THE GEnx: NEXT GENERATION AVIATION

Abstract— Jet engines have been a convenience for decades, yet General Electric has recently revolutionized this common technology with the release of their next generation jet engine, the GEnx. The GEnx has improved upon other older jet engines designs with the addition of modern composite materials, next generation compressor systems, a revolutionary high-bypass system that decreases noise pollution, and an advanced combustion chamber that improves stability and drastically reduces standard operation temperatures, and more This paper will describe, in parts, how GE has improved upon the design of their previous engine models with regards to maintenance costs and the efficiency of the operation and fuel systems. The first section will discuss how the use of modern materials and composite parts makes the fan blades and outer shell incredibly durable, while reducing the overall weight of the unit and the cost of its maintenance. The second section will describe the technical aspects of the engine and how the improved temperature stability, counter-rotation, digital diagnostics, and air-bypass systems contribute to the overall longevity, performance, and environmental benefits of the engine. In the third section, we will briefly discuss the impact of the GEnx on next-generation aircraft. GE has always been a leader in the development of jet engines and mechanical technology, and the new GEnx engine affirms that position by introducing groundbreaking advancements into the field of modern aviation science.

Key Words—GEnx, carbon fiber, titanium, composite, compressor, combustor, turbine, blisk

MECHANICS OF THE MODERN JET ENGINE

Jet engines are vital to the travel of millions of people worldwide every day. The average person, however, never explores the extremely complicated physics and mechanics behind the operation of a jet engine. The modern jet engine is composed of several main subsystems: the main fan and housing, the compressor, the combustor, the turbine, and the maintenance system. All of these systems must work together in order to perform the engine’s main function, which is providing the massive amount of thrust required to lift a standard commercial airliner. Modern companies such as General Electric are re-inventing standard jet engines by incorporating new technologies to improve efficiency, performance, and reliability.

The Main Fan

Thanks to the innovative use of carbon fiber blades with titanium edges, each of the fan blades on the GEnx is larger and more aerodynamic than those of its predecessor, allowing the GEnx to use fewer blades than older engines. Thanks to some 3D design work by General Electric, the GEnx jet engine utilizes only 18 carbon fiber blades, in comparison to 22 on its predecessor, the GE90. Each of the new carbon fiber blades is larger and moves more air than the smaller titanium blades in its predecessor. In addition, the reduction in the number of blades used on the fan of the GEnx reduces the resistance caused by the friction of air moving over the surface of each blade as it enters the turbine, which increases performance and decreases overall weight.

FIGURE 1A CLOSE UP VIEW OF THE NEW TITANIUM-TIPPED CARBON FIBER

BLADES OF THE GENX

In addition to the reduction in the number of blades in the fan, General Electric has made the fan assembly removable from the remainder of the engine, allowing a spare turbine to be mounted without removing the entire assembly from the wing of the aircraft. Using this method, GE decreases maintenance costs and downtime for all aircraft powered by its GEnx engines. In the GEnx, General Electric began to use a new high-bypass design; this involves allowing a great portion of the air coming through the fan to leave the engine, only using a small portion of the incoming air. The remaining air bypasses the rest of the engine and leaves through the honeycomb structure beyond the fan blades. A very large volume of air enters the engine at high speed, and due to the design of the engine, a large portion of this air leaves the engine without being used. This allows only clean

air into the compressor before being mixed with fuel. Use of the high-bypass design greatly reduces noise, improves airflow and air quality through the compressor, and finally increases fuel efficiency. This high-bypass setup also allows flying objects to be centrifuged to the outside of the fan duct and discarded out of the engine through variable-bleed valve door located behind the fan instead of entering the compressor, resulting in costly repairs. As seen in Figure 2, air enters through the main fan, and the majority of this incoming air exits through the back of the fan while a small portion is used for combustion.

