November 1, 2001
LINDA D. PENDLETON
Now that we've flowed the air through the engine and out the exhaust pipe in previous articles in this series, it's time to look at a few of the supporting systems on turbine engines. Most of these systems are found on the accessory pad of the engine and are driven by a tower shaft and gearing from the high-pressure compressor shaft. Common accessories are the engine-driven fuel pump, hydraulic pump, oil pump, starter-generator, and tach generator. Oil, hydraulic, and fuel pumps on turbine engines are similar to their counterparts on piston engines, but the fuel control unit and starter-generator are unique to turbines.
This engine-driven unit — which can be of the hydro-mechanical, electro-hydro mechanical, hydro-pneumatic, or electronic type — has one function in the engine fuel system: It's a very expensive air scale. A fuel control unit meters fuel on the basis of weight and provides a supply of fuel to the burner can that will ensure a 15:1 air-to-fuel ratio by weight for the proper combustion. This is the chemically correct, or stoichiometric, mixture necessary for complete combustion of the fuel. Fuel is metered on the basis of weight rather than volume because the volume of a given weight of fuel is variable based on temperature; however, fuel will provide a constant level of energy per unit weight.
The fuel control unit typically receives several signals to be used in its control of the air-to-fuel ratio. Among the most common are engine speed, inlet air pressure, compressor discharge pressure, inlet air temperature, burner can pressure, and power lever position. As you can see, if you know the temperature and pressure of the inlet air and the speed of the compressor, you will be able to calculate the weight of the airflow coming from the compressor. Of course, if you are given compressor discharge pressure or burner-can pressure the calculation is done for you. The power lever position tells the fuel control unit what power setting the pilot is requesting.
The fuel control unit, now knowing the weight of the air being provided by the compressor, can compute the proper amount of fuel to meter to the fuel nozzles to maintain the 15:1 air-to-fuel ratio in the burner can. Automatic compensation is made for the normal service bleed-air requirements of the engine and aircraft for systems such as pressurization and for engine cooling needs. Depending upon the sophistication of the fuel control unit and the electronic inputs it receives, it also may be able to compensate for the extraction of bleed air for engine anti-ice and ground air-conditioning requirements. To ignore power adjustments for these additional bleeds will risk, at the very least, a reduction in power and possibly an over-temperature condition in the burner can and at the turbine inlets. A jet engine always has much more air available than is needed for fuel combustion; however, it requires this air for cooling purposes. If bleed air is extracted without an adjustment to the fuel scheduled to the engine, the amount of cooling air will be reduced and the engine may overtemp.
Since it takes increased compressor output and consequently increased compressor rpm to convince the fuel control unit to add more fuel to the air-fuel mixture, and since it takes more fuel to increase the compressor rpm, you can see why turbojet engines have a spool-up lag of five to 10 seconds after power application. Engines with electronic fuel control units suffer less lag than hydro-mechanical types, although there will always be some lag between the request for and delivery of power. Since typical gas turbine engine compressor speeds range in the neighborhood of 34,000 rpm, you can see that inertia alone will cause a lag in spool-up time.
Systems used for starting gas turbine engines must be capable of spinning the compressor fast enough to provide combustion and cooling air for the engine start and also turn the accessory section fast enough for the engine-driven fuel pump to deliver fuel to the combustor. Neither the compressor nor the starter alone has sufficient power to accelerate the engine from rest to idle speed, but when they are used in combination the process takes place smoothly in about 30 seconds. The engine start sequence is normally initiated by a switch in the cockpit but is often terminated by a speed sensor that ends the start sequence at a speed slightly above that at which the engine is capable of self-accelerating.
If the engine is not assisted to the speed at which it is capable of self-accelerating to idle speed, a hung start may occur. The engine will stabilize at a speed below or near starter cutoff speed, often with higher-than-normal temperatures because of the lack of cooling air. The engine must then be shut down and the cause of the hung start investigated. Any attempt to accelerate the engine at this point by adding fuel will likely result in a hot start as well as a hung start. There will be sufficient airflow for combustion but insufficient airflow for cooling purposes.
A typical starting sequence is initiated in the cockpit by beginning engine rotation. At speeds of 5 to 10 percent compressor rpm, fuel will be introduced and ignition activated. Light-off should occur in 20 seconds or less (dependent upon engine type), and the engine should continue to accelerate and stabilize at idle rpm. Temperatures during start are closely monitored since any lag in acceleration or restric.ion to airflow through the engine can cause destructively high starting temperatures.
The combination starter-generator is widely used on corporate-size jets because of the weight savings involved in using one engine accessory for two purposes. This engine accessory receives electrical power from the aircraft batteries or external power source to rotate the engine for starting. Once the start sequence is terminated, the starter-generator, which has a drive gear permanently engaged to the engine, reverts to a generator. This is typically accomplished through electrical switching. When the engine is rotating the starter-generator at sufficient rpm to produce full power, the generator may be connected to the aircraft electrical buses.
The pneumatic, or air turbine, starter is a type of low-pressure air motor. This type of starter is used on almost all large commercial aircraft and some larger corporate aircraft. The starter turbine transmits power to the starter output shaft through a reduction gear and clutch mechanism. The air required to spin the turbine may be obtained from an on-board auxiliary power unit (APU), an external ground supply, or from a cross-feed pneumatic valve from a running engine. The air supply to the starter is typically regulated to begin at the initiation of the start cycle and shut off at a predetermined starter speed.
The turbine of a typical pneumatic starter rotates up to 60 to 80 thousand revolutions per minute and is geared down 20 to 30 times to achieve the high torque necessary to rotate the heavy, multistage axial compressors found on large commercial and military transport aircraft. The clutch mechanism automatically disengages the starter from the engine as the engine reaches idling rpm.
Next month we'll take a look at power settings and performance calculations for turbine engines.
Linda Pendleton, AOPA 525616, is the curriculum development manager for Eclipse Aviation. She has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.
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