Spark ignition systems for internal combustion engines
United States Patent 3900017
A spark ignition circuit for an internal combustion engine includes a transformer having a secondary winding for producing a high voltage which is rectified, stored in a capacitor and discharged through a sparking plug. The primary circuit of the transformer includes a transistor connected as a blocking oscillator with base drive derived through a resistor. A further transistor is provided the conduction of which can be varied in accordance with the variation in the supply voltage to adjust the effective value of the resistor.
Application Number:
05/482278
Publication Date:
08/19/1975
Filing Date:
06/24/1974
View Patent Images:
Export Citation:
Assignee:
Lucas Aerospace Limited (Birmingham, EN)
Primary Class:
Other Classes:
123/627, 123/655, 60/39.821, 123/654, 123/628, 60/786
International Classes:
F02P15/00; F02P15/12; F02P3/06
Field of Search:
123/148E 60/39.14,39.14R,39.82S
Primary Examiner:
Burns, Wendell E.
Assistant Examiner:
Cranson, James W.
Claims:
I claim
1. A spark ignition circuit for an internal combustion engine comprising a transformer having a secondary winding, a capacitor, a rectifier through which the capacitor is charged from the secondary winding, a pair of output terminals across which in use, is connected a spark plug, a voltage sensitive device connecting one plate of the capacitor to one of the output terminals, the other plate of the capacitor being connected to the other output terminal, a pair of DC supply terminals for connection in use, to a source of DC supply, a transistor, a primary winding on the transformer said primary winding being connected to the DC supply terminals through the collector emitter path of the transistor, a resistor for providing base drive to the transistor, a feedback winding on the transformer, said feedback winding being operable to cause switching of the transistor, and means responsive to the supply voltage for varying the effective value of said resistor, said means including a further transistor the conduction of which is dependent upon the supply voltage.
2. A circuit as claimed in claim 1 in which the collector emitter path of said further transistor is included in a path connected in parallel with said resistor.
3. A circuit as claimed in claim 2 including a potentiometer network across said supply terminals the base of said further transistor being connected to said network.
4. A circuit as claimed in claim 3 in which said collector emitter path includes a further resistor.
5. A circuit as claimed in claim 4 in which said first mentioned resistor is connected to a source of substantially constant voltage.
6. A circuit as claimed in claim 1 in which the base drive for said first mentioned transistor is supplemented at low supply voltages by current flowing through the collector emitter path of said further transistor.
7. A circuit as claimed in claim 6 in which the base drive is derived from a source of substantially constant voltage.
8. A circuit as claimed in claim 6 including a further resistor in the collector emitter path of said further transistor.
1. A spark ignition circuit for an internal combustion engine comprising a transformer having a secondary winding, a capacitor, a rectifier through which the capacitor is charged from the secondary winding, a pair of output terminals across which in use, is connected a spark plug, a voltage sensitive device connecting one plate of the capacitor to one of the output terminals, the other plate of the capacitor being connected to the other output terminal, a pair of DC supply terminals for connection in use, to a source of DC supply, a transistor, a primary winding on the transformer said primary winding being connected to the DC supply terminals through the collector emitter path of the transistor, a resistor for providing base drive to the transistor, a feedback winding on the transformer, said feedback winding being operable to cause switching of the transistor, and means responsive to the supply voltage for varying the effective value of said resistor, said means including a further transistor the conduction of which is dependent upon the supply voltage.
2. A circuit as claimed in claim 1 in which the collector emitter path of said further transistor is included in a path connected in parallel with said resistor.
3. A circuit as claimed in claim 2 including a potentiometer network across said supply terminals the base of said further transistor being connected to said network.
4. A circuit as claimed in claim 3 in which said collector emitter path includes a further resistor.
5. A circuit as claimed in claim 4 in which said first mentioned resistor is connected to a source of substantially constant voltage.
6. A circuit as claimed in claim 1 in which the base drive for said first mentioned transistor is supplemented at low supply voltages by current flowing through the collector emitter path of said further transistor.
7. A circuit as claimed in claim 6 in which the base drive is derived from a source of substantially constant voltage.
8. A circuit as claimed in claim 6 including a further resistor in the collector emitter path of said further transistor.
Description:
This invention relates to spark ignition circuits for internal combustion engines, and of the kind comprising a capacitor which is charged through a rectifier from the secondary winding of a transformer, said capacitor being connected to a pair of output terminals by means of a voltage sensitive device, the pair of output terminals in use, being connected to a sparking plug of the engine, the transformer having a primary winding connected in series with the collector emitter path of a transistor and the terminals of a source of supply, a resistor for supplying base drive to the transistor, and a feed back winding on the transformer and operable to effect switching of the transistor.
