Description

Title

PUMP FOR A FU EL I NJECTION SYSTEM OF AN I NTERNAL COM BUSTION ENGIN E WITH AN I M PROVED DELIVERY VALVE

The present invention relates to a pump comprising an improved delivery valve, and preferably to a high-pressure pump for a fuel injection system of an internal combustion engine.

A high-pressure pump is generally used in a fuel injection system which comprises a low-pressure branch, a pre-feed pump which draws the fuel from a tank and conveys it to a high-pressure pump via the low-pressure branch and a high-pressure branch which connects a delivery of the high-pressure pump to a group of injectors via an accumulator, commonly called a "common rail".

The high-pressure pump comprises a plurality of pumping cylinders operated by a driving shaft via a lobed body or via a cam. The pumping cylinders slide in a reciprocating manner inside respective compression chambers, each of which is connected in flow terms to an intake valve and a delivery or high-pressure valve. Generally the high-pressure or delivery valve is a non-return valve.

A high-pressure valve comprises a ball shut-off member biased by means of a coil spring against a conical seat. Upstream the conical seat is connected in flow to the compression chamber and downstream the high-pressure valve is connected in flow to the common rail.

The fuel flow which passes through the high-pressure valve exerts a complex hydrodynamic action on the shut-off member which results in wear and a delay in the dynamic response.

In particular, owing to both the high operating speeds of the pump and the high flow rates, the hydraulic forces acting on the shut-off member are such as to generate a torque which causes the shut-off member to rotate about the axis of the spring and thus produces both friction of the latter against the spring and a delay in the closing action.

The object of the present invention is to provide a feed pump for a fuel injection circuit which does not have the abovementioned drawbacks.

According to the invention, this object is achieved by a pump for a fuel injection system of an internal combustion engine, comprising a seat connected in flow terms to the delivery of a compression chamber, an outlet duct for connecting the seat to a plurality of injectors, a shut-off member housed inside the seat, a spring for biasing the shut-off member against a first contact surface of the seat in a closed position, the seat being provided with a first through-opening in the axial direction, arranged downstream of the shut-off member, and fuel flow deviation means being provided upstream of the outlet duct and configured so that the speed of the fuel flow is greater between the shut-off member and the opening than upstream of the shut-off member.

The fuel flow deviation means are configured so as to direct the flow towards the opening so that the speed of the fuel increases immediately downstream of the shut-off member and a negative pressure is therefore created. This negative pressure pushes the shut-off member against the spring and reduces rotation of the shut-off member. Moreover, when the fluid tends to change direction during the intake phase of the pumping member, the pressure is localized along the axis of the spring and is applied effectively to the shut-off member so as to favour a closing action.

Further details, characteristic features and advantages of the invention will emerge more clearly from the following description of a non-limiting example of embodiment provided with reference to the accompanying drawings in which:

- Figure 1 is a partially cross-sectioned view of the head of a feed pump according to the present invention; and

- Figure 2 is a prospective view of a component according to Figure 1.

In Figure 1, 1 denotes generically the head of a reciprocating fuel feed pump. The head defines a compression chamber 2 housing a pumping member 3 and a delivery valve 4 connected to the delivery of the chamber 2. The delivery valve 4 is a non-return valve.

The compression chamber 2 has an axis A and is closed axially by a threaded member 5 screwed into the head 1. The valve 4 is connected to the delivery of the compression chamber 2 via a duct 6 having an axis B perpendicular to the axis A.

The duct 6 emerges inside a seat 7 of the valve 4 defined by a cylindrical wall 8 coaxial with the duct 6 and by a frustoconical contact surface 9 axially arranged between the duct 6 and the cylindrical wall 8. The seat 7 is also connected to a duct 10 having an axis C parallel to the axis A and connected to the injectors preferably via a common rail, both not shown.

According to a preferred embodiment of the present invention, the valve 4 comprises a ball shut-off member 11 designed to cooperate selectively with the contact surface 9; a coil spring 12 for keeping the shut-off member 11 pressed against the contact surface 9 in a closed position; and a closing member 13 for pre-stressing the spring 12.

The closing member 13 is connected rigidly to the head 1 preferably by means of a threaded connection and comprises, for example in the form of a single body, a closing head-piece 14 screwed into the head 1 in a fluid-type manner and a cylindrical body 15 housed inside the seat 7.

