Ignition system dependent upon engine speed

Abstract

An internal combustion engine is comprised of at least one engine cylinder, devices for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder once per ignition cycle. An ignition transformer has a secondary winding connected across the igniting element, and also has a primary winding. A flow of current is established through the primary winding. An electronic switch is connected in the current path of the primary winding to control the flow of current through the primary winding. A triggering unit generates a train of crankshaft-synchronized triggering signals. A monostable control circuit has a trigger input for receipt of such triggering signals, and is operative when triggered into the unstable state for rendering and maintaining the electronic switch nonconductive for the duration of the unstable state, so as to interrupt the flow of current through the primary winding and induce across the secondary winding a voltage surge causing the igniting element to ignite the combustion mixture in the engine cylinder. When the monostable control circuit reverts to its stable state, it renders and maintains the electronic switch conductive for the duration of the stable state, whereby to establish a build-up of current flow in the primary winding in preparation for the next interruption of current flow. The monostable control circuit includes a timing stage operative for automatically .[.increasing.]. .Iadd.decreasing .Iaddend.the ratio of the duration of the unstable state to the duration of the ignition cycle with increasing engine speed within a predetermined range of engine speeds.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims: 
     
       1. In the ignition system of an internal combustion engine comprised of at least one electrical igniting element operative for producing an ignition spark once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding; means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering pulses; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering pulses, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element of produce an ignition spark, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative with increasing engine speed within a predetermined range of engine speeds for automatically and progressively .[.increasing.]. .Iadd.decreasing .Iaddend.the ratio of the duration of said unstable state to the duration of said ignition cycle, and wherein said timing means comprises energy-storing means, means for effecting a first change of stored energy of said energy-storing means during the intervals between said triggering pulses and thereby for time periods dependent upon engine speed, and means operative upon generation of a triggering pulse by said triggering means for causing said monostable control means to undergo a transition to said unstable state and initiating an opposite second change of stored energy of said energy-storing means and maintaining said monostable control means in said unstable state .[.until the completion of.]. .Iadd.during .Iaddend.said second change of stored energy. 
     
     
       2. The system defined in claim 1, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering .[.signal.]. .Iadd.pulse .Iaddend.for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value. 
     
     
       3. The system defined in claim 1, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles of first and second polarity, and threshold detecting means operative for generating the leading edge of a triggering pulse for said monostable control means when a voltage half-cycle of said first polarity reaches a first predetermined value and operative for generating the trailing edge of such triggering pulse when the same voltage half-cycle subsequently reaches a second predetermined value. 
     
     
       4. The system defined in claim 3, said A.C. generator means comprising means operative for generating an A.C. voltage comprised of voltage half-cycles of first and second polarity having such respective durations that the ratio of the duration of the voltage half-cycles of said first polarity to the duration of the voltage half-cycles of said second polarity decreases with increasing engine speed within said predetermined range of engine speeds. 
     
     
       5. An ignition system for an internal combustion engine, comprising in combination, a spark-producing element operative for producing an ignition spark once per ignition cycle; an ignition transformer including a secondary winding connected in circuit with said element and further including a primary winding; a controllable electronic switch connected in circuit with said primary winding and operative when conductive and non-conductive for respectively permitting and preventing the flow of current through said primary winding, said controllable electronic switch having a control input; a control transistor having a collector connected to said control input for controlling the conductivity of said switch in dependence upon the voltage of said collector, said control transistor being comprised of a control current path for the flow of control current for controlling the conductivity of said control transistor and the voltage at said collector thereof; a signal generator operative for generating an A.C. control voltage having a frequency proportional to engine speed; a threshold circuit connected to said signal generator and having .[.said.].  first and second circuit states which said threshold circuit assumes when said control voltage is of one and the opposite polarity, respectively; and energy storing means connected to said threshold circuit and to said control current path of said control transistor so as to be alternately energized through said control path and then deenergized through said threshold circuit when the latter is in said second circuit state, with the amount of energy stored by said energy storing means decreasing with increasing engine speed for automatically and progressively decreasing the duration of the .[.conduction.]. .Iadd.non-conduction .Iaddend.time of said control transistor while increasing the fraction of the ignition cycle during which said control transistor conducts, the start of the deenergizing of said energy storing means .[.through.]. .Iadd.causing via .Iaddend.said control current path .[.causing.]. said controllable electronic switch to become non-conductive, and said controllable electronic switch becoming conductive again when the amount of energy stored by said energy storing means reaches a predetermined value during said deenergizing thereof. 
     
