FR2752268A1 - DEVICE FOR IMPROVING THE FUEL INJECTION DYNAMICS FOR DIESEL ENGINES EQUIPPED WITH PULSED INJECTION PUMPS - Google Patents

Abstract

A device comprising a spring-loaded valve (21) actuated by a delay in the decrease in the pressure of an accumulator (28) supplied via a port (29), relative to the pressure in the nozzle holder inlet line (14, 15). Opening of the valve (21) controls rapid closure of the nozzle (2) by using the energy still stored in the line (14, 15) to urge the needle (5). A return port (30) enables the discharge of unused fuel. Said device is particularly useful in diesel engines fitted with pulsed-flow injection pumps.

Description

 The present invention relates to a device for improving the dynamics of fuel injection by braking the opening and anticipating the closing of the injector, which can be integrated into the injector carriers of diesel engines equipped with pulsed-flow injection pumps.

 By "injection dynamics" is meant here the law of introduction of fuel as a function of time, which is a condition for controlling combustion in the engine.

 Traditional injection devices, constituted by a simple return of the injector needle by one or more springs, present more and more difficulties in responding to the evolution of the specifications of development of Diesel engines. This constraining effect is mainly linked to the fields of the environment and the economy such as pollutant emissions (nitrogen oxides, hydrocarbons, particles, etc.), noise emitted by the engine or fuel consumption.

The operation of traditional injection systems is essentially conditioned by the adjustment of:
- The force of the injector calibration spring, which must be sufficient so that the pressure waves in the high-pressure pipe circulating after the injection do not risk causing uncontrolled reopenings of the injector and that the pressure of the high pressure line does not decrease below the gas pressure in the combustion chamber during the closing path of the injector needle.

 - The speed of the discharge of the high-pressure pipeline regulated by the characteristics of the pipeline (internal diameter, length) as well as the non-return or discharge valves of the injection pump.

The main disadvantages of traditional injection systems are as follows:
- The start of injection occurs when the pressure in the high-pressure line becomes sufficient to cause the opening of the injector, the start of injection is often abrupt. This effect is even more serious if the force of the injector calibration spring is high.

 - The closure of the injector needle can only be carried out if the pressure in the high pressure line is sufficiently low, it follows that the fuel introduced at the end of the injection period is not introduced into good conditions.

 The most conventional solution for obtaining correct operation of traditional injection systems consists in choosing the force of the injector calibration spring sufficient to stabilize the injection system and be satisfied with the poor performance of the end of injection. .

The need to optimize combustion systems adapted to both pollution control, noise reduction and engine efficiency requires a special effort on the injection process. It can be characterized by:
- The introduction of low-flow fuel at the start of the injection process in order to avoid excessive accumulation of fuel in the combustion chamber during the time necessary for the self-ignition of this fuel. The expected result is mainly the reduction of combustion noise and vibration stresses on engine components.

 - An increasing pressure and flow of fuel as combustion continues in the combustion chamber. The expected result is mainly the good adaptation to the energy requirement associated with the work provided to the engine piston at the start of expansion.

 - The clearest possible closure at the end of injection to prevent a significant amount of fuel from being introduced late in poor conditions compared to the need for rapid mixing with air, increasingly rare, contained in the combustion chamber. The expected result is mainly the reduction of soot emissions and greater flexibility on the compromise to be established between fuel consumption and pollutant emissions, including nitrogen oxides.

The practical arrangements adopted, leading to the device according to the invention, are chosen for:
- Let the injector open for low injection pressure levels, possibly modulated according to the load requested from the engine.

 - Dampen the opening of the injector in order to best limit the growth of the fuel flow rate at the start of the injection phase.

 - Close the injector before the injection takes place at too low a pressure. This implies not being satisfied with the thrust of the injector calibration spring to ensure closure. A significant advantage is that from the fact that the injector can be kept closed, it is possible to organize the discharge of the injection device over a longer period and therefore to seriously dampen the noise emitted by the equipment. 'injection.

