Abstract:
A fuel metering unit for controlling a variable displacement pump including a main metering valve in fluid communication with the pump for metering an output of the pump, a pressure regulator in fluid communication with the metering valve to create a spill return and a control valve in fluid communication with the pressure regulator and the pump for regulating the spill return flow so the spill return flow is maintained substantially constant at a low level to minimize the heat generated by recirculation by setting a displacement of the pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 60/462,652, filed Apr. 14, 2003, which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The subject invention is directed generally to a system for regulating fluid flow, and more particularly, to a system for regulating the flow of fuel from a variable displacement pump utilizing bypass flow.  
           [0004]    2. Background of the Related Art  
           [0005]    Fixed delivery fuel pumps have often been sized to provide excessive fuel flow capacity in order to insure adequate supply to the associated engine. Consequently, under many operating conditions, large amounts of pressurized fuel are returned to the pump inlet for recirculation. The return and recirculation results in significant fuel heating due to additional energy being put into the fuel which is subsequently turned into heat as the pressure drops in the recirculation path. In modern designs, fuel heating is a critical issue because the fuel is typically used as a heat exchanger to maintain proper operating temperature. Other methods of heat exchange are undesirable because of the associated size, weight and cost.  
           [0006]    Variable displacement fuel pumps have partially overcome the drawbacks of fixed delivery pumps by being able to vary the amount of fuel output. By varying the fuel output, the fuel delivered more closely matches engine demand. Thus, the recirculated flow, along with the heat generated thereby, is reduced. Variable displacement fuel pumps are known in the art, as disclosed in U.S. Pat. No. 5,833,438 to Sunberg, the disclosure of which is herein incorporated by reference in its entirety. A variable displacement pump typically includes a rotor having a fixed axis and pivoting cam ring. The cam ring position may be controlled by a torque motor operated servo valve. However, the engine operating conditions often include transients such as those caused by engine actuator slewing, start-up and the like as would be appreciated by those of ordinary skill in the pertinent art. Under such rapidly varying operating conditions, prior art pump control systems have been unable to respond quickly and adequately. So despite this, variable displacement pumps still do not respond quickly enough to varying engine demands so excess fuel flow is still common.  
           [0007]    In view of this shortcoming, control systems to fully utilize variable displacement fuel pumps have been developed. Examples of variable displacement pump control arrangements are disclosed in U.S. Pat. No. 5,716,201 to Peck et al. and U.S. Pat. No. 5,715,674 to Reuter et al., the disclosures of which are herein incorporated by reference in their entirety. Typical pump control systems attempt to maintain accurate fuel flow throughout the range of engine operating conditions. However, such systems still contain inadequacies such as instability, insufficient bandwidth. Moreover, such systems are still prone to delivering excessive fuel which must be recirculated. The pump control systems may include sophisticated electronics and numerous additional components to undesirably increase costs and complicate the pump control system.  
           [0008]    In view of the above, it would be desirable to provide a pump control system which accurately and quickly regulates the output flow of a variable displacement pump without the associated drawbacks of the prior art.  
         SUMMARY OF THE INVENTION  
         [0009]    The subject invention is directed to a pump control system for a variable displacement fuel pump such that the pump displacement exceeds the required steady state flow of the associated engine by an amount sufficient to accommodate flow transients and the bypass flow is maintained at a substantially constant acceptable level, i.e. small enough to prevent excessive heating.  
           [0010]    In accordance with a preferred embodiment of the subject invention, the advantages of the present disclosure are accomplished by employing a constant bypass flow regulator with fuel metering to set the displacement of the pump.  
           [0011]    It is an object of the present disclosure to increase the fuel metering unit bandwidth while maintaining acceptable stability at all operating conditions.  
           [0012]    It is another object to provide a hydromechanical fuel metering unit for a variable displacement pump.  
           [0013]    It is still another object to provide a fuel metering unit that achieves quick and accurate response to dynamic flow conditions.  
