Fuel injection apparatus for internal combustion engines

Abstract

There is described a pump-and-nozzle assembly which forms part of a fuel injection apparatus serving an internal combustion engine. The assembly includes a reciprocating pump piston driven by a servo piston which is intermittently exposed to fuel pressure to cause said pump piston to execute its delivery strokes. Between two delivery strokes the pump work chamber is charged with pressurized fuel through a throttle while the pump piston executes its return stroke. The said throttle causes the charging period to be substantially longer than the injection period. The admission of pressurized fuel to the servo piston is controlled by the switching positions of a valve plunger. The latter, in turn, is exposed to pressurized fuel for periods controlled by a solenoid valve. Said charging periods take place during the variable de-energized periods of the solenoid valve. In this manner the injected fuel quantities are determined.

Description

BACKGROUND OF THE INVENTION

This invention relates to a fuel injection apparatus for internal combustion engines, particularly diesel engines, and is of the type which has a separate pump-and-nozzle assembly for each engine cylinder. Each pump-and-nozzle assembly has a pump piston that is driven by a servo piston having a diameter larger than that of the pump piston. The pump-and-nozzle assembly is coupled to a pressure source which supplies both the pump work chamber and the servo pressure chamber with pressurized fuel. In the conduit between the pressure source and the pump work chamber -- bounded by the pump piston -- there is situated a supply valve. One radial face of the servo piston bounds the servo pressure chamber. A valve plunger which is driven by pressurized fuel delivered by the pressure source and controlled in phase with the internal combustion engine by means of a control device, has a first switching position in which it permits the flow of the fuel to the servo pressure chamber and a second switching position in which it allows the fuel to be discharged from the servo pressure chamber through a return or discharge conduit. In this manner the valve plunger controls the return stroke of the servo piston and thus determines the beginning of the filling or charging stroke of the pump piston. In the second switching position the speed of the charging stroke is affected by a throttle.

In a known fuel injection apparatus of the aforeoutlined type (such as disclosed, for example, in German Pat. No. 1,070,442), the control device is constituted by a mechanically driven rotary distributor which is common to all the pump-and-nozzle assemblies and which controls the admission of the pressurized liquid to the valve plunger with switching periods that are accurately and permanently set by the structure of the distributor. Because of the length of the fuel lines necessitated by the system itself (these lengths, in addition, vary widely, particularly in large engines) and because of the mechanical control of the switching periods, an exact and rapid operation of the fuel injection apparatus required for engines of recent design is very difficult to achieve. This fuel injection apparatus furthermore operates in an rpm-dependent manner since the switching periods of the distributor driven by the engine change according to the engine rpm. Thus, the throttle effect also changes in an rpm-dependent manner at the control locations, so that a uniform injected fuel quantity in case of rapidly changing rpm's can be ensured only with great difficulty, if at all.

In another known fuel injection apparatus of similar structure (such as disclosed, for example, in U.S. Pat. No. 2,598,528), the valve plunger is mechanically connected with the armature of an electromagnet and is driven thereby and controlled in phase with the operation of the internal combustion engine. This fuel injection apparatus has the disadvantage that the inertia of the armature and the valve plunger make a sufficiently rapid and accurate operation, particularly when used in fast-running diesel engines, difficult if not impossible. Since the electromagnet, for moving the valve plunger into both of its switching positions, has to execute relatively large strokes, and because of the masses to be moved has to be of relatively large volume, it is a further disadvantage of this fuel injection apparatus that the electromagnet has a relatively large delay of response and its switching periods are too long.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved fuel injection apparatus of the aforeoutlined type which is free from the aforenoted disadvantages and which, particularly when used with fast-running diesel engines, operates sufficiently rapidly and accurately and in which the injected fuel quantity may be maintained at exact values independently of the fast changing engine rpm's.

