Ignition device for a two-stroke engine

Abstract

Ignition device for triggering an ignition spark (11) at a spark plug (8) of an internal combustion engine, the crankshaft (6) of which is driven by a piston (4) reciprocating between a bottom dead center (UT) and a top dead center (OT), - in particular for a two-stroke engine in a hand-held implement, - consisting of a pole wheel (12) driven in rotation by the crankshaft (6) of the internal combustion engine, and the pole wheel (12) has two permanent magnets (20, 22) arranged at a distance (23) from one another in the circumferential direction, with a magnetic yoke (13) fixedly arranged on the pole wheel circumference - with a charging coil (16) charging an ignition capacitor (30), a primary coil (17) and a secondary coil (18) connected to the spark plug, - wherein, via one revolution of the pole wheel (12), the yoke (13) is magnetically closed in a first rotation angle range (25) around the top dead center (OT) of the piston (4) via a permanent magnet (20) and in a second rotation angle range (27) around the bottom dead center (UT) of the piston (4) via the second permanent magnet (22), - wherein a voltage is induced in each of the coils (16, 17, 18), and in order to trigger an ignition spark (11) in the first rotation angle range (25), the ignition capacitor (30) is discharged via a switching element (34), wherein the switching element (34) is controlled by an ignition control unit (33), characterized in that - that a device (40) is provided for reducing the voltage applied to the spark plug (8), which is active in the second rotation angle range of the pole wheel (12) and causes a reduced voltage, - and the device (40) comprises a switching element (36, 37, 38) which is controlled by a control unit (35).

Description

The invention relates to an ignition device for triggering an ignition spark at a spark plug of an internal combustion engine, in particular for a two-stroke engine in a hand-guided working device according to the preamble of claim 1.DE 10 2005 038 198 A1 discloses an ignition device which comprises a pole wheel rotating with the crankshaft and a fixed yoke with coils assigned to the latter. The pole wheel carries two diametrically opposed permanent magnets which close the magnetic circuit of the yoke twice over one revolution. On one leg of the U-shaped yoke, a charging coil for charging the ignition capacitor is wound, while on the other leg, the ignition coil consisting of the primary coil and the secondary coil is wound. The arrangement of two magnets enables a strong, long-burning ignition spark, which ensures reliable ignition of the mixture, and also enables a sufficient energy supply of control units, actuators or sensors.In unfavourable operating states, no ignition takes place over several revolutions of the internal combustion engine-for example due to suppression of ignition to limit the maximum rotational speed-so that-due to the scavenging principle of the two-stroke engine-the combustion chamber is filled with mixture even in the bottom dead center of the piston. At atmospheric pressure in the combustion chamber, significantly lower voltages at the spark plug are sufficient to trigger an ignition spark than in the case of a mixture compressed in the combustion chamber, i.e. under overpressure. Due to the construction of the pole wheel and the ignition device, a voltage is induced in the coils by the second permanent magnet in the region of the bottom dead center, which leads to a high voltage of 2 kV to 3 kV in the secondary coil. This can lead to an ignition spark at the spark plug at the atmospheric pressure prevailing in the region of the bottom dead center in the combustion chamber. If the combustion chamber is filled with combustable mixture due to a lack of combustion, this can lead to an ignition of the mixture in the region of the bottom dead center of the piston. This leads to uncontrolled combustions and thus to an uneasy running of the engine.It is known from DE 23 62 472 A1 to arrange a further diode in series with a charging diode, wherein an provided short-circuit switch is connected between the two diodes. As a result, the short-circuit switch is protected from high voltages.The publications DE 17 63 306A1, DE 29 20 831 A1 and DE 22 42 325 A1 show further ignition circuits of different construction.The invention is based on the object of configuring an ignition device of the generic type in such a way that ignition of the mixture in the region of the bottom dead center of the piston is reliably prevented. The object is achieved according to the features of claim 1.The device for lowering the voltage applied to the spark plug is active at least in the second angle of rotation range, within which approximately atmospheric pressure prevails in the combustion chamber and, on account of the design, a low high voltage of, for example, 2 kV to 3 kV can lead to an ignition spark at the spark plug. Expediently, the device is inactive in the remaining first angle of rotation range, so that the ignition device reliably operates in a known manner. In the first rotational angle range, the top