 - Complete the discharge, either by the valves of the injection pump, or by another device, so that the additional fuel compressed but not introduced into the combustion chamber can be evacuated and the pressure in the line returns to the rest situation.

 Observing that during the part of the injection period for which the injection pump provides flow, the pressure in the injection system is, to within a few tens of bars, always increasing, the choice of principle consists in using the detection of the start of the pressure drop in the high pressure pipe at the inlet of the injector holder to push the injector needle by exploiting the energy still stored in the injection system, more particularly in the high pressure pipe .

 The voluntary closure of the end of injection, not very effective in the case of traditional injection, means that the excess fuel present in the high pressure pipeline has to be discharged. It is therefore necessary to slightly increase the quantity delivered by the injection pump to introduce into the engine the same amount of fuel as with a traditional injection system.

 The device according to the invention makes it possible to improve the performance of the injection system in order to effectively satisfy the increasingly severe requirements, set out above.

 It includes, for this purpose, a control valve which, brought back into the closed position by a spring and actuated by the pressure of the fuel present in a volume used as a detection accumulator in communication with the high pressure pipeline by an orifice or a capillary causing a delay on the pressure by hydraulic filtering effect, controls the closing of the injector by hydraulic action on the injector needle by exploiting the energy still stored in the high pressure pipe of the injection system from the start of the pressure drop of the high pressure pipe. This action is carried out either directly, or by means of a control piston as specified below.

 Another characteristic consists in the fact that an orifice or capillary installed on the return can ensure the discharge of the excess fuel in the high pressure line, and release the hydraulic closing action applied to the injector needle during the interval between two injections.

 In the case where the hydraulic action is not applied directly to the injector needle, but is applied via a control piston, the invention provides that the diameter of the control piston is greater or equal to that of the guide of the injector needle. This piston, making it possible to apply the hydraulic action indirectly to said needle, is interposed in order to reduce the volumes subjected to sudden variations in pressure.

 According to an additional characteristic, a piston valve, recalled by an independent spring or common to that of the control piston, is installed upstream of said control piston in order to limit the volume to be compressed when the needle is opened. injector; this arrangement makes the effect of the return orifice or capillary more effective in slowing the opening of the injector needle.

Another advantageous characteristic consists in the fact that two orifices or capillaries can provide separately the braking functions of the opening of the injector and of discharge of the injection system, the opening braking or capillary discharging the very small volume located between the piston valve and the control piston, the return port or capillary discharging the volume located between the control valve and the piston valve
Another characteristic consists in the fact that a valve flap can supply the detection accumulator in order to make the pressure build-up faster and extend the time for maintaining this pressure in the accumulator by reducing the cross-section of the pressure filter port
Yet another characteristic consists in the fact that the control valve, of diameter larger than that of the control piston, can be installed directly in contact with this piston which, at rest, pushed by the injector calibration spring, does not is not in contact with the injector needle plunger, even at full opening. Preferably, a spring, the thrust of which is less than that of the injector calibration spring, is interposed between the control valve and the control piston, and ensures their separation as soon as the assembly has left its rest position.

 Another characteristic consists in the fact that the needle plunger can, instead of being centered on the nipple of the injector needle, be guided in the needle stopper, which makes it possible to use the very small volume, located between the injector needle and its stop, to ensure opening braking of the injector, the braking orifice being thus placed in the needle stop.

 A final feature consists in the fact that a return pressure adjustment valve, individual or common to several injectors, can be installed on the return circuit in order to adjust the opening pressure of the injector (s).

 Furthermore, it will be noted that the routing of the fuel to the injector is not hindered by the components of the device according to the invention.

 Each of the engine injector holders will receive the same device, only the return pressure adjustment valve could be common to several injector holders, or even unique for the engine.

