Cam shaft of fuel injection pump

Abstract: An object is to reduce noises produced by gears used to drive cams of a fuel injection pump. Conventionally, the gear (16, 17) noises are caused by a negative torque of a cam shaft. A cam profile (P) of each cam (22, 24) which lifts a plunger of the fuel injection pump is shaped like a fan. There are formed two cams (22, 24) in a single cam shaft. A lift increment segment (23) of a first cam (22) overlaps a lift decrement segment (25) of a second cam (24) to prevent a negative torque.

Description: The present invention relates to a cam shaft according to the first part of claim 1. DE-A-3 925 823 discloses a cam shaft of the afore-mentioned kind. To increase the lift of the cam without increasing the dimensions of the pump housing the cam of this shaft has a radius smaller than the radius of the shaft parts adjacent to the cam. Further as shown in Figure 3 of the accompanying drawings, a conventional fuel injection pump 1 of an in-line internal combustion engine (diesel engine) includes a plunger 2 for pressurizingly delivering fuel, a delivery valve 4 located between a fuel injection tube 3 and the plunger 2, a control sleeve 5 and a control rack 6. The control sleeve 5 and control rack 6 control in combination an amount of fuel injected. The plunger 2 contacts a cam 8 of a cam shaft 7 via a tappet 9 and the fuel injection pump 1 reciprocates up and down in accordance with a cam profile to inject fuel of predetermined pressure. Incidentally, the cam profile P of the conventional cam 8 has a lift increment segment 11 and a lift decrement segment 12. The lift increment segment 11 linearly reaches a maximum lift position 10 and the lift decrement segment 12 gently returns to a zero-lift position from the maximum lift position 10. Therefore, as shown in Figure 4, the drive torque of the cam 8 has a positive area 13 (a torque indicated by "A" in the illustration is added) in which a tappet 9 is lifted during the lift increment section 11 so as to pressurizingly transfer the fuel and a negative area 15 in which a spring force from a spring 14 pressing the tappet 9 against the cam 8 acts during the lift decrement section 12. In case of a plural-cylinder engine, plungers 2 of the same number as the cylinders are provided and the cams 8 have different phases to move the plungers 2 at predetermined timings. Incidentally, since the period of the positive and negative areas 13 and 15 of each cam 8 is short, positive and negative torques appear in a single cam shaft 7. Because of this, as shown in Figure 5, particularly in case of a gear-driven diesel engine fuel injection pump, each time the torque fluctuates between the positive and negative area, a contact face 18 of a gear 16 of the fuel injection pump side with a gear 17 of the drive side moves backward (clockwise in the drawing) by a gear lash as indicated by a double dot line, thereby raising a problem that a gear noise is produced. An object of the present invention is to provide a cam shaft of a fuel injection pump which can reduce a noise produced by gears of a fuel pump. According to the present invention, there is provided a cam shaft for driving a plunger of a fuel injection pump provided for each cylinder of an internal combustion engine comprising the features of claim 1. With this cam shaft, the drive torque for the cam shaft does not fluctuate or becomes constant and the negative area does not appear. Since the drive torque does not have the negative area, the gear noise due to the gear contact face change is greatly reduced. This and other objects, aspects and advantages of the present invention will become more apparent as the following detailed description is read with the attached drawings and the claims. Figure 1 is a view of a cam shaft of a fuel injection pump according to one embodiment of the present invention as viewed in an axial direction of the cam shaft; Figure 2 is a set of views showing cam lift curves of the cam shaft of Figure 1; Figure 3 is a sectional view of a conventional fuel injection pump; Figure 4 shows changes of cam lift and drive torque of the fuel injection pump employing a conventional cam shaft; Figure 5 shows a sectional view of gears of the conventional fuel injection pump; Figure 6 shows a modification of Figure 1; and Figure 7 also shows a modification of Figure 1. Now, an embodiment of the present invention will be described with Figures 1 and 2 of the accompanying drawings. Referring to Figure 1, a cam arrangement 21 of a cam shaft which drives a plunger provided for each cylinder of a four cylinder engine has cams having a nearly fan-shaped profile P. Although only one cam arrangement 21 is seen in Figure 1, the cam shaft has another cam arrangement in its axial direction (direction perpendicular to the drawing sheet). The cam arrangement 21 includes a first cam 22 and a second cam 24. A lift increment segment 23 of the first cam 22 overlaps a lift decrement segment 25 of the second cam 24 by a predetermined amount. In this particular embodiment, the first cam 22 serves for a #2 cylinder of the four cylinder diesel engine and the second cam 24 serves for a #3 cylinder. The profile P of the cams 22, 24 is in turn comprised of a lift increment segment 23 which linearly rises to a maximum lift position 26 from a zero lift position (true circle 31), a lift maintenance segment 27 which maintains the maximum lift position 26 and a lift decrement segment 25 which linearly drops from the maximum lift position 26 to the zero lift position. The lift increment and decrement segments 23 and 25 are respectively defined by tangential lines of the circle 31 (zero lift line) and the lift maintenance segment 27 is defined by an arc the center of which is the center axis 32 of the cam shaft. A combination of these segments 23, 27 and 25 forms an angle of beyond 180 degrees as viewed from the center 32 of the cam shaft. As understood from Figure 2, numeral 28 indicates another first cam (for a #1 cylinder) and numeral 29 indicates another second cam (for a #4 cylinder). The cam 29 is symmetrical with the cam 28. Phases of the cams are determined as follows: The cams 22 and 24 for the inside cylinders (#2 and #3 cylinders) are phase shifted by 180 degrees from the cams 28 and 29 for the outside cylinders (#1 and #4 cylinders). Specifically, where the cams 22 and 24 of Figure 1 are considered, if a pump angle is 90 degrees, the #2 cylinder cam 22 is in the lift decrement section 25 and the #3 cylinder cam 24 is in the lift increment section 23, and if the pump angle is 270 degrees, the #2 cylinder cam 22 is in the lift increment section 23 and the #3 cylinder cam 24 is in the lift decrement section 25. With this arrangement, as shown in Figure 2, the drive torque becomes constant (zero) since the positive and negative areas produced by one cam arrangement 21 are counterbalanced by those produced by the other cam arrangement. Therefore, unlike the conventional cam shaft, the drive torque of the cam shaft according to the present invention does not include the negative area. Consequently, the gear contact face change due to the positive and negative drive torque fluctuation is prevented and the gear noise is remarkably reduced. An allocation (length ratio) of the segments 23, 27 and 25 of the cam profile P is determined by a fuel injection pump and/or various performances and dimensions of the internal combustion engine. Figure 2 shows that the drive torque curve is horizontal. However, the drive torque curve may fluctuate up and down as long as the minimum torque is not the negative torque. In addition, the torque curve may have a certain plus value other than zero. In other words if the drive torque does not drop into the negative area, any torque curve may be satisfactory to eliminate the problem of the conventional arrangement. Further, the lift increment segment 23 and the lift decrement segment 25 have the linear profile. However, they may have non-linear profiles respectively, as shown in Figure 6 or 7.

