US2846846A - Fuel system for gas turbine engines having means for avoiding compressor instability - Google Patents

Description

Aug. 12, 2,846,846

FUEL SYSTEM FOR GAs TURBINE ENGINES HAVING MEANS 1958 F. c. MocK Foa AvoIDING COMPRESSOR INSTABILITY 3 Sheets-Sheet 1 Fil'ed June 14, 1951 4 6 W7 wea-.A

F v I INVENTOR. '15E FwA/A/I/Vafv/f.

Aug. 12, 1958 F. c. MocK 2,846,846

FUEL SYSTEM FOR GAS TURBINE ENGINES HAVING MEANS FOR AVOIDING COMPRESSOR INSTABILITY Filed June 14. 1951 5 Sheets-Sheet 2 N INVENToR.

Aug.. 12, 195s Filed June 14. 1951 F. c. MocK 2,846,846 FUEL sySTEMEoR GAS TUREINE ENGINES HAVING MEANS FOR AvoInING COMPRESSOR INSTAEILITY 5 Sheets-Sheet 3 United States Patent O FUEL SYSTEM FOR GAS- TURBINE ENGINES HAV"- ING MEANS FOR AVOIDING` COMPRESSOR INi- STABILITY Frank C. Mock, South Bend, Ind., assigner to. Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application June 14, 1951, Serial No. 231,556`

16 Claims. (Cl. 611-3928) This invention relates to a fuel feed and` power control system for gas turbine engines; more particularly for gas turbine engines adapted for the propulsion of aircraft, such as what are now commonly known as turbojet and turboprop engines.

It is, of course, highly desirable that a pilot or operator of a turbojet or turboprop engine be free to accelerate rapidly and smoothly at all altitudes simply by resetting a control lever or member to 'a selected power position without worrying about, or danger of, (a) exceeding the upper temperature limit for that particular engine, (b) causing ame blow-out in the burner or burners, or (c) producing surge or compressor stall, and which latter in a turboprop engine may also occur as a result of decreasing speed upon application of an external load. .Another transitional hazard that should be mentioned is flame-out or burner die-out upon deceleration or sudden closure of the throttle and reduction in fuel feed .at vhigh engine speed.

With a dynamic air compressor running at agiven speed and delivering through an orifice of fixed size, the air weight delivery tends to vary as the entering pressure and inversely as the entering temperature. lf the air be raised in temperature vafter leaving the compressor but before reaching 'the discharge orifice, this will in general cause the weight delivery to ldecrease and the delivery pressure to rise, up to a point where'the compressor stalls. If, however, the delivery conditions be such that the velocity of flow through the orifice approximates that of sound, as indicated or defined by the absolute temperature of the air approaching the orifice, this absolutely limits the weight flow through the orifice; the velocity varies with the square root of the absolute temperature, and the density inversely as the absolute temperature (and directly with the pressure), so that the net weight ow varies with the square root of the absolute temperature.

This latter is generally the case with a gas turbine in the maximum power range. The velocity is sonic through the nozzles through which the gas enters the turbine blades; the Weight ow varies with the square root of the yentering gas temperature and 'directly with its pressure; and all this is lonly remotely connected with the temperature entering the compressor.

If the weight of air passed by the Aturbine under the .condition of maximum turbine gas temperature, which is as above stated practically independent of entering'air temperature, is in excess of that quantity which the compressor could deliver at the vsame pressure (note that this last quantity does vary with .entering air temperature), the turbine gas temperature dictates the maximum amount of fuel which can be fed. But if raising the turbine gas temperature to the maximum indicated by turbine endurance causes the compressor'to stall, 'then lower combustion temperature, lower fuel ilow, and lower thrust must be tolerated. Obviously, 'the engine design must be such as to permit 'the engine tol run at steady 2,846,846 Patented Aug. 12, 1958 ice 2 speed without compressor stall; andthe loss-in perform,- ancerimposed by compressor stall characterlstics 1s-y usually confined to slower acceleration in the mid-speed range.

