EP0020249B1 - Injection pump for internal-combustion engine - Google Patents

Description

The present invention relates to an injection pump for an internal combustion engine of the type described in the preamble to the main claim.

A pump of this type is known from GB-A-571485. This pump allows a quantity of pre-injection fuel to be discharged before the main injection fuel is discharged, but has the disadvantage of not allowing a variation in the start of the pre-injection discharge.

US-A-2 810 375 relates to an injection pump which makes it possible to vary the start of the delivery of fuel from the pump to the injector. This device for varying the start of delivery includes a fuel accumulator chamber, in permanent communication with the intake chamber and of variable volume by a piston mounted movably in said accumulator chamber, against a return spring, under the effect of fuel pressure in the intake chamber.

If we used this device for varying the start of delivery in the pump according to GB-A-571 485, we would obtain a pump in which not only the start of the main injection delivery but also the start of the main injection delivery would be modified by the accumulator.

The object of the present invention is to propose an injection pump which makes it possible to vary the start of the pre-injection delivery without influencing the start of the main injection delivery.

This object is achieved thanks to the characteristics set out in the characterizing part of the main claim, and in particular the fact that the accumulator piston comes into leaktight support on a seat forming a stop limiting the stroke of the piston during the pre-injection and is maintained on this seat under the effect of a pressure equal to that produced, during the interruption of the delivery, by the pump piston continuing its stroke mounted during this interruption.

Concerning US-A-2 810 375, this document contains no teaching that during a period of interruption of delivery, the piston of the accumulator remains applied on its stop and does not reveal either that the piston of the accumulator comes into watertight support on a seat forming a stop for limiting the stroke of the piston. It is however this sealed seat provided by the invention, which prevents leaks between the piston and the wall of the accumulator chamber so that the piston can remain pressed against this seat during the period of interruption.

According to an advantageous characteristic of the invention, a rod is fixed to the rear face of the aforementioned piston and the free end of said rod is brought out of the pump structure to determine the length of the piston stroke.

• Les figs 1 à 4 sont des vues en coupe longitudinales d'un premier mode de réalisation d'une pompe d'injection et illustrent des étapes spécifiques du fonctionnement de la pompe; la fig. 5 est une vue en coupe suivant la ligne V-V de la fig. 2; la fig. 6 illustre les phases de refoulement de combustible hors de la chambre de la pompe d'injection, en fonction de la position du piston du cylindre du moteur, exprimé en position angulaire de l'arbre manivelle; la fig. 7 est une vue en coupe longitudinale d'un deuxième mode de réalisation d'une pompe d'injection; la fig. 8 est une vue en coupe suivant la ligne VIII-VIII de la fig. 7; la fig. 9 montre en une vue en coupe et de façon schématique le dispositif accumulateur selon l'invention, et les figs 10 et 11 illustrent deux modes de réalisation concrètes d'un accumulateur selon l'invention.

The invention will be better understood, and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the appended schematic drawings given solely by way of example illustrating two embodiments of the invention and in which:

• Figs 1 to 4 are longitudinal section views of a first embodiment of an injection pump and illustrate specific steps in the operation of the pump; fig. 5 is a sectional view along the line VV of FIG. 2; fig. 6 illustrates the fuel delivery phases out of the injection pump chamber, as a function of the position of the piston of the engine cylinder, expressed in angular position of the crankshaft; fig. 7 is a view in longitudinal section of a second embodiment of an injection pump; fig. 8 is a sectional view along line VIII-VIII of FIG. 7; fig. 9 shows in a sectional view and schematically the accumulator device according to the invention, and FIGS. 10 and 11 illustrate two concrete embodiments of an accumulator according to the invention.

The injection pump shown in FIGS. 1 to 5 comprises a cylindrical body 1 and a piston 2 mounted movably in this body 1 in a reciprocating rectilinear reciprocating movement, between a position of bottom dead center and a position of top dead center, under the effect of a control cam (not shown) secured to a shaft driven by the motor.

