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14.5.2. Turbine starters

The following types of turbine starters are applied to start a turbine engine (Fig. 14.9):

- air turbine (pneumatic) starters;

- fuel-air combustion starters;

- gas turbine starters;

- powder turbine starters (cartridge or solid-propellant starters);

- air-powder turbine starters (cartridge-pneumatic starters).

The air turbine starters have been widely used. The power element of an air turbine starter is an air-driven turbine, which through the reduction gear and ratchet clutch transmits a torque to accessory gear and further to an engine rotor (see Fig. 14.17). This type of starter requires a high-volume air supply, which may be provided by a ground starter unit, a compressed-air bottle on the airplane, an auxiliary power unit on board the aircraft, or compressor bleed air from other engines on board the aircraft. Usually air is supplied to an air turbine starter from an onboard auxiliary power unit – the turbogenerator of compressed air, which is constructed on the basis of a low-powered gas turbine engine with the centrifugal compressor. The auxiliary power unit is started beforehand with the help of direct acting electric starter, which receives the power from onboard storage battery or ground source.

Fig. 14.17. Schematic of an air turbine starter

In an air turbine starter the small-size air-driven turbines of impulse type are applied. To get optimum peripheral velocities the rotational speed of air-driven turbine rotors should reach 30000...40000 r/min for powerful air turbostarters and up to 60000 r/min for air turbine starters of low power. The planetary gears (one or two-stage) with gear ratio 8...15 are usually applied as the reduction gears of air turbine starters.

On reduction gear output shaft the ratchet coupling is installed, which during torque transmission from an air-driven turbine connects this shaft with an accessory gear and prevents inverse torque transmission after an air supply to an air turbine starter is stopped. At rotational speed when the starter is stopped the centrifugal forces moment, affected the rotated ratchets, provides their outlet from engagement, which eliminates itensive wearing.

In addition to the above mentioned elements the air turbine starter has a unit of air input as an air valve or shutter. This unit provides actuation or switching-off of air supply to the turbine with the help of electromagnet valves of control unit, and as well as automatic stabilization of air pressure at a given level. To control an air valve (shutter) the servodrive is used. It works on air pressure differential.

Air parameters required for air turbine starter operation are the following:

- air pressure equals рa=0,25...0,35 MPa (up to 0,4 MPa) at temperature Тa=150...180 С (up to 200 С);

- air flow rate depending on power equals Ga = 0,3...1,5 kg/s.

Air turbine starters are applied at required power range of starting devices from 20 up to 350 kW. The starting system with an air turbine starter has smaller weight in comparison with systems, in which electrical starters are used.

The turbine of a fuel-air combustion turbine starter receives energy from gas, heated in the combustion chamber (Fig. 14.18), at the expense of fuel combustion in compressed air, brought from an outside source.

The fuel-air combustion starter is essential­ly a small gas turbine engine, minus its compressor. It is completely self-contained, as is the cartridge starter system, but unlike the preceding system, requires no additional com­ponents to function. All fuel, air, and electric power needed for operation are carried on board the aircraft.

In addition to the turbine, the system consists of an air storage bottle, fuel storage bottle, and a combustion cham­ber, together with the necessary ignition and control compo­nents. During flight, an engine-driven compressor maintains 20685 kPa of air pressure in an airborne bottle. This pressure permits engine starts without the necessity of recharging the air system from an external source. The usual high-pressure bottle will provide enough air for two restarts without recharging. Provision is also made to connect an external 4137- kPa air supply. In either case, the starter receives a reduced air pressure of 2413 kPa.

The fuel-air turbine starter has such main units:

- unit of compressed air supply and its pressure stabilizer;

- combustion chamber;

- axial turbine;

- reduction gear with gear ratio 8...10;

- clutch gear of starter output shaft with accessory gearbox gears.

Fig. 14.18. Schematic of the fuel-air turbine starter

In a typical system shown in Fig. 14.18, the starter is acti­vated by a ground start switch in the cockpit. When the ground start switch is pressed, the starter air solenoid valve opens, admitting air from the storage bottle or from the exter­nal source into the combustion chamber. At the same time, the fuel valve opens to admit fuel from the accu­mulator, and the starter ignition system is momentarily ener­gized, igniting the fuel-air mixture. This action causes a rapid expansion of air, which spins the starter turbine, which in turn accelerates the engine through the reduction gear and clutch. When engine rotational speed reaches about 21 percent, starter fuel is exhausted, resulting in a drop off of burner pres­sure. The pressure switch actuates, opening the air duel to the fuel and air valves. If starter speed exceeds 22,6 percent engine rotational speed before fuel is exhausted, the centrifugal switch will open, shutting off the fuel and air valves. Exhaust from the starter combustion chamber is directed through an exhaust duct at the lower side of the starter and into the engine air-guide section. A pressure reducer and an air-con­trol valve in the starter reduce the supply air pressure to about 2275 kPa before it enters the fuel accumula­tor and the combustion chamber. The fuel accumulator con­tains enough fuel obtained from the airplane fuel system through a takeoff line at the engine fuel-flow divider to oper­ate the starter for about 4 s. The accumulator is pressurized with air pressure to ensure fuel flow to the combustion chamber. The starter has a safety clutch, which automatically disengages the starter drive shaft from the engine drive spline to prevent the engine from driving the starter turbine to destructive overspeed. In case the starter clutch fails, the safety clutch must be manually reset, which necessitates removing the starter from the engine. An air motoring switch allows the air solenoid valve to open while bypassing the fuel solenoid valve and starter ignition circuits for the purpose of motoring the engine from 4 to 6 percent.

The fuel-air turbine starters have no disadvantages, which air turbine starters have (icing of an exhaust pipe), as through a fuel-air turbine starter exhaust pipe not cooled air flows but gas with temperature of 500...600 С.

The gas turbine starter is anoth­er completely self-sufficient starting system. Relatively high output power (N=50…200 kW) is available for a comparatively low weight. The starter is actually a small, gas turbine engine, completed with a gas-generator section containing a centrifugal compressor, combustion chamber, and tur­bine to drive the compressor. It also contains its own fuel, control, starting, lubrication, and ignition systems. The gases flowing through the gas-generator sec­tion drive the turbine, which, in turn, drives the main engine through a reduction gear and clutch mechanism to automatically engage and disengage the starter’s turbine rotor from the engine. The starter is itself started by using a small electric motor, compressed air or hydraulic power from the aircraft system.

There are three types of the gas turbine starters:

- with the free turbine;

- with a hydraulic clutch;

- with the differential reduction gear.

The gas turbine starter with the free turbine scheme (see Fig.14.19, c) is similar to the turboshaft engine. The free turbine of such starter after its starting through the reduction gear and clutch gear rotates started engine rotor.

The starters of such type have:

- diagonal-axial or centrifugal compressor;

- annular combustion chamber;

- axial turbine of the compressor;

- axial free turbine;

  • exhaust unit, which is located at an angle to starter axis.

When the starting cycle is initiated, the gas turbine starter is rotated by its own starter motor until it reaches a self-sustaining speed, at this point the starting and ignition system automatically switch off. Acceleration then continues up to a controlled speed. While the gas turbine starter engine is accelerating, the exhaust gas is being directed, via nozzle guide vanes, onto the free-power turbine to provide the drive to the main engine. Once the main engine reaches a self-sustaining speed, a cutout switch operates to shut down the gas turbine starter. As the starter runs down, the clutch automatically disengages from the output shaft and the main engine accelerates up to idling rotational speed under its own power.

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