In illustration of the foregoing, the fuel feed requirementsfof a given axial compressor type turboJet engine in the. higher compression ratios at constant entering air pressure (given altitude)l but at varying or two differ ent-entering air temperatures are assumedY to be;v as plotted in-thecurve chart of Figure l, wherethefull linesrepresent the requirements for a warm entering air. temperature, say in the neighborhood of F., and the dotted. lines a cold` entering air temperature, for instance -10 F. Here the lines EA,.EA1 indicate the maximum rate of fuel feed to be observed in orden to keep within at. safe. upper turbine. temperaturelimit, and they also indicate, a trend which will avoid blow-outfparticularly at altitude. The curved lines DB and D1B1 define the surge area; they indicate the rate of fuel feed to b e observed to avoid surge, or the upper limit on the rate of fuel feed as determined by the surge. characteristic of that particular engine. The curves YX and YlXl represent the rates. of fuel feed required to maintain a steady speed; with an all-speedl governor` type4 throttle valve they represent xed throttley settings. at an equilibrium or balanced condition with respect to the rate of fuel feed.v and engine speed.

In Figure 2 an attempt is madeI to approximate the quantity of fuel required to maintaina l600= F; turbine inlet temperature for fou-r different sets of entering air pressure and temperature values showing the relationship of fuel feed rate to compressor rise.

The relations of required fuel ilow, engine speed, entering air and turbine gas temperatures, combustion ethciency, and compressor stall are both mathematically complex in theory and functionally irregular in practice. l have. found, however, that the following more or less empirical relations give a close enough approximation to permit an improved and practical scheme of control. In the following, the symbols P1 and T1 designate, respectively, the. pressure and temperature of the air entering the compressor; P2, the pressure just beyond the compressor; and T3, the temperature of the gas entering the turbine. Note that the limiting values of fuel feed given vary according to the consecutive listed conditions of engine operation:

(l) With a given absolute safe value of T3, turbine gas temperature, the amount of fuel that can be tolerated per pound of air will decrease with increase of T1, entering air temperature, or with increase of compression temperature. I have found that to hold a given turbine gas temperature under changes of entering air temperature and pressure, the fuel feed should Hvary vgenerally as .maximum engine speed under .change of entering air pressure and temperature lies between (l) and (2) above.

But when a governor is used to determine maximum speed, as in the invention here described, ,no particular correction is needed provided the maximum scheduled fuel feed is in excess of that required for steady operation at maximum speed. While compressor pressure rise has previously been proposed as a means of fuel metering, it is believed that the exponential values of pressure and temperature compensation, as herein set forth, are novel, distinctive and valuable in the art. The mechanical adaptation shown also presents the advantage of being straightforward, simple and capable of rather general application with minor change. Another novel element of the invention is a so-called -stall modulator construction by a variable orice in series with the main governor metering orifice. This provides both automatic temperature compensation for the stall range and also forms a manual adjustment for minor variations between engines of the same model. Its action is also simple and straightforward.

In the drawings:

Figures 1 and 2 are curve charts for supplementing thebrief analysis of the theory on which the invention is based;

Figure 3 illustrates schematically one form of fuel feed and power control device capable of functioning in accordance with the invention;

' Figure 3A is an enlarged longitudinal section of a temperature compensating control valve; and

Figure 4 is a performance curve for an engine equipped with the improved control.

Referring to Figure 3 in detail, a gas turbine engine is generally indicated at 19 which includes a series of combustion chambers 11, mounted in a casing having a header or air intake section 12. A dynamic compressor is indicated at 13 and is shown as of the axial fiow type driven by Vmeans of a turbine 14 through a shaft 15. Each of the combustion chambers is provided with a burner nozzle 16 to which metered fuel is supplied under pressureby way of a conduit 17, fuel manifold 18, and'individual fuel lines 19, in a manner to be described.

The parts which go to make up the fuel metering and power control system may be classified generally as a metering head regulator section 21, governor section 22, and stall modulator section 23.

