CAPACITIVE DISCHARGE IGNITION SYSTEM
United States Patent 3838328
A capacitive discharge ignition system utilizing a silicone controlled rectifier to control the application of capacitively stored energy to the primary of an ignition coil. This energy, which is supplied by a DC-to-DC convertor network, is initially stored in a capacitor. The capacitor is then discharged through the primary ignition coil circuit at predetermined times under the control of the SCR. The SCR conduction state is positively controlled, allowing for multiple energy exchange cycles to occur if desired. The DC-to-DC convertor is turned off during the time that the SCR is conducting.
US Patent References:
Semiconductor voltage regulator
Kirk - June 1964 - 3137811
Regulated voltage capacitor discharge circuit
Knak - December 1968 - 3417306
SEMICONDUCTOR VOLTAGE REGULATOR
Kirk - September 1969 - 3469175
IGNITION ARRANGEMENT FOR INTERNAL COMBUSTION ENGINES
Issler et al. - January 1970 - 3489129
ELECTRONIC CONTROL CIRCUIT
Grundberg - February 1972 - 3639826
Semiconductor voltage regulator
Kirk - June 1964 - 3137811
Regulated voltage capacitor discharge circuit
Knak - December 1968 - 3417306
SEMICONDUCTOR VOLTAGE REGULATOR
Kirk - September 1969 - 3469175
IGNITION ARRANGEMENT FOR INTERNAL COMBUSTION ENGINES
Issler et al. - January 1970 - 3489129
ELECTRONIC CONTROL CIRCUIT
Grundberg - February 1972 - 3639826
Application Number:
05/342467
Publication Date:
09/24/1974
Filing Date:
03/19/1973
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
123/598
International Classes:
F02P15/10; H02M3/338; F02P15/00; H02M3/24; H02M3/28
Field of Search:
123/148OC,148D 321/2 322/28
US Patent References:
Primary Examiner:
Shoop Jr., William M.
Attorney, Agent or Firm:
Christensen, O'Connor, Garrison & Havelka
Claims:
What is claimed is
1. An electronic ignition system, comprising:
2. An apparatus of claim 1, wherein said storing means is a capacitor.
3. An apparatus of claim 1, wherein said circuit means includes a DC-to-DC convertor.
4. An apparatus of claim 1, wherein said discharging means is a first silicon-controlled rectifier.
5. An apparatus of claim 1, including ignition transformer means having a primary winding and a secondary winding, said primary winding being in series connection with said storing means and said discharging means.
6. An electronic ignition system, comprising:
7. An apparatus of claim 6, wherein said discharging means is a first silicon-controlled rectifier.
8. An apparatus of claim 6, wherein said means for controlling includes a second silicon-controlled rectifier, saId second silicon-controlled rectifier being connected to said discharging means such that upon conduction of said second silicon-controlled rectifier said discharging means becomes nonconductive.
9. An apparatus according to claim 8, including a timer means initiated upon said discharging means becoming conductive for indicating a predetermined time therefrom, and means for rendering said second silicon-controlled rectifier conductive at said predetermined time.
10. An apparatus of claim 9, wherein said timer means includes a resistor and a first capacitor, and wherein said means for rendering said second silicon-controlled rectifier conductive is a zener diode.
1. An electronic ignition system, comprising:
2. An apparatus of claim 1, wherein said storing means is a capacitor.
3. An apparatus of claim 1, wherein said circuit means includes a DC-to-DC convertor.
4. An apparatus of claim 1, wherein said discharging means is a first silicon-controlled rectifier.
5. An apparatus of claim 1, including ignition transformer means having a primary winding and a secondary winding, said primary winding being in series connection with said storing means and said discharging means.
6. An electronic ignition system, comprising:
7. An apparatus of claim 6, wherein said discharging means is a first silicon-controlled rectifier.
8. An apparatus of claim 6, wherein said means for controlling includes a second silicon-controlled rectifier, saId second silicon-controlled rectifier being connected to said discharging means such that upon conduction of said second silicon-controlled rectifier said discharging means becomes nonconductive.
