JET ENGINE FUNDAMENTALS

a bullet he is fired from a gun and the gun kicks back a jet engine works like that a stream of water is aimed at the heart of a fire and the hose pushes back against the fireman a jet engine works like that and remember as a kid how you blew up balloons and let them sail across the room a jet engine works like that what do these seemingly unrelated things have to do with a jet engine they're all related they and the jet engine are all in the same family because each one reacts to

an action of its own creation Isaac Newton spelled it out for us to every action there is an equal and opposite reaction the air rushing out of the balloon was the action the balloon darting the other way was the reaction when the water surged from the firehose that was the action as a while the hose pushed back in the opposite direction that was the reaction when the bullet sped out of the gun and that was the action at the same moment the gun kicked back that was the reaction a jet engine works on exactly the

same principle - every action there is an equal and opposite reaction a jet engine expels a continuous high velocity stream of exhaust gases that's the action as a result the engine thrusts forward that's the reaction and now you know why jet engines and rocket engines - are often called reaction engines and you can see why the power of a jet engine is expressed in pounds of thrust rather than horsepower like your car engine now how much thrust will a jet engine developed let's look at it this way if we take the design engineers thrust equation

and put it in its simplest form it boils down to this thrust equals mass times acceleration or T equals M times a mass is a measure of the quantity of matter in this case the amount of air going through the engine acceleration is a change in velocity or how much the air is speeded up as it passes through the engine these two factors then are the main ingredients for thrust the greater the mass of air flowing through the engine the greater the thrust the greater the acceleration of the air the greater the thrust hang on

to this T equals M times a concept we'll find it's a handy tool to help us understand why engines are designed as they are now before we look at real engines and real parts let's spend just a few minutes building a very simplified engine so we can see with the major engine sections r4 we'll call it our simplex mark one jet engine picture the engine is being shaped like a big long metal tube this will form the outer case for our engine let's slice it right down the middle so we can look at the engine

in cross-section a jet engine literally lives on air it breathes huge volumes of air per minute it draws air slowly in the front of the engine and jets it out the rear at a high velocity thereby producing thrust it does all of this by burning a combustible mixture of fuel and air so with that in mind the first thing we'll install is the burner section metal lines bring fuel into the engine and specially designed nozzles turn the stream of fuel into a fine spray so it will mix thoroughly with the air edie igniter plugs is

somewhat like the spark plugs in your car when they spark we have ignition and then continuous combustion once ignited combustion continues as long as fuel is supplied the hot expanding gases escape out the rear of the engine now we have the beginnings of what we need for our thrust equation to work we have a mass mostly air and a little fuel and the burning process accelerates the mass but that's just the beginning of the story next we need a compressor we'll reshape the engine case to accommodate one that is made up of several stages each

stage looks something like a glorified blade assembly from an electric fan except that our compressor stage has many small short blades mounted around the edge of a central disc if we install it right here we'll have the first stage for our compressor now put in a second third and fourth stage bolt them all together so they'll rotate as one unit and we have a four stage compressor rotating at high speed it draws air in from the front and compresses it stage by stage it forces the air to flow into a progressively confined space until finally

it delivers high-pressure air to the burner section why have a compressor to force huge quantities of air through the engine to provide a big em in our equation and as this high-pressure air rushes out of the compressor it contributes to the a and with air under high pressure we can burn more fuel release more heat energy to get more power and do it efficiently now we need to find a way to turn the compressor to rotate it at high rpm we'll do that job with a turbine think of a turbine stage is having a disc

and blades much like the compressor only the materials are much tougher back in this very hot section of the engine and the blades are shaped differently for the job they'll have to do we'll install a single stage turbine right here behind the burner section then we built in a shaft to connect the turbine directly to the compressor so they will rotate together at the same speed the secret here is that the high velocity gases from the burner flow past the turbine blades causing the turbine to rotate like a high-speed windmill thereby causing the compressor to

rotate does this have any effect on our thrust equation you bet it does the turbine by extracting so much energy from the hot escaping gases decelerates the flow of gases a great deal which reduces the a can't be helped though we need the turbine to drive the compressor by the way if anyone tells you that a turbojet engine has only one main moving part just say sure if you mean the combined compressor turbine and shaft you're right before we move on we better install an inner case in the burner section to isolate the turbine shaft

from the combustion area next we'll change the shape of the tube to form an exhaust duct that will provide a smaller opening a nozzle at the very end remember how the nozzle on your garden hose works what adjusted right it puts out a law high-speed stream of water that's how the exhaust duct works because of its shape it speeds up accelerates the hot gases just as they leave the engine that increases the a in our thrust equation to make our machine perform as a jet engine by now I'm sure you realize that air doesn't flow

through the center of this tubular shaped power plant rather it flows through the annular space in the compressor burner and turbine sections from there it all joins together in the exhaust duct with that understanding we can now add the final touches to our engine let's put an inlet section on the front the stationary vanes and nose cone will streamline the four there as it enters the compressor and we'll add a tail cone behind the turbine to smooth the flow of gases in this area too now our engine is in pretty good shape but we have

a design problem to consider see all that flame passing across the turbine blades and into the exhaust duct that won't do it's inefficient and awfully hard on the parts we'll solve that by making two changes watch what we can do first we'll reshape the inner case in this burner section like this second we'll reshape the outer case like this now we have created the diffuser section of the engine and look what it does to the path for flowing air and gases air leaving the compressor flows into an enlarged space which slows the air down this

permits plenty of time for combustion to take place in the burner section where it ought to be notice to the outlet of the burners section the gases leaving the burner are squeezed back into a more confined space speeding the gas back up again as it flows into the turbine now our simplex mark 1 is complete and ready to go with all its major sections inlet compressor diffuser burner turbine and exhaust duct an engine of this type is often called a turbojet and you can see where it gets its name turbo it utilizes a turbine and

jet the exhaust duct jets the hot gases out through the nozzle at high velocity but then many people call it a gas turbine engine because it functions with a turbine that is driven by hot gases rather than steam or water as in some other power systems is a turbojet engine really this simple well so far we've studied the major sections of the engine but within those sections are more parts and features that need to be understood so we'll go to something more realistic a schematic of a typical turbojet engine taking first things first we'll start

at the front and work our way through the engine until we get to the exhaust nozzle so how do we get the air to enter the engine smoothly and without turbulence we do it with a well designed inlet section it's a relatively simple stationary section the outer case in your case and Inlet guide vanes are the main structural parts it has to be fairly rugged though because the bearing that supports the front of the compressor is mounted within the inner case the nose cone also contributes to the smooth flow of incoming air note that the

inlet guide vanes of a cross section that is airfoil shaped we will find many airfoil shaped parts throughout the engine some are shaped like the wing of a plane others are just streamlined shapes they are all designed to make the airflow react in some way the inlet vanes have a slight twist to turn the incoming air at an angle so that it will enter the rotating compressor efficiently remember the compressor is rotating at high rpm we're beginning to see the jet engines have a language all their own park names operating terms slang expressions acronyms abbreviations

