GB2316981A - Fuel pump for engine - Google Patents

Abstract

A fuel pump comprises a primary pumping plunger 16 reciprocable within a first bore to pressurize fluid within a primary pumping chamber, and a secondary pumping plunger 28 reciprocable within a second bore under the influence of the fluid pressure within the primary pumping chamber to pressurize fuel within a secondary pumping chamber 32. Fluid is supplied to the first bore through an inlet port 18, the port being closed by the primary plunger 16 as it moves to force fluid to drive the secondary plunger 28 against the action of a spring 34. Delivery is ended when fluid spills from the first bore through a blind drilling in the plunger 16, an inclined groove in the surface of the plunger 16, and a spill port 24. Fuel, which may be di methyl ether, is supplied to the secondary bore through a non-return valve (58, Fig.2) connected to a supply system incorporating a purge arrangement.

Description

FUEL PUMP This invention relates to a fuel pump for use in supplying fuel under pressure to the cylinders of an associated engine.

In a known fuel pump, a plunger is reciprocable within a bore under the action of a cam arrangement, the plunger and bore together defining a pumping chamber. A supply port communicates with the bore and is located such that when the plunger occupies a retracted position, fuel can be supplied from the supply port to the pumping chamber. Inward movement of the plunger results in the plunger covering the supply port thus terminating fuel supply to the pumping chamber.

Further movement of the plunger pressurizes the fuel within the pumping chamber. The pumping chamber communicates through a delivery valve with an injector which opens, once the pressure applied thereto exceeds a predetermined pressure, to commence injection.

The end of the plunger facing the pumping chamber is provided with a drilling which communicates with an angled groove provided in the surface of the plunger. The groove is located so as to be registrable with a spill port such that after a predetermined inward movement of the plunger, the pumping chamber communicates through the drilling and groove with the spill port. Such communication permits fuel to escape from the pumping chamber thus the fuel pressure therein falls. The fall in fuel pressure applied to the injector results in the injector closing.

Subsequently, the plunger is retracted, for example under the action of a spring ready for the commencement of another pumping cycle.

Such a fuel pump is particularly suitable for use in supplying diesel fuel to an engine. It is proposed to use other fuels, for example Di Methyl Ether (DME), in diesel engines. DME is gaseous at ambient temperatures and pressures and, unlike diesel, is unsuitable for lubricating the fuel pump even when in its liquid phase. Further, in order to achieve efficient pumping, it is necessary to provide a low viscosity liquid tight seal between the plunger and the housing or barrel within which the bore is provided.

It is an object of the invention to provide a fuel pump in which the disadvantages described hereinbefore are reduced.

According to the present invention there is provided a fuel pump comprising a primary pumping plunger reciprocable within a first bore, an inlet for supplying fluid to the first bore and an outlet for discharging fluid from the first bore, the primary pumping plunger defining, with the first bore, a primary pumping chamber, and a secondary pumping plunger reciprocable within a second bore under the influence of the fluid pressure within the primary pumping chamber to pressurize fuel within a secondary pumping chamber defined by the second bore and the secondary pumping plunger.

As the fuel is separated from the primary pumping plunger, the primary pumping plunger can be lubricated by the fluid supplied to the primary pumping chamber, and if the fluid is liquid at the appropriate pressures and temperatures, the sealing requirements between the primary pumping plunger and first bore are relatively easy to achieve. Indeed, a small amount of leakage may be preferred in order to assist lubrication.

The primary pumping plunger is conveniently reciprocable under the action of a cam arrangement. The outlet is conveniently communicable with the primary pumping chamber through an axially extending blind drilling provided in the primary pumping plunger, the drilling communicating with an angled groove provided in the outer surface of the primary pumping plunger. In such an arrangement the point during the pumping cycle at which communication occurs between the primary pumping chamber and the outlet is adjustable by the controlling the angular position of the primary pumping plunger relative to the housing.

The invention further relates to a fuel system incorporating a purge arrangement for removing fuel from a fuel supply line of the fuel system, the purge arrangement comprising a purge compressor arranged to draw fuel from the supply line, and supply fuel to a reservoir.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view, partly broken away, illustrating part of a fuel pump in accordance with an embodiment of the invention; and Figure 2 is a diagrammatic view of a fuel system incorporating such a fuel pump.

Figure 1 illustrates a pumping element 10 of an in-line pump 8, the pumping element 10 being arranged to supply fuel under pressure to an injector 12 (see Figure 2) of an associated engine. The in-line pump 8 includes a plurality of pumping elements 10, respective pumping elements 10 being arranged to supply fuel to respective injectors 12 of the associated engine. The pumping element 10 comprises a housing 14 within which a through bore is provided, a primary pumping plunger 16 being reciprocable within the bore under the action of a cam drive arrangement (not shown). The primary pumping plunger 16 and through bore together define a primary pumping chamber. An inlet port 18 is provided in the housing 14, the inlet port 18 communicating with the through bore when the primary pumping plunger 16 occupies a fully retracted position. Inward movement of the primary pumping plunger 16 away from the fully retracted position results in the inlet port 18 being covered by the primary pumping plunger 16 thus breaking the communication between the through bore and the inlet port 18.

