FR2459375A1 - IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINE WITH FUEL INJECTION - Google Patents

Abstract

IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINE WITH FUEL INJECTION. THIS SYSTEM IN WHICH THE FUEL IS INJECTED AND IGNITED BY A HIGH VOLTAGE ELECTRIC SPARK INCLUDES A FUEL INJECTOR CONTAINING A PLUNGER MADE OF AN ELECTRICALLY CONDUCTIVE MATERIAL. THE PLUNGER OR NEEDLE 222, 227 IS ACTUATED BY THE PRESSURE OF THE FUEL, TO OPEN THE INJECTOR 11, 78-81, 251, 276. IT HAS A FIRST ELECTRICAL CIRCUIT CONNECTING A RESISTOR 15, 91-94 WITH THE NEEDLE, A COMPARATOR 21, 104, A SECOND CIRCUIT 24, 106, A THIRD CIRCUIT 20, 113 CONNECTING THE OTHER INPUT OF THE COMPARATOR TO THE NEEDLE. APPLICATION TO IGNITION SYSTEMS FOR INTERNAL COMBUSTION ENGINES WITH FUEL INJECTION.

Description

IGNITION SYSTEM FOR INTERNAL COMBUSTION MIXER A

FUEL INJECTION

  The present invention relates to ignition systems for internal combustion engines in which the fuel is injected and the fuel mixture of this fuel is ignited by an electric spark. The system according to the invention is intended in particular for

  a diesel engine of the spark-assisted diesel type.

  The designation "diesel type engine" includes a fuel injection system in which the pressure of the latter serves to lift the fuel injectors. U.S. Patent No. 4,066,059 discloses an ignition system that utilizes the direct amplification of a signal caused by an injector to determine the generation of a high voltage spark signal. The injector described in this patent requires special members of electrically insulating material so as to isolate the needle or the plunger from the body of the injector, except when in contact with the seat

from the latter in closed position.

  U.S. Patent No. 3,942,366 proposes the use of a needle fuel injector operating as a test equipment switch in operation to control a nozzle. In this case, the body of the electrical switch is separated from the plunger and the seat of the injector, which requires an injector

  and a special switch and increases the cost of an injection

ordinary person.

  The subject of the present invention is therefore a system

  system for generating a signal for adjusting the spark-forming voltage of a motor

  internal combustion, this signal being triggered by the injection

tor.

  In accordance with the characteristics of the invention

  * tion, there is provided an ignition system for an internal combustion engine in which the fuel is injected and the fuel mixture of said fuel is ignited by a high voltage electric spark, this system comprising a fuel injector including the diver, made of an electrically conductive material, is in contact with the electrically conductive body of said plunger when the latter is closed, and this system being characterized in that said plunger is controlled by the pressure of the fuel to open the injector; said system comprises a first electrical circuit connecting in series a resistance to said plunger, a comparator

  having two inputs and an output, a second electrical circuit

  said controller applying a predetermined electromotive force to one of said comparator inputs, and a third circuit connecting the other input of said comparator to said plunger; the output of the comparator generates a signal for igniting the electric spark when said

  diver is controlled by the fuel pressure.

  The invention will be described in more detail with reference to the appended drawings as non-limiting examples and in which:

  FIG. 1 is a schematic diagram illus-

  trant the fundamental assembly of a fuel injector and a comparator; FIG. 2 is another schematic diagram showing an assembly comprising a plurality of fuel injectors connected to comparators in order to generate

  an ignition control signal transmitted to a generator

  spark or a similar organ;

  FIG. 3 is an illustrative block diagram.

  wholly incorporating a system according to the invention for applying spark ignition signals to an internal combustion engine; Fig. 4 is an elevational view of an injector and schematically illustrates the electrical assembly for producing test signals; Figure 5 is an enlarged longitudinal section with partial cutaway illustrating the components constituting a fuel injector similar to that of Figure 4; and FIG. 6 is a longitudinal section on an enlarged scale-showing another type of injector

  comprising elements according to the invention.