I'll be careful to use the right language and will point out significant terms as they come along for example rpm we all know that means revolutions per minute but igv the abbreviation for Inlet guide vanes now for the compressor and the big job it has to do draw in huge volumes of air compress it to 250 maybe 300 pounds per square inch or more tolerate temperatures of 6 or 700 degrees Fahrenheit withstand the centrifugal force created when it rotates at high rpm and above all handle the air smoothly without turbulence as it does all these

things our simplex Engine had only a four stage compressor rotating within a tubular case a real compressor is a bit more complex than that the compressor actually has two major components the rotor the rotating part and the stator the stationary part when combined they form the complete compressor assembly let's look at the makeup of the rotor first it's shown here standing vertically with the front rusting on the workbench the rotor is made up of several stages which are numbered from front to rear with the rotor disassembled the major parts are laid out for display you

can see that each stage is comprised of a disc with many blades fastened around its outer rim notice that the discs are open at the center to reduce their weight flanges on a disc like this projecting forward and rearward provide proper spacing between the disks the blades are retained in dovetail shaped slots they fit rather loosely when cold but during operation with temperature up to normal and some physical force tugging at them they fit snugly in place the number of blades the size and their shape varies from stage to stage because each stage has a

slightly different job to do in the overall scheme of compressing the air see that wing like shape I spoke of and notice how thin the blades are especially out toward the tip one very obvious point the longest blades are in the front the shortest at the rear because the air is being packed into a progressively smaller in smaller space look how short and small they are in the last stage however in some of the forward stages the blades are so long that they require mid span shrouds to provide blade to blade support this prevents blade

vibrational problems with the discs all bolted together they form the complete rotor that rotates as one unit hubs at the front and rear are supported by bearings as you can imagine the compressor rotor must not be out of balance individual rotating parts the disks and hugs are first balanced then each disk wood explains installed is and finally after assemble the complete rotor is carefully balanced now back to the stator the stationary element of the compressor the stator is also built in stages each stage consists of an outer case inner shroud and many airfoil shape vanes

there's that word again you can see the wing like contour of the vanes where they are attached to the outer case a stator stage fits behind or downstream of each rotor stage for example the first stage rotor and the first stage stator and the fourth stage rotor and stator a stator stage has a two-fold job to do first to receive the air from the rotor in front of it and redirected at the proper angle into the rotor behind it second to diffuse the air as it passes through these veins slow it down without losing any

of the pressure being developed because of the stators we find that the air doesn't swirl round and round as it is compressed in fact it flows just about straight through which brings up another term we ought to remember this is called an axial flow compressor because the air flows parallel to the axis of the compressor for the same reason we refer to this type of power plan as being an axial flow turbojet engine to maintain this axial or straight through air flow it is important that the air doesn't circle back around the tips of the

blades and veins there are two important features to prevent this the rotor blades operate in very close proximity to the surrounding state here case and the stator inner shroud forms an air seal with a spacer that fits the between disks the last stage of stators called the compressor exit vanes directs the air straight back into the diffuser section these vanes are often located in the diffuser case rather than in a separate stator case the stator cases will all join together form the stator assembly which is the main strength the backbone of the engine in this

section now how much air pressure will the compressor developed well if it increases the pressure of the incoming air by 20 times we say it has a pressure ratio of 20 to one with a ratio like that operating at sea level it would boost the incoming air to almost 300 pounds per square inch compressors today have pressure ratios of 20 to 1 or better another point the highest pressure within an engine of this type is always at the compressor exit okay next up for discussion is the diffuser section it's made up of an outer case

and inner case which are connected by several streamline struts it's very simple in its design but it has important aerodynamic and structural roles to play compressor air flows through the annular space between the outer and inner cases but notice the shape of this passage this brings us to another law of nature that's put to use in jet engine design when air flows through a convergent duct cone shape like this it increases the velocity of the flowing air speeds it up a divergent duct has just the opposite effect slows it down the full path in the

diffuser is divergent air leaves the compressor at the correct pressure but at a velocity that is too fast or complete burning in the combustion section so the diffuser has the sole purpose of slowing the air down to an acceptable speed structurally the diffuser must be good and tough the outer case forms a continuation of the complete engine case the inner case houses the bearing that supports the rear of the compressor rotor the diffuser struts in addition to being structural members straighten the airflow on its way to the burner section the burner section has the more

official designation of combustion section so we'll label it just that remember it's job is to create rapidly expanding hot gases all rushing toward the rear of the engine for two purposes to turn the turbine and then jet out through the exhaust duct the main structural members are the outer case and the inner case again the annular space between is where the action takes place however we don't just burn fuel in that open space like we did in our simplex Mach one we need a way to regulate the burning process to provide truly efficient combustion we

do this by burning the mixture of fuel and air within several individual burner cans they're placed side by side to form a circle of burner cans within that annular space the number of burner cans varies for different engines we will say that our typical engine has eight a cluster of fuel nozzles supplied by a fuel manifold introduces fuel into the front of each burner can in a spray pattern for rapid and thorough mixing with the air combustion is complete and the flame is confined within these burners fans but even with the use of special metal

alloys the cans have to be specially designed to prevent the metal from being destroyed the air which of course is essential for burning flows from the diffuser into the annular space and then enters each burner can through a myriad of holes the holes vary in size shape and spacing so that the actual flame is centered in the middle of the can by the incoming air and does not actually touch the metal walls of the can the temperature of that flame by the way is 3500 degrees Fahrenheit or hotter about 1/4 of the air is directed

