Title

PUMP UNIT FOR FEEDING FU EL TO AN INTERNAL-COMBUSTION ENGINE

AND METHOD FOR COOLING SAID UNIT

The present invention relates to a pump unit for feeding fuel, preferably diesel fuel, to an internal-combustion engine. In particular, The present invention relates to a pump unit for feeding fuel, preferably diesel fuel, to an internal-combustion engine in which an innovative cooling system has been implemented. Moreover, the present invention relates to a method for cooling such a pump unit.

More specifically, the present invention relates to a pumping unit of the type comprising a high-pressure piston pump for feeding the fuel to the internal- combustion engine and a low-pressure gear pump, or pre-feed pump, for feeding the fuel from a storage tank to the high-pressure pump.

In detail, the present invention relates to the cooling of components of the high- pressure piston pump.

As is known, a pump unit of the type described above comprises a head-piece inside which at least one cylinder housing the associated sliding pumping piston is formed. One end of the pumping piston, in particular the inner end relative to the pump unit, is connected to an actuator, usually a cam shaft, which actuates the movement of the piston. A suitable spring is provided for keeping the shaft pressed against the corresponding actuator. With a reciprocating movement along the cylinder, the piston performs an intake stroke, during which fuel is drawn into the cylinder, and a compression stroke, during which the fuel trapped inside the cylinder is compressed. The portion of the cylinder where the compression takes place is called "compression chamber" and is arranged on the opposite side to the actuator which moves the piston. Generally, feeding into the cylinder is performed through a hole, or intake hole, which is axial with the cylinder itself, while discharging of the compressed fuel is performed along a transverse hole, or delivery hole. The outer part of the head-piece where the fuel to be drawn into the cylinder is fed is called the intake chamber. Suitable valves are arranged along the feed and delivery holes for adjusting the correct flow of the fuel. Outside the head, the delivery valve is connected to the engine, preferably by means of a common header provided with a plurality of injectors.

The intake chamber, which is connected to the cylinder by means of the aforementioned intake valve, communicates with an intake duct for feeding the fuel drawn from the tank by means of a low-pressure gear pump.

Lubrication of the cam shaft actuating the movement of the piston may be performed using oil. Lubrication of the piston, instead, is performed using the fuel itself which is drawn into the compression chamber. In fact, part of this fuel advances seeping between the side wall of the piston and the cylinder and, owing to the nature of the fuel used, i.e. diesel fuel, it cools and lubricates the piston, preventing it from overheating and seizing inside the cylinder.

However, in the aforementioned solution, namely with an oil bath situated at an inner end of the cylinder and fuel which seeps from the compression chamber towards the same inner end of the cylinder, the two aforementioned lubricants must be kept separate. For this purpose, usually the inner end of the cylinder is provided with a sealing ring beyond which the seeping fuel cannot pass and remains isolated from the underlying oil.

This solution on the one hand solves the problem of separation of the lubricants, but on the other hand gives rise to the problem of disposal of the fuel which is gradually collected in the region of the aforementioned sealing ring. In fact, this fuel which collects is heated to a high temperature by the compressive movement of the piston and this overheating may in the long run damage the sealing ring coming into contact precisely with the heated fuel. In order to overcome this problem it is known to provide a drainage channel which allows the seeping fuel to be kept distant from the sealing ring. The drainage flow is generated by the movement itself of the piston during the compression stroke. According to the prior art, on the opposite side this drainage channel emerges inside the intake chamber or leads into a return circuit feeding the tank.

However, both the known solutions described above have drawbacks. For example, in the solution where the drainage channel emerges inside the intake chamber mixing of cold fuel arriving from the gear pump with fuel which is hot owing to compression of the piston occurs. The end result is an increase in the temperature of the fuel inside the intake chamber which modifies the viscous behaviour of the fuel and thermally stresses the intake valve and the outer portion of the head.

Based on this prior art, an object of the present invention is to provide an innovative pump unit and to implement a method for cooling thereof able to overcome the problems of the prior art.

