Gas turbine engines are started by rotating a high-pressure compressor. In dual-spool, axial flow engines, the high-pressure compressor and N1 turbine system are only rotated by the starter. To start a gas turbine engine, the compressor must be accelerated to provide ample air to support combustion in the combustion section or burners. Once ignition and fuel has been introduced, the starter is tasked with assisting the engine until it reaches a self-sustaining speed. Moreover, the torque supplied by the starter should be more than the torque required to overcome compressor inertia and the engine compressor’s friction loads.
From the moment the starter has accelerated the compressor to establish ample airflow through the engine, the ignition can be turned on, followed by the fuel. It is important to note that there must be sufficient airflow through the engine to support combustion prior to the fuel-air mixture being ignited. At low engine cranking speeds, the fuel flow rate is not ample enough to allow the engine to accelerate. As such, the starter continues to crank the engine until a self-accelerating speed has been reached. However, if assistance from the starter were cut off below the self-accelerating speed, the engine would fail to accelerate to an idle speed. Additionally, it may decelerate due to the fact that it is not producing enough energy to sustain rotation or accelerate during the initial phase of the starting cycle.
The starter is responsible for assisting the engine above the self-accelerating speed to avoid any delays in the starting cycle which can result in a hot or hung false start, or a combination of both. At some point in this series of actions, the starter and ignition are automatically cut off. Presently, gas turbine engines take advantage of direct current (DC) electric motors, starter/generators, and air turbine type of starters. Turbine starters utilize varying methods for turning the engine for starting, but generally, electric or air turbine starters are preferred. With an air impingement starting system, there are a series of jets containing compressed air that are piped to the inside of the compressor or turbine case, directing the jet air blast onto the compressor or turbine rotor blades so that they rotate.
A typical cartridge/pneumatic turbine engine starter can be operated as an ordinary air turbine starter through the use of a ground operated air supply or an engine cross-bleed source. Keep in mind that it can also be operated as a cartridge starter. To accomplish a cartridge start, a cartridge is placed in the breech cap. Then, the breech cap can be closed on the breech chamber and partially rotated to engage the lugs situated between the two breech sections. Meanwhile, the cartridge is ignited by applying voltage via the connector at the end of the breech handle. Once ignition takes place, the cartridge can generate gas. This gas is then forced out of the breech and into the hot gas nozzles that are directed toward buckets on the turbine rotor. It is also worth noting that rotation is generated through the overboard exhaust collector. Before making its way into the nozzle, the hot gas passes an outlet leading to the relief valve where it gets directed to the turbine as pressure rises above the preset maximum.
A fuel/air combustion starter can be used to start gas turbine engines by utilizing the combustion energy of jet A fuel and compressed air. It consists of a turbine-driven power unit and an auxiliary fuel, air, and ignition system. This starter type is usually fully automatic, meaning that the actuation of a single switch allows the starter to fire and accelerate the engine from rest to starter cutoff speed. For smaller engines, hydraulic pumps and motors can be used; however, these systems are not often employed on modern commercial aircraft due to the high power demands of large turbofan engines found on such airplanes.
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