     
       6. A system as defined in claim 5, wherein said energy storing means is capacitive, and wherein the energizing and deenergizing of said energy storing means occurs in the form of charging and discharging of said energy storing means. 
     
     
       7. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder once per ignition cycle, in combustion, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering pulses each having a crankshaft-synchronized leading edge and each having a duration dependent upon engine speed; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering signals, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative for automatically .[.increasing.]. .Iadd.decreasing .Iaddend.the ratio of the duration of said ignition cycle to the duration of said stable state with increasing engine speed within a predetermined range of engine speeds, wherein said timing means comprises first energy-storing capacitor means and second energy-storing means, means for charging said first capacitor means at a first charging rate and said second capacitor means at a higher second charging rate during the intervals between said triggering pulses and thereby for time periods dependent upon engine speed, and means operative upon generation of the leading edge of a triggering pulse by said triggering means for causing said monostable control means to undergo a transition to said unstable state if at least one of said capacitor means is charged to a respective predetermined voltage and operative for discharging said first and second capacitor means and operative for maintaining said monostable control means in said unstable state until both said first and second capacitor means are discharged to predetermined respective extents, whereby if the duration of the time intervals between triggering pulses at high engine speeds is too short to permit said first capacitor means to charge up to the respective predetermined voltage then at least said second capacitor means will charge up to the respective predetermined voltage in order to insure that said monostable control means will be triggered into its unstable state upon generation of the leading edge of a triggering pulse by said triggering means. 
     
     
       8. The system defined in claim 7, wherein said first energy-storing means and said second energy-storing means are together comprised of a first capacitor and a diode connected in series therewith and a second capacitor connected in parallel with the series combination of said first capacitor and diode. 
     
     
       9. The system defined in claim 8, wherein the capacitance of said second capacitor is approximately one hundred times the capacitance of said first capacitor. 
     
     
       10. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder, once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for penetrating a train of crankshaft-synchronized triggering signals; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering signals, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative for automatically .[.increasing.]. .Iadd.decreasing .Iaddend.the ratio of the duration of said unstable state to the duration of said ignition cycle with increasing engine speed within a predetermined range of engine speeds, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering signal for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value, wherein said monostable control means comprises a source of stabilized voltage having two terminals, a first capacitor and a diode connected in series, a second capacitor connected in parallel with the series combination of said first capacitor and said diode, a resistor connecting one terminal of said source to the junction between one terminal of said diode and one terminal of said first capacitor, a resistor connecting said one terminal of said source to the other terminal of said first capacitor, and an electronic switch element so connected to said electronic switch means as to control the conductivity of the latter and comprising a control electrode connected to said other terminal of said first capacitor, and said other terminal of said diode being connected to the output of said threshold-detecting means. 
     
     
       11. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering signals; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering signals, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative for automatically .[.increasing.]. .Iadd.decreasing .Iaddend.the ratio of the duration of said unstable state to the duration of said ignition cycle, with increasing engine speed within a predetermined range of engine speeds, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering signal for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value, wherein said monostable control means comprises a source of stabilized voltage having two terminals, a first capacitor and a diode connected in series with each other, a resistor connecting one terminal of said source to the junction between one terminal of said diode and one terminal of said first capacitor, an AND-gate so connected to said electronic switch means as to control the conductivity of the latter and having a first input connected to said other terminal of said first capacitor and having a second input, and a second capacitor having one terminal connected to the other terminal of said diode and having another terminal connected to said second input of said AND-gate, said other terminal of said diode being connected to the output of said threshold-detecting means. 
     
     
       12. The system defined in claim 11, wherein said AND-gate is comprised of two electronic switch elements having current paths connected in series with each other and being so connected to said electronic switch means as to render the latter conductive when both said two electronic switch elements are conductive, with each of said two electronic switch elements having a control electrode constituting a respective one of said two inputs of said AND-gate. 
     
