DE19714796A1 - Procedure for igniting lean air/fuel mixture thinned with air or exhaust gas - Google Patents

Abstract

A procedure for igniting lean air/fuel mixtures thinned with air or exhaust gas has enrichment of the mixture with fuel in the region of the spark plug electrodes. In a process stage introduced to the spark ignition, additional fuel is transported and sprayed on the surface of a small trough in the piston and its surrounding on the piston base and/or in the form of enriched fuel. After at least partial evaporation of the fuel still deposited on the walls and further mixing with the existing enriched mixture, after immersion of a spark ignition device with electrode into the piston trough, the additional fuel is ignited.

Description

The invention relates to a method for igniting very lean fuel-air mixtures in spark ignition engines with enrichment of the mixture with fuel in the area the ignition electrodes and also various suitable ones for carrying out the method Devices with spark ignition devices and in particular prechamber spark ignition devices with which, on the one hand, a particularly extensive mixture enrichment at the ignition point, and on the other hand an improved reproducibility of the mixture composition and Ho mogenization in the area of ignition spark is achieved.

For igniting a largely homogeneous mixture with very large dilution with air and / or exhaust gas in gasoline engines, for example from DE-A 26 11 057 and DE-A 37 09 976 known, additional fuel or a rich fuel-air mixture as a so-called layer load into an antechamber. These procedures require additional fuel Delivery system or mixture formation system with a complicated valve control. After Part of this process is also the high thermal load of the ver with the prechamber bound and controlled by the chamber pressure valve, because very high tem temperatures occur. Another disadvantage is that the mixing of the in the occurrence mer entered additional fuel or mixture only insufficiently and too slowly instead takes place so that there are fluctuating flames and thus fluctuating energy conversions comes in the main combustion chamber.

Another method with significantly improved ignition conditions is stratified charge proceed with direct fuel injection, as it is e.g. B. in DE-A 44 15 073 and EP-A 0 639 703 is described. In order to achieve the lowest possible fuel consumption, this Ver drive when the engine is operating in the partial load range, all fuel immediately before ignition injected at the time so that there is a particularly well ignitable force at the time of ignition substance-air mixture at the ignition point. This method has the disadvantage that special result in high NOx emissions and HC emissions, with both pollutants so far only can be reduced unsatisfactorily by a catalytic exhaust gas aftertreatment, since the previously developed catalysts are not yet effective enough (NOx) or the exhaust gas temperature doors are too low (HC).

By early direct injection of the bulk of the fuel during intake gangs for the combustion air, can be a lean and largely with the direct injection homogeneous mixture can be generated. In connection with another fuel injection during the compression stroke it is possible to be sure at the ignition point at the ignition point to obtain an ignitable mixture. If the fuel jet is directed directly onto the spark plug tet, then there are very high demands on the injection system with regard to the injection jet picture, fuel preparation, injection time and injection duration. So far, pollution of the Injection nozzle and thus a change in the spray pattern in long-term operation can not be prevented. There is also a risk of sooting the spark plug. Another disadvantage is that after Spark ignition of the "mixture cloud" generated by the injection jet only delays it can burn, since an acceleration of the combustion within the mixture cloud only with one increased charge turbulence in the combustion chamber can be achieved. However, such a measure leads to a reduction in the mixture enrichment in the spark area, which can only be achieved by adding a increased amount of fuel can be compensated. But this in turn leads to increased NOx emissions from the engine.  

In order to avoid these problems, EP-A 0463 613 proposed that the fuel be used to inject a semi-spherical piston recess upstream of the ignition point and with the help of Charge movement to cause the fuel to evaporate toward a spark plug the second hemispherical, downstream of the large hemispherical trough, for immersed large trough open small trough at the time of ignition. Even with this ver will drive a larger amount of fuel to ensure a satisfactory ignition needed, since only a small part of the injected fuel covers the closer area around the ignition reached candle electrodes, so that even with this method, an unsatisfactorily high NOx emission from the engine results. In addition, the speed of propagation is that after ignition resulting flame front is very low due to the low charge movement. This gives there is an unsatisfactory mixture preparation at the ignition point with the risk of soot formation. Furthermore, there is a because of the piston recesses arranged on one side in the piston of the engine asymmetrical mechanical and thermal load on the piston.