According to the present invention a pump unit for feeding fuel, preferably diesel fuel, to an internal-combustion engine is provided. This pump unit comprises:

- a low-pressure pumping unit configured to draw fuel from the tank;

- a high-pressure piston pump fed by the low-pressure gear pump and configured to feed the fuel to the internal-combustion engine;

- an intake circuit connected upstream to the low-pressure gear pump and downstream to the high-pressure piston pump;

- a discharge circuit connected upstream to the intake circuit and downstream to the tank;

- a cooling circuit comprising in series a first portion, connected upstream to the intake circuit or to the discharge circuit and downstream to the high-pressure piston pump, and a second portion connected upstream to the high-pressure piston pump and downstream to the discharge circuit.

Advantageously, according to the present invention, on the one hand a flow of "fresh" fuel to the cylinder is always guaranteed because it is drawn from the intake or discharge circuit and, on the other hand, the mixture comprising this "fresh" fuel and the fuel seeping along the cylinder are not fed to intake circuit, but injected into the discharge circuit.

This solution finds its most natural application in pumps where the cam shaft is lubricated with oil and where there is the need to avoid contamination of this oil by the fuel which has collected at the bottom of the cylinder and is used to cool the piston.

In the case where the high-pressure piston pump comprises a mechanical intake valve, the first portion of the cooling circuit is connected upstream to the intake circuit and not to the discharge circuit. In particular, this type of pump usually comprises a metering valve along the intake circuit. Downstream of this metering valve the discharge circuit or overflow circuit branches off. In this case, the first portion, or branch, of the cooling circuit is connected to the intake circuit upstream of the metering valve.

Advantageously, according to this embodiment there is always the correct quantity of "fresh" fuel to be fed to the cylinder even in the condition where there is no supply of the metering valve.

In the case where the high-pressure piston pump comprises an electrical intake valve, the first portion of the cooling circuit is connected upstream to the discharge circuit. In particular, in this type of bridge arrangement the discharge circuit usually comprises an overflow valve and the first portion of the cooling circuit is preferably connected upstream precisely to the overflow valve. The cooling circuit and the discharge circuit are joined together downstream of the overflow valve. According to the examples of the present invention described above, the cylinder is therefore fed with "fresh" fuel along a portion of the cooling circuit, and then the discharging of this "fresh" fuel - which however has now become overheated acting as a lubricant - together with the seeping fuel takes place along a different discharging portion of the cooling circuit. As is known, the piston pump comprises:

- a head-piece inside which a cylinder is formed along an axis A; and

- a pumping piston housed inside the cylinder.

In this configuration, the cooling circuit comprises an annular feed header and an annular discharge header which are arranged annularly and at different heights with respect to the axis respectively downstream of the first portion and upstream of the second portion of the cooling circuit. The two annular headers are connected to a bottom portion of the cylinder for collecting the seeping fuel respectively by a feed channel and a discharge channel.

In the case where the pump unit comprises at least two high-pressure piston pumps, the two annular feed headers and the two annular discharge headers are connected together by intermediate portions of the cooling circuit.

The present invention also relates to the method for cooling a pump unit as described above.

This method comprises therefore the steps of:

- drawing off part of the fuel passing through the intake circuit or in the discharge circuit;

- feeding the drawn-off fuel to the cylinder of the high-pressure piston pump;

- evacuating from the cylinder of the high-pressure piston pump the drawn-off fuel together with the fuel seeping along the cylinder;

- discharging the drawn-off fuel together with the fuel seeping along the cylinder along the discharge circuit.

According to one embodiment the pump unit comprises a plurality of high- pressure piston pumps. In this case both the step of feeding the drawn-off fuel to the cylinder of the high-pressure piston pump and the step of evacuating from the cylinder of the high-pressure piston pump the drawn-off fuel together with the fuel seeping along the cylinder are performed in parallel.

