GB1587959A

GB1587959A - Ignition system for internal combustion engines

Info

Publication number: GB1587959A
Application number: GB4491677A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1976-10-29
Filing date: 1977-10-28
Publication date: 1981-04-15
Also published as: DE2649732C2; ES463652A1; FR2369709A1; FR2369709B3; IT1113666B; DE2649732A1; JPS5356436A

Description

(54) IGNITION SY STEM T) Tmrn (54) IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES (71) We, ROBERT BOSCH GMBH, a German Company, of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an ignition system, which serves for the ignition of the compressed fuel-air mixture in an internal combustion engine.

An ignition system of this type is already known, as described in the German Patent Specification (O enlegungsschrift) No. 1 539 218, in which reed switches, serving to distribute the ignition voltage to the sparking plugs, are operated directly by means of a rotatable permanent magnet. A disadvantage of this system is that the rotatable arrangement of the permanent magnet, and the necessary coupling to the internal combustion engine in order to drive it, are very expensive to produce.

Moreover, misalignment of the permanent magnet, due, for example, to bearing play, may upset the predetermined sequence of switching operations, owing to the fact that opening and closing of the reed switches do not occur at identical rotational angles of the permanent magnet. Lastly, the permanent magnet must move past the reed switches at a sufficient distance, if it is not to strike against the receptacle enclosing the reed switches, which receptacle is generally made of glass. This glass, however, has an unfavourable effect on the control action of the permanent magnet.

It is an object of the present invention to provide an ignition system which avoids the above disadvantages, and which also may be accommodated, with a substantial saving of space, in- any desired position relative to the internal combustion engine.

In accordance with the invention there is provided an ignition system, for internal combustion engines, comprising a plurality of sparking plugs to which ignition voltage surges are distributed from the recondary winding of an ignition transformer to fire the plugs in repeating sequences, these ignition voltage surges being induced in the secondary windings by repeated opening of the circuit of the primary winding of the ignition transformer, the system also comprising, for each sparking plug, an electromagneticallyoperated reed switch in series with the plug, an excitation winding for controlling the reed switch, and an electrically-operable excitation switch in series with the excitation winding, the series circuits comprising the excitation- windings and the electrically-operable excitation switches being connected in parallel with one another and being adapted for connection across the power supply of the ignition system, the system being adapted to operate so that the electrically-operable excitation switches control the-excitation windings in a manner to ensure that the contacts of the reed switch in series with the spark plug next to be fired are in a closed state and the contacts of the remainder of the reed switches are in an open state before the circuit of the primary winding of the ignition transfoimer is opened to induce an ignition voltage surge in the secondary winding for firing that plug, and to ensure that the contacts of that reed switch remain in a closed state and the contacts of the remainder of the reed switches remain in an open state at least until the end of the respective ignition process.

Embodiments of the invention are hereinafter described, by way of example, with reference to the accompanying drawings, in which: . Figure 1 is the circuit arrangement of one embodiment of an ignition system according to the invention; Figure 2 is a -graph showing voltage (IJ) as a function of time (t), in clarification of control- processes; Figure 3 is a modified arrangement of the circuit shown in Figure 1; Figure 4 is a sectional structural view, along the line IV-IV in Figure 5, of the reed switches serving to distribute the ioni- tion voltage; Figure 5 is a section along the line V-V in Figure 4; Figure 6 is a front elevation of a reed switch contact which may be used in the invention;; Figure 7 is the plan view of the contacting portion of the reed switch contact, and Figure 8 is a side view of this portion.

The ignition system, whose circuit arrangement is shown in Figure 1, is intended for the internal combustion engine of a motor vehicle. This ignition system is supplied with power from a direct-current source 1, which may be the battery of the motor vehicle. Connected to the positive terminal of the current source 1 is a first lead 3, incorporating an operating switch (ignition switch) 2, and, connected to the negative terminal, is a second lead 4, which is connected to earth. A branch circuit leads from the first lead 3, first by way of the primary winding 5 of an ignition coil or transformer 6, to the collector of an NPN output transistor 7, and then from the emitter of this transistor 7 to the second lead 4. The base of the output transistor 7 is connected to the collector of an NPN transistor 8, and, by way of a resistor 9, to the first lead 3.The base of the transistor 8, whose emitter is connected to the second lead 4, is connected to the emitter of an NPN transistor 10, then to the emitter of an NPN transistor 11, and, lastly, by means of a resistor 12, to the second lead 4. The base of the transistor 10 is connected to the collector of the transistor 11, and, by means of a resistor 13, to the first lead 3. Furthermore, the collector of the transistor 10 is connected by way of a resistor 14 to the first lead 3.

