US 3835350 A
An improved ignition system for a gas turbine engine that has a high energy output capability at low input voltage at reasonable cost because of parallel arranged transistors having matched current transfer ratios (Beta).
Claims available in
Description (OCR text may contain errors)
ilnite States Patent [191 Thaltore HIGH ENERGY OUTPUT INDUCTIVE TGNITION SYSTEM  Inventor: Kaushik H. Thakore, Sidney, NY.
 Assignee: The Bendix Corporation, Southfield,
 Filed: Nov. 29, 1972  Appl. N0.: 310,281
 11.8. C] 315/209 T, 315/209 CD  Km. Cl. H05b 37/02, HOSb 39/04  Field of Search 315/209 T, 209 CD  References Cited UNITED STATES PATENTS 3,030,548 4/1962 Johnston 315/177 11] 3,835,350 [451 Sept. 10,1974
7/1967 Steams 315/209 CD 3,531,737
9/1970 Thakore 315/209 T X Primary Examiner-Nathan Kaufman Attorney, Agent, or Firm-Raymond J. Eifler [5 7] ABSTRACT An improved ignition system for a gas turbine engine that has a high energy output capability at low input voltage at reasonable cost because of parallel arranged transistors having matched current transfer ratios (Beta).
2 Claims, 1 Drawing Figure HIGH ENERGY OUTPUT INDUCTIVE IGNITION SYSTEM BACKGROUND OF THE INVENTION This invention relates to an electrical spark generating apparatus for gas turbine engines and the like. This invention is more particularly related to a transistorized ignition system for an automobile gas turbine engine.
Much difficulty has been experienced in providing a simple and inexpensive ignition system of small size, weight and with a minimum of components which will function satisfactorily to ignite so-called jet and gas turbine engines under all operating conditions. One example of a previous ignition system for a turbine engine is disclosed in US. Pat. No. 2,651,005 entitled Electrical Apparatus to T. Tognola, issued Sept. 1, 1953. However, this type of device utilizes a vibrator to create the oscillations that cause an electrical discharge across a spark gap to ignite fuel in a turbine engine. The disadvantages of such a system are (1) the short mechanical life of a vibrator, (2) the short life of the battery used to drive the vibrator because the vibrator uses so much power, and (3) the cost of the entire circuit. Because of these disadvantages, recent ignition systems have turned to solid-state oscillator circuits driven by a battery, and a transformer, the primary winding of which is connected to the oscillator, to transmit the power generated by the oscillator to the secondary portion of the transformer which is connected to a spark plug which periodically discharges the energy transferred to the secondary winding of the transformer thereby igniting fuel in the engine. In recent transistorized oscillator circuits, the power transferred from the primary winding to the secondary winding in the transformer is limited by the maximum current it can pass through the primary winding of the transformer. At the present time, emphasis is being placed in transferring greater amounts of power through the transformer which, of course, means passing more current through the primary winding of the transformer. Since the transistor of the oscillator circuit is in series with the primary winding of the transformer the current passing through the primary winding of the transformer is limited by the current transfer ratio (Beta) of the transistor. One obvious approach to increasing the current through the primary winding is to use a larger transistor i.e., one having a much, much higher Beta. However, a larger transistor costs about 50 times as much as a smaller transistor. Another approach is to use, in parallel circuit relationship, two transistors of the same type, however, in this approach the current flowing through the primary winding of the transformer is generally not equally distributed through the two transistors and they will fail due to their inability to handle the power transmitted therethrough.
SUMMARY OF THE INVENTION This invention provides a simple and reliable high energy output transistorized ignition system for an automobile gas turbine engine that assures equal current distribution through the transistors in series with the primary winding of the transformer thereby increasing, in a reliable manner, the amount of power transformed from the primary to the secondary winding of the transformer.
The ignition system is characterized by a transistorized oscillator circuit that receives power from an automobile battery 3 and applies it to a step up transformer 30 that has its secondary winding connected to a spark gap discharge device 40 that sparks at the frequency rate of the oscillator to ignite fuel in the turbine engine. The secondary portion of the circuit is characterized by a capacitor 41 in parallel with the spark gap discharge device 40 and the oscillator circuit in the primary portion of the circuit is characterized by two parallel arranged transistors 20, 50 that have approximately the same current-transfer ratio (Beta) so that power is dissipated equally between the transistors rather than having an unbalanced power dissipation which would cause failure of the transistors, and hence the ignition system.
In one embodiment of the invention the ignition system for an automobile gas turbine engine comprises: a battery 3 for supplying a dc voltage; a transformer 30 having a primary winding 31 and a secondary winding 32, said secondary winding connected to a spark plug 40 and a capacitor 41; and a transistorized oscillator electrically connected between the battery 3 and the primary winding 31 of the transformer to periodically interrupt current flow from the battery 3 to the primary winding 31 whereby the oscillating current causes periodic electrical discharges across the spark device 40 to ignite fuel in a turbine engine, the oscillator including two transistors 20, 50 arranged in parallel with respect to each other and in series with the primary winding 31 of the transformer, the transistors 20, 50 further having substantially the same current transfer ratio (Beta) during operation so that current flowing through the primary winding 31 is divided equally through the transistors thereby preventing failure of the transistors.