Briefly stated, according to the invention, each pump-and-nozzle assembly has, as a control device, a solenoid valve which is disposed in the immediate vicinity of the valve plunger. By means of varying those switching periods of the solenoid valve in which the valve plunger, due to its position, causes the servo piston to execute its return stroke, the length of the charging stroke and thus the readied fuel injection quantity may be altered.

The invention will be better understood, as well as further objects and advantages will become more apparent, from the ensuing detailed specification of a preferred, although exemplary, embodiment taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view of the pump-and-nozzle assembly according to the preferred embodiment of the invention showing the servo piston and the pump piston in their lower dead center position;

FIG. 2 is a sectional view along line II--II of FIG. 1 showing the servo piston and the pump piston in their upper dead center position;

FIG. 3 is a schematic representation of the structure shown in FIGS. 1 and 2 and including in addition, in symbolic representation, associated components of the fuel injection apparatus in a first switching position of the valve plunger at the beginning of the delivery stroke; and

FIG. 4 is a diagram illustrating the injection quantity Q.sub.E as a function of the injection, charging and control periods.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIGS. 1 and 2, the pump-and-nozzle assembly 10 is formed of three structural groups 11, 12 and 13 which are tightened together to constitute a structural unit.

The first structural group 11 includes a housing 14 containing, in a mounting bore 15 that extends normal to a transversal bore 16, an electromagnetically operated 3/2-way valve 17 which serves as a control device and which hereinafter will be referred to as the solenoid valve 17. In the transversal bore 16 there is guided a valve plunger 18, one end of which is exposed to the force of a spring 19, while its other end is exposed to a hydraulic pressure which prevails in a control pressure chamber 21 and which is controlled by the solenoid valve 17.

The second structural group is a hydraulically-operated pump 12 which is controlled by the valve plunger 18 through a control bore 22 and which has a housing 23, a servo piston 24, a pump piston 25, a throttle member 26 (FIG. 2) and a supply valve 27. The servo piston 24, with one of its radial faces 28, bounds the lower portion of a servo pressure chamber 29 while the upper portion of the latter is bounded by a sleeve 31 containing the control bore 22.

The third structural group is formed of an injection nozzle 13 which is in axial alignment with the pump 12 and which has a spring housing 32, an intermediate block member 33 and a nozzle body 34. In the latter there is guided a nozzle needle 35, on which there is inserted within the spring chamber 36 of the spring housing 32, a spring seat disc 37 supporting a closing spring 38. The closing spring 38 seeks to maintain the nozzle needle 35 in its closed position and engages with its other end an insert 39, the dimensions of which, together with the spring bias, determine in a known manner the magnitude of the stroke of the nozzle needle 35. A tightening nut 40 surrounds all components of the fuel injection nozzle 13 and is threadedly in engagement with the housing 23 of the pump 12.

The housing 14 of the first structural group 11 has, normal to the longitudinal axis of the pump-and-nozzle assembly 10, a supply bore 41 (FIG. 2) to which there is coupled a supply conduit 43 carrying fuel under a servo pressure p.sub.S from a pressure source which is schematically shown in FIG. 3 and which will be discussed in more detail hereinafter. The fuel is admitted from the fuel supply bore 41 to an annular chamber 46 in the wall of the transversal bore 16 and therefrom through a control conduit portion 47 to the solenoid valve 17 and through a charging bore 48 and the throttle member 26 to the supply valve 27. When the latter is open which is the case during the charging stroke of the pump, fuel may flow from a spring chamber 49 of the supply valve 27 through a port 51 to the pump work chamber 52. By means of serially arranged further conduits 53, 53a and 53b the pump work chamber 52 and the spring chamber 49 are in a continuous communication with an annular chamber 54 in the vicinity of the nozzle opening 55.

From a second annular chamber 56 provided in the wall of the transversal bore 16, there extends a return bore 57 outwardly to the upper face 58 of the housing 14. To the return bore 57 there is coupled a return conduit 59 which leads to a fuel tank.