Fuel from a suitable source of supply, such as a fuel tank, not shown, flows to the regulator through inlet conduit 24, having a suitable pressurizing device therein, such as a pump 25 of the by-pass type which maintains the supply at a substantially constant pressure. From conduit 24 the fuel flows through ports 26 and across regulator valve 27 to chamber 28. Valve 27 is preferably of the balanced type and is controlled by a double or dual regulator comprising a pair of diaphragms 29 and 30, the diaphragm 29 constituting a movable wall between expansible chambers 28 and 31 and the diaphragm 30 a like wall between expansible chambers 32 and 33. Chamber 31 is vented to metered fuel pressure by way of passage 34, chamber 32 to compressor inlet or P1 pressure by way of passages 35 and 36, and chamber 33 to compressor outlet or P2 by way of passage 37, variable orifice 38 and passage 37. The area of orifice 38 is controlled by a contoured needle valve 39, carried by the movable end of a pressure responsive bellows 40, mounted in a chamber 41, which is vented to compressor inlet or P1 pressure by way of passage 35. Between chambers 33 and 32 is a passage 42, having therein a variable orifice 43, the area of which` is controlled by a contoured needle valve 44, carried by the movable end ofl a temperature responsive bellows 45 connected by capillary tube 46 with a temperature bulb 47, located to sense compressor inlet or entering air (T1) temperature.

As will be more fully explained in the description of operation, the effective fiow position of the regulator valve 27 and hence the metering head between chamber 28 and fuel conduit 17 is determined by the differentials `across diaphragms 29 and 30'. The forces (pressure difference times area) at a given pressure drop between chamber 28 and metered fuel conduit 17 and at a given compressor rise and entering air pressure and temperature, will be equal and opposite, or their sum total will be zero, and balanced valve 27 will remain at a given flow position; but should there be a change in any of these variables and hence in any one of botn =of the said differentials, the regulator will become unbalanced and valve 27 will be repositioned to increase or decrease the pressure in chamber 28 until a balanced condition is again established. Since the flow through a metering orifice tends to vary as the square root of the metering head, the quantity of fuel metered to the engine will be proportionate to the square root of the compressor rise modified by entering air pressure and temperature.

From chamber 28, fuel fiows to the outlet conduit 17 through passage 48, either one or both orifices 49 and Sii, chamber 51 and metering orifice 52.

The area of the metering orifice 52 is controlled by an all-speed governor valve 53, which is preferably of the balanced type and is slidably mounted in a seat or sleeve 54. The valve 53 is provided with a headed stem 55 engaged by a governor spring 56 adapted to be variably tensioned by a lever 57 which constitutes a power control member (or may be connected to such member), said spring tending to open the valve 53 against the balancing force of governor weights 58, pivotally mounted on brackets 59, carried ny rotatable gear 60, formed on the inner end of a shaft 61. The gear 6l) is driven in relation to engine speed from a shaft 62 and gear 63. The maximum opening travel of governor valve 53 is determined by a stop 64 externally adjustable by means of screw 65, while its minimum closing travel may be determined by adjustable guide rod 64.

To 4select a desired engine speed, the pilot or power control lever 57 is reset to a position which will subject governor spring 56 to a given compression or tensioning action which will unbalance the then existing equilibrium condition of the governor weights 5S with respect to the governor spring, whereupon the throttle or governor valve 53 will either open or close to increase or decrease flow of fuel to the engine and the speed of the latter will either increase or decrease until an equilibrium condition is again attained as determined by the setting of the pilots control lever. Also, at a given setting of the pilots control lever, should be the speed of the engine tend to vary from the selected speed, the governor will become unbalanced, whereupon the governor weights will automatically adjust valve 53 to return the engine speed to the selected value, as is well understood by those having a knowledge of the art. Should the pilot reset his control lever to accelerate to full power from a low or intermediate power setting, valve 53 would open to a maximum as determined by stop 64 and the engine would accelerate to maximum governed speed with the rate of fuel feed at each point of transition determined by the metering area of orifice 52 and the fuel or metering head across said orifice. As heretofore noted, the metering head across orifice 52 is a function of compressor rise and entering air pressure and temperature; and it is further subject to modification during part of the speed range by the stall modulator system in section 23, to obtain the dip BCD or BICD, in Figure l.