An end of the primary pumping plunger 16 is provided with a blind drilling 20 which communicates through a radially extending drilling with an angled or helical groove 22 provided in the outer, cylindrical, surface of the primary pumping plunger 16, the groove 22 being located so as to be registrable with a spill port 24 provided in the housing 14, such registration occurring at a point during the inward movement of the primary pumping plunger 16 dependent upon the angular position of the pumping plunger 16 with respect to the housing 14.

A second housing member 26 abuts an end of the housing 14, the second housing member 26 including a through bore which is coaxial with the through bore of the housing 14. A secondary pumping plunger 28 is reciprocable within the through bore of the second housing member 26, an end of the secondary pumping plunger 28 being exposed to the pressure developed by the primary pumping plunger 16 in use.

The bore of the second housing member 26 communicates with a bore provided within a third housing member 30 so as to define a secondary pumping chamber 32. A helical spring 34 is located within the secondary pumping chamber 32, the spring 34 engaging an end of the secondary pumping plunger 28 so as to bias the secondary pumping plunger 28 towards a position in which a part 28a of the secondary pumping plunger 28 of enlarged diameter engages a seating defined by an end of the second housing member 26. The third housing member 30 includes an outlet which communicates with the secondary pumping chamber 32, and a fuel supply line 33 is connected to the outlet 36 in order to permit fuel to be supplied to the injector 12 associated therewith.

As illustrated in Figure 2, engine oil from the engine's oil pump is supplied to the inlet 18 through a pressure reducing valve 38, filter 40 and non-return valve 42, and the spill port 24 is connected to a return line 44 arranged to return oil to the sump or engine oil reservoir. A line 46 permits oil which leaks past the primary pumping plungers 16 to pass through a non-return valve 48 to the sump, whilst a line 47 permits oil which collects within the pump housing to return to the sump.

Fuel, for example in the form of Di Methyl Ether (DME), is supplied from a reservoir 50 through an electromagnetically operated stop valve 52 by a fuel pump 54 to a filter 56, and from the filter 56 to a first non-return valve 67 and a second non-return valve 58 through which fuel is supplied to the fuel supply line 33. The fuel pump 54 typically supplies fuel at a pressure of approximately 20 bar at which pressure DME is liquid.

In use, starting from the position shown in Figure 1, the primary pumping plunger 16 occupies its innermost position in which the primary pumping chamber communicates with the spill port 24. As the primary pumping chamber is vented via the spill port 24, the secondary pumping plunger 28 occupies a fully retracted position, the secondary pumping plunger 28 having been returned to this position under the action of the spring 34. The return of the secondary pumping plunger 28 to this position results in the line connected to the outlet port 36 being at a sufficiently low pressure that fuel is supplied thereto by the pump 54 through the non-return valve 58. From this position, motion of the cam arrangement allows the primary pumping plunger 16 to move to a retracted position under the action of a spring (not shown). Such movement of the primary pumping plunger 16 results in the communication between the groove 22 and spill port 24 being broken, and in the inlet port 18 communicating with the through bore provided in the housing 14 thus permitting oil to be supplied to the primary pumping chamber. Subsequent inward movement of the primary pumping plunger 1 6 under the action of the cam arrangement results in the primary pumping plunger 16 covering the inlet port 18 and hence in the communication between the inlet port 18 and through bore being broken, further inward movement of the primary pumping plunger 16 pressurizing the oil in the primary pumping chamber, such pressurization resulting in the application of relatively high pressure to the secondary pumping plunger 28 causing the secondary pumping plunger 28 to move against the action of the spring 34 to pressurize the fuel within the secondary pumping chamber 32. The fuel pressure applied to the injector 12 is thereby increased, and injection commences. Inward movement of the primary pumping plunger 16 continues under the action of the cam arrangement, the continued inward displacement causing further inward movement of the secondary pumping plunger 28, and hence maintaining the pressure applied to the injector 12 at a sufficiently high pressure to ensure that injection continues.

Injection of fuel by the injector 12 continues until the inward movement of the primary pumping plunger 16 results in the groove 22 registering with the spill port 24. Such registration results in engine oil being vented from the primary pumping chamber through the drilling 20 and groove 22 thus resulting in a reduction in the oil pressure within the primary pumping chamber. Such a reduction in pressure permits the secondary pumping plunger 28 to return under the action of the spring 34 towards its seating and hence reduces the fuel pressure within the secondary pumping chamber 32. The fuel pressure applied to the injector 12 is therefore reduced, the reduction in fuel pressure being sufficient that the injector 12 closes and injection is terminated. It will be appreciated that any further inward movement of the primary pumping plunger 16 under the action of the cam arrangement results in engine oil being displaced from the primary pumping chamber to the spill port 24, and not in movement of the secondary pumping plunger 28. Such further movement of the primary pumping plunger 16 does not therefore result in an increase in the fuel pressure within the secondary pumping chamber 32.