  Tests have shown that insulation

  in various places along the length of the plunge

  the injector, as proposed in the aforementioned Patent No. 4,066,059, is not necessary. As a result, the pressurized fuel that lifts the injector plunger gives rise to a thin layer

  of fuel between the plunger and the body of said injector.

  This causes a path of high ohmic resistance, which makes it possible to generate a control signal that can be

  used with the ignition system.

  On the basis of this fundamental consideration, it is possible to use an electrical system such as that illustrated in FIG. 1. Such a system comprises

  a fuel injector, shown schematically.

  This injector is connected to a battery 12 and a resistor 15 in series. It is obvious that the body of the injector (not shown in detail) is connected to the mass. This is indicated by a grounding conductor 16, and the series circuit is completed by the

grounding the battery 12.

  It should be emphasized that when the injector is not subject to the pressure of the

  fuel, its valve is not closed and, therefore,

  quent, its plunger (not shown) is in contact with the body (not shown) of said valve inside said injector 11. In this state, the electric potential

  at a junction 19 of the assembly described above is substantially

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  equal to zero, or equal to the potential of the mass, as long as the diver remains in contact with the

  body of the valve in the injector 11.

  The junction 19 is connected to a comparator 21 by a conductor 20. The latter constitutes an input of the

  comparator 21, while another driver 24 con-

  the other entrance. Driver 24 applies a

  predetermined potential or electromotive force

  The potentiometer consists of two resistors 25 and 26. The latter are connected in series with each other and with the battery 12.

  via a conductor 29 of grounding.

  It is important to note that the injector can be of any known type, as long as it comprises a plunger of electrically conductive material in contact with an electrically conductive valve body when this valve is closed and, of course, with the plunger of said valve, so that an electrical circuit not connected to ground can be established with him. Among known injectors, commercially available, those manufactured by Stanadyne of Hartford, Connecticut, and bearing the name "Roosa Master Pencil Nozzles". Another type of commercial injectors is the one made by Robert Bosch, GnIH, a company based in Stuttgart, Federal Germany, whose "KDALZ" bodies include "DLUZ" nozzles.

  "KDAL" bodies have "DLLA" nozzles.

  Those skilled in the art know that the comparator 21 is a well-known electronic component for generating an output signal that goes from a high voltage (insofar as the input voltage at one of its two inputs is lower than the predetermined voltage at its other input) at a low voltage when the voltage

  at the input under consideration exceeds the predetermined voltage.

  In other words, whenever the voltage at the first

  The first input considered exceeds the predetermined voltage at the other of the two inputs of the comparator, the latter is switched to a low output voltage, and this change causes an output signal that can be used in an ignition system. FIG. 2 illustrates an application of the basic principles given with reference to FIG. 1. This application relates to a multi-cylinder engine (not represented) equipped with several fuel injectors 33 to 36. Each of these injectors 33, 34, 35 and 36 is connected to an input of a corresponding number of comparators 39 to 42. It should be noted that, in this case, a potentiometer is connected to the terminals of a

  battery 45 which one of the terminals is grounded.

  The other terminal of the battery 45 is connected, via

  46, to a pair of resistors 49 and 50 connected in series, one end of the resistor 50 being grounded, as shown. It results

  an electromotive force or a predetermined potential

  to a driver 51 connected to. parallel to each comparator 39, 40, 41 and 42. These parallel connections lead to one of the inputs of each of the

  comparators 39 to 42. The other entry for each

  is connected to one of the injectors 33 to 36,

respectively.

  It should be noted that the outputs of all comparators 39 to 42 are connected together to a single

  output lead 53 connected to a spark generator

  those. The signal applied to this output conductor

  53 goes from high voltage to low voltage (sensi-

  equal to the potential of the mass), whenever one of the comparators 39 to 42 is switched. Such a variation of the output signal, i.e. the voltage, results from the fact that the current does not pass through, then passes through a resistor 54. It should be noted that this output signal applied to the conductor 53 does not does not go directly to a distributor, but rather plays the role of a control signal for a signal voltage

  ignition can be dispensed.

  Figure 3 schematically shows an ignition system according to. the basic idea of this

  invention, which comprises a temporal latching circuit

  corseted. The latter is intended to maintain the signal caused by the injector long enough to overcome the unevenness of the signal emitted when a diver is lifted from his seat when he is subjected to

the fuel pressure.

