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 and which includes a reciprocating pump piston performing its injection strokes by virtue of hydraulic pressure intermittently applied thereto in a servo pressure chamber. Between two injection strokes the pump work chamber is charged with pressurized fuel while the pump piston executes its return or charging stroke. During this period the fuel in the servo pressure chamber is discharged through a throttle which brakes the motion of the pump piston during its charging stroke to effect a charging period which is substantially longer than the injection period. By varying the switching periods of a solenoid valve which controls the admission of pressure into the servo pressure chamber, the duration of the charging strokes and thus the injected fuel quantities may be altered.

Description

BACKGROUND OF THE INVENTION

This invention relates to a fuel injection apparatus for internal combustion engines and is of the type which has at least one pump-and-piston nozzle assembly including a hydraulically driven pump piston having two effective radial faces of identical area. One radial face bounds a pump work chamber, whereas the other bounds a servo pressure chamber in which there is disposed a compression spring exerting pressure on the pump piston. For performing the pressure or delivery stroke, the pump piston is additionally exposed to a fuel servo pressure generated by a pressure source. The fuel injection apparatus is further provided with a supply conduit which extends from the pressure source to the pump work chamber and which contains a check valve. There is further provided a control valve by means of which the fuel supply and fuel discharge (fuel return to the fuel tank) may be controlled to and from the servo pressure chamber. The fuel injection apparatus also has a throttle member in the discharge conduit downstream of the control valve and further includes a spring biased fuel injection valve.

In a known fuel injection apparatus of the aforenoted type (such as disclosed, for example, in German Pat. No. 535,494), the control valve is mechanically driven by a cam shaft and the throttle member disposed in the discharge conduit may be controlled for varying the fuel quantities readied for injection. This regulation effects an alteration of the counterpressure in the servo pressure chamber, so that the fuel quantity prepared for injection during the charging period -- which, because of the cam shaft drive, is necessarily always of identical duration -- is variable as a function of the throttling effect of the throttle in the discharge conduit. Since the rotation of the cam shaft is proportionate to the engine rpm, the charging period controlled by the cam shaft is also rpm-dependent, whereas the aforenoted throttling effect is not. Consequently, in case of a rapidly changing engine rpm which often occurs in vehicle engines, an accurate adaptation to the rpm is required which is superposed to the arbitrary control of the throttle member. In view of the fact that to this rpm control there is added the incalculable hydraulic effect of the long fuel lines necessitated by the system itself, an accurate regulation to a predetermined fuel injection quantity is very difficult to achieve within a wide rpm range.

There is further known a fuel injection apparatus (as disclosed, for example in U.S. Pat. No. 2,598,528), which is free from the disadvantages of long fuel lines and in which the control valve is formed of a solenoid valve which is situated immediately adjacent the servo pressure chamber. This fuel injection apparatus, however, is complex and expensive, it operates with an additional servo piston and further, the solenoid valve controls only the beginning moment of the injection. In this apparatus the injected fuel quantity is determined by control edges at the servo piston or the pump piston.

OBJECT, SUMMARY AND ADVANTAGES OF THE INVENTION

It is an object of the invention to provide an improved fuel injection apparatus which is of simple and compact structure, which is economical to manufacture and which ensures the injection of accurate fuel quantities independently from the rpm.

Briefly stated, according to the invention, the throttle member inserted in the discharge conduit is provided with a permanently set throttle bore and further, the control valve is a solenoid valve which, in a manner known by itself, is arranged immediately adjacent the servo pressure chamber. In the closed position of the solenoid valve, during the charging stroke, the servo pressure chamber is in communication through the throttle member with the discharge conduit and in the open position of the solenoid valve the servo pressure chamber is in communication with the pressure source. The duration of the charging strokes and thus the fuel injection quantities readied in the pump work chamber may be altered by means of varying the switching periods of the solenoid valve.

A fuel injection apparatus as outlined above ensures a rapid and safe operation with an extremely simple structure without an additional servo piston. The fuel injection apparatus according to the invention, has the further advantage that the charging period which is rpm-independent and which is substantially longer than the period of injection, makes possible a correspondingly greater accuracy in the metering of the injection quantities. By means of the lengthened charging period even very small injection quantities can be very accurately controlled.