In the body 1, above the piston 2, is defined a fuel intake chamber 3, into which open fuel inlet orifices 4. These orifices are located at a level such that they allow the filling of the chamber 3 by the fuel, when the piston occupies its bottom dead center position.

At the top of the chamber, not shown, it is necessary to imagine a fuel delivery orifice in the direction of arrow F1, towards an injector associated with a cylinder of an internal combustion engine. This discharge orifice can be closed off by a discharge member, in a manner known per se.

The piston 2 has a first circular groove 5 delimited at its lower part by a straight edge 6 and at its upper part by a helical edge 7 forming a helical ramp and ending in a groove 8 which extends parallel to the axis of the piston . This groove establishes permanent communication between the intake chamber 3 and the groove 5.

The piston 2 is provided with a second circular groove 9, limited, in the example shown, at the top by a straight edge 10 and at the bottom by a straight edge 11. This second groove 9 communicates with the first groove 5 through axial passages, formed in the example represented by two flats 12 formed on the flange 13 which is delimited between the two grooves 5 and 9. It should be noted that the edges 6, 7, 10 and 11 each have a sharp angle of 90 °.

As shown in the figures, fuel inlet holes 14 are made in the wall of the body 1 of the pump, at a predetermined distance below the inlet ports 4. The holes 14 and the ports 4 are connected to a relatively low pressure fuel source.

The distance, in the axial direction of the piston, between the inlet orifices 4 and the inlet holes 14, the distance between the upper surface 15 of the piston 2, on the one hand, and the edges of the two grooves 5 and 9 , on the other hand, as well as the axial height of the flange 13 and the width of the second groove 9 are chosen to ensure the specific operating steps of the pump shown in FIGS. 1 to 4.

In the piston position, shown in fig. 1, the intake chamber 3 communicates with the inlet orifices 4, which allows the filling of the chamber 3 with fuel. By moving upwards, that is to say towards its top dead center position , the piston 2 reaches the position illustrated in FIG. 2, in which it begins to close the inlet orifices 4. Since the flange 13 is opposite the inlet holes 14 and closes these, no more communication exists between the chamber 3 and a conduit low pressure fuel supply. This is the start of fuel delivery to the injector in the direction of arrow F1. This start of delivery is designated in FIG. 6 with the letter A.

Continuing its climb stroke, the piston 2 arrives at the position shown in fig. 3, in which the upper edge 10 of the second circular groove 9 comes at the level of the lower edge of the inlet holes 14, which constitutes the start of the connection of these holes 14 and of the groove 9 and, at through the flats 12, of the groove 5 and of the groove 8, with the chamber 3. This results in a sudden drop in pressure inside the chamber 3. The fuel delivery stops, although the piston continues its climb stroke. This state is indicated in fig. 6 by the letter B. This communication and because of it the low pressure inside the chamber 3 persist until the piston reaches the position shown in FIG. 4, in which the lower edge 11 of the groove 9 comes to the level of the upper edge of the inlet holes 14. These are closed by the cylindrical surface of the piston, which follows the groove 9. In this position the inlet orifices 4 are still closed. The chamber 3 thus cut off from the low pressure conduits 4, 14, the fuel pressure inside the chamber increases and the fuel delivery to the injector resumes, as indicated by the letter C in FIG. 6. The discharge ends in D, when the helical ramp 7 comes opposite an inlet orifice 4. The injection is faster between C and D than between A and B, because the speed of the cam piston control is still weak between A and B, since it is located relatively to the start of the piston rise.

The position of the bottom dead center of the piston 2 with respect to the movement of the piston of the engine cylinder is chosen such that the start A of the first phase of injection into the engine cylinder is sufficiently advanced relative to the top dead center of the piston of said engine cylinder so that this first injection can constitute a pre-injection ensuring the ignition, in the combustion chamber of an engine cylinder, of the quantity of main injection combustion, practically without ignition delay. For example, the start of pre-injection could take place at an instant corresponding to an angle of rotation of about -30 ° of the crankshaft, before the top dead center of the driving piston and the main injection could be triggered at an angle d 'around -15 °. Of course, these values are given only by way of example.

