FUEL SUPPLY SYSTEM

Abstract

The fuel supply system includes mains pressurising and metering arrangement including a positive displacement ecology pump and an ecology valve piston chamber and a piston slidably movable in the chamber between de-prime and re-prime positions, the chamber forming a fuel sink to one side of the piston which increases in volume when the piston moves to de-prime position and reduces in volume when the piston moves to re-prime position. The ecology valve is fluidly connected to the ecology pump for pilot-only operation reverse direction operation of the ecology pump causes the piston to move to de-prime position removing mains fuel from injectors through a mains fuel distribution pipework and into the fuel sink, and for pilot and mains operation the forward direction operation of the ecology pump causes the piston to move to re-prime position refilling injectors with mains fuel from the fuel sink.

Claims

1. A fuel supply system for fuel injectors of a multi-stage combustor of a gas turbine engine, the fuel supply system including: a pilot pressurising and metering arrangement which receives a first portion of a low pressure fuel flow, and pressurises and controllably meters the first portion of the low pressure fuel flow into a high pressure metered pilot flow for injecting at pilot discharge orifices of the injectors; a mains pressurising and metering arrangement including a positive displacement ecology pump, the mains pressurising and metering arrangement being configured to pressurise and controllably meter a second portion of the low pressure fuel flow into a high pressure metered mains flow for injecting at mains discharge orifices of the injectors, wherein the ecology pump is operable in a forward direction for pilot and mains operation in which there are pilot and mains supplies to the combustor from the injectors, the ratio of the metered pilot flow to the metered mains flow determining a staging control split of the pilot and mains flows, and wherein the ecology pump is operable in a reverse direction to provide a zero metered mains flow for pilot-only operation in which there is a pilot supply to the combustor from the injectors but no mains supply to the combustor; pilot fuel distribution pipework distributing fuel from the pilot pressurising and metering arrangement to the pilot discharge orifices, and mains fuel distribution pipework distributing fuel from the mains pressurising and metering arrangement to the mains discharge orifices; wherein the mains pressurising and metering arrangement further includes an ecology valve having a piston chamber and a piston slidably movable in the chamber between de-prime and re-prime positions, the chamber forming a fuel sink to one side of the piston which increases in volume when the piston moves to its de-prime position and reduces in volume when the piston moves to its re-prime position; and wherein the ecology valve is fluidly connected to the ecology pump such that for pilot-only operation the reverse direction operation of the ecology pump causes the piston to move to its de-prime position thereby removing the mains fuel from the injectors through the mains fuel distribution pipework and into the fuel sink, and such that for pilot and mains operation the forward direction operation of the ecology pump causes the piston to move to its re-prime position thereby refilling the injectors with mains fuel from the fuel sink.

2. A fuel supply system according to claim 1, wherein the mains pressurising and metering arrangement further includes a flow sensing valve which senses the mains flow rate.

3. A fuel system according to claim 2, wherein the mains pressurising and metering arrangement further includes a restricted orifice bypass line in parallel to the flow sensing valve, the bypass line being configured such that, when the flow sensing valve is closed, the mains fuel removed from the injectors into the fuel sink during de-priming passes through the bypass line, and the mains fuel from the fuel sink which refills the injectors during re-priming passes through the bypass line.

4. A fuel supply system according to claim 1, wherein the mains pressurising and metering arrangement is configured to fluidly isolate the mains fuel distribution pipework from the low pressure fuel flow during pilot-only operation.

5. A fuel supply system according to claim 1, wherein the mains fuel distribution pipework includes a mains fuel manifold distributing fuel from the mains pressurising and metering arrangement to the mains discharge orifices.

6. A fuel supply system according to claim 5, wherein moving the piston of the ecology valve to its de-prime position also removes mains fuel from the mains fuel manifold, and moving the piston to its re-prime position refills the mains fuel manifold with mains fuel.

