Abstract:
A high pressure fuel system includes tapped high pressure flows ( 22,24 ) with restrictors ( 44,46 ) to provide low pressure flows with which, via a valve ( 50 ) to enable either a low pressure flow to bias a main, high pressure fuel cut off valve ( 16 ) to close at a steady given rate, or a high pressure flow across a further valve ( 30 ), to close the valve ( 16 ) at a faster rate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the control of a high pressure flow of fuel to combustion equipment. 
     BACKGROUND OF THE INVENTION 
     When fuel is burned to create an expanding gas, and the forces generated thereby are utilised to rotate a turbine, or to drive a piston, which in turn rotates a crankshaft, a situation called overspeed can develop. It is then necessary to obviate the flow of fuel to the combustion zone as quickly as possible, so as to avoid mechanical damage to the associated powerplant. This means that it is desirable to provide fuel shut off capability which will satisfy both normal and abnormal running speeds of the apparatus being driven. 
     It is known, to take a plurality of tappings from the high pressure fuel system, and provide each tapping with an outlet and a dedicated blocking device, with which to selectively block the outlet as and when appropriate. This is in order to achieve the fuel flow control as described hereinbefore, so as to protect the associated driven apparatus. It is an object of the present invention to provide an improved fuel flow control apparatus. 
     SUMMARY OF THE INVENTION 
     According to the present invention, in a main, high pressure fuel system including fuel flow cut off means, wherein fuel flows tapped from said system are connected to said cut off means and utilised to apply differing main flow cut off rates by said cut off means, a single, tapped fuel flow blocking device positioned between tapped flow outlets and selectively movable to block one or other of said outlets, the arrangement being such that blocking of one said outlet effects a low pressure fuel flow to said main fuel flow cut off means to move it into the high pressure fuel line to close it, and blocking of said other outlet effects a high pressure fuel flow to said fuel cut off means, to move it more rapidly into said high pressure fuel line than said low pressure fuel flow, to close it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 a diagrammatic layout of a high pressure fuel system incorporating a single, tapped flow blocking device in accordance with the present invention. 
     FIG. 2 is an alternative single, tapped flow blocking device in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a fuel pump  10  receives fuel from a low pressure fuel supply (not shown) via a line  12 . The pump  10  raises the fuel pressure to a high value and forces it along line  14 , partly illustrated in chain dotted arrow form, to and past a fuel cut off valve  16 , whereafter, the fuel reaches combustion equipment (not shown). The piston  18  of valve  16  is spring loaded in a direction across the fuel flow, but the spring  19  is rated such that during normal operation of an associated powerplant (not shown), a fuel pressure holds the valve  16  open. 
     In order to provide selective boost biasing of the valve  16  so as to close it when desired, a tapping  20  is taken from line  14 , at a position adjacent the outlet of pump  10 . Two further tappings  22  and  24  are taken from tapping line  20  and have outlet nozzles formed at their end extremities, numbered  26  and  28  respectively. Tapping  22  is extended and shaped so that the outlet nozzles  26  and  28  oppose each other in spaced relationship. 
     Tapping  20  is extended and connects to a spool valve  30 , at about the mid point thereof. The spool valve  30  consists of a cylinder  32 , containing a piston  34 , which has a reduced diameter central portion  36 , as is known per se, and during normal running and stopping of the associated powerplant (not shown), a spring  38  urges the piston  34  to the left hand end of cylinder  32 , as viewed in the drawing. High pressure fuel is thus prevented from crossing the valve  30 , into a further line  40 , which is in fuel flow connection with the interior of the piston  18  of valve  16 . 
     A further tapping  42 , this from tapping line  24 , connects high pressure fuel to the left hand end of valve  30 , but in the operating conditions described immediately hereinbefore, has no affect on the position of valve  30 , as is depicted in FIG.  1 . 
     A restrictor  44  is positioned in tapping line  24 , upstream of the tapping point of line  42  and, and a further restrictor  46  is positioned in tapping line  22 , upstream of nozzle outlet  28  and a line  48 , which connects tapping line  22  with line  40 , valve  16  and valve  30 . Restrictors  44  and  46  serve to drop the high pressure of the fuel flow from pump  10 , to a lower pressure, for reasons which are explained hereinafter. 
     The design of the apparatus described herein, is such as to selectively enable a flow of low pressure fuel to the interior of valve  16 , so as to boost the spring bias load, and so shut off the high pressure fuel flow to the powerplant (not shown) in a normal manner, and alternatively, should overspeed occur by relevant rotating parts in the powerplant (not shown), to enable a flow of high pressure fuel to the interior of valve  16 , so as to boost the bias of the spring by a magnitude which will achieve a relatively rapid shut off of the flow of high pressure fuel to the powerplant (not shown). These modes of operation are achieved by the provision of a reed valve  50 , a free end of which, when not actuated, is positioned between and equi distant from, nozzle outlets  26  and  28 . 