The emitter of the transistor 11 is connected to the anode of a diode 15, whose cathode is connected to the base of this transistor 11. The base of the transistor 11 is also connected, by means of a resistor 16, to the first lead 3. The transistors 10, 11, together with their respective resistors 12, 13, 14, form a threshold value switch, whose operation is similar to that of a Schmitt trigger circuit. The base of the transistor 11, which forms the input of the threshold value switch, is influenceable by means of a signal generator 17. The signal generator 17 here used is of the inductive type. It comprises a disc 18, made of magnetically impermeable material, such as plastics material, the disc 18 being mounted on a shaft 19, and having, on its outer perimeter, a conductor 20, of magnetically permeable material, such as soft iron.The shaft 19 is coupled to the internal combustion engine. The signal generator 17 has two magnetic circuits 21, 22, shown by broken lines, the magnetic circuit 21 incorporating a permanent magnet 23, and the magnetic circuit 22 incorporating a permanent magnet 24. The magnetic circuit 21 is inductively coupled to a transducer winding 25, and the magnetic circuit 22 is inductively coupled to a transducer winding 26. One end of each transducer winding 25, 26 is connected to the second lead 4, the other end of each winding being connected to the cathode of a respective one of two diodes 27, 28, whose anodes are connected to the base of the transistor 11. Thus, a respective one of the two magnetic circuits 21, 22 is closed by means of the conductor 20 whenever the disc 18 rotates through 1800.

During the individual ignition processes, ignition voltage surges are produced in the secondary winding 29 of the ignition coil 6, these surges being applied alternately to one of the two sparking plugs 31, 32.

For this purpose, a reed switch 33 is serially connected to the sparking plug 31, and a reed switch 34 is serially connected to the sparking plug 32. The reed switch 33 comprises a tubular envelope 35, which is sealed at both ends and contains gas or a vacuum, and into which there extends, through one of its ends, a reed contact 36, disposed roughly on the longitudinal central axis of the envelope 35, and through the other of its ends a reed contact 37, also disposed roughly on the longitudinal central axis of the envelope 35. The flat end portions of the contacts 36, 37 which extend inside the envelope 35, are placed with their flat sides facing each other, and with a gap between them.Encircling the surface of the envelope 35 is an excitation winding 38, which, when current is flowing through it, effects the contact between the ends of the contacts 36, 37, which extend inside the envelope 35, these ends being made of magnetically permeable material.

The reed switch 33 thus operates as a contact maker. One end 39 of the excitation winding 38 is connected to the first lead 3, its other end 40 being connected by way of an electrically operable excitation switch 41 to the second lead 4. Preferably, the excitation switch 41 is formed by the emitter-collector path of an NPN transistor 42, the emitter being connected to the second lead 4. The base of the transistor 42 is connected to the collector of a transistor 43, and, by way of a resistor 44, to the first lead 3. Between the base and the emitter of the transistor 43 there is connected a diode 45, whose anode is connected to the emitter.The base of the transistor 43 is connected also, by means of a resistor 46, to the first lead 3, and, by way of a diode 47, to the end of the transducer winding 26 remote from the second lead 4, the cathode of this diode 47 being connected to the transducer winding 26. The construction of the reed switch 34 is the same as that of the reed switch 33. Its envolope is designated by the reference numeral 48, its reed contact, connected to the secondary winding 29, by the reference numeral 49, its reed contact, connected to the sparking plug 32, by the reference- numeral 50, and its excitation winding by the reference numeral 51.

