FR2886689A1 - Internal combustion engine ignition system for e.g. aircraft, has hollow body, and head forming electrode which surrounds end portion so that corona discharge takes place in precombustion chamber between head and central electrode - Google Patents

Abstract

The system has a hollow body (11) approximately symmetrical with respect to a central axis (X-X`) and a longitudinal and through hole (15) for passage of a central electrode (16). The body is covered with a hollow head (18) at the interior of compression chamber and a combustion chamber. The holes (20, 21) pierced in the head communicate with the chamber. The head forms an electrode, which surrounds an end portion (22) so that corona discharge takes place in a precombustion chamber (19) between the head and the electrode (16). Independent claims are also included for the following: (1) an internal combustion engine comprising an ignition system (2) a method for igniting an internal combustion engine.

Description

The present invention relates to the field of combustion engines

  two or four-stroke engines, such as motorcycles, automobiles, aircraft and boats, or

  generators or tools. More specifically, the invention relates to a system and a method of ignition of an internal combustion engine, and an internal combustion engine provided with such an ignition system. It finds particular application in piston and crankshaft engines, it being understood that its application to motors of the rotary type must not be excluded.

  Among the internal combustion engines, one of the best known and most used includes at least one piston, a connecting rod and a crankshaft transform the rectilinear movement back and forth in a cylinder in a rotary motion. This cylinder and the piston jointly delimit a combustion chamber and combustion of a fuel mixture of a fuel and an oxidizer, which is ignited by an electric arc still called spark and produced between two electrodes. Products result from the combustion of the fuel mixture in the combustion chamber. An evacuation system for these products is added to an intake system for fuel and oxidant in the compression and combustion chamber.

  Internal combustion engines have been the subject of many improvements. Research to further improve these engines is still conducted. One of them relates to the use of an ignition device comprising a prechamber which is separated from the combustion chamber by a wall. A candle is placed in the antechamber to ignite the mixture. An example of such an ignition system is described in the French patent application FR-2 781 840.

  Pre-chamber ignition devices have been developed to improve the initiation and / or flow of combustion in the compression and combustion chamber. In practice, however, there have been ignition defects, probably associated with the accumulation in the prechamber of residual combustion products, with consequent dilution of the air-fuel mixture in the prechamber. A fortiori, these devices are not compatible with recycling strategies to reinject a portion of the exhaust gas including to minimize pollutants.

  In addition, it occurs over time fouling inside the antechamber, where carbon deposits accumulate in particular on the insulating ceramic of the candle. These deposits of carbon can lead to cases of sliding sparks, that is to say divert the electric arcs which, therefore, do not necessarily appear in the middle of the prechamber but in areas where the evacuation of combustion products and their replacement by new fuel mixture are incomplete and where the fuel mixture ignites more difficult due to its dilution. In other words, the trapping of combustion products in the antechamber makes the occurrence of sliding sparks more critical in prechamber ignition systems than in the case of conventional candles.

  The object of the invention is at least to enable a reduction of the ignition defects in the prechamber chambers of prechamber ignition devices.

  For this purpose, the subject of the invention is an ignition system for an internal combustion engine, this ignition system comprising at least one igniter, a prechamber and at least one hole adapted to put in communication this prechamber and a compression and combustion chamber of the engine, characterized in that the igniter is configured to produce a breakdown discharge in the prechamber.

  The concept of breakdown of an insulator corresponds to a well-known notion in static electricity, and corresponds to the discharge that occurs when an insulator such as air is subjected to an electric field greater than a threshold of the order 30 kV / cm. A first electron is accelerated by the electric field, hits an atom or a molecule, then tearing a new electron, into an avalanche phenomenon until the multiplication of the ionized species leads to the formation of a conductive plasma. If the phenomenon occurs at the end of a tip, there is a so-called corona discharge (also called corona discharge).

  A peculiarity of these breakdown discharges is that they occur over periods of time of the order of a few nanoseconds, an order of magnitude of about 100 compared to the sparks generated by traditional candles.

