Abstract:
Mechanical complexity and high cost in a fluid flow control system are avoided through the use of a pulse width modulated (PWM) valve ( 20 ) to meter a fluid flow to an inlet ( 50 ) of a pump ( 16 ) that pumps the metered flow to an outlet ( 56 ) of the fuel pump ( 12 ). The system utilizes a pulsating vapor core in the pump ( 16 ) to dampen the pulses in the fluid flow generated by the PWM valve ( 20 ). A regulator valve ( 22 ) is provided to maintain a relatively constant pressure drop across the PWM valve ( 20 ). The control system is ideally suited for controlling the flow of fuel to a gas turbine engine.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to the art of fluid controls and, more particularly, to fuel controls for combustion engines such as gas turbine engines that provide primary or secondary power to a vehicle. 
     BACKGROUND OF THE INVENTION 
     Cost and size of engine components are of constant concern in vehicular engine applications. This is particularly true for small turbojet engines that are designed for use in missiles and other short-life/disposable applications. 
     It is known to use a pulse width modulated valve (PWM valve) on the high pressure side of a fuel pump to meter the fuel flow to a gas turbine engine by cycling the PWM valve between an on and off position. Fuel flow is determined by the time period that the valve is open during each cycle and by the cycle frequency. Typically, such systems utilize a regulator valve to control the inlet pressure to the PWM valve by bypassing fuel flow from the high pressure side of the fuel pump back to the fuel tank. Examples of such systems are shown in U.S. Pat. Nos. 3,568,495 to Fehler et al.; 3,936,551 to Linebrink et al.; and 4,015,326 to Hobo et al. 
     Two disadvantages associated with these systems are the size and cost of the PWM valve components which must be designed to withstand the output pressure of the fuel pump, which commonly is in the range of 100-200 psig to provide adequate fuel injection pressure to the combustor. 
     Another disadvantage associated with these systems is the wasted power input into the pressurized fuel flow that is bypassed by the regulator valve from the high pressure side of the fuel pump back to the fuel tank. The wasted power is particularly critical in missiles and other vehicles having a limited fuel capacity and a mission profile that may be determined by the time required to deplete the stored fuel. 
     Yet another disadvantage associated with these systems is the pulsating flow generated by the PWM valve as it cycles between its open and closed positions. Such pulsating flow can result in combustor flameout and/or deleteriously affect the combustor stability. Accordingly, depending on the engine and combustor parameters, these systems typically require some form of accumulator/damper in the high pressure fuel line connecting the PWM valve to the combustor to dampen the pulses in the fuel flow to the combustor. The accumulator/damper is an additional component that adds cost, complexity and weight to the system and introduces a potential failure point in the system. 
     Thus, it can be seen that there is a need for a small, low-cost, and efficient fuel control system for gas turbine engines and, in particular, for small turbojet engines. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the invention to provide a new and improved fluid flow control system. 
     More specifically, it is an object to provide a small, low cost fluid flow control, and particularly a small, low-cost fuel control system for a gas turbine engine and, in particular, for small turbojet engines. 
     It is a further object of the invention to provide a fluid flow control system that utilizes a PWM valve to meter the fluid flow without requiring any additional components dedicated to damping pulses in the fluid flow generated by the PWM valve. 
     It is a further object of the invention to provide a fuel control system that reduces or eliminates the energy wasted in bypassing pressurized fuel flow from a pump outlet back to a fuel tank. 
     These and other objects of the present invention are attained in a fluid flow control in the form of a fuel control system that utilizes a PWM valve to meter a fuel flow to the inlet of a fuel pump that pumps the metered fuel flow to an engine. By virtue of this construction, the PWM valve is not subjected to the output pressure of the fuel pump. This allows the fuel control system to utilize a small, low-cost PWM valve, such as is commonly used in connection with automotive fuel injectors. Further, because the fuel is metered prior to entering the fuel pump, the fuel pump only pumps the precise amount of fuel required for the engine and no energy is wasted in pumping a fuel flow that must be bypassed back to a fuel tank. Additionally, because the PWM valve is on the inlet side of the fuel pump, the fuel pump can be utilized to dampen the PWM valve generated pulses in the fuel flow by operating with a vapor core wherein fuel is vaporized at the pump inlet and reformed back to liquid at the pump outlet, thereby damping the pulses. 