The flow passage section of the throttle member 26 threadedly engaged in the charging bore 48 affects the supply speed of the fuel to the supply valve 27 and thus to the pump work chamber 52. Stated differently, the said flow passage section determines the charging period t.sub.F of the pump-and-nozzle assembly 10. The charging period t.sub.F is, by virtue of the throttle member 26, substantially lengthened (for example, 18 fold) with respect to the injection period t.sub.E and, accordingly, a correspondingly greater accuracy in the metering of the injected fuel quantity Q is achieved. By virtue of the lengthened charging period t.sub.F, even very small injection quantities (smaller than 3 mm.sup.3 per stroke) may be accurately controlled.

The solenoid valve 17 which is disposed in the bore 15 of the housing 14 immediately adjacent the valve plunger 18 and which in FIG. 1 is shown in section and in a simplified manner, is a known, pressure-equalized 3/2-way valve actuated by an electromagnet 61 and having a valve housing 62 and, as its movable valve member, a sphere 63. The latter engages, in its shown closed position, a valve seat 64 and thus blocks fuel admission from the control conduit portion 47 to a control conduit 65 leading to the control pressure chamber 21. Simultaneously, the control conduit 65 and thus the control pressure chamber 21 are, by means of an open second valve seat 66 in communication with the return or discharge conduit 59 through a bore 67 and a return bore 57 in the housing 14.

The electromagnet 61 has an armature 69 which is guided in the valve housing 62 and which, urged by the force of a spring 71, presses the sphere 63 against the valve seat 64 when the electromagnet 61 is in a deenergized condition. To permit the use of a spring 71 of moderate force, the solenoid valve 17 is pressure-equalized by providing a channel 72 through which the servo pressure p.sub.S prevailing in the control conduit portion 47 is communicated to a chamber 73 which is located behind the armature 69 and which accommodates the spring 71. The faces on the sphere 63 and the armature 69 which are exposed to the pressure of the fuel are at least approximately of the same magnitude so that the forces exerted on the sphere 63 in the opening and closing directions are also at least approximately identical. For this reason the spring 71 merely has to be strong enough to maintain the sphere 63 at the valve seat 64.

The second, non-illustrated, open position of the solenoid valve 17 is achieved when the electromagnet 61 is energized, for example, by means of an electronic control apparatus (shown only symbolically in FIG. 3). In this manner the force of the spring 71 is overcome by the electromagnet forces and the armature 69 is displaced. The fuel then flows through the first valve seat 64 and presses the sphere 63 against the second valve seat 66 so that the fuel may then flow from the control conduit portion 47 through the first valve seat 64 into the control chamber 21 where it drives the valve plunger 18 against the force of the spring 19 to the right and brings an annular groove 74 provided on the valve plunger 18 into such a position that the annular chamber 46 which is under the servo pressure p.sub.S will be connected with the control bore 22. As a result, the fuel flows through the control bore 22 into the servo pressure chamber 29. The servo piston 24 and the pump piston 25 are, for example, in case of the largest possible fuel injection quantity, moved by the servo pressure from their position shown in FIG. 2 (upper dead center) into their lower dead center position illustrated in FIG. 1. During this displacement of the pistons 24, 25, fuel is delivered from the pump work chamber 52 through the channel 51 (FIG. 2) and the conduits 53, 53a and 53b to the annular chamber 54 and to the nozzle opening 55 of the fuel injection nozzle 13 and thus fuel injection takes place.

The sections of the valve housing 62 which are exposed to different large pressures are isolated from one another in the stepped mounting bore 15 by means of sealing rings 76, 76a and 76b.

The use of an abovedescribed solenoid valve is particularly advantageous, since the small mass of the moving components permits an almost delay-free switching operation which is a desideratum for a rapid and accurate operation of the solenoid valve.