7. A fuel system according to claim 1, wherein the ecology pump is electrically powered.

8. A fuel system according to claim 1, wherein the ecology pump is positioned between the ecology valve and the mains fuel distribution pipework, the ecology pump pumping fuel from the mains fuel distribution pipework into the fuel sink to move the piston to its de-prime position prior to pilot only operation, and the ecology pump pumping fuel from the fuel sink into the mains fuel distribution pipework to move the piston to its re-prime position prior to pilot and mains operation.

9. A fuel system according to claim 1, wherein the ecology valve has a position sensor which senses the position of the piston, the position sensor sending signals to vary the operation of the ecology pump when the piston reaches its de-prime and re-prime positions.

10. A fuel system according to claim 1, wherein the mains pressurising and metering arrangement further includes a relief valve in parallel to the ecology pump, the relief valve being configured such that, when the piston reaches its de-prime position, a rise in pressure in the fuel sink causes the relief valve to open whereby the ecology pump can continue to operate in the reverse direction pumping fuel in a circuit around the ecology pump and the relief valve, whilst maintaining the piston at its de-prime position.

11. A fuel system according to claim 1, wherein the mains pressurising and metering arrangement further includes a low cracking pressure check valve in parallel to the ecology valve, the check valve being configured such that, when the piston reaches its re-prime position, a reduction in the pressure in the fuel sink causes the check valve to open whereby the second portion of the low pressure fuel flow is delivered through the check valve to the ecology pump for pressurisation thereby.

12. A gas turbine engine having a multi-stage combustor and the fuel supply system according to claim 1 for supplying fuel to and performing staging control in respect of pilot and mains fuel discharge orifices of fuel injectors of the combustor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0041] FIG. 1 shows schematically a combustion staging system for a gas turbine engine in pilot and mains operation mode;

[0042] FIG. 2 shows a longitudinal cross-section through a ducted fan gas turbine engine;

[0043] FIG. 3 shows schematically a pump system and a fuel supply system for fuel injectors of a multi-stage combustor of the gas turbine engine with the fuel supply system providing pilot-only operation;

[0044] FIG. 4 shows schematically the pump system and the fuel supply system of FIG. 3 but with the fuel supply system providing pilot and mains operation; and

[0045] FIG. 5 shows schematically a variant of the pump system and the fuel supply system of FIGS. 3 and 4 with the fuel supply system providing pilot and mains operation.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

[0046] With reference to FIG. 2, a ducted fan gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

[0047] During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate-pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate-pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high-pressure compressor 14 where further compression takes place.

[0048] The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

[0049] The combustion equipment 15 of the engine 10 includes a multi-stage combustor. FIGS. 3 and 4 show schematically a pump system 24 and a fuel supply system for fuel injectors of the multi-stage combustor. In FIG. 3 the fuel supply system is shown in pilot-only operation with mains supply off and the mains fuel passages of the injectors and a connecting mains manifold de-primed. In FIG. 4 the fuel supply system is shown in pilot and mains operation with the mains injector fuel passages and mains manifold re-primed and mains supply on.

[0050] The pump system 24 comprises typically a low pressure (LP) pumping stage 41 which draws fuel from a fuel tank of the aircraft and supplies a first portion of the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage 42. The LP stage may be a centrifugal impeller pump, while the HP pumping stage may comprise one or more positive displacement pumps, e.g. in the form of twin pinion gear pumps. The LP and HP stages can be connected to a common drive input, which is driven by the engine HP or IP shaft via an engine accessory gearbox.

[0051] The HP pumping stage 42 also forms the first part of a pilot pressurising and metering arrangement of the fuel supply system, the pilot pressurising and metering arrangement (described in more detail below) controllably metering the high pressure flow from the HP pumping stage to provide a high pressure metered pilot flow for injecting at pilot discharge orifices of the injectors 33. An offtake from the pump system 24 between the LP and HP pumping stages directs a second portion of the boosted pressure fuel from the LP pumping stage 41 into a separate and parallel mains pressurising and metering arrangement 26 of the fuel supply system, the mains pressurising and metering arrangement (also described in more detail below) providing a high pressure metered mains flow for injecting at mains discharge orifices of the injectors. In addition, the mains pressurising and metering arrangement provides the capability to de-prime/re-prime the mains fuel passages of the injectors and the mains manifold for fuel staging.

