Abstract:
An electronic fuel control system for varying the fuel flow to a gas turbine engine during the engine&#39;s acceleration. The reference speed set point of the engine&#39;s fuel controlling governor is increased as a function of the elapsed time from engine start-up and the engine characteristics so that the engine accelerates substantially along its &#34;required to run line.&#34;

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to fuel control systems, and more particularly to fuel control systems having electronics for controlling the amount of fuel delivered to the gas turbine engine as a function of elapsed time. 
     Prior art turbines have attempted to design fuel control systems so that the rate of the engine acceleration closely approaches but does not reach a maximum fuel schedule line (plotted as fuel flow rate vs. speed) so that maximum acceleration of the engine can be obtained without overheating or causing damage to the engine. Prior art fuel control systems monitor various engine parameters, such as temperature, pressure, and fuel flow, controlling the fuel flow and the rate of acceleration in relation to these parameters. The problem with these prior art fuel control systems is the difficulty in obtaining accurate measurements of such parameters as the temperature, pressure and viscous drags. 
     The present invention overcomes these disadvantages by providing a fuel control system which electronically controls the amount of fuel flow to the engine as a function of the elapsed time from the engine start-up to deliver the optimum fuel flow to the engine. 
     SUMMARY OF THE INVENTION 
     The fuel control system of the present invention includes a timer for increasing the rate of fuel flow to the gas turbine engine as a function of elapsed time from engine start-up. The system further includes a hung start detector which senses when the rate of fuel flow is too great for the engine speed, and means for inhibiting the timer to allow the engine speed to catch up to the rate of fuel flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the fuel control and power system constructed in accordance with the principles of the present invention. 
     FIG. 2 is a schematic of the fuel control system of the present invention. 
     FIG. 3 is a graphical representation of a typical fuel flow rate versus engine speed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A power system 10 of a turbine engine as shown in FIG. 1 includes an auxiliary power unit (APU) 12, coupled to a speed sensor 14, for sensing the turbine engine&#39;s speed. The speed sensor 14 is in turn coupled through a speed conditioner 15 to the fuel control system 16 of the present invention. The speed conditioner 15 converts the A.C. signal from the speed sensor 14 to a D.C. signal to be delivered to the fuel control system 16. A reference voltage, +V, three reference speeds, namely, N1, N2 and N3 are coupled to the fuel control system 16, along with a temperature conditioner 18 which amplifies the signal from a temperature sensor 20, which indicates the engine temperature. Finally, the output of the fuel control system 16 is coupled through a fuel control torque motor 22 back to the APU 12. The fuel control system 16 monitors the engine speed and temperature and regulates the fuel control torque motor 22 to deliver the optimum amount of fuel flow to the APU 12. 
     FIG. 2 illustrates the fuel control system 16 in detail. The speed signal from the speed conditioner 15 is delivered through a resistor 24 to a summing junction 26, and a reference voltage (+V) is delivered to summing junction 26 through a resistor 25. The signal from the temperature conditioner 18 is delivered through a resistor 30 to the summing junction 26, which sums the two signals and sends the summed signal through an amplifier 32 to a first input 33 of a summing amplifier 34. The speed signal from the speed conditioner 15 is delivered through a resistor 36 to a second input 37 to the summing amplifier 34. 
     A timer-hung start detector section 39 controls the amount of fuel to the turbine engine between the reference speeds N1 and N2. The hung-start detector section 39 includes a summing junction 38 which receives the speed signal through a resistor 40, the speed reference signal N1, and the output signal from a timer 42. The summed signal from junction 38 is coupled to a third input 43 to the summing amplifier 34 through an amplifier 44. The output of the amplifier 44 is connected to a hung start detector 46, which is connected to a device 48 for inhibiting the timer 42. The hung start detector 46 sends a signal to activate the inhibiting device 48 and thereby inhibit the timer 42 when the speed deviates from the required to run line. The reference voltage (+V) is delivered through a resistor 50 to a summing junction 52. The junction 52 sums the reference signal +V and the signal from the inhibiting device 48, and this summed signal is coupled to and inhibits the timer 42. When the inhibiting device 48 is activated by the hung start detector 46, the timer 42 can consist of a timing device such as an operational amplifier with an integrator feedback loop; the hung start detector 46 also can consist of an operational amplifier; and the inhibiting device 48 can consist of a device such as a current limiting resistor. 
     A summing junction 54 sums the signal from the timer 42 and a second reference speed signal N2, so that when the signal from the timer 42 is equal to the reference signal N2, an error signal is amplified by an amplifier 56 and delivered to the inhibiting device 48 to activate it and thereby inhibit the timer 42. 
     The speed signal from the speed conditioner 15 is delivered through a resistor 58 to a summing junction 60. A third reference speed signal N3 is also summed by the junction 60 whose summed signal passes through and is amplified by an amplifier 62 into a fourth input 63 to the summing amplifier 34. A clamp 64 is coupled between the input and output of the amplifier 62. The second speed reference signal N2 is also connected to the clamp 64 so that the clamp 64 holds the amplifier 62 at a zero error signal until the speed reaches the reference speed N2. 