With such circuits it is found that the spark discharge rate varies with the supply voltage, and if the circuit is designed to provide an adequate spark discharge rate, when the supply voltage is low as when the engine is being started, it is found that the spark discharge rate becomes excessively high thereby impairing the life of the sparking plug, as the supply voltage increases.
It is therefore an object of the invention to provide a circuit of the kind specified in a form in which the spark discharge rate is less influenced by variations in the supply voltage.
According to the invention, a circuit of the kind specified comprises means responsive to the supply voltage for varying the effective value of said resistor.
Examples of spark ignition circuits in accordance with the invention will now be described with reference to the accompanying circuit diagrams.
Referring to the drawing, there is provided a transformer 10 having a secondary winding 11, one end of which is connected to one plate of a capacitor 12, whilst the other end of the winding is connected to the other plate of the capacitor by way of a high voltage rectifier 13. Moreover, one plate of the capacitor is connected to one output terminal 14, and the other plate of the capacitor is connected to another output terminal 15 by way of a voltage sensitive discharge device 16. In use, a spark discharge plug 17 is connected between the terminals 15 and 14, and the arrangement is such that when the capacitor voltage attains a predetermined value, the device 16 breaks down and the capacitor discharges through the sparking plug. The sparking plug is mounted in a combustion chamber of a gas turbine engine, and serves to ignite an air fuel mixture therein.
The transformer 10 is also provided with a primary winding 18 one end of which is connected to a positive supply terminal 19 by way of a safety diode 20. The other end of the winding 18 is connected to the collector terminal of an n-p-n transistor 21, the emitter of which is connected to a negative supply terminal 22.
A feed back winding 23 is provided on the transformer and one end of the winding is connected to the terminal 22, whilst the other end of the winding is connected to the cathode of a diode 24, the anode of which is connected to the base of transistor 21.
A potentiometer chain comprising resistors 25, 26 is connected between the terminal 22 and the cathode of the diode 20. A point intermediate the resistors 25 and 26 is connected to the base of a p-n-p transistor 27, the collector of which is connected to the base of transistor 21. Moreover, a Zener diode 28 is provided, the anode of which is connected to the terminal 22 and the cathode of which is connected by way of a resistor 29 to the cathode of the diode 20. A point intermediate the resistor 29 and Zener diode 28 is connected by way of a resistor 30 to the emitter of the transistor 27 and the same point is connected by way of a resistor 31 to the base of transistor 21.
In operation, and ignoring for the moment the effect of resistors 25, 26 and 30 and transistor 27, the circuit operates as a blocking oscillator in a conventional manner, with constant base drive current over the working voltage range, and a high voltage is induced in the secondary winding 11. This is rectified, and the capacitor 12 is charged. When the voltage across the capacitor attains a predetermined value, the device 16 breaks down as explained, and the capacitor is discharged through the sparking plug 17, whereafter the process is repeated. The spark discharge rate varies in accordance with the voltage of the supply, and with increasing voltage, the spark discharge rate increases. As previously explained, this can have a damaging effect upon the sparking plug.
In order to minimize this difficulty, the circuit components are designed to provide a substantially correct sparking rate at the highest expected supply voltage, and when the supply voltage falls, the base of transistor 27 is biased so that the combined resistance of the transistor 27 and resistors 30 and 31 falls. This has the effect of increasing the base drive current of the transistor 21 as the input voltage is reduced and thereby the sparking rate is increased above the value which would normally obtain without the provision of the transistor 27, and its associated components.
It will be appreciated that this circuit does not maintain a constant sparking rate over the full variation in the supply voltage. It does however effect a control and it will be appreciated that a further transistor 27 together with associated circuits could be added to the circuit if so desired, the added transistor or transistors being rendered effective at different supply voltages.
In the circuit shown in FIG. 2 identical reference numerals are utilized wherever possible. In the circuit of FIG. 2 a resistor 32 is connected in parallel with the diode 24 and the base of transistor 21 is connected to the collector of a P.N.P. transistor 33. The emitter of transistor 33 is connected through a resistor 38, to a point intermediate a resistor 34 and a Zener diode 35 connected in series across the source of supply. The base of transistor 33 is connected to a point intermediate a potentiometer chain including resistors 36 and 37 connected across the supply terminals. At high supply voltages the base drive current to transistor 21 is supplied from the transformer winding by way of resistor 32. At lower supply voltages this is supplemented by current from the supply flowing through the collector emitter path of transistor 33. The spark rate can therefore be held substantially constant over a wide range of input supply voltages.
