GB2321935A - Snubber valve for location in the outlet of a fuel injection pump - Google Patents

Abstract

A snubber valve comprises a valve member 24 slidable within a bore 18, the valve member 24 and bore 18 defining therebetween a first flow path, the valve member 24 being engageable with a seating defined around the bore 18 to control fuel flow along the first flow path, the valve member 24 defining a second restricted flow path 28, 30 by-passing the first flow path, wherein in the rest position of the snubber valve, the valve member 24 is not biased into engagement with its seating. The snubber valve may further comprise means biassing the valve member 24 out of engagement with the seating, for example by way of a spring 38.

Description

SNUBBER VALVE This invention relates to a snubber valve for location in an outlet port of a high pressure fuel pump to limit the rate at which fuel is able to escape from a supply line connecting the outlet port to a fuel injector.

In a known arrangement a valve member is slidable within a bore, the valve member being fluted to define a fuel flow path between the valve member and the bore. The valve member includes an enlarged region which is engageable with a seating defined around the bore to control fuel flow along the flow path. A spring biases the valve member into engagement with the seating. The valve member further defines a restricted flow path which by-passes the seating, thus fuel is able to flow through the snubber valve at a restricted rate when the valve member is in engagement with its seating.

In use, at the termination of fuel injection, the valve member engages its seating thus the pressure of fuel in the supply line falls at a relatively low rate, the fuel escaping from the supply line through the restricted flow path. Such an arrangement is advantageous in that the low rate of pressure reduction avoids a large cavity being drawn in the supply line.

However, the low rate of pressure reduction also results in a poor termination of fuel injection and consequent emissions.

According to the present invention there is provided a snubbe v.åhre comprising a valve member slidable within a bore, the valve member and bore defining therebetween a first flow path, the membier being engageable with a seating defined around the bore to controí fuel flow along the first flow path, the valve member definingusecond restricted flow path by-passing the first flow path, wherein in the rest position of the snubber valve, the valve member is not biased into engagement with its seating.

The snubber valve preferably further comprises means biassing the valve member out of engagement with the seating, for example by way of a spring.

The invention will further be described, by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a snubber valve in accordance with an embodiment of the invention; Figure 2 is an end view of the valve element of the snubber valve of Figure 1; and Figure 3 is a view similar to Figure 1 of an alternative embodiment.

Figure 1 illustrates a snubber valve 10 located in an outlet port 12 provided in the housing 14 of a high pressure fuel pump. The snubber valve 10 comprises a valve body 16 which is of generally cylindrical form and includes an axially extending bore 18. The valve body 16 is secured within the outlet port 12 by means of an end part of a connector 20 which is in screw-threaded engagement within the port 12 of the housing 14 and which secures an end of a supply line to the housing 14.

A sealing ring 22 is located between an end part of the valve body 16 and the pump housing 14 to form a fluid-tight seal therebetween. The end of the connector 20 also seals against the valve body 16.

A valve member 24 is slidable within the bore 18, the valve member 24 including an enlarged part 26 including a frustoconical portion which is engageable with a seating defined by a frustoconical end part of the bore 18. The diameter of the part of the valve member 24 remote from the part 26 is substantially equal to that of the bore 18, thus the valve member 24 is guided for sliding movement within the bore 18. The peripheral surface of the valve member 24 is provided with flutes which define with the wall of the bore 18 a first flow path whereby fuel is able to flow through the snubber valve 10 in a direction substantially parallel to the axis of the valve member 24. The flutes do not extend as far as the enlarged part 26 thus fuel is unable to flow between the enlarged part 26 and the seating when the valve member 24 occupies the position illustrated in Figure 1.

The valve member 24 is provided with an axially extending passage 28 which, partway along its length, includes a region 30 of restricted diameter, the bore 28 and region 30 defining a restricted flow path which by-passes the seating with which the enlarged part 26 is engageable thus fuel is able to flow at restricted rate from the supply line through the snubber valve 10 when the valve member 24 occupies the position illustrated in Figure 1.

