GB2276921A

GB2276921A - Fuel injection pump for an internal combustion engine

Info

Publication number: GB2276921A
Application number: GB9406969A
Authority: GB
Inventors: Theodor Stipek; Stanislaw Bodzak
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-04-08
Filing date: 1994-04-08
Publication date: 1994-10-12
Anticipated expiration: 2014-04-08
Also published as: GB2276921B; DE4311628A1; JPH06299931A; GB9406969D0

Description

A 2276921

DESCRIPTION FUEL INJECTION PUMP FOR AN INTERNAL COMBUSTION ENGINE

The invention relates to a fuel injection pump for an internal combustion engine. In a fuel injection pump of the type known from DE OS 36 24 134r a pump piston driven by a cam drive is axially guided in a cylindrical bore of a cylinder liner, and extends into the cylinder linerwhere its end face defines a pump working chamber connectable to the combustion chamber of the internal combustion engine to be supplied. The pump piston is divided into a piston body and a piston head by an annular groove and its outer surface has in the region of the piston head a -control recess having an inclined edge which is constantly-connected to the pump working chamber by way of a longitudinal groove and which-, for the purpose of controlling the quantity of fuel delivered, cooperates with a radial discharge bore incorporated in the wall of the cylindrical bore.

In fuel injection pumps delivering very high working pressures for internal combustion engines, socalled "piston scores" cause by direct metal to metal contact between the piston head and the wall of the cylindrical bore frequently occur in the region of the piston head, and particularly at control edges of the -2control recesses, wherein the lubricating film (generally diesel fuel) otherwise maintained between these parts is displaced by high lateral forces acting upon the piston or by geometrical inaccuracies of shape of the piston head.

In order to avoid the above-described wear phenomenon, which frequently results in the jamming of the piston in the cylinder liner and the mechanical destruction of the components, the piston head in the known fuel injection pump is coated with a layer of titanium nitride. However, this coating has the disadvantage that it results in high costs owing to the high-grade material and relatively expensive production.

In contrast to this. the fuel injection pump in accordance with the invention has the advantage the metal to metal contact between the pump piston and the cylinder liner may be reliably avoided by deliberately increasing the clearance between the outer surface of the head part of the pump piston and the wall of the cylindrical bore. At the same time, it is particularly advantageous to retain the small clearance, necessary for guidance of the piston head in the cylinder liner, in the unendangered region beyond the control recesses. whose wear does not lead to "piston scores", since no sharp-edged profiles are 4k -3incorporated in the outer surface of the pump piston in this region. Thus, it is possible to increase the clearance between the piston head and the wall of the cylindrical bore in the critical region of the control recesses without impairing the axial-guidance of the piston head by definite line guidance due to the small clearance in non-critical regions.

At the same time, it is particularly advantageous to realise the intendedlines of the smallest clearance by providing the piston head with an elliptical cross-sectional shape or with a convex or barrel-shaped longitudinal section, since in these cases existing manufacturing equipment may be used.

A further possible method of increasing the clearance is that of uniformly reducing the cross section of-the entire head of the pump piston, which can be done in a particularly simple manner. The pump piston is then guided in the cylindrical bore by way of the circumferential surface of its body part.

Thus, by increasing the clearance betweenthe pump piston head and the cylindrical wall in the critical region, and without complicated and costintensive coating of the pump piston, it is possible, in fuel injection pumps operating with high pump pressures, to reliably avoid "piston scores" which,, in particular, impair the function of controlling the -4quantities of fuel injected.

Further advantages and advantageous developments of the subject of the invention may be inferred form the drawings, the description and the claims.

Three embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a first embodiment of a portion of a fuel injection pump in accordance with the invention, having a pump piston whose head diameter is uniformly reduced relative to the diameter of the body part; Fig. 2 shows, in accordance with a second embodiment, a cross-section through the head of an elliptical pump piston; and Fig. 3 shows a third embodiment of a pump piston head whose profile is barrel-shaped or convex.

The illustration and description of the fuel injection pump in accordance with the invention are confined to the components material to the invention.

In Fig. 1, a pump piston 1 is axially guided in cylindrical bore 3 of a cylinder liner 5 which is in turn inserted into a pump housing (not shown). This may be a single-cylinder injection pump or an in-line injection pump. The pump piston 1 is axially reciprocated in a known manner by a cam drive (not shown) against a restoring force, and its end face 7 a A within the cylinder liner 5 defines a pump working chamber 9 which is connected by way of a fuel line 11 to an injection point in the combustion chamber of the internal combustion engine to be supplied. The pump piston 1 has an annular groove 13 which divides the pump piston 1 into a body part 15 and a head part 17.

Control recesses 19, in the form of inclined grooves or of a widening of the annular groove 13 with an inclined boundary edge,- are incorporated in the outer surface of the head part 17 of pump piston 1 and, like the annular groove 13, are constantly connected to the pump working chamber 9 by longitudinal grooves 21.