9. An apparatus according to claim 8, including a timer means initiated upon said discharging means becoming conductive for indicating a predetermined time therefrom, and means for rendering said second silicon-controlled rectifier conductive at said predetermined time.
10. An apparatus of claim 9, wherein said timer means includes a resistor and a first capacitor, and wherein said means for rendering said second silicon-controlled rectifier conductive is a zener diode.
Description:
BACKGROUND OF THE INVENTION
This invention relates primarily to the art of semiconductor ignition systems, and more specifically, to capacitive discharge ignition systems utilizing a silicone controlled rectifier to control the application of a capacitively stored voltage to the primary of an ignition coil.
The ignition system of an internal combustion engine plays a critical role in the proper operation of an engine, particularly in a modern, emission-controlled automobile. The iignition system must supply a large, properly timed, spark to the spark plugs to properly ignite the fuel/air mixture contained in the engine's combustion chambers. Until recently, nearly all ignition systems used the sequential opening and closing of mechanical distributor points to directly control the application of a battery voltage to the primary of an ignition coil having a high secondary-to-primary turns ratio. The resulting high voltage at the secondary of the coil was then applied to the spark plugs in a predetermined sequence to achieve a smooth running of the engine.
With the advent of semi-conductor devices, however, there has resulted a great deal of experimentation with respect to ignition systems, including oscillator-excited ignition systems, gate controlled ignition systems, piezoelectric ignition systems, and capacitive discharge ignition systems, to name some varieties currently being investigated as well as commercially used.
The capacitive discharge ignition system utilizing a silicone controlled rectifier is currently a popular and practical implementation of a semi-conductor ignition system. Present capacitive discharge systems, however, have significant operational disadvantages. In a typical prior art capacitive discharge system, the charging capacitor charges to a specified maximum value, independent of the dwell time of the distributor points. When the points open, a positive pulse is applied to the gate element of an SCR, turning the SCR on, and a single energy exchange then occurs between the primary of the ignition coil and the charging capacitor. The collapsing field of the primary of the ignition coil when the capacitor is completely discharged will then tend to establish a charge on the capacitor opposite to that of its original charge. The SCR is typically rendered non-conducting, or commutated, by a reverse potential, which is established by the reverse current in the circuit when the oppositely charged capacitor begins to discharge back through the primary of the coil. This reverse current is sufficient to commutate tbe SCR, and thus, one complete cycle of energy exchange between the capacitor and the coil typically occurs before the SCR becomes non-conducting. During the time that he SCR is conducting, the power converter which charges the capacitor is under a very heavy load, and does not produce a useful output for the operation of the ignition system. The battery, however, is still supplying the input to the power convertor, resulting in continued heat loss in the input section of the convertor.
Another significant disadvantage with present capacitive discharge systems utilizing distributor breaker points concerns the phenomenon of breaker point bounce, which may result in spurious triggering of the SCR and subsequent engine misfire.
Additionally, since the commutation of the SCR depends upon a reverse current sufficient to overcome the small forward current of the power convertor and reach a threshold reverse potential, a partial failure in the coil or the charging capacitor, or abnormalities in the coil load, may result in a failure of the SCR to commutate at all, ultimately resulting in complete engine failure and possible ignition burn-out.
In view of the above, it is an object of the present invention to provide a capacitive discharge ignition system wherein the operational state of the silicone controlled rectifier is controlled by means other than the energy stored in the primary of the coil.
It is another object of the present invention to provide a capacitive discharge ignition system wherein the entire power convertor circuit is turned off when the SCR is in a conducting state.
It is a further object of the present invention to provide a capacitive discharging system wherein more than one energy exchange cycle may take place between the charging capacitor and the primary of the coil during the distributor breaker point cycle.
It is a still further object of the present invention to eliminate or reduce the effect of breaker point bounce producing spurious triggering of the silicone controlled rectifier.