into the flame area for combustion the other 3/4 is directed along the wall of the burner can to provide a blanket of cooling air burned and unburned air mixed together at the burner can exit substantially reducing the temperature ignition to start the burning is provided by igniter plugs in two of the burner cans the burner cans are interconnected by small flame tubes so that during the starting process flame started in two of the cans developed rapidly in all the others once started burning is continuous during engine operation and it's not necessary for the igniters to

continue firing the burner cans are supported at the front by brackets attached to the eighth diffuser struts they are supported at the rear by an outlet duct that provides a transition from eight circular openings at the front to a single annular opening at the rear notice that the outlet duct forms a convergent flow path this causes the rush of exhaust gases to accelerate even more as they flow into the turbine section and now we've arrived at the hottest section in the whole engine the total output of the burner impacts directly the turbine temperatures here may

well exceed 2000 degrees if the best in metallurgy was ever needed this is the place so let's look closely at the makeup of the turbine section the burner case and now the turbine case continue to form the outer shell strength in these areas and by the way you can always recognize a turbine case because of its unique shape see how it overlaps the burner section that Ross voiced shape provides the convergence we need at the burner exit and the in large diameter at the rear as we'll soon see accommodates the size of the turbine rotors

now we've set it before the turbine has a single-minded purpose to rotate the compressor so it can do its job and like the compressor it also has a rotating and stationary element the turbine rotor spins like a glorified windmill as the exhaust gases flow past fact is it extracts about 3/4 of the hot gas energy just to drive the compressor being joined to the compressor by the long turbine shaft the turbine and compressor both spin at the same high rpm an engine like this will run at about 10,000 rpm or higher each rotor stage or

turbine wheel as they are sometimes called is made up of a disc in blades much like we found in the compressor the unusual shape of the blades especially the great curvature is designed to convert the energy of the gas flow into rotational power you will find that the longer turbine blades frequently have tip shrouds to prevent blade vibration problems and also to reduce gas flow leakage around the tips of the blades some turbine blades because of the tougher and therefore heavier materials required are retained in the disk using a fir-tree base rather than the dovetail

shape we spoke of before now how about the stationary element of the turbine each stage consists of many vanes attached to the turbine case these nozzle guide vanes as they are called precede each rotor stage nozzle guide vanes good name for them they guide the hot gases at the most forceful angle against the rotor blades and since the openings between the vanes form a series of small convergent nozzles they speed the gas up as it flows against the rotor you'll hear people speak of a complete assembly of these vanes as being a turbine nozzle our

engine then has a first stage turbine nozzle and a second stage turbine nozzle are you beginning to see that while the turbine looks similar in some respects to the compressor it is really quite different the compressor is forcing the air pressure to rise by packing it into a smaller and smaller space the compressor can only do this in small easy steps our typical engine has nine stages the turbine however is permitting the hot gases to escape exchanging this source of energy for rotational power turbines can do this in large steps ours has two stages turbine

materials are different - they must be tougher and highly heat resistant in many of our engines the forward stages of the turbine have air-cooled blades and vanes so they can tolerate these extreme temperatures relatively cool air is routed from the compressor through special passages and then right through these hollow blades and vanes the escaping air then Joe into the mainstream of exhaust gas have you wondered why the second stage turbine is larger in diameter than the first stage no matter how many stages in a turbine each succeeding stage is larger because it has less hot

gas energy to work with put simply the windmill has to be larger because there is less breeze to turn it to more points and our turbine is complete turbine exit vanes straighten the airflow as it passes into the exhaust duct and provide the structure to support the turbine rear hub bearing and just as in the compressor you'll find the need here to prevent flow leakage around the tips of the blades and vanes special air seals and minimum blade tip clearances are part of the turbine scheme too let's take a final look at the turbine by

reviewing the stage numbering first stage nozzle first stage rotor second stage nozzle second stage rotor and exit vanes last comes the simplest part of the engine the exhaust duct because of its convergent shape it accelerates the gases to the highest possible velocity just as they leave the engine because of this jet affect the final opening in the duct is called the exhaust nozzle the nozzle is sized and measured carefully in square feet of opening to provide the exact gas velocity required to produce a specified thrust a tail cone streamlines the inner boundary of the gas

flow and now our engine is complete we've covered a lot of ground in short time so let's summarize by reviewing some of the highlights the tubular outer cases of the engine when all joined together at their bolted flanges form the main strength of the engine the one main rotating part of the engine is the combined compressor shaft and turbine all rotating at the same rpm this huge mass of rotating machinery is supported by bearings in the inlet case diffuser case and turbine case air at atmospheric temperature and pressure enters the inlet at a relatively low

velocity the inlet section ensures a smooth entry of air into the first stage of the compressor the compressor draws in a huge mass of air and compresses it to a high pressure the diffuser slows the air down while retaining the high pressure the burner increases the velocity of the air by adding the energy of burning fuel the turbine extracts energy from the hot gases as they rush by and converts it into rotational power to turn the compressor the exhaust duct and nozzle boosts the gases to a final high velocity so you see air and gas

don't flow through the center of the engine rather they flow through the annular space in the various engine sections now let's take a final look at our thrust equation did it work you bet it did the compressor assured us of a huge mass of air and the total engine created a final high velocity at the exhaust nozzle that gave us the acceleration we needed we'll have to leave it up to our design engineers to pick it up from there to account for the fuel passing through the engine and other important factors needed to arrive at

a real and accurate thrust value the engine we've been studying is typical but there are some variations in the design of turbojet engines you can watch for here are five common ones you should know about first to build more efficient engines we need higher compression ratios often this requires more stages in the compressor suppose we change our nine stage compressor to two sixteen stages and add the necessary turbine stages to power it that looks good but the problem is all 16 stages are rotating at the same rpm and that would result in inefficiency the big

long blades up front are trying to grab a lot of air and pull it in the smaller shorter blades toward the rear are working under pressure and trying to create even more pressure this results in poor engine performance and would likely result in an operating problem called compressor stall this is a situation where some of these airfoil shaped blades actually quit pumping air more on that in a moment the solution to this is a concept called the dual rotor engine it has to mechanically independent rotor systems two compressors each driven by its own turbine the

low-pressure compressor draws in the atmosphere and starts the compression process it's driven by the low-pressure turbine it gets its name from the lower pressures that make it turn the high-pressure compressor boosts the air up to maximum pressure it's driven by the high pressure turbine so called because of a high pressure is delivered from the burner that make it revolve you can see how the one turbine shaft revolves independently inside the other now what does the dual rotor design do for us it has many advantages openers the two compressors operate at different rpms so that each