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

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view of a pump-and-nozzle assembly constituting the preferred embodiment shown in cooperation with symbolically illustrated further components of the fuel injection apparatus;

FIG. 2 is a longitudinal sectional view of one component of the pump-and-nozzle assembly shown in FIG. 1 and

FIG. 3 is a diagram illustrating the injection quantities Q.sub.E as a function of the injection, charging and control periods.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIG. 1, the fuel injection apparatus shown therein has at least one pump-and-nozzle assembly 10, in the housing 11 of which there is guided, in a cylinder bore 12, a pump piston 13, having opposite radial faces 14 and 15 of identical area. One radial face 14 bounds a pump work chamber 16 while the other radial face 15 bounds a servo pressure chamber 17. The latter has a greater inner diameter than the cylinder bore 12 and is closed by a cap screw 18 which, through an attachment 19 supports one end of a spring 21 compressed between the attachment 19 and a spring seat disc 22.

The spring seat disc 22 is supported on a spherical attachment 23 which projects from the frontal face 15 of the pump piston 13 and thus transmits the force of the compression spring 21 to the pump piston 13 which, in addition to the aforenoted spring force, is also affected by a hydraulic pressure prevailing in the servo pressure chamber 17 and controlled by a solenoid valve 24.

The solenoid valve 24, the internal structure of which is illustrated in FIG. 2, is disposed in a mounting bore 25 in the head of the housing 11 immediately adjacent the servo pressure chamber 17. The solenoid valve 24 is formed as a 3/2-way valve and closes, in the position shown, the flow of fuel from a supply channel 26 which extends from a fuel supply bore 27 in a coupling nipple 28 to the servo pressure chamber 17 and, at the same time, connects a control conduit 29 coupled to the servo pressure chamber 17 with a discharge bore 31 into which there is inserted a throttle member 32 having a throttle bore 32a.

The coupling nipple 28 is threadedly engaged in a stepped transversal bore 33 of the housing 11 and fixedly positions in the transversal bore 33 a check valve 34 which permits fuel flow only to the pump work chamber 16. The check valve 34 may, as shown, also contain a throttle 40, by means of which the supply of the fuel to the pump work chamber 16 may be additionally affected. The latter and the check valve 34 are interconnected by means of a bore 35 which extends from a pressure conduit section 36 disposed parallel to a spring chamber 37 in the lower part of the housing 11. The pressure conduit section 36 terminates in an annular channel 38 provided in a radial terminal face 39 of the housing 11.

The spring chamber 37 accommodates a closing spring 41 for a fuel injection nozzle 42. The latter, together with an intermediate block 43, is tightened by means of a sleeve nut 44 in a known manner against the terminal face 39 of the housing 11 and is thus securely clamped thereto. Thp pressure conduit portion 36 continues as a pressure conduit portion 36a in the intermediate block 43 and then as a pressure conduit portion 36b in the fuel injection nozzle 42. All three portions together form the pressure conduit 36, 36a, 36b through which the fuel quantity readied in the pump work chamber 16 and driven by the pump piston 13 may flow to the nozzle opening 45.

To the coupling nipple 28 there is connected a supply conduit 51 which delivers fuel to the pump-and-nozzle assembly 10 from the pressure source 52 at the supply pressure p.sub.Z. The pressure source 52, as well as the accessory components, are in general known and therefore are shown only symbolically.

The pressure source 52 includes a gear pump 54 which is driven by the engine 53 and the output pressure of which is maintained by means of a pressure limiting valve 55 at the desired supply pressure p.sub.Z which may be, for example, 100 kg/cm.sup.2. In order to compensate for the pressure fluctuations, the pressure source 52 is provided with a pressure accumulator 56. The gear pump 54 draws fuel through a suction conduit 57 and a filter 58 from a fuel tank 59 into which the fuel released by the pressure limiting valve 55 may return. Further, in the shown position of the solenoid valve 24, the fuel displaced by the pump piston 13 from the servo pressure chamber 17 may flow through the return bore 31 and the throttle 32 through a discharge conduit 61 back into the fuel tank 59. The leakage fuel accumulating in the spring chamber 37 may also flow back to the fuel tank 59 through the return conduit 61, since the spring chamber 37 is connected by means of a conduit 62 (shown in broken lines) with that portion of the bore 25 with which the return bore 31 communicates.

Additional pump-and-nozzle assemblies (not illustrated) are connected with the supply conduit 51 through supply conduit portions 51a, 51b and 51c and with the return conduit 61 through return conduit portions 61a, 61b and 61c.