With such circuits it is found that the spark discharge rate varies with the supply voltage, and if the circuit is designed to provide an adequate spark discharge rate, when the supply voltage is low as when the engine is being started, it is found that the spark discharge rate becomes excessively high thereby impairing the life of the sparking plug, as the supply voltage increases.
It is therefore an object of the invention to provide a circuit of the kind specified in a form in which the spark discharge rate is less influenced by variations in the supply voltage.
According to the invention, a circuit of the kind specified comprises means responsive to the supply voltage for varying the effective value of said resistor.
Examples of spark ignition circuits in accordance with the invention will now be described with reference to the accompanying circuit diagrams.
Referring to the drawing, there is provided a transformer 10 having a secondary winding 11, one end of which is connected to one plate of a capacitor 12, whilst the other end of the winding is connected to the other plate of the capacitor by way of a high voltage rectifier 13. Moreover, one plate of the capacitor is connected to one output terminal 14, and the other plate of the capacitor is connected to another output terminal 15 by way of a voltage sensitive discharge device 16. In use, a spark discharge plug 17 is connected between the terminals 15 and 14, and the arrangement is such that when the capacitor voltage attains a predetermined value, the device 16 breaks down and the capacitor discharges through the sparking plug. The sparking plug is mounted in a combustion chamber of a gas turbine engine, and serves to ignite an air fuel mixture therein.
The transformer 10 is also provided with a primary winding 18 one end of which is connected to a positive supply terminal 19 by way of a safety diode 20. The other end of the winding 18 is connected to the collector terminal of an n-p-n transistor 21, the emitter of which is connected to a negative supply terminal 22.
A feed back winding 23 is provided on the transformer and one end of the winding is connected to the terminal 22, whilst the other end of the winding is connected to the cathode of a diode 24, the anode of which is connected to the base of transistor 21.
A potentiometer chain comprising resistors 25, 26 is connected between the terminal 22 and the cathode of the diode 20. A point intermediate the resistors 25 and 26 is connected to the base of a p-n-p transistor 27, the collector of which is connected to the base of transistor 21. Moreover, a Zener diode 28 is provided, the anode of which is connected to the terminal 22 and the cathode of which is connected by way of a resistor 29 to the cathode of the diode 20. A point intermediate the resistor 29 and Zener diode 28 is connected by way of a resistor 30 to the emitter of the transistor 27 and the same point is connected by way of a resistor 31 to the base of transistor 21.
In operation, and ignoring for the moment the effect of resistors 25, 26 and 30 and transistor 27, the circuit operates as a blocking oscillator in a conventional manner, with constant base drive current over the working voltage range, and a high voltage is induced in the secondary winding 11. This is rectified, and the capacitor 12 is charged. When the voltage across the capacitor attains a predetermined value, the device 16 breaks down as explained, and the capacitor is discharged through the sparking plug 17, whereafter the process is repeated. The spark discharge rate varies in accordance with the voltage of the supply, and with increasing voltage, the spark discharge rate increases. As previously explained, this can have a damaging effect upon the sparking plug.
In order to minimize this difficulty, the circuit components are designed to provide a substantially correct sparking rate at the highest expected supply voltage, and when the supply voltage falls, the base of transistor 27 is biased so that the combined resistance of the transistor 27 and resistors 30 and 31 falls. This has the effect of increasing the base drive current of the transistor 21 as the input voltage is reduced and thereby the sparking rate is increased above the value which would normally obtain without the provision of the transistor 27, and its associated components.
It will be appreciated that this circuit does not maintain a constant sparking rate over the full variation in the supply voltage. It does however effect a control and it will be appreciated that a further transistor 27 together with associated circuits could be added to the circuit if so desired, the added transistor or transistors being rendered effective at different supply voltages.
In the circuit shown in FIG. 2 identical reference numerals are utilized wherever possible. In the circuit of FIG. 2 a resistor 32 is connected in parallel with the diode 24 and the base of transistor 21 is connected to the collector of a P.N.P. transistor 33. The emitter of transistor 33 is connected through a resistor 38, to a point intermediate a resistor 34 and a Zener diode 35 connected in series across the source of supply. The base of transistor 33 is connected to a point intermediate a potentiometer chain including resistors 36 and 37 connected across the supply terminals. At high supply voltages the base drive current to transistor 21 is supplied from the transformer winding by way of resistor 32. At lower supply voltages this is supplemented by current from the supply flowing through the collector emitter path of transistor 33. The spark rate can therefore be held substantially constant over a wide range of input supply voltages.