  According to other advantageous features of this ignition system: the discharge is produced for a time less than 10 ns; the discharge is produced for a time less than the time required for the formation of an electric arc; - The igniter power supply circuit is configured to apply to the igniter a voltage rise having a duration less than, preferably of the order of 100 ns; - The igniter power supply circuit is configured to apply to the igniter a voltage greater than, preferably greater than 30 kV. Note however that this voltage remains of the same order of magnitude as that of a standard ignition, without the need for modification of the electrical circuit of the vehicle; - The igniter power supply circuit is configured to apply to the igniter a voltage in the form of pulses; it comprises several holes adapted to put in communication the prechamber and the compression and combustion chamber of the engine; the igniter comprises a first electrode placed in the prechamber and a second electrode which surrounds at least a portion of the first electrode; the first electrode terminates in several points located inside the prechamber; the second electrode forms at least a portion of the peripheral wall of the prechamber; it comprises an electrical supply circuit of the igniter, this circuit being configured so as to apply to the igniter a voltage with a duration of discharge less than the time necessary for the formation of an electric arc between the first and second electrodes; - It comprises a body which is provided with means for mounting a sealed seal in a motor hole and which is extended by a wall at least partially defining the prechamber, the first and second electrodes being configured to produce the corona discharge at the level of a half before the prechamber, that is to say half of that body.

  The invention also relates to an internal combustion engine, comprising at least: - a combustion chamber and combustion of a fuel mixture; an admission system for at least one fuel and an oxidant of the fuel mixture in the compression and combustion chamber, and a system for discharging combustion products out of the compression and combustion chamber, characterized in that it is provided with a system, as defined above, for igniting the fuel mixture in the compression and combustion chamber.

  According to other advantageous characteristics of this engine: - it is of the direct fuel injection type in the compression and combustion chamber; - it is fed at least partly with recycled gases from the exhaust; - The ignition system comprises a hollow head which at least partially delimits the prechamber and which is pierced with said hole, at least a portion of the fuel injected into the compression and combustion chamber describing a path on which this hollow head is disposed of. to be wetted by this fuel when it is directly injected into the compression and combustion chamber.

  The subject of the invention is also a method of igniting an internal combustion engine as defined above, this method comprising steps in which: a) fuel and oxidant are introduced into the compression chamber and compression, then b) compressing the combustible mixture of fuel and oxidizer in the combustion chamber, then c) in the prechamber, is made to ignite the ignition a breakdown by ignition that causes ignition of the fuel mixture from the prechamber, into the compression and combustion chamber.

  According to other advantageous features of this process: the discharge is produced by applying a rise in voltage over a period of less than 100 ns; the discharge is produced by applying a voltage greater than on the order of 30 to 40 kV; the discharge is produced by applying a voltage in the form of pulses.

  The invention furthermore relates to a method for igniting an internal combustion engine according to which: a) fuel and oxidant are introduced into a compression and compression chamber, and b) the mixture is ignited in a combustion chamber. prechamber with an ignition system as defined above.

  In this process, the mixture advantageously comprises recycled gases.

  The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings, among which: FIG. 1 is a diagram of an internal combustion engine according to a first embodiment of the invention and a system for igniting the fuel mixture admitted into a compression and combustion chamber of this engine; FIG. 2 is a diagrammatic and axial section of an ignition device of the ignition system shown in FIG. 1; - Figure 3 is a schematic and partial section, along a longitudinal plane, of a combustion chamber and combustion engine shown in Figure 1 and illustrates a step of the operation of the engine; - Figure 4 is a section similar to Figure 3 and illustrates another step of the operation of the motor of Figures 1 and 3; FIG. 5 represents a head of the ignition device of FIG. 2 and illustrates the initiation of the combustion in the compression and combustion chamber, through orifices of this head, which is seen from the compression chamber and combustion; - Figure 6 is a section similar to Figure 2 and shows an ignition device according to a second embodiment of the invention; and - Figure 7 is a view similar to Figure 5 and shows the end of a head of an ignition device according to a third embodiment of the invention.

