Low-flow Contaminated Fuel Transfer System For A Fuel Control

Abstract

A fuel control system, adapted to handle contaminated fuel, has a primary fuel supply line incorporating a pump and a wash flow filter downstream of the pump. A bypass line containing a bypass valve communicates with the primary line at respective positions upstream of the pump and downstream of the filter. A secondary fuel supply line communicates with the filter to receive a flow of filtered fuel. A main fuel supply line includes a window-type metering valve which receives fuel from either the primary or secondary line via a transfer valve. At low flows the metering valve receives fuel from the secondary line so that it will not become clogged by contaminants in the fuel and at high flows contaminated fuel is delivered to the metering valve through the primary line.

Description

BACKGROUND OF THE INVENTION

This invention relates to fuel control systems which embody a metering valve.

Metering valves of the window type are quite common in prior art fuel control systems. A requirement frequently imposed upon these systems is that they be able to accommodate a flow of contaminated fuel. At low flow settings the outlet opening of the metering valve is rather small and therefore is likely to become obstructed by contaminants in the fuel.

In order to overcome the problem alluded to above, metering valves in the prior art are frequently contoured to provide a minimum opening which is larger than the largest of the anticipated contaminants. A prominent drawback to this approach is that large metering heads are not feasible because of minimum flow requirements. It is highly desirable to impress a large metering head across the metering valve to minimize the effects of metering head regulator friction and flow forces and thereby enhance the accuracy of the fuel control.

SUMMARY OF THE INVENTION

The invention utilizes a standard wash flow filter positioned in a primary fuel supply line downstream of a pump. A secondary fuel supply line communicates with the filter. A main fuel metering valve is positioned in a main fuel supply line to receive either a filtered or contaminated flow of fuel, depending on the position of a control valve, which functions to fluidly interconnect the primary, secondary and main fuel supply lines. The position of the control valve is dependent on the position of the main metering valve such that filtered flows and contaminated flows are delivered to the main metering valve when it occupies respective low-flow and normal-flow positions.

The invention is unique in that it allows the minimum metering valve opening to be reduced to the extent necessary to meter filtered fuel and thus provides a means to precisely control low fuel flows.

Accordingly, it is a primary object of the invention to provide a fuel control system for contaminated fuel having a metering valve, in which the system is adapted to direct either a filtered or unfiltered flow through the valve.

Another object is to provide a fuel control system for contaminated fuel in which the valve is capable of metering low fuel flows.

Yet another object is to provide a wash flow filter arrangement in a fuel control system for contaminated fuels, in which the filtered fuel not only serves as servo fluid and wash fluid, but also is directed to a main fuel supply line which incorporates a metering valve.

Other objects and advantageous features of the invention will become apparent from the following description and the accompanying drawing, wherein for purposes of illustration only, a specific form of the invention is set forth in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a portion of a fuel control system incorporating an embodiment of the invention.

FIG. 2 is a schematic view of a portion of a fuel control system incorporating another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring in detail to FIG. 1, wherein there is shown only a portion of a fuel control system adapted to control the flow of a contaminated fuel to an engine (not shown), a primary fuel supply line 10 receives a flow of fuel from an inlet conduit (not shown) which may incorporate a boost pump such as a centrifugal impeller pump. An engine-driven gear pump 12 mounted within primary fuel supply line 10 delivers fuel through the primary line. Pump 12 is drivingly connected to the engine by means of pump drive 14.

A conventional cylindrical wash flow filter 16 is secured within an annular recess 18 in primary line 10 in such a manner that the filter's inner surface is flush with the inner surface of the primary line. Recess 18 fluidly communicates with a secondary fuel supply line 20 which carries a filtered flow of fuel emerging from the outer surface of filter 18. Secondary line 20 fluidly interconnects with a branch conduit 22 which directs filtered fuel to the remainder of the fuel system for use as servo fluid in the various components thereof and for use as wash fluid in the system's valving (including the main metering valve) to prevent contaminants in the fuel from hindering movement of movable elements in the system.