Further characteristic features and advantages of the present invention will become clear from the description below of a non-limiting example of embodiment thereof, with reference to the figures of the attached drawings, in which:

- Figure 1 is a schematic view of a hydraulic diagram of a mechanically actuated pump unit of the known type;

- Figure 2 is hydraulic diagram showing a schematic view of an example of embodiment of a mechanically actuated pump unit according to the present invention;

- Figure 3 is a hydraulic diagram showing a schematic view of an electrically actuated pump unit of the known type;

- Figure 4 is a hydraulic diagram showing a schematic view of an example of embodiment of an electrically actuated pump unit according to the present invention;

- Figures 5 and 6 are schematic cross-sectional views of two examples of embodiment of ducts connected to the high-pressure pump according to the present invention. With reference to the list of figures indicated above Figure 1 is a hydraulic diagram showing a schematic view of a mechanically actuated pump unit of the known type.

In particular, according to this hydraulic diagram a tank 2 is connected, upstream, to an internal-combustion engine 3, situated downstream. The terms "upstream" and "downstream" relate to the direction of feeding of the fuel flow from tank 2 to the engine 3.

The first section of the hydraulic diagram in Figure 1 joins the tank 2 to a low- pressure gear pump 4. This first section has been schematically indicated in Figure 1 by the reference number 23 and includes a plurality of known devices, such as a filter 24. For the purposes of the present invention the low-pressure gear pump 4 is also of the known type. An intake circuit 6 starts from the low- pressure gear pump 4 and terminates at the intake valves 11 associated with the high-pressure piston pumps 5. These intake valves 11 regulate the feed flow to the cylinders of the high-pressure piston pumps 5 where compression takes place. The compressed fuel is then discharged from the high-pressure piston pumps 5 along delivery circuits 25 which downstream are connected to a single high-pressure header 26 which in turn feeds the internal-combustion engine 3 via a plurality of injectors 27. Figure 1 also shows a discharge or overflow circuit 7 which has a first end connected to the intake circuit 6 and a second end which emerges inside the tank. According to the example of the prior art shown in Figure 1, the intake circuit 6 comprises a metering valve 10 and therefore the first end of the discharge circuit 7 intercepts the intake circuit 6 upstream of the metering valve 10. The lubrication of the piston inside the associated cylinder of the high-pressure pump 5 is performed by part of the fuel drawn into the cylinder which seeping advances along the walls of the cylinder as far as a seal 22 which may be arranged on the opposite side to the intake valve 11. According to the example shown in Figure 1 there is also a recirculation duct 28 which is schematically shown in broken lines and which feeds again the intake valve 11 with the fuel seeping at the base of the cylinder.

The opposite end of the piston to the intake valve 11 is connected to an actuator, in the example a cam shaft - schematically indicated by the reference number 29 with an axis orthogonal to the axis of the pumping pistons - which is lubricated by an oil bath. In Figure 1 the chamber containing the oil is schematically indicated by the reference number 30, while the reference numbers 31 and 32 indicate the channel for feeding and discharging this oil.

Parts which have been described with reference to Figure 1 and are also present in the remaining figures are indicated by the same reference numbers and will not be described again for the purposes of easier description.

Figure 2 is a hydraulic diagram showing a schematic view of an example of embodiment of a mechanically actuated pump unit according to the present invention.

Unlike Figure 1, the example of embodiment of the invention shown in Figure 2 is not provided with channels 28 for feeding back to the valves 11 the fuel collected at the base of the cylinder and used as coolant/lubricant. Differently, in an innovative manner this collected fuel is discharged into a circuit 8" which downstream leads into the discharge circuit 7. However, according to the invention this circuit 8" is not fed solely with the fuel seeping along the cylinder, but with a mixture comprising partly this seeping fuel and partly a quantity of fuel which has reached the cylinder not passing via the valve 11. This fuel, which may be defined as being a cooling fuel because it performs the function of lubricant for the piston, is fed to the cylinder by means of a circuit 8' which, together with the preceding circuit 8", forms two portions or branches of a cooling circuit 8 which are situated upstream and downstream of the high-pressure pumps 5. According to the example shown in Figure 2, this cooling circuit 8 is connected to the intake circuit 6 upstream of the metering valve 10.