     
       13. The system defined in claim 12, wherein said two electronic switch elements are both transistors each having a base constituting the respective control electrode and an emitter and a collector, the base of the first of said transistor being connected to said other terminal of said first capacitor and the base of the second transistor being connected to said other terminal of said second capacitor via a diode, the collector of said second transistor being connected to the emitter of said first transistor and being furthermore connected via a resistor to said one terminal of said source of stabilized voltage, and further including a resistor connecting said other terminal of said second capacitor to said one terminal of said source. .Iadd. 14. An ignition system for an internal combustion engine, comprising in combination, a spark-producing element operative for producing an ignition spark once per ignition cycle; an ignition transformer including a secondary winding connected in circuit with said element and further including a primary winding; a controllable electronic switch connected in circuit with said primary winding and operative when conductive and non-conductive for respectively permitting and preventing the flow of current through said primary winding, said controllable electronic switch having a control input; a control transistor having a collector connected to said control input for controlling the conductivity of said switch in dependence upon the voltage of said collector, said control transistor being comprised of a control current path for the flow of control current for controlling the conductivity of said control transistor and the voltage at said collector thereof; a signal generator operative for generating an A.C. control voltage having a frequency proportional to engine speed; a threshold circuit connected to said signal generator and having first and second circuit states which said threshold circuit assumes when said control voltage is of one and the opposite polarity, respectively; and energy storing means connected to said threshold circuit and to said control current path of said control transistor and forming with said threshold circuit a monostable control means, said energy storing means being alternately energized through said control path and then deenergized through said threshold circuit when the latter is in said second circuit state, with the amount of energy stored by said energy storing means decreasing with increasing engine speed, the start of the deenergizing of said energy storing means causing via said control current path said controllable electronic switch to become non-conductive, and said controllable electronic switch becoming conductive again when the amount of energy stored by said energy storing means reaches a predetermined value during the deenergizing thereof. .Iaddend..Iadd. 15. In the ignition system of an internal combustion engine comprised of at least one electrical igniting element operative for producing an ignition spark once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding; means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering pulses; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering pulses, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to produce an ignition spark, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative with increasing engine speed for automatically and progressively controlling the duration of said unstable state in such a manner that the time interval during the ignition cycle for which said electronic switch means is conductive will be sufficiently long for effecting predetermined current build-up in said primary winding irrespective of the increasing engine speed, and wherein timing means comprises energy-storing means, means for effecting a first change of stored energy of said energy-storing means during the intervals between said triggering pulses and thereby for time periods dependent upon engine speed, and means operative upon generation of a triggering pulse by said triggering means for causing said monostable control means to undergo a transition to said unstable state and initiating an opposite second change of stored energy of said energy-storing means and maintaining said monostable control means in said unstable state during said second change of stored energy. .Iaddend..Iadd. 16. The system defined in claim 15, wherein said triggering means comprises A.C. generator operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering pulse for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value. .Iaddend..Iadd. 17. The system defined in claim 15, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles of first and second polarity, and threshold-detecting means operative for generating the leading edge of a triggering pulse for said monostable control means when a voltge half-cycle of said first polarity reaches a first predetermined value and operative for generating the trailing edge of such triggering pulse when the same voltage half-cycle subsequently reaches a second predetermined value. .Iaddend..Iadd. 18. The system defined in claim 17, said A.C. generator means comprising means operative for generating an A.C. voltage comprised of voltage half-cycles of first and second polarity having such respective durations that the ratio of the duration of the voltage half-cycles of said first polarity to the duration of the voltage half-cycles of said second polarity decreases with increasing engine speed within said predetermined range of engine speeds. .Iaddend..Iadd. 19. An ignition system for an internal combustion engine, comprising in combination, a spark-producing element operative for producing an ignition spark once per ignition cycle; an ignition transformer including a secondary winding connected in circuit with said element and further including a primary winding; a controllable electronic switch connected in circuit with said primary winding and operative when conductive and non-conductive for respectively permitting and preventing the flow of current through said primary winding, said controllable electronic switch having a control input; a control transistor having a collector connected to said control input for controlling the conductivity of said switch in dependence upon the voltage at said collector, said control transistor being comprised of a control current path for the flow of control current for controlling the conductivity of said control transistor and the voltage at said collector thereof; a signal generator operative for generating an A.C. control voltage having a frequency proportional to