SUMMARY OF THE INVENTION
Briefly, in accordance with a preferred embodiment, the present invention includes a voltage supplying circuit, which converts a given low voltage generated by other components in the automobile to a significantly higher voltage; a series-circuit connected to the voltage supply circuit, including a charging capacitor, a discharge circuit for the capacitor, and the primary winding of the ignition coil; and a control circuit to control both the operation of the discharging circuit, and the voltage supply circuit such that the voltage supply circuit is turned off during the time that the discharging circuit is conducting, and such that a predetermined number of energy cycle exchanges may take place between the charging capacitor and the ignition coil during one ignition cycle.
DESCRIPTION OF THE DRAWINGS
A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which,
FIG. 1 is a block diagram of the capacitive discharge ignition system of the present invention.
FIG. 2 is a schematic diagram of the capacitive discharge ignition system of the present invention.
FIG. 3 is timing diagrams for the charging capacitor relative to the operational condition of the DC-to-DC convertor, and the distributor breaker point cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a nominal 12-volt voltage supply 11 for a standard automobile ignition system is shown with respect to a block diagram of the present invention. It should be understood, however, that although the present invention is disclosed in the context of an automobile ignition system, its principles are applicable in any system employing a semi-conductor control with capacitive discharge voltage circuit.
Furthermore, it should also be understood that if the system is utilized in an automobile ignition system, triggering sources other than breaker points, such as photochoppers or the like, may be utilized. In FIG. 1, however the 12-volt voltage supply 11 is connected to a standard ignition switch 12 which is in turn connected to a standard DC--DC convertor 13, which has a typical DC output of approximately 400 volts. This output is applied to a charging capacitor 17, which is discharged periodically under the control of a silicone controlled rectifier 18. The operating state of SCR 18 is in turn positively controlled by trigger network 21 and control circuit 23.
The triggering sequence is initiated by tbe breaker points 22 in the distributor. Additionally, the breaker points initiate the operation of a control circuit 23 which in turn controls the operation of the DC-to-DC convertor 13. CIrcuits 21 and 23 operate in conjunction to control the operational cycle of the entire ignition system.
Referrring to FIG. 3, timing diagrams for several important portions of the ignition system are shown, and which will be referred to in the following paragraphs, as the description of the preferred embodiment proceeds. Diagram A is an idealized charging cycle of the capacitor 17. Diagram B is the operational sequence of the DC-to-DC convertor, and Diagram C is the timing sequence for the opening and closing of the distributor points. Each of the timing sequences is referenced to the opening of the distributor points.
When the distributor points 22 open, and capacitor 17 is charged to a peak value, a positive gate current is established by the triggering network 21, turning SCR 18 on, thus allowing the charging capacitor 17 to discharge through the primary of the auto transformer 25, resulting in a high secondary voltage spike being applied to the electrodes of the spark plug 26. At the same time, the control circuit 23 turns the DC-to-DC convertor entirely off, effectively isolating it from the rest of the circuit.
Several cycles of energy exchange then occur between the capacitor 17 and the primary of the ignition coil or auto transformer through the SCR 18, which is held on by the trigger network 21, in the forward direction, and a diode bridge circuit in the reverse direction. At some predetermined point in time, after a number of energy exchanges has occurred, independent of whether the breaker points 22 remain open, the trigger network 21, in conjunction with the control circuit 23, will commutate the SCR, thus interrupting the current flow to the transformer 25. The trigger network 21 supplies a negative gate current to the SCR 18 in conjunction with a lack of forward current through the SCR, and commutation of the SCR occurs. When the SCR commutates, capacitor 17 will begin to charge again, because of the energy remaining in the system.
Sometime after the SCR 18 commutates, as determined by a timing circuit in control circuit 23, control circuit 23 will turn the DC-to-DC convertor back on, and the capacitor 17 will once again charge to a peak value.