The connector 20 defines a recess 32 within which an end part 34 of the valve member 24 is received. The end part 34 of the valve member 24 is of frustoconical shape, the axially extending bore 28 extending therethrough. As best shown in Figure 2, a cross-slot 36 is provided in the end part 34, thus when the valve member 24 is lifted away from its seating to a position in which the end part 34 engages the connector 20, communication is still permitted between the recess 32 and the supply line through the cross-slot 36.

As shown in Figure 1, a spring 38 is engaged between the valve member 24 and the end of the outlet port 12, the spring 38 biasing the valve member 24 away from the position shown in Figure 1 towards a position in which the end face of the end part 34 engages the connector 20.

In use, at rest the valve member 24 is biased away from the position shown in Figure 1. When injection is to commence, fuel at high pressure is supplied to the outlet port 12 by the high pressure fuel pump.

As the enlarged part 26 of the valve member 24 is not in engagement with its seating, fuel at high pressure is able to flow along the flow path defined by the fluted part of the valve member 24 and the wall defining the bore 18, past the seating and into the recess 32. The fuel can flow from the recess 32 through the cross-slot 36 to the supply line. The supply line communicates with a pressure actuated fuel injector, and the application of high pressure fuel to the supply line results in the fuel injector opening, and hence in fuel being delivered to a cylinder of an associated engine.

When injection is to be terminated, fuel at high pressure is no longer supplied to the outlet port 12, and instead the outlet port 12 communicates with a suitable low pressure drain. The reduction in pressure applied to the outlet port 12 results in a pressure drop across the snubber valve 10, and fuel flows from the supply line past the snubber valve 10 to the low pressure drain. Initially, the rate of fuel flow through the snubber valve 10 from the supply line to the low pressure drain is relatively low due to the inertia of the fuel, the velocity of fuel flow in this direction increasing with time. At relatively low fuel velocity, the flow of fuel through the valve 10 is substantially unhindered, the inertia of the valve member 24 resulting in the valve member 24 initially moving slowly.

The flow of fuel from the supply line results in the fuel pressure applied to the injector falling. When the fuel pressure falls below a predetermined level, the injector closes thus terminating injection. As the initial rate of fuel flow is low, the force applied to the valve member 24 due to the momentum of the fuel is also low. As the velocity of the fuel increases, the force applied to the valve member 24 also increases, and will reach a point beyond which the spring force is overcome, the valve member 24 moving to the position illustrated in Figure 1. It will be appreciated that in this position, the enlarged part 26 of the valve member 24 engages its seating thus fuel is only able to escape from the supply line at a restricted rate through the passage 28 and region 30.

The inertia of fuel within the supply line is such that once the valve member 24 occupies the position shown in Figure 1 in which fuel can only flow through the snubber valve 10 at a restricted rate, the continued movement of the fuel within the supply line generates a sufficiently high pressure within the recess 32 to ensure that the valve member 24 remains in the position shown in Figure 1. The diameter of the region 30 is selected to reduce the magnitude of any reflected pressure wave resulting from the pressure build-up in the recess 32, and also to reduce the risk of drawing large cavities in the supply line. Rather than reflecting undesirably large pressure waves along the supply line, the kinetic energy of the fuel from the supply line is conveniently damped or absorbed into the snubber valve.

It will be appreciated that once the valve member 24 occupies the position shown in Figure 1, the rate of fuel flow from the supply line is restricted thus the risk of formation of large cavities within the supply line is reduced. The flow of fuel through the passage 28 and region 30 results in the pressure within the recess 32 falling, and subsequently the valve member 24 will return to its rest position under the action of the spring 38 ready for the commencement of the next injection cycle. Prior to commencement of the next injection cycle, the outlet port 12 may be supplied with fuel at transfer pressure, thereby reducing the quantity of fuel which must be supplied to the outlet port 12 by the high pressure pump in order to raise the pressure applied to the injector to a level sufficient to cause commencement of injection, and permitting an improvement in the injection rate profile. Filling the supply line with fuel at transfer pressure is also advantageous in that any cavities which had been drawn in the supply line are removed prior to the application of high pressure fuel to the supply line. The initial high pressure wave which is transmitted along the supply line would otherwise be reflected, undesirably, by such cavities.

