Abstract:
An automatic engine protection system for use when electronic parts of the control system are exposed to overtemperature conditions. A thermally sensitive component, such as an engine electronic control or an electronic overspeed control, is mounted on the engine. A thermal fuse is mounted adjacent, or in thermal contact with, the speed control. The thermal fuse is placed in electrical series with a valve which controls fuel delivery to the engine. If the temperature of the fuse exceeds its melting point, indicating a possible danger to the electronic control, the fuse melts, thereby terminating fuel to the engine.

Description:
TECHNICAL FIELD  
         [0001]    The invention concerns an engine protection system for preventing anomalous engine behavior due to erroneous control system behavior when electronic parts of the control system are exposed to overtemperature conditions.  
         BACKGROUND OF THE INVENTION  
         [0002]    Gas turbine engines are traditionally equipped with some type of control system, speed governor, or both. Early control systems or speed governors were mechanical or hydromechanical. FIG. 1 is a simplified schematic which shows operative principles used by a common type of mechanical speed governor.  
           [0003]    A shaft  3 , on the left side of the Figure, is connected to a linkage  6 , which supports weights  9 . The shaft  3  and linkage  6  rotate as indicated by arrow  12 . As speed increases, the weights  9  are driven radially outward, in the directions of arrows  15  shown on the right side of the Figure. This radial motion withdraws piston  18  from a valve  21 , thereby closing the valve  21  and either (1) shutting down the engine or (2) limiting the speed of the engine.  
           [0004]    Advancements in modern electronics, and particularly in integrated circuits, have greatly (1) reduced cost, (2) increased reliability, and (3) increased the amount of functionality which can be contained in relatively small packages. For these reasons and others, the traditional mechanical control system or speed governor is being replaced by electronic control systems and overspeed protection systems.  
           [0005]    However, despite the great benefits offered by modern electronic systems, they nevertheless suffer some disadvantages. One disadvantage is sensitivity to heat. For example, certain types of transistors can experience “thermal runaway,” wherein a high temperature promotes excessive numbers of carriers into the transistor&#39;s conduction band, thereby turning the transistor into a short circuit. The short circuited transistor attempts to conduct a very large current, and destroys itself.  
           [0006]    Related phenomenon can occur with solid-state diodes. In addition, printed circuit boards, upon which the solid-state components are mounted, cannot withstand excessive temperatures.  
           [0007]    Therefore, when an electronic circuit is used as part of a control system or as an overspeed protection device, in a gas turbine engine for example, the engine must be protected from erroneous control system behavior when the electronic parts of the system are exposed to overtemperature conditions.  
         SUMMARY OF THE INVENTION  
         [0008]    In one form of the invention, temperature of a temperature-sensitive component, or a region near the w component, is sensed in a gas turbine engine. If the temperature exceeds a limit, fuel flow to the engine is terminated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a simplified schematic of a mechanical speed governor.  
         [0010]    [0010]FIG. 2 illustrates a system implementing one form of the invention for an engine control system using an electronic control.  
         [0011]    [0011]FIG. 3 is a perspective, cutaway view of several steps undertaken in assembling one type of thermal fuse  45  in FIG. 2.  
         [0012]    [0012]FIG. 4 is a cross-sectional view of the type of fuse shown in FIG. 3.  
         [0013]    [0013]FIG. 5 shows the apparatus of FIG. 4 contained within a housing.  
         [0014]    [0014]FIG. 6 is an enlarged view of housing  63 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Block  30  in FIG. 2 represents a generalized propulsion system as indicated. A gas turbine engine (not shown) represents one such propulsion system. Fuel  33  is delivered to servovalve  36 , which delivers metered fuel  39  to the engine within the propulsion system  30 , as indicated.  
         [0016]    [0016]FIG. 2 also shows a temperature-sensitive component  42 , such as an engine electronic control, which monitors engine speed and controls fuel flow to control engine speed. Thermal fuse  45  is mounted adjacent the component  42 . In one arrangement, the thermal fuse  45  is mounted in a primary thermal path between a source of heat and the component  42  itself.  
