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What is a Gas Turbine Engine and How Does it Work?

I. What is a Gas Turbine?

i. Intro

ii. Types of Gas Turbines

II. How Does it Work?

i. Intro

ii. Compressor

iii. Burner

iv. Turbine

v. Additional Gas Turbine Components

vi. Exhaust System

III. Starting a Gas Turbine

IV. Modulating Power in a Gas Turbine

 

 

WHAT IS A GAS TURBINE?

A gas turbine engine is an internal combustion engine, designed to convert latent energy in a fuel (kerosene, jet fuel, diesel, gasoline, etc.) into mechanical energy or kinetic energy to do work. This energy can be used in the form of thrust, or mechanical horsepower to drive a propeller, a rotor, a transmission, a generator, or just about anything else.

The definition of a turbine is a device which converts the kinetic energy of a fluid into a rotational energy through the reaction or impluse of the moving fluid upon paddles, buckets, or blades. A common example of a turbine is a windmill. A windmill, or wind turbine, converts kinetic energy present in moving air into a rotation. In a wind turbine, the working fluid is air. In a gas turbine, the working fluid is gas. Hot combustion gases, at high dynamic pressure and temperature, pass through one or more turbine wheels, causing them to rotate, typically at very high speed. This gas turbine wheel forms the basis for what are known as gas turbine engines.

There is more to a gas turbine engine than the turbine wheel, however. Every gas turbine engine has three fundamental elements in common; the compressor, the combustor, and the turbine. These three elements form the basis of what is known as the gas producer, also referred to as the gasifier, the gas generator, or the core. The purpose of the gas producer is simply to create gas at high dynamic pressure and temperature. What is done with this high energy gas determines what specific type of gas turbine engine is being used.

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Types of Gas Turbines

Gas turbine engines generally fall into one of four categories. These are turbojet, turbofan, turboprop, and turboshaft; Although a fifth category would cover alternative uses for a gas producer core, such as a turbogenerator, turbo water pump, a jet dryer or snow melter, turbocompressor, etc.

A turbojet is the simplest form of gas turbine engine. In a turbojet, the hot gas produced is channelled through a jet pipe and jet nozzle, which increase the exhaust velocity tremendously. This gas is then expelled from the rear of the engine, creating reaction force that is referred to as thrust. This thrust was used to power the earliest jet aircraft, as well as older fighter aircraft. Turbojet engines are now almost exlusively used in small engine applications such as model jet aircraft, and various other specialized or recreational applications. The Concorde is an example of a modern aircraft that utilizes turbojet engines, although the Concorde is no longer in service.

A turbofan is similar to a turbojet, except that instead of using the hot gas produced to exclusively provide high velocity thrust, some of the gas energy is expanded through a second set of turbine wheels, often referred to as a low pressure turbine, to drive a large ducted fan in front of the engine. The low pressure turbine rotates completely independent of the gas producer, allowing the fan to turn at a slower speed than the engine core. The fan draws air in from the front of the engine, accelerates it, and feeds some of the air into the gas producer core, while the rest of it actually travels around the engine core, helping to cool the engine and quiet it as well. The air that travels through this bypass duct mixes with the core engine exhaust, where it is expelled out of the rear of the engine in the form of thrust. Depending upon the bypass ratio of the engine, the fan can contribute as much as 90% of the overall engine thrust. The bypass ratio is the ratio of air drawn in by the fan that is distributed to the bypass duct rather than the engine core. The bypass ratio can vary from as little as .2:1 on modern military fighter aircraft, to as much as 9:1 on modern commercial aircraft. Turbofans are much more efficient at high subsonic aircraft speeds, and have almost totally replaced turbojets in modern aircraft. Specific fuel consumption of a large commercial jet liner engine is dramatically lower than a turbojet engine in a fighter aircraft. Turbofans range in power output from as little as 600 lbs of thrust, to as much as 115,000 lbs of thrust as in the General Electric GE90 ultra high bypass turbofan.

A turboprop is similar to a turbofan, except that instead of turning a ducted fan, the engine turns a traditional aircraft propeller through a gearbox. Also, while in a turbofan, a significant portion of the core engine exhaust is often used to produce thrust, the core exhaust of a turboprop produces very little thrust, typically no more than around 20%. In most cases, the propeller is driven by a separate turbine wheel, usually referred to as the free power turbine. The free power turbine spins completely independent of the gas producer, so that the propeller can turn at a constant speed while the compressor rpm can be varied to increase or decrease power. However, some turboprop engines are referred to as a single spool design, where there is no free power turbine. The propeller is driven via a reduction gear by the gas producer. In most turboprops, the reduction gear is integral with the engine, although some turboprops require a separate propeller reduction gearbox.

