METHOD AND APPARATUS FOR STARTING AN AIRCRAFT ENGINE

Abstract: A pair of electrical generators (14,16) are installed on a jet engine (10). The generators (14,16) are coupled to each other in a manner so that together they can be used as crank motors to start the engine.

Description: Technical Field This invention relates to a method of starting an aircraft jet-engine as described in the preamble of claim 1, and to an apparatus for starting an aircraft jet-engine as described in the preamble of claim 3. Such method and apparatus are known from US-A-4,456,830. Background Art Aircraft are much like automobiles in at least one respect. If an aircraft can be constructed that is lighter in weight, or if certain weight penalties on an aircraft can be eliminated, the aircraft's range and/or fuel efficiency can be increased. For this reason, there is a continuing trend among aircraft designers to build lighter and more efficient aircraft at reduced cost. Part of this trend includes attempts to significantly reduce or eliminate aircraft weight penalties normally associated with the utilization of hydraulic or pneumatic power extraction systems. At least one approach toward eliminating these kinds of penalties has been to implement, as replacement systems, electric systems which are generally lighter and lower in cost. For this reason, as new generations of aircraft are designed, it is anticipated that such aircraft will become "more electric" and will utilize hydraulic and pneumatic systems to a lesser extent. Most present-day commercial aircraft have only one electrical generator installed on each engine. Each generator is typically integrated with a hydro-mechanical constant speed drive (IDG) that converts variable engine speed to a constant output speed for the purpose of driving the generator. This permits a generator to operate at a constant frequency, which is typically 400 Hz, for example. A known problem with the IDG is that it precludes integration of engine and generator cooling systems. Further, there are known cost and reliability problems associated with using the IDG. Because of this, IDGs which provide 400 Hz power are being replaced by electronic power convertors, which are often referred to as "variable speed constant frequency" or VSCF power convertors. When VSCF convertors are used, the associated aircraft generator is hard coupled to the high pressure rotor of the engine by means of a fixed gear ratio linkage. The gear ratio is usually selected so that it will limit the maximum generator speed to approximately 20,000-30,000 rpm. The recent development of high current power transistors have made it possible, with only minor modifications, to also use a VSCF power convertor to control electrical power at the current level required to make the "generator" operate as an engine crank motor. An onboard electric crank or starter motor has not yet been utilized in large commercial aircraft; perhaps because of excessive cost, weight and lack of the above-mentioned high current power transistors. For example, modern large-sized commercial aircraft (exceeding one hundred passengers in capacity) typically use a pneumatic ground power cart which supplies pressurized air to a dedicated air starter motor. As would be familiar to a person skilled in the art, the air starter motor cranks an aircraft motor on the ground. If, for some reason, an engine is shut down or fails during flight, the usual way to restart the engine is to crank the engine by means of ram air feeding into the engine duct; which has a wind milling effect on engine turbine blades. However, sometimes a low airspeed/altitude condition requires crank assistance by crossfeeding bleed air from another operating engine to the failed engine's air starter motor. An advantage to having a generator act as a crank motor would be that a separate dedicated pneumatic starter may be deleted from the engine accessory box. Recent improvements in jet engine efficiency and the recent introduction of the prop-fan engine have reduced engine inertia and drag torque characteristics. This means that generator ratings (based on normal load demands) are becoming more compatible for use as engine crank motors. This, in conjunction with the development of new transistors, has made the potential use of generators as engine crank motors more attractive. This is exemplified by the disclosure of US-A-4,456,830, which describes a method and apparatus for starting an aircraft jet-engine by supplying electrical current at a controlled voltage/frequency ratio to a generator which is coupled mechanically to the engine. The generator thus functions as an electric start motor, and cranks the engine to start it. During the first phase of the engine start in sequence, the electrical power for the generator is supplied by an external power source, and at a given moment during the starting sequence, a switch is made to power provided by the aircraft's APU. However, the method and apparatus of US-A-4,456,830 have a disadvantage in that the generator ratings required for servicing the various aircraft electrical systems are not sufficient for cranking the engine under all circumstances. For example, if a General Electric GE36P prop-fan engine is used on an aircraft, each engine may have two 75 KVA generator/convertor system for providing power to the various aircraft electrical systems. However, it is anticipated that it will be a design requirement to start an engine at minus 65° F, and it has been estimated that to meet this requirement using a generator as a crank motor may require a generator terminal rating in excess of 90 KVA. Therefore, a single generator in the above-mentioned 75 KVA system would be inadequate for starting this engine during minus 65° F conditions. Still further, if the engines used in the next generation of aircraft are fan jet-engines, which have increased inertia and drag torque characteristics, it is likely that the generator terminal rating required to crank such an engine could be as high as approximately 150 KVA. Because it is anticipated that two generator systems per engine will be required to provide essential power source