         [0017]    The term primary thermal path can be explained by an example. Assume that the source of heat is a candle (not shown). If the component  42  is located one foot directly above the candle, then, in the arrangement under consideration, the thermal fuse  45  would be located between the component  42  and the candle flame. That is, the thermal fuse would be located in the primary thermal path between the flame and the component  42 .  
         [0018]    This situation is different from another possible situation, wherein the thermal fuse  45  is located above the component  42 , that is, the component  42  now lies between the thermal fuse  45  and the candle flame. This arrangement is not precluded by the invention, but the previous arrangement is preferred, wherein the thermal fuse  45  is located between the component  42  and the heat source, in a primary heat path.  
         [0019]    The thermal fuse  45  is connected electrically in series with a coil  48 , which represents one torque motor coil which operate servovalve  36 . Thermal fuse  45  is removably connected by connectors  49  and  50 , which can take the form of standard pin-and-socket connectors.  
         [0020]    If more than one torque motor control is present, then a separate thermal fuse  45  is preferably provided for each coil.  
         [0021]    Servovalve  36  is designed such that, when no current flows through coil  48 , the servovalve  36  closes, and no fuel  39  is delivered to the propulsion system  30 . A control  51 , known in the art, controls the current through the coil  48 , thereby controlling the amount of fuel  39  delivered to the propulsion system.  
         [0022]    If the temperature at thermal fuse  45  reaches its melting point, thermal fuse  45  melts, thereby becoming an open circuit. The open circuit blocks current to the coil  48 , thereby closing servovalve  36 . The now-closed servovalve  36  blocks fuel delivery to the propulsion system  30 , and the propulsion system  30  shuts down.  
         [0023]    It should be observed that component  42  is designed to operate properly in the presence of all normal sources of heat, such as heat produced by engine operation, sunlight or the heating system, HVAC, of an aircraft hangar within which the propulsion system  30  is housed.  
         [0024]    [0024]FIG. 3 illustrates a perspective, cutaway view of several steps undertaken in assembling one type of thermal fuse  45  in FIG. 2. It is emphasized that the steps illustrated in FIG. 3 are presented in order to conveniently illustrate structural aspects of the assembled fuse  45  of FIG. 2. These steps are not presented to represent an optimal mode of assembly. For example, housing  63  is shown as a cylinder, but could take the form of two half-cylinders, arranged clamshell style.  
         [0025]    In FIG. 3, Fuse element  60  is inserted into a cylindrical housing  63 , which contains internal bulkheads  66  which define three chambers  68 . After insertion, fuse element  60  and housing  63  form an assembly  72 . That assembly  72  is inserted into a second cylindrical housing  75 , to form a second assembly  78 . Second housing  75  contains perforations  81 , which allow ambient air to contact the fuse element  60 , to thereby heat the fuse element  60 .  
         [0026]    Connectors  84  are inserted into the second housing  78 , to form a third assembly  87 . FIG. 4 is a cross-sectional view which includes the third assembly  87 . It is emphasized that elements  90  compose a cylindrical shell, and that perforations  81  are merely holes in that shell. That is, the three components labeled  90  do not represent three individual components separated by annular spaces  81 . Elements  81  are holes.  
         [0027]    Spaces  100  within connectors  84  are diagrammatic, and are not drawn to scale. Those spaces  100  may be filled with solder (not shown), to make contact with wires  105 . Alternately, the connectors  84  can take the form of standard crimp-type butt connectors, which are deformed by crimping in order to make contact with wires  105 . Deformation is not shown. Other modes of making electrical attachment between wires  105  and connectors  84  are possible.  
         [0028]    The wires  105  which connect to the fuse element  60  contain bends  110 , which accommodate differential thermal expansion.  
         [0029]    [0029]FIG. 5 shows the apparatus of FIG. 4, but contained in a hard protective package  115 . The package  115  contains perforations  118  which allow ambient air to communicate with perforations  81  (only two perforations  118  are shown).  