A turboshaft engine is similar to a turboprop, exept that instead of turning a propeller, it is used to turn some other type of load. Turboshafts are most commonly used in helicopters. In a turboshaft, the hot gas produced by the gasifier is used to drive a separate free power turbine wheel that spins independently from the gas producer. The engine is designed so that the free power turbine extracts as much energy from the combustion gas as possible, and the remaining exhaust is expanded through a diffuser so that the engine exhaust produces no thrust. A turboshaft could theoretically be used to drive any load, and works equally well when running with a constant output shaft speed as it does with a variable output shaft speed. Some turboshaft engines have integrated reduction gearboxes, which convert the high speed/low torque of the free power turbine into low/speed high torque, which is more suitable to drive a load, while some engines are direct drive; output is taken off at free power turbine speed. Auxiliary power units are a type of turboshaft engine that are used in aircraft to drive the accessories when the main engines are shut down. They can drive generators, pumps, air compressors, and any number of other ancillary loads. APUs can either be single shaft, or free shaft engines.

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HOW DOES IT WORK?

As stated above, a gas turbine consists fundamentally of three elements; a compressor, a burner, and a turbine. These elements work together to produce usable power. A gas turbine is a heat engine. It produces power basically in two steps. First, it converts fuel energy into heat energy, and then it harnesses as much of that heat as possible and converts it into mechanical energy. The more heat the engine can produce, combined with the larger mass of air it can flow through the combustor, the more power it can extract.

The heat is produced by burning fuel in the burner, or combustion chamber. The more fuel that is burned per unit time, the more heat is produced. Under ambient pressure, fuel will only burn at a certain rate. This would not be sufficient to produce enough power to drive the engine cycle. In order to increase the fuel burn, air under high pressure must be forced into the burner. This is the first job of the compressor. The compressor draws in ambient air, increases its pressure, and feeds this pressurized air to the burner, where the air and fuel are mixed to provide a very high temperature combustion. In combustion, the fuel's potential energy is converted to heat energy through a chemical reaction. It should be noted that the hotter the combustion temperature, the more complete the burning of the fuel is.

The second job of the compressor is to flow large quantities of air, referred to as the air mass flow, through the combustor. The air mass serves two purposes: It cools the gas temperature of combustion down, to a value that is safe for the materials downstream of the primary combustion zone, by absorbing the combustion heat; Then, by absorbing that heat, the large mass of air expands and increases its dynamic pressure dramatically. The rapid increase in temperature due to combustion, combined with the large mass of air, creates a tremendous flow of high energy gas which provides the power to drive the turbine. This hot gas under high pressure seeks to escape the burner, and finds its exit via the burner nozzle. The burner nozzle has vanes that accelerate the gas and direct it onto the turbine wheel, or wheels, causing the turbine to spin at a high rate of speed. The turbine drives the compressor via a fixed shaft, and the cycle is complete, and self sustaining. The cycle that governs the operation of a gas turbine engine is referred to as the Brayton constant pressure cycle. The engine compressor typically requires about 2/3 of the usable heat energy produced in the burner to turn at maximum speed; the remaining energy can then be used to produce thrust or mechanical power, or a combination of the two.

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Compressor

The purpose of the compressor is twofold; It is to provide the burner with high pressure air at large air mass flows. A compressor will increase the pressure ratio anywhere from 3:1 on some of the smallest engines, to as much as 50:1 on the largest commercial turbofan engines. The compressor is probably the single most important component in a gas turbine engine, because its performance governs how powerful and how efficient the complete engine can be. Thermal efficiency of the engine is almost directly related to the pressure ratio, and engine power is very closely linked to the compressor's air mass flow. A General Electric GE90 turbofan's fan and compressor can pull in around 3,000 lbs. of air every second.

There are two basic types of compressors; axial and centrifugal. Centrifugal compressors are generally used on smaller engines. They are compact and rugged, and have a high pressure ratio increase per stage, up to 9:1 on some of the newer engines. However, very high compression ratios become difficult to accomplish with a centrifugal compressor because having multiple stages becomes awkward from a packaging and airflow standpoint. And because of the centrifugal compressor's lower efficiency, using more than one or two stages requires too much of the power produced in the burner to drive.