The fuel which leaks through the nozzle needle 35 of the fuel injection nozzle 13 and which accumulates in the spring chamber 36 of the spring housing 32 may, as indicated in FIG. 2, flow through a conduit 77 and an adjoining conduit 78 to the return conduit 59, since the conduit 78 merges into that portion of the bore 15 which is upwardly and downwardly isolated by sealing rings 76 and 76a and from which, as shown in FIG. 1, there extends the bore 67 to the return bore 57.

The fuel leaking through the pistons 24 and 25 is collected in a chamber bounded by an annular groove 79 (FIG. 2). Said chamber is located in the zone of contact between the servo piston 24 and the pump piston 25 and is in communication with the conduit 78. A check valve 81 provided in the conduit 78 prevents the fuel from being drawn back from the return conduit 59 and the conduit 78 during the suction stroke of the servo piston 24 since in this manner the downward motion of the servo piston 24 during the injection stroke would be hindered.

In order to affect the course of the stroke speed of the pump piston 25 (FIG. 1) during the delivery stroke and thus affect the course of the fuel injection, the servo piston 24 driving the pump piston 25 has a conical extension 82 protruding from its radial face 28. The extension 82 projects into the control bore 22 and defines there a flow passage section 83 which is variable in cross section and which is dependent upon the stroke position H of the servo piston 24. In this manner the stroke speed of the pump piston 25 and thus the course of the fuel injection may be varied. It is also feasible to provide the pump piston 25 with a cylindrical extension, in which case the control bore has a correspondingly matching configuration.

In the lower dead center position of the pump piston 25 illustrated in FIG. 1, for the depressurization of the fuel injection nozzle 13, the pump work chamber 52 is connected through a channel 85 in the pump piston 25 with the annular chamber 86 which, in turn, is coupled through a connecting bore 87 (FIG. 2) to the conduit 77. In this manner a depressurization down to the pressure prevailing in the return chamber 59 (practically 0 kg/cm.sup.2) may be achieved. If it is desired to maintain a higher residual pressure in the pump work chamber 52, the annular chamber 86, instead of being connected with the conduit 77, may be connected with the charging bore 48, so that in the pump work chamber 52 and in the fuel injection nozzle 13 there will remain a residual pressure which will equal the servo pressure p.sub.S of the pressure source.

In the upper dead center position shown in FIG. 2, the servo piston 24, at the end of the charging stroke and at the beginning of the delivery stroke, engages with its radial face 28 an abutment 88 which is formed by the radial face of a cylindrical extension 89 of the sleeve 31 and which bounds from above the servo pressure chamber 29. The length of the cylindrical extension 89 determines the maximum stroke H.sub.max of the servo piston 24 and pump piston 25 and thus determines the maximum injection quantity Q.sub.max. By proper setting of the stroke H.sub.max one may limit the greatest injection quantity Q.sub.max to the maximum permissible full load injection quantity, so that even in case of a defective control of the fuel injection apparatus, a fuel quantity larger than the full load quantity can never be injected. This feature ensures that the maximum permissible injected fuel quantity cannot be exceeded. This is particularly advantageous in diesel engines, in case the maximum permissible injected fuel quantity is identical to the full load fuel quantity. In this manner an emission, during excess fuel injection, of uncombusted pollutants prohibited with ever increasing severity by the clean air laws, is effectively prevented.

In FIG. 3 the pump-and-nozzle assembly 10 is shown with the associated and known components of the fuel injection apparatus in a simplified manner. The pistons 24, 25 are in their upper dead center position as shown in FIG. 2, whereas the solenoid valve 17 which is shown only symbolically, is in its open position in which it connects the supply conduit 43 through the control conduit 65 (shown here in broken lines) with the control pressure chamber 21. Consequently, the valve plunger 18 is moved against the force of the spring 19 towards the right and its annular groove 74 connects the supply conduit 43, 41 through the annular chamber 46 and the control bore 22 with the servo pressure chamber 29. In this switching position the pressure or injection stroke is initiated which takes place until the pump piston 25 engages its lower abutment 90 (FIG. 1). The supply conduit 43 is coupled to a pressure source 91.