  The portion of the illuminated ignition system for generating a high voltage electric spark is similar to that illustrated and described in copending application No. 885,844 filed March 13, 1978 and having the same object as the present invention. request. However, it should be noted that the ignition system causing spark signals may also be similar

  to various other systems such as that described and

  No. 4,022,177 issued May 10, 1977 and in other prior patents cited above. All of the above-mentioned ignition systems use a square-wave, high-frequency, square-wave spark signal during the intervals

between sparks.

  The aforementioned part of the system is illus-

  on the upper right of Figure 3.

  Since such a system producing such a spark signal is described in detail in the various patents and in the above-mentioned application, it will be briefly described herein. Thus, in FIG. 3, an output transformer 57 delivers high voltage and high frequency spark signals from secondary or high output windings.

  tension 58 and 59, with several candles 60-

  57 has a primary winding 63 which is centrally

  and a reaction winding 64, used together in an oscillator circuit as clearly described in the patents mentioned above. In addition, there is provided a control winding 65 which determines the

  duration of a continuous spark of the alternating current type

  native, depending on the beginning and end of oscillation periods of the oscillator. Such control of the stopping and resumption of oscillations is provided by an electronic switch, that is to say by a transistor 68. This transistor (spark control switch) is connected in series with a source of direct current which is a battery 69. The control circuit leads to the upper end of the control winding 65 through a diode 70 and a resistor

  71 (see Figure 3). The lower end of the coil

  65 reaches the transistor switch 68 by

  via another diode 74.

  As shown in the figure, a relay 75 is connected to the control winding 65. However, it is not useful to describe the details since they are irrelevant to the present invention. This relay is intended to interrupt the oscillator at each

  once the ignition system is interrupted.

  The basic principles of the present invention have been described above with reference to Figs. 1 and 2. Fig. 3 schematically illustrates a concrete embodiment of mounting an ignition system for a four-cylinder engine. Several injectors 78, 79, 80 and 81 are connected in parallel with the battery 69

  through the illustrated drivers. These

  conductors consist of a conductor 84 from the positive terminal of the battery 69 and reaching another conductor 85 which leads to conductors 86, 87 and 88, the latter being connected to a common conductor 89 for all the injectors 78 to 81; the assembly further comprises breakers which will be described below. These

  conductors are connected via resistance

  these individuals described below, the electrically conductive plunger (not shown) of each of the four injectors 78 to 81. In addition, individual resistors 91, 92, 93 and 94 are interposed between the

  conductors 88 and 89 and each of the divers injecting

  78 to 81. Similarly, the 88 and 89 conductors

  at one end of another resistor 97, the other end of which is connected, via the contacts of a switch 98 actuated by relay, to switches 101 which act as auxiliary members of the injectors 79 to 81 , as will be seen later in

detail.

  Note the presence of two comparators 104 and 105. As explained above, a comparator is a well-known electronic component. A predetermined electromotive force is connected in parallel to an input of each of these comparators. This circuit for connecting an electromotive force of predetermined amplitude comprises a conductor 106 cooperating with the comparator 104 and an associated conductor 107

  105. These conductors 106 and 107 are

  both are at one end of a resistor 109 whose other end is connected to. a lead-output of a potentiometer interposed between two resistors and 111. The lower end of the resistor 111 (by observing Figure 3) is connected to the ground, while the upper end of the resistor 110 is

  connected to the driver 87 described above, through

  driver's license 114. Driver 87 returns

  to the positive terminal of the battery 69 through

  drivers-86, 85 and 84.