A centrifugal compressor consists of an impeller, a diffuser, and a compressor manifold. Air is draw in at the center of the rotating impeller, known as the inducer, and is accelerated radially toward the outside of the wheel by vanes on the face of the impeller. This air is collected in the diffuser and compressor manifold, where it is allowed to slow down, converting its velocity into static pressure. The air is then directed to the burner.

An axial compressor is more commonly used on larger engines, which require greater air flows and pressure ratios. Large commercial turbofan engines are typically axial flow engines. Axial compressors have a lower pressure rise per stage, more along the lines of 1.2-1.5:1, although some large fans have pressure rises of 3:1, but for the most part, an axial compressor is much more efficient, meaning that for the amount of compression it provides, it uses a lot less engine power than a centrifugal compressor. Also, an axial compressor is much more amenable to multi-stage use. Axial compressors with ten stages or more are very common. The General Electric J79 turbojet engine has a 17 stage axial compressor with a pressure ratio of 13:1.

An axial compressor is like a fan with many thin, angled blades along its hub. Each set of blades makes up one stage. Each compressor stage must be followed by a set of similar, stationary blades called stators, which slow down the airflow, increasing its static presssure, while directing the airflow to the next set of rotating blades. Many axial compressors feature variable geometry stator vanes, for more fine control of the airflow, especially during acceleration and deceleration of the compressor. The airflow travels from stage to stage, increasing its pressure as it goes. The compressor blades and stator vanes get subsequently shorter in length to assure the pressure increases as the air mass tries to fill a smaller volume. The high velocity airflow after the final compressor stage is converted into static pressure in the diffuser before it enters the burner.

Axial-Centrifugal and Split Compressors

It is also possible to combine both types of compressors into an engine design. In just about every case, an axial compressor of any number of stages is followed by a single centrifugal stage. The Lycoming T53 as well as the Allison T63 both feature a multi-stage axial compressor followed by a single stage centrifugal. This design takes advantage of the strong points of both compressor designs. To further improve efficiency, some engines use a split compressor, where there are actually two separate compressors, spinning on completely independent spools. Each compressor spins at its own optimum speed, driven by a separate set of turbine blades. The compressors are then usually referred to as a low and high pressure compressors. The Rolls Royce Gem has a four stage axial low pressure compressor, followed by a single stage centrifugal high pressure compressor. The Rolls Royce Olympus, which powers the Concorde, also features a split compressor, although it is completely axial. Finally, the Pratt & Whitney PW100 series of engines feature a two stage split compressor that is completely centrifugal.

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Burner

The burner is the section of the engine where air and fuel are mixed and ignited to create heat energy to drive the turbine and to provide usable power. A gas turbine engine is a continuous combustion device. As long as the engine is running, fuel is continually being burned. During engine start up, some outside ignition source, such as a spark or a pilot burner, must be present to start the combustion process, but once the engine has accelerated up to self sustaining speed, the flame will keep itself lit as long as there is fuel flow, and the ignition source can be deactivated. The burner consists of an outer combustion liner and an inner combustion liner. Compressor discharge air first enters the outer combustor, surrounding the inner combustor in a blanket of air that is designed to conduct heat away from the metal parts of the combustion chamber, which would otherwise melt from the extreme combustion temperatures, which can get as high as 4,000 degrees F or more. Since thermal efficiency and specific power are directly related to combustion temperature, engineers are constantly striving to make the burner run hotter, increasing the requirements for cooling and increasing the need for exotic, high temperature materials. Actually, only a very small portion of the compressor discharge air gets used in the combustion reaction; typically around 20%. The rest is used to cool the combustor and turbine. Air enters the inner combustor via a number of holes in the liner. The combustor is divided into three sections; the primary zone, the secondary zone, and the dilution zone. Fuel is continuously injected at the front of the burner. In the primary zone, air is admitted and recirculated to create a stable low pressure area that acts as a continuous pilot for the whole combustor. Air that enters through the secondary holes feeds the flame to accelerate the reaction and increases the temperature to the highest levels, to assure the most complete combustion of the fuel and reduced hydrocarbon formation. The dilution zone is where the large air mass is mixed with the combustion gas, to put out the flame in the rear part of the burner and cool the gases down to a lower localized temperature which is around half the maximum temperature in the primary and secondary zone, assuring that the turbine nozzle and blades don't melt, at the same time increasing the average temperature of the air mass flow to create the high volume gases to drive the turbine. The high energy gases are accelerated through the turbine nozzle where they will impinge on the turbines.