Referring to FIG. 2, a schematic diagram of the present invention is shown. Transistors 30 and 31, transformer 32, diode bridge network 33, resistors 34, 35, 36, 37, 38, and capacitors 39, 40 and 41 comprise a DC-to-DC convertor. As explained above, this circuit converts ordinarily the nominal 12-volt battery voltage to a high DC voltage, which is used to charge the capacitor 17 during a controlled interval. FIG. 3 shows an idealized charging curve of the capacitor 17 with respect to the timing sequence of the distributor points and the timing of the DC-to-DC convertor.
The ignition cycle begins when the points 22 open, the capacitor 17 at this time being charged to a peak value. A current will flow from the positive battery terminal through resistor 50, resistor 51 and diode 52, and will then divide between resistor 53 and the gate connection of the SCR 18. This positive current turns the SCR on, placing it in the conducting state. Capacitor 17, which had been charged to a peak value, now discharges through the SCR 18 and the primary winding of the coil 56, which is connected as shown. As current flows through the primary winding 56 an expanding electromagnetic field is created around the winding, and the energy initially stored in the capacitor 17 is now stored in the electromagnetic field, less the energy dissipated in the spark plugs and through normal heat and other losses.
At some point, the capacitor 17 will become completely discharged, and the electromagnetic field around the coil will be at a maximum. The field will then begin to collapse and in turn will tend to reinforce the original current flow, thus charging the capacitor 17 in the opposite direction. The capacitor 17 will be charged to its maximum opposite or negative value when the field around the primary of the coil is at a minimum. The energy exchange then continues, as a reverse current through the coil 56 is established, which flows through the diode bridge 33 and tends to charge the capacitor 17 in the original, or positive direction. This continuous energy exchange would continue until the energy is depleted due to the natural dampening effect of energy losses in the circuit elements or until the SCR is commutated. During a predetermined number of these cycles, the SCR 18 is continuously conducting, provided with a positive gate current from the triggering circuit, also allowing conduction through the bridge diode consistent with the reverse current of energy exchange cycle.
The commutation of the SCR 18 is initiated, after a predetermined number of energy exchanges, by the triggering network 21, comprising resistors 60, 61, 62, 51, 53, diodes 65, 66, 67, 52, capacitors 70, 71 and SCR 72. When the points 22 open, resulting in a positive current flow to the SCR 18, a positive current was also established in resistor 60, which charges capacitor 71 toward the nominal voltage rating of zener diode 66. When the charge on capacitor 71 exceeds the critical zener voltage, a positive current flow occurs at the gate of SCR 72, turning SCR 72 on. When SCR 72 conducts, SCR 18 is deprived of positive gate current. The gate-to-cathode voltage of SCR 18 becomes effectively negative because of the small positive voltage at its cathode provided by the voltage divider of resistors 62 and 68. Since the SCR 18 thus has an effective negative gate voltage, and since there is no forward current at this time through the SCR either by the DC-to-DC convertor 13 or C17, the SCR 18 commutates. At this point, the capacitor 17 begins to charge as shown in FIG. 3, because of the remaining energy in the system.
The timing of the commutation depends on the RC time constant established by resistor 60 and capacitor 71. This RC time constant is chosen such that several high voltage spikes may be delivered to an engine combustion chamber during a single firing sequence. This positive control over the number of firing spikes allows for more complete combustion, as well as positive SCR 18 commutation, during each combustion cycle.
The opening of the contact point 22 also initiates the operation of the control circuit 23, which comprises resistors 77, 78, 79, transistors 80, 81, capacitor 76, and diode 75. A positive current flow is established by the opening of the points through diode 75, in turn charging capacitor 76, flowing through resistor 77, and then dividing between resistor 78 and the base of transistor 80. This heavy base current drives transistor 80 into saturation. This saturation condition in transistor 80 deprives transistor 81 of its positive base current, which results in transistor 81 turning off. Since transistor 81 is now off, the base current to transistors 30 and 31 through resistors 34 and 35 will also be blocked. The DC-to-DC convertor 13 is hence turned off. This is shown at point 57 in Diagram B of FIG. 3. By thus preventing current flow into the DC-to-DC convertor circuit when the SCR 18 is conducting, heat loss in the primary winding of the transformer 32 and transistors 30 and 31 is eliminated.