The end 52 of the excitation winding 51 is connected to the first lead 3, and the end 53 of the winding is connected by way of an electrically operable excitation switch 54 to the second lead 4. In this case also, the excitation switch 54 is formed preferably by the emitter-collector path of an NPN transistor 55, the emitter being connected to the second lead 4. The base of the transistor 55 is connected to the collector of an NPN transistor 56, and, by means of a resistor 57, to the first lead 3. The transistor 56, whose emitter is connected to the second lead 4, is provided, between its emitter and its base, with a diode 58, whose cathode is connected to the base.Furthermore, the base of the transistor 56 is connected by means of a resistor 59 to the first lead 3, and, by way of a diode 60, to the end of the transducer winding 25 remote from the second lead 4, the cathode of this diode 60 being connected to the transucer winding 25.

The method of operation of the ignition system described above is as follows: When the operating switch 2 is cloesd, the ignition system is ready for operation.

If it is assumed that the emitter-collector path of the transistor 11 is conducting, so that the emitter-collector path of the transistor 10 is non-conducting, the emittercollector path of the transistor 8 is also non-conducting, and the emitter-collector path of the output transistor 7 is conducting. Hence, current flows through the primary winding 5, whereby energy for the ignition process is stored in the ignition coil 6. If, as shown in the drawing, the magnetic conductor 20 is now moved through the magnetic circuit 21, an alternating voltage cycle is Droduced in the transducer winding 25. The half-wave S, shown in Figure 2, of this alternating voltage cycle, is used for triggering the ignition process.First, the reed switch 34 - is turned on so that an ignition voltage surge subsequently .produced in the -secon- dary winding 29 may cause an electrical discharge (an ignition spark) across the sparking plug 32. The reed switch 34 is turned on when the value U1 of the voltage half-wave S is reached as now described.

The bias voltage across the base - of the transistor 56 is reduced by the diode 58 to such a degree that the emitter-collector path of this transistor 56 is blocked. Control current can therefore flow by way of the base-emitter path of the transistor 55, whereby the emitter-collector path forming excitation switch 54 of this transistor 55 is rendered conducting, and the excitation winding 51 of the reed switch 34 is energised. Consequently, the reed switch 34 is closed, that is, contact is made between its contacts 49, 50. As long as the value of the voltage half-wave S is greater than U1, the operating mode, just described, of the reed switch 34, is maintained.When the voltage half-wave S reaches the value U2, the bias voltage across the base of the transistor 11 is so reduced by the voltage applied to the base by the diode 15. that the emitter-collector path of this transistor 11 is blocked. Consequently, the emitter-collector path of the transsistor 10, and also, in dependence thereon, the emitter-collector path of the transistor 8, are rendered conducting. In consequence, the emitter-collector path of the output transistor 7 is blocked, and the current in the primary winding 5 is interrupted. An ignition voltage surge is thereby induced in the secondary winding 29. This ignition voltage surge is transmitted by the previously closed reed switch 34 to the sparking plug 32, whereby an electrical voltage discharge is produced across the sparking plug.When the voltage óf the voltage half-wave S falls again to the value U2, the emitter-collector path of the transistor 11 again becomes conducting, the emitter-collector path of the transistor 10, and also the emitter-collector path of the transistor 8, become non-conducting, and therefore the emitter-collector path of the output transistor 7 also again becomes conducting, whereby the primary winding 5 is again energised, and energy for the next ignition process is once more stored in the ignition coil 6. On a further decrease in the voltage half-wave S to the value U1, the emitter-collector path of the transistor 56 again becomes conducting, and hence the emitter-collector path of the transistor 55 again becomes non-condúcting, whereby the excitation winding 51 is deenergised.

The reed switch 34 is thereby opened.

When the magnetic conductor 20 is moved through the magnetic circuit 22, and when the voltage half-wave induced in the transducer winding 26 readhes the value U1, the emitter-collector path of the transistor 43 is rendered non-conducting, and the emitter-collector path of the transistor 42, forming the excitation switch 41, is rendered conducting. Hence, the excitation winding 38 is energised, whereby the reed switch 33 is closed, that is, contact is made between its contacts 36, 37. The reed switch 33 is therefore now turned on, in order to ensure that the sparking plug 31 receives the ignition voltage surge. When the voltage half-wave S reaches the voltage U2, the ignition process is triggered in the above-desciibed manner since transistor 7 ceases to conduct, and the ignition spark is produced across the sparking plug 31.On the decay of the voltage half-wave S to the value U2, the primary winding 5 is again turned on by the output transistor 7, in the manner described above, and the energisation of the reed switch 33 is cancelled again by means of the excitation switch 41, formed by the emitter-collector path of the transistor 42. The above-described cycle is repeated when the magnetic conductor 20 passes once again through the magnetic circuit 21.