runs at its best most efficient speed because it handles the air flow more effectively higher compression ratios are attainable and stall problems are greatly reduced structurally the individual parts vanes blades discs and so forth follow the same design as a single rotor engine however this has one additional major part the intermediate case a rugged structure that joins the two compressors and provides bearing support for both the struts straighten the airflow as it passes through obviously the bearing system must be more elaborate in an engine like this the one shown here has eight bearings before we

move on let's resolve a point of terminology on this dual rotor engine here's a situation where we find that people in the jet engine industry have three names for the same thing the low pressure compressor is also called the low compressor or front compressor no confusion the terms make sense the high pressure compressor is also called the high compressor or rear compressor the same trend in language also follows for the turbine and you'll find the dual rotor concept itself called by other names twin school dual axial take your pick they all mean the same thing

since most of our engines today have a dual rotor we'll use this type in illustrations coming up let's digress for a moment and talk about compressor stall what is it it's a momentary condition most often where airflow over the compressor blades becomes turbulent the blades actually stall just as the wing of an airplane might if it approaches the air at too great an angle air no longer flows back through the compressor as it should in fact the air does a complete turnaround while the condition exists the normal a higher pressure at the rear of the

compressor causes the air to actually flow forward it can happen to one blade or many to one stage or several stages to one compressor or both the result of compressors stall is engine surge air flow through the entire engine is momentarily disrupted vibration is likely metal temperatures climb for lack of air cooling combustion is irregular and loud bangs may be heard flameouts can occur sound mysterious like a demon that intrudes in the night not really compressor stall is an inherent potential problem in all high performance jet engines compare it with the piston engine in your

car it has an inherent but different problem with the wrong grade of gas it will ping maybe you call it knock or detonation the point is all gasoline-powered four cycle engines are prone to do this with a high air flows and high pressures demanded of a modern-day compressor compressor stalls and ever-present characteristic that our designers must deal with head-on it can be caused by conditions that drastically disturb the smooth flow of air through the compressor unusual aircraft attitude flying in turbulent air ice buildup in the inlet Jam acceleration things like that what do we do

about it how do we build high-performance compressors and yet avoid this condition we do a number of things the dual rotor compressor we've already discussed that decreases the engine susceptibility to stall a great deal compressor air bleeds provide another answer most all of our engines have these air bleed ports release part of the compressed air overboard during those periods of operation where stall is most likely to occur this relieves the bottleneck to air flow that can cause the stall the bleed ports and they're controlling valves are usually located between the low and high compressors they

are open while starting the engine to let it gain RPM more easily and they're closed during high thrust operation during other periods of performance acceleration deceleration low thrust levels and so forth the bleed valves are scheduled to open or close automatically depending on operating conditions another solution on our later engines lies in the design of the compressor stator vanes up to now we've seen stator vanes that are manufactured at a fixed angle variable angle stator things stator stages in which the vanes can be angled a little more a little less tailor the airflow and pressure

to avoid stall situations a sink ring controlled by hydraulic actuators synchronizes the movement of the vanes they're used in the forward stages of a compressor and it's common to find a similar arrangement used for the engine Inlet where they are called variable Inlet guide vanes VI G V for short for example on our typical engine we find them used at the inlet the first two stages of the low compressor and the first two stages of the compressor these variable Staters are frequently used in conjunction with an air bleed system to prevent compressor stall and like

the air bleeds they are scheduled to function automatically as engine condition the signal the need so three of these variations in design the dual rotor overboard air bleeds and variable angle stator vanes have to do with that all-important smooth steady flow of air through the compressor another major difference in our family of engines is in the combustion section as you'll recall our typical engine had multiple burner cams eight of them where the burning actually took place our more recently designed engines have an annular burner one continuous circular burner you can recognize many of the design

features used in the multiple can set up the brackets for attachment to the diffuser case the fuel nozzles evenly spaced around the forward edge the holes for combustion air and cooling air are used in the same way with the annular burner you look for them on the outer surface and inner surface that notice that the design eliminates the need for the separate rather complex outlet duct required for the eight burner cams advantages compared to the multiple can set up smaller lighter it provides more efficient combustion and yet it takes up less space in some of

our later engines the annular burner requires so little room that the diffuser and burner cases are combined as one now the last of these design variations we want to cover provides us with an entirely different type of compressor rather than the multiple stage axial flow type that we've Illustrated so far this is a single stage centrifugal flow compressor the impeller it's a high rpm with in a closely fitting case the incoming air trapped between the veins of the impeller is pressurized as it is slung outwards and packed into the confined space at the rim from

there it flows through the diffuser and into the burner jet engines seldom use and typical compressors alone usually they are used in combination with the actual flow compressor in this engine for example air is first compressed by a three-stage axial flow compressor and then compressed further by a single stage centrifugal compressor as you have just seen jet engines do vary in design and that's mainly because of the customer's requirements and the mission an aircraft is destined to fly you see the design of each engine is tailored to meet the demands of a specific type of

plane now that we understand the principles used in our typical jet engine we're ready to see how this thing we call a gas turbine engine can be applied in different ways you see the combined sections of the engine that really generate the power the compressor diffuser burner and turbine are called the gas generator the gas generator not only functions on its own but it generates hot exhaust gases energy in the form of heat and pressure that can be used in a variety of ways we've already seen that the addition of a convergent exhaust duct turns

it into a turbojet engine let's take the gas generator now and add the necessary parts to build an afterburning turbojet engine the afterburner is in effect one huge burner can where we burn more fuel to accelerate the gases to an even higher rate to make the a in our thrust equation much larger fighter and interceptor aircraft require that their engines produce more than the normal power for short periods of time takeoff climb may be a burst of speed for combat well the afterburner serves this need it provides thrust increases of 50% or more the amount

varies with different engine models however they're used only for short periods of time because they double or triple the engines fuel consumption while they're in operation the afterburner called a B for short is a large addition to the engine but it's not complicated first off it requires a diffuser case in combination with a tail cone it provides a divergent flow path to slow the exhaust gases down a bit prior to combustion just as in the gas generator the four gases into the afterburner has to be slowed to the extent that complete burning can take place

within the afterburner the afterburner ducked or a main case within which all combustion takes place notice the diameter and length of this thing it's the largest single component in the engine and on engines like this you'll find a variable area exhaust nozzle one that can be scheduled who automatically vary its opening larger or smaller to help create the amount of trust that the pilot calls for circular fuel spray rings several of them each with built-in fuel nozzles they're scheduled so that with one or two rings providing fuel the pilot can get partial power from the