When the positive current flow through diode 75 is interrupted by the conduction of SCR 72, which is at the predetermined time of the commutation of SCR 18, as explained above, the DC-to-DC convertor will not immediately revert to an "on" condition. The DC-to-DC convertor remains off until time is allowed to assure that output SCR 18 has indeed commutated. This time is determined by the time constant of the discharged current of capacitor 76 flowing through resistor 77 and the base of transistor 80. When this current is low enough for transistor 80 to come out of saturation and turn off, the DC-to-DC convertor will then revert to an on condition. The capacitor 17 will then again charge to peak value, with a slight ripple caused by the DC--DC convertor, as shown in FIG. 3. When the points again open, the cycle of operation will be repeated.
Another feature of the invention concerns protection against the effect of point bounce, which refers to the condition in which the points 22, upon initial closing, will bounce slightly open again for a very brief period before closing for the rest of the normal cycle time. Point bounce is undesirable because it Is possible that the SCR 18 may immediately fire again. This would be, of course, a spurious triggering of the output SCR, and unless prevented, results in misfiring and inefficient operation of the engine. To prevent this, the commutation of the SCR 72 is delayed for a very short period of time. Energy is stored in capacitor 70 while the points are open to provide for this delay in the commutation of the SCR 72. When the points 22 initially close, capacitor 70 will start to discharge through SCR 72 and resistor 71, holding SCR 72 in a conducting state, which maintains the negative gate current for SCR 18 and prevents a possible false trigger. When the discharge curve in capacitor 70 drops below the forward threshold current of SCR 72, SCR 72 will commutate. At this point in time, points 22 have already closed, the time for point bounce is over, the DC-to-DC convertor 13 is in an on condition, the capacitor 17 is once again charging toward a peak value, and SCR 72 and 18 are both commutated. When the points 22 again open, at point 57 in FIG. 3, the cycle repeats, with the capacitor 17 immediately discharging through the SCR 18, the DC-to-DC convertor being turned off by the control circuit.
Although a preferred embodiment of the invention has been disclosed herein for purposes of illustration, it will be understood that various changes, modifications and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims which follow:
This invention relates primarily to the art of semiconductor ignition systems, and more specifically, to capacitive discharge ignition systems utilizing a silicone controlled rectifier to control the application of a capacitively stored voltage to the primary of an ignition coil.
The ignition system of an internal combustion engine plays a critical role in the proper operation of an engine, particularly in a modern, emission-controlled automobile. The iignition system must supply a large, properly timed, spark to the spark plugs to properly ignite the fuel/air mixture contained in the engine's combustion chambers. Until recently, nearly all ignition systems used the sequential opening and closing of mechanical distributor points to directly control the application of a battery voltage to the primary of an ignition coil having a high secondary-to-primary turns ratio. The resulting high voltage at the secondary of the coil was then applied to the spark plugs in a predetermined sequence to achieve a smooth running of the engine.
With the advent of semi-conductor devices, however, there has resulted a great deal of experimentation with respect to ignition systems, including oscillator-excited ignition systems, gate controlled ignition systems, piezoelectric ignition systems, and capacitive discharge ignition systems, to name some varieties currently being investigated as well as commercially used.
The capacitive discharge ignition system utilizing a silicone controlled rectifier is currently a popular and practical implementation of a semi-conductor ignition system. Present capacitive discharge systems, however, have significant operational disadvantages. In a typical prior art capacitive discharge system, the charging capacitor charges to a specified maximum value, independent of the dwell time of the distributor points. When the points open, a positive pulse is applied to the gate element of an SCR, turning the SCR on, and a single energy exchange then occurs between the primary of the ignition coil and the charging capacitor. The collapsing field of the primary of the ignition coil when the capacitor is completely discharged will then tend to establish a charge on the capacitor opposite to that of its original charge. The SCR is typically rendered non-conducting, or commutated, by a reverse potential, which is established by the reverse current in the circuit when the oppositely charged capacitor begins to discharge back through the primary of the coil. This reverse current is sufficient to commutate tbe SCR, and thus, one complete cycle of energy exchange between the capacitor and the coil typically occurs before the SCR becomes non-conducting. During the time that he SCR is conducting, the power converter which charges the capacitor is under a very heavy load, and does not produce a useful output for the operation of the ignition system. The battery, however, is still supplying the input to the power convertor, resulting in continued heat loss in the input section of the convertor.