The pump piston 13 is shown in its upper dead center position in which its spring seat disc 22 is in engagement with an abutment 63 forming part of the attachment 19. The length of the abutment 63 determines the maximum stroke H.sub.max of the pump piston 13 and thus fixes the magnitude of the maximum injection quantity Q.sub.max. By a proper setting of the stroke H.sub.max, the maximum injection quantity Q.sub.max may be limited to the admissible full load injection quantities. Thus, even in case of a defective control of the fuel injection apparatus, the injected fuel quantity cannot exceed the full load quantity. This feature is of interest particularly in diesel engines and is very advantageous, since an emission of uncombusted exhaust gases resulting from excess fuel and prohibited in an ever increasing extent by the clean air laws is effectively prevented.

The solenoid control valve 24 shown in a simplified manner in FIG. 2, is a known, pressure-equalized, 3/2-way valve actuated by an electromagnet 64 and has a valve housing 65 and a sphere 66 as the movable valve member. The sphere 66 is adapted to assume a closed position (shown in FIG. 2) and an open position.

In the closed position the sphere 66 engages a valve seat 67 and thus blocks the supply of fuel from the pressure source 52 to the servo pressure chamber 17. At the same time, the servo pressure chamber 17 is connected through a second, open valve seat 68, the return bore 31, and throttle member 32 with the return conduit 61. The electromagnet 64 has an armature 69 which is guided in the valve housing 65 and which, urged by a spring 71, presses the sphere 66 against the valve seat 67 when the electromagnet 64 is in a de-energized condition. To permit the use of a spring 71 of moderate strength, the solenoid valve 24 is pressure-equalized by providing a channel 72 which communicates the supply pressure p.sub.Z prevailing in the supply channel 26 to a chamber 73 which accommodates the spring 71 and which is located behind the armature 69. The effective surface of the sphere 66 and of the armature 69 exposed to the pressure of fuel are at least substantially identical, so that the hydraulic forces exerted on the sphere 66 in the opening and closing directions are also at least substantially equal to one another. For this reason the force of the spring 71 is needed only to maintain the sphere 66 against the seat 67.

The solenoid valve 24 is in its second, open position (not illustrated) as a result of the energization of the electromagnet 64, for example, by means of an electronic control apparatus 75 shown only symbolically. Upon energization of the electromagnet 64, the force of the spring 71 is overcome by the electromagnetic forces and thus the armature 69 is displaced. The inflowing fuel then presses the sphere 66 against the second valve seat 68, whereupon the fuel may flow from the supply channel 26 through the first valve seat 67 into the servo pressure chamber 17 where, together with the force of compression spring 21, it drives the pump piston 13 downwardly and thus effects the fuel injection. The sections of the valve housing 65 which are under different high pressures are sealed from one another in the stepped bore 25 by packing rings 76, 76a and 76b. The use of the above-described solenoid valve 24 is particularly advantageous since the mass of its moving components is small and thus its response to switching commands is almost instantaneous.

A fully satisfactory operation of the fuel injection apparatus may be achieved if the pressure of the compression spring 21 exerted on the pump piston 13 is smaller than the fuel supply pressure p.sub.Z exerting a force in the pump work chamber 16 on the pump piston 13 and the nozzle opening pressure p.sub.O of the fuel injection nozzle 42 is greater than the fuel supply pressure p.sub.Z, but smaller than the sum of the fuel supply pressure p.sub.Z and the spring pressure p.sub.F. Thus,

(p.sub.Z + p.sub.F) > p.sub.O > p.sub.Z and

p.sub.F < p.sub.Z .

As an example, the fuel injection apparatus may operate at the following pressures: p.sub.Z = 100 kg/cm.sup.2, p.sub.O = 115 kg/cm.sup.2, and p.sub.F = 80 kg/cm.sup.2.

In FIG. 3 in the lower portion of the diagram there is shown the course of the stroke H of the pump piston 13 and thus the injected quantity Q as a function of the charging, injection and control periods (t.sub.F , t.sub.E, t.sub.S). The largest possible fuel injection quantity Q.sub.max (highest point on curve A) is achieved with a stroke H.sub.max and for a charging period t.sub.F. The associated injection period is t.sub.E. In case of curve A, the two periods add up to a 360.degree. cam angle which corresponds to one revolution of the cam shaft. In case of a four-cycle engine, to one cam shaft rotation there correspond two crank shaft rotations, that is, a crank shaft angle of 720.degree.. The injection and charging periods (t.sub.E + t.sub.F) total, for example, in case of an engine rpm n = 4,500, according to curve A, 26.7 milliseconds (ms). This duration corresponds to the cycle period T of one work cycle of the engine, since T = 2.60/4,500 = 2.360/6.4500 = 26.7 .times. 10.sup.-.sup.3 sec. = 26.7 ms, of which t.sub.F = 25.2 ms and t.sub.E = 1.5 ms. The equality T = t.sub.F + t.sub.E holds true only when the moment t.sub.2 designates both the end of the injection period and the beginning of the charging period.