In contrast, the object of the present invention is to record a method gene and to create suitable ignition devices for carrying out the method by or which makes it possible to use a particularly small amount of fuel and nevertheless a safe ignition by a mixture enrichment at the ignition point and a fast and even combustion of the entire combustion chamber, on average very much lean mixture, to achieve a very smooth running and a low NOx emission of the engine is reached. In addition, this creates a catalytic exhaust gas aftertreatment sufficiently high exhaust gas temperature. Other tasks are: The reduction of thermal Load the ignition device, a simple, inexpensive and in terms of accuracy requirements Robust control of the fuel supply and the ignition angle as well as the achievement symmetri shear ignition and combustion conditions, which reduces the thermal load of the piston and the tendency of a motor to knock.

The solution to the problem is a method for igniting very lean fuel-air Ge mix with enrichment of the mixture with fuel in the area of the ignition electrodes. That invented The method according to the invention is characterized in that upstream of the spark ignition Process stage additional fuel on the surface of a small piston bowl and its vice Exercise on the piston crown and / or in the form of a fuel-enriched mixture in the Piston recess or in its vicinity is transported or sprayed and after at least part wise evaporation of the fuel still attached to the wall and further mixing with the existing, fuel-enriched mixture after immersion of a spark ignition device with an ignition electrode in the piston recess.

The local enrichment of the very lean and largely homogeneous in the combustion chamber NEN basic mixture with fuel is either by targeted control of the intake manifold spraying (late injection of a partial quantity into the open inlet valve) or by direct injection injection of a small amount of fuel into the combustion chamber towards the piston bowl Lich generated before the ignition point. During the further compression process the evaporates partially still wall-mounted fuel as a result of the hot piston surface, one due to the increase in pressure in the combustion chamber, large parts of the air in the area of the bowl, fuel-enriched mixture is transported into the piston bowl.

A spark ignition device dips into the piston recess before the ignition point is reached a, at the time of ignition in the area of the ignition spark compared to the very lean Basic mixture (with e.g. 70% excess air) a mixture significantly enriched with fuel and thus better ignitable mixture with z. B. stoichiometric composition at the ignition job available. By temporarily storing the fuel in the piston bowl and its proximity An extensive homogenization of the mixture at the ignition point is achieved in the environment.  

The invention further relates to various devices suitable for carrying out the method with spark ignition devices and in particular prechamber spark ignition devices, with on the one hand a particularly extensive mixture enrichment at the ignition point, and others on the other hand, an improved reproducibility of the mixture composition and homogenization is achieved in the area of the ignition spark. At the same time, by the prechamber ignition device created a particularly quick and even combustion of the ignition torches particularly lean fuel-air mixture can be achieved with low pollutant emissions.

The invention is explained in more detail below, including the accompanying subclaims.

The advantages of the method come into play particularly when generating the Use a direct fuel injection in the fuel-enriched mixture in the piston recess is detected, in which a slim injection jet is directed directly at the piston bowl.

An additional advantage in terms of fuel consumption and engine performance is given if the direct injection simultaneously to produce a largely homogeneous mixture the intake stroke of the engine is used, d. H. if the injection system to generate a "early" and a "late" injection is used.

For spark ignition of the mixture in the piston bowl, you can use a conventional spark plug at least one finger-shaped ground electrode, an arrangement can also be selected in which the ground electrode is embedded in the piston in the form of a pin. In the latter variant ent the influence of the mass electrode that disrupts combustion in the initial phase falls. For ignition dung can of course also another spark ignition device without disruptive influence of Ground electrode are used, such as. B. a sliding spark ignition device, in which the ignition spark is generated on the ceramic stone base of the spark plug.

Special advantages in terms of fuel processing at low piston temperatures surface and / of the fuel-air mixture result when an occurrence occurs for ignition mer spark ignition device is used. Here, the ignition point is within one with play adapted to the piston bowl, essentially cylindrical prechamber. When on immersing the prechamber in the piston bowl develops between it and the outer chamber turned very high flow velocities. This will possibly be on the surface the remaining part of the liquid force remaining before the prechamber is immersed material removed and atomized.

Another advantage of the prechamber spark ignition device is that after ignition of the in the mixture located ignition flare rays are generated, which radially from the Spread the antechamber into the further combustion chamber. This process leads to a special one uniform and rapid combustion of the remaining very lean fuel-air mixture and low NOx emissions.

There are particular advantages if the antechamber is designed as a swirl chamber, in which the mixture is wholly or largely via tangential overflow channels into the swirl chamber. In this for the ignition of a homogeneous mixture be known process arises from the currents rotating around the main axis in the swirl chamber centrifugal force, which leads to a mixture enrichment with fuel in the area of the inside wall of the vortex chamber leads spark ignition sections.