  The other input of the comparator 104 is connected by a conductor 113 to a common conductor which starts from the cathodes of several diodes 115, 116, 117, 118 and 119. The other electrodes of these diodes are each connected, via capacitors 121, 122, 123, 124 and 125, respectively, to each of the plungers of the injectors 78 to 81, as well as the switches 101. It should be noted that the assemblies of each fuel injector are substantially similar. Their

  operation leads to a change in the

  when the electrically conductive diver

  The driver (not shown) of each of the valves is lifted under the effect of the pressure exerted by the injected fuel. Such operation can be described by

  example for the injector 78. When the injection pressure

  When the plunger lifts the plunger, the connection to the mass, ensured by the seat and the body of the injector, is interrupted. As a result, the electrical potential at a junction 128 of the circuit goes from a value equal to the ground potential to a high potential, produced by the positive terminal of the battery 69. This change is reflected by the capacitor 121 on the side. not connected to the ground of a resistor 129 and, by the diode 115, on the conductor 113 which leads to one of the inputs of the comparator 104. It will be noted that each of the assemblies of the other injectors 79, 80 and 81 and the breakers 101 are identical to the one just described, and that the separation between these assemblies is ensured by the

diodes15 to 119.

  The output of the comparator 104 is connected to the base electrode of a transistor 132 which transmits a signal intended to cause the spark signals

  electrical generated by the oscillator described above.

  Of course, the outputs of this oscillator are

  connected to the candles 60, via the transformer

57.

  An important feature of the present invention is the operation of the second comparator

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  and drivers associated with it. This comparator 105 is connected so that its other input conductor (different from the conductor 107) is connected to the output of the comparator 104 through the transistor 132 and a timing circuit.

  This montage works as a temporal device

  to maintain the output signal from the comparator 104 for a predetermined time. This duration is defined by an RC circuit (resistance-capacity)

  which comprises a capacitor 135 and a resistor 136.

  These two components of the RC circuit are part of a circuit which passes through a resistor 139 to join the other input of the comparator 105. The output of this comparator 105 is connected to the other conductor

  113 of the first comparator 104, via

  conductor 141 and a resistor 142.

  As indicated above, such an assembly aims at keeping the comparator 104 in its modified state as long as the comparator 105 remains in the modified state, the duration of this phenomenon being determined by the circuit

  RC (capacitor 135 and resistor 136).

  The output of the transistor 132 is connected by means of a capacitor 145 to an inverter (frequency) circuit 146. The output of this circuit 146 is connected via a conductor 14 to the base electrode of the transistor 68 described herein. above and constituting an electronic switch. The inverter circuit 146 does not in itself play an important role in the present invention. It is only intended to vary the pulse duration of the output signals from the injectors, so as to establish an inverse relationship with respect to the speed (repetition frequency) of the internal combustion engine. Such an inverter circuit is known per se and, therefore, it is not necessary to describe its operation in the context

of this memoir.

  It should be noted that the interrupters 101 and the conductors associated therewith have a particular utility when cold starting the engine when the pressure exerted by the injected fuel

  is not sufficient to operate the injectors 78 to 81.

  Therefore, the breakers 101 are stalled relative to each other.

  to the operation of the engine, so as to open appro-

  approximately 10 to 150 before the top dead center, or, in any case, to open slightly before the injection

  fuel that would have occurred under normal conditions.

  the operation. In this way, it is possible to obtain an effective spark signal during the start-up of the engine, whether or not there is fuel injection, and also when the cylinders are primed for a cold start. It will be noted that the switch

  98 is controlled by a coil 152 of the switching relay.

  The latter can be actuated by a special cold start switch (not shown) to control the excitation of the coil 152 which actuates the switch

  98. A second pole 151 of the switch 98 controls the inter-

  resistance of a resistor 154 in the inverter circuit

  146 under start-up conditions at. cold.

  It is now necessary to describe the functioning

  According to the invention, the normal operation of the ignition can be examined in support of one of the assemblies of the fuel injectors. Therefore, if we consider that these injectors are not actuated since no pressure exerted by the fuel has been applied to their divers, they directly establish a connection to the mass so that the voltage at the junction 128 of the circuit is substantially zero. When the plunger of the injector 78 is raised, the voltage at said junction 128 increases to reach that of the battery 69. This increase in voltage is transmitted by the capacitor 121 and the diode 115 to the input conductor 113 of the comparator 104 When this increase of the input voltage exceeds the predetermined voltage of the conductor 106, the comparator is switched and an output signal is applied to the base of the transistor 132 via a conductor 155 and a resistor 156. As a result, this transistor 132 drives and

  sends a signal to one end of the resistor

  the signal 135 connected to ground via the capacitor 135 and a conductor 159, and also to the other input of the comparator 105, through the other resistor 139. This signal forces the comparator to perform a switching immediately after the

comparator 104.

