Emergency lubrication system for a gas turbine engine

<u>Technical Field</u>

This invention relates to lubrication systems and particularly to a turbine engine lubrication system capable of supplying lubricant for a limited time following a failure or malfunction of a lubrication system component.

<u>Background of the Invention</u>

Lubrication systems, such as those used in aircraft gas turbine engines, supply lubricant to bearings, gears and other engine components that require lubrication. The lubricant, typically oil, cools the components and protects them from wear. A typical oil lubrication system includes conventional components such as an oil tank, pump and filter.

If one of the lubrication system components fails, malfunctions or sustains damage, the oil supply to the lubricated component may be disrupted resulting in irreparable damage to the component and undesirable corollary consequences. For example, if an engine oil pump fails, the resulting loss of oil pressure could disable the engine by causing overheating and/or seizure of the bearings that support the engine shaft. An aircraft engine that becomes disabled in flight is obviously a concern, especially if the engine powers a single-engine military aircraft operating in hostile airspace.

It is known to accommodate the possibility of a failure in the oil system by configuring the system so that it continues to supply oil to the lubricated components for a limited time, thereby enabling continued temporary operation of the engine. Such a system allows the aircraft crew time to safely shut down the engine or to take other appropriate actions to safeguard the aircraft and its occupants. In a military aircraft, such a system can provide the crew with valuable additional time to return to friendly airspace.

An emergency lubrication system for an aircraft engine should possess several desirable attributes. The system should provide emergency lubrication for as long as possible, but should not add significantly to aircraft weight or consume precious space by requiring that the aircraft carry a large reserve of emergency lubricant. Moreover, it is desirable that any reserve quantity of lubricant be continuously replenished. Otherwise the properties of the reserve lubricant could degrade over time, rendering the lubricant unsuitable for use when called upon in an emergency. The emergency system should also make as much use as possible of the existing non-emergency lubrication system hardware and lubricant flowpaths, thus avoiding the weight, cost and complexity of dedicated emergency hardware. The system should also operate autonomously, i.e. without requiring that the aircraft crew take any action to engage the system.

Some known lubrication systems use high pressure to deliver a high velocity stream of lubricant during normal operation, but provide a low velocity mist or gravity induced flow of lubricant during abnormal or emergency conditions. Despite the merits of these systems, they may not be satisfactory for all applications. For example, intershaft bearings, such as those used between corotating or counterrotating shafts of a turbine engine, normally receive lubricant by way of a lubricant flowpath that includes scoops projecting from the exterior of a shaft and lubricant passageways (which may include the shaft bore) extending axially through a shaft. The scoops receive pressurized lubricant from the lubricant pumps mounted on the nonrotating structure of the engine and convey that lubricant to the passageways, which then guide the lubricant to the intershaft bearings. Because the scoops and passageways are designed to receive and transport high velocity lubricant, they may not work satisfactorily with low velocity lubricant such as a lubricant mist or a gravity induced lubricant stream. Hence, if it is desired to use the existing lubricant flowpath under emergency operating conditions, it is advantageous to maintain lubricant pressure despite the failure of one or more of the lubrication system components. Although it might be possible to include auxiliary features and hardware that allow low pressure emergency lubricant ( e.g. a mist or gravity induced stream) to bridge the interface between the nonrotating structure and the rotating shafts and adequately lubricate the bearing, doing so would introduce undesirable weight, cost and complexity.

<u>Summary of the Invention</u>

It is, therefore, an object of the invention to provide an emergency lubrication system for a gas turbine engine that allows continued operation of the engine for as long as possible following failure of a system component and accompanying loss of oil pressure. Further objects are that the system be light weight and compact and that it operate autonomously.