[0052] Focusing initially on the pilot pressurising and metering arrangement, this typically has a hydro-mechanical unit (HMU) 25 which receives the high pressure flow from the HP pumping stage 42. The HMU can comprise a fuel metering valve operable to control the rate at which the pilot fuel flow is sent to the combustor. The HMU further typically comprises: a pressure drop control arrangement (such as a spill valve and a pressure drop control valve) which is operable to maintain a substantially constant pressure drop across the metering valve, and a pressure raising and shut-off valve at the fuel exit of the HMU which ensures that a predetermined minimum pressure level is maintained upstream thereof for correct operation of any fuel pressure operated auxiliary devices (such as variable inlet guide vane or variable stator vane actuators) that receive fuel under pressure from the HMU. Further details of such an HMU are described in EP 2339147 A (hereby incorporated by reference).

[0053] The pilot flow is continuous. Total metered flow (pilot+mains) is metered in response to engine control parameters via an Engine Electronic Controller (EEC). Similarly, the pilot/mains flow split is set in accordance with the fuel staging laws, e.g. to reduce emissions.

[0054] The EEC commands the HMU fuel metering valve to supply pilot fuel to the combustor at a given flow rate. The metered pilot fuel leaves the HMU 25 into pilot fuel distribution pipework 34, and can be split by this pipework between first and second (or more) segments of a pilot manifold 31. A lean blow out protection valve actuated for example by a solenoid-operated control valve or similar device (both not shown) may be located between the pilot fuel distribution pipework and the second pilot manifold segment. Such a valve can be used to restrict the portion of total pilot flow passing to one of the pilot manifold segments such that the other segment receives a higher proportion of the flow to ensure that some of the injectors remain lit when there is a threat of lean blow out at certain operating conditions. Each fuel injector 33 of the combustor of the engine has a fuel spray nozzle (FSN) containing a pilot (primary) discharge orifice and a mains (secondary) discharge orifice. If the pilot manifold is split into two segments, the injectors are split into two groups. The pilot discharge orifices of the FSNs of the injectors of one group are connected to the first pilot manifold segment, while pilot discharge orifices of the FSNs of the injectors of the other group are connected to the second pilot manifold segment. The mains flow feeds the mains discharge orifices of the FSNs of both groups of the fuel injectors. The pilot and mains discharge orifices may have respective weight distribution valves (WDVs) to reduce gravitational head effects between the injectors.

[0055] Turning to the mains pressurising and metering arrangement 26, the second portion of boosted pressure fuel from the LP pumping stage is delivered to a low cracking pressure check valve 27 and to a spring chamber of an ecology valve 28. The ecology valve comprises a piston chamber and a piston slidably movable in the piston chamber between de-prime and re-prime positions. The piston chamber provides on one side the spring chamber, and on the other side a fuel sink which increases in volume when the piston moves to its de-prime position and reduces in volume when the piston moves to its re-prime position.

[0056] At steady state pilot and mains conditions, the flow rate to the spring chamber of the ecology valve 28 is zero as the fuel line is dead headed. The mains flow path is through the check valve 27 to an electrically driven ecology pump 30 (shown here as a gear pump but it can be a different type of positive displacement pump). The mains flow rate is varied by controlling the forward speed of the motor driven pump in response to a demand signal from the EEC. Downstream of the pump, the mains flow passes through a flow sensing valve 35 (or similar flow measurement device). This can be a single or two-stage device, e.g. as described in U.S. Pat. No. 5,795,998, hereby incorporated by reference. The flow sensing valve may comprise a piston moveable within a sleeve against a spring load, the piston opening/closing a flow port in the sleeve depending on the level of flow. Since the piston position is a measurement of mains flow rate, a position sensor is used to provide a feedback signal to the EEC. This facilitates closed loop control of mains flow. Open loop control, based on speed scheduling of the ecology pump and with no flow sensing device is also possible but is less accurate.