A window-type main fuel metering valve, generally indicated at 24, is positioned in main fuel supply line 26, the main line embodying a conventional pressurizing valve (not shown) and a metered fuel outlet (not shown). The structure of the main metering valve, which is generally conventional, includes a spool 28 having two spaced lands 30 and 32 thereon. Spool 28 functions to meter the fuel flow in main line 26 through a metering orifice or opening defined by window 34 and land 32. A pressure balance conduit 36 interconnects the outboard portions of the spool 28 to axially balance the spool. The valve spool is positioned by a positioning mechanism 29.

A transfer valve, generally designated at 37, fluidly interconnects the primary line 10 and secondary line 20 with the main line 26. Valve 37 comprises a spool 38 having lands 42 and 44. Spool 38 is axially positionable between an upper limit of travel (illustrated in the drawing), in which land 44 prevents a flow from primary line 10 to metering valve 24, while permitting a flow from the secondary line 20 to the main line 26, and a lower limit of travel, in which land 42 blocks flow from secondary line 20 to main line 26, while allowing a flow from primary line 10 to main line 26.

Transfer valve 37 is operable in a "bang-bang" manner by means of a differential pressure imposed upon the outboard faces of lands 42 and 44. Pressure conduits 46 and 48 respectively communicate with these faces to transmit pressure thereto from a suitable valve operator 50, which is connected to sources of high and low pressure. The valve operator 50 causes pressure conduits 46 and 48 to either respectively communicate with the low- and high-pressure sources or the high- and low-pressure sources, in accordance with signals conveyed thereto by metering valve position indicator 51. At small openings of window 34, the transfer valve 38 occupies the illustrated position in which the pressure conduits 46 and 48 respectively communicate with the low- and high-pressure sources, thereby allowing only filtered fuel to pass therethrough. Conversely, at a predetermined larger opening, the order of communication between the pressure conduits and sources is reversed such that spool 38 is impelled to its lower limit of axial travel, whereby only unfiltered fuel traverses the transfer valve.

It will be apprehended that the invention is not limited to mutually exclusive flows of filtered and unfiltered fuel, but that a transfer valve may be fashioned to provide an intermixture of filtered and unfiltered fuel in the main fuel supply line, should design considerations sanction such operation.

A bypass line 60 fluidly communicates with portions of primary line 10 at respective locations upstream and downstream of pump 12. A conventional bypass valve 62 is positioned within bypass line 60 to control the flow of bypassed fuel to the inlet side of pump 12. The bypass valve serves to regulate the pressure differential (metering head) across the main metering valve 24. Lower member 63 of bypass valve 62 is fixedly and sealingly secured to a diaphragm 64 which is sealed to the walls of a chamber 66 so as to divide the chamber into an upper chamber and a lower chamber. The upper and lower chambers defined by diaphragm 64 each communicates with respective upstream and downstream locations in main line 26, as referenced to metering valve 24, by means of interconnecting conduits 68 and 70. Bypass valve 62 is resiliently urged to a seated position in the bypass line by spring 72.

In operation, contaminated fuel enters primary line 10 and is accelerated by gear pump 12. Transfer valve 37 remains in its illustrated position until the opening in window 34 is enlarged to a predetermined extent. Fuel pressure rises rapidly within primary line 10 after rotation of the pump 12 is initiated, thus causing bypass valve 62 to unseat and permit a flow in bypass line 60 back to the inlet side of pump 12. As a result of this bypass flow, a recirculation of contaminants in the fuel develops. Contaminants, which would otherwise adhere to filter 15, are washed from the inner surface thereof and progressively accumulated in the recirculating flow as long as land 44 blocks primary line 10.