According to one aspect of the invention, a lubrication system for a turbine engine includes a reservoir containing a reserve quantity of lubricant and having a lubricant inlet and a lubricant outlet. During a normal mode of operation, the reservoir receives lubricant through the inlet while concurrently releasing lubricant from the outlet at a normal rate. The system also includes a means for resisting backflow of reserve lubricant through the inlet and for effecting a subnormal rate of lubricant release during abnormal operation, such as when the lubricant pressure becomes abnormally low. The subnormal rate of release persists until the lubricant reserve is substantially depleted. The subnormal rate is ideally achieved by intermittently releasing lubricant from the reservoir, but may also be achieved be releasing a continuous stream of lubricant at a rate less than the normal rate of release.

In one detailed embodiment of the invention, a lubricant supply line and a lubricant outlet line have a lubricant inlet valve and a lubricant outlet valve, respectively, for regulating lubricant flow into and out of the reservoir. A fluid supply line employs a fluid inlet valve to selectively establish communication between the reserve quantity of lubricant and a source of pressurized motive fluid. During normal operation, the fluid inlet valve isolates the reservoir from the pressurized fluid while the lubricant outlet valve continuously releases lubricant to the component requiring lubrication and the lubricant inlet valve concurrently admits fresh lubricant into the reservoir. During abnormal operation, the lubricant inlet valve closes in response to abnormally low lubricant pressure outside the reservoir thereby resisting backflow of reserve lubricant out of the reservoir. The fluid inlet valve opens to admit pressurized fluid into the reservoir thus pressurizing the reserve lubricant. The lubricant outlet valve cycles open and closed to intermittently release lubricant under the influence of the pressure applied by the pressurized fluid.

One advantage of the invention is that it affords the aircraft crew an interval of time to shut down the affected engine and/or take other actions necessary to safeguard the aircraft and its occupants. The subnormal rate of lubricant release conserves lubricant, thus prolonging the time interval. Another advantage is that the emergency system takes advantage of existing non-emergency components, rather than requiring additional auxiliary components. This ensures that the system is simple, reliable, compact, light weight and inexpensive.

<u>Brief Description of the Drawings</u>

<ul id="ul0001" list-style="none"><li>Figure <b>1</b> is a schematic illustration of a turbine engine showing an exemplary embodiment of the emergency lubrication system of the present invention.</li><li>Figure <b>2</b> is a schematic illustration showing an emergency lubrication system similar to that of Figure <b>1</b>, but arranged in series with a main lubrication flowpath.</li><li>Figure <b>3</b> is a schematic illustration showing an emergency lubrication system similar to that of Figure <b>1</b>, but with an alternate valving arrangement.</li><li>Figure <b>4</b> is a schematic illustration showing another alternate embodiment of the inventive lubrication system.</li></ul>

<u>Preferred Embodiments of the Invention</u>

Referring to Figure <b>1</b>, an aircraft gas turbine engine <b>10</b> includes low pressure and high pressure compressors <b>12</b>, <b>14</b>, a combustor <b>16</b>, and high pressure and low pressure turbines <b>18, 20</b>. A low speed shaft <b>22</b> couples the low pressure compressor to the low pressure turbine. A high speed shaft <b>24</b> couples the high pressure compressor to the high pressure turbine. An engine controller <b>26</b> issues control signals C to various engine valves and actuators in response to demand signals <b>D</b> from the aircraft pilot and feedback signals <b>F</b> from the engine.

Bearings <b>30</b> support the low speed shaft from the nonrotatable engine structure. Intershaft bearings <b>32</b> support the high speed shaft from the low speed shaft. The bearings are enclosed in bearing compartments <b>34</b>. A lubrication system, which typically includes an oil tank, pump, filters and deaerators, supplies a lubricant to the bearing compartments, and thus to the bearings, by way of a main lubricant flowpath, which is schematically represented in the illustration as a conduit <b>36</b>. The lubricant cools and lubricates the bearings.

An annular, working medium fluid flowpath <b>38</b> circumscribes an axially extending centerline <b>40</b> and extends axially through the engine. During operation, the compressors pressurize a working medium fluid, specifically ambient air. The pressurized air supports combustion of a fuel in the combustor. The resulting products of combustion flow through the turbines, which power the compressors by way of shafts <b>22, 24</b>.