The inclined boundary edges, facing the pump working chamber, of the control recesses form control edges 23 which, for the purpose of controlling the quantity of fuel delivered, cooperate with radial discharge bores provided in the wall of the cylinder liner 5.

- In this connection, the diameter of the head part 17 of the pump piston 1 in Fig. 1 is reduced by a small amount relative to the diameter of the body part 15, preferably by an amount between 0.01 and 0.03% of the diameter of the body part, so that the entire periphery of the head part 17 has a greater amount of clearance relative to the wall of the cylindrical bore 3 than that of the body part 15.

The pump piston head 17 has an elliptical cross- -6sectional shape in the second embodiment shown in Fig. 2, wherein the minor principal axis of the ellipse lies in the region of the longitudinal groove 21, and the corresponding smaller diameter D2 is approximately 0.001 to 0.003 mm smaller than the larger diameter D1 of the ellipse at the major principal axis, disposed at right angles to the axis extending through the longitudinal grooves 21, which corresponds to the diameter of the body part 15 of the pump piston 1. It is thus ensured that the pump piston head 17 is guided along an axial line in the cylindrical bore 3 which is at a distance from the critical region of the control recesses 19, so that it is possible, with reliable guidance of the pump piston 1. to increase the clearance between the pump piston head 17 and the wall of the cylindrical bore 3 in the critical.region of the control recesses 19.

The third embodiment, shown in Fig. 3, differs from the second embodiment in the design of the piston head 37, and Fig. 3 is therefore also limited to the illustration of the pump piston 1.

In this connection, the profile of the circumferential surface of the piston head 37 is barrel-shaped or convex and the diameter D2 of the piston head at its top and bottom boundary edges 31 is in each case approximately 0.001 to 0.003 mm smaller A -7than the diameter D1 which lies exactly between the edges 31 and which corresponds to the diameter of the body part 15 of the pump piston 1, so that the pump piston head 37 is guided axially in the cylindrical bore 3 along an axial line with small clearance only in the region of the diameter D1. while the rest of the region of the outer surface of the piston head 37 has an increased amount of clearance relative to the wall of the cylindrical-bore 3.

Claims

-8CLAIMS

1. A fuel injection pump for an internal combustion engine, having a pump piston which is guided axially in a cylindrical bore of a cylinder and is reciprocated by a cam drive and whose end face defines a pump working chamber in the cylinder, the outer surface of which pump piston has control recesses which are constantly connected to the pump working chamber and cooperate with discharge bores in the wall of the cylinder, wherein an annular groove on the pump piston separates a head part from a body part and a clearance is provided between the outer surface of the head part of the pump piston and the wall of the cylindrical bore and has, along one or two lines on the outer surface of the head part, a constant minimum value which is smaller than the clearance between the outer surface of the remainder of the head part and the wall of the cylindrical bore.

2. A fuel injection pump as claimed in claim 1 wherein the control recesses are in the form of inclined grooves with inclined control edges.

3. A fuel injection pump as claimed in claim 1 or claim 2, wherein the lines on the outer surface of the pump piston head with the minimum value of the clearance between the head part and the wall of the cylindrical bore lie outside a region of the outer -9surface forming the control edges.

4. A fuel injection pump as claimed in claim 3 wherein the head part is elliptical in a radial sectional plane.

5. A fuel injection pump as claimed in claim 4 wherein the ellipse has a minor principal axis (D2) and a major principal axis (D1), wherein the minor principal axis (D2) of the ellipse extends through a region of the outer surface accommodating a longitudinal groove.

6. A fuel injection pump as claimed in claim 4 or 5, wherein the diameter (D2) in the region of the minor principal axis of the ellipse is 0.001 to 0.003 mm smaller than the diameter (D1) in the region of the major principal axis of the ellipse lying at right angles to-the minor principal axis.

7. A fuel injection_pump as claimed in claim 3, wherein the circumferential contour of the head part of the pump piston is convex in an axial section.

8. A fuel injection pump as claimed in claim 7, wherein the diameter of the head part in the region of a top and a bottom boundary edge is in each case 0.001 to 0.003 mm smaller than in a cross section lying between these edges.

9. A fuel injection pump as claimed in claim 8, wherein the largest diameter cross section of the head -10part is disposed in the centre between the top and the bottom boundary edge.

10. A fuel injection pump for an internal combustion engine, having a pump piston which is axially guided in a cylindrical bore of a cylinder liner and is reciprocated by a cam drive and whose end face defines a pump working chamber in the cylinder liner, the outer surface of which pump piston has control recesses, preferably inclined grooves with inclined control edges, which are permanently connected to the pump working chamber and cooperate with discharge bores in the wall of the cylinder liner, wherein an annular groove on the pump piston separates a head part from a body part and the diameter of the pump piston in the region of the head part is smaller by a uniform amount that the diameter of the pump piston in the region of the body part.

11. A fuel injection pump as claimed in claim 10, wherein the difference in diameter between the head part and the body part is between 0.01 and 0. 03%.

12. Kfuel injection pump substantially as herein described, with reference to and as illustrated in the accompanying drawings.

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