The cylindrical body 5 defines preferably a radial through-hole 16 coaxial during use with the axis C and an axial cavity 17 communicating with the hole 16 via an

opening 18 and open towards the contact wall 9. The axial cavity 17 houses the spring 12 and is cylindrical with an internal diameter smaller than that of the shut- off member 11.

The opening 18 is situated on the opposite side to the shut-off member 11 relative to the spring 12 and is situated in an axial position lying between the axis C and the cavity 17.

Towards the contact wall 9, the cylindrical body 15 comprises an annular end portion 19 which defines a flaring 20 and at least one radial through-groove 21.

The spring 12 is housed inside the axial cavity 17 so as to be coaxial with the axis B and the shut-off member 11 makes contact with the contact surface 9 when the valve 4 is closed. The valve 4 opens when the shut-off member 11 separates from the contact surface 9 and is able to reach the fully open condition when the shut-off member 11 makes contact with the flaring 20.

Moreover, according to a preferred embodiment of the present invention, the cylindrical body 15 is mounted with radial play of its diameter inside the seat 7 so that the hydraulic resistance defined by the aperture 22 between the cylindrical body 15 and the cylindrical wall 8 is greater than that defined by the groove 21, the coils of the spring 12, the opening 18 and the hole 16.

Moreover, the diameter of the duct 6 and the dimensions of the groove 21 are such as to ensure that the speed of the fuel inside the cavity 17 is greater than that inside the duct 6.

During use, the flow of fuel pumped by the member 3 opens the shut-off member 11 when the pressure upstream of the valve 4 is greater than the sum of the pressure downstream of the shut-off member and the equivalent, in terms of pressure, of the pre-stressing force of the spring 12.

When the valve 4 is open, in the fully open condition, the shut-off member 11 makes contact against the flaring 20 and most of the flow is directed along a path

which leads from the groove 21 to the hole 16 through the coils of the spring 12 and the opening 18 since the hydraulic resistance of this path is less than that defined by the aperture 22 between the annular end portion 19 and the duct 10.

In this way the fuel flow generates a negative pressure inside the cavity 17 and the shut-off member 11 is pressed with an additional force against the flaring 20. Consequently, the frictional force increases and the rotational movement of the shut-off member decreases when the valve 4 is open.

When the pumping member 3 retracts in order to start the intake phase, the sum of the pressure downstream of the valve 4 and the equivalent pressure of the spring 12 is greater than the pressure inside the compression chamber 2. The response time of the shut-off member 11 decreases and the pressure signal present inside the duct 10 is transmitted effectively to the shut-off member 11 via the through-hole 16 and the opening 18.

From that described with reference to the accompanying figures the operating principle and advantages of the pump according to the present invention clearly emerge.

It has been established that the reduction in the rotational movement of the shut- off member 11 during the opening phase of valve 4 reduces the delay in the dynamic response of the said valve 4 and also the wear of the shut-off member 11.

In fact, in particular operating conditions, the speed of rotation of the shut-off member 11 about the axis B is so high that the shut-off member continues to rotate even when the pumping member 3 reverses its movement and starts the intake phase, preventing closure of the valve also - in the worst of cases - for the entire duration of the intake phase.

The reduction in the rotational movement of the shut-off member 11 is obtained by means of a particular deviation of the fuel flow inside the cavity 17. This deviation of the flow requires machining of the closing member 13 which is not

expensive and does not affect other components and the assembly procedure. In this way, the general configuration of the pump remains unvaried and the costs associated with modification of the apparatus and the assembly cycles are avoided.

Finally, it is clear that the fuel pump described and illustrated here may be subject to modifications and variations without thereby departing from the protective scope of the present invention, as defined in the accompanying claims.

In particular, the number of grooves 21 may be more than one, for example up to five, so as to ensure that the speed of the fuel flow inside the cavity 17 is greater than that inside the duct 6.

The aperture 22 may also be completely closed in the case where the part 15 engages precisely with the cylindrical surface 8 of the seat 7. In this case, the hydraulic resistance is practically infinite and the entire flow of fuel which passes through the valve 4 flows through the coils of the spring 12 and the opening 18.