dead center of the piston is situated; in the second rotational angle range, the bottom dead center of the piston is situated.The device for lowering the voltage present at the spark plug is a switching element which is controlled by a control unit as a function of the angle of rotation of the pole wheel. The control unit can be formed by the ignition control unit.The switching element can be arranged parallel to the primary coil, i.e. the primary coil is short-circuited via the switching element with or without a load. This results in attenuation of the voltage induced at bottom dead center, with the effect that the high voltage developing in the secondary coil is lower by factors, so that an ignition spark at the spark plug can be ruled out even at atmospheric pressure in the combustion chamber.It can be advantageous to arrange the switching element in series with the spark plug and thus to interrupt the voltage branch of the spark plug in the predefined second angle of rotation range. If the switching element is in parallel with the spark plug, the secondary coil is short-circuited, which leads to a corresponding attenuation.The switching elements for lowering the voltage present at the spark plug are advantageously electronic switching elements such as thyristors, MOSFETs or other transistors.Further features of the invention are evident from the further claims, the description and the drawing, in which exemplary embodiments of the invention described in detail below are illustrated. The following are shown: FIG. 1 shows a schematic view of a working device using the example of a power chain saw, FIG. 2 is an enlarged view of the arrangement of the rotating pole wheel and the stationary yoke, FIG. 3 is a schematic circuit diagram of the ignition device, FIG. 4 shows a diagram of the voltages induced in the secondary coil over one pole wheel revolution.The working device illustrated in FIG. 1 is a portable, hand-guided working device 1 with an internal combustion engine arranged in the housing 2. The internal combustion engine is in particular a two-stroke engine, preferably a single-cylinder two-stroke engine having a cylinder 3 in which a piston 4 is arranged. The piston 4 reciprocating between a top dead center OT and a bottom dead center UT drives a crankshaft 6 via a connecting rod 5. The piston 4 delimits a combustion chamber 7 into which mixture is conveyed by the two-stroke method known per se. A spark plug 8 projects into the combustion chamber, which spark plug emits a controlled ignition spark in the region of the top dead center of the piston 4 in order to ignite the mixture compressed in the combustion chamber 7. At the bottom dead center of the piston, the combustion chamber 7 is connected to the atmosphere via an outlet, so that the combustion gases of a preceding combustion can flow away.The spark plug 8 is actuated by an ignition device 10, which triggers an ignition spark 11 at the spark plug 8 (FIG. 3 ) as a function of the rotational speed and the load of the internal combustion engine. For this purpose, an ignition control unit 33 is provided.The energy for the ignition is generated by an ignition generator 9 which consists of a pole wheel 12 and a yoke 13 which is fixedly assigned to the pole wheel and on whose legs 14 and 15 coils 16, 17 and 18 are arranged. The coils 16 to 18 can be cast together with the yoke 13 to form a structural unit 19 with a small installation space. Furthermore, weight is saved due to this compact design.The pole wheel 12, which in the exemplary embodiment is advantageously the fan wheel of the air-cooled two-stroke engine, carries, for example, two permanent magnets 20 and 22 In the exemplary embodiment shown, the permanent magnets 20 and 22 are arranged diametrically opposite one another with respect to the axis of rotation 21 of the pole wheel 12, the magnets 20 and 22 being magnetized in opposite directions. In the exemplary embodiment shown, the permanent magnets 20 and 22 are thus located in the circumferential direction of the pole wheel 12 at a distance 23 of 180° KW from one another. Other distances between the permanent magnets may be advantageous. It can also be expedient to provide more than two permanent magnets 20, 22.1, 22.2 over the circumference of the pole wheel 12 (FIG. 2 ), e.g. three or more magnets.In the exemplary embodiment, the pole wheel 12 is driven in rotation by the rotating crankshaft 6 of the internal combustion engine; preferably, the pole wheel 12 is flanged onto the end of the crankshaft 6 and rotates therewith. The distance 23 between the permanent magnets 20 and 22 therefore corresponds to 180° crankshaft angle, wherein the permanent magnet 20 magnetically closes the yoke 13 at the top dead center OT of the piston 4 and the permanent magnet 22 magnetically closes the yoke 13 at the bottom dead center UT.The yoke 13, preferably the one leg 14 of the yoke 13, carries the charging coil 16 which serves to charge an ignition capacitor 30. Furthermore, the primary coil 17 and the secondary coil 18 of the ignition coil 31 are arranged on the yoke 13, preferably on the other leg 15 of the yoke 13, wherein