FIGURE 2

A CUT AWAY SIDE VIEW OF THE GENX JET ENGINE ILLUSTRATING AIR FLOWING THROUGH THE ENGINE

The new design increases airflow, and has the highest bypass ratio of any commercial engine available today at 19:2. This number means that the volume of air bypass the compressor and combustor of the engine compared to the volume of air entering the engine is in a 19:2 ratio. This process decreases noise produced by the aircraft while improving the quality of air flowing into the compressor, which causes an increase in fuel efficiency. The air being mixed with fuel in the GEnx has less debris than in similar engines due to the design of the fan housing and variable-bleed ducts. This innovation will help to decrease noise pollution in areas surrounding commercial airports, while also decreasing fuel costs.

The Compressor

After the main fan, comes the compressor of a jet engine. This is the stage of the engine where the air from the main fan is compressed before it is mixed with fuel and ignited. The GEnx has one of the highest compression ratios of any commercial jet engines, coming in at 23:1. This means that the pressure of air leaving the compressor is 23 times that of the air that enters. Compressing the air that enters the jet engine allows the jet engine to optimize the burn of fuel to maximize power output. To achieve this high level of compression, General Electric uses a seven stage low pressure compressor which differs from the older six stage compressor. The stages of a compressor, as shown in figure

3, are disks with varying number of blades that increase the pressure of the air as it passes through each individual disk on its path through the engine. In the GEnx, General Electric added the additional stage, while reducing the number of blades for each individual stage. The use of fewer blades with an additional stage kept the weight the same, but increased fuel efficiency.

FIGURE 3A CUT AWAY SHOWING THE BLISKS WITHIN THE COMPRESSOR OF THE

NEW GENX JET ENGINE

As seen in Figure 3, the GEnx also utilizes a new high-pressure turbine that was increased from nine stages to ten, creating the final 23:1 compression ratio within the engine. General Electric also designed the two compressor sections to spin in opposite directions. The low-pressure compressor spins counter-clockwise while the high-pressure turbine spins clockwise. The counter rotation of these two turbines actually improves fuel efficiency by improving the transition between the two stages.

FIGURE 4A CLOSE UP OF A STANDARD BLADED DISK. NOTE THE DOVETAIL-SHAPED

JOINT WHERE THE BLADE AND DISK MEET

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In each of these stages, “blisks” or blade-disks are used to increase the pressure of the outside air to the pressures needed for efficient combustion. Instead of using standard blades attached to a central disk, a blisk is one solid piece of material that is both the blade and the disk, removing the dovetail joints where blades are normally attached to the disk. This dovetail joint can be seen in Figure 4, and the blisk replacement of this part can be seen in Figure 5. Removing this joint improves overall strength and reliability of the blisk, which increases the longevity of the engine itself. These blisks spin at very high rates, and at these rates, cavitation becomes a severe concern. Cavitation occurs when a fluid is subjected to differences in pressure, causing small vacuum bubbles of low pressure to form, which then implode when entering areas of high pressure. In a jet turbine, the fluid is air, and the areas of low and high pressure are created from the spinning of the fan blades and blisks. Inside the compressor, these small implosions of bubbles can wear down the surface of the blisk, causing cracks and failure.

FIGURE 5ONE OF THE TITANIUM-REINFORCED BLISKS (BLADED-DISKS) OUT OF

THE GENX

Cavitation is lessened due to the removal of the weaker joint between the two pieces, but can still cause failure of a blisk, shooting debris through the rest of the engine. If one of these blisks fails, the rest of the engine is put in danger of being shredded by metallic debris. To combat this problem, General Electric uses very strong metal blisks, covered in a special coating designed for use in very high temperatures, paired with a state of the art tracking system that can predict when a part will fail. This strong metals and heat resistant coating improves the reliability of the blisks under the high temperature and high pressure conditions present in the GEnx jet engine. The state of the art tracking system records the serial number of every part within the engine, and tracks usage to determine when one specific part could become damaged to prevent a total loss. Instead of replacing a part after it fails and destroys the parts around it, GE replaces the part before it fails, saving other costly components downwind.

