Internal-combustion engines – Charge forming device – Fuel injection system
Reexamination Certificate
2002-09-09
2004-07-20
Miller, Carl S. (Department: 3747)
Internal-combustion engines
Charge forming device
Fuel injection system
C123S495000, C123S467000, C417S216000
Reexamination Certificate
active
06763808
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fuel pump comprising a plurality of plungers and a camshaft having a plurality of drive cams. Each of the drive cams corresponds to one of the plungers, and the camshaft with the drive cams engages the plungers in reciprocal movement by rotating the camshaft. The present invention also relates to a fuel feeding device that employs this fuel pump.
BACKGROUND ART
A fuel feeding device adopting the so-called common rail system comprises a fuel pump, a common rail in which high-pressure fuel force-fed from the fuel pump is stored, and fuel injection valves each provided in correspondence to one of the cylinders of an internal combustion engine to enable a fuel feed of the high-pressure fuel stored in the common rail to enter the cylinders. The fuel pump normally includes two plungers, and these plungers are caused to move reciprocally by drive cams provided as separate units at the camshaft to supply pressurized fuel to the common rail. In a standard common rail system through which fuel is fed to, for instance, a six-cylinder engine by employing two plungers, three cam lobes are provided over constant intervals at each of the drive cams that drive the individual plungers. The phases of the drive cams are offset from each other by 60° so as to achieve six injections by allowing the plungers alternately to make three reciprocal movements as the camshaft rotates over 360° once, as disclosed in Patent Official Gazette No. 2797745.
As is understood from the cam lift characteristics and the shape of the cams described in the publication, a fuel pump such as the one described above normally assumes the shape shown in FIG.
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(A). Namely, cam lobes formed at the individual drive cams &agr; and &bgr; are each formed to achieve a shape having portions used for a forward stroke. (i.e., movement from bottom dead center to top dead center) and a backward stroke (i.e., movement from top dead center to bottom dead center) of the corresponding plunger, and the shapes are symmetrical with respect to each other so that each drive cam takes on a triangular shape overall.
As a result, the lift characteristics manifesting at each of the plungers driven by the drive cams &agr; and &bgr; during the forward stroke, in which the plunger travels from the bottom dead center to the top dead center, (i.e., away from the center of rotation of the drive cams) and the lift characteristics manifesting at the same plunger during the backward stroke, in which the plunger travels from the top dead center to the bottom dead center (i.e., toward the center of rotation of the drive cams) are symmetrical. In addition, the lift characteristics of one of the plungers manifest as sine waves whose phase is offset by 60° from the sine waves representing the lift characteristics of the other plunger, as shown in FIG.
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(B). Since one of the plungers first ascends from the bottom dead center to the top dead center and completes an injection, and then the other plunger starts ascending from the bottom dead center in the structure of the related art described above, the geometric injection rate (GIR) achieves characteristics whereby the value of the geometric injection rate continuously changes between 0 and the peak value every 60° of the cam rotating angle (cam angle), as shown in FIG.
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(C). Since the cam speed and the drive torque are roughly in proportion to the characteristics of the geometric injection rate, the plunger lift speed (i.e., the cam speed) and the drive torque, too, manifest characteristics whereby they fluctuate in a similar manner.
While a fuel pump having the symmetrical cams described above is normally utilized in an injection pump employed in a common rail system in the related art, a number of problems discussed below arise with regard to a fuel feeding device that employs such a fuel pump.
Namely, if the drive cams illustrated in FIG.
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(A) are utilized, the geometric injection rate (GIR) constantly fluctuates between 0 and the peak value over every 60°, causing a significant fluctuation in the pressure within the common rail. In addition, since the plunger must be lifted to the highest lift position while rotating the drive cam by 60°, the fluctuation in the cam speed, too, is bound to be great, which, in turn, requires a large drive torque.
Furthermore, since the plunger must be lifted to the highest lift position over a small cam rotating angle (60° in the example described above), it becomes necessary to form the cam nose with a small radius of curvature. This results in a large force being applied onto the cam surface while lifting the plunger, so that the surface pressure becomes a problem.
When the fuel pump in the prior art with the problems discussed above is utilized in a common rail system, the range of application in engines becomes limited, and the durability of the overall system is lowered.
Namely, while the pressure-withstanding performance of the product is normally designed to provide an ample margin to comfortably tolerate even the upper limit of the pressure fluctuation to maximize the service life of the product, the pressure-withstanding level of the overall system, including the fuel injection valves, the common rail, the piping connecting the fuel pump with the common rail, and the piping connecting the common rail with the fuel injection valves must be extremely high if the fluctuation of the pressure of the fuel let out from the fuel pump is great. For this reason, a significant fluctuation in the pressure gives rise to problems in that the weight of the product is bound to increase since the components need greater wall thickness, and in that the structure of the product becomes more complicated in order to achieve better pressure-withstanding performance.
In addition, since ignitions normally occur over irregular intervals in the engine combustion chamber of an engine having 10 or more cylinders, the timing with which the fuel is injection into the engine and the timing with which the fuel is fed from the fuel pump to the common rail cannot match each other if a fuel pump having drive cams corresponding to 6 cylinders is used as a replacement in conjunction with such an engine in which ignitions occur over irregular intervals. As a result, if the injection rate of the fuel pump fluctuates greatly, as illustrated in FIG.
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(C), inconsistency occurs between the injection characteristics manifesting as the fuel is injected from a fuel injection valve while the injection rate is low and the injection characteristics manifesting as the fuel is injected while the injection rate is high. For this reason, the injection pump in the prior art and a fuel feeding device that utilizes the injection pump cannot be employed in conjunction with engines in which ignitions occur over irregular intervals.
It is conceivable to increase the number of cam lobes in correspondence to a larger number of cylinders provided in the engine, or to increase the numbers of plungers and drive cams if the first option is not feasible, in order to solve the problem. However, it is difficult to secure a sufficient angle range for forming each cam lobe when the number of cam lobes formed at the drive cams is increased. Accordingly, it becomes necessary to increase the diameter of the drive cams to achieve the required lift quantity, or to increase the wall thickness of the drive cams to withstand the pressure applied to the cam surfaces. Thus, the dimension of the camshaft along the radial direction increases if the diameter of the drive cams is increased, or the dimension of the camshaft along the axial direction increases if the wall thickness of the drive cams is increased. Furthermore, the dimension of the camshaft along the axial direction increases instead when the numbers of plungers and drive cams are increased.
Moreover, when the drive cams in the related art described above are utilized, the drive torque constantly fluctuates between 0 and the peak valve. As a result, the load on the drive system and the noise occurring in the system are bound t
Bosch Automotive Systems Corporation
Miller Carl S.
Wenderoth , Lind & Ponack, L.L.P.
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