Referring to the modulator section 23 of Figure 3, the variable orifice 49 is regulated by a contoured valve 70 which is positioned automatically as a function of engine speed by a speed-sensing element generally indicated at 71 and comprising centrifugal weights 71' which are pivotally mounted on brackets 72 carried by the gear 63 and having inner fingers or shoes engaging a collar or bearing surface 73 on the adjacent end of a shaft or rod 74, said rod being slidably mounted in a bushing` 75 and having its opposite end provided with spaced collars 76 between which the adjacent or upper forked end of a lever '77 engages. The lever 77 is pivotally mounted or fulcrumed at 78, and at its opposite or lower forked end engages a pair of spaced collars or bosses 79 formed on quired. A light stabilizing spring 83 may be provided for the valve 70.

It has been found that on different engines of the same, model or design (and supposedly having the same operational characteristics) the magnitude of the stall dip or curve BCD, BlCDl with respect to fuel requirements may be different, while the respective speeds at which deviation from the line AE, A1B takes place or is required may not change. A convenient adjustment for adapting the control to such variations is provided by the contoured valve 85, which regulates the area of the orifice 50 and is externally adjustable by screw 86.

As will be more fully explained in the description of operation, at some predetermined point in the mid-speed"- range of the engine, depending upon the setting of governor 71 by spring 81 and/or the contour of valve 70, the latter becomes effective to reduce the metering head across the governor valve 53; and by predetermining the vfixed area of orifice 50 with respect to variable parallel orifice 49, the magnitude of curves BCD or BlCDl may be varied without varying the point of departure (speed function) from lines AE or A1B. This results from the fact that the magnitude of the stall fuel head varies as,

the total combined area of the orifices 49 and 50, while the particular engine speed at which the stall head starts depends upon the beginning of the .restricting action'by valve 70. In this connection it will be understood that .the total or combined area of parallel orifices 49 and,r

50V is substantially in excess of the area of the metering orifice 52.

The temperature modifying action on the fuel head by valve 44 is proportionately effective throughout the range of engine speed, but such modication inthe mid-..

speed range may constitute a variable; usually less tem-` perature modification in the surge area or mid-speed range being required. This compensation is provided for by the valve unit or yassembly generally indicated at 87, which may be used in conjunction with or substituted for the valve 85. An orce 88, Figure 3A, in parallel With orifice 49 and/ or orifice 50, is regulated by a valve 89 having its stem slidable in bushing 90 and connected to the free or movable end of a bellows 91, in lluid communication by way of capillary tube 92 with a temperature bulb 93, so located as to sense compressor or en-l gine inlet temperature, the lbellows, tube and bulb being loaded with a suitable temperature responsive iiuid or gas. A spring 94 tends to urge valve 89 to closed position.

It may be assumed that the control is set or calibrated-' for a relatively warm atmospheric condition at sea level (warm day), -then should the entering air becomeA appreciably colder, bellows 45 will shrink and produce an increase in the metering head across orifice 52. Simultaneously, however, the bellows 91 shrinks and valve 8927 reduces the area of orifice 88, thereby reducing the total flow area provided by parallel orifices 49 and 88 and proportionally reducing the metering head across orifice 52. By properly contouring valves and 89, the compensating action may be restricted to the stall range. Itiv will be noted that the contour o-f valve 70 is such that at the upper and lower ends of the speed range BA and ED, the area of orifice 49 is so great as to constitute no appreciable restriction to fuel flow, so that the position of valve 89 at this time has little effect on the meteringv head.

Operation In the respective positions of the vvarious parts `as shown in Figure 3, lthe engine may bel assumed to be 'operatingv at a steady speed in the low speed range, say at point in Figure 4. If now the pilot desires to accelerate from 95 to 98, lever 57 would be rotated counterclockwise and governor valve 53 opened to the limit determined by stop 64. The suddenly increased metering area of orice 52 would result in a sharp increase of the rate of fuel feed to, for example, point 96, but at this point the metering head takes effect and causes fuel flow to follow the arrows to point 97 where the al1-speed governor begins to cut olf fuel and flow decreases to point 98..

