GB2197392A

GB2197392A - Differential power system for a multi-spool turbine engine

Info

Publication number: GB2197392A
Application number: GB08725602A
Authority: GB
Inventors: Arthur Paul Adamson; Richard Paul Johnston
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1986-11-03
Filing date: 1987-11-02
Publication date: 1988-05-18
Also published as: IT8722496A0; SE8704293L; GB8725602D0; JPS63159627A; SE8704293D0; DE3736984A1; FR2606077A1; IT1223046B

Description

1

SPECIFICATION

Differential power system for a multi-spool turbine engine The invention concerns a power generating device driven at a speed related to a difference between rotational speeds of two spools in a multispool turbine engine.

BACKGROUND OF THE INVENTION

A multi-spool turbine engine has a high pressure spool that generates a high velocity gas stream, and a low pressure, power spool that extracts mechanical energy from the gas stream with a power turbine. The power turbine drives, for instance, an alternator to power auxiliary equipment on board an aircraft. However, the speed of each spool in a multi-spool engine varies greatly from ground.idle to full speed, driving the alternator over a wide range of rotational speeds. Accordingly, the alternator yields an output of many frequencies. Because the output frequency of the alternator should be stable, within ten percent of 400Hz, for instance, chopping and rectifying circuits have been used to compensate for fluctuating frequencies, and transmission and clutch devices have been used to regulate the rotational speed of the alternator. These circuits and devices add undesirable weight to an aircraft, for instance. Unexpectedly, it has been found that the rotational speed difference between the high pressure spool and the low pressure, power spool is relatively constant from flight idle to maximum flight speed.

SUMMARY OF THE INVENTION

The invention concerns two rotatable spools of a multi-spool turbine engine, which are connected to a means that generates power related to a speed difference between the two spools. In one embodiment, a gear means is mechanically connected to the two spools for producing an output related to this speed difference. The gear means output can be connected to an alternator, for instance. In another embodiment, an alternator's stator is mounted to one spool, and the alternator's rotor is mounted to the other spool. A speed difference between the rotor and stator yields an output that is taken through slip rings or by a rotating transformer. Embodiments of the invention may reduce percent operating speed ranges of the alternator and allow use of a smaller and cheaper alternator.

One embodiment uses the rotational speed difference of two spools in a multi-spool engine to mechanically drive a power generating device at a relatively constant speed.

The other embodiment uses the rotational speed difference of two engine spools to provide rotation between an alternator's stator and rotor to yield a relatively constant fre- quency output.

GB2197392A 1 Embodiments of the invention may drive an alternator at a relatively constant speed with a multi-spool engine, without transmissions and clutches or frequency compensating circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of an example of a twin-spool turbofan engine.

Figure 2 shows the rotational speed of two spools, and the speed difference between these spools, as a function of thrust in a multi-spool turbine engine.

Figure 3 shows a side view of one embodiment of the invention.

Figure 4 is a generalized perspective view of the Fig. 3 system.

Figures 5 through 8 show other embodiments of the invention.

Figure 9 shows turbine engine stator vanes controlled in accordance with an embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 schematically shows one type of twin-spool turbine engine with a high speed, high pressure spool surrounding a coaxial low speed, low pressure, power spool. This turbine engine is an example of one environment for the invention which may be used with any multi-spool turbine engine. A differential power system (detailed in Figs. 3 through 9) is generally located at 8 to receive inputs from each spool. The power system produces an output related to a speed difference between the spools. The output is an electrical signal with a relatively constant frequency as discussed below.

Fig. 2 shows, as a function of engine thrust, the rotational speeds in RPM for a high pres- sure, high speed spool (N2) and a low pressure, low speed, power spool N 1 in a CF6-80A twin-spool turbofan engine used in Boeing Company's 767 aircraft, for instance. Also, shown is the difference in rotational speeds (N 1 -N 1) of the two spools which rotate in the same direction. From ground idle (CIDL) to flight idle (FIDL) the difference in rotational speeds between the high speed spool and the low speed, power spool varies with thrust. This variation occurs as the high speed spool accelerates faster, in response to an increase in thrust, than the low speed, power spool, prior to take-off. To correct for this dip at ground idle, compressor stator vanes at the high speed spool (N2) can be closed to speed up the spool. However, for thrust greater than 10,000, this difference in rotational speeds is substantially constant as a function of thrust for high bypass engines such as the CF6-80A engine.

Fig. 3 illustrates one embodiment of a power system 10 according to this invention in a twin-spool turbine engine. In such an engine, air enters a receiver channel 12, passes high pressure stator vanes 14 and rotor vanes 2 GB2197392A 2 16 before proceeding to a combustor (not shown). A low speed, power spool shaft 18 is coaxial with and surrounded by a high speed spool shaft 20 that rotates in the same direction, but much faster than the power shaft 18 (see Fig. 2). The power system 10 includes gear teeth 22 and 24, respectively enciruling the shafts 18 and 20. The number of teeth 22 and the number of teeth 24 are selected so that one revolution of either shaft 18 or 20 will yield the same mechanical output (but in opposite directions, as discussed below) from a differential gear 26. Preferably, for the CF6-80A engine the ratio of teeth 24 to the teeth of a gear 34 is twice the ratio of teeth 22 to the teeth of a spider 44. However, for other engines, the number of teeth 22 and 24 are unequal to yield different amounts of mechanical output from the differ- ential gear 26. The ratio of teeth depends on the shape of the N2-M curve for a particular engine.