The invention will in any case be better understood with the aid of the description which follows, with reference to the appended diagrammatic drawing representing, by way of examples, embodiments of this device for improving the dynamics of injection of combustible
- Figure 1 is a block diagram of an injector holder with traditional injection system.

 - Figure 2 shows an example of integration into an injector holder, seen in section on II-II of Figure 3, the basic solution of the device according to the invention.

 - Figure 3 is a sectional view along III-III of Figure 2.

 - Figure 4 shows an overall diagram of a traditional injection system.

 - Figure 5 shows an overall diagram of an injection system equipped with the basic solution of the device according to the invention.

 - Figure 6 shows a partial diagram of a variant of the device without return port.

 - Figure 7 shows a diagram of a variant of the device equipped with a valve for adjusting the return pressure.

 - Figure 8 shows a diagram of a variant of the device with control piston.

 - Figure 9 shows a diagram of a variant of the device with control piston, equipped with a valve for adjusting the return pressure.

 - Figure 10 shows a diagram of a variant of the device with control piston and piston valve.

 - Figure 11 shows a diagram of a variant of the device with control piston and piston valve equipped with a valve for adjusting the return pressure.

 - Figure 12 shows a diagram of a variant of the device with control piston, piston valve and separate brake and return ports.

 - Figure 13 shows a diagram of a variant of the device with control piston, piston valve and separate braking and return ports, equipped with a valve for adjusting the return pressure.

 - Figure 14 shows a diagram of a variant of the device with control piston, piston valve, valve valve and separate brake and return ports.

 - Figure 15 shows a diagram of a variant of the device with control piston, piston valve, valve valve and separate braking and return ports, equipped with a valve for adjusting the return pressure.

 - Figure 16 shows a diagram of a variant of the device with control piston, piston valve, valve valve and braking and return ports in cascade.

 - Figure 17 shows a diagram of a variant of the device with control piston, piston valve, valve valve and braking and return ports in cascade, equipped with a valve for adjusting the return pressure.

- Figure 18 shows an example of construction of the device with control piston and return orifice incorporated in the control piston according to the diagrams of Figures 8 and 9, seen in section along XVIII
XVIII of figure 19.

- Figure 19 is a sectional view along XIX
XIX of figure 18.

 - Figure 20 shows an example of construction of the device with control piston, piston valve and return port incorporated in the control piston according to the diagrams of Figures 10 and 11, seen in section.

 - Figure 21 shows an example of construction of the device with control piston, piston valve, opening braking orifice incorporated in the control piston and separate return orifice according to the diagrams Figures 12 and 13, seen in section.

 FIG. 22 represents an example of a control valve with an integrated pressure filtering orifice according to the diagrams of FIGS. 5 to 13.

 FIG. 23 represents an example of a control valve with an integrated pressure filtering capillary which can be used in place of the control valve shown in FIG. 22.

 FIG. 24 represents an example of a control valve with a valve valve with an integrated pressure filtering orifice according to the diagrams of FIGS. 14 to 17.

 FIG. 25 represents an example of a control piston with piston valve and return orifice incorporated in the control piston according to the diagrams of FIGS. 10 and 11.

 FIG. 26 represents an example of a control piston with a piston valve, an opening braking orifice incorporated in the control piston and a separate return orifice with parallel flow according to the diagrams in FIGS. 12 to 15.

 FIG. 27 represents an example of a control piston with a piston valve, opening braking orifice incorporated in the piston valve and separate return orifice, installed in cascade according to the diagrams in FIGS. 16 and 17.

 - Figure 28 shows a diagram of a variant of the device with stacked control valve and control piston.

 - Figure 29 shows a diagram of another variant of the device with control valve and control piston stacked, equipped with a valve for adjusting the return pressure.

 FIG. 30 shows in section an example of integration in a nozzle holder of the device with control valve and control piston stacked according to the diagrams of FIGS. 28 and 29.

 - Figure 31 shows an operating diagram corresponding to a traditional injection device.