Another significant disadvantage with present capacitive discharge systems utilizing distributor breaker points concerns the phenomenon of breaker point bounce, which may result in spurious triggering of the SCR and subsequent engine misfire.
Additionally, since the commutation of the SCR depends upon a reverse current sufficient to overcome the small forward current of the power convertor and reach a threshold reverse potential, a partial failure in the coil or the charging capacitor, or abnormalities in the coil load, may result in a failure of the SCR to commutate at all, ultimately resulting in complete engine failure and possible ignition burn-out.
In view of the above, it is an object of the present invention to provide a capacitive discharge ignition system wherein the operational state of the silicone controlled rectifier is controlled by means other than the energy stored in the primary of the coil.
It is another object of the present invention to provide a capacitive discharge ignition system wherein the entire power convertor circuit is turned off when the SCR is in a conducting state.
It is a further object of the present invention to provide a capacitive discharging system wherein more than one energy exchange cycle may take place between the charging capacitor and the primary of the coil during the distributor breaker point cycle.
It is a still further object of the present invention to eliminate or reduce the effect of breaker point bounce producing spurious triggering of the silicone controlled rectifier.
SUMMARY OF THE INVENTION
Briefly, in accordance with a preferred embodiment, the present invention includes a voltage supplying circuit, which converts a given low voltage generated by other components in the automobile to a significantly higher voltage; a series-circuit connected to the voltage supply circuit, including a charging capacitor, a discharge circuit for the capacitor, and the primary winding of the ignition coil; and a control circuit to control both the operation of the discharging circuit, and the voltage supply circuit such that the voltage supply circuit is turned off during the time that the discharging circuit is conducting, and such that a predetermined number of energy cycle exchanges may take place between the charging capacitor and the ignition coil during one ignition cycle.
DESCRIPTION OF THE DRAWINGS
A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which,
FIG. 1 is a block diagram of the capacitive discharge ignition system of the present invention.
FIG. 2 is a schematic diagram of the capacitive discharge ignition system of the present invention.
FIG. 3 is timing diagrams for the charging capacitor relative to the operational condition of the DC-to-DC convertor, and the distributor breaker point cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a nominal 12-volt voltage supply 11 for a standard automobile ignition system is shown with respect to a block diagram of the present invention. It should be understood, however, that although the present invention is disclosed in the context of an automobile ignition system, its principles are applicable in any system employing a semi-conductor control with capacitive discharge voltage circuit.
Furthermore, it should also be understood that if the system is utilized in an automobile ignition system, triggering sources other than breaker points, such as photochoppers or the like, may be utilized. In FIG. 1, however the 12-volt voltage supply 11 is connected to a standard ignition switch 12 which is in turn connected to a standard DC--DC convertor 13, which has a typical DC output of approximately 400 volts. This output is applied to a charging capacitor 17, which is discharged periodically under the control of a silicone controlled rectifier 18. The operating state of SCR 18 is in turn positively controlled by trigger network 21 and control circuit 23.
The triggering sequence is initiated by tbe breaker points 22 in the distributor. Additionally, the breaker points initiate the operation of a control circuit 23 which in turn controls the operation of the DC-to-DC convertor 13. CIrcuits 21 and 23 operate in conjunction to control the operational cycle of the entire ignition system.