the secondary coil 18 lies on the primary coil 17, which in turn is wound on the leg 15.The yoke 13 is magnetically closed via the permanent magnets 20 and 22, so that a magnetic flux is formed in the yoke 13 via the permanent magnet. This flux is greatest when the permanent magnet magnetically closes the free ends of the yoke 13; this corresponds to a maximum induction voltage. When the yoke 13 is opened, the induction voltage collapses again.While the permanent magnet 20 leads to a positive voltage pulse 26 in the secondary coil 18, a negative voltage pulse 28 of the same magnitude is generated when the permanent magnet 22 magnetized in the opposite pole passes the yoke 13 (FIG. 4 ) if a permanent magnet of the same thickness is used.As the circuit diagram of the ignition device 10 according to FIG. 3 shows, the ignition capacitor 30 is charged via the diode 32 and the primary coil 17. The voltage resulting from the induced voltage in the ignition coil 31 on the secondary side is shown in FIG. 4. The voltage occurring on the secondary coil 18 of the ignition coil 31 as a result of the induction is in the range from approximately 2 kV to 3 kV, which is also present at the electrodes of the spark plug 8. The maximum voltage pulse 26 or 28 is preceded by a small voltage wave which occurs as the permanent magnet approaches the yoke. If the yoke 13 is magnetically closed via the permanent magnet, the maximum voltage pulse 26 or 28 shown (FIG. 4 ) is produced; if the permanent magnet moves away from the yoke again, a decaying voltage wave occurs. The voltage pulse 26 or 28 is thus always surrounded by a voltage wave lying ahead and a subsequent voltage wave.In the region of top dead center OT (FIG. 4 ), the mixture in the combustion chamber is highly compressed, so that the voltage of 2 kV to 3 kV resulting from the induction on the secondary side of ignition coil 31 is not sufficient to form an ignition spark 11. Therefore, at the desired ignition time, a switching element 34, in the exemplary embodiment a thyristor, is switched on by the ignition control unit 33, which switching element closes a circuit formed from ignition capacitor 30 and primary coil 17; the ignition capacitor 30 can discharge via the primary coil 17. The discharge leads to an ignition voltage 50 of more than 20 kV on the secondary side of the ignition coil, which is sufficient for triggering an ignition spark 11 and a reliable ignition of the compressed mixture in the combustion chamber 7. In this case, the ignition takes place approximately at TDC, i.e. is placed as a voltage peak on the high-voltage pulse of approximately 2 kV to 3 kV triggered by the induction of the permanent magnet 20.The ignition control unit 33 is provided with a control unit 35 which serves to control further switching elements 36, 37 and / or 38. Advantageously, the control unit 35 is integrated into the ignition control unit 33, so that only one control unit has to be kept available.The control unit 35 controls a device 40 for lowering the voltage applied to the spark plug 8 in predetermined rotational angle ranges. The device 40 is activated whenever the magnet group following an ignition pulse, i.e. for example the permanent magnet 22, passes the yoke 13. In the exemplary embodiment shown, the induced voltage is greatest at bottom dead center (UT) due to the diametrical arrangement of the permanent magnets 20 and 22 relative to the axis of rotation 21 of the pole wheel 12. The induced voltage, as in the case of the first permanent magnet 20, leads to a voltage peak of approximately 2 kV to 3 kV in the secondary coil 18 of the ignition coil 31. Since the combustion chamber 7 is open to the atmosphere in the bottom dead center UT-the outlet is open for discharging the combustion gases from the combustion chamber-substantially atmospheric pressure prevails in the combustion chamber 7. Under these pressure conditions, a voltage of 2 kV to 3 kV applied to the spark plug 8 may result in a spark spark 11. This is without consequences if no combustible mixture is present in the combustion chamber 7. If the two-stroke engine runs at high speed, for example, and if one or more ignitions are suppressed to control the speed, then a plurality of cycles result without combustion, which is why combustible mixture can be present under atmospheric pressure even when the outlet is open in the combustion chamber 7. Under these conditions, however, a spark occurring in the angle of rotation range around UT can lead to ignition, which is undesirable. Therefore, according to the invention, it is provided that the control unit 35 for lowering the voltage present at the spark plug 8 switches on a switching element 36 in the form of a thyristor, which short-circuits the primary coil 17. As a result, the primary coil is attenuated, so that the secondary voltage 28 still arising on the secondary side is significantly reduced, as is illustrated by the dotted line 41 in FIG. 4. The control unit 35 actuates the switching element 36 at least whenever the second permanent magnet 22 passes the yoke 13. As a result, no ignition spark can form on the spark plug 8 