In the case just described, the excitation switches 41, 54 are .normally blocked.

During the ignition process, only that excitation switch 41 or 54 is rendered conducting which is associated with the excitation winding 38, 51 of the respective reed switch 33, 34, connected between the secondary winding 29 of the ignition transformer 6 and the respective plug 31, 32 which is to be fired.

Alternatively, if desired, as shown in Figure 3, distribution of the ignition volt age surges may be effected in such a manner that the excitation switches 41, 54 are normally conducting, and that, during the ignition process, only that excitation switch 41 or 54 remains conducting, which is associated with the excitation winding 38, 51 of the respective reed switch 33, 34 connected between the secondary winding 29 of the ignition transformer 6 and the respective plug 31, 32 which is to be fired.

To relate this to the circuit arrangement shown in Figure 1, the transistors 42, 55 and resistors 44, 57 are omitted, and the excitation windings, 38, 51 are inserted between the respective emitter-collector path of one of the transistors 43, 56 and the first lead 3. The emitter-collector path of the transistor 43 then forms the excitation switch 41, and thte emitter-collector path of the transistor 56 forms the excitation switch 54.

Operatively, the only difference compared with the circuit arrangement shown in Figure 1 is that, after the closing of the operating switch 2, the emitter-collector paths of both transistors 43, 56 are rendered conducting, both excitation windings 38, 51 being therefore energised, and, on the triggering of the ignition process, the excitation winding 38, 51 is switched off, whose reed switch 33, 34 is associated with the respective plug 31, 32 which is not to be fired. Driving of the transistors 43, 56, and triggering of the ignition process, are effected in the same way as described with reference to Figure 1. In the cases so far described the reed switches 33, 34 act as contact makers.

Alternatively, it may sometimes be advantageous to construct the reed switches 33, 34 so that they operate as contact breakers. How this may be achieved will be described with reference to Figures 4 and 5. The reed switches 33, 34, which, in the case under consideration, are enclosed in a single common envelope 60, are provided with respective contacts 36, 37; 49, 50, of magnetically permeable material, which are closed when the excitation windings 38 and 51 are deenergised. The excitation winding 38 is divided into two parts 38a and 38b, of which the part 38a is disposed on the portion of the contact 36 extending externally of the envelope 60, and the part 38b is disposed on the portion of the contact 37 extending externally of the envelope 60.The winding sense of the parts of the excitation winding 38, or the direction of the current flowing through them, is so selected that the ends of the contacts 36, 37, which extend inside the envelope 60, and which serve to make the contact, have the same polarity. In the example, a south pole appears at these ends. Owing to the fact that the ends have the same polarity, they repel each other, whereby the current can be interrupted. The reed switch 34 is of similar construction, the excitation winding 51, in this case, being divided into two parts 51a, 51b, the part 51a being disposed on the portion of the contact 49, extending externally of the envelope 60, and the part 51b being disposed on the portion of the contact 50 extending externally of the envelope.In the case also of the excitation winding 51, the current direction, or the winding sense, is so selected that a south pole appears, that is, the same polarity exists, at the contactmaking ends of the contacts 49, 50.

If, now the reed switches 33, 34, as shown in Figure 4, are considered in situ in the circuit arrangement shown in Figure 1, that is, the ends 39 of the excitation winding 38 being connected to the first lead 3, and the end 40 of the winding being connected to the collector of the transistor 42, whose emitter-collector path forms the excitation switch 41, and, similarly, the ends 52 of the excitation winding 51 being connected to the first lead 3, and the end 53 of the winding being connected to the collector of the transistor 55, whose emitter-collector path forms the excitation switch 54, and if it is also remembered that, in the case of the circuit arrangement shown in Figure 1, the excitation switches 41, 54 are normally blocked, then it follows that, during the ignition process, only the reed switch 33, 34, associated with the excitation winding 38, 51 which is not switched on by means of the respective excitation switch 41, 54, is maintained conducting.