Fuel within the primary line 10 also passes radially through the filter 16 to emerge at the outer surface thereof, from where it enters secondary line 20 as a filtered flow. A portion of this filtered flow in secondary line 20 enters branch conduit 22 and thence is distributed to the fuel system to wash the valves and function as servo fluid. Filtered flow emerges from secondary line 20 to pass through transfer valve 38 and enter main line 26.

Fuel flowing in main line 26 is metered through window 34 by land 32 of metering valve 24. Since the metering head across the valve is maintained at a constant value by the bypass valve, the rate of flow through the opening 34 is substantially a function of the metering valve's area. Because of the fact that the metering orifice, defined by opening 34 and land 32, for low flows, may be smaller than that which would ordinarily be required to accommodate contaminated fuel at low flows, the bypass valve may be designed to maintain a metering head larger than that which would ordinarily be permissible. This is a significant advantage as it is highly desirable to maintain a metering head of a large magnitude in order to mitigate the effects of bypass valve friction and thereby enhance the accuracy of the fuel control system.

When main metering valve positioning mechanism 29 repositions the metering valve to a higher flow position, in which contaminants passing through the metering orifice are not likely to cause a clogging thereof, main metering valve position indicator 51 transmits a signal to valve operator 50. Valve operator 50, which is in constant communication with the sources of high and low pressure, functions to communicate pressure conduits 46 and 48 respectively with the sources of high and low pressure. The pressure differential occasioned across the spool 38 results in downward axial movement of the spool until it reaches its lower limit of downward axial travel. In this position, the secondary line 20 is blocked by land 42 and the primary line 10 freely communicates with the main line 26.

Referring now to FIG. 2, wherein like numerals designate elements similar to those of FIG. 1, another embodiment of the invention is shown which operates in a manner similar to that of the previous embodiment, but differs slightly therefrom in construction. Instead of the transfer valve 37 of FIG. 1, a pressure-responsive spring-loaded shut-off valve 80 is positioned between primary line 10 and main line 26 to prevent contaminated flow from entering main line 26 until the metering orifice is sufficiently enlarged. Bypass valve 82 (metering head regulator) is of the well-known "hour glass" variety. This type of bypass valve provides flow force compensation which enables it to regulate the metering head more accurately than the bypass valve 62 of FIG. 1. Spool 84 of bypass valve 82 is biased by a spring preload exerted by an adjustable spring 86. The spring preload determines the nominal value of the metering head and consequently may be varied to compensate for changes in fuel density and maintain the proper fuel flow schedules. Secondary line 20 incorporates a check valve 88 to permit only a one-way flow of filtered fuel therein from the filter 16 to the main line 26. Secondary line 20 also incorporates a variable resistance (orifice) 89, which may be employed to adjust the pressure drop (P.sub.2 -P.sub.3).

Since P.sub.3 -P.sub.4 is held constant by valve 82, the pressure drop (P.sub.2 -P.sub.3) increases as flow increases through secondary line 20. Consequently, the differential pressure (P.sub.2 -P.sub.3), produced by the filter element 16 and orifice 89, can be used as a parameter of metered flow and hence the area of the metering orifice, as the metered flow is essentially a function of this area. Therefore, the spring preload acting on valve 80 is selected so that the valve 80 cracks open when P.sub.2 -P.sub.3 attains a value which is indicative of a metering orifice area large enough to permit contaminants to pass freely therethrough. The valve 80 is thus responsive to the position of the metering valve.

If it is necessary to connect a branch conduit to the line 20 of FIG. 2, in order to draw filtered flow therefrom for operation of servos associated with the fuel control or for other purposes, differential pressure (P.sub.2 -P.sub.3) may no longer be representative of the metered flow unless orifice 89 is adjusted to effect a pressure drop thereacross, which is large in comparison to the pressure drop across the filter. If orifice 89 is so adjusted, the pressure drop P.sub.2 -P.sub.3 is representative of the metered flow, but may be of a magnitude which is unacceptable for the fuel control. If the pressure drop is impractical, two solutions are available:

A. install a second wash flow filter for servo flow only; or

B. replace valve 80 with either an appropriate spool or poppet valve and provide suitable pressure conduits on respective sides of the orifice to communicate pressures to the selected replacement valve. In solution B, as the communicated pressure differential will be indicative of the area of the metering orifice, the replacement valve should be openable to permit a flow from primary line 10 to main line 26 after the pressure differential attains a sufficient value.