  FIG. 1 is a schematic representation of a spark ignition internal combustion engine 1, of which only one of several substantially identical compression and combustion chambers is visible and referenced 2. A system 3 for igniting a fuel mixture admitted into these compression and combustion chambers comprises several ignition devices and a circuit 4 for supplying electrical power to these ignition devices, each of which equips one of the compression and combustion chambers and only one of which is visible and referenced 5 in Figure 1.

  The engine 1 is known per se, except that it is provided with the ignition system 3, so that only this system 3 is described in detail in the following. The engine 1 comprises a plurality of cylinders, in each of which slides a piston connected by a connecting rod to a common crankshaft 6. In FIG. 1, only the cylinder 7 and the piston 8 which jointly delimit the compression and combustion chamber 2 are visible. Also, only one, referenced 9, connecting rods of the engine 1 is shown in Figure 1, for the sake of clarity. Opposite the pistons, a cylinder head 10 closes the cylinders of the engine 1.

  As can be seen in Figure 2, the ignition device 5 comprises a hollow body 11 which is made of an electrically conductive material, such as a metal or a metal alloy. This body 11 has a proximal portion 12, which is threaded and which, for this reason, can be screwed tightly into a threaded through hole 13 pierced in the cylinder head 10. The device 5 has the advantage that it can be configured from such that the tapped hole 13 may have the characteristics of the holes in which the conventional spark plugs are traditionally mounted. A distal portion 14 of the body 11 extends the proximal portion 12 and forms a coupling head of a screwing tool of the portion 12 in the cylinder head 10. In the example shown, the distal portion 14 has a cross sectional shape hexagonal, so that you can put a key into it.

  The body 11 is substantially symmetrical with respect to a central axis X-X ', in which there is provided a longitudinal and through hole 15 for the passage of a central electrode 16. In this hole 15, a portion of the electrode 16 is embedded in a ceramic 17, which secures the electrode 16 to the body 11 while electrically insulating it from the body 11. A dielectric material other than the ceramic 17 may be used instead of or in addition to the latter.

  Opposite its distal portion 14, the body 11 is capped with a hollow head 18, provided to be inside the compression and combustion chamber 2. This head 18 and the ceramic 17 together define a pre-chamber 19, that several holes 20 and 21 pierced in the head 18 put in communication with the chamber 2. Preferably, it is provided between one and twenty holes 20 and 21. An end portion 22 of the central electrode 16 emerges ceramic 17 and enters the prechamber 19, where it is protected by the head 18.

  In the case represented here, the prechamber 19 is substantially centered on the central axis X-X ', which extends at least the end portion 22 of the electrode 16. In addition, it partially delimits the prechamber 19 , the head 18 forms an electrode which surrounds the end portion 22 so that a corona discharge can take place in the antechamber 19, between this head 18 and the central electrode 16. The head 18, with which the electrode 16 forms an igniter, thus fulfills two functions, which is advantageous. It is electrically connected to the body 11 which, once screwed into the cylinder head 10, is electrically connected to the latter. In order to promote the appearance of a filament discharge, the corona discharge of which is a special case, the end portion 22 has the shape of a tapered tip, whose radius of curvature is less than 1 mm.

  The distance d between the end portion 22 and the head 18 is advantageously greater than 1 mm.

  It follows from the above that the ignition device has a simple structure so that it can easily be robust and its manufacturing cost can be low.

  In addition, the end portion 22 may be short, so that it is easy to adapt the thermal index of the igniter 5 relative to the engine. The thermal index is an indicator of the ability of the candle to remove heat to the cylinder head, and thus to maintain a certain temperature at the electrodes.

