Method for igniting a fuel/air mixture, ignition system and glow plug

Abstract

What is described is a method for igniting a fuel/air mixture in a combustion chamber of an engine, wherein a pencil, which is electrically insulated with respect to walls of the combustion chamber and contains a heating resistor, is electrically heated to a temperature of at least 800 C. in the combustion chamber of the engine by applying a heating voltage, and a high voltage of at least 500 V, which is different from the heating voltage, is applied to the pencil and thereby ions are generated in the combustion chamber by field emission of electrons. The invention additionally relates to an ignition system and a glow plug.

Claims

1. A method for igniting a fuel/air mixture in a combustion chamber of an engine, comprising: providing a pencil which is electrically insulated with respect to walls of the combustion chamber and contains a heating resistor; applying a heating voltage to the heating resistor and thereby electrically heating the pencil to a temperature of at least 800 C. in the combustion chamber of the engine; and applying a high voltage of at least 500 V, which is different from the heating voltage, to the heating resistor and thereby generating ions in the combustion chamber by field emission of electrons.

2. The method according to claim 1 wherein the pencil is a ceramic pencil.

3. The method according to claim 1 wherein the high voltage is a high-frequency AC voltage.

4. The method according to claim 3 wherein the high-frequency AC voltage has a frequency of at least 10 kHz.

5. The method according to claim 1 wherein the high voltage is at least one hundred times higher than the heating voltage.

6. The method according to claim 1 wherein the pencil is electrically heated by applying DC voltage pulses of less than 25 V.

7. The method according to claim 6 wherein the high voltage is applied between the DC voltage pulses.

8. The method according to claim 7 wherein the heating voltage has an effective value of less than 10 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 shows a schematic illustration of an example of a corona ignition system;

(3) FIG. 2 shows an illustrative embodiment of an igniter for such a corona ignition system; and

(4) FIG. 3 shows a detailed view of FIG. 2.

DETAILED DESCRIPTION

(5) The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure.

(6) FIG. 1 shows a combustion chamber 1, which is delimited by walls 2, 3 and 4, which are connected to earth potential. An igniter 20, which is illustrated in FIG. 2, protrudes from above into the combustion chamber 1 and has an ignition electrode 5, which is surrounded at least over part of its length by an insulator 6, by means of which it is guided in an electrically insulated manner through the upper wall 2 into the combustion chamber 1. The ignition electrode 5 and the walls 2 to 4 of the combustion chamber 1 are part of a resonant circuit 7, to which a capacitor 8 and an inductor 9 also belong. The series resonant circuit 7 may comprise further inductors and/or capacitors and other components, which are known to a person skilled in the art as possible parts of series resonant circuits.

(7) To excite the resonant circuit 7, a high-frequency generator 10 is provided, which has a DC voltage source 11 and a transformer 12 with a center tap 13 on its primary side, whereby two primary windings 14 and 15 meet at the center tap 13. The ends of the primary windings 14 and 15 distanced from the center tap 13 are connected alternately to earth by means of a high-frequency switch 16. The switching frequency of the high-frequency switching unit 16 determines the frequency at which the series resonant circuit 7 is excited and can be altered. The secondary winding 17 of the transformer 12 feeds the series resonant circuit 7 at the point A. Thus the high frequency switching unit 16 is part of a controller which sets the high frequency AC voltage.

(8) The series resonant circuit is excited in the vicinity of its resonance frequency, which is generally between 10 kHz and 1 GHz. The AC voltage of the series resonant circuit is applied to the ignition electrode 5 and is generally at least 10 kV, for example 20 kV to 100 kV. The high-frequency AC voltage leads at the ignition electrode 5 to the discharge of electrons by field emission and to the formation of a corona discharge.

(9) A particular feature of the illustrated corona ignition system lies in the fact that a ceramic glow pencil is used as ignition electrode 5 and is electrically heated. In the illustrated illustrative embodiment, a heating voltage is applied to the glow pencil and is supplied by a DC voltage source 18, for example the on-board network of the vehicle. The DC voltage source may be identical to the DC voltage source 11; however, two separate DC voltage sources may also be provided. The heating voltage can be applied as DC voltage or is applied in the form of pulse width-modulated voltage pulses to the glow pencil. A switch 19 that is part of a controller of the ignition system determines when the DC voltage is applied to the pencil 5. The AC voltage can be applied to the glow pencil between the DC voltage pulses. It is also possible, however, to simultaneously apply both the heating voltage and the AC voltage to the glow pencil.

