GB2056716A

GB2056716A - A fuel injection pump for an internal combustion engine

Info

Publication number: GB2056716A
Application number: GB8025551A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-08-07
Filing date: 1980-08-06
Publication date: 1981-03-18
Also published as: DE2931987A1; JPS5629028A; GB2056716B

Abstract

The injection pump 1 has a timing piston 3 which, for the purpose of timing the instant of injection, is adjustable against a restoring force 5 by fluid from a pump 6 which delivers in proportion to the engine speed. The injection pump 1 having speed-proportional control of the pressure of the fluid by means of a pressure-control valve 11 which has a control piston 17 which is displaceable between a pressure-chamber and a restoring chamber 15 against a restoring force 16 and which determines a shut-off cross section, the injection pump 1 having a pressure-holding valve 13 which additionally influences the pressure of the fluid, in dependence upon characteristic quantities of the engine and which has a valve spring. The opening pressure of the pressure holding valve being variable by way of an electrically heatable bimetal spring 28, 27 which serves as a control member and which influences the effective force of the valve spring 26 or (32). <IMAGE>

Description

SPECIFICATION A fuel injection pump for an internal combustion engine The present invention relates to a fuel injection pump for an internal combustion engine.

In particular the invention relates to a fuel injection pump having a timing piston which, for the purpose of timing the instant of injection, is adjustable against a restoring force by fluid from a pump which delivers in proportion to the engine speed, and having speedproportional control of the pressure of the fluid by means of a pressure-control valve.

The pressure control valve having a control piston which is displaceable between a pressure chamber and a restoring chamber against a restoring force which determines a shut-off cross section. A pressure valve being provided which additionally influences the pressure of the fluid and is controlled by a control member in dependence upon characteristic quantities of the engine and operate with at least one valve spring.

In a known fuel injection pump of this kind, the pressure which varies the commencement of injection, and which normally varies with the rotational speed, is varied by a valve operating in dependence upon characteristic quantities of the engine. During cold starting for example, this change of pressure advances the commencement of injection to such an extent that sufficient time remains for the preparation of the fuel, and the ignition or combustion operations are correspondingly improved. Either the movable member of the pressure valve is directly opened by the control member, or the initial stress of the spring is changed. However, the control accuracy is inadequate for many requirements of the exhaust gases, since the system exhibits a certain inertia with respect to readjustment only leads to an average value in the pressure control in the case of rapid changes of load.

According to the present invention there is provided a fuel injection pump for an internal combustion engine having a timing which, for the purpose of timing the instant of injection, is adjustable against a restoring force by fluid from a pump which delivers in proportion to the engine speed, and having speed-proportional control of the pressure of the fluid by means of a pressure-control valve which has a control piston which is displaceable between a pressure chanber and a restoring chamber against a restoring force and which detemines a shut-off cross section, and a pressure-holding valve which additionally influences the pressure fluid and is controlled by a control member in dependence upon characteristic quantities of the engine, and which has a valve spring, the opening pressure of pressure holding valve being variable by way of an electrically heatable bimetal spring which serves as the control member and which influences the effective force of the valve spring.

In contrast to the prior art the fuel injection pump in accordance with the invention has the advantage that the reaction time is relatively short and, moreover, the pressure-holding valve can be subsequently fitted in a unitcomposed manner. The mutually opposing action of the bimetal spring and the valve spring renders it possible to adjust the pressure in an extremely accurate manner with a very fine and relatively large reaction capacity.

The present invention will now be further described by way of example only, with reference to the accompanying drawings, in which Figure 1 is a diagrammatic illustration drawn to an enlarged scale, of an injection pump having a bimetal-spring-controlled pressure-holding valve.

Figure 2 is a function graph, and Figure 3 shows a bimetal valve which operates in a somewhat different manner.

In the system illustrated in Fig. 1, a timing piston 3 acts upon a cam drive of fuel injection pump 1 by way of a pin 2 for timing the instant at which the injection operation commences. The timing piston is displaceable against the force of a return spring 5 by hydraulic fluid located in a working chamber 4, the instant of injection being advanced with respect to the top dead centre of the engine piston the further the piston displaced towards the spring. A feed pump 6 draws fuel from a fuel tank 7 and delivers it into a suction chamber 8 of the injection pump 1 from which (not illustrated) the actual fuel injection pump is supplied with fuel and which is connected to the working chamber 4 by way of a bore 9 in the timing piston 3.

The bore 9 has a restrictor 1 0. The feed pressure of the feed pump 6, and thus the pressure in the suction chamber 8, are controlled in dependence upon rotational speed by a pressure-control valve 11, the pressure increasing proportionally as the rotational speed increases. This speed-dependent pressure also prevails in the working chamber 4, so that, as the rotational speed and thus the pressure, increases, the injection-timing piston 3 is displaced in a direction to advance the instant of injection.

Fig. 2 is a graph in which the stroke s (ordinate) of the timing piston is plotted against the rotational speed n (abscissa).

I is the characteristic curve for the timing of the commencement of injection for which the stroke, and thus the adjustment to advance the instant of injection, increase lineary as the rotational speed increases. In many engines, it is desirable or necessary to advance or retard the commencement of injection at least up to a predetermined speed when the engine is cold or under other conditions, particularly to accelerate the engine rapidly or to improve the exhaust gas. Curve II of Fig. 2 shows how the commencement of injection can be temporarily advanced to an increased extent, particularly at low engine speeds.