engine speed; a threshold circuit connected to said signal generator and having first and second circuit states which said threshold circuit assumes when said control voltage is of one and the opposite polarity, respectively; and energy storing mens connected to said threshold circuit and to said control current path of said control transistor so as to be alternately energized through said control path and then deenergized through said threshold circuit when the latter is in said second circuit state, with the amount of energy stored by said energy storing means decreasing with increasing engine speed for automatically and progressively controlling the duration of the conduction time of said control transistor in such a manner that the time interval during the ignition cycle for which said controllable electronic switch is conductive will be sufficiently long for effecting predetermined current build-up in said primary winding irrespective of increasing engine speed, the start of the deenergizing of said energy storing means causing via said control current path said controllable electronic switch to become non-conductive, and said controllable electronic switch becoming conductive again when the amount of energy stored by said energy storing means reaches a predetermined value during the deenergizing thereof. .Iaddend. 20. A system as defined in claim 19, wherein said energy storing means is capacitive, and wherein the energizing and deenergizing of said energy storing means occurs in the form of charging and discharging of said energy storing means. .Iadd.21. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering pulses each having a crankshaft-synchronized leading edge and each having a duration dependent upon engine speed; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering pulses, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including means operative with increasing engine speed for automatically and progressively controlling the duration of said unstable state in such a manner that the time interval during the ignition cycle for which said electronic switch means is conductive will be sufficiently long for effecting predetermined current build-up in said primary winding irrespective of increasing engine speed, wherein said timing means comprises first energy-storing capacitor means and second energy-storing means, means for charging said first capacitor means at a first charging rate and said second capacitor means at a higher second charging rate during the intervals between said triggering pulses and thereby for time periods dependent upon engine speed, and means operative upon generation of the leading edge of a triggering pulse by said triggering means for causing said monostable control means to undergo a transition to said unstable state if at least one of said capacitor means is charged to a respective predetermined voltage and operative for discharging said first and second capacitor means and operative for maintaining said monostable control means in said unstable state until both said first and second capacitor means are discharged to predetermined respective extents, whereby if the duration of the time intervals between triggering pulses at high engine speeds is too short to permit said first capacitor means to charge up to the respective predetermined voltage then at least said second capacitor means will charge up to the respective predetermined voltage in order to insure that said monostable control means will be triggered into its unstable state upon generation of the leading edge of a triggering pulse by said triggering means. .Iaddend. .Iadd.22. The system defined in claim 21, wherein said first energy-storing means and said second energy-storing means are together comprised of a first capacitor and a diode connected in series therewith and a second capacitor connected in parallel with the series combination of said first capacitor and said diode. .Iaddend. .Iadd.23. The system defined in claim 22, wherein the capacitance of said second capacitor is approximately one hundred times the capacitance of said first capacitor. .Iaddend..Iadd. 24. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder, once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for penetrating a train of crankshaft-synchronized triggering signals; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering signals, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative with increasing engine speed for automatically and progressively controlling the duration of said unstable state in such a manner that the time interval during the ignition cycle for which said electronic switch means is conductive will be sufficiently long for effecting predetermined current build-up in said primary winding irrespective of increasing engine speed, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering signal for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value, wherein said monostable control means comprises a source of stabilized voltage having two terminals, a first capacitor and a diode connected in series, a second capacitor connected in parallel with the series combination of said first capacitor and said diode, a resistor connecting one terminal of said source to the junction between one terminal of said diode and one terminal of said first capacitor, a resistor connecting said one terminal of said source to the other terminal of said first capacitor, and an electronic switch element so connected to said electronic switch means as to control the conductivity of the latter and comprising a control electrode connected to said other terminal of said first capacitor, and said other terminal of said diode being connected to the output of said threshold-detecting means. .Iaddend..Iadd. 25. In the ignition system of an internal combustion engine comprised of at least one engine cylinder, means for introducing a combustion mixture into the cylinder, and an electrical igniting element operative for igniting the combustion mixture in the cylinder once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding means for establishing a flow of current through said primary winding and including electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding; crankshaft-synchronized triggering means operative for generating a train of crankshaft-synchronized triggering signals; monostable control means connected to said electronic switch means and having a trigger input connected to said triggering means for receipt of said triggering signals, and operative when triggered into the unstable state thereof for rendering and maintaining said switch means non-conductive