Having described the invention, what is claimed is
1. In combination with a gas turbine engine of the type having a spark gap for igniting fuel in the engine and an ignition circuit for producing an electrical discharge at the spark gap, the improvement wherein the ignition circuit comprises:
2. The electrical circuit recited in claim 1 wherein said source of electrical energy is a battery.
3. An electrical circuit for generating a plurality of discharges across a spark gap which comprises:
4. The electrical circuit recited in claim 3 wherein said source of electrical energy is a battery.
BACKGROUND OF THE INVENTION
This invention relates to an electrical spark generating apparatus for gas turbine engines and the like.
Much difficulty has been experienced in providing a simple 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 U.S. 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.
SUMMARY OF THE INVENTION
This invention provides a simple and reliable transistorized ignition system for an automobile gas turbine engine.
The ignition system is characterized by a 2-transistor oscillator circuit that receives power from an automobile battery and applies it to a step-up transformer that has its secondary winding connected to a spark gap discharge device that sparks at the frequency rate of the oscillator to ignite fuel in the turbine engine.
In one embodiment of the invention the ignition system for an automobile gas turbine engine comprises: a battery for supplying a DC voltage; a transformer having a primary winding and a secondary winding, with the secondary winding connected across a spark gap discharge device for igniting fuel in the engine; and a transistorized oscillator circuit that is connected to the battery and the primary winding of the transformer to periodically interrupt current from the battery to the primary winding whereby the oscillating current causes periodic electrical discharges across the spark gap device to ignite the fuel in the turbine engine. The circuit described provides high energy output pulses at the spark discharge device while requiring a low input voltage equal to the automobile battery voltage. The circuit is further capable of operating under short or open circuit conditions at the secondary winding of the step-up transformer.
Accordingly, it is an object of this invention to provide a 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 requires only a spark gap discharge device in the secondary winding circuit of a step-up transformer.
It is still another object of this invention to provide a transistorized ignition system for a gas turbine engine that has a minimum amount of components and preferably no more than two transistors.
It is still another object of this invention to provide a relatively inexpensive ignition system for a gas turbine engine.
A still further object of this invention is to provide a novel simplified ignition or spark-producing system which is useful for igniting automobile gas turbine engines.
Another object of this invention is to provide an electrical apparatus for creating electrical sparks or arcs that are adapted for igniting combustible materials.
Yet another object of this invention is to provide a transistorized ignition system for a gas turbine engine that is capable of operating under open circuit and short circuit conditions in the secondary circuit of the step-up transformer without damage to the remaining components of the ignition circuit.
It is yet another object of this invention to provide a constant power ignition system whereby regardless of variations of input voltages within a predetermined range of 8-24 volts the output power remains constant.
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 this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an ignition system for an automobile turbine engine.
FIG. 2 is a schematic diagram of a battery-powered transistorized ignition circuit that accomplishes the objects of this invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to FIG. 1, there is shown a block diagram of an ignition system for an automobile turbine engine which includes an ignition circuit 1 and a turbine engine 2 that includes a spark gap 40 therein for igniting the fuel fed to the engine.
FIG. 2 is a schematic diagram of a preferred embodiment of the ignition circuit 1 shown in FIG. 1. The source of electrical energy for the circuit is a battery 3 which is an ordinary automobile battery or a DC power supply having a voltage between 8 and 24 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 40 is a spark plug or the like which receives energy generated by the transistorized circuit and transformer. This causes a plurality of electrical discharges across spark gap 40 which occur at the same frequency rate as the oscillations in the primary portion of the transformer 30.
The first portion of the oscillator circuit includes a resistor 11 in series with the emitter of transistor 10 which has its collector in series with resistor 12 and its base in series with resistor 15 and diode 23. Blocking diode 23 is connected to the collector of transistor 20. The collector of transistor 10 is connected to the base of transistor 20 through lead 21 to provide a base current (drive) to transistor 20 when transistor 10 is conducting.