(10) The glow pencil is heated by the heating voltage to a temperature of 800 C. or more, for example 1000 C. or more. The discharge of electrons from the ignition electrode 5 is facilitated, and the field emission is consequently strengthened. The creation of a corona discharge is thus facilitated.

(11) An illustrative embodiment of an igniter with an ignition electrode 5 in the form of a ceramic glow pencil is illustrated in FIG. 2. FIG. 3, in a detailed view of FIG. 2, shows the front, combustion-chamber-side part of the igniter with the glow pencil as ignition electrode 5.

(12) The glow pencil plugs into a metal housing 21. As is shown in particular in FIG. 3, the glow pencil consists of a number of ceramic layers. The glow pencil has a core formed from a conductive ceramic. This core is the inner conductor 22 of the glow pencil. The inner conductor 22 is surrounded by a ceramic insulator layer 23. A layer formed from conductive ceramic material is arranged on the insulator layer 23 and will be referred to hereinafter as an outer conductor layer 24. The outer conductor layer 24 and the inner conductor 22 are electrically conductively connected by a heat conductive layer 25 at the end of glow pencil remote from the metal housing 21. The ceramic heat-conducting layer 25 covers an end face of the glow pencil and contacts there the inner conductor 22. The heat-conducting layer 25 may additionally cover the insulator layer 23 in an end portion of the glow pencil. In this case, the outer conductor layer 24 ends at a distance from the end of the glow pencil remote from the metal housing 21 and is electrically contacted there by the heat-conducting layer 25. It is also possible, however, for the outer conductor layer 24 to extend as far as the end of the glow pencil and for the heat-conducting layer 25 to cover only the end face of the glow pencil.

(13) The heat-conducting layer 25 in the shown illustrative embodiment has a higher electrical resistance than the outer conductor layer 24. The heat-conducting layer 25 and the outer conductor layer 24 are preferably made of different material. A higher electrical resistance of the heat-conducting layer 25 can also be achieved alternatively or additionally by a lower layer thickness.

(14) The outer conductor layer 24 is covered by a further insulator layer 26. The insulator layer 26 causes an electrical insulation of the outer conductor 24 and therefore of the glow pencil from the metal housing 21. This insulation is important so that the glow pencil can serve as an ignition electrode 5 and a corona discharge can form at said glow pencil in the event of application of a high-frequency AC voltage. The heat-conducting layer 25 is uncovered by the insulator layer 26 at least in an end portion.

(15) Instead of the insulator layer 26, a ceramic sleeve for example, from which the glow pencil protrudes, can also be used as ceramic insulation of the glow pencil from the metal housing 21. It is important that the insulator layer of the glow plug from the metal housing 21 has a dielectric strength of at least 500 V, e.g., 1000 V or more.

(16) In the embodiment described above a corona discharge is created by applying a high frequency AC voltage. A significantly improved ignition and better combustion can also be achieved if the applied high voltage is too low to cause a corona discharge and merely causes an increased ion concentration in the combustion chamber by field emission.

(17) Instead of an AC voltage of a resonant circuit a DC voltage or a pulsed DC voltage of 500 V may be applied to the pencil 5.

(18) While exemplary embodiments have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of this disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

(19) TABLE-US-00001 LIST OF REFERENCE NUMERALS 1. combustion chamber 2. wall of the combustion chamber 3. wall of the combustion chamber 4. wall of the combustion chamber 5. ignition electrode 6. insulator 7. resonant circuit 8. capacitor 9. inductor 10. high-frequency generator 11. DC voltage source 12. transformer 13. center tap 14. primary winding 15. primary winding 16. high-frequency switching unit 17. secondary winding 18. DC voltage source 19. switch 20. igniter 21. metal housing 22. inner conductor 23. outer conductor layer 24. insulator layer 25. heat-conducting layer 26. insulator layer