     When the powerplant (not shown) is running normally, the reed valve  50  stays centrally of the space between outlets  26  and  28 . The fuel flow is then such, that high pressure fuel flows through line  20 , to the spool valve  30 , where it is prevented from crossing spool valve  30 , and via lines  20 ,  24  and  42 , to the spool valve  30 , where it is again stopped, and has no affect thereon. Simultaneously, the high pressure fuel flows, via line  20 , along lines  22  and  24 , and respective restrictors  44  and  46 , at which point, its pressure drops to a relatively low level. 
     The outlet nozzles  26  and  28  are connected in flow series with a low pressure fuel return line  52 , and via a tapping line  54 , to the spring loaded end of spool valve  30 . The pump  10  sucks low pressure fuel out of nozzles  26  and  28 , through line  52 , to the inlet side thereof. 
     Whilst the recirculatory system described immediately hereinbefore is operating, the pump  10  sends high pressure fuel to combustion equipment via line  14  and valve  16 . However, should it be desired to stop operation of the powerplant (not shown), the electrical winding  56  of a stator pole (not shown) is activated to generate a magnetic force on the ferrous head  58  of the reed valve  50 . This results in the reed valve  50  pivoting about an axis  60 , in an accurate manner, to block low pressure fuel outlet  28 . The low pressure fuel flow in line  22  is thus barred from return line  52 , and instead, flows via line  40  to the interior of valve  16  where, in combination with the spring therein, forces the piston  18  across line  14 , to close it against the high pressure fuel flow, and thereby stops the powerplant. 
     In the event that overspeed occurs in the powerplant, during its otherwise normal operation, it is necessary to cut off the fuel supply more rapidly than is achievable by use of low pressure fuel as described hereinbefore. In the present example, rapid shutdown is achieved by electrically energising the winding  64  of a further stator pole (not shown) which is arranged in opposition to the winding  56 . The reed valve  50  is thus caused to pivot to block outlet  26 . Low pressure fuel from restrictor  44  is thus prevented from entering return line  52 , and the resulting back pressure in line  24  diverts fuel therefrom, into line  42 , with a force sufficient to move the piston  34  of spool valve  30  to the right as viewed in FIG. 1, against the resistance of the spring  38 . The high pressure fuel is assisted in this, by the suction action of the pump  10  generating a relatively low pressure in lines  52  and  54 . 
     Movement of the piston  34  as described hereinbefore, aligns space  36  with the outlet  66  of high pressure fuel line  20 , and consequently, high pressure fuel therefrom flows across the valve  30 , into line  40 , and thus to the interior of valve  16 , with a force sufficient to slam piston  18  across, the high pressure fuel line  14 , and thereby obviate drive to the rotary members (not shown) of the powerplant (not shown), which then freewheels to a stop. 
     Stator devices are unavoidably bulky, heavy structures. If the invention described and claimed in this specification is used to control a group or ship mounted powerplant (not shown), such characteristics may not be a drawback. However, if the invention is used to control a powerplant mounted in an aircraft, wherein the achievement of the lowest possible weight commensurate with safety is paramount, it may then be desirable to substitute another device, with which to selectively block outlets  26  and  28 . 
     Referring now to FIG. 2, a rotary valve  68  is substituted for the reed valve  60  and associated stator windings  56 ,  64  of FIG.  1 . Low pressure fuel flows into both sides of valve  68 , and then enters line  52 , to be sucked back to the pump  10 , (not shown in FIG.  2 ). When it is desired to stop the associated powerplant (not shown) in the normal manner, the central portion  70  of valve  68  is rotated, by any suitable means (not shown) in a clockwise direction, as viewed in FIG. 2, so as to block outlet  28 . Low pressure fuel from line  20  is then prevented from being sucked back to the pump  10 , but is diverted to the valve  16  (not shown in FIG. 2) exactly as described with respect to FIG.  1 . 
     When it is desired to slam shut valve  16 , because an overspeed condition has arisen in an associated powerplant (not shown), the central portion  70  of valve  68  is rotated anti clockwise as viewed in FIG. 2, so as to block outlet  26  and thus prevent low pressure fuel from line  24  entering line  52 . A back pressure results, by virtue of the fuel in line  24  now being unable to pass through restrictor  44 , and fuel at tapped high pressure flows down line  42 , to move piston  34 , as described with respect to FIG.  1 . 
     Actuation of either valve described hereinbefore can be achieved manually, by an operator or a pilot, when a normal stopping procedure is required. However, when overspeed occurs, a speed sensing device, of which many types are known, may be used. The device would observe, preferably electronically, the speed of rotation of the relevant parts, and on sensing the occurrence of a speed more than a pre-set maximum, would send a signal to the valve actuation apparatus (not shown) to actuate the valve, so as to shut valve  16 .