Although most of the air flowing through the compressors enters the combustor, a portion of the air may be extracted from the compressors for other uses. For example, it is customary to divert a quantity of pressurized air <b>42</b> to the vicinity just outside the bearing compartments <b>34</b>. The diverted air is at a pressure slightly higher than the prevailing pressure inside the bearing compartments and therefore helps to resist lubricant leakage out of the compartments. This diverted air <b>42</b> is referred to as buffer air.

A lubricant reservoir <b>50</b> is mounted on the nonrotating structure of the engine. The reservoir holds a reserve quantity of lubricant <b>52</b>. The reservoir has a lubricant inlet <b>54</b>, a lubricant outlet <b>56</b> and a fluid inlet <b>58</b>. A lubricant supply line <b>62</b>, which branches off the main lubricant flowpath <b>36</b>, extends to the lubricant inlet <b>54.</b> A lubricant inlet valve <b>64</b>, represented by check valve <b>64a</b>, regulates lubricant flow through the lubricant inlet <b>54</b>. The check valve operates passively in response to local lubricant pressure, i.e. its position is directly governed by local influences acting on the valve, specifically the difference in lubricant pressure on either side of the valve.

A lubricant outlet line <b>66</b> extends from the lubricant outlet <b>56</b> to convey lubricant to the bearings. A lubricant outlet valve <b>68</b>, represented by electrically powered solenoid operated valve <b>68a</b>, regulates lubricant flow through the lubricant outlet <b>56</b>. The solenoid valve is a nonpassive valve, i.e. its position is governed by the solenoid acting in response to control signal <b>C</b>_<b>1</b>, not by local influences acting directly on the valve. Although the solenoid valve is a two position valve, variants of the invention may benefit from or even require a modulating valve. Moreover, other types of valves, such as mechanically, hydraulically or pneumatically driven valves may also be used.

A fluid supply line <b>70</b> extends from the fluid inlet <b>58</b> to a source of pressurized fluid. A fluid inlet valve <b>72</b>, represented by check valve <b>72a</b>, selectively establishes communication between the reserve quantity of lubricant <b>52</b> and a source of pressurized fluid. In the illustrated example, the pressurized fluid is a portion of the bearing compartment buffer air <b>42</b>.

During a normal mode of engine operation, a main lubrication circuit provides high pressure lubricant to the bearing compartments. The pressure of the lubricant in the main circuit opens lubricant inlet check valve <b>64a</b> so that a fraction of the main lubricant supply also enters the reservoir <b>50</b> to establish a reserve quantity of pressurized lubricant <b>52</b> therein. The reserve quantity need be established only on a new or recently serviced engine because, as the reader will soon appreciate, the reserve quantity remains confined in the reservoir, although it is continuously extracted and replenished during normal engine operation. The check valve <b>64a</b> remains open as long as the lubricant supply pressure exceeds the pressure in the reservoir.

During normal operation, the solenoid valve <b>68a</b> is commanded to its open position by the controller. As a result, reserve lubricant is continuously released from the reservoir at a normal rate. Replenishment of the reserve lubricant occurs as a result of fresh lubricant admitted to the reservoir through inlet <b>54</b>. Consequently, the reserve lubricant is not susceptible to age related degradation. The fluid inlet check valve <b>72a</b> remains closed because the pressure in the reservoir exceeds that of the buffer air <b>42</b>.

In the event that a malfunction or failure in the main lubricant circuit disrupts the lubricant supply to the bearing, the attendant reduction or complete loss of main lubricant pressure causes check valve <b>64a</b> to close, thus resisting backflow of reserve lubricant through the lubricant inlet <b>54</b>. To the extent that the solenoid valve <b>68a</b> remains open, reserve lubricant continues to flow from the reservoir to the bearing. The continued release of lubricant begins depressurizing the reservoir so that fluid inlet check valve <b>72a</b> opens, thereby establishing communication between the reserve lubricant <b>52</b> and the pressurized buffer air <b>42</b>. The buffer air maintains pressure on the reserve lubricant to force it through the outlet line <b>66</b> to the bearing. As pointed out previously, maintaining pressure on the lubricant may be highly advisible if the lubricant must pass through a lubricant flowpath leading to an intershaft bearing or other lubricant flowpath not designed to accept low pressure lubricant. The pressurization of the lubricant by the buffer air causes a pressurized stream of lubricant to be released from the reservoir.