The pressure source 91 may be formed, as indicated in this example, of a gear pump 93 which is driven by the internal combustion engine 92 and the output pressure of which may be maintained by means of a pressure regulating valve 94 at the desired servo pressure, for example, p.sub.S = 50 kg/cm.sup.2.

The servo pressure p.sub.S, by virtue of appropriate design of the pressure regulating valve 94, may be regulated in an rpm and/or load-dependent manner. The load-dependency may be achieved, for example, in a known and not illustrated manner by changing the spring bias as a function of the position of the accelerator pedal or by turning the movable valve member which forms part of the regulator valve 94 and which is provided with an oblique overflow control edge.

In order to compensate for pressure fluctuations, the pressure source 91 is provided with a pressure accumulator 95. The gear pump 93 draws fuel through a suction conduit 96 and a filter 97 from a fuel tank 98 into which the fuel may flow back from the pump-and-nozzle assembly 10 through the return conduit 59.

In case the fuel injection apparatus is used with a multicylinder internal combustion engine, a plurality of pump-and-nozzle assemblies are used which are coupled to the supply conduit 43 by means of branches 43a, 43b and 43c and to the return or discharge conduit 59 by means of branches 59a, 59b and 59c.

The control of the solenoid valve 17 is effected by means of a known and only symbolically shown control apparatus 99 which transmits the switching signals of variable length to the electromagnet 61 of the solenoid valve 17.

In FIG. 4 in the lower portion of the diagram there is shown the course of the stroke H of the pump piston 25 and thus, there is illustrated the fuel injection quantity Q as a function of the charging, injection and control periods t.sub.F, t.sub.E, and t.sub.S, respectively. The greatest possible fuel injection quantity Q.sub.max (the highest point on the curve A) is achieved with a stroke H.sub.max. At an engine rpm of, for example, 4500, the charging period t.sub.F for obtaining Q.sub.max is 25.2 milliseconds (ms). The associated injection period t.sub.E then amounts to 1.5 ms. In case of an operation according to curve A, these two periods add up to a cam angle of 360.degree., thus, to one revolution of the cam shaft. In case of a four-cycle engine, one cam shaft rotation corresponds to two crankshaft rotations, that is, to a crank angle of 720.degree.. Then, the injection and charging periods (t.sub.E + t.sub.F) add up to 26.7 ms which corresponds to the cycle period T of one work cycle of the engine in case of an engine rpm of 4500, since T = 2.60/4500 = 2.360/6.4500 = 26.7 .times. 10.sup..sup.-3 sec = 26.7 ms. The equality T = t.sub.F + t.sub.E holds true only if moment t.sub.2 is simultaneously the terminal moment of the injection period and the beginning moment of the charging period.

The smaller injection quantity Q.sub.1 (partial load injection quantity) is achieved with a stroke H.sub.1 and with an injection course according to the curve B shown in broken line. The associated charging period is t.sub.F1 and the corresponding injection period is t.sub.E1. To the injection quantity Q.sub.1 there belongs a pump piston stroke H.sub.1. In FIG. 4 it is assumed that for Q.sub.1 too the engine rpm is 4500, since a smaller engine rpm would result in a correspondingly larger cycle period T (not shown). Between the terminal moment of t.sub.E1 and the beginning of t.sub.F1 there is a dwelling period t.sub.R1, during which the pump piston 25 is in contact with its lower abutment, that is, it is in its lower dead center position. In this case the cycle period T is composed of periods t.sub.E1 + t.sub.R1 + t.sub.F1. The switching periods of the electromagnet 61 of the solenoid valve 17 are illustrated by the solid line curve C for the largest possible fuel injection quantity Q.sub.max and by the broken line curve D for the partial load fuel quantity Q.sub.1. At C.sub.1 and D.sub.1 the solenoid valve 17 is in its closed position, whereas at C.sub.2 and D.sub.2 it is in its open position. The beginning and the end of the energizing periods t.sub.S and t.sub.S1 determine the beginning moment t.sub.1 of the injection period and the beginning moment t.sub.2 and, respectively, t.sub.4 of the charging period. The period between two energizing periods t.sub.S or t.sub.S1 in which the electromagnet 61 is in a de-energized condition and thus, the solenoid valve 17 is in its closed position C.sub.1 or D.sub.1, is designated as de-energized period and identified at t.sub.A or t.sub.A1. The moment of the termination of the injection period is indicated at t.sub.2 and t.sub.3 ; it is affected basically only by the prepared fuel injection quantity Q.sub.max or Q.sub.1, because the other influencing magnitudes, such as the servo pressure p.sub.S and the characteristic magnitudes of the injection nozzle 13 remain constant. If desired, the fuel servo pressure p.sub.S may be varied, for example, in an rpm-dependent manner, for altering the injection periods within limits.