afterburner with all the Rings applying fuel maximum power from the a/b is obtained and of course with this condition the total engine is producing its maximum power the flame holder it sits right in the path of the gasses as they enter the combustion area its purpose to create turbulent gas flow maybe that's why it's the ugliest part of the entire engine after looking at so many smoothly contoured in airfoil shaped parts this is truly a strange looking site it's a network of angular and radial gutter shaped pieces that slow the gases down even more and

provide thorough mixing of a fuel and air so the flame will be held within the afterburner and not be blown out the exhaust nozzle the igniter plug is frequently mounted right in the flame holder as in the gas generator the high-intensity spark is required just long enough to get the fuel and air mixture lit after that the flame is continuous and self-sustaining the liner has a rough-and-tumble job to do in fact it serves a dual purpose our portion of the exhaust gases cooled considerably by the time they leave the sermon flows between the duct and

liner and then passes through thousands of tiny holes in the liner and then into the combustion area this confines burning to the center and helps keep the a be duct cool enough to survive the extremely high temperatures its second purpose is to reduce noise the a B makes a howling or screeching sound in fact this part is often referred to as the screech liner because it is designed to do an acoustical job by reducing the amount of noise the afterburner creates and there you have it the afterburner serves the military pilot well when he positions

his throttle between idle and intermediate the gas generator alone delivers the thrust he needs the afterburner merely acts as a long exhaust duct with the exhaust nozzle serving its usual function when the pilot wants more thrust from intermediate up to max power the gas generator continues to deliver its utmost and the a/b cuts in and contributes to the total thrust produced fuel flow from the spray rings and the exhaust nozzle opening vary according to the amount of thrust called for okay let's look at a second way of applying the gas generator for propulsive force suppose

you managed an airline and your air routes were all milk runs up and down out of small fields all the time you need engines that will provide quick takeoff and rapid climb out what's the best engine for that a turboprop on gas turbine engine that turns a huge propeller notice that the prop is driven by the low-pressure turbine but not directly the low turbine drives the low compressor the low compressor drives a reduction gearbox and the gearbox rotates the prop a gear reduction of about ten to one is typical with that the engine might be

turning at 10,000 rpm and the prop would turn at 1,000 rpm you see props can't be turned at real high rpm if they did those long blades would exceed the speed of sound above which conventional propellers operate inefficiently and are subject to stress beyond their limitations and notice too that the low-pressure turbine in this engine must be relatively larger or have more stages because it has more work to do it has to extract enough of the hot gas energy to rotate the low compressor and the prop a turboprop gets about 90 percent of its propulsive

power from the prop and ten percent from the jet effect of the exhaust nozzle and by the way a propeller produces thrust using that very same thrust equation only this time the prop is handling a huge mass of air but imparting a relatively small acceleration that's why when comparing it to a turbojet engine it has spectacular acceleration and takeoff characteristics and proves to be more efficient at aircraft speeds of 400 miles per hour or less however it doesn't do well at higher altitudes where air is too thin for the prop to perform at its best

what we've just said about thrust is true but you won't find turbo props rated in pounds of thrust like the turbojet engine because most of its power is extracted from the shaft rather than the jet exhaust it is rated in shaft horsepower SHP or sometimes equivalent shaft horsepower es HP which is the power from the shaft plus the equivalent horsepower gained from the small jet effect of the exhaust let's consider a type of engine that performs along side the turbojet in terms of high aircraft speeds and high altitude performance and yet provides a rapid takeoff

and climb that the turboprop is known for this is the turbofan engine is it obvious what the designer was trying to do with this one sure it's a hybrid the best of two worlds it has all the elements of a turbojet but it's also a little like a turboprop in that it has many fan blades incorporated as part of the front compressor this provides a prop like thrust without having the complexity and weight of a propeller and gearbox take a closer look the forward stages of the low compressor have extra-long blades and vanes and they

serve two distinct purposes the inner portion of the blades and vanes function as part of the low compressor in the gas generator the outer part serves as a multitude of miniature propeller blades properly called fan blades they draw in an additional mass of air accelerated and then expel it from the exit of the fan section thereby producing a separate additional source of thrust it takes a lot of power to rotate this low compressor with the added fan blades so the low-pressure turbine must be large enough to have enough stages to do the job so the

gas generator with its jet nozzle contributes to the overall thrust and the fan contributes to how much for a ballpark figure let's say 5050 but it's hard to generalize because the amount of thrust generated by the fan depends on its size and the number of stages some engines have single stage fans our schematic shows two others have more and this brings us to the term bypass ratio the ratio of fan air to gas generator air our PW 2037 commercial engine has a bypass ratio of about 6 to 1 because it's fan handles a great share

of the air if our schematic appears to show that the fan and gas generator handle equal amounts of air then our engine has a bypass ratio of 1 to 1 the fan exhaust is handled in different ways depending mainly on the needs of the aircraft installation on some the short fan case Sean is all that is needed and on others our long fan duct encircling the whole engine carries the fan air back to where it joins with the gas generator exhaust in this configuration the engine is referred to as a ducted turbofan our commercial jt8d

is built like this now if we develop this engine one step further we'll have today's typical military engine the Augmented turbofan the augmenter is nothing new to us yesterday's afterburner is today's augmenter the same thing they just changed names on us the terms are interesting though afterburner describes what it does odd matter tells us why we have it from our discussion on the afterburner I'm sure you'll recognize the fuel spray rings the flame holder the liner and so forth look how the concept is applied to our fan engine when running a throttle settings up to

intermediate the exhaust gases in fan air joined together downstream of the turbines and exit through the exhaust nozzle the engine performs as a ducted turbofan above the intermediate power setting the augmenter cuts in and provides infinite variation in power up to maximum the big difference between this engine and the afterburning turbojet is that the turbofan gets its oxygen for burning from two sources the unburned air and the gas generator Rost gases and the supply of fan discharge air with this design the fan provides an excellent source of relatively cool air to reduce augmenter case temperatures