  When the comparator 105 is switched, its output is connected to the input conductor 113 of the comparator 104 by the conductors 141 and the resistor 142. Under these conditions, the comparator 104 is maintained

  in its modified state for the duration of the

  It is obvious that this maintenance eliminates the difficulty that would manifest itself in the opposite case because of the unequal nature of the signal generated when the plunger of the injector is lifted by the pressure of the radiator. fuel. Thus, during the cold start, when a plunger is raised, it generates the signal that controls the comparator 104, and the break signal appears during the timeout period so that

  is eliminated as a result with all unwanted signals.

  FIG. 4 illustrates an injector 211 of the type

  current on the market. It is connected to an electrical circuit

  to connect a test apparatus such as a cathode ray oscilloscope 212 for

  issue a dynamic test signal during operation.

  It will be apparent to those skilled in the art that a signal thus

  can also be used if desired in an ignition spark signal system such as

  that described with reference to FIGS. 1 to 3.

  The embodiment of the injector 211 shown in Figures 4 and 5 is the same as that manufactured by Stanadyne, Inc. Windsor, Connecticut. It is known as the pencil-shaped "Roosa Master" injector. At the top, this injector

  211 comprises a protective sealing envelope 215.

  In FIG. 4, this envelope is shown exploded above the body of the injector 211. Of course, when this injector 211 is fully assembled, it has the configuration shown in FIG. 5. Therefore, the protective envelope 215 is threaded on a screw 216

  intended to limit the lifting of the valve.

  The arbor screw 216 is threadedly inserted into a metal sleeve 217 which has at one end (inside the injector 211) a pressure adjusting spring 220, shown in FIG. 5. This spring 220

  is in contact with a seat 221 at the upper end

  a needle 222 (see Figure 5). This needle 222 is axially movable, against the pressure exerted by the spring 220, by the hydraulic force resulting from the fuel pressure and applied by a conduit 223 from a fuel pump (not

  shown) actuated by the motor (not shown).

  A locking nut 225, adapted to adjust the lift, is screwed into the stop screw 216 and an upper spring 226 is housed inside the casing 215 when the injector is fully assembled. This spring 226 establishes a good electrical contact against the nut 225 at its lower end (see FIG. 5), while its other end is in contact with the head of a screw 227 in order to establish an electrical circuit (including the

  spring 226) coming out of the upper end of the enve-

  loppe 215 through a pair of nuts 230, between

  which is fixed an electrical connector 231.

  The additional organs constituting the injection

  Further, FIG. 4 illustrates an electrical circuit for emitting a test signal. A conductor 232 is connected to. one of the ends of a resistor 233 whose other end is connected by a conductor 236 to a terminal of a battery 237. The other terminal of the latter is grounded, as indicated by a corresponding conductor 238. This circuit continues via a conductor 241 which, connected to the ground, leads to the conduit 223 and a fuel inlet connector 242, secured to a metal body 243 of the injector 211. This electrical circuit is also provided via the needle 222 (see Figure 5)

  when he is in contact with his headquarters at the

  of the jet. This circuit is finally established by

  intermediate needle 222 and seat 221 of the spring, to the spring 220 for adjusting the pressure. The end of this spring 220 is in contact with the metal sleeve 217, where it follows that the electrical circuit extends to the spring 226 it passes through, the upper end of said spring 226 establishing a contact with the head of the screw 227 o the electrical connector 231 is fixed

by the pair of nuts 230.

  Of course, when the electrical circuit described above is closed because the needle is in contact with its seat, this circuit, comprising the resistor 233, is traversed by the current from the battery. However, the voltage at a junction 246 is equal to the ground potential or zero. Thus, when the fuel pressure lifts the needle 222 from its seat, an electrically insulating layer (not shown) is formed around said needle in the region where the fuel is injected. As a result, the electrical circuit described above, in which the current flows through the resistor 233, is open. This results in an increase of the potential at the junction 246 and this variation is transmitted to the oscilloscope 212 by

  via a driver 247.