OPERATION OF THE PREFERRED EMBODIMENT

In the description that follows, there will be set forth the mode of operation of the pump-and-nozzle assembly 10 during one work cycle T of the engine with reference to FIGS. 1-4.

Prior to the start of the injection of the full load quantity Q.sub.max (FIGS. 2, 3 and curves A, C in FIG. 4) the servo piston 24 is, upon completion of a preceding charging stroke, in its upper dead center position at H.sub.max where its radial face 28 engages the upper abutment 88. At moment t.sub.1 the solenoid valve 17 switches from the closed position C.sub.1 into the open position C.sub.2. The sphere 63 (FIG. 2) rapidly moves from the first valve seat 64 to the second valve seat 66 and the fuel, which is pressurized to the servo pressure p.sub.S by the pressure source 91, is admitted to the control pressure chamber 21 which causes the valve plunger 18 to shift so that pressurized fuel is admitted to the servo pressure chamber 29 (FIG. 3). At this time the fuel exerts a pressure on the radial face 28 of the servo piston 24 and drives the same, together with the pump piston 25, downwardly until the latter, at moment t.sub.2, arrives in its lower dead center position as shown in FIG. 1. During the course of the aforenoted downward motion of the pump piston, it travels its maximum stroke H.sub.max (FIG. 2) and delivers the fuel prevailing in the pump work chamber 52 through the port 51 and conduits 53, 53a and 53b to the nozzle opening 55 of the fuel injection nozzle 13. During this step, in the pump work chamber 52 there is generated an injection pressure p.sub.E of, for example, 300 kg/cm.sup.2, which correspondingly to the transformation ratio between the servo piston 24 and the pump piston 25, is greater than the servo pressure p.sub.S of, for example, 50 kg/cm.sup.2. The closing spring 38 of the fuel injection nozzle 13 is, in this example, biased at 150 kg/cm.sup.2 of nozzle opening pressure. Thus, the pressure p.sub.E of the fuel in the nozzle chamber 54 exerts a pressure on the nozzle needle 35 overcoming the opening pressure of the spring 38 and lifting the nozzle needle off its seat. In this manner a fuel injection quantity Q.sub.max is injected in a known manner by the pump piston 25.

At the terminal moment t.sub.2 of the injection period, the solenoid valve 17, after an energizing period of t.sub.S, switches back from its open position C.sub.2 into its closed position C.sub.1. The sphere 63 now blocks the fuel supply to the control pressure chamber 21 and depressurizes the same through the control conduit 65, the presently open second valve seat 66, the bore 68 and the return conduit 59 which leads to the fuel tank 98. The spring 19 moves the valve plunger 18 into its initial position shown in FIG. 1, whereby the annular groove 74 establishes communication between the servo pressure chamber 29 and the return conduit 59. As a result, the pressure in the servo pressure chamber 29 and the pump work chamber 52 drops suddenly, the supply valve 27 opens, and the fuel at servo pressure p.sub.S moves the pump piston 25 and the servo piston 24 away from its lower dead center position during a charging period t.sub.F, braked by the throttle member 26. This charging step occurs between moments t.sub.2 and t.sub.1 in the charging period t.sub.F until, at moment t.sub.1, the solenoid valve 17 again switches into its already described switching position C.sub.1, so that the successive work cycle T may begin. In the above-described case, the charging period t.sub.F is identical to the de-energized period t.sub.A of the solenoid valve 17.