The differential gear 26 is mounted to a wall 28, defining the receiver channel 12, by a flange 30. An axle 32 is secured to the flange 30, and carries an input gear 34 that meshes with teeth 24. The input gear 34 is integral with a pinion 36. An idler gear 38 is mounted on the axle 32, coaxial with the input gear 34 and the pinion 36. The pinion 36 drives the idler gear 38 through two or more perpendicular pinions 40 and 42. A spider 44 meshes with teeth 22, and carries the pinions 40 and 42 around the axial 32. An output gear 46 is driven through the idler gear 38 which is beveled on two sides. The output gear 46 is mounted to turn an output shaft 48. The output shaft 48 thus rotates at a speed related to the difference in rotational speeds between the power shaft 18 and high speed shaft 20. Thus, an alternator 50 is driven by shaft 48 at a constant speed, and requires no transmission/clutch devices or frequency compensating circuits.

The alternator 50 can be replaced with a d.c. generator. Prior d.c. generators were designed to operate over a wide range of speeds, and were, therefore, large and heavy. According to this invention, shaft 48 rotates at a constant speed, allowing a generator to be smaller, which results in a substantial weight savings. The operation of the power system is explained below.

Fig. 4 shows a similified view of the Fig. 3 apparatus, with the teeth of each gear omitted for clarity. Normally, the power shaft 18 and high speed shaft 20 rotate in the same direction at different speeds. However, for clarity, the rotations of the two shafts (18, 20) are explained separately.

If the power shaft 18 is stationary, and the high speed shaft 20 turns clockwise, the input gear 34 and the integral pinion 36 turn counterclockwise, due to the mesh of the teeth 24 with the input gear 34. The pinion 36 turns the pinion 40 counterclockwise, about the pin 52 (and turns the pinion 42 clockwise) which drives the idler gear 38 clockwise. In response, the output gear 46 and the shaft 48 also rotate clockwise.

Conversely, if the power shaft 18 rotates clockwise and the high speed shaft 20 is stationary, the spider 44 rotates counterclockwise about the axle 32. The spider 44 carries the pin 52 counterclockwise about the axle 32. The pinion 40, revolves about the axle 32, but rotates clockwise about the pin 52 against the stationary pinion 36. Similar action occurs with the horizontal pinion 42.

The clockwise rotation of the pinions 40 and 42 turns the idler gear 38 counterclockwise. Accordingly, the output gear 46 and shaft 48 rotate counterclockwise.

During operation of a turbine engine, the power shaft 18 and the high speed shaft 20 rotate in the same direction but at different speeds. However, the differential gear 26 rotates the output shaft 48 at a speed related to a speed difference between the shafts 18 and 20. Thus, if the shafts 18, 20 rotate at equal speed and in the same direction, the output shaft 48 does not spin.

Fig. 5 show a modification of the Fig. 4 apparatus for use with engines having oppo- sitely rotating spools. The operation is unchanged in Fig. 5 for the power gear, pinions, idler gear, output gear and output shaft, which are not shown, for clarity. For engines with oppositely rotating spools, the difference be- tween absolute rotational speeds (i.e., [N,]-[Nj]) of the two spools is constant. In a twinspool engine with counter-rotating spools, an idler 54 is placed between the input gear 34 and teeth 24. The idler 54 spins freely on a shaft 56 which is secured to the flange 30 (shown in Fig. 3). Thus, counterclockwise rotation of the high speed shaft 20 in Fig. 5 (opposite power shaft's 18 spin) drives the idler 54 clockwise which spins the input gear 34 counterclockwise. Though the rotation of the high speed shaft 20 in Fig. 5 is opposite that of the Fig. 4 version, the input gear 34 revolves in the same direction in both Figs. 4 and 5.

Fig. 6 is another version of the power sys- tem of this invention, also for use in engines having counter-rotating spools (or shafts). An input gear 34 is mounted on and is coaxial with the high pressure shaft 20. A power gear 58 is mounted on the shaft 18. The rotational speed difference of counter-rotating spools is the difference between the absolute values of rotational speed for each spool ([N2]-[N,]). The output gear 46 is directly driven by a spider 60.

Fig. 7 shows another power system, shown in a schematic cutaway view. Again, the power shaft 18 is coaxial with and surrounding the high pressure shaft 20. Permanent magnets 62 are attached to the inside surface 3 GB2197392A 3 & 15 of the high pressure shaft 20. The high pressure shaft 20 and permanent magnets 62 comprise a rotor and rotate as a unit. Rotating permanent magnets 62 induce an alternating current in coils 64 mounted on the power shaft 18. Conductor pair 66 conducts the induced current to a slip ring 68 that is contacted by an output brush 70. The power shaft 18, coils 64, conductor pair 66 and slip ring 68 comprise a rotating "stator", within the rotor (20, 62).