redundancy, using more powerful generators, having a rating enabling them to crank the engine under all circumstances, would result in a considerable increase in generator weight and cost. Disclosure of the Invention. In view of the above, it is the object of the invention to provide a method of starting an aircraft jet-engine by using the engine's electrical generator as an electric start motor, by which the above-mentioned disadvantages are overcome. According to the invention, this is achieved in that said generator system comprises a plurality of redundant generators that are coupled mechanically in parallel to said engine, and said generators are coupled together electrically in series when being used for cranking said engine to start it. By electrically coupling the plurality of generators that is provided on each engine for reasons of power source redundancy, the required rating for cranking the engine to start it is achieved in an efficient manner. The apparent loss of reliability that results from coupling the generators, as compared to using a single generator from a plurality of independent, redundant generators for cranking the engine is irrelevant, in view of the remoteness of the possibility of an engine failure and a generator failure occuring at the same time. Preferably, when using the starting method according to the invention, the coupled generators are used together to crank the engine until the engine reaches 50% engine idle rpm, and then only one of the generators is used until the engine reaches a starter "cut-out" rpm, such that the engine can continue to run on its own without further cranking. This provides an efficient and wear-limiting starting sequence. It is also an object of the invention to provide an improved apparatus for starting an aircraft jet-engine by using the above method. The improved apparatus according to the invention is characterized in that said generator system comprises a plurality of redundant generators, connected mechanically in parallel to said engine, each generator having a terminal rating which is insufficient for cranking said engine, switching means are provided for electrically coupling said generators in series in a manner so that said coupled generators have a combined terminal rating which is sufficient for cranking said engine, and a VSCF power convertor is placed between said power source and said start bus, and forms the connection therebetween. The use of a VSCF power convertor allows precise control of the voltage/frequency ratio as the back EMF increases with engine speed during starting, and thus allows efficient starting of an aircraft jet-engine by the above method. The rotor and stator portions of each coupled generator may be coupled mechanically and electrically in phase with rotor and stator portions of the other generators, and each generator may thus function as a synchronous crank motor. This eliminates the need for a modification of the generator's iron core and amortisseur winding for operation as a squirrel cage induction motor, and thus allows the use of a lightweight generator. In an alternative embodiment of the invention, each generator has a modified iron core and amortisseur winding, and thus functions as a squirrel cage induction motor. This eliminates the need for a rotor field current, and thus allows a simplification of the motor controls. Brief Description of the Drawings In the drawings: Fig. 1 is a schematic drawing showing two generator stator windings connected electrically in series, and connected mechanically in parallel via their respective rotors to an accessory gear box of either a fan-jet or prop-fan engine, in accordance with a preferred embodiment of the present invention. Best Mode for Carrying out the Invention The description which follows is directed toward a specific application wherein two identical generators are mechanically connected in parallel to an engine accessory gear box that has the capability of functioning as an engine crank gear box. A person skilled in the art would be familiar with this type of connection, and further, a person skilled in the art would be familiar with the connection of a generator rotor to this type of engine gear box. Referring now to Fig. 1, therein is shown a schematic representation of either a fan-jet or prop-fan engine 10. The engine has a high pressure rotor 12 and a low pressure rotor 13. The high pressure rotor 12 is normally cranked during an engine start operation - - in contrast to the low pressure rotor 13 which is not cranked. Operatively attached to the high pressure rotor 12 via an accessory gear box 18 are first and second generators 14, 16. The high pressure rotor 12 is mechanically connected to the accessory gear box 18 at a fixed gear ratio. As would be well familiar to a person skilled in the art, two output shaft splines in the accessory gear box are connected to the two generators 14, 16. If the previously-mentioned synchronous crank motor approach is used to couple the generators 14, 16 together, then these shaft splines are keyed accordingly for phase control of the generators. To start the engine 10, the two generators 14, 16 are first disconnected from their related power system buses 32, 34, by means of generator circuit breakers 28, 30 (GCB1, GCB2). Those power buses 32,34 normally transmit power to various aircraft electrical systems when the generators 14, 16 are in the generating mode. In the engine start mode, contactor 24 is moved downwardly into the "S" position as shown in Fig. 1. Then, a variable speed constant frequency (VSCF) power convertor 35 converts electricity from an input power source 22 to a controlled voltage/frequency ratio, which energizes a start bus 37. The controlled voltage/frequency electric power is transmitted to the generators 14, 16 via contactors 20, 24 which cause the generators to create crank torque. This torque is delivered to the rotor 12 by means of the accessory gear box 18. The circuit represented in Fig. 1 is a single phase representation for each additional generator electrical phase (usually a total of three). Another contactor 36 is normally open as shown when both