Referrring to FIG. 3, timing diagrams for several important portions of the ignition system are shown, and which will be referred to in the following paragraphs, as the description of the preferred embodiment proceeds. Diagram A is an idealized charging cycle of the capacitor 17. Diagram B is the operational sequence of the DC-to-DC convertor, and Diagram C is the timing sequence for the opening and closing of the distributor points. Each of the timing sequences is referenced to the opening of the distributor points.
When the distributor points 22 open, and capacitor 17 is charged to a peak value, a positive gate current is established by the triggering network 21, turning SCR 18 on, thus allowing the charging capacitor 17 to discharge through the primary of the auto transformer 25, resulting in a high secondary voltage spike being applied to the electrodes of the spark plug 26. At the same time, the control circuit 23 turns the DC-to-DC convertor entirely off, effectively isolating it from the rest of the circuit.
Several cycles of energy exchange then occur between the capacitor 17 and the primary of the ignition coil or auto transformer through the SCR 18, which is held on by the trigger network 21, in the forward direction, and a diode bridge circuit in the reverse direction. At some predetermined point in time, after a number of energy exchanges has occurred, independent of whether the breaker points 22 remain open, the trigger network 21, in conjunction with the control circuit 23, will commutate the SCR, thus interrupting the current flow to the transformer 25. The trigger network 21 supplies a negative gate current to the SCR 18 in conjunction with a lack of forward current through the SCR, and commutation of the SCR occurs. When the SCR commutates, capacitor 17 will begin to charge again, because of the energy remaining in the system.
Sometime after the SCR 18 commutates, as determined by a timing circuit in control circuit 23, control circuit 23 will turn the DC-to-DC convertor back on, and the capacitor 17 will once again charge to a peak value.
Referring to FIG. 2, a schematic diagram of the present invention is shown. Transistors 30 and 31, transformer 32, diode bridge network 33, resistors 34, 35, 36, 37, 38, and capacitors 39, 40 and 41 comprise a DC-to-DC convertor. As explained above, this circuit converts ordinarily the nominal 12-volt battery voltage to a high DC voltage, which is used to charge the capacitor 17 during a controlled interval. FIG. 3 shows an idealized charging curve of the capacitor 17 with respect to the timing sequence of the distributor points and the timing of the DC-to-DC convertor.
The ignition cycle begins when the points 22 open, the capacitor 17 at this time being charged to a peak value. A current will flow from the positive battery terminal through resistor 50, resistor 51 and diode 52, and will then divide between resistor 53 and the gate connection of the SCR 18. This positive current turns the SCR on, placing it in the conducting state. Capacitor 17, which had been charged to a peak value, now discharges through the SCR 18 and the primary winding of the coil 56, which is connected as shown. As current flows through the primary winding 56 an expanding electromagnetic field is created around the winding, and the energy initially stored in the capacitor 17 is now stored in the electromagnetic field, less the energy dissipated in the spark plugs and through normal heat and other losses.
At some point, the capacitor 17 will become completely discharged, and the electromagnetic field around the coil will be at a maximum. The field will then begin to collapse and in turn will tend to reinforce the original current flow, thus charging the capacitor 17 in the opposite direction. The capacitor 17 will be charged to its maximum opposite or negative value when the field around the primary of the coil is at a minimum. The energy exchange then continues, as a reverse current through the coil 56 is established, which flows through the diode bridge 33 and tends to charge the capacitor 17 in the original, or positive direction. This continuous energy exchange would continue until the energy is depleted due to the natural dampening effect of energy losses in the circuit elements or until the SCR is commutated. During a predetermined number of these cycles, the SCR 18 is continuously conducting, provided with a positive gate current from the triggering circuit, also allowing conduction through the bridge diode consistent with the reverse current of energy exchange cycle.