If, on the other hand, the reed switches 33, 34, shown in Figure 4, are considered in situ in the circuit arrangement shown in Figure 3, that is, the ends 39, 52 of the windings being connected to the first lead 3, and the ends 40, 53 of the windings being connected to a respective one of the two collectors of the transistors 43, 46, and if it is also remembered that the excitation switches 41, 54 in the circuit arrangement shown in Figure 3 are normally conducting, then it follows that, dur ing the ignition process, the reed switch 33, 34, associated with the excitation wind ing 38, 51 which is switched off by means of the respective excitation switch 41, 54, is rendered conducting.

Each reed switch 33 or 34, together with its respective excitation winding 38 or 51, and the envelope 35 or 48 enclosing its respective contacts 36, 37 or 49, 50, may advantageously be combined to form a separate module or, as shown in Figure 4, all the reed switches may form a single module in a common envelope 60. Such a module may be fitted at any-desired point of the internal combustion engine, and, if this engine serves to drive a motor vehicle, may be fitted at any desired point of the vehicle. It is advantageous, however, if a housing, for accommodating other com ponents necessary for ignition, is used as a receptacle or supporting member for accommodating this module, or these modules. The housing of the ignition trans former (ignition coil) 6, in particular its cover, which serves to seal the housing, may be used for this purpose.The housing, which accommodates the electronic modules serving to control the ignition system, or the insulating body enclosing the sparking plug connection, is also very suit able for this purpose. Alternatively, of course, it is possible to use the conven tional ignition distributor to house the signal generator 17, to retain the ignition timing by means of a fluid-pressure sensi tive device connected to the inlet manifold, or by means of a centrifugal timing device, and then to use the cover, in which previously a distributor rotor rotated past fixed- contacts, to accommodate the above mentioned module or modules.

Reliability of the voltage discharge via the reed switches 33, 34 may be increased by filling the envelopes 35, 48, 60, at least partially, with a substance having a high dielectric strength, which is liquid at room temperature. A fluorine-substituted, prefer ably cyclic, hydrocarbon, or a chlorine substituted, preferably cyclic, hydrocar bon, are particularly suitable materials for this purpose. Alternatively, however, the electric strength of the blocked reed switches 33, 34 may advantageously be increased by filling the envelope 35, 48, 60 with a substancehaving a high dielectric strength, which is gaseous at room temperature, the filling pressure being at least 2 bar, and preferably 6 bar. Sulphur hexafluoride is especially suitable for this purpose.Hydrogen and/or nitrogen and/or carbon dioxide, either alone, or combined with sulphur hexafluoride, are also suitable, however, as a gaseous charge. When sulphur hexafluoride is used in combination with hydrogen, the proportion of hydrogen should preferably be up to 30% by volume.

When sulphur hexafluoride is used in combination with hydrogen and nitrogen, the overall amount of hydrogen and nitrogen should preferably be up to 30% by volume. In the event of sulphur hexafluoride being used in combination with hydrogen, nitrogen and carbon dioxide, the overall amount of hydrogen, nitrogen and carbon dioxide should preferably be up to 30% by volume.

Introduction of the gaseous or liquid charges into the envelopes 35, 48, 60 may be facilitated if the portions of the respective contacts 36, 37; 49, 50, extending from the outside through the envelopes 35, 48, 60, are tubular in form, as illustrated by the contact 36 in Figure 4 and also in Figure 6. Following the introduction of the charge into the envelopes 35, 48, 60, the outer ends of the contacts 36, 37; 49, 50 are pinched off, so that the filling pressure is retained, and the charge is sealed in.

Alternatively, of course, it is possible for the portion, extending from the outside into the respective envelope 35, 48, 60, of only one of the contacts 36, 37; 49, 50 to be tubular in form, and for the remainder of the contacts to be in the form of simple reeds.

An iron-cobalt-nickel alloy, or an iron cobalt-chromium alloy, is a particurly suitable material for the manufacture of the contacts 36, 37; 49, 50.