With the proportion of fuel pumped from the piston bowl into the swirl chamber enriched mixture are in the front area of the vortex chamber to the outside walls longitudinal grooves arranged from the front of the vortex chamber to the overflow  channels are enough. In addition, it is very advantageous in the central front area of the vertebrae chamber to arrange a hole through which the fuel-enriched mixture from the lower area of the piston bowl centrally towards the cross-section with the igniters electrodes is steered. By both measures, it is possible to charge in the Keep the amount of fuel required in the area of the piston bowl low.

As a result of the vortex flow in the vortex chamber, there is a rapid energy transfer settlement. Accordingly, ignition torches are generated with a high exit speed. Total together with this, the compatibility with larger changes in the air ratio at the ignition point is very high Well. Even with large air ratios or in connection with an additional exhaust gas recirculation a particularly good uniformity in energy conversion. This also applies to use an extremely late ignition point, which is used to generate a particularly high exhaust gas temperature may be necessary during warm-up in order to start the catalytic converter particularly quickly to reach after the cold start.

The following are exemplary embodiments and important features of the invention with reference to the drawing described. These show in

Fig. 1 shows an arrangement for the ignition with conventional spark ignition device with a particularly long electrode at the time of fuel injection,

Fig. 2 shows the arrangement for the ignition with conventional spark ignition device with a particularly long electrode of Fig. 1 in the piston top dead center (OT) immediately after the initiation of the ignition,

Fig. 3 shows an arrangement for ignition with a piston bowl disposed in the metal pin, which serves as a ground electrode,

FIG. 4a shows an arrangement with a cylindrical pre-chamber spark ignition device with ignition in the central area of the prechamber,

FIG. 4b shows a section through the arrangement of a cylindrical pre-chamber spark ignition device with ignition in the central area of the pre-chamber of FIG. 4a,

FIG. 5a, an arrangement with a cylindrical swirl chamber with spark ignition multiple of the vortex chamber ignition in the wall area with tangentially extending transfer ducts,

5b shows a section through the arrangement with a cylindrical vortex chamber-Funkenzündeinrich device with multiple ignition in the wall region of the vortex chamber with tangential overflow channels according to FIG. 5a.

In Fig. 1 an exemplary arrangement for the method for igniting very lean fuel-air mixtures is shown, in which a spark ignition device with particularly long elec trodes ( 1 ) is used, which is essentially central to one through the cylinder head ( 2 ) , The piston ( 3 ) and the cylinder wall ( 4 ) formed combustion chamber of the engine is arranged. The Kol ben ( 3 ) is shown in a position during the compression process, in which the fuel is injected from an injection nozzle ( 5 ) substantially in the direction of the piston bowl ( 6 ). That can e.g. B. depending on the combustion process, valve arrangement, fuel jet angle, etc. at a crank angle of the engine between 40 and 80 ° KWv OT and z. B. pull over 15 ° KW.

Within the limits of the fuel jet, the fuel density distribution is generally to be described approximately by a normal distribution ( 8 ), particularly large fuel drops being located in the core area of the fuel jet. In connection with a short injection duration z. B. 0.5-1.0 ms can be ensured that up to the high speed range, the piston passes through the injection process only a small stroke, so that most of the fuel within the piston bowl or in the immediate vicinity of the bowl be attached to the wall or are in the piston bowl as a fuel-enriched mixture or the piston bowl are stored in front.

During the further compression process, i.e. H. the further compression of the fuel-air mixture mixes in the combustion chamber, it heats up on the piston surface inside and outside the piston trough wall-mounted fuel and evaporates. There is also a pronounced charge movement out of the main combustion chamber towards the bottom of the bowl, causing the The majority of the upstream fuel components are also transported into the piston recess will.

In order to increase the proportion of fuel entering the piston bowl through the fuel injection, the piston bowl can have a bevel ( 9 ) at the upper edge in the direction of injection of the fuel.

FIG. 2 shows the arrangement according to FIG. 1 at top dead center (TDC) of the piston immediately after the ignition has been initiated. As a result of the rapid piston movement in the direction of the top dead center of the piston (TDC) immediately before the ignition, there is an intensive charge movement in the piston recess, but a mass exchange with the main combustion chamber is largely suppressed, so that in the area of the ignition electrodes Ignition point has formed a comparatively homogeneous and fuel-enriched mixture compared to the mixture composition in the main combustion chamber.

In the further example shown in FIG. 3, a metal pin arranged centrally in the piston recess serves as the ground electrode for the spark ignition device. With this arrangement, the electrodes have a particularly low disruptive effect on the flame propagation. The arrangement shown in the TDC position of the piston leads to a variable and, on average, particularly large electrode spacing, depending on the ignition angle as a function of the engine operating point. By choosing an adapted ignition system with a high voltage supply and low sensitivity to shunts, particularly good ignition properties are achieved.