The smaller fuel injection quantity Q.sub.1 (partial load injection quantity) is obtained with a stroke H.sub.1 and with a course of injection as indicated with the curve B drawn in broken lines. The associated charging period and injection period are t.sub.F1 and t.sub.E1, respectively.

In FIG. 3 it is assumed that for Q.sub.1 the rpm n is also 4,500, since a smaller rpm would result in a correspondingly greater cycle period T (not shown). Between the time interval defined by the end of t.sub.E1 and the beginning of t.sub.F1, the pump piston 13 dwells in contact with its lower abutment. The said interval is designated as the dwelling period t.sub.R1. Thus, in such a case the cycle period T is composed of t.sub.E1 + t.sub.R1 + t.sub.F1.

The switching periods of the electromagnet 64 of the solenoid valve 24 are illustrated by the curve C drawn in solid line for the largest possible fuel injection quantity Q.sub.max and by the curve D drawn in broken line for the partial load quantity Q.sub.1. In C.sub.1 and D.sub.1, respectively, the valve 24 is in its closed position, while in C.sub.2 and, respectively, 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 duration between two energizing periods t.sub.S or t.sub.S1 in which the solenoid valve 24 is in a de-energized condition (closed position C.sub.1 and D.sub.1) is designated as de-energized periods t.sub.A and, respectively, t.sub.A1. The terminal moment t.sub.2 and, respectively, t.sub.3 of the injection period is affected practically only by the readied fuel injection quantities Q.sub.max and, respectively, Q.sub.1 because the other influencing magnitudes, such as the fuel supply pressure p.sub.Z and the characteristics of the fuel injection nozzle 42 are constant. If desired, the fuel supply pressure p.sub.Z may be varied to alter the length of the injection period within limits, for example, in a rpm-dependent manner.

OPERATION OF THE PREFERRED EMBODIMENT

In the description that follows there will be set forth the operation of the abovedescribed pump-and-nozzle assembly 10 for one work cycle T of the engine 53.

Prior to the beginning of the injection of the full load fuel quantity Q.sub.max (curves A and C in FIG. 3), the pump piston 13 is, upon completion of its charging stroke, at H.sub.max in which its spring seat disc 22 is in engagement with the upper abutment 63.

At moment t.sub.1 the solenoid valve 24 switches from its closed position C.sub.1 into its open position C.sub.2, whereby the sphere 66 (FIG. 2) quickly moves from the first valve seat 67 to the second valve seat 68 and the fuel pressurized to the supply pressure p.sub.Z by the pressure source 52 is admitted to the servo pressure chamber 17. There, together with the force of the compression spring 21, it exerts a force on the radial face 15 of the pump piston 13 and drives the same downwardly until, at moment t.sub.2, it reaches its lower dead center position. During this downward motion, the pump piston 13 travels its maximum stroke H.sub.max and drives the fuel from the pump work chamber 16 through the pressure conduit 36, 36a, 36b to the fuel injection nozzle 42. Since the fuel supply pressure p.sub.Z exerting a force on the upper radial face 15 of the pump piston 13 is, together with the pressure exerted by the compression spring 21, greater than the opening pressure p.sub.O of the fuel injection nozzle 42, and further, since the check valve 34 is closed, the fuel injection nozzle 42 injects in a known manner the injection quantity Q.sub.max -- delivered by the pump piston 13 during its pressure stroke -- into the cylinder of the engine.

At moment t.sub.2 which is the end of the injection, the solenoid valve 24, after an energizing period t.sub.S, switches back from its open position C.sub.2 into its closed position C.sub.1. The sphere 66 now blocks the fuel flow to the servo pressure chamber 17 and, at the same time, depressurizes the latter through the open second valve seat 68 through the return bore 31, the permanently adjusted throttle bore 32a of the throttle member 32 and the return conduit 61. Simultaneously, the pressure in the servo pressure chamber 17 and the pump work chamber 16 very suddenly drops, the check valve 34 opens and the fuel which is at the fuel supply pressure p.sub.Z, moves the pump piston 13 during a charging period t.sub.F -- delayed by means of the throttle member 32 -- against the force of the compression spring 21 away from its lower dead center position. This charging step occurs during a period t.sub.F between moments t.sub.2 and t.sub.1 until, at moment t.sub.1, the solenoid valve 24 again switches into its switching position C.sub.2 and the successive work cycle T is ready to start. In this case the charging period t.sub.F is identical to the de-energized period t.sub.A of the fuel valve 24.