In accordance with the invention, this intervention is effected by way of a pressureholding valve 1 3 which, in the illustrated embodiment, is disposed in an outlet passage 14 of a pressure-control valve 1 1. This outlet passage 1 4 branches from- a chamber 1 5 in which a return spring 1 6 of a control piston 1 7 is disposed. The control piston 1 7 is subjected to the fuel pressure produced by the pump 6 and is displaced against the force of the return spring 1 6 in conformity with the pressure, control port 1 8 being opened to a greater or lesser extent.A bore 19 is disposed in the control piston 1 7 and has a restrictor 20 through which the fuel can flow to the spring chamber 1 5. Thus, the control piston 1 7 is displaced to the left as soon as the throttling action of the pressure-holding valve 1 3 in the outlet passage 1 4 is intensified, whereby the size of the control port 1 8 is reduced and the pressure in the suction chamber 8, and thus in the working chamber 4 of the injection-timing piston 3; is increased.

However, as described above, an increase in the pressure means that the instant of injection is advanced.

The pressure-holding valve 1 3 has a control diaphragm 21 which covers an annular chamber 22 into which an inlet conduit 23 opens, which diaphragm also controls the mouth 24 of an outlet conduit 25. The diaphragm is biased towards the mouth 24 of the annular chamber 2 by a spring 26 whose force is transmitted to the diaphragm 21 by means of a rod 27. A bimetal spring 28 engages over the rod 27 in a fork-like manner and its initial stress is variable by a heating resistor 29. The bimetal spring 28 is actuated in an operative direction either against or in the direction of the force of the spring 26 occording as to whether the opening pressure of the valve is to be determined by a force which is larger or smaller than the force of the spring 26.By way of example, if the pressure in the working chamber 4 is to be reduced as the- temperature increases, the dynamic pressure in the spring chamber 1 5 has to be reduced as the temperature increases, that is to say, the dynamic pressure effected by the pressureholding valve 1 3 has to be reduced. This is acheived by virtue of the fact that the bimetal spring 28 opposes the force of the valve spring 26 upon a rise in the temperature of the heating resistor 29 or, if the function is reversed, upon the cooling of the heating resistor 29, so that the ultimate force of the valve spring is decreased. It is thereby possible to effect a corresponding transition from, for example, the characteristic curve II of Fig.

2 to the characteristic curve 1 for which the throttling resistance of the pressure-holding valve 1 3 is virtually zero.

Fig. 3 shows a bimetal-valve which operates in a somewhat different manner. In the present instance, the valve is not a diaphagm valve, but is a ball valve in which the ball 51 serving as a moveable valve member is pressed onto a valve seat 33 by a valve spring 32. The spring 32 abuts against a slider 34 which is axially moveable and which is sealed relative to the housing by means of an annular seal 35. The slider 34 is loaded by a support spring 36 which acts against the force of a bimetal spring 37. The position of the bimetal spring 37 is determined by a heating coil 38 which is connected to the electrical connection in 39. The position of the bimetal spring 37 shown by broken lines corresponding with the heated state of the bimetal spring.When the bimetal spring is in this heated state, i.e. in the position shown by broken lines, the slider 34 is displaced by the support spring 36, When the slider is in this displaced position, the valve spring 32 is relieved of stress such that the moveable valve member 31 offers virtually no resistance to the flow of fuel. Compared with the forces of the support spring 36 or even the forces of the bimetal spring 37, the forces of the valve spring 32 are relatively small even when the latter is in its illustrated initially stressed state, so thaw'the valve spring 32 has no effect on the actual temperature-dependent position of the slider 34. In the embodiment shown in Fig. 3, the flow-through resistance is small when in the hot state, that is to say, the pressure prevailing in the suction chamber 8 or in the working chamber 4 is relatively low, so that the instant of injection is retarded.

Here also, the pressure-holding valve 1 3 operates as a hot-running regulator. However, in accordance with the invention, it is conceivable to subject the pressure-holding valve 1 3 directly to the feed pressure of the feed pump 6, that is to say, to dispose the pressureholding valve in parallel with the pressurecontrol valve 11.

Claims

1. A fuel injection pump for an internal combustion engine, having a timing piston which, for the purpose of timing the instant of injection, is adjustable against a restoring force by fluid from a pump which delivers in proportion to the engine speed, and having speed-proportional control of the pressure of the fluid by means of a pressure-control valve which has a control piston which is displacea ble between a pressure chamber and a restor ing chamber against a restoring force and which determines a shut-off cross section, and a pressure-holding valve which additionally influences the pressure of the fluid and is controlled by a control member in dependence upon characteristic quantities of the engine, and which has a valve spring, the opening pressure of pressure holding valve being variable by way of an electrically heatable bimetal spring which serves as the control member and which influences the effective force of the valve spring.

2. A fuel injection pump as claimed in claim 1, in which, the pressure-control valve has a throttle connection between a pressure chamber and a spring chamber, and an outlet passage for the spring chamber, and that the pressure-holding valve is connected in the outlet passage.

3. A fuel injection pump as claimed in claim 1 or 2, in which a slider is radially sealingly guided in a housing of the pressureholding valve to serve as a stop for the valve spring, and that end of the slider is remote from the valve spring is acted upon by the bimetal spring.

4. A fuel injection pump as claimed in claim 3, in which the slider is acted upon by a support spring which abuts against the housing by way of an abutment and which opposes the force of the bimetal spring.

5. A fuel injection pump as claimed in claim 4, in which the initial stress of the valve spring is reducible to zero by the support spring and the bimetal spring.

6. A fuel injection pump as claimed in claim 1, or 2, in which the pressure-holding valve has a control diaphragm which is acted upon by the fuel in the opening direction.

7. A fuel injection pump as claimed in claim 6, in which an outlet controlled by the diaphragm is in the form of a flat seat valve.

8. A fuel injection pump as claimed in claim 6 or 7, in which a valve inlet opens into an annular chamber which is defined at one end by the diaphragm.

9. A fuel injection pump contructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.