even under unfavourable operating conditions in UT.A device 40 for lowering the voltage present at the spark plug 8 is formed in that a switching element 37 is arranged in the voltage branch 42 of the spark plug 8, which switching element opens the voltage branch 42. Whenever the permanent magnet 22 passes the yoke 13 in the region of the bottom dead center, the voltage branch 42 is opened, so that the spark plug 8 is free of stress. The switching element 37, which is designed as a thyristor, is only closed again when the mixture in the combustion chamber 7 is compressed, since with increasing compression of the mixture the voltage required for an ignition spark 11 at the spark plug 8 increases.A pre-spark gap 43 can be formed between the secondary coil 18 and the spark plug 8, which gap blocks voltages of up to 3 kV, for example. Only when the voltage is greater than 3 kV can the spark gap 43 be bypassed and the high voltage thus applied to the spark plug 8.A high-voltage diode 44 can also be provided, which is inserted into the voltage branch 42 in the reverse direction. The high-voltage diode 44 acts in the reverse direction in a similar manner to the pre-spark gap 43; only when the breakdown voltage of, for example, 2 kV to 3 kV is overcome can a high voltage be present at the spark plug 8.Damping of the coils at the bottom dead center of the piston 4 can also be achieved by a device 40, by means of which the charging coil 16 is loaded, preferably short-circuited, by means of a switching element 38, in the exemplary embodiment a thyristor. This also achieves a lowering of the voltage occurring in UT in the secondary coil 18.In principle, it is sufficient if the device 40 is inactive in the first rotational angle range 25 in which the TDC of the piston 4 lies and is actively switched in the second rotational angle range 27 in which the BDC of the piston 4 lies. In an advantageous development, it is provided that the device 40 is inactive within a first widened angle of rotation range 45, wherein the first angle of rotation range 25 lies in the first widened angle of rotation range 45; the first widened angle of rotation range comprises approximately 90° crankshaft angle and extends in particular from approximately 70° crankshaft angle before TDC up to approximately 20° crankshaft angle after TDC.In a corresponding manner, a second widened angle of rotation range 47 is advantageously provided, which comprises the second angle of rotation range 27. The second extended rotational angle range 47 can extend over a range of approximately 90° crankshaft angle. Advantageously, the second widened angle of rotation range 47 forms approximately the supplementary angle to the first widened angle of rotation range 45 and thus extends over a range of approximately 20° crankshaft angle after TDC up to approximately 70° crankshaft angle before TDC. The second widened rotational angle range 47 is thus larger, preferably larger by a factor of 3, than the first widened rotational angle range 45.Accordingly, the control unit 35 is designed such that in a first extended angle of rotation range 45 of approximately 70° before TDC to 20° after TDC the device 40 for lowering the voltage is switched inactive and in the remaining extended angle of rotation range of 20° after TDC to 70° before TDC the device 40 for lowering the voltage is active. The first widened angle of rotation range 45 thus extends over approximately 90° crankshaft angle, while the second widened angle of rotation range 47 extends over approximately 270° crankshaft angle.In order to reduce the voltage occurring in the secondary coil in the second rotational angle range, the magnetic flux occurring in the yoke 13 can be weakened. This is possible because the air gap 24 between the free ends of the yoke legs 14 and 15 and the permanent magnet 22 is formed larger than the air gap between the free legs of the yoke 13 and the first permanent magnet 20.Weakening of the magnetic flux in the yoke 13 can also be achieved in that the second permanent magnet is magnetized more weakly than the first permanent magnet 20.In order to reduce the voltage applied to the spark plug in the second angle of rotation range, the geometry of the second permanent magnet 22' can also be changed in comparison with the geometry of the first permanent magnet 20. In the exemplary embodiment shown, the second permanent magnet 22' extends over a circumferential angle v which is greater than the circumferential angle u, measured in the circumferential direction of the pole wheel 12, of the open ends of the yoke 13. Owing to the greater extent of the permanent magnet 22', the yoke 13 cannot be closed in an optimum manner, so that the maximum of the magnetic flux is less than in the case of the first permanent magnet 20 which closes the magnetic yoke 13 with an exact fit over its magnetic poles.If a higher energy yield is required, a plurality of permanent magnets 20, 22.1, 22.2 can also be arranged over the circumference of the pole wheel 12, wherein the device 40 for lowering the voltage at the spark plug is always then active in the second angle of rotation range.