It will be observed that the embodiments of FIGS. 1 and 2 operate in two basic modes, namely, a filtered flow and a contaminated flow mode. It will be appreciated that during periods of operation in the former mode, the entire filtered output flow (through secondary line 20) equals the contaminated inlet flow, while the stored contaminants recirculate in the flow path defined by primary line 10 and bypass line 60.

It will be understood that the invention is not limited to the construction specifically illustrated. For example, the metering valve need not be of the window type but may embody other varieties of valves such as a poppet valve. Also, the spool-type transfer valve illustrated could readily be replaced with a rotary transfer valve. Further, the disclosed bypass valves could be replaced with other available varieties and need not function as metering head regulators, inasmuch as a separate and distinct valve would be suitable for that purpose.

While I have shown a particular embodiment of my invention it will be understood, of course, that I do not wish to be limited thereto, since many modifications may be made, as fall within the scope and spirit of my invention.

Claims

I claim:

1. In a fuel control system for an engine, the combination comprising:

a primary fuel supply line for receiving a flow of fuel;

a pump mounted in said primary line for pumping fuel therethrough;

a main fuel supply line adapted to receive a flow of fuel from said primary line;

secondary fuel supply line means to communicate with said primary line for carrying a flow of filtered fuel to said main line upstream of said metering valve;

a wash flow filter positioned in said primary line to fluidly interconnect said primary line and said secondary line means;

a fuel bypass line fluidly connected to said primary line downstream of said filter and upstream of said pump for bypassing fuel to the inlet side of said pump;

a bypass valve located in said bypass line to control the flow in said bypass line;

a metering valve for defining a metering opening located in said main line for metering a flow of fuel therethrough;

a control valve positioned between said primary line and said main line to prevent the flow of fuel from said primary line to said metering valve when said metering opening does not exceed a predetermined area; and

means responsive to the position of said metering valve to position said control valve when the metering opening exceeds said predetermined area so as to permit the flow of fuel from said primary line to said metering valve.

2. The combination of claim 1, wherein said position-responsive means comprises:

a valve operator operatively connected to the control valve for imposing a differential pressure thereacross, said differential pressure serving to position said valve; and

a metering valve position indicator operatively connected to said operator to transmit a signal thereto.

3. The combination of claim 1, wherein said position-responsive means comprises:

means to bias said control valve to a position in which flow from said primary line to said main line is prevented, said control valve being positionable by a sufficient differential pressure between said primary line and said main line to permit a flow of fuel from said primary line to said main line; and

a restriction in said secondary line means to produce said sufficient differential pressure.

4. In a fuel control system for an engine, the combination comprising:

a primary fuel supply line for receiving a flow of fuel;

a pump mounted in said primary line for pumping fuel therethrough;

a main fuel supply line adapted to receive a flow from said primary line;

secondary fuel supply line means to communicate with said primary line for carrying a flow of filtered fuel to said main line upstream of said metering valve;

a wash flow filter positioned in said primary line to fluidly interconnect said primary line and said secondary line means;

a fuel bypass line fluidly connected to said primary line downstream of said filter and upstream of said pump for bypassing fuel to the inlet side of said pump;

a bypass valve located in said bypass line to control the flow in said bypass line;

a metering valve for defining a metering opening located in said main line for metering a flow of fuel therethrough; and

means responsive to the position of said metering valve to prevent the flow of fuel from said primary line to said metering valve when said metering opening does not exceed a predetermined area and to permit the flow of fuel from said primary line to said metering valve when said metering opening exceeds the predetermined area.