Figure 3 is a hydraulic diagram showing a schematic view of an electrically actuated pump unit of the known type. In particular, this example shows a filter 33 along the intake circuit 6 downstream of which the discharge circuit 7 starts. This discharge circuit 7 is provided with an overflow valve 12. The known example shown in Figure 3 also has channels 28 for feeding the fuel stored at the base of the cylinder back to the valves 11.

Figure 4 is a hydraulic diagram showing a schematic view of an example of embodiment of an electrically actuated pump unit according to the present invention.

Unlike Figure 3, the example of embodiment of the invention shown in Figure 4 is not provided with any channel 28 for feeding the fuel stored at the base of the cylinder back to the valves 11. In the same way as for the example shown in Figure 2, in an innovative manner this stored fuel is discharged into a circuit 8" which leads into the discharge circuit 7 downstream of the overflow valve 12. According to the invention this circuit 8" is not fed solely with the fuel seeping along the cylinder, but also with a mixture comprising partly this seeping fuel and partly a quantity of fuel which has reached the cylinder not passing via the valve 11. This fuel, which may be defined as being a cooling fuel because it performs the function of lubricant for the piston, is fed to the cylinder by means of a circuit 8' which, together with the preceding circuit 8", forms two portions upstream and downstream of the high-pressure pumps 5 of a cooling circuit 8. According to the example shown in Figure 4, this cooling circuit 8 is connected to the overflow valve 12.

Figures 5 and 6 are schematic cross-sectional views of two examples of embodiment of the ducts connected to the high-pressure pump according to the present invention. In particular, Figures 4 and 5 shows how the branches 8' and 8" of the cooling circuit 8 are connected to the cylinders 14 of the high-pressure pumps 5. In these figures, as in the preceding figures, the reference number 13 indicates the head-piece inside which the cylinders and at least part of the circuits described above are formed. Figure 5 shows two cylinders 14 with an axis A having associated pistons 15 in a parallel configuration. In this example the upstream branch 8' of the cooling circuit 8 leads in the region of the cylinder into an annular header or space 16. This annular header 16 is, on the one hand, connected to a feed duct 18 which feeds a collection chamber at the base of the cylinder 14 in the region of a seal 22, and on the other hand, is connected to a corresponding annular header 16 of the second cylinder 14 via an intermediate duct 20. The collection chamber at the base of the cylinder 14 is also connected by means of a discharge channel 19 to a second annular header 17 from which the downstream branch 8" of the cooling circuit 8 extends. In a similar manner the collection chamber of the second cylinder is also connected to an annular discharge header 17 connected by means of an intermediate duct 21 to the aforementioned annular header 17. According to this example the flow path of the fuel may now be described. "Cold" fuel drawn from the intake circuit or the discharge or overflow circuit is fed along the upstream branch 8' of the cooling circuit 8. At the first annular header 16 part of this "cold" fuel is conveyed to the collection chamber of the first cylinder, while the remaining part is fed via the duct 20 to the second annular header 16 and from there to the collection chamber of the second cylinder.

The collection chamber of the first cylinder is connected by means of a channel 19 to as second annular header 17 which leads into the downstream branch 8" of the cooling circuit 8. In a similar manner the collection chamber of the second cylinder is connected by means of a channel 19 to a second annular header 17 which is joined to the first header by means of a channel 21.

Figure 6 shows an alternative variant of the embodiment shown in Figure 2 in which the collection chamber 34 is visible and the sealing ring 22 is not present.

Obviously it is possible to provide a single cylinder or more than two cylinders joined together in a cascade arrangement in the same way as shown in Figures 5 and 6.

It is clear that the present invention described here may be subject to modifications and variations without departing from the scope of the accompanying claims.