 FIG. 32 represents an operating diagram corresponding to an injection device according to the diagrams of FIGS. 16 and 17, with the details as shown in FIGS. 24 and 27.

 FIG. 33 represents an operating diagram corresponding to an injection device according to the diagrams of FIGS. 28 or 29, with the details as shown in FIG. 30.

 In general, with reference to these figures, we consider an assembly comprising an injector holder 1 and an actual injector 2. The injector holder 1 consists of an injector holder body 3 and possibly a injector holder cover 4. The injector 2 comprises an injector needle 5, housed and guided in translation in an injector body 6. A needle stopper 7 is interposed between the injector holder body 3 and the body d injector 6. The needle 5 comes to cooperate with a needle seat 8, to control the supply of injector holes 9. The injection system also conventionally comprises a fuel supply circuit, with an injection pump assembly 10.

The traditional injection system, shown in Figures 1 and 4, still includes
- a low pressure supply 11,
an injection pump 12 proper, with pulsed flow,
- a non-return valve 13,
a high pressure pipe 14, external to the injector holder 1,
- following this pipe 14, a portion of high pressure pipe 15 inside the injector holder 1, making it possible to create fuel jets 16, starting from the holes 9 and propagating in the combustion chamber,
an injector calibration spring 17,
- a low pressure return 18,
- an injection pump discharge valve 19, optional, and
- an injector needle plunger 20, by means of which the spring 17 acts on the injector needle 5.

The device according to the invention comprises essential additional elements, which are
- a control valve 21, with spring (not marked),
a detection accumulator 28, and
- A pressure filtering orifice 29, these elements being associated constructively and functionally.

 Figures 2, 3 and 5 show the basic solution, practically using only these elements.

The variant embodiments of the device may require optional additional elements such as
- a valve stop 22,
- a control piston 24,
- a piston valve 25,
- a valve valve 26,
- a return pressure adjustment valve 27,
a return orifice 30 (present in the case of FIGS. 2 and 3),
an opening braking orifice 31,
- These elements can be used alone or in various combinations, the figures already defined above giving a broad illustration of the many possible variants. It will be noted that, in the case of FIG. 6, therefore for a variant of the device without return orifice, the discharge must be carried out exclusively by the valves of the injection pump, during the time when the control valve 21 remains open .

Some elements are only functional characteristics (concerning Figures 28 to 30) such as
the relay valve 23 resulting from the effect of the contact of the control valve 21 on the control piston 24, as well as
- the control clearance 32, between the control piston 24 and the injector needle plunger 20.

The curves shown on the operating diagram of a traditional injection system (Figure 31) represent
- The internal discharge pressure 33 of the injection pump.

 - The pressure 34 at the inlet of the injector holder in the main high pressure pipe.

 - The fuel flow 35 actually injected.

 - The displacement 36 of the injector needle.

The curves appearing on the operating diagram of the injection system equipped with the device according to the invention (FIG. 32) represent:
- The internal discharge pressure 37 of the injection pump.

 - The pressure 38 at the inlet of the injector holder, in the main high pressure pipe.

 - The pressure 39 in the detection accumulator.

 - The pressure 40 applied to the control piston.

 - The fuel flow 41 actually injected.

 - The displacement 42 of the injector needle and the control piston.

 - Displacement 43 of the control valve.

 The abscissa of the diagrams representing time is, for the part represented, limited to 10 to 12% of the period of a complete cycle. The opening time of the injector represents, in these examples, only 4 to 4.5% of this same period. These curves apply more particularly to the diagrams of FIGS. 16 and 17.

The operation of a traditional type injection device, used as a reference, can be described as follows:
- At rest, the injector is closed by the effect of the injector calibration spring 17.

 - At the beginning of the discharge, when the injection pump 12 provides flow to the high pressure line 14 through the non-return valve 13, the pressure increases until the injector opens by pushing on the injector needle 5.

