Unit injector

Abstract

 A unit injector system includes a rotary cam (22A) for imparting inward movement to a plunger (19) to displace fuel from a pumping chamber to a fuel injection nozzle (12). A spill valve (29) is utilised to control the amount of fuel supplied. The leading flank of the cam has a first portion (B1) which after initial inward movement of the plunger effects inward movement of the plunger at a substantially constant rate, a second portion (B2) during which the rate of movement is increased and a third portion (B3) during which the rate of movement of the plunger decreases until the crest of the cam is reached.

Description

[0001] This invention relates to a so called unit injector system for supplying fuel to a compression ignition engine and of the kind comprising a body in which is defined a bore and in which is mounted a reciprocable plunger, a cam for actuating the plunger, the plunger and bore defining at least in part, a pumping chamber from which fuel is expelled during inward movement of the plunger, an outlet passage extending from said pumping chamber and communicating with the inlet of a fuel injection nozzle which is mounted on the body and a spill valve mounted on the body and which controls the flow of fuel through a spill passage communicating with said pumping chamber, and means through which the pumping chamber is completely filled with fuel during the outward movement of the plunger.

[0002] Unit injectors are well known in the art, the cam being driven by the associated engine and the operation of the spill valve being controlled by an electro-­magnetic device the supply of electric current to which is effected by an electronic control system. The electro-magnetic device includes an armature which may be coupled to the valve member of the spill valve although other arrangements are known in which a servo system is provided.

[0003] Whatever the arrangement the spill valve takes a finite time to move from its open position to the closed position and back to its open position and this means that as the speed of operation of the associated engine increases the minimum amount of fuel which can be supplied to the engine increases. At high engine speeds this minimum amount of fuel may be greater than is required to operate the engine at light load. Moreover, in some engine installations where the injection of fuel takes place in two stages, the aforesaid minimum quantity of fuel may be in excess of the desired pilot quantity of fuel at high engine speeds.

[0004] The object of the present invention is to provide a unit injector of the kind specified in a simple and convenient form.

[0005] According to the invention the profile of the leading flank of the cam has a first portion which after the initial inward movement of the plunger effects inward movement of the plunger at a substantially constant rate, a second portion during which the rate of movement of the plunger is increased and a third portion during which the rate of movement of the plunger decreases until the crest of the cam is reached, the rate of movement being considered in terms of the angular rotation of the cam.

[0006] An example of a unit injector in accordance with the invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a sectional side elevation of one example of a unit injector,

Figure 2 is a diagram showing the conventional form of cam
and

Figure 3 is a diagram similar to Figure 2 showing the cam modified in accordance with the invention

[0007] Referring to Figure 1 of the drawings the unit injector comprises a body 10 which defines a stepped cylindrical portion 11 to which is secured the body 12 of a fuel injection nozzle, there being interposed between the nozzle body and the body 10, a distance member 15.

[0008] The valve member of the nozzle is of conventional construction and is biased to a closed position in which it contacts a seating, by means of a coiled compression spring 14 which is housed within a chamber defined in the body 10. The valve member of the nozzle mounts a spring abutment 13 against which the spring 14 bears. The nozzle body is retained relative to the body 10 by means of a cap nut 16 which has a skirt portion 17 in screw thread engagement with a complementary thread formed on the portion 11 of the body.

[0009] The body also mounts a high pressure fuel pump which is generally indicated at 18 and which includes a reciprocable plunger 19 which is housed within a bore formed in a pump barrel 20, the latter being retained within a complementary recess in the body 10 by means of a retarding nut 21. The plunger is connected to a tappet assembly generally indicated at 22 and which in use is engaged by a cam 22A driven by the associated engine. The tappet assembly and plunger are moved inwardly by the cam against the action of a return spring 23. Formed in the wall of the barrel 20 is a pair of filling ports 24 which communicate with a gallery surrounding the barrel and which in turn communicates with a fuel supply passage 25 which in use, communicates with a fuel supply channel formed in the wall of the bore in the associated engine, in which the unit injector is located.

[0010] The plunger and bore define a pump chamber which communicates by way of a passage 28, which extends through the cylindrical portion 11 of the body, and the distance piece 15 with a nozzle inlet. The nozzle inlet is connected in the usual manner, with a fuel supply gallery which surrounds a part of the valve member of the nozzle and when fuel under pressure is supplied through the inlet, the fuel pressure acting upon the valve member lifts the valve member against the action of the spring 14 to permit fuel flow through an outlet orifice. The flow of fuel can occur providing a spill control valve generally indicated at 29 is in the closed position. The spill valve is connected to the passage 28 by a spill passage which is shown in dotted outline at 30 and when the valve 29 is open the fuel pressure in the passage 28 is insufficient to lift the valve member in the nozzle, the fuel returning by way of a passage 31 to a spill outlet 32 formed in the cylindrical portion of the body 10. The spill valve is controlled by an electro-magnetic actuator which is generally indicated at 33 and the supply of current to the actuator is controlled by an electronic control system not shown.