Accordingly, it is an object of this invention to provide an inexpensive battery powered transistorized ignition system for an automobile gas turbine engine.
It is another object of this invention to provide a transistorized ignition system for a gas turbine engine that has a high output energy pulse at a low input voltage without the necessity of an expensive transistor.
It is yet another object of this invention to provide an ignition system that operates over a wide range of input voltages (6 to 30 volts) while maintaining high efficiency.
Another object of this invention is to provide a battery operated ignition system that increases battery life because of a current regulated oscillator that reduces the drain on the battery.
It is still another object of this invention to provide a transistorized ignition system for a gas turbine engine that is capable of withstanding a short circuit or an open circuit load condition without detriment to the circuit.
A still further object of this invention is to increase the power output of a transistorized ignition circuit by utilizing two parallel arranged transistors that have substantially the same current transfer ratios in series with the primary winding of the transformer that transfers the energy to a spark gap.
Another object of this invention is to provide an ignition system that operates at a low sparking rate with high energy thereby increasing spark plug life.
The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS A single FIGURE is the schematic diagram of a bat- 5 transformer and a particular spark plug, the turnoff tery powered transistorized ignition system that accomplishes the objects of this invention.
DETAILED DESCRIPTION OF THE DRAWINGS The single FIGURE is a schematic diagram of a preferred embodiment of an ignition to be used on a jet or gas turbine engine. The source of electrical energy for the circuit is a battery 3 which is an ordinary automobile battery or a d-c power supply having a voltage between 6 and 30 volts. The switch 5 is operable to connect and disconnect the battery to the circuit and is preferably a part of or associated with the ignition switch of an automobile. The spark gap discharge device e.g., spark plug or igniter plug 40 is a spark plug or the like which receives energy generated by the transistorized circuit and transformer 30. This causes a plurality of electrical discharges across spark gap 40 which occur at the same frequency rate as the oscillation in the primary portion of the transformer 30. The secondary coil 32 of the transformer 30 has capacitor'4l in parallel therewith to increase the peak power applied to the spark gap 40.
The first portion of the oscillator circuit includes a resistor 11 in series with the emitter of a transistor which has its collector in series with resistor 12 and its base in series with resistor 15 and diode 23. Diode 23 is a blocking diode connected to the collector of transistor and transistor 50. The collector of transistor 10 is connected to the base of transistor 20 and 50 through lead 21 to provide a base current (drive) to transistors 20 and 50 when transistor 10 is conducting. Diodes 9 and 26 connected across the emitter and base terminals of the transistors 10, 20, 50 protects transistors from exceeding their ratings V The resistor 11, the transistor 10 and the diodes 13 and 14' are connected in the configurations shown to form a constant current regulator. This provides a constant current during the increasing input voltages. The constant current output of this configuration provides the base current drive through lead 21 for transistors 20 and 50. Hence, the basecurrent of transistor 20 and 50 is fairly constant over the entire input voltage range. Since the collector current of transistor 10 is relatively constant, the ON time of the transistors 20 and 50 will decrease as the input voltage increases. Since the ON time of transistors 20 and 50 decreases as the input voltage increases, the input average current will also decrease if the. OFF time of the transistors 20 and 50 is constant. In this system, the OFF time of transistors 20 and 50 is a function of the ratio of the inductance of the secondary winding 32 of the transformer 30 to the resistance of the secondary winding 32 and the voltage drop across the spark discharge device 40. The OFF time T may then be expressed by the following equation:
where L, Inductance of the secondary winding 32 R, Resistance of the secondary winding 32 Ln Natural log time will be constant. Similarly the ON time may be expressed by the following equation:
T L,, 1 /E.
where L Inductance of primary 1,, Peak input current E Battery voltage The key feature of this circuit is transistors 20 and 50 which must be of the same type and have a current transfer ratio substantially the same. One method of chosing the proper transistors 20 and 50 is to measure the current transfer ratio of each transistor and pairing together only those transistors which have substantially the same beta or which have betas that do not differ by more than 10. To improve the performance of the circuit even further (equal current distribution) a resistor 25, 55 is placed in series with the emitter of each transistor, the resistors having substantially the same resistance preferably in the order 0.1 ohm.