There are essentially four types of burners. The can type burner, the annular burner, the can-annular burner, and the toroidal, or scroll type burner. The can burner consists of one or more cylindrical "cans" with an outer wall and an inner wall which features the combustion holes. An annular burner is a continuous dual walled ring surrounding the engine rotor shaft. The combustion holes are on the inner wall, which surrounds the inner combustor. A can annular burner features multiple can burners for the inner combustion chambers, sitting inside of an annular outer combustion chamber. These burners can be what is referred to as through-flow or reverse-flow. The through flow design is favorable for aircraft because it allows the engine to have a smaller frontal area, but it also takes up more engine length. Much shorter and compact packaging can usually be realized by utilizing a reverse flow layout, with attendant increases in efficiency under most circumstances, due to more favorable mixing of air and fuel due to the counter-flow. The toroidal burner is a somewhat unique design; one which was used to good effect on many of the automotive turbine designs of the late 60's and 70's. The combustor is a round scroll or ring that sits around the gas producer shaft in between the compressor and turbine. The burner is fed air from holes in the scroll, in a reverse flow manner, while fuel is actually sprayed out through tiny nozzles in the rotating gas producer shaft. The hot gas is ducted 90 degrees out to the turbine nozzle. Advantages of the scroll combustor is an ultra compact design and slightly higher efficiency, due to the excellent atomization of the fuel by the spinning action of the gas producer shaft and the natural reverse flow effect of the airflow and the fuel flow opposing one another. A major disadvantage is the complexity of plumbing the fuel from the fuel controller to the center of the rotating engine shaft. The Textron Lycoming AGT 1500 Engine in the M1A1 Abrams Main Battle Tank is an example of an engine with a scroll combustor.

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Turbine

The turbine is the section of the engine where the heat energy is converted into mechanical energy, to drive the compressor, the engine accessories, and the engine load on a turboprop or turboshaft. A turbine can be single stage or multi-stage, depending on how much power it is required to absorb from the gas stream, but typically a gas turbine engine will have far fewer turbine stages for a given number of axial compressor stages, due to the fact that the high energy gas expands across a turbine stage much more naturally than the air can be compressed by a single compressor stage.

There are basically two types of turbines; the axial flow and radial flow turbine. Axial flow turbines are much more common, while radial flow turbines are rarely used, and only on very small engines.

A radial turbine is similar to a centrifugal compressor working in reverse. High velocity gas flow travels radially from the outside toward the center of the turbine wheel, impinging upon vanes and causing the wheel to rotate. The gas expands and collects at the eye of the radial turbine wheel, where it ultimately travels out of the exhaust duct.

An axial flow turbine is similar in appearance to an axial compressor. There are two types of axial turbine blades, although most turbine blades are actually a mixture of the two types. An impulse turbine has concave u-shaped blades often referred to as buckets. High velocity gas passes through a stationary set of blades referred to as the turbine nozzle vanes, which redirect the gas flow so that it strikes the concave section of the buckets almost directly. Each axial turbine stage is preceeded by a stationary set of nozzle vanes, to accelerate and turn the gas flow so that it strikes the buckets with the most force. The other type of blade is the reaction blade. Once again, the gas is turned by the nozzle to strike the blades, but it does so in more of a true axial direction. The reaction blades are designed so that when the gas strikes them, the gas molecules are deflected in a particular direction. This deflection of gas causes a reaction jet effect, which tends to turn the turbine wheel in the opposite direction to the gas flow, in much the same way a child's pinwheel spins when air is blown through it. Most axial turbines are a combination of the impulse and reaction type. The blades are blended so that the tips are reaction type blades while the roots are impulse type.

Some turbine engines feature variable geometry turbine nozzles, which automatically change position to control the way in which the gas strikes the turbine blades, while also controlling the effective nozzle area. Controlling the nozzle area allows for higher turbine inlet temperatures to be maintained as engine power is decreased, which can greatly improve engine part power efficiency. The variable nozzles can also greatly improve engine acceleration and even provide a great degree of engine braking. This feature is particularly useful when using a turboshaft engine to drive a variable speed load, such as an automobile or a tank.