If the power shaft 18 and high pressure shaft 20 are stationary, or rotate the same direction and speed, no current is induced in the coils 64. However, in twin-spool turbine engines, as discussed concerning Fig. 2, the high speed shaft 20 rotates faster than the power shaft 18. The speed difference between these shafts is relatively constant. The permenent magnets 62 thus induce an alternating current with a substantially constant frequency in the coils 64. The resulting electric power is taken from the slip ring 68 and the brush 70 to power auxiliary equipment on the aircraft.

The slip ring 68 and brush 70 of Fig. 7 may be replaced with the rotating transformer of Fig. S. A magnetic iron housing 72 is attached to the power shaft 18, and carries a winding 74 that is electrically conne cted to the coils 64 (see Fig. 7) by conductors 76 (schemati cally shown). Current from the coils 64 ener gizes the winding 74. A stationary iron hold ing member 78 supports a secondary winding 80, and is attached to the wall 28 (see Fig. 4) 100 by brackets (not shown). The energized wind ing 74 induces a magnetic flux 82 in the housing 72 and the holding member 78. The magnetic flux of the holding member 78 in duces a voltage in the secondary winding 80. 105 A pair of conductors 84, connected to the secondary winding 80, pass through a hole 86 in the member 78. Electrical power is taken from the conductors 84 to run electrical equipment onboard an aircraft, for instance. 1 Fig. 9 shows a closed loop system for controlling the output of the alternator 50. A speed differential 26 (electrical or mechanical, as in Figs. 3-8) yields the rotational speed difference, N2-Nl, between the high speed 1 shaft 20 and the low speed shaft 18. The speed differential 100 drives the alternator 50 at a speed related to this speed difference. The alternator's output is alternating current power with a frequency related to the alternator's speed. This power frequency is converted to a digital signal in an A/D converter 88. In turn, the digital signal is input to a comparator 90.

A range of values, stored in memory 92, is also fed to the comparator 90. This stored range represents a desired output frequency for the alternator. To run aircraft equipment, a frequency of 40OHz 0.5% is desirable, for in- range, there is no output from the comparator 90. Otherwise, a digital comparison signal is converted to analog in a D/A converter 94. This analog signal is input to a controller 96.

The controller 96 can include amplifiers, servo-mechanisms, hydraulic devices and linkages. The controller 96 opens and closes stator vanes 98 associated with a high speed, high pressure compressor, for instance. One example of a controller that positions stator vanes is found in U.S. Patent 3,314,595 to Burge et al.

When these stator vanes are modulated, the speed of shaft 20 '(N2) changes. As a result, the rotational speed difference (N2-Nl) between the two shafts 18 and 20 changes, and the alternator's rotational speed changes. Thus, the alternator's speed is controlled in a closed-loop, and the alternator's output fre- quency is maintained within 0.5% of a desired frequency, for example. Between ground idle and flight idle, for instance, the N2:N1 ratio can be increased to drive the alternator at a relatively constant speed.

Other modifications are apparent. For instance, the positions of the permanent magnets 62 and the coils 64 can be reversed. Instead of permanent magnets, electro-magnets may be used. In turbine engines with fluid flow controllers like variable pitch turbine fans, blades of the fan can be positioned to speed up or slow the low speed spool 18 (N1) to alter the N2:N1 ratio. The system can be used in triple-spool engines in which the speed difference of two spool shafts is relatively constant with 'increasing thrust.

The differential power system generates electrical power in response to the relatively constant rotational speed difference between two spools of a multi-spool engine. The electrical power is generated with an alternator or d.c. generator in a mechanical or electrical version of the system.

Numerous substitutions and modifications can be undertaken without departing from the scope of the present invention.

Claims

CLAIMS 1. A system for a multi-spool turbine en- gine, comprising:

first and second rotatable spools; and a means, connected to the first spool and the second spool, for generating power related to a rotational speed difference between the first and second spools.
2. The system of claim 1, also comprising:

a gear means, mechanically connected to the first spool and the second spool, for producing an output related to the rotational speed difference of the first and second spools.
3. The system of claim 2, the generating means comprising an alternator means for producing an alternating current power having stance. If the digital signal is within the stored 130 a frequency related to the rotational speed dif- 4 GB2197392A 4 ference, the alternator means connected to the gear means output.
4. The system of claim 1, comprising an alternator with a magnet mounted to the first spool and a coil mounted to the second spool adjacent the magnet.
5. The system of claim 4, the generating means comprising a brush contracting a slip ring that is connected to the coil and is mounted to the second spoof.
6. The system of claim 4, the generating means comprising a stationary coil means mounted about the second spool.
7. The system of claim 1, comprising a means for controlling fluid flow through the engine, connected to the power generating means and controlled in response to the generated power.
8. The system of claim 7, the fluid flow controlling means comprising: a means for storing a desired range of values related to the rotational speed difference, and a means for comparing the generated power to the range of values.
9. The system of claim 1, the power generating means comprising a d.c. generator.

Published 1988 at The Patent Office, State House, 66/7 1 High Holborn, London WC1R 4TP Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87