     In operation, the present invention can be better understood by reference to FIG. 3 which provides a plot of the engine fuel flow rate versus the engine speed. The line &#34;A&#34; represents an illustrated maximum fuel schedule line which indicates the maximum rate of fuel flow for a given engine speed allowable without overheating or causing damage to the engine. Therefore, the engine must operate under the line &#34;A&#34;. The line &#34;R&#34; represents the required to run line of the engine, which is indicative of the minimum amount of fuel flow required to maintain the engine at a certain speed. The fuel control system 16 of the present invention causes the gas turbine engine to accelerate along the required to run line regardless of changes in the engine parameters, such as temperature, pressure or load. 
     In operation, an external power source such as an electric motor is used to start up the gas turbine engine and the engine speed begins to increase. The speed sensor 14 senses the engine speed, and the signal indicating the speed is sent through the resistor 36 to the summing amplifier 34. The current through line represents the engine speed; and as the engine speed increases, the current output of summing amplifier 34 increases, thereby increasing the rate of fuel flow to the engine. 
     Upon engine start-up the engine temperature is monitored. The summing junction 26 sums the signal indicative of engine temperature from the temperature sensor 20 and the signal indicative of the engine speed from the speed sensor 14. If the temperature is too high for the particular speed, the error signal from the summing junction 26 will decrease the rate of fuel flow to the engine, thereby reducing the engine temperature. 
     Once the engine has reached the reference speed N1, the timer-hung start detector section 39 increases the rate of fuel flow to the engine as a function of elapsed time from engine start up. If, because of changes in loads and variations in other parameters, the rate of fuel flow to the engine becomes too great for the engine speed the timer (which controls the increase in the rate of fuel flow) must be inhibited until the engine&#39;s speed is allowed time to catch up. When the rate of fuel flow does exceed the maximum fuel schedule (line &#34;A&#34;) of FIG. 3 which is called a &#34;hung start,&#34; the hung start detector 46 causes the timer 42 to be inhibited, thereby preventing the rate of fuel flow from increasing until the engine speed catches up to the particular rate of fuel flow on the maximum fuel schedule line &#34;A&#34;. 
     From the engine start-up until the engine reaches the reference speed N1 only the first and second inputs 33 and 37 respectively, to the summing amplifier 34 are operative. When the engine reaches the speed N1, the timer-hung-start detector section 39 of the fuel control system 16 is activated. 
     The reference voltage +V causes a reference current to pass through the resistor 50 to the summing junction 52 to be summed with the signal from the inhibiting means 48. When the signal from the inhibiting means exceeds the reference current, the junction 52 produces an error signal which inhibits the timer 42. 
     The signal from the timer 42 is summed with the first reference speed signal N1 and with the speed signal at the summing junction 38; and when the speed is equal to or greater than the reference speed N1, the timer 42 runs and the speed is increased with elapsed time. If the output current from the timer is greater than the current from the speed sensor, an error signal from the summing junction 38 decreases the amount of fuel to the engine. 
     When the timer current (or output) exceeds the sum of the first reference speed signal or current (N1) and the speed signal, an error signal is delivered from the summing junction 38 causing the hung start detector 46 to activate the inhibiting means 48 and thereby inhibit the timer 42. This means that too much fuel is being delivered to the engine for the speed that the engine is presently at so the timer 42 is inhibited, thereby giving the engine speed time to catch up. 
     Referring to FIGS. 2 and 3, the current through input 37 to summing amplifier 34 controls the rate of fuel flow from engine start-up (a zero speed) until the reference speed N1 is reached; as the speed increases, the amount of fuel to the engine likewise increases. 
     When more time has elapsed and the engine speed has increased to the reference speed N1, the timer-hung start section 39 is activated and controls the rate of fuel flow until the engine reaches the reference speed N2. Between the reference speeds of N1 and N2 the timer-hung start section 39 generates a proportional error signal when the system does not follow the maximum fuel line &#34;A&#34;. 
     After still more time has elapsed and the engine has reached the reference speed N2, the section of the fuel control system containing the summing junction 60, the amplifier 62 and the clamp 64 operate. The clamp 64 holds the error signal from the junction 60 at zero until the engine reaches the reference speed N2. Due to the excess torque available at these higher speeds, the fuel flow can be increased at a higher rate (greater engine acceleration) and still retain low turbine temperatures between the reference speeds N2 and N3. Reference speed N3 represents the maximum desired speed so the rate of fuel flow is regulated to prevent the speed from exceeding N3. 
     Although the device which has just been described appears to afford the greatest advantages for implementing the invention, it will be understood that various modifications can be made thereto without going beyond the scope of the invention, it being possible to replace certain elements by other elements capable of fulfilling the same technical functions therein.