The flow passage section of the throttle member 26 threadedly engaged in the charging bore 48 affects the fuel supply speed of the fuel to the pump work chamber 52 and thus determines the charging period t.sub.F of the pump-and-nozzle assembly 10. By virtue of the throttle member 26 the charging period t.sub.F is substantially (for example, 18 fold) lengthened with respect to the injection period t.sub.E and thus there is achieved a correspondingly greater accuracy in the metering of the fuel quantity Q. By virtue of the lengthened charging period t.sub.F even very small (smaller than 3 mm.sup.3 /stroke) injection quantities may be very accurately controlled.

An automatic safety control is achieved if the throttle bore of the throttle member 26 is so designed that the charging period t.sub.F at the maximum permissible rpm (n.sub.max) is extended to the entire period between the terminal moment t.sub.2 of the injection period of one work cycle T and the beginning moment t.sub.1 of the injection period of the successive work cycle T, as it is the case in an operation according to curve A. When the maximum rpm n.sub.max is exceeded, an automatic reduction of the injection quantity is effected because in case of an increasing rpm, the charging period which is rpm-independent, is no longer sufficient for a complete charging of the pump work chamber 52.

In case of delivery of the partial load injection quantity Q.sub.1 according to the curves B and D in FIG. 4, during a charging period t.sub.F1 between moments t.sub.4 and t.sub.1, there is readied only an injection quantity Q.sub.1 for a charging stroke H.sub.1. At moment t.sub.1 when the solenoid valve is switched from D.sub.1 to D.sub.2, the injection stroke begins and terminates at moment t.sub.3. Until the end of the energizing period t.sub.S1 at t.sub.4, the pump piston 25 remains in its lower dead center position during a dwelling period t.sub.R1. As the solenoid valve 17 is switched from its open position D.sub.2 into its closed position D.sub.1 at moment t.sub.4, the charging stroke starts which lasts for the charging period t.sub.F1 until the moment t.sub.1. At t.sub.1 the next injection begins and the abovedescribed operation is repeated.

Claims

What is claimed is:

1. A fuel injection pump-and-nozzle assembly forming part of a fuel injection apparatus serving an internal combustion engine, said assembly being of the type that has (a) a pump piston executing alternating delivery strokes and return or charging strokes, (b) a pump work chamber bounded by said pump piston, (c) a servo piston connected to said pump piston to drive the latter, said servo piston having a diameter greater than that of said pump piston, (d) a pressure source externally of said assembly for delivering fuel under pressure to said assembly, (e) first supply conduit means extending from said pressure source to said pump work chamber, (f) a supply valve disposed in said first supply conduit means, (g) a servo pressure chamber bounded by said servo piston, (h) second supply conduit means extending from said pressure source to said servo pressure chamber, (i) discharge conduit means extending from said servo pressure chamber, (j) a housing including a bore, and (k) a valve plunger disposed within said bore for controlling said second supply conduit means and said discharge conduit means, said valve plunger being adapted to assume a first switching position for admitting pressurized fuel from said pressure source to said servo pressure chamber for effecting said delivery strokes and a second switching position for establishing communication between said servo pressure chamber and said discharge conduit means for effecting said return strokes, the improvement comprising,

A. a control pressure chamber formed within said bore and bounded by said valve plunger,

B. third supply conduit means extending from said pressure source to said control pressure chamber,

C. a solenoid valve disposed in and forming part of said third supply conduit means at a location immediately adjacent said valve plunger for intermittent energization in phase with the operation of the engine, so that pressurized fuel is admitted through said third supply conduit to said control chamber to move said valve plunger from one of its switching positions to the other, and

D. a throttle member for affecting the speed of said return strokes, said throttle member being disposed in said first supply conduit means between said supply valve and said pressure source, said throttle member including means defining a permanently set flow passage section so dimensioned as to substantially lengthen said charging period with respect to the injection period during which fuel is injected into the engine from said pump work chamber.