In most cases, turbine nozzles and blades must be cooled somehow, or the high temperatures of the engine would melt or damage the blades, especially in modern engines which are running incredibly high burner temperatures. Cooling air comes from the compressor. As stated earlier, a majority of the air that enters the combustor is used to cool the combustion gas down, as opposed to actually being used in the combustion itself. In active turbine cooling, some of the compressor air is bled off before it reaches the burner and is circulated to the turbine blades, where it can provide additional cooling. Transpiration cooling is one modern technique for cooling the turbine blades and nozzle vanes. Compressor bleed air is actually circulated through the inside of the vanes and blades, and the air is made to escape through thousands of tiny pores in the skin of the blade. As the air escapes, it provides an insulating blanket of air around the blade, directing heat away from the skin and allowing for higher turbine inlet temperatures to be used for greater thermal efficiency.

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Additonal Gas Turbine Components

Apart from the compressor, burner, and turbine, there are many other systems involved that make a turbine work. Like any engine, there are auxiliary systems, such as the fuel system, the lubrication system, and electrical system, a starting system, etc. These systems are almost always driven by the engine's accessory gearbox. The accessory gearbox is a geartrain which is always driven off the rotation of the gas producer spool. Various gear sets in the accessory gearbox drive output pads to which the accessories are mounted. The oil system can be of either a wet sump or dry sump design. An oil pump will circulate oil to the engine's main bearings. Then, either a scavenger pump or gravity will return the oil back to the oil sump. The fuel system on a gas turbine engine is usually fairly complex. It consists of a number of pumps and some kind of fuel control unit. FCU's can be either mechanical, electronic, or a combination of the two. The FCU controls power output by varying fuel flow to the burner. FCU's are equipped with complicated engine speed governors that are designed to maintain a desired turbine speed based on power setting, engine load, ambient temperature and pressure, and a host of other factors. The electrical system consists of a generator, which sometimes also doubles as an electric engine starter motor, and an ignition system which is designed to begin the continuous combustion cycle. The ignition is typically only needed during engine start. After that, the burner will remain lit on its own. Hydraulic pumps and external air pumps are also sometimes driven off of the accessory gearbox.

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Exhaust System

The engine exhaust is treated differently depending upon the type of gas turbine engine that is being used. In a turboshaft, the exhaust is essentially just waste heat. Thrust is usually not desirable in a turboshaft, so a diffuser is used to slow down the velocity and kinetic energy, and let it vent to the atmosphere. In some turboshafts, the hot exhaust is channeled through a heat exchanger which is used to pre-heat the compressor discharge air, so that less fuel is required to be burned to achieve a target turbine inlet temperature, thus saving fuel. These heat exhangers are referred to as recuperators or regenerators. In a turboprop, the exhaust is usually ducted so that its energy can contribute some thrust. The exhaust duct is sized to maintain the relatively high exhaust gas velocity. In a turbofan, the core exhaust is also used to provide some portion of thrust, as little as 10% to as much as 90%, depending upon the bypass ratio. At any rate, it is allowed to escape at high velocity and mix with the fan discharge, or bypass air. Some turbofans mix the hot and cold exhaust streams in a jet pipe before expelling it out of a nozzle, while others exhaust the hot and cold streams individually. In a turbojet, the exhaust gas is collected in a jet pipe, and then forced to accelerate through a nozzle, to maximize thrust. Some turbojet engines have variable area jet nozzles. At low power settings, the nozzle area is large, so that the engine does not produce residual thrust. As power increases, the nozzle area decreases to further increase jet velocity and thrust while maintaining a high turbine inlet temperature. If an afterburner is used, the nozzle area must again increase, to allow the much greater volume flow to escape the jet nozzle unrestricted.

Some shaft and prop engines feature exhaust heat recuperators or regenerators. A recuperator is essentially an exhaust heat to intake air heat exchanger. The compressor discharge air is made to circulate through numerous ducts in the recuperator, as exhaust gas is allowed to flow around these ducts. This transfers some of the heat of the exhaust gases to the compressor air. Because the air is higher in temperature when it enters the burner, less fuel is required to be burned to develop the same burner pressures and turbine inlet temperatures, thus resulting in a fuel savings, especially at part load, where turbine inlet temperature can be maintained near peak while compressor rpm and mass flow decreases. A regenerator is a rotating ceramic honeycomb disc, driven at very low rpm by the high pressure spool. As a given part of the regenerator disc rotates toward the rear of the engine, exhaust is made to pass through the honeycomb elements, heating up the disc. That side of the disc then rotates back toward the intake side, where compressor discharge air is made to circulate through the disc, cooling the disc and heating the air, again improving fuel economy. This type of heat exchanger was favorable in automotive applications because of its more compact size, and its inherent self cleaning capability.

