GB2306999A - I.c. engine fuel supply system with an accumulator and pressure control means - Google Patents

Abstract

A fuel system, for example as shown in the accompanying drawing, comprises a pump 10 arranged to charge an accumulator 30, a delivery valve 34 arranged to control fuel delivery from the accumulator 30, and pressure control means 46 arranged to reduce the pressure of the initial supply of fuel from the accumulator 30 to reduce noise and improve efficiency. The pressure control means 46 conveniently takes the form of a spring biased piston 50.

Description

FUEL SYSTEM This invention relates to a fuel system for supplying fuel under pressure to the cylinders of an internal combustion engine. In particular, this invention relates to a fuel system of the type comprising a fuel pump arranged to charge an accumulator with fuel at high pressure, and a distribution arrangement for delivering fuel from the accumulator to the cylinders of the associated engine, in turn. The distribution arrangement may include one or more valves for use in controlling the timing of fuel delivery.

One disadvantage associated with such fuel systems is that on the valve opening, the initial supply of fuel to the engine is at a high rate resulting in noisy and inefficient operation of the engine. It has been found that where the fuel is delivered in such a way that the initial delivery rate is relatively low, such disadvantages can be reduced.

According to the present invention there is provided a fuel system comprising a pump arranged to charge an accumulator, a delivery valve arranged to control the delivery of fuel from the accumulator to a delivery passage, and pressure control means arranged to communicate with the delivery passage for reducing the pressure of the initial supply of fuel from the delivery passage.

The pressure control means conveniently comprises a piston arrangement.

Such a fuel system is advantageous in that on opening the delivery valve, part of the initial flow of fuel along the delivery passage is extracted from the delivery passage, flowing to the piston arrangement, thus reducing the rate of the initial flow of fuel.

The piston arrangement is preferably biased such that when fuel is not delivered through the delivery passage, fuel is expelled from the piston arrangement.

The invention will further be described, by way of example, with reference to the accompanying drawing which is a diagrammatic view of a fuel system in accordance with an embodiment of the invention.

The fuel system illustrated in the accompanying drawing comprises a rotary fuel pump 10 which includes a distributor member 12 which is rotatable within a sleeve 13. The distributor member 12 includes a diametrically extending through bore 14 within which a pair of plungers 16 are reciprocable. The outer end of each plunger 16 carries a shoe and roller arrangement 18, the roller of which is engageable with the cam surface of a cam ring 20.

The distributor member 12 includes a passage 22 which is arranged to register with ports 13a, 13k provided in the sleeve 13 upon rotation of the distributor member 12.

A transfer pump 24 is arranged to supply fuel from a suitable reservoir through a throttle arrangement 26 to an inlet port 1 3a of the sleeve 13, an outlet port 13h of the sleeve 13 communicating through a non-return valve 28 with an accumulator 30. The throttle arrangement 26 is arranged to control the quantity of fuel supplied to the inlet port 13a, and hence to control the quantity of fuel supplied to the accumulator 30 in order to maintain the pressure of fuel within the accumulator 30 within a predetermined range of pressures, in use.

Starting from the position illustrated in the accompanying drawing, the plungers 16 have been moved to substantially their innermost positions due to the engagement of the shoe and roller arrangements 18 with the cam lobes of the cam ring 20, and the passage 22 is in communication with the outlet port 13k of the sleeve 13. Subsequent rotation of the distributor member 12 results in the communication between the passage 22 and the outlet port 13k being broken, and in the passage 22 registering with the inlet port 1 3a of the sleeve 13. Once the passage 22 aligns with the inlet port 13a, fuel from the transfer pump 24 is supplied through the throttle arrangement 26 to the passage 22, the fuel being at a sufficiently high pressure to push the plungers 16 and shoe and roller arrangements 18 radially outward.Continued rotation of the distributor member 12 results in the communication between the passage 22 and the inlet port 13a being broken, and in the passage 22 aligning with one of the outlet ports 13k of the sleeve 13. After such communication has commenced, the rollers of the shoe and roller arrangements 18, engage with respective lobes of the cam surface, pushing the plungers 16 radially inward and pumping fuel from the bore 14 through the passage 22 to the outlet port 13k. From the outlet port 13k, the fuel is supplied through the non-return valve 28 to the accumulator 30. Such pumping of fuel continues until the plungers reach their radially innermost position as illustrated in the accompanying drawing. The above described cycle is repeated in order to maintain the pressure within the accumulator 30 at a suitable operating pressure, for example 400 to 1,500 bar.

When the fuel system is initially started, the pressure of fuel within the accumulator 30 may be lower than the output pressure of the transfer pump 24. In order to raise the pressure of the fuel within the accumulator 30 to substantially the output pressure of the transfer pump, a non-return valve 32 is provided, thus provided the fuel pressure at the outlet of the transfer pump 24 is sufficient to overcome the pressure of the fuel within the accumulator 30 and the force of the spring of the non-return valve 32, fuel is supplied directly from the transfer pump 24 through the throttle arrangement 26 to the accumulator 30.

Although in the drawing, the transfer pump 24 is illustrated separately to the distributor member 12, the transfer pump 24 may be mounted on an end of the distributor member 13 so as to be driven therefrom.

The accumulator 30 communicates through a valve 34 and supply passage 35 with an annulus 36 provided on the outer periphery of the distributor member 12. A delivery passage 38 communicates with the annulus 36, the delivery passage 38 being arranged to communicate, in turn, with delivery ports 13g provided in the sleeve 13, on rotation of the distributor member 12. Each of the delivery ports 13g communicates through delivery lines with respective fuel pressure actuated injectors 40 provided in the cylinders of the engine.