     According to one aspect of the invention, a method for controlling a fluid flow rate from a pump is provided and includes the steps of providing a pump having a pump inlet and a pump outlet, and a fluid flow path to the pump inlet. The fluid flow path is cyclically restricted to achieve a fuel flow to the pump inlet that cycles between a first flow rate for a time period T 1  and a second flow rate for a time period T 2 , with the second flow rate T 2  being greater than the first flow rate. The fluid flow to the pump inlet is pumped by the pump from the pump inlet to the pump outlet. 
     According to another aspect of the invention, the method further includes the steps of vaporizing at least a portion of the fluid flow at the pump inlet for at least a portion of the time period T 1  and reforming the vaporized fluid flow back to liquid at the pump outlet. 
     According to another aspect of the invention, an improvement is provided in a method for controlling the fluid flow rate from a pump including the steps of providing a pump having a pump inlet and a pump outlet, providing a substantially liquid fluid flow to the pump inlet, pumping the fluid flow with the pump from the pump inlet to the pump outlet while creating a pressure at the pump outlet that is above the vapor pressure of the fluid flow at the outlet. The improvement includes repetitively reducing the pressure at the pump inlet to a value below the vapor pressure of the fluid flowing into the pump inlet to provide a vapor core within the pump sufficient to dampen pulses in the fluid flow. 
     Other objects, advantages and novel features of the present invention will be apparent to those skilled in the art upon consideration of the following drawing and detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a diagrammatic illustration of a fluid flow control unit in the form of a fuel control system embodying the present invention in combination with a gas turbine engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the FIGURE, an exemplary embodiment of a fluid flow control system made according to the invention is described and illustrated in connection with a fuel control system for a gas turbine engine, shown generally at  12 . However, it should be understood that the invention may find utility in other applications, and that no limitations to use as a fuel control system for a gas turbine engine is intended except insofar as expressly stated in the appended claims. 
     The fuel control system includes a pressurized fuel storage device or fuel tank  14 ; a fuel pump  16 ; a fuel flow path  18  from the fuel storage device  14  to the fuel pump  16 ; a restricting means, shown in the form of a PWM valve  20 , for cyclically restricting the fluid flow path  18  to achieve a fuel flow to the pump inlet that cycles between a first flow rate for a time period T 1  and a second flow rate for a time period T 2 , with the second flow rate being greater than the first flow rate; and means, shown in the form of a regulator valve  22 , for regulating pressure in the storage device  14  to achieve a desired average pressure differential in the fuel flow across the PWM valve  20 . 
     The gas turbine engine  12  may be of any known construction and includes a compressor section  24 , a turbine section  26 , and a combustor assembly  28 . As is known, the compressor section  24  supplies a pressurized airflow to the combustor assembly  28  where the airflow is mixed with fuel and combusted to produce a hot gas flow that is expanded through the turbine section  26  to produce shaft power and/or thrust from the gas turbine engine  12 . It is anticipated that the fuel control system will be particularly useful with gas turbine engines  12  in the form of small turbojets, such as those disclosed in U.S. Pat. Nos. 5,207,042, issued May 4, 1993 to Rogers et al. and 4,794,742, issued Jan. 3, 1989 to Shekleton et al., the entire disclosures of which are herein incorporated by reference. 
     The pressurized fuel storage device  14  may be of any known construction and is shown in the form of a pressure tank or chamber  30  and a fuel bladder  32  contained within the pressure chamber  30 . The pressure chamber  30  includes a pressure port  34  for receiving a regulating air pressure flow from the compressor section  24 . The pressure chamber  30  further includes a fuel outlet port  36  for supplying fuel from the fuel bladder  32  to the fuel flow path  18 . 
     The PWM valve  20  includes a valve inlet  40 , a valve outlet  42 , and an electromagnetically actuated spool assembly  44  including a solenoid  46  and a metering spool  48 . It should be appreciated that any known type of PWM valve  20  may be utilized in the fuel control system and that the valve  20  selected will depend upon the environment and installation requirements, the fuel flow requirements and the operating parameters of the particular engine  12  selected for use with the system. 
     The fuel pump  16  may be of any known type and is shown in the form of a centrifugal pump including a pump inlet  50 , a pump outlet  52 , and a centrifugal impeller  54  that is driven by a shaft  56  powered by the gas turbine engine  12 . The pump outlet  52  is connected to the combustor assembly  26  by a high pressure fuel conduit  58 . 