           [0014]    In a preferred embodiment, the present invention is directed to a fuel metering unit for controlling a variable displacement pump including a metering valve in fluid communication with the pump for metering an output of the pump, a pressure regulator in fluid communication with the metering valve to create a spill return flow and a control valve in fluid communication with the pressure regulator and the pump for regulating the spill return flow so the spill return flow is maintained substantially constant by setting a displacement of the pump. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    So that those having ordinary skill in the art to which the subject invention appertains will more readily understand how to make and use the same, reference may be had to the drawing wherein:  
         [0016]    The Sole FIGURE is a schematic representation of the fuel control system of the subject invention which includes a variable displacement vane pump, a bypassing pressure regulator and a control valve that maintains substantially constant bypass flow at a sufficient level to accommodate flow transients encountered during engine operation while minimizing the heat generated by recirculation. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    Referring now to Sole FIGURE, there is illustrated a schematic representation of the fuel control system of the subject invention which is designated generally by reference numeral  10 . For clarity throughout the following description, arrows are shown within the lines of system  10  to indicate the direction in which the fuel flows and an annotated letter “P” is shown to indicate a pressure at certain locations. All relative descriptions herein such as left, right, up, and down are with reference to the system  10  as shown in Sole FIGURE and not meant in a limiting sense. Additionally, for clarity common items such as filters and shut off solenoids have not been included in the schematic representation of Sole FIGURE. System  10  is illustrated in association with a variable displacement vane pump  12 . System  10  maintains the output flow of the pump  12  to meet engine needs yet advantageously minimizes recirculation, e.g., spill return flow which prevents excessive energy from being imparted to the fuel.  
         [0018]    Pump  12  includes a rotor  14  and a pivoting cam ring  16 . For a detailed description of a variable displacement vane pump, see U.S. patent application Ser. No. 09/867,359 filed May 29, 2001 which is incorporated herein by reference in its entirety. Pump  12  receives fuel flow from line  15  at an inlet pressure P AF , and delivers fuel flow at an output pressure P F  into line  37 . A piston  18  is operatively connected to the cam ring  16  to control the position of the cam ring  16  relative to the rotor  14 , and in turn vary the output flow of the pump  12 . A half area servo  17  positions piston  18  within housing  11 . It would be appreciated by those of ordinary skill in the art that other types of servos similarly and differently arranged would perform this same function and are, therefore, considered mere design choices. For example without limitation, an equal area servo could be utilized as servo  17 . The maximum flow setting of pump  12  occurs when the piston  18  is moved the maximum distance to the left. A feedback line  30  provides fuel at pressure P F  to one inlet of the half area servo  17 . An orifice  31  in line  30  dampens the motion of the piston  18 . It will be appreciated by those of ordinary skill in the art that line  30  may connect the half area servo  17  to a variety of sources while still maintaining the required performance for the preferred embodiment. Line  44  provides pressure to the other inlet of half area servo  17  as is described below. Spring  19  is sized and configured to bias piston  18  to maximum flow for start up of pump  12 . Throughout system  10 , springs are sized as a function of the product of piston area and fuel pressure as would be appreciated by those of ordinary skill in the art and therefore not further described herein.  
         [0019]    Main metering valve  20  is disposed in line  37  between the pump  12  and engine (not shown) for providing fuel to the engine at a selected rate and pressure P M . Suitable main metering valves  20  are well known in the prior art and therefore not further described herein. A variety of metering valves  20  may be utilized as long as the selected valve performs the function of selectively varying the amount of fuel which may pass through to the engine.  