It has been found that the effect of the pre-injection varies as a function of the speed of rotation of the engine. At relatively low speeds, the illustration of the first delivery phase, that is to say the pre-injection delivery, in the form of a rectangle according to FIG. 6, corresponds to the phenomena that actually occur.

However, at higher speeds, the delivery and the pre-injection, as a function of time, take on the appearance of a bell, because the establishment of the relatively high pressure inside the chamber 3 during the delivery of fuel out of the chamber and the drop in pressure in the chamber due to the communication between the latter and the inlet holes 14 require a certain period of time, since the inlet orifices 4 and the inlet holes 14 have cross sections of limited value.

Thus, at high rotational speeds, the pressure will increase in the chamber before closing the inlet orifices 4 and the discovery of the inlet holes 14 by the flange 13 will not immediately cause the fall of the pressure in the room. Consequently, a fuel backflow and an injection effect will occur even at high engine speed, when the opening of the holes 14 occurs before the orifices are closed. This delivery of fuel out of the chamber by the “rolling” effect can occur, for example, for a piston provided with a notch in its upper surface 15 and operating at an angular position in which the notch comes opposite the inlet orifices. 4, which delays the closing of these orifices. If the notch is deep enough, the groove 9 can come in front of the holes 14 before closing the inlet orifices. At low speed, the delivery phase A-B (fig. 6) would not occur. However, at full speed, thanks to the rolling effect, there will still be a backflow and a pre-injection.

In the example described and shown, the upper and lower edges 10, 11 of the second groove 9 are straight and circular. However, it is possible to configure at least one edge, for example the upper edge, at least partial helical ramp. This would make it possible to vary the instant of the end of the pre-injection delivery (B in FIG. 6), by rotation of the piston 2 around its longitudinal axis. An appropriate configuration of the lower edge of the groove 9 would make it possible to modify the start of the main injection delivery (C).

The embodiment represented by FIGS. 2 to 5 comprises a circular groove 9 and twice two inlet holes 14, and this in order to reduce the height of the groove 9 and the diameters of said holes 14, because these two dimensions decide the distance between B and C in fig. 6, that is to say the time of the pre-injection with respect to the main injection, on the one hand, and the lifting of the cam corresponding to this period, on the other hand. These measures make it possible to reduce the length of the inactive part, that is to say without the piston stroke being pushed back, to advance the start of the main injection delivery and / or to delay the start of the pre-injection delivery.

Figs 7 and 8 show another embodiment of the injection pump, in which the inlet holes 14 '- two in number in the example shown - are located approximately at the same level as the inlet orifices 4. Two grooves 9 ', the function of which corresponds to that of the circular groove 9 of the first embodiment, are formed in the peripheral surface of the piston, at a predetermined distance from the upper surface 15 of the piston, substantially parallel to the latter.

As is clear from FIGS. 7 and 8, each groove 9 'opens at one end in a vertical groove 8 and thus communicates, permanently, with the interior of the intake chamber 3. According to an alternative embodiment, each groove could be in communication with this chamber by its two ends.

The operation of this embodiment is the same, in principle, as that illustrated in FIGS. 1 to 4. The pre-injection delivery begins when the orifices 4 are closed by the piston. The inlet holes 14 'are closed by the part of the peripheral surface 13' of the piston which is located above the grooves 9 '. This discharge is finished at the moment when the grooves 9 'come opposite the holes 14'. The main injection delivery starts after the holes 14 'are closed by the peripheral surface of the piston located below the grooves 9' and ends when the ramp 7 discovers the orifices 4.