Claims

Ignition device for triggering an ignition spark (11) at a spark plug (8) of an internal combustion engine, the crankshaft (6) of which is driven by a piston (4) which reciprocates between a bottom dead point (UT) and a top dead point (OT), - in particular for a two-stroke engine in a hand-guided working device, - comprising a pole wheel (12) which is driven in rotation by the crankshaft (6) of the internal combustion engine, and the pole wheel (12) has two permanent magnets (20, 22) which are arranged at a distance (23) from one another in the circumferential direction, having a magnetic yoke (13) which is arranged at a fixed location on the pole wheel circumference, - having a charging coil (16) which charges an ignition capacitor (30), a primary coil (17) and a secondary coil (18) which is connected to the spark plug, - wherein, via a revolution of the pole wheel (12), the yoke (13) is magnetically closed in a first rotational angle range (25) about the top dead center (OT) of the piston (4) via a permanent magnet (20) and in a second rotational angle range (27) about the bottom dead center (UT) of the piston (4) via the second permanent magnet (22), - wherein a voltage is in each case induced in the coils (16, 17, 18), and, for triggering an ignition spark (11) in the first rotational angle range (25), the ignition capacitor (30) is discharged via a switching element (34), wherein the switching element (34) is controlled by an ignition control unit (33), characterized - in that a device (40) for lowering the voltage present at the ignition plug (8) is provided, which device is active in the second rotational angle range of the pole wheel (12) and brings about a lowered voltage, and the device (40) comprises a switching element (36, 37, 38) which is controlled by a control unit (35).Ignition device according to Claim 1, characterized in that the control unit (35) is formed by the ignition control unit (33).Ignition device according to Claim 1 or 2, characterized in that the switching element (36) is arranged parallel to the primary coil (17).Ignition device according to Claim 1 or 2, characterized in that the switching element (37) is connected in parallel or in series with the spark plug (8).Ignition device according to one of Claims 1 to 4, characterized in that the switching element (36, 37, 38) is an electronic switching element, in particular a thyristor.Ignition device according to one of Claims 1 to 5, characterized in that the internal combustion engine is a single-cylinder two-stroke engine and the first rotational angle range (25) lies in a first widened rotational angle range (45) in which the device (40) is inactive and the second rotational angle range (27) lies in a second widened rotational angle range (47) in which the device (40) is active.Ignition device according to Claim 6, characterized in that the first widened angle of rotation range (45) extends over a crankshaft angle of approximately 90° and the second widened angle of rotation range (47a) extends over a crankshaft angle of approximately 60° to 80°, preferably the second angle of rotation range (47) extends up to a supplementary angle of the first widened angle of rotation range (45).Ignition device according to one of Claims 1 to 7, characterized in that the device (40) is inactive in the first angle of rotation range (25).Ignition device according to Claim 1, characterized in that the device (40) is formed by weakening the magnetic flux occurring in the yoke (13) in the second angle-of-rotation region (27, 47a, 47).Ignition device according to Claim 9, characterized in that the weakening is formed by a larger air gap (24) between the second permanent magnet (22) and the yoke (13).Ignition device according to Claim 9 or 10, characterized in that the weakening is formed by a weaker magnetization of the second permanent magnet (22.1, 22.2).Ignition device according to one of Claims 9 to 11, characterized in that the weakening is formed by a geometry of the second permanent magnet (22') which is modified in relation to the first permanent magnet (20).Ignition device according to one of Claims 9 to 12, characterized in that a plurality of second permanent magnets (22.1, 22.2) are provided over the circumference of the pole wheel (12).

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