generators 14, 16 are used together for cranking. An airframe ground plane is shown at 25 which is functional for either the engine start mode or the generating mode. Two optional start modes are also illustrated in the drawing, wherein either one of the two generators individually may be used as a crank motor at any one time. For example, one of the generators 14 may be used when only light cranking loads are required by operating the contactor switch 24 to the "N" position, as shown by the dashed lines 26 and retaining contactor switch 20 in the noted position. As an alternative, the other generator 16 could also be used as a single crank motor by closing the contactor switch 36 in the manner indicated by the dashed lines 39. In this case, however, the contactor switch 20 would also be moved into an open position as shown by the dashed lines 41. It should be appreciated that if a single generator is used to crank the engine 10, the VSCF convertor 35 would be required to provide twice the electric current level and twice the wasted heat dissipation of a more efficient arrangement wherein both generators are series connected to crank the engine. It is anticipated that both generators 14, 16 will be required for starting the engine 10 during cold ambient conditions. Obviously, both could be used during warm ambient conditions in order to reduce heat dissipation in the generators 14, 16 and the VSCF convertor 35. Certain engine starting conditions may arise where it is useful to use both generators 14, 16 when cranking the engine 10 from zero rpm to approximately 50% of engine idle rpm. At 50% engine idle, two generator cranking may continue at reduced field current and reduced torque or the cranking may continue with one only generator. To crank with one generator, contactor 24 is returned to position "N" and only one of the generators 14, 16 completes cranking the engine 10 up to a higher starter cutout rpm. When the starter cutout rpm is reached, the contactor 20 is opened (contactor 36 would already be open) and the generator circuit breakers 28 and 30 are subsequently returned to the dotted positions 29 and 31. The generators 14, 16 are then operative to provide electrical power to the aircraft power system buses 32, 34. The above-described switching changes or changes in contactor positions would be familiar to a person skilled in the art. The actual control of the contactors would be accomplished by suitable utilization of a microprocessor (not shown in the drawings) located in the engine start control module onboard the aircraft. Along this line, any additional non-baseline electronics dedicated for use only in conjunction with the invention would be minimal in nature, and consequently, so would any weight penalty associated with such electronics.

Claims: Method of starting an aircraft jet-engine (10) having an electrical generator system (14,16) coupled mechanically thereto, said generator system (14,16) having a generating mode, in which it functions to transform power from said engine (10) into electrical power, and an engine start mode, in which it functions as an electric start motor for starting said engine (10), said method comprising disconnecting said generator system (14,16) from power buses (32,34) which transmit electrical power to aircraft systems when in the generating mode, connecting said generator system (14,16) to a power source (22) and supplying said generator system (14,16) with electrical current at a controlled voltage/frequency ratio to cause said generator system (14,16) to rotate and through said mechanical coupling crank said engine (10) to start it, characterized in that said generator system (14,16) comprises a plurality of redundant generators (14,16) that are coupled mechanically in parallel to said engine (10), and said generators (14,16) are coupled together electrically in series when being used for cranking said engine (10) to start it. Method according to claim 1, characterized by using said coupled generators (14,16) together to crank said engine (10) until said engine (10) reaches 50% engine idle rpm; and followed by using only one of said generators (14,16) until said engine (10) reaches a starter "cut-out" rpm such that said engine (10) can continue to run on its own without further cranking. Apparatus for starting an aircraft jet-engine (10), comprising: an electrical generator system (14,16) operatively connected to said engine (10), and being electrically reversible between a generating mode, in which it functions to transform power from said engine (10) into electrical power, and an engine start mode in which it functions as an electric start motor for starting said engine (10); an engine start bus (37); switching means (20,36) for electrically coupling said generator (14,16) to said engine start bus (37); and an electrical power source (22) connected to said engine start bus (37); characterized in that : said generator system (14,16) comprises a plurality of redundant generators (14,16), connected mechanically in parallel to said engine (10), each generator (14,16) having a terminal rating which is insufficient for cranking said engine (10); switching means (24) are provided for electrically coupling said generators (14,16) in series in a manner so that said coupled generators (14,16) have a combined terminal rating which is sufficient for cranking said engine (10); and a VSCF power convertor (35) is placed between said power source (22) and said start bus (37), and forms the connection therebetween. Apparatus according to claim 3, characterized in that rotor and stator portions of each said generator (14,16) are coupled mechanically and electrically in phase with rotor and stator portions of other generators (16,14), whereby each said generator (14,16) functions as a synchronous crank motor. Apparatus according to claim 3, characterized in that each said generator (14,16) has a modified iron core and amortisseur winding, whereby each said generator (14,16) functions as a squirrel cage induction motor.

Inventors: NORDLUND, LESTER H.
Applicants: THE BOEING COMPANY
Application year: 1986
Main classification: F02C7
IPC codes: F02C7