In use, with the valve 34 in the position illustrated in the drawing, fuel is being supplied from the accumulator 30 to the injector 40. In order to terminate injection, the valve 34 is switched whereby the communication between the accumulator 30 and the injector 40 is broken, the switching of the valve 34 resulting in the injector 40 being arranged to communicate through a suitable pressure regulation valve 42 with a low pressure drain 44 to maintain the lines therebetween at a suitable, relatively low, pressure.

A piston arrangement 46 communicates with the supply passage 35, the piston arrangement 46 comprising a bore 48 within which a piston 50 is reciprocable, the piston 50 being biased by means of a spring 52 towards a position in which the piston 50 engages a seating 54. In this position, the volume of the bore 48 available to be occupied by fuel from the accumulator 30 is minimised, and a relatively small surface area of the piston 50 is exposed to the pressure of fuel within the supply passage 35.

The region of the bore 48 within which the spring 52 is provided communicates through a suitable passage with a source of relatively low pressure, for example the outlet of the transfer pump 24 as illustrated in the drawing, and is therefore at a relatively low pressure compared to the pressure of the fuel within the accumulator 30 under normal operating conditions.

In use, in order to supply fuel to the injector 40, the valve 34 is switched to the position illustrated in the drawing, thus high pressure fuel from the accumulator 30 is supplied through the valve 34 resulting in the pressure of the fuel within the supply passage 35 increasing. Even when the accumulator 30, valve 34 and distributor member 12 are as close together as is practical, pipe pressure wave effects occur in the supply passage 35. Thus, on opening the valve 34, a pressure wave front defining the boundary between the high pressure fuel from the accumulator 30 and the relatively low pressure fuel occupying the supply passage before the valve 34 is opened travels along the supply passage 35.On reaching the connection to the piston arrangement 46, high pressure fuel is applied to the piston 50 and when this pressure acting on the exposed area of the piston 50 exceeds the force exerted by the spring 52, the piston 50 is pushed against the action of the spring 52 thus providing an increased volume in the piston arrangement 46 for occupation by fuel from the accumulator 30. Once the piston 50 is lifted from the seating 54, the area of the piston exposed to the high pressure fuel is increased, thus the force pushing the piston 50 against the action of the spring 52 is increased, movement of the piston 50 occurring until the piston 50 engages movement stops provided in the piston arrangement 46.Such movement of the piston 50 results in a reduction in the pressure of the initial flow of fuel from the accumulator 30 to the annul us 36 and from there to the injector 40, or in a reduction in the rate of pressure increase across the pressure wave front. It will be recognised that once the piston 50 has moved to the position in which the volume available for high pressure fuel from the accumulator 30 is maximised, the piston 50 engaging the movement stops, the piston arrangement 46 has minimal effect in controlling the rise in pressure of fuel supplied to the injector 40. As the high pressure acts upon a relatively large area of the piston 50, the piston 50 remains in this position until the pressure of fuel in the passage 35 drops at the end of injection.It will be recognised that by providing a relatively large diameter piston, a relatively small movement of the piston permits a relatively large quantity of fuel to be drawn from the passage 35.

In order to terminate injection, the valve 34 is switched so as to break the communication between the accumulator 30 and the injector 40.

With the valve in this position, high pressure fuel in the supply line 35 is permitted to flow through the valve 42 to drain 44, the consequent reduction in the fuel pressure acting on the injector 40 resulting in the injector 40 closing. The reduction in the pressure applied to the piston 50 results in the piston 50 moving under the influence of the spring 52 expelling fuel from the piston arrangement 46 until the piston 50 engages the stop 54, returning the piston 50 to the position illustrated in the accompanying drawing.

After termination of injection, continued rotation of the distributor member 12 results in the communication between the delivery passage 38 and the respective delivery port provided in the sleeve being broken, continued rotation of the distributor member 12 resulting in the delivery passage 38 communicating with a different one of the delivery ports 13Ç.

The delivery cycle is then repeated.

It will be recognised that although the process of charging the accumulator 30 and the process of delivering fuel from the accumulator 30 to the injector 40 are described separately herein, both processes may occur simultaneously in use.

The pressure which must be applied to the piston 50 in order to result in movement of the piston 50 is dependent, amongst other things, upon the diameter of the exposed part of the piston 50 when the piston 50 engages the seating 54 and upon the preloading of the spring 52. It is intended that the pressure which must be applied to the piston 50 in order to produce such movement should be between the pressure at which the injector nozzle opens and the maximum intended operating pressure of the accumulator 30. The exact selection of the pressure at which such movement occurs and the volume of fuel withdrawn by the piston arrangement is dependent upon the desired variation of the initial rate of fuel supply to the injector 40. The pressure may be greater than or less than the minimum operating pressure of the accumulator in order to provide such a variation in injection rate over the intended ranges of engine speed and load, the accumulator pressure not necessarily being constant, in use. It will be recognised that as the chamber of the piston arrangement which houses the spring 52 is in communication with the outlet of the transfer pump 24, the pressure at which movement of the piston 50 first occurs is also dependent upon the pressure at the outlet of the transfer pump.

Claims (4)

1. A fuel system comprising a pump arranged to charge an accumulator, a delivery valve arranged to control the delivery of fuel from the accumulator to a delivery passage, and pressure control means.

arranged to communicate with the delivery passage for reducing the pressure of the initial supply of fuel from the delivery passage.

2. A fuel system as claimed in Claim 1, wherein the pressure control means comprises a piston arrangement.

3. A fuel system as claimed in Claim 2, wherein the piston arrangement is biased such that when fuel is not delivered through the delivery passage, fuel is expelled from the piston arrangement.

4. A fuel system substantially as hereinbefore described with reference to the accompanying drawing.