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STARTING A GAS TURBINE

Starting a gas turbine is somewhat different than starting a piston engine, although the basic operations are the same; they are just carried out in a different way. Unlike a piston engine, a gas turbine engine does not spring to life when provoked by the starter and ignition system. A gas turbine must be spooled up and allowed to accelerate to a particular speed before fuel can be introduced and ignited. Even after ignition, the engine still is not producing enough of its own power to accelerate to idle. Fuel must be carefully metered while the starter continues to accelerate the engine to a point where the engine can then continue to accelerate itself up to a governed idle speed.

Requirements for the starter motor are also very different. A piston engine's starter motor requires a quick and sudden burst of starting torque to move the crankshaft and begin the combustion cycle. A turbine engine, on the other hand, should have a very soft initial engagement, and motor torque should then build as rpm builds, to overcome the increasing compressor load. The turbine engine starter must be able to provide steadily increasing and then decreasing power over a longer period of time.

Gas turbine engines use many types of starters. The most common starter is the pneumatic starter. The pneumatic starter is basically a compact radial inflow turbine that is spun up to very high rpm through a high volume air source. Reduction gears convert the high rpm into high torque to spool up the engine. Electric starters, jet fuel starters, hydraulic starters, hand crank starters, combustion turbine starters, and combination starter generators may also be used. The most basic type of turbine starter is impingement starting. In this method, very high pressure air is injected through special nozzles that allow the air to impinge directly onto the engine's turbine, causing it to spool up. Immense quantities of air at very high pressures are required for impingement starting.

An ignition source is also needed during starting. Normally, this ignition source is only active while the starter motor is energized. Once the engine has reached self sustaining speed, the ignition source is no longer needed, as combustion is continuous and self-sustaining. Typically, a high energy spark plug is used as the ignition source, although some engines use glow plugs, or a torch igniter, also referred to as a pilot burner, or any combination of the above. A pilot burner is a very small stream of fuel that is sprayed into the combustor and ignited by a low energy spark or glow plug, as soon as the starter motor is energized. The fuel spray immediately ignites into a small flame, which then becomes the ignition source to light off the main fuel flow when it is introduced into the burner.

In most cases, the igniter is de-energized once the engine is running. However, there are certain applications where a low energy igniter runs continuously to pre-empt a flame out. Some modern engines have an auto-relight feature, which automatically fires the igniter in the even that a sudden loss of combustion temperature or pressure occurs, indicating a flameout.

Starting a typical gas turbine engine, such as the Allison T63-A-700, goes something like this:

1. The electric fuel boost pump must be activated to provide the fuel controller and fuel pump with a source of pressurized fuel.

2. The igniter must be enabled, so that when the starter circuit is closed, the igniter will begin to produce a spark.

3. The starter circuit is closed, and the starter motor and ignition unit are energized. The starter motor begins to accelerate the compressor while the igniter begins to spark continuously.

4. At lightoff speed, between 12% and 15% N1, about the same time that the starter peaks and cannot accelerate the engine any further on its own, the throttle lever is moved from the cutoff position into the ground idle position. This fully opens the fuel cutoff valve, immediately upstream of the fuel metering valve. The metering valve has been in the minimum flow position, and so it introduces a relatively small amount of fuel to the fuel nozzle in the burner. The atomized fuel spray is immediately ignited by the hot spark of the igniter.

5. The operator continues motoring the starter, and the compressor continues to accelerate beyond lightoff speed. At this point, the combustion gases are contributing minimally to the acceleration of the engine; the starter motor is still providing a bulk of the starting torque. When the engine reaches approximately 25% N1, due to the increase in airflow, the acceleration limiter allows the fuel metering valve to open further than the minimum flow position, and the engine begins to accelerate more quickly. Turbine inlet temperature climbs very rapidly, and must be monitored carefully. At this point, the combustion gases are accelerating the engine, and the starter motor is providing less starting torque.

