As is well known in the art, gas turbine engines are started by rotation of their compressors to provide sufficient pressurized air to support combustion of fuel in burners of the engines. In a typical gas turbine engine starting sequence, a starter first rotates the compressor at a constantly increasing speed. Next, when rotation of the compressor has established adequate air flow through the engine, a fuel ignition system is turned on, and then fuel flows into the burners and is ignited by the ignition system. At this stage of the starting sequence, the force generated by the fuel combustion is inadequate to permit the engine to accelerate to a self-sustaining, or self-accelerating speed. Consequently, the starter must continue contributing torque to assist the engine in achieving a self-sustaining speed. After the engine accelerates beyond its self-sustaining speed and approaches its idle speed, the ignition system and the starter are cut out, and the engine continues on to its idle speed.
To achieve the shortest possible total starting time, or "optimal start time", the starter remains engaged for the longest possible time. That is because, after combustion of the fuel commences, the engine and starter are working together to furnish the torque necessary for engine acceleration. Consequently, to decrease total engine start times, modern gas turbine engine starters are designed to provide substantial surplus torque beyond a minimum torque level required for an engine start.
The most common starters utilized with gas turbine engines on modern "jet" aircraft are pneumatic starters (also referred to as "air-turbine" starters). Pneumatic starters include a control valve to admit compressed air into the starter. The air passes through one or more nozzles that direct the air onto a turbine rotor. Resulting rotation of the rotor drives a sequence of starter gears affixed to a starter output shaft that engages an engine starter shaft within the engine's accessory drive gear box. An overrunning clutch is affixed to the starter output shaft to functionally disconnect the starter upon completion of the starting sequence. The compressed air is supplied to the starter by way of starter air ducts, and originates from either a source exterior to the aircraft; from on board, stored compressed air; or, from bleed air fed from other on board engines that have already been started.
Consistent, reliable starter performance is critical to efficient operation of a modern aircraft. Any starter failure often results in prolonged grounding of an aircraft until the failed starter is repaired or replaced. Additionally, failure of a starter during a starting sequence may result in either a "hot" start, wherein ignited fuel produces exhaust gas temperatures in excess of allowable limits due to inadequate air flow, or a "hung" start, wherein the engine is ignited, but unable to accelerate. Both "hot" and "hung" starts will result in an unacceptable delay in engine start up, and may even damage an engine. Because known pneumatic starters do not include a redundant back-up system, any failure of the control valve, turbine rotor, or starter gears, etc. will result in total starter failure.
Accordingly, it is the general object of the present invention to provide a redundant engine starting system that overcomes the reliability problems of the prior art.
It is a more specific object to provide a redundant engine starting system that is capable of starting a gas turbine engine despite failure of some of the system's components.
It is another specific object to provide a redundant engine starting system that increases starter reliability while primarily using known starter components.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.