 - The injection continues, the fuel jets 16 advancing in the combustion chamber. The pressure continues to increase in the high pressure line 14.

 - At the end of the delivery, when the injection pump 12 no longer supplies flow to the high pressure pipe 14, the latter discharges through the injector 2 to the combustion chamber and to the pump discharge valve d injection 19 to the low pressure supply 11.

 - The pressure in the high pressure line 14 decreases to a low level which is no longer sufficient to keep the injector needle 5 open. The thrust of the injector calibration spring 17 then closes the injector while the pressure in the high pressure pipe 14 is very low. The fuel jets 16 are interrupted.

 - The end of the discharge of the high pressure pipe 14 through the injection pump discharge valve 19 returns the injection system to its rest position, ready for a new injection.

The operation of the device according to the invention, illustrated by one of the diagrams in FIGS. 5 to 17, can be described as follows:
- At rest the injector is closed by the effect of the injector calibration spring 17. The control valve 21 is closed under the effect of its return spring.

The assembly is therefore closed as for a traditional injection system.

 - At the start of the discharge, when the injection pump 12 supplies flow to the high-pressure line 14 through the non-return valve 13, the pressure increases until the injector opens by pushing on the injector needle 5.

 - The opening of the injector, braked by the return orifice 30 or the opening braking orifice 31, is progressive. This has the effect of limiting the flow rate at the start of injection and increasing the pressure in the high pressure pipe 14.

 - The injection continues, the fuel jets 16 advancing in the combustion chamber. The pressure continues to increase in the high-pressure line 14. The very small flow rate passing through the pressure filtering orifice 29 or the valve valve 26 supplies the detection accumulator 28. The pressure in this accumulator 28 increases with a slight lagging behind the high pressure line 14.

 - At the end of the discharge, when the injection pump 12 no longer supplies flow to the high pressure pipe 14, the latter begins to partially discharge through the injector 2 towards the combustion chamber and the discharge valve. from injection pump 19 to low pressure supply 11.

 - The pressure in the high pressure line 14 decreasing, this no longer suffices to maintain the control valve 21 which, pushed by the pressure in the detection accumulator 28, opens suddenly. The pressure in the high-pressure pipe 14 is communicated either directly to the injector needle 5 or to the control piston 24, possibly through a piston valve 25. The effect of the hydraulic thrusts cumulated with that of the spring of injector calibration 17 rapidly closes the injector while the pressure has only slightly decreased in the high pressure pipe 14. The fuel jets 16 are very clearly interrupted.

 - During the end of the discharge of the high pressure line 14 through the injection pump discharge valve 19 and / or the return orifice 30, the detection accumulator 28 discharges very gradually towards the high line pressure 14 through the pressure filtering port 29. The control valve 21 is kept open for a longer time than that necessary for the complete discharge of the high pressure pipe 14. The injector 2 cannot in any case reopen uncontrollably. The end-of-discharge pressure can optionally be controlled by a single return pressure adjustment valve 27, placed on the common return of the injectors, when it exists.

 - When the pressure difference between the detection accumulator 28 and the high pressure line 14 becomes too small, the return spring closes the control valve 21. The device, returned to its rest position, is ready for a new injection .

 In this device, more particularly if the control valve 21 is kept open for a long time after the injection period by the effect of the combination of a valve valve 26 and a fairly small pressure filtering orifice 29 , the residual pressure of the high-pressure line 14 can then be controlled by the return pressure adjustment valve 27 through the return orifice 30, the injection pump relief valve 19 no longer d interest in controlling this pressure. It may suffice to use a non-return valve 13 having a breathing volume, possibly small, or a simple non-return valve.

The operation of the variant of the device according to the invention with control valve 21 and control piston 24 stacked, according to one of the diagrams in Figures 28 or 29, can be described as follows:
- The start of operation is identical to that of the basic device until the injector opens. At rest, under the thrust of the injector calibration spring 17, the control piston 24 is supported on the control valve 21 and releases a control set 32 from the injector needle plunger 20, this play must be greater than the maximum path of the injector needle 5.