The resistor 11, the transistor 10 and the diodes 13, 14 are connected in the configuration shown to form a constant current regulator. This provides a constant current during increasing input voltages. The constant current output of this configuration provides the base current drive through lead 21 for transistor 20. Hence the base current of transistor 20 is fairly constant over the entire input voltage range. Since the collector current of transistor 10 is relatively constant, the "ON" time of the transistor 20 will decrease as the input voltage increases. Since the "ON" time of transistor 20 decreases as the input voltage increases, the input average current will also decrease if the "OFF" time of the transistor 20 is constant. In this system the "OFF" time of transistor 20 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 voltage drop across the spark discharge device 40. The "OFF" time (T) may then be expressed by the following equation:
Toff = Ls /R s Ln (1 + Io . Rs /Vs)
Ls = Inductance of the secondary winding 32
Rs = Resistance of the secondary winding 32
Ln = Natural log
Io = Initial current in the secondary winding 32 when the energy in the transformer 31 begins to discharge through the spark gap device 40
Since these parameters are fixed for a particular transformer and a particular spark plug, the turnoff time will be constant. Similarly the "ON" time may be expressed by the following equation:
Ton = Lp Ip /E
Lp = Inductance of primary
Ip = Peak input current
E = Battery voltage
When switch 5 is closed the battery 3 applies electrical power to the circuitry causing capacitor 7 to charge through diode 13. The capacitor eventually attains the voltage equal to the battery voltage less the forward voltage drop of the diode 13. A current flows from the base of transistor 10 through resistors 11, 15 and 16 to ground 4. This turns transistor 10 "ON" which permits a collector current to flow to ground through resistor 12 and through lead 21 to provide a base current to transistor 20, thereby turning transistor 20 "ON." When transistor 20 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. With transistor 20 "ON" a linearly rising current begins to flow through the primary winding 31 of the 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 the transformer 30. This voltage across the primary 31 causes diode 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 to saturate quickly. A linearly rising current flows through the primary winding 31 and transistor 20 until the current reaches a peak value equal to the current through the base of transistor 20 times the gain of the transistor, at which time the transistor 20 comes out of saturation. When transistor 20 comes out of saturation the voltage across the transistor 20 increases and the voltage across the primary winding 31 will drop toward zero. As the voltage across the transistor 20 increases, it charges capacitor 7 through resistor 22. When the capacitor 7 charges to 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 transistor 20, turning transistor 20 "OFF." Transistor 10 stays "OFF" as long as the voltage across the transistor 20 is high. During the "OFF" time the voltage across the transistor 20 is approximately equal to the voltage across the spark gap divided by the turns ratio of the transformer 30 plus the voltage of the battery 3. Turning transistor 20 "OFF" results in a sudden decrease of the current flowing through the primary winding 31 and collector of transistor 20. 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 30 also reverses, and the secondary winding 32 becomes a current source. The high voltage produced by the secondary winding causes an electrical discharge across the spark gap device 40 and the energy stored in the transformer 30 is dissipated in the electrical discharge in the spark gap discharge device 40.
When the energy stored in the transformer 30 is delivered to the spark discharge device 40, the current in the secondary winding 32 goes to zero. Simultaneously, a current starts to flow through resistor 11, emitter-base junction of transistor 10, resistor 15 and resistor 16, turning transistor 20 "ON." The capacitor 7 discharges through resistor 22 and collector emitter junction of transistor 20 until the voltage of the capacitor 7 reaches the battery voltage 3 minus the voltage drop across diode 13. The circuit is now in a state to repeat the above-described cycle.
In one satisfactorily operable system the ignition system described in FIG. 2 had the values or were of the types indicated below:
battery 3 8-24 volts DC capacitor 7 .15 microfarads - 50 volts diodes 13, 14, 23 1N645 resistor 11 8.5 ohms - 1 watt resistor 12 82 ohms -0.5 watt resistor 15 1.5 K ohms -0.5 watt resistor 16 10 K ohms - 0.5 watt resistor 22 1.5 K ohms -0.5 watt transistor 20 (NPN) TIP 3055 transistor 10 (PNP) TIP 30A transformer 30 primary 180 turns--No.18 secondary 18,000 turns--No.40 Bendix Part No. 10-372561-1 switch 5 single pole 2-contact
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 inventions 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.