     The fuel flow path  18  is shown in the form of a first conduit  60  that directs flow from the fuel outlet port  36  to the valve inlet  40 , and a second conduit  62  that directs flow from the valve outlet  42  to the pump inlet  50 . 
     The regulator valve  22  is basically conventional and is to provide a regulated, constant pressure differential across the PWM valve  20 . The regulator valve  22  includes an air inlet  64 , an air outlet  66 , and a regulating spool  68  for metering the airflow from the air inlet  64  to the air outlet  66 . The valve  22  further includes pressure chambers  70  and  72  separated by a piston or diaphragm  73 . The regulating spool  68  is controlled by the pressure differential between pressure chambers  70  and  72  acting upon the diaphragm  73  and by a biasing spring  74 . The pressure chamber  70  is connected by a pressure tap  75  to the conduit  62  between the valve outlet  42  and the pump inlet  50 . The pressure chamber  72  is connected by a pressure tap  76  to an airflow conduit  78  between the air outlet  66  and the pressure port  34 . The air inlet  64  is connected to the compressor section  24  by an airflow conduit  80 . 
     A controller  90  in the form of a digital electronic controller provides control signals  92  to the PWM valve  20  based on engine speed and power command signals  94  and engine parameter signals  96 , as is known. The controller  90  preferably utilizes conventional digital techniques for providing the control signal  92  to the PWM valve  20 , as is known. Accordingly, further description of the constructional details of the controller  90  are not required, it being sufficient to note, that to increase the fuel flow rate from the valve outlet  42  to the pump inlet  50 , the controller  90  adjusts the control signal  92  to cause an increase in the time period T 2  for the second flow rate and a decrease in the time period T 1  for the first flow rate. Conversely, to decrease the fuel flow rate from the valve outlet  42  to the pump inlet  50 , the controller  90  adjusts the control signal  92  to cause a decrease in the time period T 2  for the second flow rate and an increase in the time period T 1  for the first flow rate. 
     An alternative gas pressurization supply  100  is provided for engine starting. A check valve  101  in the airflow conduit prevents reverse flow of the gas from the supply  100  into the compressor section  24 . Preferably, the supply  100  is in the form of compressed air tank or a start squib. During engine starting, the pressure port  34  receives a pressure flow from the supply  100  for pressurizing the storage device  14 . 
     In operation, fuel flow is supplied to the valve inlet  40  at a pressure P u  via the fuel bladder  32  and the conduit  60 . Fuel flow is supplied to the pump inlet  50  at a pressure P 1  via the PWM valve  20  and the conduit  62 . The fuel flow through the PWM valve  20  is controlled by a signal  92  from the controller  90  which causes the spool assembly  44  to cycle between a first position that allows a first flow rate for a time period T 1  and a second position that allows a second flow rate for a time period T 2 . Typically, the first flow rate will be equal to zero or substantially equal to zero, and the second flow rate will be equal to or greater than the maximum fuel flow rate required for the gas turbine engine  12 . Preferably, the spool assembly  44  is cycled at a fixed frequency and the fuel flow rate from the valve outlet  42  to the pump inlet  50  is controlled by adjusting one or both of the time periods T 1 , T 2 , as is known. 
     In order to insure that the flow through the PWM valve  20  has a relatively predictable relationship to the control signal  92 , it is important to maintain a relatively constant pressure drop ΔP (ΔP=P u− P i ) across the PWM valve  20 . This function is performed by the regulator valve  22  which senses the pressures P u  and P i  and controls the pressure P u  to maintain a relatively constant ΔP. More specifically, the pressure chamber  70  is pressurized to P i  by the pressure tap  74  and the pressure chamber  72  is pressurized to the pressure P u  by the pressure tap  76 . The position of the metering spool  68  is controlled by the pressure differential, ΔP=P u− P i , in the pressure chambers  70 , 72  to regulate a bleed airflow from the compressor section  24  to the pressurized fuel storage device  14 . It should be noted that the above explanation assumes that the pressure P u  at the valve inlet  40  is equal to the pressure in the airflow conduit  78  and the pressurized fuel storage device  14 . It is believed that this assumption is essentially correct for most pressurized fuel storage devices utilizing a fuel bladder. However, the regulator valve  22  will still perform satisfactorily in any system where the pressure P u  at the valve inlet  40  is dependent upon the pressure inside the storage device  14 . Preferably, the regulator valve  22  has sufficient damping to accommodate any pressure pulses generated by the PWM valve  20  in the conduit  62  while maintaining a relatively constant ΔP across the PWM valve  20 . 