and permit the burning of greater quantities of fuel the addition of more heat energy to get more thrust the exhaust muzzle yes it is shaped differently than on our previous illustrations remember we discussed how convergent and divergent shapes influence air flow the convergent areas accelerate flow and the divergent decelerate flow well that's true with air or gas that is flowing subsonic Li below the speed of sound flow through the gas generator is subsonic and we've seen how these principles are applied now it may sound strange but this principle reverses itself when air or gas flows

above the speed of sound a convergent duct slows the air down and a divergent duct speeds it up and now we can see how our designers put this knowledge to work using the CD nozzle the convergent divergent nozzle velocities are below the speed of sound Mach 1 until they reach the exhaust nozzle to gain the greatest possible acceleration through the nozzle the convergent section brings the gases up to Mach 1 and then the divergent portion accelerates the gas is even more above Mach 1 the same understanding of airflow is used in designing the aircraft Inlet

that part of the airframe that guides air into the engine with the aircraft flying at Mach 2 two times the speed of sound you may wonder why air doesn't Ram itself through the engine at Mach 2 if it did it would be destructive the engine wouldn't tolerate that so a CD inlet is used in this case to slow the air down the convergent section slows the air from Mach 2 down to Mach 1 at the three and the divergent section slows it even more before it enters the engine just as the exhaust nozzle is scheduled

to vary the size of its opening for varying amounts of thrust the aircraft Inlet is often designed to vary its configuration to accommodate different Inlet velocities okay one more family member you should know about this application of the gas generator is a blood relative of the engines we've seen so far but it doesn't have to fly to her and it's keep this is the turbo shaft or free turbine engine the one that is used for a variety of power requirements this time we mount a free turbine just downstream of the gas generator turbine by free

I mean it's free to rotate independently of the gas generator picture it is if it were a windmill free to spin then the blast of hot gases the output shaft connected directly to the free turbine makes the rotational power readily available for work to be done the exhaust gas its energy pretty well spent in driving the free turbine is directed out and away from the installation to any convenient location these engines are rated in shaft horsepower like the turboprop can you imagine 40 to 50 thousand horsepower available in one package and they can be used

as a power source for a variety of applications turning huge electrical generators rotating natural gas pumps and by the way our engine designs can be adapted to run on a variety of fuels including natural gas for powering boats and ships and for airborne purposes too they're used to power helicopters and when the output shaft drives a prop for conventional aircraft it becomes another form of turboprop turboshaft engine design varies the output shaft doesn't necessarily have to project out from the exhaust end of the engine in this version the free turbine shaft revolves within the gas

generator shaft so that the rotational power of the free turbine is available at the front of the engine as it might be for a turboprop application well you can see now that the gas generator is a very versatile power source it is the heart of every engine we build whether it be turbojet turboprop turbofan or turbo shaft up until now we've been dealing with bare-bones engines just plain engines with no way to run them or control them maybe you've seen all the plumbing wiring valves and so forth on the outside of the engine now we'll

find out what they're for for a comparison to what we'll be talking about look under the hood of your car you'll find a carburetor alternator power steering pump air conditioning compressor etc well the jet engine needs it's accessories to some similar to your car and others are good bit different let's find them a gearbox mounted below the engine provides the rotational power for most of the accessories that must be driven the gearbox gets its rotary power from the compressor as the compressor rotates a pair of bevel gears and tower shaft transfer the motion down to

the gearbox then the gear train within the gearbox comes alive to provide a rotary power to reach accessory drive pad what kinds of accessories are driven fuel pump oil pumps hydraulic pump electrical generator things like that and on some a tachometer generator and fuel control are linked to the gearbox but not all accessories are driven mechanically by a gearbox some accessories can be driven by compressed air from the compressor or diffuser section of the engine for example when of our engines has an afterburner fuel pump that incorporates a small air turbine works on the same

principle as the gas turbine in the engine itself compressed air from the diffuser section flows through the air turbine and that causes the pump to rotate on other engines air pressure is used as the muscle power for controlling the opening of the variable area exhaust nozzle compressor air is also used to drive accessories on the airplane you may hear of a customer ear bleed for example to power an air conditioning system however we can't overdo this business of robbing compressor air for such purposes we have to limit it to a small percentage of the engine

airflow more than this would reduce engine performance too much ok now that we have found ways of powering the accessories we need let's look very briefly at seven major engine systems first of all the lubrication system and what do we have to lubricate the engine bearings the bearings and gears for the tower shaft and the gears within the gearbox the oil used in jet engines is not your usual 10w30 type you might put in your car it takes a special synthetic oil to withstand the extremes from the colder spots around the globe when the engines

are shut down to the high temperatures created when operating at max power when you walk up to an engine the only obvious part of the loop system is the oil supply tank usually a saddle-shaped affair to fit the contour of the engine it's martyred alongside the compressor section up forward where it's not too hot those metal lines you see carry the oil from the tank into the guts of the engine to those bearings and gears and then back to the tank but during this continuous operation several other components in the system get into the act

our pressure pump and pressure regulator to ensure an adequate supply of oil at each lubrication point scavenge pumps that's a good name for them they grab the oil from each of the Lube points after it's done its job and send it back to the supply tank oil coolers remove the heat from the oil it gets mighty hot during that round-trip especially to the bearings back in the turbine area oil filters clean the oil remove foreign particle add a special green port so we can inspect samples of the oil for contamination periodically and our system is

complete second the fuel supply system to start with when you fill up your car you pick regular or premium or maybe diesel these are types of automotive fuels when you hear of jet engines running on JP 4 JP 5 or JP 7 these are different types of jet engine fuels but they're not gasoline jet fuels are in the kerosene family the aircraft stores the jet fuel most often in wing tanks it also has boost pumps and filters to provide a clean steady supply of fuel to our engine the engine fuel pump that's driven by the

gearbox receives the fuel and delivers it under pressure to the gas generator fuel system and after burning engine will have a separate pump for the afterburner fuel supply most likely it will be the air driven type we spoke of earlier third many of our engines require a hydraulic system much like the brake system on your car when you apply your brakes hydraulic fluid under pressure forces the piston to move in the activating cylinder for each wheel brake in a very similar way we use hydraulic actuated cylinders to move things on the engine remember those variable

angle stator vanes hydraulic power can be used to set these at the right position and the variable area exhaust nozzle some of our engines have hydraulic actuated cylinders to provide this movement unusual thing about our hydraulic systems though we don't use the regular automotive or aircraft hydraulic fluid that you might be familiar with instead the hydraulic pump driven by that gearbox picks up jet fuel right from the wing tanks and the fuel becomes our Golic fluid being a circulating type of system the fuel is cycled right back into the fuel system and is eventually burned