In Fig. 4a the cut is indicated by a pre-chamber spark ignition device with ignition in the central area of the antechamber in the TDC position of the piston. When the cylindrical ignition chamber penetrates into the piston bowl, the fuel-enriched mixture in the piston bowl and in particular in the bottom area of the piston bowl is pressed through the openings ( 13 ) in the floor into the prechamber and into the area of the ignition point ( 12 ) directed. Any liquid fuel that may still be on the walls of the piston bowl is removed and atomized by the high flow velocity between the prechamber and the piston bowl, this mixture then being directed either into the ignition chamber or laterally past the ignition chamber. Then a part of the laterally escaping fuel-enriched mixture still reaches the prechamber via the radial bores of the ignition chamber ( 14 ).

To improve the ignition of the mixture in the prechamber, the ignition electrodes ( 15 , 16 ) can be thinned in the front area and z. B. consist of precious metal or be designed as a noble metal coated electrodes.

After the ignition has been initiated, ignition flares are produced radially from the ignition chamber via the bore ( 14 ), which cause the mixture in the main combustion chamber to be burned quickly and uniformly. At the same time, unburned and subsequently burned, fuel-enriched mixture emerges through the openings ( 13 ) in the base area of the ignition chamber and first presses the unburned and later the burned mixture through the annular gap between the ignition chamber and the piston recess. This mixture and the fuel-enriched mixture accumulated in the vicinity of the prechamber during the compression stroke, as well as the large charge movement, result in intensive combustion in the further environment of the prechamber, which also contributes to a safe and uniform ignition of the very lean To reach fuel-air mixture in the wider environment of the ignition chamber.

The combustion gases escaping from the front of the ignition chamber also have an effect on low Rige piston temperatures, such as. B. during engine warm-up, one complete combustion of all possible before starting the ignition in the piston bowl liquid fuel residues still present. At the same time there is rapid warming the piston bowl, so that very early after a cold start of the engine an extremely lean Engine operation can be performed.

Fig. 4b shows a section through the arrangement according to 4a at the level of the radial bores ( 14 ).

Fig. 5a shows a further variant of a prechamber spark ignition device in the form of a wirbelkammer spark ignition device with which by generating a swirl flow through tan gential arranged overflow channels ( 17 ) and use of a multiple center electrode device ( 18 ) with which several spark gaps ( 19 ) for the essentially cylindrical inner wall of the swirl chamber ( 20 ), particularly lean mixtures can be ignited safely. This makes it possible to make larger fluctuations in the air ratio at the ignition point, as z. B. may occur in connection with unsteady driving conditions to control safely.

The mechanism of the vortex chamber ignition is u. a. in EP-A 0 675 272.

In order to achieve a particularly extensive mixture enrichment at the ignition point and on the other hand to achieve a particularly good reproducibility of the mixture composition and homogenization in the area of the ignition spark, the central chamber ( 21 ) from the bottom area of the pistons is used to immerse the ignition chamber in the piston recess well ( 22 ) fuel-enriched mixture directed into the central area of the cross section in the area of the ignition electrodes. On the other hand, via the tangential overflow channels ( 17 ), fuel-enriched mixture from the bottom and side region of the piston bowl is directed tangentially into the swirl chamber, this mixture rotating about the main axis of the swirl chamber and the central one, formed by the inlet openings ( 21 ) Beam surrounds. This results in a mixture that is particularly well enriched with fuel in the entire cross section of the ignition level.

In order to enrich the mixture flowing over the tangentially arranged overflow channels ( 17 ) with fuel to a particularly large extent, grooves ( 23 ) are arranged on the outside of the lower region of the swirl chamber, which channels the enriched mixture from the bottom region during the penetration of the ignition chamber into the Steer the piston bowl in front of the overflow channels ( 17 ).

FIG. 5b shows a section of the arrangement according to FIG. 5a at the level of the tangential overflow channels.

Claims (25)