  It is clear that the oscilloscope 212 has two input leads 248 through which a trigger signal is transmitted to the oscilloscope. Such a trip signal (not shown) can be generated by a magnetic connection (not shown) mounted on the crankshaft (not shown) of the motor, or by another connection to said motor, so that a trigger signal is generated at the desired moment during the operation of the combustion engine

  internal, illustrated for example in Figure 3.

  Referring to Figure 5, it should be noted that among the bodies constituting the injector 211, there is a metal spring 220 acting on a needle 222 movable axially, as in commercial injectors. This needle 222 is part of the injector 211 and it cooperates with a seat 251 formed on the inner wall of a metal tip 252. This spring 220 acts on the end of the needle 222 opposite

at headquarters 251.

  It will be observed that there is no electrical insulation at the lower part, that is to say along the entire length of the needle 222 relative to the body 243 of the injector 211. However, at the upper end of the spring 220 (by observing FIG. 5), a sleeve 255 of electrically insulating material is interposed between the metal sleeve 217 described above and a threaded sleeve 257. The latter is screwed into the upper end of the body 243 (see FIG. Figure 5). The locking nut 225 is screwed onto the stop screw 216, and it bears against the upper end of the sleeve 217 which consequently blocks the external thread of the sleeve 217.

relative to the thread of the screw 216.

  A guide washer (not shown), wedged between the locking nut 225 and the upper end of the sleeve 217, guides and centers the sealing envelope 215 on the upper end of the injector 211. As can be seen in FIG. as seen in FIG. 5, an electrically isolated conductive path is established from the end

  upper spring 220- (taking support against the man-

  internal slot 217) to the locking nut 225, via said sleeve 217 and the stop screw 216. This path continues to the lower end of the spring 226 whose upper end touched the screw 227 establishing the electrical contact. From there, this journey continues with the connector 231 wedged between the two nuts

230.

  A bushing 261 of insulating material surrounds the screw 227 passing through the sealing envelope 215. There is also provided a rubber O-ring 262

  constituting a conventional element of the injector 211. Natural

  when the fuel return conduits (not shown) threaded onto the projecting tips 265 and 266 are made of an electrically insulating material, the bushing

It is no longer indispensable.

  Figure 6 shows another type of commercially available fuel injector manufactured by the German company Robert Bosch, GinLH, Stuttgart, Federal Germany. In this embodiment, the injector has a metal body 271 in which is housed a metal pin 272. The latter cooperates with a metal spring 273 in contact with a seat 274. The spring 273 forces the needle 272 to its closed position against a seat 276 housed at the lower end of the body 271 of the Bosch type injector. Just above seat 276 is a cavity 277 for receiving

  the pressurized fuel to be injected. THE SUPPLY TERMINAL

  of this fuel to be injected is ensured by a

  conduit 278 formed inside the body 271.

  The upper end of the spring 273 (by observing FIG. 6) bears against a metal washer 282 whose thickness determines the bandage of said spring 273. A washer 283 made of electrically insulating material separates the washer 282 from a metal washer 275 and thus electrically isolates the upper end of the spring 273 and said washer 275 from the other metal elements. There is also provided a sleeve 284 of electrically insulating material, which surrounds the spring 273 and the washer 275 to provide electrical insulation with respect to the body 271 of the Bosch type injector. Preferably, this sleeve 284 consists of a well-known tube of "Teflon"

  narrowed under the effect of heat.

  In this type of injector, the flow of fuel takes place from a central open space 286 by a central channel 287 and exits by traversing the internal space delimited by a conduit 288 for the flow of fuel. An insulated electrical conductor 292 traverses the conduit 288 and a connecting conduit 289 to terminate in the central channel 287. This conductor 292 is welded or electrically secured in another manner to

the washer 275.