in the engine the ignition system number four in the list of systems we need we spoke of the igniter plugs before always two plugs redundant system in the burner section the odd matter may have one or two an electrical generator driven by the gearbox is the usual source of power ignition exciters take this low voltage supply and boost it to provide the high energy spark we need at the igniter plugs v we need a starter we'll talk about how we start the engine shortly the job of the starter is to get the high and low

pressure rotors revolving up to a desired rpm the starter is adapted to the gearbox and its power is applied up through the tower shaft and from there to the high rotor as it rotates and draws air through the engine the flow of air causes the low rotor to rotate there are a variety of power sources for jet engine starters some are electric like your car most are pneumatic air pressure driving a small air turbine is the motivating source number six we need instrumentation the pilot needs enough gauges to tell him that the engine is operating

properly and safely things like rotor rpm fuel flow and pressure oil pressure and temperature and that all-important egt exhaust gas temperature to make sure the turbine isn't exposed to excessive heat but additional instrumentation is needed for another purpose as part of the scheme for controlling thrust the heart of the system is the engines fuel control this is the seventh of our systems and I saved it for now because it ties so many of the other systems together the system for controlling thrust with the fuel control calling the signals is a very complex system because of

the very great demands we put on aircraft and their missions you see in today's jet aircraft the pilot can pay only little attention to his engine his attention must be outside the cockpit as much as possible for that reason they don't give him a lot of knobs to pull whether to twist or switches to flip darn few so the pilots single main control over the engine operation is the power lever or throttle with this one lever he commands the engine to provide the performance he calls for and everything that happens in response to his request

is automatic and this is where the fuel control comes in the pilots power lever is linked directly mechanically or electrically to the fuel control now the fuel control is a brain of sorts besides knowing what the pilot is asking for it senses everything that's going on and that's where the additional instrumentation is needed from instrument probes at strategic points it senses the pulse of the engine the critical temperatures and pressures and things like rpm and fuel flow and moment by moment it sorts all these things out and sets up the conditions by regulating other systems

to give the pilot the thrust he is called for on a typical afterburning engine it schedules fuel flow to the burner section fuel flow to the afterburner variable area nozzle opening the opening and closing of the compressor bleeds and the position of the variable angle Inlet and compressors stator vanes the fuel control even schedules the igniters to fire when the pilot moves the power to start the engine and when he pulls it all the way back for an engine shut down it shuts off the fuel it's like having a genie at his command all he

has to do is ask oK we've got enough to run the engine now we took the bear engine and added seven necessary systems we have the power lever to operate it with and enough dials and gauges to keep track of its operation one more point before we start it up though take a look at the tachometer for engine rpm there are some things that we ought to know here you can expect axial-flow engines to operate in the ballpark of eight to fourteen thousand rpm somewhere in there you may find a tachometer for the low compressor

the high compressor or both in either case they operate at slightly different rpm with the low compressor running slower than the high take a closer look at the tach in the cockpit and you'll find that it's not graduated in rpm but in percent of full rpm the jet aircraft industry agreed long ago they would go this route this means that when the engine is running at the four rpm it was designed to run at the gauge will read 100% rpm with a readout like this the pilot can speak in terms of an over speed the

engine went to 103 percent or the engine should idle at 52% on our test stands however you'll find tachometers of both types those that read in rpm and others that read in percent rpm that business about the idle rpm is another interesting point jet engines don't idle slowly like automobiles no they idle ed 40% rpm or maybe even higher and one last point before we move on did I mention which way our engines turn they are I'll turn the same way viewed from the rear stand at the back of the engine and look forward they

turn clockwise unless you have a digital watch that is okay now let's put our engine on a test stand and see if it will run the cycle of events required to start a jet engine is the same on all engines whether the engine is mounted on a test stand where separate actions may be required by the operator or installed in an aircraft where everything happens automatically in this C level control room these engineers and stand operators monitor engine performance three things must occur in the proper sequence first we need air flow through the engine and

we get that by engaging the starter and bringing the rotor speed up to 10% rpm then 15% and then 20 next the igniter plugs are scheduled to start firing and last as the rpm continues to increase the fuel is scheduled to burn falling to the burner and we get a light off the engine starts to run but we're not done yet the engine is running at a low rpm but not to a point where it will sustain itself so the starter continues to be applied and the combination of the starter and the engine power bring

the RPM on up to its idle of about 60% that's where the engine will sustain operation by itself then the starter is disengaged and the starting cycle is completed it's running now and we can check it out does it require a warmup time no way piston engines require warmup time but not jet engines once that fire gets lit in the burner it heats up in a hurry like it or not oh that's not to say that every bit and piece of the engine reaches its operating temperature in a split second they won't your friends on

the test stand will speak of stabilizing the engine to get accurate test data they'll set the engine at the power setting they want and then leave it there for three or four minutes maybe even more so temperature effects will be complete then they check their data and you'd be amazed at the expansion that takes place when a stone-cold engine is cranked up and then push that up to full power any doubt in your mind as to where the greatest expansion take place sure back in the burner and urban areas and of course the afterburner especially

when it's lit if you work with the drawings or parts for these engines it's interesting to watch poor design features that permit the expansion and contraction when the engine is shut down without causing buckling or fracture of the parts look at this burner can for example the front of it is firmly attached to the diffuser case yet with flame temperatures of 3,500 degrees or more it's going to expand like gangbusters so it's designed so it can expand toward the rear see how it mates with the outlet duct how it can slide back and forth and

check this zband support for the afterburner liner you can imagine how hot this gets when the afterburner is operating if these supporting members were radial like the spokes on a wheel they would buckle when the liner grows larger from the heat instead these even pieces can give a little without braking while we have the engine running let me show you what goes on inside the engine in terms of temperature and pressure these are the numbers we might see if we ran our f100 engine for example at sea level on a standard day quote-unquote I'll explain

that one a little later on first let's look at internal pressures air enters being led at fourteen point seven pounds per square inch that's the air pressure that you and I live in the fan section functioning as the low compressor compresses it to almost 45 pounds and the high compressor brings it up to about 360 pressure drops only a little going through the burner section but drops way down as the turbine converts the energy of the gases into rotational power and it continues to drop as it flows through the afterburner and out the nozzle now