1. A process for the ignition of very lean, air and / or exhaust gas-air mixtures with enrichment of the Ge mixture with fuel in the area of the ignition electrodes, characterized in that benmulde upstream of the spark ignition process stage additional fuel on the surface of a small Kol and their surroundings are transported or sprayed onto the piston crown and / or in the form of a mixture enriched with fuel into the piston recess or in the vicinity thereof, and at least partial evaporation of the fuel still attached to the wall and further mixing with the existing fuel-enriched mixture Immersion of a spark ignition device with ignition electrode is ignited in the piston recess. 2. The method according to claim 1, characterized in that in and around the small piston bowl wall-mounted fuel and the mixture enrichment in and around generated by a partial injection of fuel through the open intake valve be, the transport of the fuel or the enriched mixture in particular by a targeted charge movement is generated. 3. The method according to claim 1, characterized in that the in and around the small piston bowl wall-mounted fuel and the mixture enrichment in and around of the piston bowl are generated by direct fuel injection. 4. The method according to claim 3, characterized in that the in and around the small piston bowl accumulated fuel and the mixture enrichment in and around the piston bowl by at least twice direct fuel injection during a work cycle of the engine are generated. 5. The method according to claim 3 or 4, characterized in that in and in the area the small piston bowl accumulated fuel and the mixture enrichment in and around Exercise the piston bowl by a main injection, which at least during the intake stroke of the Piston is started, and by injecting a small amount of fuel during the Ver sealing strokes are generated. 6. The method according to any one of claims 3 to 5, characterized in that the in and in the Fuel accumulated around the small piston bowl and the mixture enrichment in and in the vicinity of the piston bowl mainly by injecting a small amount of force fabric are generated during the compression phase. 7. The method according to any one of claims 3 to 6, characterized in that the in and in the Fuel accumulated around the small piston bowl and the mixture enrichment in and essentially in the vicinity of the piston bowl by injecting a small amount of fuel Lichen before starting the ignition in an angular range between 80 and 40 ° before the upper one Dead center (OT) are generated. 8. The method according to any one of claims 3 to 7, characterized in that the in and in the Fuel accumulated around the small piston bowl and the mixture enrichment in and in the vicinity of the piston bowl mainly by injecting a small amount of force be generated that is less than 10% of the total amount of fuel injected.   9. Ignition device for carrying out according to one of claims 1 to 8, characterized in that the piston bowl is dimensioned so that essentially only the Spark ignition device with sufficient play to the walls and the bottom of the piston bowl fits into it. 10 Ignition device according to claim 9, characterized in that the piston bowl in essence Lichen has a cylindrical shape. 11. Ignition device according to claim 9 or 10, characterized in that the piston recess is arranged essentially centrally in the piston. 12. Ignition device according to one of claims 9 and 11, characterized in that the Kol benmulde essentially has a closed shape on all sides. 13. Ignition device according to one of claims 9 to 12, characterized in that the Kol benmulde has a bevel at the upper edge in the direction of injection of the fuel. 14. Ignition device according to one of the preceding claims, characterized in that for Spark ignition, a spark plug with at least one finger ground electrode is used. 15. Ignition device according to one of the preceding claims, characterized in that for Spark ignition within the piston bowl uses a spark plug without a ground electrode is and the ground electrode is formed by a substantially cylindrical pin which is embedded in the bottom of the piston bowl. 16. Ignition device according to one of the preceding claims, characterized in that for Spark ignition uses a substantially cylindrical prechamber spark igniter that plunges into the piston bowl. 17. Ignition device according to claim 16, characterized in that the ignition electrodes in are arranged substantially axially parallel in the center of the ignition chamber. 18. Ignition device according to claim 17, characterized in that the substantially zylin Drische prechamber spark ignition device designed as a swirl chamber spark ignition device is. 19. Ignition device according to claim 17 or 18, characterized in that on the end face a hole is arranged in the swirl chamber, which contains the fuel-enriched mixture the piston bowl directs centrally in the form of a beam in the direction of the ignition electrodes. 20. Ignition device according to claim 19, characterized in that a multiple means for ignition Electrode is used which sparks essentially radially to the axis of the swirl chamber stretch to the substantially cylindrical inner wall of the vortex chamber. 21. Ignition device according to one of claims 17-20, characterized in that the vortex Chamber contains at least one overflow channel opening tangentially into the swirl chamber. 22. Ignition device according to one of claims 17-21, characterized in that the vortex chamber in the front area on the outside parallel to its main axis contains lateral grooves, through which the fuel-enriched mixture from the piston bowl in front of at least one of the openings are opened tangentially into the ignition chamber.   23. The method according to any one of claims 1 to 8 using the ignition device according to one of the Claims 16 to 22, characterized in that when the prechamber is immersed in the piston trough fuel-enriched mixture from the piston trough enters the prechamber and there leads to an enrichment of the mixture, the mixture subsequently by spark ignition is ignited. 24. The method according to claim 23, characterized in that ignition flares by the prechamber are generated, which are substantially radially from the axis towards the main combustion chamber spread. 25. The method according to claim 23 or 24, characterized in that when immersing the essentially cylindrical prechamber spark ignition device into the piston bowl Boreholes in the forehead area jet-enriched mixture from the piston bowl in the direction is directed to the ignition electrodes.