engine starter

5A-ALI · 09:59 PM

STARTERS

Gas turbine engines are started by rotating the compressor. In the case of dual axial-flow compressor engines, the high-pressure compressor is usually the only one rotated by the starter. First, compressor accelerates to provide sufficient air under pressure to support combustion in the burners. Second, after fuel is introduced and the engine is fired, starter continues to assist the engine above the self-sustaining speed. The torque must be in excess of that required to overcome rotor inertia and engine friction and air loads.

Basic types of starters developed for gas turbine engines are air turbine (pneumatic), electric motor, hydraulic, combustion, and cartridge pnuematic. Air turbine starters are the most commonly used. Smaller engines generally use electric starters. Hydraulic starters are frequently found in helicopters and some marine gas turbine installations. Combustion and cartridge-pneumatic starters are generally special application devices that may require a self-contained starting system. An impingement starting system is sometimes used An impingement starter consists of simple jets of compressed air piped to the inside of the compressor or turbine case. The jet air blast is directed onto the compressor or turbine rotor blades and causes them to rotate.

As the starter accelerates the compressor sufficiently to establish airflow through the engine, the ignition and then the fuel are turned on. The exact sequence of the starting procedure is important. There must be sufficient airflow through the engine to support combustion when the fuel-air mixture is ignited The fuel flow rate will not be sufficient to enable the engine to accelerate until after the self-sustaining or self-accelerating speed has been reached If assistance from the starter were cutoff below the self-sustaining speed, the engine would either fail to accelerate to idle speed or decelerate. Deceleration occurs because sufficient energy was not produced to sustain rotation or acceleration during the initial phase of the starting cycle. The starter must continue to assist the engine above the self-staining speed to avoid a delay in the starting cycle. This would result in a hot or hung (false) start, or a combination of both. In a hot start, the engine lights up, but the exhaust gas temperature exceeds that allowed for an engine start. In hung or false start the engine lights up normally but, instead of increasing to idle speed, the RPM remains at some lower value.

At the proper points in the sequence, the starter and the ignition cut off. The higher the RPM before the starter cuts out, the shorter the total time required for the engine to attain idle RPM. The engine and starter work together to furnish the torque necessary for engine acceleration.

The most important requirement of a starter is to produce sufficient torque to start the engine properly. Engines must be rotated and accelerated above a certain minimum rate if consistently good starts are to be achieved. The torque characteristics of an acceptable starter must be well above the required minimum.

Air Turbine

Air-turbine starters (which are also called pneumatic starters) are used more than any other for starting jet aircraft engines, particularly larger engines (Figure 8-7). A small geared air turbine is attached to the engine starter pad located at the accessory drive gearbox. Air-turbine starters must receive compressed air from an external power source. A compressor mounted on a ground unit or onboard the aircraft is one such source. A small turbine engine usually drives these units. On multiengine aircraft, air is often bled from the first engine started and used to operate the starters for the remaining engines. With an air-turbine starter, the air supply must be of sufficient volume and pressure to meet starter requirements. Otherwise, the starter torque may not produce consistently successful starts within an acceptable time limit. When bled air from another operating engine is used, the engine being used for a compressed air supply must be turning over fast enough to ensure adequate air pressure to the starter of the engine being started .

Still another form of pneumatic starter is when the starter itself is part of the ground power unit. The end of a flexible shaft from the starter is placed in a connection box on the engine to turn the compressor. When the engine starts, the flexible shaft is removed This type starter might be used on small gas turbine engines. It might also be used on a gas turbine engine for a missile or where airborne weight must be held to the minimum.

Combustion

A combustion starter is essentially a small turboshaft gas turbine engine (Figure 8-8). Its chief advantages are quick starts because of the high torque produced and portability. The complete starting system may be carried aboard the aircraft. This starter obtains its power from hot expanding gases. These gases are generated in the starter combustion chamber. They occur by burning a combustible mixture either fuel, air, or monopropellant. A monopropellant is a specially compounded solid or liquid slow-burning substance.

The quantity of fuel or monopropellant required by the starter is proportional to the length of time the starter is operating. Aircraft weight and size limitations require that the quantity of fuel or monoporpellant be held to a minimum in an airborne, combustion-starter system. The starter burning time must be held to a minimum. This is determined by the starter torque and the starter cutout speed. The starter cutout speed depends on the self-sustaining speed of the engine. The self-sustaining speed determines, to a large extent, the amount of fuel or monopropellant required. Ordinarily, sufficient fuel or monopropellant to provide a minimum of two starts will be carried aboard the aircraft. Some combustion starters operate as simple air turbines. When an outside air source from a ground unit or another engine which is already started is connected to the starter, the combustion starter functions as a pneumatic starter.

Cartridge Pneumatic

A cartridge-pneumatic starter is a combustion starter that operates from monopropellant contained in a cartridge. Prior to starting, the monopropellant cartridge is inserted in the starter. Like a regular combustion starter, a cartridge starter fires electrically from a switch in the aircraft. Cartridge-pneumatic starters can be operated as pneumatic starters when provided with outside air.

Electric Starting

Direct cranking and starter generator are the two electric starting systems for gas turbine aircraft (Figure 8-9).

Direct-cranking electric starting systems are similar to those used on reciprocating engines. Starter-generator starting systems are also similar to direct-cranking electrical systems. Electrically, the two systems may be identical. But the starter generator is permanently engaged with the engine shaft through the necessary drive gears. The direct-cranking starter must use some means of disengaging the starter from the shaft after the engine has started.

Direct-Cranking Gas Turbine Starters. On some gas turbine engines, no overload release clutch or gear reduction mechanism is used. This is because of the low-torque and high-speed requirement for starting. A reduced voltage mechanism is used, however, in the starting assembly during starting.

Starter-Generator Starting System. Many gas turbine aircraft are equipped with starter-generator systems. These starting systems use a combination starter-generator which operates as a starter motor to drive the engine during starting. After the engine reaches a self-sustaining speed, the starting system operates as a generator to supply the electrical system power.

The starter-generator unit is basically a shunt generator with an additional heavy series winding. This series winding is electrically connected to produce a strong field and a resulting high torque for starting. Start-generator units are desirable from an economical standpoint. One unit can perform the function of both starter and generator. Additionally, the total weight of starting system components is reduced, and fewer spare parts required.

Reference :

https://www.globalsecurity.org

/