OPERATION When switch 5 is closed the battery 3 supplies electrical power to the circuitry causing thecapacitor 7 to charge through diode 13. The capacitor eventually attains the voltage equal to the battery voltage less the forward voltage drop of diode 13. Simultaneously, a current flows through resistor 11, the base of transistor 10 and through resistors 15 and 16 to ground 4. This turns transistor 10 ON which permits a collector current to flow through resistor 12 and through lead 21 to provide a base current to transistor 20 and transistor 50, thereby turning transistors 20 and 50 ON. When transistor 20 and 50 is on, current from the battery 3 flows through the primary winding 31 of transformer 30 and through the collector and emitter of transistor 20 and 50 and resistors 25 and 55 establishing a dc. bias (voltages) in each transistor 20, 50. With transistor 20 and 50 ON, a linearly rising current begins to flow through the primary 31 of transformer 30. Due to the inductance of the primary winding, this current develops a constant voltage (approximately equal to the input voltage) across the primary winding 31 of transformer 30. This voltage across the primary 31 causes diodes 23 to conduct ON and causes more current to flow through resistor 11 and the emitter base junction of transistor 10. This causes transistor 20 and transistor 50 to saturate quickly. A linearly rising current flows through the primary winding 31 and transistor 20 and transistor 50 until the current reaches a peak value equal to the current through the base of transistors 20 and 50 times the gain of the transistors, at which time the transistors 20 and 50 come out of saturation. When transistors 20 and 50 come out of saturation, the voltage across the transistors 20 and 50 increases and voltage across the primary winding 31 will drop towards zero. As the voltage across the transistors 20 and 50 increases, it charges capacitor 7 through resistor 22. When the capacitor 7 charges through a voltage that overcomes the base voltage on transistor 10, the transistor 10 will stop conducting. This removes the current flowing in lead 21 to the base of transistors 20 and 50 Turning transistors and 50 OFF results in a sudden decrease of the current flowing through the primary winding 31 and collectors of transistors 20 and 50. During this time the rate of change of current (di/dt) becomes sharply negative, the high voltage induced in the secondary winding 32 of the transformer also reverses and the secondary winding 32 becomes a current source. The high voltage produced by the secondary winding charges the capacitor 41 to the voltage equal to the ionizing potential of the spark plug which causes an electrical discharge across the spark gap device 40 and the energy stored in the transformer 30 and capacitor 41 is dissipated in the electrical discharge in the spark gap discharge device 40. During open circuit operations (when the plug orleads are open), the capacitor 41 connected across the secondary winding of the transformer forms a tank circuit which transfers the energy back and forth from the transformer to the capacitor (ringing) thereby dissipating most of the energy in the circuit rather than in the components of the primary portion of the circuit which may cause damage. Since, under the open circuit operation, energy stored in the core of the transformer 30 is released in the capacitor 41 the reflected load on the primary is light. This reduces the power dissipation in the transistors 20 and 50. Thus, the unit requires less of a heat sink and makes it possible for the unit to run on a continuous duty cycle even under open circuit conditions.
As previously emphasized, the two transistors 20 and 50 will receive current from the primary winding 31 and therefore must meet the requirements of this invention. Otherwise, current will not be equally distributed between the transistors 20 and 50, causing one of them to fail and then the other to fail. To assure that the transistors 20 and 50 pass the same amount of current, the transistors 20 and 50 should be tested to assure that the current transfer ratios are substantially the same and that each of the transistors 20 and 50 have a resistor in series with the emitter so as to minimize the differences between the collector-emitter currents of transistors 20 and 50. While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims and in some cases certain features of the invention may be used to advantage without corresponding use of other features. For example, different types of semiconductors or solid-state control devices may be substituted for the types illustrated. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.
Having thus described the invention, what is claimed is:
1. An electrical circuit for generating a plurality of discharges across a spark gap which comprises:
a source of d-c electrical energy;
a transformer having a primary and a secondary winding, said secondary winding connected across the spark gap;
a capacitor connected directly across the secondary winding of the transformer;
switching means connected between said d-c energy source and the primary winding of said transformer for connecting and disconnecting said direct current source to and from said transformer; and
transistorized switching oscillator means connected between said direct current source and the primary winding of said transformer to periodically interrupt current flow from said source through said primary winding when said switching means connects said d-c energy source to said transformer, said transistorized switching oscillator including:
a first and second transistor having their collector and emitter terminals connected in series with the primary winding of said transformer, and their bases connected together, said transistors having alternate conductive and nonconductive intervals to periodically interrupt the current flowing from said primary winding of said transformer; each of said first and second transistors having a Beta characteristic in the range from 40 to 50 to divide current equally between them; first voltage divider network connected across said source of d-c energy and said switching means, said first divider network including first and second series connected diodes connected in series to first and second series connected resistors;
a first resistor in series with the emitter of said first transistor and said d-c energy source;
a second resistor in series with the emitter of said second transistor and said d-c energy source; said second resistor having substantially the same resistance as said first resistor;
energy storage means connected between the junction between the diodes of said first voltage divider network and the negative side of said d-c energy source;
diode means connected between the junction between said first and second resistors of said first voltage divider network and the collectors of said first and second transistors;
a resistor connected between the junction between the diodes of said first voltage divider network and the collectors of said first and second transistors;
a second voltage divider network connected across said d-c energy source and said switching means, said second voltage divider network including a third transistor having a fourth resistor connected in series with its collector, a fifth resistor connected in series with its emitter and having its base connected to the junction between the diodes and resistors of said first voltage divider network; and
means for connecting the collector of said third transistor to the bases of said first and second transistors whereby when said switching means connects said electrical source to said transformer, said transistors are periodically rendered conductive, thereby periodically causing a discharge across said spark gap connected across the secondary winding of said transformer 2. The electrical circuit recited in claim 1 wherein said source ofelectrical energy is a battery; and including a diode connected between the base and emitter terminals of said third transistor; and another diode is connected to the base of said first and second transistors and to the negative side of the d-c source.