2. An improvement as defined in claim 1, wherein the maximum permissible fuel injection quantity is determined by the maximum stroke of said pump piston.

3. An improvement as defined in claim 1, wherein said flow passage section of said throttle member is so dimensioned that for the maximum permissible engine rpm each charging period during which fuel is supplied to said work chamber from said pressure source extends to the entire period between the terminal moment of the injection period of one work cycle and the starting moment of the injection period of the successive work cycle.

4. An improvement as defined in claim 1, wherein said solenoid valve is an electromagnetically operated, pressure-equalized 3/2-way valve having a sphere as its movable valve member.

5. An improvement as defined in claim 1, including means for varying the pressure of fuel prevailing in said second conduit means as a function of at least one engine parameter.

6. An improvement as defined in claim 6, wherein said engine parameter is the engine rpm.

7. An improvement as defined in claim 5, wherein said engine parameter is the engine load.

8. An improvement as defined in claim 1, wherein said first, second and third supply conduit means intersect said bore.

9. An improvement as defined in claim 1, wherein said bore is disposed substantially transverse to the direction of injection.

10. A fuel injection pump-and-nozzle assembly forming part of a fuel injection apparatus serving an internal combustion engine, said assembly being of the type that has (a) a pump piston executing alternating delivery strokes and return or charging strokes, (b) a pump work chamber bounded by said pump piston, (c) a servo piston connected to said pump piston to drive the latter, said servo piston having a diameter greater than that of said pump piston, (d) a pressure source externally of said assembly for delivering fuel under pressure to said assembly, (e) first supply conduit means extending from said pressure source to said pump work chamber, (f) a supply valve disposed in said first supply conduit means, (g) a servo pressure chamber bounded by said servo piston, (h) second supply conduit means extending from said pressure source to said servo pressure chamber, said second supply conduit means having a bore forming a part thereof which opens into said servo pressure chamber, said servo piston having an extension rigidly affixed to said servo piston and projecting axially outwardly from that face of said servo piston that bounds said servo pressure chamber, said extension projecting into said bore and defining therewith a variable flow passage section, the magnitude of which is a function of the momentary axial position of said servo piston, (i) discharge conduit means extending from said servo pressure chamber, (j) a housing including a bore, and (k) a valve plunger disposed within said bore for controlling said second supply conduit means and said discharge conduit means, said valve plunger being adapted to assume a first switching position for admitting pressurized fuel from said pressure source to said servo pressure chamber for effecting said delivery strokes and a second switching position for establishing communication between said servo pressure chamber and said discharge conduit means for effecting said return strokes, the improvement comprising:

A. a control pressure chamber formed within said bore and bounded by said valve plunger;

B. third supply conduit means extending from said pressure source to said control pressure chamber;

C. a solenoid valve disposed in and forming part of said third supply conduit means at a location immediately adjacent said valve plunger for intermittent energization in phase with the operation of the engine, so that pressurized fuel is admitted through said third supply conduit to said control chamber to move said valve plunger from one of its switching positions to the other; and

D. a throttle member for affecting the speed of said return strokes, said throttle member being disposed in said first supply conduit means between said supply valve and said pressure source, said throttle member including means defining a permanently set flow passage section so dimensioned as to substantially lengthen said charging period with respect to the injection period during which fuel is injected into the engine from said pump work chamber.