  In this embodiment of the injector, the only significant modification introduced by the present invention with respect to the injector or nozzle type Bosch is to add the insulating washer 283 and the sleeve 284. As a result, the upper end spring 273 is electrically isolated from the body 271 and other electrically conductive elements. Of course, the electrical connection must be made from the electrically conductive washer 275 and it is provided via the insulated electrical conductor 292. It should be noted that the present invention allows a minimized modification and / or addition of additional elements necessary to obtain the desired electrical installation. Such an assembly opens the circuit when the needle 272 is no longer in contact with its seat 276 at the lower end.

  of the injector. It is the fuel under pressure,

  storing a film surrounding said needle 272, which confers

  the insulation necessary to generate the electrical signal

  desired. The fuel injector that comes

  to be described makes it therefore possible to emit an electrical signal when the needle is no longer

  contact with its seat, subject to a minimum of

  compared to injectors available on the market.

  It goes without saying that many modifications can be made to the system described and represented,

  without departing from the scope of the invention.

Claims (10)

  1. Ignition system for an internal combustion engine in which the fuel is injected and wherein the fuel mixture of said injected fuel is ignited by a high voltage electric spark, canmortant a fuel injector comprising a plunger which, made of a material electrically conductive, is in contact with an electrically conductive metal body of the injector when said injector is closed, characterized in that said plunger or needle (222, 272) is actuated by the pressure exerted by the fuel to open said injector ( 11, 78-81, 251, 276), and in that it comprises a first electrical circuit connected in series a resistor (15, 91-94) with said needle, a comparator (21, 104) comprising two inputs and a an output circuit, a second circuit (24, 106) for applying a predetermined electromotive force to one of the inputs of said comparator, and a third circuit (20, 113) connecting the other input of said comparator to said needle, the output (132) of said comparator emitting a signal for igniting said electric spark when said needle   is actuated by the pressure exerted by the fuel.   2. System according to claim 1, characterized in that a delay device (105) is connected to the output of said comparator (104) to maintain the signal long enough to substantially eliminate the dynamic variations manifested in the electrical resistance. between the needle and the body of the injector after its initial opening by the pressure exerted by fuel.   3. System according to claim 2, characterized in that the delay device consists of a second comparator (105) having two inputs and one   output, one (106) of said inputs of said second compa-   the generator being connected in parallel with an input of the first comparator to obtain the predetermined electromotive force, the other input of said second comparator being connected to the spark ignition signal by an RC circuit (135, 136) for determining the time delay, the output (141) of said second comparator being connected to the other input of the first comparator to maintain said spark initiation signal long enough to substantially eliminate dynamic variations.   4. System according to any one of the   cations 1 to 3, characterized in that it comprises several injectors and in that its third circuit comprises a diode (115-118) connected in series with each of said   Needles to ensure electrical separation.   5. The system of claim 4, characterized   characterized in that it comprises an auxiliary device (101) actuated by the motor and generating a spark timing signal and in that its third circuit also comprises a diode (119) connected in series.   with said auxiliary device (101) to ensure its separation electric ration.   6. System according to claim 5, characterized in that said auxiliary device (101) actuated by the engine and generating a delay signal of   the spark consists of breakers.   7. System according to any one of the   1 to 6, wherein the injector comprises a metal pin (222, 272) housed in a metal body (243, 271) in which is formed a seat (251, 276) and a metal spring (220, 273) acting at the end of said needle opposite said seat to normally close said injector, characterized in that it comprises means (217, 284) for electrically isolating said spring from said metal body, as well as a connector   electric (231) connected to said metal spring at.   the end of the latter opposed to said needle.   8. System according to any one of the   cations 1 to 6, wherein the injector comprises a metal pin (222, 272) housed in a metal body (243, 271) in which is formed a seat   (251, 276), as well as a mechanical member of   elastic citation (220, 273) acting at the end of said needle opposite said seat to normally close said injector, characterized in that it comprises   means (217, 284) for the purpose of   each of said mechanical members with respect to said metal body, and a connection means (231) for electrically connecting a test apparatus to said needle for the purpose of generating a test signal when the contact is interrupted between said needle. and said seat, this needle being isolated from the metal body when injecting fuel.   9. System according to claim 8, characterized in that the mechanical member of elastic biasing   consists of a metal spring (220, 273).   10. System according to one of the claims   8 and 9, characterized in that said connection means   consists of an electrical connector (231) connected to the end   mite of the metal spring opposite the needle.