Claims: A cam shaft for driving a plunger of a fuel injection pump provided for each cylinder of an internal combustion engine, comprising a first cam (22) lifting a first plunger and a second cam (24) lifting a second plunger, the first cam (22) having a cam profile (P) of nearly fan shape for lifting the plunger characterized in that the second cam has a cam profile (P) of nearly fan shape symmetrical with the first cam (22) with respect to a center (32) of the cam shaft and in that the lift increment segment (23) of one of the first and second cams (22, 24) overlaps the lift decrement segment (25) of the other cam (24, 22) by a predetermined amount. The cam shaft of claim 1, characterized in that the cam profile (P) includes a linear or non linear lift increment segment (23), a linear or non linear lift decrement segment (25) and an arcuate maximum lift maintenance segment (27) connecting the lift increment and decrement segments (23, 25). The cam shaft of claim 1 or 2, characterized in that the lift increment and decrement segments (23, 25) are respectively defined by tangential lines connecting a maximum lift position (26) with a true circle (31) defining a zero lift position or non linear lines connecting the maximum lift position (26) with a true circle (31) defining the zero lift position. The cam shaft of claim 1, 2, or 3, characterized in that the maximum lift maintenance segment (27) is defined by an arc the center of which is a center (32) of the cam shaft. Internal combustion engine having a cam shaft according to any one of foregoing claims, characterized in that the internal combustion engine is a four cylinder diesel engine. The cam shaft of any one of foregoing claims, characterized in that the cam shaft further includes another first cam (28) and another second cam (29), and wherein the first cam (22) is a cam for a #2 cylinder of a four cylinder diesel engine, the second cam (24) is a cam for a #3 cylinder and the second cam (24) is 180 degree phase shifted from the first cam (22), the another first cam (28) is a cam for a #1 cylinder and the another second cam (29) is a cam for a #4 cylinder and the another second cam (29) is 180 degree phase shifted from the another first cam (28). The cam shaft of any one of foregoing claims, characterized in that the cam profile (P) extends over 180 degrees as measured from the center (32) of the cam shaft.

Inventors: MURATA YUICHI C O ISUZU MOTORS, NAKADA TERUO C O ISUZU MOTORS, MURATA, YUICHI, C/O ISUZU MOTORS LIMITED, NAKADA, TERUO, C/O ISUZU MOTORS LIMITED
Applicants: ISUZU MOTORS LTD, ISUZU MOTORS LIMITED
Application year: 1992
Main classification: F02M59
IPC codes: F02M39, F02M59