         [0030]    Package  115 , as well as housings  63  and  78 , are preferably constructed of a material which is an electrical insulator. If this material is also thermally conducting, then the response time of the fuse will be shorter. Such materials are known in the art.  
         [0031]    The housing  63  in FIG. 3 contains internal chambers  68 . The inner surfaces of these chambers  68  will become contacted by melted material emanating from fuse element  60 , if it melts. It is not desired that the melted, and possibly re-solidified, material form a conductive path through housing  63 .  
         [0032]    Consequently, the internal bulkheads  66  act to form a labyrinthine structure. More precisely, any molten material is expected to attempt to form a film which will adhere to the internal surfaces of housing  63 . In so doing, that material will be required to spread over surfaces  150  shown in FIG. 6. Those surfaces represent a longer pathway between points A and B, than the original fuse element  60  (not shown) occupied. Thus, since the material is required to span a longer distance, it will necessarily be much thinner, and thus will probably contain gaps.  
         [0033]    Further, the statistical likelihood of the material forming a continuous film between points A and B is considered highly unlikely, especially given the fact that several sharp, 90-degree corners  155  are present. Thin films typically do not cover sharp corners well.  
         [0034]    In addition, the material of surfaces  150  of the housing  63  in FIG. 6 is constructed is preferably non-wettable by the molten material of which fuse element  60  is constructed. For example, Teflon (™) is one such material.  
         [0035]    With the two expedients of (1) non-wetting material and (2) a labyrinthine passage from points A to B in FIG. 6, it is considered extremely unlikely that the molten fuse material will form a conductive bridge between points A and B.  
         [0036]    Dashed lines  121  in FIG. 5 represent a woven wire sleeve which surrounds the structure shown in FIG. 4, and acts as electrical shielding. Wires  105  terminate with electrical connectors  124 , shown as sockets. These connectors  124  mate with mating connectors, which would be pins in this case, contained in connector  50  in FIG. 2. Pins are not shown.  
         [0037]    In one embodiment, the woven wire sleeve  121  may be grounded, in which case an additional connector  124  would be added, and connected to a system ground.  
         [0038]    The invention has been described in the context of a gas turbine engine. However, the invention is applicable to numerous apparatus in which (1) fuel is delivered through an electrically controlled valve which blocks fuel flow when current is terminated to the valve and (2) a temperature-sensitive component can be affected by excessive heat due to a fault condition.  
         [0039]    The invention places a thermal fuse at a position which represents the temperature environment of the temperature-sensitive component, and places the thermal fuse in electrical series with the valve. When the thermal fuse opens, current is terminated to the valve, thereby terminating fuel flow, and shutting down the engine in an orderly manner.  
         [0040]    A thermal fuse is shown in FIGS. 2 and 4. It is not strictly necessary that the fuse melt in order to block current. Thermal circuit breakers are available, and such breakers, or similar apparatus can be used. Stating the preceding another way, one form of the invention focuses on the architecture shown in FIG. 2, and not upon the particular type of thermal fuse used.  
         [0041]    One type of thermal fuse used by the invention melts at a temperature of 150° C. In other modes of operation, melting temperatures of 175° C., 200° C., 225° C., 250° C., 275° C., and 300° C. can be used. In yet other modes of operation, different thermal fuses having melting points below the respective temperatures just identified can be used.  
         [0042]    An issue of terminology will be addressed. It could be said that any electrical conductor acts as a thermal fuse, because at some temperature that conductor will melt, and thereby become an open circuit. However, the term “thermal fuse” is a term-of-art. It refers to an element which melts, or becomes open-circuited, while the remaining conductors with which it is connected remain fully operative.  
         [0043]    In one form of the invention, connectors  84  are not used, but wires  105  are continuous from the fuse element  60  to the connectors  124 .  
         [0044]    It is not necessary that the fuse  45  in FIG. 2 terminate current to a fuel metering valve. Some, and possibly all, gas turbine engines also contain a main shut-off valve, which is not used for metering. The fuse  45  can control the main shut-off valve. Alternately, two fuses can be used, one for the main shut-off valve, and another for the metering valve, if present.  
         [0045]    Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.