  The circuit 4 comprises a transformer 25 whose function is to raise a supply voltage V. At the output terminals of this transformer 25 is connected a capacitor 26, a terminal of which is connected to ground. A spark gap 27 is connected between the other terminal of this capacitor 26 and the electrode 16. The head 18, which forms the other electrode of the ignition device 5, is connected by the body 11 to the cylinder head 10, which is to the mass.

  In operation, the transformer 25 charges the capacitor 26, the spark gap 27 serves to discharge only when the voltage across the capacitor 26 is large enough. During the discharge of the capacitor 26 via the spark gap 27, a corona discharge takes place in the antechamber 19, between the tapered and pointed end 22 of the central electrode 16 and the head 18.

  The circuit 4 applies a pulsed pulsed high voltage between the electrodes 16 and 18 of the device 5. The rise in the voltage between these electrodes is very fast, since it takes place in a time of less than 100 ns and, for example, of the order of 10 ns. At its maximum, the voltage applied between the electrodes 16 and 18 of the device 5 is very high since it can be greater than 40 kV and, for example, of the order of 30 kV, while being able to reach 100 kV.

  The duration of the pulse, that is to say the discharge of the capacitor 26 via the spark gap 27 and the device 5, is preferably less than 100 ns. In any case, it is less than the time required for the formation of an electric arc between the electrodes 16 and 18.

  Advantageously, several corona discharges take place during a motor cycle. When this is the case, the total energy input is increased, while avoiding the formation of an electric arc. The frequency with which the corona discharge takes place can, for example, reach 1000 Hz.

  The electrical energy supplied to the device 5 at each engine cycle can be of the same order of magnitude as that provided by a conventional candle, which is advantageous. For example, it can be of the order of 30 mJ.

  In the example shown, the engine 1 is a direct injection of the fuel contained in the fuel mixture. It therefore comprises an injector, which is visible and refers to FIGS. 3 and 4. In FIG. 3, fuel 31 is injected into the chamber 2 by the injector 30. The head 18 is on the path of a part of the fuel injected, which so wet. In FIG. 4, fuel 31 previously injected is present on the head 18 and in a recess at the end of the piston 8. It vaporises because of the high temperature of the engine 1. An oxidizer, such as air atmospheric, is also admitted in the chamber 2, where he and the fuel 31 mix and form the fuel mixture. During the compression phase by the piston 8, part of this fuel mixture enters the prechamber 19.

  Then, a corona discharge takes place in this prechamber 19, where it ignites the fuel mixture. Unstable species result from this inflammation. Part of them ignites the fuel mixture present in the chamber 2, having migrated to this chamber 2 through the holes 20 and 21 which, however, have stopped the flame front in the prechamber 19. For this purpose, each these holes 20 and 21a, preferably a smaller diameter or of the order of 1 mm and preferably between 0.5 mm and 1 mm. For the same purpose, each of the holes 20 and 21 preferably has a length less than its diameter.

  Figure 5 illustrates the initiation and progression of combustion in chamber 2, in which unstable species from prechamber 19 enter different points. The probability of very favorable ignition conditions at at least one of these points is high. The ignition of the fuel mixture in the chamber 2 occurs substantially simultaneously at several points, so that the progression of this combustion is then very fast and stable. Among the advantages resulting from the use of the ignition device 5, there will therefore be a gain on the total duration of the combustion in the chamber 2. This gain can notably improve the idling stability of direct injection engines.

  In addition, the time between the end of the combustion and the opening of the exhaust valves being longer, the combustion products are less hot from the chamber 2, which is advantageous. In particular, it is possible to reduce the fuel enrichment of the fuel mixture, when this enrichment has the function of producing a cooling by evaporation. However, lower enrichment results in lower consumption and a reduction of pollutants emitted.

  It has been found that the corona discharge produces a safe and robust ignition of the mixture present in prechamber 19, in accordance with the object of the invention. In particular, this ignition is insensitive to a depletion of fuel and / or oxidant because of the residual presence of combustion products and takes place even if the fuel and / or the oxidant present in the prechamber 19 are highly diluted.