When the throttle is suddenly opened, there is a momentary drop in pressure in chamber- 28 and the decreased differential across diaphragm 29 will tend to open the regulator valve 27. The extent to which this valve opens, however, is also controlled :by the relative pressures in chambers 32 and 33, or the differential across diaphragm 30, which is a function of compressor rise and compressor inlet pressure and temperature. As the speed of the engine begins to increase and the rise across the compressor increases, the differential across diaphragm 30 acts in a direction to open the regulator valve 27, but this opening travel and hence the increase in metering head is proportional to compressor rise modified by inlet pressure and temperature, and hence the rate of fuel feed is maintained within a predetermined upper limit and follows the arrows from 96 to 96' in Figure 4. At this point the stall modulator governor 71 has moved valve 70 to the right to where it begins to restrict orifice 49 and reduce the fuel metering head across valve 53, whereupon the rate of fuel feed is reduced to avoid the surge area. At the lowest point in the surge dip, the maximum diameter of valve 70 is interposing a maximum restriction to flow through orifice 49, and as said valvev moves further to the right, flow through said orifice, `and hence the metering head across valve 53, gradually increases to where the rate of fuel feed again reaches the upper limit for acceleration as it passes the surge area, at which time valve 70 will have opened orifice 49 to its maximum area or to where it has no appreciable effect on said metering lhead. As acceleration continues along the upper temperature limit, the metering head again becomes solely a function of compressor rise, modified by compressor inlet pressure and temperature. At point 97, the speed of the engine has substantially attained the selected value, or to where the all-speed governor weights 58 are substantially in 'balance with the setting of the governor spring 56, whereupon the rate of fuel feed decreases to the point 98 along the steady speed curve.

Should the pilot wish to decelerate from point 98 back to point 95, power control lever 57 is rotated clockwise, thereby relieving the tension on governor spring 56', whereupon the governor weights 58 close valve 53 to where it is brought up against the adjacent end of guide` rod 64', and the rate of fuel feed decreases sharply to point 99, whereupon the metering head takes effect and the rate of fuel feed gradually reduces along with the compr-essor rise, until at point 10i) the all-speed governor weights 5S are substantially in balance with the setting of the governor spring 56, whereupon fuel feed increases slightly to point 95 along the steady speed curve.

An attempt to brieliy explain the operation of the regulator 21 follows:

First considering the fuel or hydraulic section comprising chambers 2S and 31 and diaphragm 29, it will be seen that at a given or fixed area of the metering restriction 52 and a given drop from chamber 28 to metered fuel conduit 17, the pressure differential between chambers 28 and 31, or between passage 4S a'nd conduit 17, equals or is proportional to the compensated pressure differential across diaphragm 30. Should the metering area of either orifice 49 or 52 be increased or decreased, there will be a momentary increase or decrease in pressure in chamber 28, whereupon valve 27 will open or close to a new position where fuel ow compensates .for the change in drop across orifice 49 or 52, at which position the differential across diaphragm 29 again equals the differential across diaphragm 30.

Considering now the compressor rise or air section of the regulator, comprising the chambers 32 and 33 and diaphragm 30, here the differential across the diaphragm 30 may be considered the prime controlling factor of the regulator since any change therein will operate to modify the differential across diaphragm 29 at any engine operating condition. Without the pressure bellows 40 and temperature bellows 45 and associated circuits, the differential across diaphragm 30 would be proportional to compressor rise only, Pz-Pl, but said latter differential is biased with respect to the differential across the compressor by the variable leak pressure and temperature orifices 38 and 43, so that any change in these parameters operates to modify the differential across the diaphragm 30. Thus, the differential across diaphragm 3i), and hence also across diaphragm 29, varies with variations in P2 minus P1 and with P1 divided by T1, as does also the metering head; and since the flow across the metering orifice 52 tends to vary as the square root of the metering head, the quantity of fuel metered to the engine becomes proportional to the square root of Pl times the square root of Pz-Pl divided by T1. Along the surge dip, the fuel feed rate is further modified as a function of engine speed by valve 85 and/ or as a function of T1 by the temperature compensating valve 89, as heretofore explained.