for the duration of said unstable state, whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to ignite said combustion mixture, and operative upon reverting to the stable state thereof for rendering and maintaining said switch means conductive for the duration of said stable state, whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow, said monostable control means including timing means operative with increasing engine speed for automatically and progressively controlling the duration of said unstable state in such a manner that the time interval during the ignition cycle for which said electronic switch means is conductive will be sufficiently long for effecting predetermined current build-up in said primary winding irrespective of increasing engine speed, wherein said triggering means comprises A.C. generator means operative for generating an A.C. voltage having a frequency proportional to engine speed and comprised of voltage half-cycles, of first and second polarity and threshold-detecting means operative for generating a triggering signal for said monostable control means when a voltage half-cycle of said first polarity reaches a predetermined value, wherein said monostable control means comprises a source of stabilized voltage having two terminals, a first capacitor and a diode connected in series with each other, a resistor connecting one terminal of said source to the junction between one terminal of said diode and one terminal of said first capacitor, a resistor connecting said one terminal of said source to the other terminal of said first capacitor, an AND-gate so connected to said electronic switch means as to control the conductivity of the latter and having a first input connected to said other terminal of said first capacitor and having a second input, and a second capacitor having one terminal connected to the other terminal of said diode and having another terminal connected to said second input of said AND-gate, said other terminal of said diode being connected to the output of said threshold-detecting means. .Iaddend..Iadd. 26. The system defined in claim 25, wherein said AND-gate is comprised of two electronic switch elements having current paths connected in series with each other and being so connected to said electronic switch means as to render the latter conductive when both said two electronic switch elements are conductive, with each of said two electronic switch elements having a control electrode constituting a respective one of said two inputs of said AND-gate. .Iaddend..Iadd. 27. The system defined in claim 26, wherein said two electronic switch elements are both transistors each having a base constituting the respective control electrode and an emitter and a collector, the base of the first of said transistor being connected to said other terminal of said first capacitor and the base of the second transistor being connected to said other terminal of said second capacitor via a diode, the collector of said second transistor being connected to the emitter of said first transistor and being furthermore connected via a resistor to said one terminal of said source of stabilized voltage, and further including a resistor connecting said other terminal of said second capacitor to said one terminal of said source. .Iaddend..Iadd. 28. In the ignition system of an internal combustion engine comprised of at least one electrical igniting element operative for producing an ignition spark once per ignition cycle, in combination, an ignition transformer having a secondary winding connected across said igniting element and having a primary winding;   a threshold circuit having a control-signal input and a triggering-signal output and assuming either a first state or a second state in dependence upon the control signals received at said control-signal input;   crankshaft-synchronized control-signal-generating means operative for applying to said control-signal input of said threshold circuit control signals causing said threshold circuit to alternately assume said first and second states for respective first-state and second-state time intervals t1 and t2, where t1 + t2 equals the duration of one ignition cycle, the ratio t2/(t1 + t2) undesirably decreasing with increasing engine speed within a predetermined range of engine speeds;   monostable control means having a trigger input connected to said trigger-signal output of said threshold circuit and triggered into the unstable state thereof in response to assumption of said first state by said threshold circuit and subsequently reverting into the stable state thereof, the unstable-state and stable-state durations of said monostable control means being equal to t3 and t4, respectively, where t3 + t4 equals the duration of one ignition cycle;   electronic switch means in the current path of said primary winding operative for controlling the flow of current through said primary winding,   said electronic switching means being connected to said monostable control means and being rendered non-conductive when said monostable control means is triggered into said unstable state and remaining non-conductive for the duration of said unstable state,   whereby to interrupt the flow of current through said primary winding and induce across said secondary winding a voltage surge causing said igniting element to produce an ignition spark,   said electronic switch means being rendered conductive when said monostable control means reverts into said stable state and remaining conductive for the duration of said stable state,   whereby to establish a build-up of current flow in said primary winding in preparation for the next interruption of current flow,   said monostable control means including timing means operative with increasing engine speed within said predetermined range of engine speeds for automatically and progressively decreasing the duration of said unstable state while simultaneously causing the ratio t4/(t3 + t4) plotted with respect to engine speed to have within said predetermined speed range a slope which is more positive than the ratio t2/(t1 + t2) plotted with respect to engine speed within said predetermined speed range,   said monostable control means including a control transistor having a control current path,   said timing means being energy-storing timing means connected to said control current path of said control transistor and to said threshold circuit for alternating energization of said energy-storing timing means through said control current path of said control transistor and deenergization of said energy-storing timing means through said threshold circuit. .Iaddend.