Patent Publication Number: US-8523534-B2

Title: High-pressure pump for supplying fuel to an internal-combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2008/057265 filed on Jun. 11, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In particular, the present invention relates to a high-pressure piston pump for supplying fuel in a common-rail circuit of an internal-combustion engine. 
     2. Description of the Prior Art 
     A piston pump of this type generally includes a pump body. A shaft, which extends along a longitudinal axis, is supported rotatably about the longitudinal axis by the pump body and has an eccentric portion and a prismatic jacking end. A first pumping station has a gear engaged with the prismatic jacking end. A second pumping station has at least one piston, which is slidable inside the pump body transversely with respect to the longitudinal axis and which is actuated by the eccentric portion of the shaft. 
     The first pumping station essentially has a gear pump which produces a first relatively small pressure difference, while the second pumping station generally comprises three pistons which produce a large pressure difference, also greater than 1600 bar in the high-pressure pumps which are currently manufactured, and destined to increase in order to improve further the performance features of internal-combustion engines. 
     The first pumping station essentially comprises a gear pump which produces a first relatively small pressure difference, while the second pumping station generally comprises three pistons which produce a large pressure difference, also greater than 1600 bar in the high-pressure pumps which are currently manufactured, and destined to increase in order to improve further the performance features of internal-combustion engines. 
     High-pressure pumps pose problems of wear of certain components such as the prismatic jacking end of the shaft which, during use, is engaged with a gear generally made of sintered material. At present, the actuating shaft of a high-pressure pump is made of 16MnCrS5 steel which undergoes a surface hardening heat treatment. However, the prismatic jacking end is subject to greater wear than the remainder of the shaft and is the main cause of a relative short working life of the high-pressure pump. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a high-pressure pump for an internal-combustion engine which does not have the drawbacks of the known art and which, in particular, is particularly low-cost. 
     According to the present invention a high-pressure pump for an internal-combustion engine is provided. The pump includes a pump body. A shaft, which extends along a longitudinal axis, is supported rotatably about the longitudinal axis by the pump body and has an eccentric portion and a prismatic jacking end. A first pumping station has a gear engaged with the prismatic jacking end. A second pumping station has at least one piston, which is slidable inside the pump body transversely with respect to the longitudinal axis and which is actuated by the eccentric portion of the shaft. The high-pressure pump according to the invention further providing that the prismatic jacking end is made of a first material and the remainder of the shaft is made of a second material, where the first material being harder than the second material. 
     According to the present invention, the wear of the shaft is limited substantially and uniformly spread over the various parts. Consequently, the working life of the high-pressure pump is increased as a whole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristic features and advantages of the present invention will become clear from the description of an exemplary embodiment thereof which follows, provided with reference to the accompanying figures in which: 
         FIG. 1  is a perspective view, with parts cross-sectioned and parts removed for the sake of clarity, of a high-pressure pump provided in accordance with the present invention; and 
         FIG. 2  is an exploded perspective view, on a larger scale and with parts removed for the sake of clarity, of a detail of the high-pressure pump of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 ,  1  denotes in its entirety a high-pressure pump which is able to compress the fuel to pressures greater than 2,200 bar in order to feed the fuel to a common rail of an internal-combustion engine not shown in the accompanying figures. 
     The pump  1  comprises a pump body  2  defined by three metallic bodies  3 ,  4  and  5  assembled together; a low-pressure pumping station  6  and a high-pressure pumping station  7 ; and a shaft  8  which extends along a longitudinal axis A 1  and is able to actuate simultaneously the low-pressure pumping station  6  and the high-pressure pumping station  7 . 
     The low-pressure pumping station  6  is arranged in the pump body  2  and comprises a gear pump  9 , a gear  10  of which, arranged in a seat  11  of the pump body  2 , is shown in  FIG. 1 . 
     The high-pressure station  7  comprises three pistons  12 , each of which extends along an axis A 2  in a substantially radial direction with respect to the longitudinal axis A 1  and is slidable inside a cylinder  13  formed in the pump body  2 . 
     Each piston  12  is actuated along the axis A 2  of the shaft  8  which, via the intervening arrangement of a hub  14  and a cup  15 , produces compression of the fuel against the action of an opposition spring  16 . 
     The feed conduits  17 , delivery conduits  18 , feed valves  19  and the delivery valves  20  are formed inside the pump body  2 . 
     The shaft  8  is supported rotatably about the longitudinal axis A 1  by the pump body  2  and comprises in succession a jacking pad end  21 , a conical portion  22 , a cylindrical portion  23 , an eccentric portion  24 , a cylindrical portion  25 , a cylindrical portion  26  with a diameter smaller than the portion  25  and a prismatic jacking end  27  which, during use, is inserted in the gear  10 . 
     With reference to  FIG. 2  the shaft  8  is formed by joining together the prismatic jacking end  27  with the remainder of the shaft  8 . The prismatic jacking end  27  is made of sintered carbide, more specifically sintered tungsten carbide, while the remainder of the shaft is made of steel, more specifically 16MnCrS steel. 
     The joint between the prismatic jacking end  27  and the remainder of the shaft  8  is performed by means of a braze-welding method. The shaft  8  is then subjected to a heat treatment in order to obtain surface hardening. 
     The remainder of the shaft  8 , more specifically the cylindrical portion  26 , has a pocket  28  able to house partly the prismatic jacking end  27 . 
     The prismatic jacking end  27  comprises a prismatic body  29  and an end lug  30  with a circular cylindrical form which, during use, is coaxial with the cylindrical portion  26 . 
     The pocket  28  has a seat  31  for housing the end lug  30  and a seat  32  for housing the prismatic body  29 . 
     More specifically, the seat  31  is delimited by a surface  33  matching the end lug  30 , while the second seat  32  is delimited by a bottom surface  34  and by two lateral surfaces  35  parallel to and facing each other and matching the prismatic body  29 . 
     Production of the shaft  8  is performed as follows: the prismatic jacking end  27  is formed by means of sintering of tungsten carbide powders, while the remainder of the shaft  8  is produced by means of lathe-machining and milling. The prismatic jacking end  27  is inserted in the pocket  28 . The lug end  30  engaged with the first seat performs centring of the prismatic jacking end  27  with respect to the remainder of the shaft along the longitudinal axis A 1 , while insertion of the prismatic body  29  between the lateral walls  35  prevents rotation of the prismatic jacking end  27  about the longitudinal axis A 1  relative to the remainder of the shaft  8 . 
     The shaft  8 , after definition of its form, is braze-welded so as to produce an irreversible joint between the prismatic jacking end  27  and the remainder of the shaft  8  and subsequently subjected to a surface-hardening heat treatment. 
     The foregoing relates to the 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. 
     The shaft  8  thus produced has a prismatic jacking end  27  which has a hardness greater than the remainder of the shaft and able to limit substantially the wear of the parts of the prismatic jacking end  27  in contact with the gear  10 , which is preferably made of sintered carbide.