the temperatures with air entering the inlet at 59 degrees Fahrenheit it's a little over 1,000 the compressor exit and that's just from compressing the air the hottest spot in the gas generator just behind the burner or at the turbine Inlet the gasses are over 2,500 degrees we expect the temperature to drop as the gas is passed through the turbine because the turbine extracts energy from the gasses and then temperatures soon back up with the augmenter operating to over 3200 degrees is it any wonder we're continually searching for the best in metals and consider the exterior

parts the components the wiring the plumbing this will give you an idea what the trend of temperatures is on the outside of the engine the cooler sure but now you know where the hottest places are and why they mount these exterior parts up around the compressor and diffuser sections or it's not so darned hot let's get back to that business of testing our engines on a standard day remember we said way back that the amount of thrust developed by a jet engine depends to a great extent on the amount of air the engine is handling

the mass of the air flowing through the engine well it makes sense then that as the weather changes the density of the air changes changes in barometric pressure and temperature change the amount of mass going through the engine and therefore it changes the amount of thrust produced hence with a low temperature or a high parameter the engine will produce more thrust and vice-versa the jet engine industry established 59 degrees Fahrenheit and 14.7 pounds per square inch as being a theoretical standard day for engine testing but we can't sit around waiting for that standard day to

happen so our test engineers run the engines on any kind of day and then mathematically correct the engine data to what it would be if it will run on a standard day now we can calibrate our engines in Nome Alaska West Palm Beach Florida or Timbuktu calibration that's when we run the engine to see if it performs two specified standards the environment has other effects on a jet engine - a jet engine is like a giant vacuum cleaner and it will suck in anything it can dust and dirt if it goes up enough on the

blades and veins it can hurt engine performance with the aircraft running on the ground rocks and debris on the runway get sucked in if you leave a wrench laying in an aircraft Inlet in it goes fog they call it foreign object damage the heavier objects can damage the engine and jet engine people have an eternal vigil to make sure that does not happen on our test stands we install an inlet screen they call it a five screen to prevent such damage but on the airplane everything goes in the inlet of the engine the rain the

snow the ice that brings up the necessity for our Eddy ice system number eight on our list you see with atmospheric conditions just right ice can form on the inlet veins nose cone and even our first stage of the compressor a buildup of ice could distort air flow to the point that compressor stall is inevitable it's easily prevented however with an anti-icing system the system ingredients are simple we have a plentiful supply of hot high-pressure air at the compressor exit in the diffuser case and the nose cone and Inlet guide vanes are hollow does that

make the lights flash and the bells ring add a few feet of tubing and a control valve and one more essential part a nice detection probe in the aircraft Inlet since is the first formation of ice and infants the control valve hot air from the diffuser flows through the valve to the inlet section where it passes through and warms up the nose cone and Inlet guide vanes from there it flows into the main stream of compressor air now ice can't form on these heated parts of the inlet when icing conditions cease to exist the detector

probe closes the control valve and our icing problem is solved they've just finished making some of their preliminary checks on the stand so let's watch how they put the engine through its paces the engine is an idle that's about 60 percent rpm now the stand operator accelerates the engine on up to intermediate that's full power from the gas generator with the augmenter fog that puts it at about 13,000 rpm and over 14,000 pounds of thrust of course that's uncorrect it data and the engine is trimming itself trimming is the fine tuning adjustments automatically made by

the engine now he's taking it up to maximum that's max AV did you see how that nozzle responds as the power setting is changed what a beautiful sound that makes that's her power from the gas generator plus everything the augmenter can put out the thrust gauge is reading a little under twenty three thousand nine hundred fifty now back to Idol we're trimming adjustments occur if required that brings up another term you're here cycle we just observed one type of cycle idle to max and back to idle another type might be intermediate to max and back

to intermediate again you see we judge the durability of our engines and their components not only in terms of how many hours they will operate but also in terms of how many cycles they will endure when military pilots start putting their fighter aircraft through the hoops these cycles really begin to add up and this affects the lifespan of the engine looks like they're shutting down so let's talk for a while about engine performance nothing nitty-gritty numbers the test engineers deal with but a broader picture for yardsticks by which our engines are measured sure there are

many more but these are the ones you often read about and hear about first of all engine thrust what is the maximum thrust the engine will produce under certain specific conditions we've pretty well covered that but I haven't spoken of how big our engines are or how small one of the smaller jet engines we have is made by our Canadian cousins at PW at Canada it's the JT 15 rated at a max of about 2200 pounds of thrust in a fly on a Boeing 767 that's powered by two of our pw4000 s rated right close

to sixty thousand pounds of thrust for takeoff then there's the thrust to weight ratio how much thrust does it produce compared to its weight it doesn't do much for our image if we build a fine engine but it weighs 16 tons if I build an engine that puts out 15,000 pounds of thrust in a weighs 3,000 pounds that's a ratio of 5 to 1 let's try harder and build one that develops 16,000 pounds of thrust and weighs only 2,000 pounds 8 to 1 that's much better now we're becoming more competitive and it's a continual battle

to make that ratio better and better how about fuel economy with that limousine of yours you might say I get 24 miles to the gallon the higher the number the better the mileage not so with a jet engine we speak in terms of tsfc thrust specific fuel consumption and that turns out to be pounds of fuel used per hour divided by pounds of thrust produced our example shows a tsfc of 0.8 and if you look at the arithmetic you'll see that the lower the number the better your engine having at tsfc of 0.4 is better

than mine which has at tsfc of 0.6 these examples by the way are typical of some of our commercial engines military engines because of their missions run a little higher and when the augmenter is turned on the fuel consumption doubles or even triples the fourth one time between overhaul or tbo this one's simple how many hours can you operate the engine before you have to remove it from service and totally overhaul it our commercial engines go 10 15 and some 20,000 hours before overhaul with a commercial jetliner flying at 500 miles an hour in 20,000

hours it will fly 10 million miles why that's let's see 400 laps around the world now the TV o4 fighter aircraft engines much much lower that's expected because of the mission they fly some of the alarm slap on your helmet push the goal lever car man straight up if need be chase the enemy or be chased on top of that they're designed to put out the last inch of performance when they're called on it's like comparing the Indy 500 with a drive to the church picnic that's the way it is with engines for fighter aircraft

but whatever the TBL is our people don't stop there they refine and improve boost ft do again and again in fact they continue to do it for the life of the engine