[0057] Downstream of the flow sensing valve 35, the total metered mains flow passes from the mains pressurising and metering arrangement 26, through mains fuel distribution pipework 32 and into a mains manifold 29. This manifold feeds mains flow to each injector 33, the flow passing through the mains fuel passages of the injectors and out through the mains discharge orifices to the combustion chamber (at pressure P40). As previously mentioned, WDVs at the injector heads can ensure an even distribution of flow, compensating for manifold head effects.

[0058] In pilot-only operation (FIG. 3), the HMU 25 meters pilot flow to the pilot discharge orifices. Mains flow is de-selected by powering the electrically driven ecology pump 30 in a reverse sense direction, thereby stopping further flow to the mains discharge orifices. Initially, the pump performs an ecology function, draining a fixed volume of fuel from the mains fuel passages of the injectors 33, and typically also from the mains manifold 29, into the fuel sink (non-spring chamber) of the ecology valve 28 at pressure Pev. As the pump draws fuel, Pev rises and is set by the ecology valve spring and piston diameter to be above the delivery pressure (LP) of the low pressure pumping stage in the pump system 24 (for example, PevLP 410 kPa (60 psi)). The pressure level is sufficient to cause the low pressure check valve 27 to close (for example, closure can occur when LPPev<140 kPa (20 psi) approx.). A drip tight seal in the check valve prevents fuel flow, or the ingress of any hot combustion gases (at P40) back into the low pressure fuel system. At the same time, pressure Pev is insufficient to crack open a low cracking pressure relief valve 36 which is in parallel to the ecology pump (for example, the relief valve may crack at PevPm1.0 MPa (150 psi), where Pm is the line pressure on the mains fuel distribution pipework side of the ecology pump).

[0059] When the low pressure check valve 27 is closed, the flow from the reverse-direction flowing ecology pump 30 displaces the piston of the ecology valve 28 to the left, as illustrated in FIG. 3. As the piston moves, it draws the fixed volume of mains fuel into the valve's fuel sink so that, (i) the mains fuel passages of the injectors 33 are fully emptied, and (ii) the mains manifold 29 is sufficiently emptied to avoid subsequent ingress of mains fuel into the injectors should the remaining fuel in the mains manifold/mains fuel distribution pipework expand under temperature during pilots-only operation or be displaced during aircraft maneuvers. Draining the injectors of mains fuel protects their narrow internal mains passages during pilot-only operation. With no residual mains fuel present in the injectors, it cannot breakdown under temperature (coke) which would cause blockage of the passages and increase injector-to-injector maldistribution with an overall reduction in injector life. Some external cooling (for example, air cooling) may be required to prevent coking in the de-staged mains manifold).

[0060] When the ecology valve 28 reaches its left hand stop (de-prime position), the ecology pump 30 is dead-headed and Pev rises to crack open the low cracking pressure relief valve 36 (i.e. PevPm>1.0 MPa (150 psi)). At the same time, the low cracking pressure check valve 27 is held closed since Pev>LP. In this state, the mains pressurising and metering arrangement 26 allows any flow displaced by the ecology pump to recirculate via the low cracking pressure relief valve 36. The speed of the pump can be reduced to reduce any heat input into the fuel, as it only has to provide sufficient pressure to hold the ecology valve on its left hand stop to maintain the mains passages of the injectors 33 and the mains manifold 29 de-primed.

[0061] A position sensor on the ecology valve 28 can provide an indication of the valve reaching its stop, and when this is confirmed the speed of the ecology pump 30 can be reduced. The flow left recirculating around the pump causes no net change in the volume of fuel left in the mains fuel distribution pipework 32, so the mains passages of the injectors 33 and the mains manifold 29 remain de-primed throughout the duration of pilot-only operation.

[0062] The piston of the ecology valve 28 sits against a face seal to achieve a drip tight seal. In combination with the drip tight seal of the low pressure check valve 27, this achieves isolation of the de-primed mains passages/mains manifold from the pump system 24. The seals prevent ingress of LP fuel into the de-primed mains passages/mains manifold when LP>P40, and also prevent ingress of hot combustion gas (at P40) back into the pump system at conditions where P40>LP.