In order to conserve lubricant, a suitable sensor, such as a pressure sensor in the main lubricant circuit, detects the disruption of the lubricant supply and informs the controller <b>26.</b> The controller then issues a command <b>C</b>_<b>1</b> to lubricant outlet valve <b>68</b> to effect a subnormal rate of lubricant release through the lubricant outlet <b>56.</b> The command <b>C</b>_<b>1</b> may cause the valve <b>68</b> to cycle between its open and closed positions, in which case the subnormal release of lubricant is an intermittent release. The intermittent release is desirable because it is highly effective at conserving lubricant, and therefore provides for a long interval of emergency operation in the abnormal mode. Moreover, we have discovered that the intermittent application of lubricant, although not satisfactory for normal operation, is satisfactory for emergency operation. Alternatively, the command <b>C</b>_<b>1</b> may cause the valve <b>68</b> to remain open or partially open, in which case the subnormal release of lubricant is a continuous release at a rate lower than the rate of release in the normal mode of operation. Either way, the subnormal release persists until the reserve quantity of lubricant <b>52</b> is substantially depleted,

Because the above described system enables the engine to continue operating after failure of a system component, pressurized air <b>42</b> continues to be available to pressurize the reserve lubricant and force lubricant to the bearing. As a result, the operation of the engine is sustained, albeit in an abnormal operating mode. The sustained operation of the engine ensures an uninterrupted supply of pressurized air to the reservoir so that the bearings continue to receive lubricant and the engine continues to operate in the abnormal mode. Because the lubricant flows from the reservoir at a subnormal rate, the bearing will likely experience highly accelerated wear and require replacement. Nevertheless, the subnormal quantity of lubricant is adequate to keep the engine operating safely until the lubricant reserve in the reservoir is depleted. The aircraft crew can take advantage of this additional interval of operation to systematically follow the procedures necessary to safeguard the aircraft and its occupants and to safely shut the engine down before the bearings sieze or overheat. If the engine powers a military aircraft operating in hostile airspace, the crew can use the interval of extended operation to proceed toward friendly airspace.

Although Figure <b>1</b> depicts a parallel flow configuration, a series flow configuration, as depicted in Fig <b>2</b>, may also be employed. In the series flow configuration the supply line <b>62</b> to the reservoir and the lubricant outlet line <b>66</b> are the exclusive vehicle for supplying lubricant to the bearing. All lubricant destined for the bearing traverses through the reservoir during the normal mode of operation.

Figure <b>3</b> illustrates an alternate embodiment of the invention in which the fluid inlet valve <b>72</b> is a nonpassive valve. In the illustrated example, the nonpassive valve is a solenoid valve <b>72b</b>, but may be a modulating valve if necessary or desirable. No valve is present in the lubricant outlet line <b>66</b>. During the normal mode of operation, the lubricant inlet check valve <b>64a</b> is open as in the first embodiment. Solenoid valve <b>72b</b> is closed. Lubricant enters the reservoir through inlet <b>54</b> and is released through outlet <b>56</b>. During abnormal operation, abnormally low lubricant pressure causes check valve <b>64a</b> to close. The controller <b>26</b> commands the solenoid valve <b>72b</b> to cycle open and closed. When the valve <b>72b</b> opens, the pressurized buffer air <b>42</b> acts on the reserve lubricant and forces some of the lubricant through the lubricant outlet <b>56</b>. When the valve <b>72b</b> closes, pressurized lubricant flow soon ceases. In the present example, the subnormal rate of lubricant release is an intermittent release. However a continuous subnormal rate of lubricant release may be achieved by keeping valve 72 in an open or partially open position.