It will be appreciated that the presence of the non-return valve 58 prevents high pressure fuel from the second pumping chamber 32 being returned to the fuel reservoir 50 during injection, although leakage from the injector 12 is able to return to the reservoir side of the non-return valve 58 through a suitably connected backleak fuel line 60.

When the pump is to be switched off, it is desirable to depressurize the high pressure part of the fuel system, removing residual DME vapour therefrom to prevent the vapour passing the seating of the injector. The pump can be switched off by changing the angular positions of the primary pumping plungers to a 'no fuel' position in which the grooves 22 are continuously in communication with the spill ports 24 resulting in no movement of the secondary pumping plungers, and hence is no fuel being delivered. Alternatively, the supply of oil to the primary pumping chambers may be cut off. The stop valve 52 is switched off and the position of a change over valve 62, which may take the form of an electrically controlled valve, is changed to permit fuel communication between the supply line 33 and a compressor 64 through the injector 12 and backleak line 60. The compressor 64 is switched on to draw fuel from the supply line 33 through the injector 12, and supply the fuel through a non-return valve 66 to the reservoir 50. At the instant at which the pump is switched off, the valve 58 may be partly open, thus some fuel may be drawn from the supply line 33 through the valve 58.

This operation continues until the pressure within the supply line 33 is less than atmospheric pressure. As the fuel system is of essentially fixed volume, the duration of the purge operation may be controlled using a simple timing arrangement.

During the part of the pumping cycle in which the enlarged part 28a of the secondary pumping plunger 28 engages its seating, it will be appreciated that the risk of fuel mixing with oil is reduced. During the purge operation and when the fuel pump is not being used, the pressure within the primary pumping chamber is low, thus the enlarged part 28a engages its seating, thus the risk of mixing of fuel and oil is further reduced.

The purge operation described hereinbefore is merely illustrative, and it will be appreciated that other purge arrangements could be used with the fuel pump of the present invention.

It will be appreciated that the presence of engine oil in the primary pumping chamber is sufficient to lubricate movement of the primary pumping plunger 16, and a sufficiently good seal is provided between the primary pumping plunger 16 and housing 14 to limit leakage of oil to an acceptable level, such a seal not being easily maintained if certain low viscosity, non-lubricating fuels are to be pumped by way of the primary pumping chamber. The engine oil may further lubricate movement of the secondary pumping plunger 28. The fit of the secondary pumping plunger 28 in the through bore provided in the second housing part 26 is sufficiently close that substantially no transfer of fuel or engine oil between the respective pumping chambers is permitted.

In the fuel system illustrated in Figure 2, the connection of the reservoir 50 to the fuel supply line 33 is close to the centre of the fuel supply line 33 thus avoiding excessive heat soak from the pump 10 and from the injector 12. If desired, the fuel may be supplied to the fuel supply line 33 through a suitable metering device, and such fuel supply may be controlled in order to minimise the time during which the fuel occupies the fuel supply line 33 and secondary pumping chamber 32 prior to the commencement of injection.

Although the description hereinbefore is of the use of the pump element 10 in conjunction with other, similar, pump elements an in-line pump arrangement, it will be appreciated that the pump element 10 could be used in a spill type distributor pump or in a unit injector. The invention may also be applicable to pumps in which the quantity of fuel supplied thereto is metered or the plunger stroke is controlled, rather than controlling the quantity of fuel delivered using a spill arrangement.

Further, although the injector 12 described is of the fuel pressure operated type, it may be possible to use the pump element in fuel systems including electricaily actuated injectors.

Although the description hereinbefore refers to the use of engine oil being supplied to the primary pumping chamber, it will be appreciated that other fluids could be used, for example diesel fuel or any other hydraulic fluid. Further, other low viscosity fuels may be supplied under pressure using the pump element.

Claims (5)

1. A fuel pump comprising a primary pumping plunger reciprocable within a first bore, an inlet for supplying fluid to the first bore and an outlet for discharging fluid from the first bore, the primary pumping plunger defining, with the first bore, a primary pumping chamber, and a secondary pumping plunger reciprocable within a second bore under the influence of the fluid pressure within the primary pumping chamber to pressurize fuel within a secondary pumping chamber defined by the second bore and the secondary pumping plunger.

2. A fuel pump as claimed in Claim 1, wherein the primary pumping plunger is reciprocable under the action of a cam arrangement.

3. A fuel pump as claimed in Claim 1 or Claim 2, wherein the outlet is communicable with the primary pumping chamber through an axially extending blind drilling provided in the primary pumping plungei which communicates with an angled groove provided in the outer surface of the primary pumping plunger.

4. A fuel system comprising a fuel supply line and a purge arrangement for removing fuel from the supply line, the purge arrangement comprising a purge compressor arranged to draw fuel from the supply line and return the fuel to a fuel reservoir.

5. A fuel pump substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.