varies in diameter along its length. The main section  306  of the spring  38  is in the form of a conventional cylindrical helical spring. The free end of this main section  306  locates against the ball  32  that the spring  38  biases to close the aperture  36 . At the other end of this main section  306 , the free diameter of the spring  38  increases to greater than the diameter of the outlet bore  31 . The spring is put in place by pressing it into the outlet bore  31  and forcing it to a position sufficient to provide the necessary biasing force on the ball  32 . The end of the spring  38  is closed and ground to form a flat end surface  312 —this allows force to be applied evenly in positioning of the spring  38 , enabling the spring to be accurately positioned. 
         [0030]    At the increased diameter end  302  of the spring  38 , the outer surface  304  of the spring  38  forms an interference fit with the inner surface of the outlet bore  31 . To provide an effective interference fit, at least two turns of the spring  38  are in contact with the outer surface of the outlet bore  31 . This provides rigidity to the increased diameter end  302  of the spring  38 , and the number of turns may be further increased if greater rigidity is required, for example for a significantly heavier duty valve. 
         [0031]    As shown in  FIG. 2 , the increased diameter end  302  of the spring  38  and a transitional zone  310  of the spring (between the increased diameter end  202  and the main section  306 ) are both close wound, with one turn of the spring adjacent to and touching the next turn. For the transitional zone  310 , this close winding is desirable to minimize stress in the transitional zone (this may be particularly significant for heavier duty valves). For the increased diameter end  302 , the close winding further increases the rigidity of the spring and hence improves the interference fit with the outlet bore  31 . This issue is more significant for the outlet valve  30  than for the inlet valve  20  in the fuel pump of  FIG. 3 , as two factors can increase the requirements on the interference fit and the transitional zone for the outlet valve  30  relative to the inlet valve  20 . One is that the diameter of the spring  38  on the outlet valve  30  is greater at the interference fit, whereas the diameter of the spring  28  on the inlet valve  20  is less at the interference fit. If the spring were to support the same load, then in outlet valve  30  the turns of the spring (or coils) by the interference fit would have a higher stress, with consequent modification of the spring rate. The other is that the elements of the outlet valve  30  are exposed to higher pressures (at high pressures the valve is forced open), whereas the elements of the inlet valve  20  are not (where there is a high pressure in the fuel chamber  50 , the inlet valve is forced to close). In the particular embodiment illustrated, the change of diameter is relatively large on the outlet valve spring  38  relative to the inlet valve spring  28 , so a more closely wound transition will improve alignment. 
         [0032]    These and other valve assemblies according to embodiments of the invention can be used in other fuel pump assemblies, and in assemblies for other forms of pump.