[0063] When mains flow is required for pilot and mains operation, the ecology pump 30 is powered in a forward sense to drive the piston of the ecology valve 28 to the right, as illustrated FIG. 4. This displaces fuel from the ecology valve fuel sink to re-prime the mains injector passages and mains manifold.

[0064] Advantageously, the electrically driven ecology pump 30 can be accelerated rapidly to provide rapid re-prime capability. When the pump begins to rotate in the forward sense, Pev falls (e.g. to PevLP410 kPa (60 psi)) as determined by the ecology valve piston diameter and spring. This causes the low pressure relief valve 36 (having e.g. a cracking differential1.0 MPa (150 psi)) to close, and at the same time the low pressure check valve 27 and a mains pump relief valve 37 (also in parallel to the ecology pump) remain closed. As the ecology pump draws flow from the fuel sink of the ecology valve, the valve piston moves to the right so that the fixed volume of fuel is displaced via the pump back into the mains fuel distribution pipework/manifold/injector passages, fully re-priming these volumes prior to mains flow being demanded. The WDVs limit any pre-leakage to the mains discharge orifices.

[0065] The ecology valve position sensor provides indication of the valve 28 reaching its right hand stop (i.e. its re-prime position corresponding to the mains manifold/injector passages being fully re-primed). On reaching the stop, with the ecology pump 30 still rotating in a forward sense, Pev falls towards vapour pressure (<LP) until the low pressure check valve 27 cracks open (e.g. when LPPev140 kPa (20 psi)) to feed the pump from the LP pumping stage 41 of the pump system 24. At this point, the speed of the ecology pump can be varied under closed loop control to meter the correct mains flow to the fully primed mains passages of the injectors 33.

[0066] The downstream flow sensing valve 35 provides a flow measurement signal to the EEC, which responds to engine control laws to set the ecology pump speed for the required flow level. Pilot flow is metered by the HMU 25 so that the two flow streams are controlled independently. This ensures that there are no significant dips and spikes in the pilot flow at the mains staging points or when mains flow is modulated.

[0067] A restricted orifice bypass line 40 can be provided in parallel to the flow sensing valve 35. When the flow sensing valve is closed, the bypass line allows the mains fuel removed from the injectors/mains manifold into the fuel sink to pass around the closed valve and similarly allows the mains fuel from the fuel sink which refills the injectors/mains manifold to pass around the closed valve. The restricted orifice can be configured such that, during pilot and mains operation, only a relatively small and compensatable portion of the mains flow bypasses the flow sensing valve.

[0068] The ecology valve 28 does not require a drip tight seal when the piston is on its right hand stop as any leakage from LP to Pev merely results in a slightly lower flow through the low pressure check valve 27, with no overall effect on performance. The mains pump relief valve 37 prevents any over-pressurisation of the ecology pump 30 specifically and the mains pressurising and metering arrangement 26 generally in the event of a blockage occurring downstream of the pump.

[0069] The fixed volume of fuel displaced during de-priming and re-priming is determined by the ecology valve diameter and travel, and can be set to be: [0070] (i) Sufficiently large to ensure that a full de-prime of the mains passages of the injectors 33 and mains manifold 29 is achieved for pilot-only operation; [0071] (ii) Sufficiently large so that in pilot-only operation, any expansion of the residual fuel in the mains fuel distribution pipework 32 at high temperatures does not result in fuel ingress into the injectors; [0072] (iii) Sufficiently large so that in pilot-only operation, any displacement of the residual fuel in the mains fuel distribution pipework 32 during aircraft maneuvers does not result in fuel ingress into the injectors; and [0073] (iv) As low as possible to minimise re-prime time, so that the engine can achieve acceleration performance requirements.