 - The opening of the injector, braked by the return orifice 30 or the opening braking orifice 31 which increases the pressure in the housing of the injector calibration spring 17, or between the needle 5 and its stop 7 is progressive.

- The injection continues, the fuel jets 16 advancing in the combustion chamber. Pressure continues to increase in the high pressure line 14
The very small flow rate passing through the pressure filtering orifice 29 feeds the detection accumulator 28. The pressure in this accumulator increases with a slight delay compared to that of the high pressure pipe 14.

 - At the end of the discharge, when the injection pump 12 no longer supplies flow to the high pressure pipe 14, the latter begins to partially discharge through the injector 2 towards the combustion chamber and the discharge valve. from injection pump 19 to low pressure supply 11.

 - The pressure in the decreasing high-pressure pipe 14 is no longer sufficient to maintain the control valve 21, which, pushed by the pressure in the detection accumulator 28, leaves the valve stop 22, moves the piston by as much control 24, fills the control set 32, and brings the control piston 24 and the injector needle plunger 20 into contact. The pressure of the detection accumulator 28 having decreased, the control valve 21, under the effect of the spring separates from the control piston 24 and comes back into contact with the valve stopper 22. The pressure of the high pressure pipe 14 is communicated to the control piston 24 through the fictitious relay valve 23, produced by the clearance between the control piston 24 and the control valve 21. The effect of the hydraulic thrusts quickly closes the injector while the pressure has only slightly decreased in the high pressure line 14. The fuel jets 16 are interrupted very cleanly e.

 - During the end of the discharge of the high pressure line 14 through the injection pump discharge valve 19 and / or of the return orifice 30, the detection accumulator 28 rapidly discharges towards the high pressure line 14 and towards the thrust volume of the control piston 24. The control piston 24 keeps the injector 2 closed as long as the pressure of the high-pressure pipe 14 remains greater than a fraction of the opening pressure of the injector. This fraction depends on the guide diameters and the seat 8 of the injector needle 5, the diameter of the control piston 24 and the ratio between the forces of the springs placed on either side of said piston 24.

 - When the pressure in the high pressure line 14 has decreased sufficiently, the injector calibration spring 17 returns the piston d of the control valve 21. The device, returned to its rest position, is ready for a new injection.

The diagram in FIG. 33 illustrates the operation of the injection system according to the variants of FIGS. 28 or 29, the curves appearing on this diagram representing
- The internal discharge pressure 44 of the injection pump.

- The pressure 45 at the inlet of the injector holder,
in the main high pressure pipeline.

 - The pressure 46 in the detection accumulator.

 - The pressure 47 applied to the control piston.

- The pressure 48 observed in the volume of
braking (in this case: spring housing
taring).

- The fuel flow rate 49 actually
injected.

 - The displacement 50 of the injector needle.

 - The movement 51 of the control piston.

 - The displacement 52 of the control valve.

 It will be noted that, despite the complex appearance of the diagrams, the variants according to the diagrams of FIGS. 28 and 29, and the corresponding embodiment of FIG. 30, are the most compact. The passage section of the control valve 21 is here the interval between the valve body and the valve stopper 22, while the fictitious relay valve 23 is the interval between the control piston 24 and the valve 21.

Depending on the integration possibilities, the construction choices or the operating modes it is possible
- Whether or not to install the control piston 24 making it possible to push on the injector needle 5 in a less compressible manner than by pressurizing the cavity serving to house the injector calibration spring 17. In this case, the section of the control piston 24 may be greater than that of the guide of the injector needle 5 in order to increase the closing thrust.

 - To incorporate into the injector holder 1 all or part of the elements added to the traditional injection system which constitute the device, with the exception of the return pressure adjustment valve 27 which may be common.