The flow passage section of the throttle bore 32a of the throttle member 32 threadedly engaged in the return bore 31 affects the discharge flow speed of the fuel from the servo pressure chamber 17. Stated differently, the said flow passage section determines the charging period t.sub.F of the pump-and-nozzle assembly 10. By means of the charging period t.sub.F which is lengthened substantially (for example 18 fold) with respect to the injection period t.sub.E by the throttle member 32, a correspondingly greater accuracy in the metering of the fuel injection quantity Q is possible. By means of the lengthened charging period t.sub.F, even extremely small injection quantities (smaller than 3 mm.sup.3 per stroke) may be controlled very accurately.

An automatically operating safety control is achieved if by appropriate dimensioning of the throttle bore 32a of the throttle member 32, the charging period t.sub.F (FIG. 3) for a 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 period T, as it is the case for the curve A in FIG. 3. If the maximum rpm (n.sub.max) is exceeded, an automatic reduction of the fuel quantities takes effect because as the rpm becomes greater, the charging period, which is rpm-independent, is no longer sufficient for the full charging of the pump work chamber 16.

During delivery of a partial load fuel quantity Q.sub.1 according to the curves B and D, during the charging period t.sub.F1 between t.sub.4 and t.sub.1 there is readied, with a stroke H.sub.1, an injection quantity Q.sub.1 only. At moment t.sub.1, when the solenoid control valve 24 switches from D.sub.1 to D.sub.2, the injection stroke begins and terminates at moment t.sub.3. From this moment until the end t.sub.4 of the energized period t.sub.S1 of the electromagnet 64, the pump piston 13 dwells in its lower dead center position. This interval is the dwelling period t.sub.R1. As the solenoid valve 24 is switched from its open position D.sub.2 into its closed position D.sub.1 at moment t.sub.4, the charging stroke begins which takes place during the charging period t.sub.F1 until the moment t.sub.1. At t.sub.1 the successive injection starts and the operation described hereinbefore is repeated.

Claims

What is claimed is:

1. A fuel injection apparatus serving an internal combustion engine and having at least one pump-and-nozzle assembly of the type that includes (a) a hydraulically operated pump piston having two opposite radial faces of identical area, (b) a pump work chamber bounded by one of said radial faces of said pump piston, (c) a servo pressure chamber bounded by the other of said radial faces of said pump piston, (d) a compression spring disposed in said servo pressure chamber and exerting a force on said pump piston, (e) a pressure source externally of said assembly for pressurizing fuel, (f) first supply conduit means extending from said pressure source to said pump work chamber, (g) a check valve disposed in said first supply conduit means to permit a fuel flow from said pressure source to said pump work chamber, (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 control valve for controlling the flow in said second supply conduit means and in said discharge conduit means for intermittently causing pressurized fuel to be admitted from said pressure source into said servo pressure chamber to effect, together with said compression spring, the delivery strokes of said pump piston, (k) a throttle member disposed in said discharge conduit means downstream of said control valve and (1) a spring-biased fuel injection valve communicating with said pump work chamber, the improvement comprising

A. means defining a permanently set throttle opening in said throttle member,

B. a solenoid valve situated immediately adjacent said servo pressure chamber and constituting said control valve, said solenoid valve adapted to assume a closed position and an open position,

C. means establishing communication between said servo pressure chamber and said discharge conduit means through said throttle member in the closed position of said solenoid valve,

D. means establishing communication between said servo pressure chamber and said pressure source in the open position of said solenoid valve and

E. means controlling the periods of closed and open positions of said solenoid valve for determining the duration of the charging strokes of said pump piston during which fuel is supplied to said pump work chamber from said pressure source, said duration determining the fuel quantity readied for injection in said pump work chamber.

2. An improvement as defined in claim 1, wherein said throttle opening of said throttle member is so dimensioned that the duration of the charging period during which fuel is supplied to said pump work chamber from said pressure source extends, for the maximum permissible engine rpm, 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.

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

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 the movable valve member.

5. An improvement as defined in claim 1, including means defining an additional throttle of permanently set flow passage section, said additional throttle is disposed in said first supply conduit means between said check valve and said pressure source.