[0074] Advantageously, the fuel supply system: [0075] 1. Permits removal of FSVs and hence mitigates associated risks (i.e. mal-scheduling due to failed open FSV; reduces injector-to-injector fuel maldistribution due to component and frictional variation; avoids lifing issues such as FSV seal wear/degradation leading to fuel dribbling and consequent nozzle coking); provides combustion efficiency benefits at low mains flow conditions. [0076] 2. Avoids complex mains manifold cooling flow recirculation architectures. [0077] 3. Avoids ingress of hot combustion gases (P40) into the fuel system. The ecology valve 28 and low cracking pressure check valve 27 isolate the reverse flowing ecology pump 30 from the LP system when the main manifold 29 is de-primed. [0078] 4. Provides cost and mass benefits associated with 1 & 2 above. [0079] 5. Provides via the ecology pump 30 and the ecology valve 28 an accurate means of controlling the volume of fluid displaced during de-priming/re-priming. In particular, excessive de-priming can be avoided, so that the re-prime time can be minimised. There is also no risk of drawing hot P40 gases into the LP system. Similarly, the mains manifolds and mains passages of the injectors are not significantly over-filled during re-priming so there is only a low risk of pre-fuelling mains combustor zones before mains is selected. [0080] 6. Provides via the electric drive to the ecology pump 30 a rapid re-prime capability, making it practical to de-prime/re-prime a large fixed volume (e.g. that empties the entire mains manifold 29) whilst ensuring that the engine can still meet its acceleration performance requirements, particularly at take-off and go-around scenarios. [0081] 7. Allows use of the ecology pump as a dual purpose device, providing a means of metering the mains flow and also, a means of priming/de-priming when the mains is staged in/out. [0082] 8. Independently controls the pilot and mains flow streams to reduce cross-talk so that significant dips or spikes in the pilot flow can be avoided at the mains staging points or when mains flow is modulated. Similarly, significant dips or spikes in the mains flow can be avoided when pilot flow is modulated. [0083] 9. Provides redundancy capability if the ecology pump 30 or its electric drive fails. In particular, if the pilot flow control does not have an electrical pump drive, control can be maintained via the shaft driven pump system 24 and the HMU 25. [0084] 10. Allows the system to be extended to include circumferential staging, i.e. if more than one mains stage is required, the mains pressurising and metering arrangement 26 can be replicated for the additional stages. [0085] 11. Reduces heat input into the fuel when mains is selected by avoiding a spill flow recirculating around the variable speed ecology pump 30. This in turn allows the engine oil system to pass more heat to the fuel, thereby allowing a reduction the size of other oil coolers on the engine. [0086] 12. Provides independent pilot and mains metering systems which can accurately set the total burner flow and pilot/mains flow split with minimal sensitivity to any of the downstream restrictions e.g. blocked burner discharge nozzles, WDVs etc.

[0087] The system can be re-configured to replace the shaft driven HP pumping stage of the pump system 24 with a variable speed, motor driven pump used for pressuring and metering the pilot flow. This configuration is illustrated in FIG. 5 as a variant of the pump system and a fuel supply system of FIGS. 3 and 4 in pilot and mains operation. The mains pressurising and metering arrangement 26 remains unchanged, taking flow from a conventional shaft driven LP pumping stage. However, in the variant HP pumping stage, the shaft driven HP pumping stage and the HMU for pilot flow control are replaced by a motor driven, variable speed pilot pump 38 (shown here as a gear pump but it could be a different type of pump such as a piston pump) and a pilot flow sensing device 39 located downstream of the pilot pump. Closed loop control of the pilot flow can be achieved via the EEC, using a flow measurement signal from the pilot flow sensing device 39, comparing it to the demanded flow and then adjusting the motor/pump speed to deliver the required flow pilot flow.

[0088] A benefit of this variant is the avoidance of a spill system associated with a shaft driven pump/metering control arrangement. The electrically driven pilot pump 38 delivers only the amount of flow required by the pilot fuel passages of the injectors 33, so there is no excess spill flow recirculating around the pump, adding heat to the fuel. This helps to further reduce the size of any additional oil coolers on the engine, as more heat that is generated in the oil system can be passed into the fuel system.

[0089] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.