A high degree of durability of the contacts, and good contacting, may be obtained if at least one of the two contacts 36, 37; 49, 50 of the individual reed switches is provided, at.least in the region of con tact, with a coating consisting of at least one metal of the first or eighth sub-groups of the periodic system of the elements, with the exception of iron. Alternatively, carbides of titanium, zirconium, hafnium, tantalum or niobium are very suitable materials for such a coating.

Reliability of the voltage discharge via the reed switches 33, 34 may be further increased if the portions of the contacts 36, 37; 49, 50, extending inside the respective envelopes 35, 48, 60, have no angular edges, but are rounded, in particular on their contacting portions, as shown in Figures 6, 7 and 8, wllere the contacting portion of one of the contacts 36, 37; 49, 50 is shown.

In the simplest instance, of course, it is possible, alternatively, to provide a vacuum inside the envelope 35, 48, 60.

It is preferable for the signal generator used to be an inductive type signal generator, but it is also possible to use an optical signal generator, a Hall signal generator, or a conventional mechanical contact brcaker, if a control voltage signal, of finite slope with respect to time, is provided by means of a serially connected pulse shaping circuit.

It would also be possible to assist the opening and closing of the reed switches 33, 34 by means of small permanent magnets, attached to the contacts 36, 37; 49, 50, namely, in the region of their contacting portions.

WHAT WE CLAIM IS:- 1. An ignition system for internal combustion engines, comprising a plurality of sparking plugs to which ignition voltage surges are distributed from the secondary winding of an ignition transformer to fire the plugs in repeating sequences, these ignition voltage surges being induced in the secondary winding by repeated opening of the circuit of the primary winding of the ignition transformer, the system also comprising, for each sparking plug, an electromagnetically-operated reed switch in series with the plug, an excitation winding for controlling the reed switch, and an electircally-operable excitation switch in series with the excitation winding, the series circuits comprising the excitation windings and the electrically-operable excitation switches being connected in parallel with one another and being adapted for connection across the power supply of the ignition system, the system being adapted to operate so that the electrically-operable excitation switches control the excitation windings in a manner to ensure that the contacts of the reed switch in series with the spark plug next to be fired are in a closed state and the contacts of the remainder of the reed switches are open before the circuit of the primary winding of the ignition transformer is opened to induce an ignition voltage surge in the secondary winding for firing that plug, and to ensure that the contacts of that reed switch remain in a closed state and the contacts of the remainder of the reed switches remain in an open state at least until the end of the respective ignition process.

2. An ignition system according to claim 1, in which triggering of the ignition voltage surges and operation of the excitation switches are effected in dependence upon a single signal generator, coupled to the internal combustion engine.

3. An ignition system according to claim 2, in which by means of the signal generator, control voltage signals, having a finite slope with respect to time, are generated, and in which the switching voltage threshold for the triggering of the ignition voltage surges is higher than the switching voltage threshold for reversal of the excitation switches.

4. An ignition system according to claim 1, 2 or 3 in which each of the excitation switches is formed by the emittercollector path of a respective one of a plurality of transistors.

5. An ignition system according to any of claims 1 to 4, in which the reed switches are in the form of contact makers.

6. An ignition system according to claim 5, in which the excitation switches are normally blocked, and in which during the ignition voltage surge, only the excitation switch associated with the respective excitation winding of the reed switch, connected between the secondary winding of the ignition transformer and the plug to be fired, is conducting.

7. An ignition system according to claim 5, in which the excitation switches are normally conducting, but, during the ignition voltage surge, only the excitation switch associated with the respective excitation winding of the reed switch, connected between the secondary winding of the ignition transformer and the plug to be fired remains conducting.

8. An ignition system according to any of claims 1 to 4 in which the reed switches are in the form of contact breakers.

9. An ignition system according to claim 8, in which the excitation switches are normally blocked, and in which, during the ignition voltage surge, only the excitation switch associated with the respective excitation winding of the reed switch, connected between the secondary winding of the ignition transformer and the plug to be fired, remains blocked.