The TAPS Combustor System

Inside of the GEnx, after the compressor, the air enters the specially designed combustion chamber of the engine called a TAPS combustor. The use of a Twin Annular, Pre-mixing Swirler (TAPS) combustor in the GEnx has greatly improved its fuel efficiency and reduced its emissions. This technology uses a system of pre-mixing the fuel and air before combustion to improve efficiency. In the GEnx, air leaves the compressor stages and is directed into two high-energy “swirlers” near the fuel nozzles. These swirlers, as seen in Figure 6, create a vortex where air and fuel are mixed before entering into the combustion chamber to be ignited. Air enters the TAPS combustor, and is swirled through the circular white holes while fuel is injected into the region. This swirling of the air allows the fuel and air to mix more homogenously, which creates a more concentrated mixture that burns at a much lower temperature than the mixtures used in its predecessors. The lower combustion temperature, which “reduces exit temperature variation by 205°C (401°F)” reduces the strain on the components after the combustor in the GEnx jet engine, which decreases overall maintenance costs. In older jet engines, high temperatures after the combustor can wreak havoc on the blades of the turbine. In the GEnx, the lower combustion temperatures reduce the strain due to heat, saving these vital components. The lower temperature within the engine also reduces pollution because certain types of air pollution, especially nitrogen oxides (NOx), increase exponentially at higher temperatures. NOx, a pollutant which generates acid rain and smog, is created when oxygen and nitrogen react at high temperatures. Reducing these emissions is extremely important when building a sustainable future, as a reduction in atmospheric pollution leads to less contamination of rainwater, air, and the ozone layer. When fired at comparable thrust levels, the GEnx produced over 30 percent lower NOx emissions than General Electric’s widely-used CF6 engine. In addition, the damage reduction due to lower temperatures in the TAPS combustor allows the combustor to last longer than comparable engines which fire at higher temperatures. By allowing combustor components to last longer, GE is saving the energy and raw resources that would be used in the production of new parts.

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. FIGURE 6

A CUT AWAY SHOWING THE TAPS COMBUSTION SYSTEM IN THE COMBUSTOR OF THE GENX

Maintenance System

Along with all of the innovative technologies used in the mechanics and materials of the GEnx, there is also a state of the art monitoring system that tracks every part in each jet engine by serial number. This system warns technicians to replace parts within the engine before they actually fail. The system tracks the life of individual parts by their usage, and monitors the strains placed on the entire system to calculate when a part could crack or warp. As mentioned earlier, this system allows airlines to do preventative maintenance on their engines, saving time and money. The system includes multiple sensors throughout each subsystem of the engine that track real-time data and compare it against a General Electric model of the ideal operation of the engine. This model took years to develop and consists of thousands of hours of operational data on the ideal operation of the jet engine. This real-time comparison allows the maintenance crew to look at each engine’s usage and history and analyze anomalies. If any issues arise, the crew can look into each subsystem at that particular time to isolate which part caused the issue. The computer system saves approximately $7 per hour of flight time due to the more efficient use of both parts and labor. The lowering of costs and increase in performance and sustainability of the engine helps companies to maintain lower operating costs and more reasonable prices for everyone using their service.

MATERIALS IN THE MODERN JET ENGINE

The materials used in the construction of a jet engine and its component parts are equally as important as the mechanics themselves. Without the proper materials, the engine parts simply will not function as efficiently as possible. As such, important considerations must be made when considering the materials of a jet engine. The components must be thermally stable, corrosion-resistant, durable, easily produced, and easily obtained, among many other concerns. In order to meets these needs, modern companies are

designing their next-generation engines to utilize some of the finest, most reliable materials available. These materials will make the engine components more durable than ever, while greatly improving operation performance and reducing maintenance costs.

Titanium Alloys

Titanium has been recently used in jet engine construction because of its strength, durability, and other properties that make it useful in the aviation industry. Other materials, such as aluminum, stainless steel, and aluminum-and-nickel-based alloys are lacking in many of the benefits inherent to titanium, making it a material of substantial use in modern aviation science. For example, titanium is about 40% less dense than other alloys and steels, which leads to an increase in titanium’s strength-to-weight ratio. This term simply means that a small quantity of titanium alloy has the equivalent strength of a larger quantity of an alloy of steel or aluminum and nickel. This is crucial to aviation design, because it means that less material can be used in development of engine components. Thus, the resulting parts will be lighter in weight than yet just as efficient as similar parts made with other, heavier alloys. Titanium alloys can be far stronger than iron- and cobalt-based superalloys (alloys that exhibit incredible strength, thermal stability, and corrosion resistance) and certain iron-based steels as well. In addition, titanium is very easily forged and cast into many forms for mechanical and industrial use. It can be made using standard, non-special industrial techniques and fused together in a variety of ways. Commercially pure (CP) titanium and its alloys are the most easily welded, allowing them to be formed into component engine parts with little difficulty and less cost than more non-malleable materials. It should be noted, though, that titanium can cost up to as four times as much as some steels. However, such a price is comparable to that of some superalloys, which, with respect to the design of jet engine components, are not as stable and effective as titanium.

Heat and Corrosion Resistance

Titanium alloys, especially commercially pure (CP) types, exhibit strong thermal stability (resistance to decomposition at high temperatures) up to temperatures of roughly 538-595°C (1000-1100°F), and it is possible for certain titanium-aluminum samples to resist even higher temperatures. This is important in jet engine design because the components of the engine that are exposed to these extreme temperatures must be made of materials that can withstand such heat without fail. Also, titanium is known for its exceptional corrosion resistance; it is far more resistant to atmospheric corrosion and degradation than stainless steel alloys. This is because titanium is naturally reactive with and has a strong affinity for oxygen, so a thin yet incredibly protective and durable film of oxide molecules naturally

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forms on titanium surfaces when they are exposed to air and moisture. This protective film continually forms and reforms itself so long as the metal remains exposed to moisture and open air. Thus, titanium and titanium alloys are very resistant to open-air corrosion and oxidation (degradation and chemical changes, such as rusting, due to a loss of electrons) at high temperatures (up to 650°C/1200°F). This is incredibly important in aeronautics because jet engine components must be able to resist the negative effects of air being combusted at very high temperatures and the unstable chemical reactions that occur therein.

Titanium in the GEnx

General Electric has incorporated titanium alloys into many of the components of their new GEnx engine model. Where older models utilized other, more inferior metals and alloys, such as aluminum, in engine design, General Electric has introduced stronger, more reliable titanium alloys. The fan blades of a jet engine are crucial to the system’s proper operation. However, they are very prone to damage from flying debris, such as birds that stray too close to the engine. The GEnx’s new fan blades have titanium-tipped edges, giving them numerous advantages in structural stability and performance. The blades themselves are made with composite materials, but these materials can be easily frayed by the force of the air passing over them. In order to prevent the tips from fraying and compromising the safety of the engine, they have been covered in titanium (see Figure 8), the durability of which will prevent the blade tips from needing frequent maintenance. Also, these titanium-reinforced blade tips redirect the force of debris collisions to the main body of the blades themselves. In other words, the shock damage caused by debris hitting a blade tip will not overly damage it; the energy is transferred downward into the far stronger, more durable composite material of the

blade’s main body. Titanium alloys are also used in the later-stage turbine blisks of the engines’ compressor. Given that these turbines are subject to rotation at high speeds, temperatures, and volume of air flow, the blisks must be made of a highly heat- and corrosion-resistant material. Titanium, as previously mentioned, is very resistant to damage from heat and oxidation, allowing it to perform tasks that require resistance to extreme conditions.

CARBON FIBER & PLASTIC COMPOSITES

While the use of titanium in industry has been a mainstay for some time, the use of carbon fibers in industrial design and manufacturing has been a rising trend in recent years. The numerous advantages that carbon fibers have over traditional materials make them a very attractive option for the structure of many industrial products, including jet engines. As a design material, carbon fiber filaments are durable,

lightweight, electrically and thermally stable, and easily formed into final products.

Composition and Structural Benefits

A carbon fiber is a fiber filament that is composed of at least 92 percent carbon; the carbon is in the form of long filaments which are woven together and sealed with a resin that, when dried, provides incredible strength. Since carbon is a very light element, the fibers are very thin and lightweight, with fibers measuring roughly 5-6 micrometers (1 micrometer = 10-6 meters) in diameter. As a result, carbon fiber filaments are far less dense than other materials, causing them to contribute much less weight to any engine components that they are a part of. Also, they are a very elastic material; a high elasticity means that they are able to very effectively resist deformation when stress is applied, allowing them to retain their shape under extreme physical conditions. As a result, they can be easily constructed into various jet engine components without the negative side-effects of stress damage. This allows for more experimental component design and a greater degree of reliability and structural stability for parts made of carbon fiber. The structure of a crystalline carbon fiber is most closely related to that of graphite; multiple layers of carbon atoms, bonded together in a flat, honeycomb-style arrangement, as shown in Figure 7. These layers, due to the weak intermolecular forces that hold them together, easily slide across each other horizontally.

FIGURE 7THE MOLECULAR STRUCTURE OF GRAPHITE, IN WHICH MULTIPLE

LAYERS OF BONDED CARBON ATOMS ARE BONDED TOGETHER VERTICALLY (AS SHOWN IN THE INSET)

However, carbon fibers, when looked at on a larger scale, are not made entirely of parallel layers of carbon, but rather longer ribbons. While the surface layers are technically parallel, the inner layers are warped into hairpin folds. These folds prevent the ribbon layers from sliding past each other

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as they do in graphite, granting the fiber strength, durability, and elasticity parallel to the horizontal plane. Also, the more aligned the layers are, the stronger the structure becomes; this type of arrangement is known as a fiber texture. The stronger the fiber texture is, the greater the carbon fiber’s properties, such as density, elasticity, and tensile strength (the amount of stress due to stretching that a material can handle before it begins to deform). Hence, a carbon fiber that possesses these qualities would have tremendous strength, elasticity, and durability when compared to other, more conventional industrial materials such as metal alloys.

Electrical and Thermal Conductivity

In addition to strong physical properties, carbon fibers are also very stable in terms of electrical and thermal energy. With the increasing strength of the fiber texture, a carbon fiber becomes more electrically and thermally conductive. The more conductive a material is, the more energy it can stand to safely transfer. For example, a thermally-conductive material may be used in the application of a heat sink, which is a device that absorbs heat in a system (such as in a computer or an engine) and transfers it to the surrounding air, thereby cooling off the system and keeping it from overheating. Thus, fibers used in engine design that are structured this way are able to prevent electrical- and heat-based damage in the areas of the engine that they are incorporated into. With regard to the aforementioned molecular layer structure of the fibers, the bonds that hold the carbon atoms of each layer together are caused by delocalization of certain electron orbitals (areas of negative charge around an atom). When an electron is delocalized, it is no longer associated with a single atom, but instead roams freely around the space of the structure. This results in increased electrical and thermal conductivity along the horizontal plane since the material is more receptive to energy and electrical charges.

CARBON FIBER IN THE GENX

General Electric’s GEnx engine utilizes the best properties of carbon fiber material technology in its redesign of traditional jet engine components. Numerous parts of the engine have been constructed using carbon fiber materials in place of previously-used metal alloys, all with outstanding benefits and results. By replacing aluminum and titanium alloys with carbon fibers and fiber-reinforced plastics, GE has greatly enhanced the performance of its newest jet engine.

Fan Blades

Formerly, the fan blades of a jet engine were composed of a titanium or aluminum alloy, mostly for the durability those materials provided. However, such metals are heavy, cumbersome, and prone to damage. During flight, engines may encounter debris and foreign objects that sporadically

fly into the blades, either damaging them or moving past them and into the combustor, where they can cause massive damage. Fortunately, the GEnx utilizes the aforementioned variable-bleed valve doors, which remove debris that passes the fan blades, preventing it from entering the later stages of the engine’s combustion process. These valve doors are built with carbon fiber plastics so as to improve their durability and reliability, as their operation is essential to the safety of the engine. In addition, the blades themselves are made of carbon fiber materials; a far cry from metal-based blades of past models (see Figure 8). With the replacement of traditional metal alloy blades, GE had greatly enhanced the blades’ strength and lifespan as well as their general efficacy. The composite blades are more shock-resistant and able to withstand much more foreign object damage (FOD) from debris than their metal predecessors. Also, “their inherent low density makes solid composite blades 10% lighter than a hollow titanium blade,” and “overall, composites are 66% lighter than titanium …and 100% stronger”.

FIGURE 8THE CARBON FIBER FAN BLADES OF THE GENX, WITH TITANIUM-COATED

EDGES

Composite fan blades are stronger and lighter than traditional titanium blades, and General Electric has also designed the engine to work with fewer blades—and thus less weight—than previous models; the count has been reduced from 22 to 18. This change increases efficiency and decreases noise by reducing the amount of “scrubbing” on the blades; a phenomenon that involves the drag force of air and air resistance wearing down the blades as air moves across their surfaces. When the blades were tested on the earlier GE90 model, they were given a 30-year lifespan, and “after a decade of performance and 6.5 million flight hours…only three composite blades [had been] removed from service” due to debris damage. This not only serves as a testament to the blades’ incredible performance, but also

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shows that costs of maintenance are drastically reduced with the switch to composite fan blades.

Fan Blade Chamber Housing

The outer casing of the fan, as shown in Figure 9, is also made of carbon fiber plastic composites, and it provides drastic improvements over the older aluminum casings. The design of the casing utilizes a specific braid-and-weave pattern of carbon composite fibers and a coating of resin in order to replace the traditional aluminum alloy material. In addition, a special thickened middle weave improves durability by “[providing] a fan-blade containment layer, eliminating the need for the Kevlar blanket used on aluminum cases”. Since this containment layer is incorporated directly into the blade housing, the casing contains a durable extra layer without the need of additional materials such as Kevlar.

FIGURE 9THE COMPOSITE OUTER HOUSING OF THE GENX FAN BLADE CHAMBER

This design greatly reduces the weight of the unit while also improving its resistance to damage; after over 100 ballistics test were performed on the casing design, it was shown that the composite case “was more resistant to ballistic FOD damage than aluminum cases”. Through the use of carbon fibers in the design of their newest jet engine, General Electric has significantly reduced the risk presented by airborne debris, the cost of maintenance, and the overall weight of then engine, all while greatly improving its performance. In comparison to the earlier GE CF6 engine model, the GEnx will use 15 percent less fuel, which is an important factor in lowering costs for airlines and customers. With the new, more durable fan blades and housing, General Electric has greatly reduced its need for extra parts and equipment. This allows the company to save the raw materials and energy necessary for the construction of replacement parts. These resources saved on replacements can go directly into the development of new engines, allowing for more fully-functional engines to be built out of

materials once used for backup fan blades and chamber housing.

RE-INVENTING THE JET ENGINE

General Electric has spent millions of dollars designing and building their next generation GEnx jet engine, but what good is it to the average person? The GEnx allows airlines to spend less money annually on fuel and maintenance, lowering the operating costs of all of their aircraft. This, in turn, keeps flight costs competitive for the consumer. The GEnx also helps the environment world-wide due to its decrease in air pollution thanks to its more efficient fuel mixture and lower burn temperatures. Planes are not going to stop flying, and providing a safer and more environmentally friendly solution to propel them will allow the newest generation of aircraft to operate safely. The GEnx is a part of General Electric’s “Ecomagination” product line, which stresses the use of technology that improves operational and consumer performance while paying heed to the ecological concerns of the industry. Thanks to General Electric’s innovative use of modern composites, metals can be saved for use in more vital parts of the aircraft such as the turbines and compressor instead of being wasted on costly fan blades and housings. The use of various specialized coatings and lightweight materials increases fuel efficiency while reducing weight, which saves airlines money, and reduces the amount of fuel consumed each year. Fossil fuels are a non-renewable resource, and providing the most efficient ways to use these fuels is vital to continuing their use in the years to come. Finally, the high-bypass ratio of the GEnx decreases the noise output of the engine by 30%, reducing noise pollution in areas surrounding large airports. Along with all of the other innovations General Electric has utilized in the GEnx, the engine will stay on aircraft 30% longer than previous engines, will increase fuel efficiency, decrease various types of pollution, and decrease operating costs in many ways. All of the perks of the newest generation of jet engines makes airlines, passengers, and regulatory officials eager for these engines to become the standard.

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