  Moreover, even if, in the antechamber 19, there is a fouling capable of deflecting a spark and which may result from the deposition of carbonaceous material, it is very unlikely that this will lead to the corona discharge not crossing an area where the fuel mixture is sufficiently undiluted to ignite easily. In other words, the ignition device 5 is reliable, even in case of fouling. The fouling and the wear of this device 5 are also reduced, because of the quality of the combustion of the mixture in the prechamber 19.

  The structure of the ignition device 5 offers a great possibility of choice as to the position of the corona discharge inside the prechamber. In particular, it is easy to ensure that the corona discharge takes place at one half before the prechamber 19, that is to say at the level of the half adjacent to the body 11 and to the ceramic 17, where the rate of residual combustion products is low.

  In summary, the ignition defects found with the system 3 are rare or nonexistent.

  The duration of a corona discharge in the prechamber 19 can be much lower than that of the combustion of the fuel mixture in the chamber 2. Thanks to this, it is possible to reduce the cyclic variations within the engine 1, which is advantageous. . Indeed, for a given advance, the time between the command and the actual ignition is shorter in the case of a discharge mode as in the invention. Fluctuations (in time) are eliminated or at least reduced by the same factor (if identical as a% of the initiation delay), which reduces cyclical fluctuations accordingly.

  The use of a corona discharge in the prechamber 19 can improve the plasma-flame transition. When this is the case, the energy efficiency and thus the initial speed of ejection of the gases out of the prechamber 19 are increased. The energy efficiency is at least two times higher for a breakdown type discharge (quality of the conversion between electrical energy and energy for ignition).

  The head 18 protects the electrode 16 against contamination by liquid fuel 31 injected into the chamber 2. The inner wall of the head 18 is further protected from such contamination. This results in less fouling of the ignition device 5 and an improvement of the cold start of the engine 1.

  In the case of stratified combustion, the injection of fuel on the hot head 18 makes it possible to increase the robustness of the advance ratio to the injection on ignition advance, by a vaporization effect of the fuel near the ignition. igniter.

  The improvement of the combustion by the use of the ignition system 3 makes it possible to improve the permeability at the level of the cylinder head 10 by degrading the aerodynamics of the chamber 2, that is to say by accepting that the flows within this chamber 2 are less optimized.

  Tests have shown that the use of the ignition device 5 has made it possible to reduce the rattling phenomenon encountered with direct injection engines.

  FIG. 6 shows an ignition device 105 according to a second embodiment of the invention. In what follows, only what distinguishes this device 105 from the ignition device 5 will be described. In addition, a reference used hereinafter to designate a part of the ignition device 105 similar or equivalent to a referenced part of the device 5 will be constructed by increasing by 100 the reference identifying this part on igniter 5.

  The electrode 116 ends not only with one but with several end portions 122, each of which has substantially the same tapered tip shape as the portion 22 of the electrode 16. In the example shown, the portions of FIG. end 122 are three in number.

  In Figure 7 is shown a head 218 of an ignition device according to a third embodiment of the invention. This head 218 is similar to the head 18 except that there is drilled a through hole 220A large enough to pass a flame front from the prechamber. Preferably, this hole 220A has a diameter greater than 1 mm and less than 3 mm. Other holes 220B are also drilled in the head 218. They have the same dimensions as the holes 20 of the head 18. The holes 220A and 220B therefore have diameters of between 0.5 and 3 mm. Advantageously, there is between one and twenty holes 220B. In the example shown, there is only one hole 220A, which is the preferred solution. However, there can also be several. In any case, it is preferred that there be between one and five holes 220A.

  The invention is not limited to the embodiments described above. In particular, it can be incorporated in conventional engines or engines with or without supercharging, while being particularly interesting in the case of engines operating with a lean mixture, such as direct injection engines. The invention can also be used in diesel engines and in homogeneous diesel combustion engines, whether a supercharging is provided or not. In addition, the body 11 and the head 18 or 218 can be in one piece.