10. An ignition system according to claim 8, in which the excitation switches are normally conducting, and in which, during the ignition voltage surge, the excitation switch associated with the respective excitation winding of the reed switch, connected between the secondary winding of the ignition transformer and the plug to be fired, is blocked.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Reliability of the voltage discharge via the reed switches 33, 34 may be further increased if the portions of the contacts 36, 37; 49, 50, extending inside the respective envelopes 35, 48, 60, have no angular edges, but are rounded, in particular on their contacting portions, as shown in Figures 6, 7 and 8, wllere the contacting portion of one of the contacts 36, 37; 49, 50 is shown. In the simplest instance, of course, it is possible, alternatively, to provide a vacuum inside the envelope 35, 48, 60. It is preferable for the signal generator used to be an inductive type signal generator, but it is also possible to use an optical signal generator, a Hall signal generator, or a conventional mechanical contact brcaker, if a control voltage signal, of finite slope with respect to time, is provided by means of a serially connected pulse shaping circuit. It would also be possible to assist the opening and closing of the reed switches 33, 34 by means of small permanent magnets, attached to the contacts 36, 37; 49, 50, namely, in the region of their contacting portions. WHAT WE CLAIM IS:-

1. An ignition system for internal combustion engines, comprising a plurality of sparking plugs to which ignition voltage surges are distributed from the secondary winding of an ignition transformer to fire the plugs in repeating sequences, these ignition voltage surges being induced in the secondary winding by repeated opening of the circuit of the primary winding of the ignition transformer, the system also comprising, for each sparking plug, an electromagnetically-operated reed switch in series with the plug, an excitation winding for controlling the reed switch, and an electircally-operable excitation switch in series with the excitation winding, the series circuits comprising the excitation windings and the electrically-operable excitation switches being connected in parallel with one another and being adapted for connection across the power supply of the ignition system, the system being adapted to operate so that the electrically-operable excitation switches control the excitation windings in a manner to ensure that the contacts of the reed switch in series with the spark plug next to be fired are in a closed state and the contacts of the remainder of the reed switches are open before the circuit of the primary winding of the ignition transformer is opened to induce an ignition voltage surge in the secondary winding for firing that plug, and to ensure that the contacts of that reed switch remain in a closed state and the contacts of the remainder of the reed switches remain in an open state at least until the end of the respective ignition process.

11. An ignition system according to any

preceding claim, in which a housing, which accommodates further components necessary for ignition, serves as a receptacle or supporting member for the reed switches, together with their respective excitation windings and their respective envelopes, composed of non-magnetic material, which envelopes enclose the switch contacts.

12. An ignition system according to claim 11, in which the contacts of all the reed switches are enclosed within a single envelope.

13. An ignition system according to claim 11 or 12, in which the envelope is filled, at least partially, with a substance having a high dielectric strength, which is fluid at room temperature.

14. An ignition system according to claim 13, in which the envelope is filled with a fluorine-substituted hydrocarbon.

15. An ignition system according to claim 13, in which the envelope is filled with a chlorine-substituted hydrocarbon.

16. An ignition system according to claim 11 or 12, in which the envelope is filled with a substance having a high dielectric strength, which is gaseous at room temperature, and in which the filling pressure is at least 2 bar.

17. An ignition system according to claim 15, in which the envelope contains sulphur hexafluoride.

18. An ignition system according to claim 16 or 17, in which the envelope con tains hydrogen.

19. An ignition system according to any of claims 16 to 18, in which the envelope contains nitrogen.

20. An ignition system according to any one of claims 16 to 19, in which the envelope contains carbon dioxide.

21. An ignition system according to any of claims 11 to 20, in which the contacts of the reed switches are composed of an iron-cobalt-nickel alloy.

22. An ignition system according to any of claims 11 to 20, in which the contacts of the reed switches are composed of an iron-cobalt-chromium alloy.

23. An ignition system according to any of claims 11 to 20, in which at least one of the two contacts, pertaining to the respective reed switch, is provided, at least in its contacting region, with a coating, which, with the exception of iron, is composed of at least one metal of the first or eighth subgroups of the periodic system of the elements.

24. An ignition system according to any of claims 1 to 20, in which at least one of the two contacts, pertaining to a respective reed switch is provided at least in its contacting region, with a coating compound of a carbide of titanium, zirconium, hafnium, tantalum or niobium.

25. An ignition system according to claims 1 to 11, in which at least the regions of the contacts of the reed switches which extend inside the envelope have rounded edges.

26. An ignition system, for internal combustion engines, substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.