6. At around 50% N1, the engine reaches self sustaining rpm, the point at which the starter is no longer needed to continue accelerating the engine. The starter and igniter can be de-energized. The engine will continue to self-accelerate toward governed idle speed.

7. As the engine approaches ground idle, around 58% N1, the governing valve opens which moves the fuel metering valve in a closing direction, reducing fuel flow as rpm settles into an equilibrium, maintaining a governed 58% N1 idle.

8. The engine is allowed to stabilize momentarily before it can be put to use.

Shutting down the turbine is simple by comparison. The operator moves the throttle lever from the ground idle position to the cutoff position. This action mechanically closes the cutoff valve, which cuts all fuel flow to the burner. The engine then gradually coasts down to a halt.

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MODULATING POWER IN A GAS TURBINE

Controlling the power output of a gas turbine engine is often compared to a captain controlling the engine power on a large ship. The captain does not directly control engine power; instead, he calls down to an engineer in the engine room. The engineer then calculates what parameters need to be changed to meet the captain's request while at the same time being sure that those changes will not over exceed any limits put on the engines or the ship. Then the engineer carries out the required engine power change.

Similarly, the operator of a gas turbine engine is not directly controlling engine power. When he moves the power lever, he is essentially making a request for a certain level of power. Then, either through a complex mechanical control system, an electronic control system, or a combination of the two, the change is made in such a way that the engine will not exceed certain limits such as temperature, speed, and/or torque.

From the most basic standpoint, the amount of engine power being produced is a function of gas producer rpm. The higher the compressor rpm, the higher the air mass flow, pressure ratio, along with an increase in the turbine inlet temperature, and ulitmately, the more power is being produced. Gas producer rpm is modulated by increasing or decreasing fuel flow to the burner. Increase the fuel flow to the burner, and the gas producer rpm will increase. In some of the earliest jet engines, engine rpm, and therefore power, was controlled by a fuel valve directly connected to the throttle lever inside the cockpit. This arrangement is referred to as a full authority power control, and engineers and pilots alike quickly learned that this wasn't a safe or effective way to control a gas turbine engine. If the pilot moved the throttle lever too quickly in either direction, he could either over temp the engine or cause it to flame out. The pilots had to move the power lever very smoothly and very slowly, which was sometimes difficult in the heat of combat. With a full authority control, it was also very difficult to set a steady engine power setting, as engine rpm would have a tendency to slowly creep upward or downward.

These problems led to the development of a system called the fuel control unit, or FCU. The earliest fuel control units consisted of a complex set of mechanical linkages, flyweights, and springs connected to a fuel metering valve. The FCU was incorporated into the engine driven fuel pump, and was driven by the fuel pump drive on the accessory gearbox. An FCU basically consists of an engine speed governor, to maintain a set speed based on power lever position, and an acceleration limiter, to control the rate at which fuel flow is increased or decreased, to prevent overtemp, flameout, or even compressor surge or stall. As technology developed, Further improvements were incorporated, including bimetallic temperature compensators, and pneumatic bellows systems to compensate for varying ambient pressures and temperatures, as well as to make for more precise governing and acceleration limiting. Eventually these hydromechanical fuel control units were supplemented with electronic systems to sense temperature, compressor discharge pressure, compressor surge, and countless other parameters. This gradual shift toward electronic control finally led to the FADEC systems that are used on just about every modern gas turbine engine in production today.

FADEC stands for Full Authority Digital Engine Control, and it is essentially a fully electronic fuel control unit. A full authority fuel metering valve controls all fuel flow to the engine, but the fuel valve is actuated by an electric servomotor, which receives all of its commands from an electronic processor. The electronic processor is constantly monitoring hundreds of engine parameters including rpm, temperatures, pressures, torque, as well as power lever position. When the operator moves the power lever, the FADEC makes numerous calculations and then increases or decreases fuel flow to modulate engine power. Because so many parameters are constantly monitored, the FADEC can make engine power changes more precisely and more rapidly than the old hydromechanical systems. The incorporation of the FADEC system has improved gas turbine engine response, time between overhaul, and fuel consumption dramatically.

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Required Reading:

Aircraft Gas Turbine Engine Technology, by Irwin Treager

Jet Engines: Fundamentals of Theory, Design and Operation, by Klaus Hunecke

Gas Turbine Theory (5th Edition), by Herb Saravanamuttoo

 

 
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