The commutation of the SCR 18 is initiated, after a predetermined number of energy exchanges, by the triggering network 21, comprising resistors 60, 61, 62, 51, 53, diodes 65, 66, 67, 52, capacitors 70, 71 and SCR 72. When the points 22 open, resulting in a positive current flow to the SCR 18, a positive current was also established in resistor 60, which charges capacitor 71 toward the nominal voltage rating of zener diode 66. When the charge on capacitor 71 exceeds the critical zener voltage, a positive current flow occurs at the gate of SCR 72, turning SCR 72 on. When SCR 72 conducts, SCR 18 is deprived of positive gate current. The gate-to-cathode voltage of SCR 18 becomes effectively negative because of the small positive voltage at its cathode provided by the voltage divider of resistors 62 and 68. Since the SCR 18 thus has an effective negative gate voltage, and since there is no forward current at this time through the SCR either by the DC-to-DC convertor 13 or C17, the SCR 18 commutates. At this point, the capacitor 17 begins to charge as shown in FIG. 3, because of the remaining energy in the system.
The timing of the commutation depends on the RC time constant established by resistor 60 and capacitor 71. This RC time constant is chosen such that several high voltage spikes may be delivered to an engine combustion chamber during a single firing sequence. This positive control over the number of firing spikes allows for more complete combustion, as well as positive SCR 18 commutation, during each combustion cycle.
The opening of the contact point 22 also initiates the operation of the control circuit 23, which comprises resistors 77, 78, 79, transistors 80, 81, capacitor 76, and diode 75. A positive current flow is established by the opening of the points through diode 75, in turn charging capacitor 76, flowing through resistor 77, and then dividing between resistor 78 and the base of transistor 80. This heavy base current drives transistor 80 into saturation. This saturation condition in transistor 80 deprives transistor 81 of its positive base current, which results in transistor 81 turning off. Since transistor 81 is now off, the base current to transistors 30 and 31 through resistors 34 and 35 will also be blocked. The DC-to-DC convertor 13 is hence turned off. This is shown at point 57 in Diagram B of FIG. 3. By thus preventing current flow into the DC-to-DC convertor circuit when the SCR 18 is conducting, heat loss in the primary winding of the transformer 32 and transistors 30 and 31 is eliminated.
When the positive current flow through diode 75 is interrupted by the conduction of SCR 72, which is at the predetermined time of the commutation of SCR 18, as explained above, the DC-to-DC convertor will not immediately revert to an "on" condition. The DC-to-DC convertor remains off until time is allowed to assure that output SCR 18 has indeed commutated. This time is determined by the time constant of the discharged current of capacitor 76 flowing through resistor 77 and the base of transistor 80. When this current is low enough for transistor 80 to come out of saturation and turn off, the DC-to-DC convertor will then revert to an on condition. The capacitor 17 will then again charge to peak value, with a slight ripple caused by the DC--DC convertor, as shown in FIG. 3. When the points again open, the cycle of operation will be repeated.
Another feature of the invention concerns protection against the effect of point bounce, which refers to the condition in which the points 22, upon initial closing, will bounce slightly open again for a very brief period before closing for the rest of the normal cycle time. Point bounce is undesirable because it Is possible that the SCR 18 may immediately fire again. This would be, of course, a spurious triggering of the output SCR, and unless prevented, results in misfiring and inefficient operation of the engine. To prevent this, the commutation of the SCR 72 is delayed for a very short period of time. Energy is stored in capacitor 70 while the points are open to provide for this delay in the commutation of the SCR 72. When the points 22 initially close, capacitor 70 will start to discharge through SCR 72 and resistor 71, holding SCR 72 in a conducting state, which maintains the negative gate current for SCR 18 and prevents a possible false trigger. When the discharge curve in capacitor 70 drops below the forward threshold current of SCR 72, SCR 72 will commutate. At this point in time, points 22 have already closed, the time for point bounce is over, the DC-to-DC convertor 13 is in an on condition, the capacitor 17 is once again charging toward a peak value, and SCR 72 and 18 are both commutated. When the points 22 again open, at point 57 in FIG. 3, the cycle repeats, with the capacitor 17 immediately discharging through the SCR 18, the DC-to-DC convertor being turned off by the control circuit.
Although a preferred embodiment of the invention has been disclosed herein for purposes of illustration, it will be understood that various changes, modifications and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims which follow: