|Written by Sabc|
The title can be misleading somewhat. It actually designates two different models of Pratt & Whitney engines: one is F100-PW-220 and the other is F100-PW-229, this latter being the newer, more powerful and more reliable version. Today's F-15E jets are running on these two types of engines.
Pratt & Whitney's F100 series fighter engines has accumulated more than 20 million flight hours in service. In 2005, the F100-PW-229 reached one million flight hours. This record of flight hours is unmatched by any other fighter aircraft engine. It's reliability is among the best: there was not a single aircraft which was lost due to the failure of its F100-PW-229 engine.
Operational F-15E squadrons use the following engine types:
The afterburning turbofan jet engine is a masterpiece of modern engineering. Today's engines are extremely complex systems requiring precise and high-tech manufacturing. However their basic working principles are quite simple. It releases fuel stored energy by burning the fuel and transfers this to kinetic energy by letting hot fuel exhaust gases leave through the back of the airplane at very high speeds.
The following figure is a schematic illustration of how an afterburning turbofan engine works.
To supply the engine with oxygene for burning, it inputs huge quantities of air via its air intakes (6). This air gets compressed in several steps. First it goes through the stages of the low pressure compressor (1), often called simply as 'fan'. This compressor consists of several rows of blades. Typically one row is a 'static' row, which means that its blades are not moving, the next row is a 'rotating' row, which means that its blades are rotating at high speeds. Rotating blades force the air speed towards the back of the engine. Rotating blades has a cross-section very similar to the wings of an airplane, this helps increasing the pressure of the air. Air pressure increase is also helped by the effect of the static blades, which also help regulate and deflect the airflow to the proper direction.
After the low pressure compressor the airflow is split. One part of it is directed outwards (this is called the fan bypass airflow) while the other part of it is directed more inwards of the engine. The 'outer' airflow (7) avoids the combustion chamber, it goes directly towards the back of the engine and rejoins with the rest of the air before the afterburning stage. This outer airflow is of much lower pressure and temperature, hence it helps cooling the engine thus reducing wear of engine parts.
The 'inner' airflow (8) goes into the high pressure compressor (2). The high pressure compressor works the same way as its low pressure counterpart and produces a high pressure and high temperature air which flows directly into the combustion chamber (9). Fuel is added into the air and gets ignited there. The intense burning results in very high temperature and high pressure mixture of air, fuel and exhaust gases.
These go through and spin the high pressure turbine (3) which extracts some energy from the flow to power the high pressure compressor. Rotating on a co-axial way, the low pressure turbine (4)is the next, it powers the low pressure compressor.
The gases leaving the 'inner' parts of the engine meet the outer 'airflow' here and leave the engine at high speeds, thus producing thrust. In a normal operation the afterburner section is not used. However when extreme thrust is required, extra fuel is sprayed into the leaving airstream by tiny nozzles of the afterburner spray ring (10). This fuel immediately gets ignited in the very high temperature airflow and burns in the afterburner area (11). The end result is an even higher temperature and higher speed gas mixture which go through the exhaust nozzles (5) producing a drastic increase in engine thrust at the price of dramatic fuel consumption and increased wear on engine parts. Note that while during normal operation only hot gases leave the exhaust nozzles (barely visible), the usage of afterburner produces a long, bright flame which is highly visible, especially by night.
The following figure shows a cutaway drawing of the F100-PW-220 engine. Parts discussed above can be seen in their 'natural' form here.
The low pressure compressor in the F100-PW-220/229 consists of 3 stages (that is 3 rotating rows of blades, with rows of static blades standing between them. All blades are made from titanium. The high pressure compressor is a 10-stage axial design, constructed primarily from titanium, Inconel and other high-temperature alloys. The first 3 stages of the high pressure compressor are equipped with variable stator blades to allow optimum airflow shceduling.
The annular combustion chamber is fabricated of Haynes-188 cobalt-based alloy with film cooling. The F100-PW-220/229 is designed to be smokeless during normal operation, this is achieved by utilizing extremely high operating temperatures and concentrating combustion on the front of the burner. The afterburner produces flame and trail of smoke, though, but this is unavoidable.
The high pressure turbine is a 2-stage design and it uses directionally solidified alloy blades. The low pressure turbine has 2 stages, too, it uses PWA-73 coated alloy blades. Note that the low pressure turbine is rated for up to 10,400 rpm (rotations per minute). First and second stage turbine blades are made from single crystals that dramatically increases engine life.
Its predecessor was the Pratt & Witney F100-PW-100, an engine developed especially for F-15A and B models. When it was introduced, PW-100 represented a quantum leap in modern turbofan engine design over the previous engines. It introduced computer technology (it was an analogue computer) the first time in jet engine design - previous engines used mechanical linkages to control the fuel flow. However it's service life was not without problems, to say the least. It often suffered stagnations and compressor blade stalls, not mentioning afterburner fires, this latter posing a considerable risk of losing the entire airframe.
Needless to say, USAF was not happy with the reliability of the engines, they even seriously considered switching to General Electric engines in their F-15 jets. Pratt & Whitney realized the risk of losing a well-paying customer, so they pushed on with developmental work and the result was the PW-220, the first jet engine featuring Digital Electronic Engine Control ( DEEC). The PW-220 proved to be more reliable and needed less maintenance. From October 1986, all F-15E's were equipped with this engine (and later C and D models got it too).
PW-220 produced slightly less thrust than PW-100 (23,450 lbs against 23,830 lbs), but the DEEC system reduced fuel consumption and wear and tear on engine components as well. DEEC automatically trims to maintain performance as the engine deteriorates, and produces much quicker reactions to pilot's input than the previous analogue control system.
The most visible difference between PW-220 and PW-229 is the color of the flame coming out from them in afterburner mode. PW-220 produces a yellowish flame, while PW-229's afterburner flame is bluish (see images below).
Differences in afterburner flame color between -220 engines (above) and -229 engines (below).
The PW-229 variant was introduced in 1992, the first jet to be equipped with it was 90-0233. Many people say that the PW-229 was not as reliable as the PW-220, but these criticisms are often dismissed in the light of the sheer power of PW-229. In fact the power of the 229 engine is such that an F-15E flying in a clean configuration (i.e. no external ordnance and pods) and without CFT's can even reach supersonic speed without using afterburner. This is called 'supercruise' ability, a term that was introduced with reference to the ultramodern F-22 Raptor!
PW-229 features an Improved DEEC ( IDEEC) and 22% more take-off thrust than its predecessor. It reacts more quickly to pilot inputs (only 4 seconds from minimum power to maximum power, compared to 7 seconds of PW-220). Its greater power and quicker reactions make the PW-229 the engine of choice among Strike Eagle pilots - especially when they are talking about missions flown with heavy weapons loads.
The PW-229 has bigger cooling requirements, hence CFT's had to be redesigned to equip with cooling scoops that reached further than the relatively slower CFT boundary layer airflow. The PW-229 was also capable of powering an increased capacity generator.
Another big difference between these two engine types is the presence of ATDPS (Assymmetric Thrust Departure Prevention System). Only PW-229's are equipped with this. You can read more about this further below in this article.
Implementation in the F-15E
The engine system in the F-15E is made up of a collection of elements, namely the following:
How engines are built in the F-15E airframe is illustrated in the figure below. Numbered parts in the cutaway drawing will be discussed and referred by their number in the following text.
Engines are mounted to the airframe by standard mounting links (2) resting on all-titanium mounting frames (8). This way engine mounting and engine change is a relatively simple event, which can be done by front-line maintainers as well. Engine mounting links were design to incorporate not only the Pratt & Whitney F100-PW-220/229 engines, but the General Electric F-110- GE-129 engine as well, this latter engine being a realistic alternative to the P&W engines, which however never made it to the F-15E in operational service.
Each engine is equipped with a completely self-contained engine oil system. Oil is supplied to the main pump element by gravity feed. Return of the engine oil to the pump reservoir is severely limited during 0 or negative G flight, that's why the duration of these kinds of flights is limited.
The ignition system is a capacitor-discharge type and contains an independent engine mounted generator and four igniter plugs (two for the engine combustor, two for the afterburner). During engine start (it is discussed in detail in the JFS article - see 'Engines' section in the left menu) moving the throttle from OFF to IDLE causes the engine igniter plugs to discharge. Ignition then continuous during engine operation. When the throttle is moved into afterburner, afterburner ignition is activated for approximately 1 - 1.5 seconds in co-operation with the LOD system (see further below).
The engine control system ( ECS) comprises a primary (PRI) digital control and a secondary (SEC) hydromechanical control mode. PRI offers all the advantages of DEEC/ IDEEC and runs the engine scheduling across the full range of throttle positions. SEC is a backup mode and will be discussed later on. Since the PW-229 is more powerful than the PW-220, IDEEC incorporates a ground idle thrust ( GIT) setting to mimic the PW-220 performance when taxiing on the ground. IDEEC also incorporates a transient idle control logic that gives the pilot freedom to snap the throttles to idle whilst the engines maintain about 79% rpm. Thrust is reduced in accordance with requested throttle settings, but the engines maintain core rpm momentum for 20 seconds - after this they return to idle if no further throttle commands are given. This elongates engine life and improves subsequent throttle response times. The pilot has override authority on the operating mode of the ECS vie engine control switches, so the pilot can re-enter PRI mode and enter SEC mode if he desires so. This is achieved by the engine control switches: ON is for PRI mode, OFF is for SEC mode.
The DEEC schedules engine and afterburner fuel flows, compressor inlet variable vanes ( CIVV), rear compressor variable vanes ( RCVV), start bleed position, anti-ice and nozzle position. The DEEC controls engine performance by scheduling engine fuel flow to control airflow and nozzle position to control engine pressure ratio ( EPR), this latter being a ratio between exhaust and inlet pressures. By controlling airflow and EPR, the engine performance remains consistent for a new or deteriorated engine until the FTIT limit is reached.
If a fault is detected by in the PRI mode software, then SEC mode is entered automatically. Note that SEC mode can be entered manually by placing the engine control switch to OFF position. SEC mode inhibits afterburner use and limits MIL power to 70-80% of its normal value. The CIVV are in a fully closed position, the nozzle is closed near to its minimum area (below 5%). In the meantime L or R ENG CONTR caution is displayed. RCVV, start bleeds and engine fuel flow are scheduled by the MFC. The engine remains in SEC mode until the fault is cleared or the engine control switch is put back to ON position.
Note that engine start can be accomplished with the engine control switches in either ON or OFF position, but after engine start they should be left in their position for at least 1 minute, otherwise DEEC will switch to SEC mode. If the engine is started with its engine control switch in OFF position then SEC mode will be entered immediately of course - this way ground starting time will be longer.
The F100-PW-220/229 incorporates a built-in engine monitoring system ( EMS) which consists of DEEC/ IDEEC software and the EDU (Engine Diagnostics Unit). This continuously monitors engine parameters and system states to detect engine operating failures. Engine operation failures (along with engine and aircraft data) are detected and logged in memory for further analysis by maintenance personnel. The EDU also maintains engine life-cycle information.
The afterburner has 5 concentric fuel spray rings in the flow coming out from the core engine (see image below: only 4 is visible, 1 is hidden), plus 2 more rings further outwards in the fan bypass airflow (see image below: these rings have a shiny, metallic color).
The afterburner is equipped with a high-energy ignition system which allows a modulated light-up. A light-up detector ( LOD) is attached to the system which senses afterburner ignition and along with the DEEC permits faster throttle transients. If the LOD does not sense a light-up, it automatically retards the throttles to MIL, terminates fuel flow to the afterburner and checks all systems. If everything checks good, the LOD will automaticall attempt two additional relights. If these are too unsuccesful, the LOD is disabled by the DEEC and one additional relight is attempted, using tailpipe pressure values for sensing afterburner light-up.
In PW-220 engines the afterburner has 5 stages which are progressively selected as the throttle moves from MIL to MAX. PW-229 engines have 11 afterburner stages for MIL to MAX. During snap accelerations (when the pilot agressively snaps the throttle forward) the first afterburner segment may be lighted just above IDLE rpm and more segment may be lighted advancing towards MIL setting, depending on flight conditions.
All major afterburner assemblies are made of titanium, while the interior liner is coated with Haynes-188. There is a small fuel drain on the bottom of the afterburner, this is to gather unburnt fuel (for example after an unsuccessful light-up) thus avoiding fuel accumulation and probable flame-out.
The exhaust nozzles (12) are axially symmetrical and they follow a convergent-divergent profile. They give a wide range both in area and in profile. The nozzles can be moved pneumatically by utilizing engine bleed air. The nozzles themselves are moved by titanium rods (11) driven by actuators (9). On the ground (when WOW is sensed) and the engine is in IDLE, the nozzles are 80% open with PW-220 engines and 90-100% open with PW-229 engines (provided that DEEC/ IDEEC is on). As the throttles advance toward MIL, the nozzles are moving to their fully closed position.
During flight the nozzles are at their minimum area at all times, except at MIL power or on afterburner. At MIL nozzles are 5-10% open with PW-220 engines and 6-20% open with PW-229 engines. The nozzles reach their fully open state only in full afterburner to compensate for increasing afterburner fuel flow.
The engine anti-ice system is covered in another article, see it under 'Miscellaneous' in the 'Technology' menu.
The Assymmetric Thrust Departure Prevention System ( ATDPS) is present only in the F100-PW-229 engine. ATDPS's main purpose is to reduce the possibility of a directional departure following loss of a single engine at high airspeeds (that is 500+ KCAS or Mach 1.1+). When activated ATDPS puts both engines into their secondary hydromechanical control mode (thus disabling DEEC/ IDEEC) and tries to equalize thrust between the two (enabling DEEC/ IDEEC again once it returns back to a state what it considers to be 'normal'). The system reacts very quickly and in the majority of cases it prevents loss of control of the aircraft due to a sudden dramatic increase of yaw rate. Sudden and big sideslips can be dangerous, since air would be coming from the side of the intakes, thus not letting enough air into the engines, resulting in a possible flameout (not mentioning the structural stress the aircraft faces).
The engines are controlled by direct linkage between two sets (one for the pilot and one for the WSO) of split throttles, mounted on the left hand console in both cockpits. The throttle can be freely moved and be put to any setting in between the two limits, OFF and full afterburner (MAX). For OFF setting the throttle must be pulled fully backward, afterburner is reached when the pilot pushes the throttle fully forward. There are two detents in between the two limits, one for IDLE power (minimal thrust possible with the engines running) and one for MIL military power (maximum thrust available without afterburner). The friction of throttle movement can be adjusted manually by a small lever at the base of the quadrant. Control is electronic, moving the throttle provides input to DEEC/ IDEEC, which controls engines based on its software (for more detailed description of the throttle quadrants see section 'Cockpit' in the left menu).
The secondary power system is discussed in detail in another article.
Engine Displays and Cautions
Both the pilot and the WSO can monitor engine status in the engine display called up on one of the MPD/MPCDs. The pilot also have a smaller display called the Engine Monitor Display ( EMD) near his right knee. This constantly provides essential engine information as illustrated on the following figure:
Information provided on this display is the following:
The same engine information can also be displayed by any of the MPDs/MPCDs in the following format:
If a parameter exceeds its minimum or maximum value then the min/max value will be displayed and the parameter will be displayed in yellow (on an MPCD) and boxed. Yellow color is not possible on an MPD, so a greater intensity will be used instead). If any of the data is invalid or cannot be acquired then an 'OFF' text is displayed.
On airframes 90-0233 and up an additional test is available for the ATDPS. It can be tested only during low speed ground operation. When ATDPS test is selected, switching one engine control to OFF will result in both engines transferring to SEC mode. The engines will return to PRI mode only after engine control switches are set to ON and ATDPS test is deselected.
Engine start and shutdown is a fairly simple process, which is described in the article on the Jet Fuel Starter (JFS).
The engine caution system captures and displays engine caution situations. In case a caution situation occurs, the master caution light will be illuminated together with the engine caution light both in the front and the rear cockpits. The exact cause of the caution can be determined by invoking the caution display on any of the MPD/MPCDs. The following cautions may be displayed:
The engine control panel contains the necessary switches and lights for the pilot to control the engine and its accessories. The engine control panel is located on the pilot's right console, near his right knee. See the illustration below.
The generator switches (1) turn generators ON and OFF. These are lever-locked switches which must be raised before they are moved to a new position. Generators are discussed in more details in another article under the 'Power Systems' section in the left menu.
The external power control switch (2) controls application of external power to the aircraft's electrical buses. It has three positions: NORM allows aircraft electrical buses to be energized by external power, RESET established external power if it's not on the line (this position is spring loaded back to NORM), OFF disconnects external electrical power from the aircraft. This topic is discussed in more details in another article under the 'Power Systems' section in the left menu.
The emergency generator switch (3) controls the emergency generator, a utility hydraulic motor driven AC/ DC generator. The switch has three positions: AUTO provides automatic activation of the emergency generator, MAN provides manual activation, while ISOLATE restricts the emergency generator to powering to a couple of critical systems only. This is discussed in more details in another article under the 'Power Systems' section in the left menu.
The engine control switches (4) are used to place the engines (left and right) either in PRI or in SEC mode. More detailed descriptions on these modes can be found above where the engine control system ( ECS) is discussed. Note that afterburner usage is inhibited with the switch in OFF position.
The engine master switches (5) are guarded. They are to allow/deny the operation of their respective (left/right) engine. The engine can be operated only if its master switch is ON. Placing the switch to ON opens the airframe mounted engine fuel shutoff valve of its respective engine and directs power to the fuel transfer pumps. Placing the switch to OFF decouples the engine from the JFS and closes the airframe mounted fuel shutoff valve (provided that engine control/essential power is available).
The STARTER switch and light (also on this console) are for the Jet Fuel Starter (JFS), hence they are discussed in another article. See the JFS article in the 'Engines' section in the left menu.
PW-220 engines have another switch (not visible on the illustration above since it is located below the front cockpit left canopy sill), this is called the VMAX arm switch. It has a wired down guard and its use is prohibited in peace time. With the VMAX system armed, the throttle in MAX afterburner and airspeed above Mach 1.1, the engine control schedules a 22 Celsius increase in FTIT and a 2% increase in rpm. Main engine and afterburner fuel flow is increased by approx. 4% and thrust is increased by approx. 4% as well. Not more than 6 minutes is allowed to stay in VMAX mode. Each VMAX use is logged and a hot section borescope inspection may be performed after flight. Maximum total VMAX time is 60 minutes until engine overhaul. Note that PW-229 engine do not respond to the VMAX switch.
Technical Data (PW-229)
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|Last Updated on Monday, 30 May 2011|
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