Claims (17)

  An ignition system of an internal combustion engine (1), said ignition system comprising at least one igniter (16, 18; 116; 218), a prechamber (19) and at least one hole (20, 21; 121; 220A, 2208) adapted to put in communication this prechamber (19) and a compression and combustion chamber (2) of the engine (1), characterized in that the igniter (16, 18; 116; 218 ) is configured to produce a breakdown discharge in the prechamber (19).   2. Ignition system according to claim 1, characterized in that the discharge is produced for a time less than 10 ns.   3. Ignition system according to one of the preceding claims, characterized in that the discharge is produced for a time less than the time required for the formation of an electric arc.   Ignition system according to one of the preceding claims, characterized in that the igniter power supply circuit is configured to apply to the igniter a voltage rise having a duration of less than 100 ns.   Ignition system according to one of the preceding claims, characterized in that the igniter power supply circuit is configured to apply to the igniter a voltage greater than 30 kV.   Ignition system according to one of the preceding claims, characterized in that the igniter power supply circuit is configured to supply the igniter with pulse voltage.   7. Ignition system according to one of the preceding claims, characterized in that it comprises a plurality of holes (20, 21; 120, 121; 220A, 220B) adapted to put in communication the prechamber (19) and the chamber of compression and combustion (2) of the engine (1).   Ignition system according to one of the preceding claims, characterized in that the igniter comprises a first electrode (16; 116) placed in the antechamber (19) and a second electrode (18; at least one portion (22; 122) of the first electrode (16; 116).   9. Ignition system according to claim 8, characterized in that the first electrode (116) terminates in several points (122) located inside the prechamber.   10. Ignition system according to claim 8 or claim 9, characterized in that the second electrode (18; 218) forms at least a portion of the peripheral wall of the antechamber (19).   11. Ignition system according to any one of claims 8 to 10 characterized in that it comprises a circuit (4) for power supply of the igniter (16, 18; 116; 218), this circuit (4 ) being configured to apply to the igniter a voltage with a discharge duration shorter than the time required to form an electric arc between the first and second electrodes (16, 18; 116, 218).   12. Ignition system according to any one of claims 8 to 10 characterized in that it comprises a body (11) which is provided with means (12, 5 14) for mounting a seal in a hole (13). ) of the motor (1) and which is extended by a wall (18) defining at least partially the prechamber (19), the first and second electrodes (16, 18; 1 16; 218) being configured to produce the corona discharge at the half a front of the prechamber (19), that is to say half of the adjoining body (11).   13. Internal combustion engine, comprising at least: - a combustion chamber and combustion chamber (2) of a fuel mixture, -an admission system (30) of at least one fuel (31) and a fuel oxidant of the combustible mixture in the compression and combustion chamber (2), and - a combustion product evacuation system out of the compression and combustion chamber (2), characterized in that it is provided with a system (3) according to any one of the preceding claims for igniting the fuel mixture in the compression and combustion chamber (2).   14. Internal combustion engine according to claim 13, characterized in that it is of the direct fuel injection type (31) in the compression chamber and combustion (2).   15. Internal combustion engine according to claim 14, characterized in that the ignition system (3) comprises a hollow head (18; 218) which at least partially delimits the antechamber (19) and which is pierced with said hole (20). , 21; 120, 121; 220A, 220B), at least a portion of the fuel (31) injected into the compression and combustion chamber (2) describing a path on which said hollow head (18; 218) is disposed to be wetted by this fuel (31) during its direct injection into the compression chamber and combustion (2).   16. A method of igniting an internal combustion engine according to which: a) fuel (31) and oxidant are introduced into a compression and compression chamber (2), and (b) the mixture is ignited in a prechamber with a system   ignition device according to one of claims 1 to 12.   17. The ignition method according to claim 16, characterized in that the mixture comprises recycled gases.