In the above described embodiments, the lubricant inlet valve <b>64</b>, fluid inlet valve <b>72</b> and the lubricant outlet valve <b>68</b> (when present) and the coordinated operation of these valves offer an effective way to resist backflow of reserve lubricant out of the reservoir and to effect a subnormal rate of lubricant release during the abnormal mode of operation.

Figure <b>4</b> shows another alternate embodiment of the invention in which the reservoir <b>50</b> is a tank that holds a reserve ration of lubricant <b>52</b>. The tank has an opening <b>76</b> sealed by a removeable cap <b>78</b>. A maintenance technican introduces the reserve ration of oil into the tank through the opening prior to use of the engine. Means are provided for confining the lubricant ration in the tank during a normal mode of operation and for releasing lubricant from the tank at a regulated rate during the abnormal mode of operation. In the illustrated variant, a fluid inlet valve <b>72</b> in the form of a solenoid valve <b>72b</b> or other nonpassive valve selectively establishes communication between the reserve ration of lubricant and the buffer air <b>42</b>. A lubricant outlet valve <b>68</b> represented by check valve <b>68b</b> regulates lubricant release through the outlet line <b>66</b>. During normal operation both valves are closed, thus confining the lubricant ration in the tank. During abnormal operation, the valve <b>72b</b> is commanded to open or to cycle open and closed, thus introducing pressurized buffer air <b>42</b> into the tank and forcing lubricant past the check valve <b>68b</b> at a regulated rate. Another variation has a valve arrangement similar to that of Fig. <b>1</b>. The fluid inlet valve <b>72</b> is passive, the lubricant outlet valve <b>68</b> is nonpassive and the tank holds an initially pre-pressurized ration of lubricant. During normal operation, both valves are closed. During abnormal operation, the nonpassive lubricant outlet valve opens or cycles open and closed in response to a controller command to effect a regulated release of lubricant under the influence of the initial pre-pressurization. The release of lubricant causes a decay of the pressure inside the tank, which allows the passive fluid inlet valve to open, thus admitting pressurized buffer air to maintain adequate pressurization. Other variants employ only a single, nonpassive valve, either a fluid inlet valve or a lubricant outlet valve.

The embodiment of Figure 4 does not continually refresh the lubricant in the reservoir. Nevertheless, lubricant freshness can be ensured by periodic draining and refilling of the tank by maintenance personnel. In some applications, such as engines for single-use vehicles, periodic refreshment may not be required.

As is evident from the foregoing, the invention in its various embodiments supplies a primary quantity of lubricant to the bearing during the normal mode of operation. For the embodiments of Figures <b>1, 3</b> and <b>4</b>, This primary quantity is the aggregate of the quantities flowing through main lubricant flowpath <b>36</b> and the normal amount of lubricant released through lubricant outlet <b>56</b>. For the embodiment of Figure <b>2</b>, the primary quantity is the amount of lubricant released through the outlet <b>56</b> because all the lubricant passes through the reservoir <b>50</b>. During the abnormal mode of operation, which occurs if the primary supply of lubricant is disrupted, the invention provides for the release of a secondary quantity of lubricant. This secondary quantity is the amount released from the reservoir or tank <b>50</b>. The secondary quantity is a subnormal quantity, less'than the normal quantity released during the normal mode of operation. The subnormal quantity may be the result of an intermittent release of lubricant, or may be the result of a continuous release of lubricant at a rate of flow less than the normal rate of flow through lubricant outlet <b>56</b>. For the embodiment of Figure <b>4</b>, the secondary, subnormal quantity is a quantity less than that provided by the main lubrication circuit. This subnormal quantity would typically approximate the subnormal quantities provided by the embodiments of Figure <b>1-3</b>.

Although this invention has been shown and described with reference to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the invention as set forth in the accompanying claims.