It will be appreciated that this arrangement is advantageous in that when termination of injection is desired, the initial rate at which fuel is able to flow from the supply line is relatively high, thus there is a rapid fall in the pressure applied to the injector and termination of injection is improved. After the initial rapid rate of fuel pressure reduction within the supply line, the valve member 24 moves to the position shown in Figure 1 thus the velocity of fuel within the supply line is reduced thus reducing the formation of large cavities within the supply line.

Figure 3 illustrates an arrangement which is similar to that of Figures 1 and 2 but in which the spring 38 is omitted. In this arrangement, at rest the valve member 24 can occupy any one of a number of positions, but may be biased into an open position by transfer pressure flow. When injection is to commence, the application of high pressure fuel to the outlet port 12 results in the lower part of the valve member 24 being exposed to fuel at high pressure whilst the upper part of the valve member 24 is exposed to the relatively low pressure within the supply line. This pressure imbalance causes the valve member 24 to move to the position in which its end face engages the connector 20, if the valve member 24 does not already occupy that position. Once the valve member 24 is in engagement with the connector 20, operation of the snubber valve is as described hereinbefore until termination of injection is required.

When injection is to be terminated, the outlet port 12 is connected to a low pressure drain, thus the pressure applied to the lower end of the valve member 24 is reduced. As in the arrangement illustrated in Figure 1, fuel is able to flow past the snubber valve 10 to the low pressure drain resulting in a relatively rapid rate of fall in the pressure within the supply line, the rate of fuel flow through the snubber valve 10 initially being low due to the inertia of fuel within the supply line, and gradually increasing. After the initial flow of fuel past the snubber valve 10, the valve member 24 will move to the position illustrated in Figure 3 due to the pressure imbalance across the snubber valve 10. Once this position has been attained, further flow of fuel from the supply line is at a restricted rate thus avoiding or reducing the risk of formation of large cavities within the supply line. It will be appreciated that as the valve member 24 is not spring biased away from its seating, movement of the valve member 24 into engagement with its seating will occur earlier in this arrangement than in the arrangement illustrated in Figure 1.

In the arrangement of Figures 1 and 2, a relatively high pressure builds up within the recess 32 as the fuel momentum required to move the valve member 24 against the action of the spring is relatively high compared with the embodiment illustrated in Figure 3 which does not include such a spring. The high pressure build up may cause a pressure wave to be transmitted along the supply line sufficient to cause undesirable secondary injections. Such secondary injections are less likely to occur in the Figure 3 arrangement. However the absence of the spring in this arrangement may give rise to irregular operation, thus the injection characteristics may be irregular.

In both the arrangement illustrated in Figures 1 and 2 and the arrangement of Figure 3, the dimensions of the region 30, and in the Figure 1 arrangement the pre-loading of the spring 38, are chosen in order to ensure that the fuel pressure within the supply line does not fall to too low a level prior to the injector closing in order to ensure that combustion gases are not able to flow past the injector needle, hence avoiding an effect known as gas blow-by.

Although the description hereinbefore is of a spill-type pump in which in order to terminate injection the outlet port is connected to a low pressure drain, the invention is also suitable for use in, for example, a fuel pump in which the pressure reduction at the outlet port results from the rollers associated with pumping plungers of the pump riding "over the nose" of the cam lobes, i.e. in which the pressure reduction results from allowing the pumping plungers to move outwards thus allowing the fuel pressure in the pumping chamber to the pump to fall.

Claims (5)

1. A snubber valve comprising a valve member slidable within a bore, the valve member and bore defining therebetween a first flow path, the valve member being engageable with a seating defined around the bore to control fuel flow along the first flow path, the valve member defining a second restricted flow path by-passing the first flow path, wherein in the rest position of the snubber valve, the valve member is not biased into engagement with its seating.

2. A snubber valve as claimed in Claim 1, further comprising means biassing the valve member out of engagement with the seating.

3. A snubber valve as claimed in Claim 2, wherein the means biassing the valve member out of engagement with the seating comprises a spring.

4. A snubber valve as claimed in Claim 2, wherein the valve member is biased out of engagement with the seating by the application of fuel under pressure thereto.

5. A snubber valve substantially as hereinbefore described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.