[0011] The spill valve takes a finite time to move from its open position to the closed position and back to its open position and this means that for a given engine speed there is a minimum amount of fuel which can be supplied through the outlet orifice. This minimum amount increases as the engine speed increases. It is usual in unit injectors of this type to use a constant rate cam which means that after initial inward movement of the plunger the rate at which the plunger moves inwardly will be constant for a given angular movement of the cam and normally delivery of fuel to the associated engine is arranged to take place during the period of constant inward movement of the plunger. Figure 2 illustrates at A the profile of the leading flank of such a cam lobe, it being understood that it is the leading flank which causes inward movement of the plunger. At B there is indicated the rate of movement of the plunger considered in terms of Degrees of rotation of the cam and at C the acceleration of the plunger. As will be seen from the curve B, there is a period during which the plunger moves at a constant rate and it is during this period that delivery of fuel is arranged to take place.

[0012] Figure 3 shows at A the profile of the leading flank of a cam lobe as modified in accordance with the invention and as will be seen following initial inward movement of the plunger there is a first period B1 during which the plunger moves at a constant rate but it will be observed that in the example shown, the rate of movement of the plunger is half that obtained with the standard cam. Following the first period during which the plunger moves at a constant rate, there is a second period B2 during which the speed of the plunger increases and this is followed by a third period B₃ during which the speed of the plunger is decreased as the crest of the cam lobe is approached. In the example the second portion B2 is also shaped to provide constant rate of acceleration. It will be understood that the rates quoted are by way of example only.

[0013] At light load operation of the engine that is to say when only a small quantity of fuel is supplied, the spill valve 29 is operated during the first period B1 so that for a given spill valve operating as quickly as possible, with the cam modified in accordance with the invention a reduced minimum quantity of fuel is supplied as compared with the conventional cam. In engine installations which provide for the fuel to be delivered to the engine in two distinct stages, the first stage of delivery or "pilot" delivery takes place during the first period B1.

[0014] In the example the rate of movement of the plunger during the first period B1 is constant, it will be understood however that the rate of movement of the plunger can be arranged if so desired, to vary slightly in this period.

[0015] A further problem which occurs with unit injectors and which is minimised by the form of cam described, is the high rate of fuel delivery due to the fact that the injectors have little "dead" volume and hydraulically are very "rigid". During the ignition delay period the large amount of fuel which can be delivered to the engine cylinder can cause a sharp rise in cylinder pressure. Providing the spill valve is closed whilst the plunger is being moved inwardly by the constant rate portion of the modified cam, the rate of supply of fuel will be less than with the conventional cam and the quantity of fuel delivered during the ignition delay period will be reduced. As a result the rate of rise of cylinder pressure will be reduced and the engine noise reduced.

[0016] By retarding the closure of the spill valve more of the fuel can be arranged to be delivered in the aforesaid second period B2 and this can be used to obtain higher pumping pressures as compared with the conventional cam, in the intermediate speed range of the engine.

Claims

1. A unit injector system for supplying fuel to a compression ignition engine and of the kind comprising a body (10) in which is defined a bore and in which is mounted a reciprocable plunger (19), a cam (22A) for actuating the plunger, the plunger (19) and the bore defining at least in part, a pumping chamber from which fuel is expelled during inward movement of the plunger (19), an outlet passage (28) extending from the pumping chamber and communicating with an inlet of a fuel injection nozzle (12) mounted on the body (10), a spill valve (29) mounted on the body and which controls the flow of fuel through a spill passage (30) communicating with the pumping chamber, and means (24) through which the pumping chamber is completely filled with fuel during the outward movement of the plunger (19) characterised in that the leading flank of the cam (22A) has a first portion (B1) which after the initial inward movement of the plunger effects inward movement of the plunger at a substantially constant rate, a second portion (B2) during which the rate of movement of the plunger is increased and a third portion (B3) during which the rate of movement of the plunger decreases until the crest of the cam (22A) is reached, the rate of movement being considered in terms of the angular rotation of the cam.

2. A system according to Claim 1, characterised in that the second portion (B2) of the leading flank of the cam is shaped to provide constant acceleration.

Drawing