     The fuel pump  16  pumps the fuel from the pump inlet  50  to the combustor assembly  28  via the conduit  58  at a pressure P b.  The fuel pump should be designed to attain the maximum pressure required by the combustor assembly  28 . For a small turbojet engine, P b  will typically vary from 25-160 psia during operation. 
     To prevent combustor flame-out or deleterious effects on combustor stability, it is preferred that the pulsating fuel flow output from the PWM valve  20  be damped to closely approximate steady state flow. In the preferred embodiment, this damping is primarily provided by a pulsating vapor core in the fuel pump  16 . More specifically, the damping is provided by vaporizing a portion of the fuel flow at the pump inlet  50  for at least a portion of the time period T 1  and re-forming the vaporized fuel back to liquid at the pump outlet  52  throughout the time periods T 1  and T 2 . Fuel is vaporized at the pump inlet  50  during the time period T 1  because the PWM valve  20  is essentially closed at this time while the pump  16  continues to operate. This causes the pressure at the pump inlet  50  to drop, resulting in such vaporization which forms the vapor core within the pump  16 . When the PWM valve  20  again opens, fuel at about the pressure at the pressure port  34  is available at the inlet  50 . This pressure is sufficiently close or above the vapor pressure of the fuel with the result that vaporization is reduced or ceases altogether, causing pulsating of the vapor core within the pump  16 . 
     At the same time, the geometry of the pump  16  is such that pressure at its outlet  52  is always above the vapor pressure of the fuel. Consequently, only liquid fuel flows from the outlet  52 . This flow is at a relatively constant pressure because the changing length of the vapor core within the pump as the vapor core forms and collapses in pulsating fashion acts as a damper for the pulsating liquid fuel flow through the PWM valve  20 . The ability of centrifugal pumps to reform slugs of vaporized fuel back into liquid form is known and is dependent upon the flow characteristics of the pump and the pump inlet and outlet pressures. Accordingly, it is preferred that the pump  16  be a centrifugal pump and that the components  14 ,  16 ,  18 ,  20 , and  22  of the fuel system be designed to provide a pressure Pi at the pump inlet  50  that allows for sufficient amount of vapor damping in the fuel pump  16 . 
     While the exact amount of damping in the fuel flow required will be highly dependent upon the particular engine  12  selected for use with the system, it has been determined that for some systems and engines  12  the damping should be sufficient to reduce the pulse amplitude of P b  to approximately 10% of the mean value of P b  based on an operating frequency of 50 hertz for the PWM valve  20 . 
     From the foregoing, it will be appreciated that, by placing the PWM valve  20  on the low pressure side of the fuel pump  16 , the fuel control system may utilize a relatively small and low-cost PWM valve, such as is commonly used in connection with automotive fuel injectors. 
     It should further be appreciated that, by metering the fuel flow to the inlet  50  of the fuel pump, rather than from the outlet  52  of the fuel pump, the energy required to pressurize the fuel flow to the combustor is minimized because excess flow at high pressure does not exist and therefore need not be returned to the tank as in prior art systems. 
     It should also be appreciated that the placement of the PWM valve  20  on the inlet side of the fuel pump  16  provides the beneficial advantage of utilizing the fuel pump  16  to provide damping via a pulsating vapor core thereby to minimize the effects of the pulsated fuel flow from the PWM valve  20 . 
     While a PWM valve  20  is preferred, any electromechanical or solenoid valve  20  capable of metering fuel flow by cyclically restricting the fuel flow path  18  to achieve a fuel flow to the pump inlet  50  that cycles between a first flow rate for a time period T 1  and a second flow rate for a time period T 2  may be utilized. Further, while pulse width modulated control is preferred, any form of control, including cycle frequency control, capable of causing a valve  20  to provide the desired cyclical restriction of the flow path  18  may be utilized. By way of further example, it is anticipated that some systems may utilize a fuel storage device  14  that is not pressurized and, further, may not require a relatively constant pressure differential ΔP across the valve  20 .