 - Integrate the pressure filtering orifice 29 into the control valve 21, the valve plug 26 or the body 3 of the injector holder 1.

 - Integrate the return orifice 30 into the control piston 24 or into the body 3 of the injector holder 1.

 - Integrate the opening braking orifice 31 into the control piston 24, the piston valve 25 or the body 3 of the injector holder 1.

 - To make each of the orifices in the form of a single hole, an attached nozzle, or a capillary, the capillaries possibly being a helical groove on a cylindrical guide or fitting part, or a spiral on a part plane in contact with another or by the effect of the leak associated with the guide clearance.

The device according to the invention allows:
- The clear closure of the injection by forced closing of the injector. This is the main function of the device.

 - The adjustment of the injector opening pressure by the effect of the return pressure adjustment valve 27, the basic level being controlled by the injector calibration spring 17 and the characteristics of the injector needle 5. The closing pressure does not depend on the opening pressure, the setting of the injector can be more moderate than that of traditional injectors.

 - The possible adjustment of the residual pressure in the injection device by the return pressure adjustment valve 27, coupled to the adjustment of the injector opening pressure.

 - Control of the flow rate at the start of injection, through the section of the opening braking orifice 31 or the return orifice 30.

 The device according to the invention is particularly intended to supplement and improve the injection devices of diesel engines using injection pumps with pulsed flow.

Claims (10)

 1- Device making it possible to improve the dynamics of fuel injection for diesel engines equipped with pulsed-flow injection pumps, characterized in that it comprises a control valve (21) which, brought back to its position closing by a spring and actuated by the pressure of the fuel present in a volume used as a detection accumulator (28) in communication with the high pressure pipe (14, 15) by an orifice or a capillary (29) causing a delay in the pressure by hydraulic filtering effect, controls the closing of the injector (2) by hydraulic action on the injector needle (5) by exploiting the energy still stored in the high pressure pipe (14, 15) of the injection from the start of the pressure drop in the high-pressure pipe (14, 15).  2- Device according to claim 1, characterized in that an orifice or a capillary (30) installed on the return (18) ensures the discharge of excess fuel in the high pressure line (14, 15) and releases the action closing hydraulics applied to the injector needle (5) during the interval between two injections.  3- Device according to claim 1 or 2, characterized in that the hydraulic action is not applied directly to the injector needle (5), but via a control piston (24) whose diameter is greater than or equal to that of the guide of the injector needle (5).  4- Device according to claim 3, characterized in that a piston valve (25), recalled by an independent spring or common to that of the control piston (24), is installed upstream of said control piston (24) so limit the volume to be compressed when opening the injector needle (5).  5- Device according to claim 3, characterized in that the control valve (21), of diameter greater than that of the control piston (24), is directly in contact with this piston (24) which, at rest, pushed by the injector calibration spring (17) does not come into contact with the injector needle plunger (20), even at full opening.  6- Device according to claim 5, characterized in that a spring, whose thrust is less than that of the injector calibration spring (17), is interposed between the control valve (21) and the control piston ( 24) and ensures their separation as soon as the assembly has left its rest position.  7- Device according to any one of claims 4 to 6, characterized in that two orifices or capillaries (30, 31) separately provide the braking functions of the opening of the injector (2) and discharge of the system d 'injection.  8- Device according to any one of claims 1 to 7, characterized in that the needle plunger (20) is guided in the needle stopper (7), in order to use the volume located between the needle d injector (5) and its stop (7) to ensure the opening braking of the injector (2).  9- Device according to any one of claims 1 to 8, characterized in that a valve valve (26) ensuring the supply of the detection accumulator (28) makes it possible to shorten the pressure rise delay while lengthening the time for maintaining this pressure in the accumulator (28).  10- Device according to any one of claims 1 to 9, characterized in that a return pressure adjustment valve (27), individual or common to several injectors, is installed on the return circuit (18) in order '' adjust the opening pressure of the injector (s) (2).