The injection pump according to the present invention also comprises means which allow the variation of the start of the pre-injection delivery. These means are formed by an accumulator device. According to the schematic representation given in FIG. 9, the accumulator comprises an accumulator chamber 16 in which is mounted a piston 17 movable against a return spring 18. The chamber 16 communicates permanently with the inlet chamber 3 of the injection pump by a passage 19 which opens into the chamber 3 at a level above the top dead center of the piston. The rear part 20 of the chamber, which houses the spring 18, is in permanent communication with the space 21 from which the inlet orifices leave. This ensures that a relatively low pressure acts on the rear face of the piston 17. The stroke of the piston 17 is limited by a stop 22 formed for example by a shoulder of the wall of the chamber 16. The piston 17 is movable over a length e. The stop 22 is shaped as a sealed seat to produce a sealing barrier between the chamber 16 and the part 20 of the chamber containing the spring 18, when the piston is pushed against the stop under the effect of the high pressure prevailing in chamber 3 of the pump and chamber 16, during the delivery of fuel to the injector.

This accumulator operates as follows: When after closing the inlet orifices 4, the piston 2 continues its upward stroke, the pressure inside the receiving chamber 3 increases only slowly since the decrease in the volume of the receiving chamber 3 is compensated by an increase in the volume of the accumulating chamber 16 due to the fact that the piston 17 moves back under the effect of the increase in pressure. Consequently, the pressure which is necessary for the opening of the discharge valve 23 can only be established after the piston has come into contact on its sealed seat stop 22. This means that point A, in FIG . 6, moves to point B and the amount of pre-injection fuel delivered decreases. On the other hand, the interruption of the discharge at point B is independent of the accumulator and determined only by the communication of the additional holes 14 and of the groove 9 for interrupting the discharge. Although the pressure inside the chamber 3 falls, it remains however, due to the piston which continues its upward stroke, at a value sufficient to maintain the piston 17 of the accumulator on its seat.

This means that the accumulator has no influence on the start of the main injection delivery, but allows to vary the start of the pre-injection delivery (point A in fig. 6), and thus the quantity of the pre-injection fuel. .

Figs 10 and 11 illustrate two embodiments of the accumulator 15 according to the invention. In both cases, the accumulator is placed next to the device forming a discharge member 23, above the inlet chamber 3 of the injection pump. Thus, the accumulator is perfectly incorporated into the structure of the pump.

The accumulator shown in fig. 11 is made to allow adjustment of the outside of the start of the pre-injection delivery. To this end, a rod 24 is fixed to the rear face of the piston 17. The free end of said rod comes out of the pump. The rod 24 can be controlled from outside the pump, which makes it possible to easily vary the final position of the piston 17. It is also possible to deactivate the pre-injection.

Claims (2)

1. Injection pump for an internal combustion engine adapted to deliver an amount of pre-injection fuel before the delivery of the main injection fuel, of the type comprising a fuel receiving chamber (3) defined by a body (1) which is cylindrical and variable in volume by the displacement within this body (1) of a piston (2) in a rectilinear reciprocating motion, the said chamber (3) being provided with fuel admission orifices (4) connected to a source of fuel of relatively low pressure, and obturable by the said piston (2) during its displacement towards its upper dead center to allow the delivery of fuel from the said chamber (3), the said piston (2) having a head provided with at least one helical edge (7) generally intended to define the end of the delivery by the pump and a groove (9) for interrupting the delivery communicating with the said chamber (3), the interruption of the delivery taking place when the groove (9) is opposite at least one passage (14) through the said body (1) and communicating with the said source of fuel, the said passage (14) having a predetermined flow cross-section, characterized in that, in a manner known per se, the pump comprises a device for varying the beginning of the delivery which includes a fuel accumulating chamber 16 in permanent communication with the admission chamber (3) and the volume of which is varied by a plunger (17) mounted movably in the said accumulating chamber (16), counter to a return spring (18), under the action of the fuel pressure in the admission chamber (3) and in that the plunger (17) is sealingly forced onto a seat (22) constituting a stop for limiting the travel of the plunger (17) during the pre-injection and is retained on the seat (22) under the action of a pressure equal to the one produced, during the interruption of the delivery, by the pump piston (2) continuing its upward travel during the said interruption without influencing the beginning of the main injection delivery.

2. An injection pump according to claim 1, characterized in that a stem (24) is secured to the rear face of the said plunger (17) and in that the free end of the said stem (24) projects outwardly of the pump structure to determine the length of the plunger travel.

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