In Figure 4, a sea level condition as regards altitude is assumed. As altitude is gained, pressure compensation for the rate of fuel feed is had by the aneroid bellows 40, which acts to reduce the differential across regulator diaphragm 30 upon a decrease in pressure so that the respective curves of Figure 4 would swing downwardly until critical altitude for the particular aircraft is attained.

Although only one physical embodiment of the invention has been schematically illustrated and described, such disclosure obviously constitutes a teaching which will readily enable those skilled in the art to practice the invention and to make the required changes in form and relative arrangement of parts to adapt the improved control to engines having different characteristics.

I claim:

l. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, means for metering fuel to the engine at a rate proportional to pressor discharge pressure, and T1 compressor inlet temperature, and means for modifying the effects of compressor inlet temperature on the rate of fuel feed in the intermediate speed range of the engine.

2. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, means for varying fuel iiow to the engine as a function of cornpressor rise modified by the temperature of the air iiowing to the compresor, and compressor stall control means in fiow controlling relation with said first mentioned means and responsive to an engine operating condition related to power output for substantially varying the normal rate of change of fuel feed controlled by said first mentioned means at a predetermined engine speed during an acceleration of the engine in such a manner that compressor instability is avoided` 3. A system as claimed in claim 2 plus means operatively connected to said last mentioned means for varying the engine speed at which said variation in normal fuel feed rate occurs.

4. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, adjustable governor means for maintaining a selected engine speed, control means for adjusting said governor means adapted to be reset to different positions to select an engine operating speed, means operable automatically as a function of compressor rise to maintain the rate of fuel feed within predetermined limits during a transition of engine speed and means responsive to engine speed for automatically modifying the rate of fuel feed controlled by said latter means during an acceleration of the engine.

5. A system for controlling the rate of fuel [feed to a gas turbine engine having a compressor, adjustable governor means for maintaining a selected engine speed, means in flow controlling relation with said governor means and operable automatically as a function of compressor rise modified by compressor inlet temperature to maintain the rate of fuel feed within predetermined limits during a transition of engine speed, means in flow controlling relation with said last mentioned means for automatically modifying the rate of fuel feed as a function of engine speed during an acceleration of the engine, and means for automatically reducing the effects of compressor inlet temperature compensation during acceleration of the engine, whereby the engine may be accelerated without encountering compressor instability.

6. In a system for controlling the rate of fuel feed to a gas turbine engine having a. compressor, the combination of a fuel metering valve, means responsive to a pressure generated by the compressor for controlling the fuel metering head across said metering valve, means responsive to a variable quantity related to power o utput of said engine for automatically modifying said fuel metering head during an acceleration of the engine to avoid compressor instability, and means responsive to the temperature of the air iiowing to the compressor for modifying the pressure to which said fuel metering head controlling means responds.

7. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, a metering valve, means for selectively positioning said valve to control the metering area, a regulator valve for controlling the fuel metering head across said metering area, means for automatically positioning said regulator valve as a function of compressor rise modified by compressor inlet pressure, and means responsive to changes in engine speed for automatically modifying said fuel metering head at a preselected point in the midspeed range of the engine to avoid compressor instability.

8. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, a fuel conduit having a metering orifice therein, means for varying the area of said orifice to select an engine speed, a regulator valve in said conduit in series wit-l1 said metering orifice for controlling the metering head across said orifice, pressure responsive means connected to said regulator valve, means for subjecting said pressure responsive means to compressor pressure rise modified by compressor inlet pressure and temperature, a compressor stall modulator valve also in series with said metering Iorifice and movable to different positions to modify the metering head, engine speed responsive means for controlling said latter valve, another valve in parallel with said stall modulator valve, and means responsive to changes in compressor inlet temperature for controlling said latter valve, whereby fuel flow to the engine is controlled during an acceleration thereof to avoid compressor stall.

9. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, a fuel conduit having a metering restriction therein, a metering valve and associated all-speed governor for varying the area of said restriction to select an operating speed for the engine, valve means for varying the fuel metering head across said restriction, means for sensing the pressure rise across the compressor, means for sensing changes in compressor inlet pressure and temperature, means operatively connecting said sensing means to said valve means to effect variation of the fuel metering head at a rate proportional to the square root of compressor pressure rise times a function of compressor inlet pressure modified by compressor inlet temperature, and means for automatically 9 modifying the action of said valve means as a function of engine speed in the intermediate speed range of the engine during acceleration following resetting of said governor.

10. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, a fuel conduit having a metering restriction therein, means for varying the area of said restriction to select an operating speed for the engine, a iirst valve in flow series with said restriction, a second valve in ow series with said restriction, means for sensing the pressure rise across the compressor, means for sensing changes in compressor inlet pressure and temperature, means operatively connecting said sensing means to said rst valve to eifect variation of the fuel metering head at a rate proportional to the square root of compressor pressure rise times a function of compressor inlet pressure modified by compressor inlet temperature, and means responsive to changes in engine speed operatively connected to said second valve and arranged to automatically modify the fuel metering head in the intermediate speed range of the engine during acceleration to avoid compressor instability.

11. A system as claimed in claim 10 wherein means are provided for modifying the eect of temperature compensation on the rate of fuel ow in the intermediate speed range of the engine.

12. In a system for feeding fuel to a gas turbine engine having a compressor and a burner or generator to which air and liquid fuel is supplied under pressure, a fuel supply conduit having a metering restriction therein, governor means for controlling the area of said restriction, means for resetting said governor to accelerate and decelerate the engine, valve means for regulating the fuel metering head across said restriction, means for sensing changes in compressor inlet pressure and temperature, means operatively connecting said sensing means to said valve means, second valve means responsive to engine rotational speed in flow controlling relation with said valve means, and means operatively connected to said second valve means for modifying the eiects of temperature compensation on the rate of fuel feed during a portion of the acceleration range to avoid compressor instability.

13. In a system for controlling the rate of fuel feed 10 to a gas turbine engine having a compressor, a conduit for conducting fuel to the engine, means for controlling the ow of fuel through said conduit at a rate which is proportional to p N/P Q-P 1 times a function of P1 with an exponent between 1 and 1/2, where P1 denotes a pressure of the air flowing to the compressor, P2 denotes a pressure which is generated by the compressor, and T1 denotes a temperature of the air owing to the compressor,.and means for modifying the said flow of fuel during an acceleration of the engine as a function of an engine operating condition related to power output.

14. In a system for controlling the rate of fuel feed to a gas turbine engine having a compressor, a fuel conduit having a metering restriction therein, valve means responsive to an engine operating condition related to power output for reducing the flow area of said restriction to avoid compressor stall during an acceleration of the engine, and means responsive to a pressure derived from the compressor and in ow controlling relation to said valve means for varying the fuel ilow through said restriction with variations in said pressure. v

15. A system as claimed in claim 14 plus means for modifying the derived pressure as a function of the temperature of the air flowing to the compressor.

16. A system as claimed in claim 14 wherein said valve means responds to engine speed and said pressure is derived from the discharge side of the compressor.

References Cited in the iile of this patent UNITED STATES PATENTS 2,422,808 Stokes .Tune 24, 1947 2,474,033 Chamberlin June 21, 1949 2,479,813 Chamberlin Aug. 23, 1949 2,531,664 Bolt Nov. 28, 1950 2,705,047 Williams et al. e Mar. 29, 1955 FOREIGN PATENTS 646,780 Great Britain Nov. 29, 1950 941.556 France July 19, 1948 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,846,846 August l2, 1958 Frank C Mook l lt is hereby certified that error appears in the printed specification i of the above numbered patent requiring correction and that the said Letters l Patent should read as corrected below. v Column 4, line 5, for Hone of both" read one on both n; line 44,

strike out HbeH Signed and sealed this 17th day of March 1959.,

(SEAL) Attest:

KARL H *AX-LINE ROBERT c. wATsoN Atbeetng Officer Comissioner of Patents

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