         [0020]    A bypassing pressure regulator  22  is connected to line  37  through spill return flow line  32  and static sensing line  34 . Regulator  22  includes a housing  21  defining an interior with a spring-biased spool  23  operatively disposed therein. Spill return flow line  32  contains fuel flowing therethrough in accordance with the relationship (P F −P M ), e.g., the spill return flow. Static sensing line  34  has no flow but provides pressure to the spool  23  of regulator  22  at pressure P M . The flow exits from the pressure regulator  22  into line  39  at a pressure P AF   ′ , and passes through a bypass flow sensing orifice  48  into line  38 . Fuel in line  38  recirculates to the pump  12  by line  45 , and passes into the half area servo  17  by line  44 . Orifice  46  is disposed in line  38  to limit the fuel flow therethrough. Under static conditions, the pressure in line  44  is substantially half the pressure within line  30  hence the moniker “half area servo”  17  is appropriate.  
         [0021]    The flow from pressure regulator  22  is also directed by lines  41 ,  43  to inputs of a control valve  26  that is in direct communication with the output flow from pump  12  by line  36  at a pressure P F . Control valve  26  includes a housing  27  that defines an interior with a spring-biased spool  29  operatively disposed therein. During steady-state conditions, the control valve  26  maintains the displacement of the pump  12  and, in turn, the relationship (P AF″ −P AF ) across bypass flow sensing orifice  48 . Thus, the bypass fuel flow from the pressure regulator  22  through the orifice  48  remains substantially constant. The fuel flow through orifice  48  is set at a sufficient level to accommodate transient events such as bleed actuators, engine slewing from maneuvers such as terrain avoidance, engine surging due to missile launching, and other like demands. The primary output flow from control valve  26  exits into line  42  at a servo pressure P S  and is delivered to the half area servo  17  to act on the piston  18 . The position of the piston  18  moves the cam ring  16  relative to the rotor  14  to determine the output of the pump  12 .  
         [0022]    During steady-state operation, the control valve  26  maintains bypass flow through orifice  48  at a relatively small level to prevent significant heating in the system  10 . When a transient event occurs whereby the engine requires more fuel, main metering valve  20  responds by opening to immediately increase flow to the engine and starts a chain of events which leads to an increase in the output of the pump  12 . The pump  12  cannot immediately respond with increased displacement so the incremental demand is filled by a reduced spill return flow in line  32 . In effect, the control system  10  immediately responds. In response to the spill return flow decrease in line  32 , spool  23  in pressure regulator  22  strokes up. As a result, the output in line  39  is decreased and, in turn, the pressure differential (P AF″ −P AF ) across orifice  48  decreases. When (P AF″ −P AF ) decreases, the spool  29  in control valve  26  strokes to the right to decrease the flow in line  42  and thereby the pressure in line  44  decreases which causes the piston  18  to move to the left. As a result, the output of pump  12  increases until the spill return flow in line  32  returns to the desired level and a steady-state condition is reached across orifice  48 .  
         [0023]    In the alternative, when the engine requires less fuel, main metering valve  20  responds by closing to decrease flow to the engine. As a result, the spill return flow in line  32  increases to start a chain of events which leads to a decrease in the output of the pump  12 . In particular, spool  23  in pressure regulator  22  strokes down increasing the output in line  39  and, in turn, increasing the pressure differential (P AF″ −P AF ) across orifice  48 . Spool  29  in control valve  26  strokes to the left and the pressure in lines  42 ,  44  increases which causes the piston  18  to move to the right. When the piston  18  moves to the right, the output of pump  12  decreases. Ultimately, the piston  18  shifts to the right until the spill return flow in line  32  returns to the desired steady-state level. When the spill return flow is at the desired level, (P AF″−P   AF ) returns to the substantially constant steady-state level. Thus, control valve  26  reacts to the pressure differential across bypass sensing orifice  48  to reposition the pump  12  to maintain a desired spill return flow level and a substantially constant pressure across bypass sensing orifice  48 . Accordingly, system  10  is a stable hydromechanical unit which can quickly respond to engine transients without unnecessary recirculation flow.  
         [0024]    While the subject invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention as defined by the appended claims.