3

Aircraft Gas Turbine Engines

INTRODUCTION

All commercial aircraft designed in the last 40 years (other than aircraft with fewer than a dozen passengers) are powered by gas turbine engines, either turbofan or turboprop. Thus, any discussion of reducing carbon emissions from commercial aircraft will need to consider the potential for improvement of gas turbine engines. To that end, this chapter will delineate the current state of the art of aircraft engines, discuss the potential for and constraints on gas turbine improvement over the next three decades, and suggest research directions to achieve such improvement. Unless otherwise noted, the discussion in this chapter refers to gas turbine engines for large commercial aircraft, as discussed in Chapter 1.

BACKGROUND

Engine Metrics

For this discussion, engine refers to the device that converts the energy in fuel into shaft power and the shaft power into propulsive power. In current implementations, engines are highly integrated and take the form of a turbofan engine or a turboprop engine with propeller. With a modern turbofan (see Figure 3.1), the fan draws air through the inlet, 80-90 percent of which is exhausted through the fan nozzle to provide most of the thrust produced by the engine. The rest of the fan air is pressurized in the compressor and is either (1) used for cooling or (2) mixed with fuel and burned in the combustor. Exhaust gases from the combustor pass through the turbine, generating the mechanical energy that turns the shaft that drives the fan and compressor. The gases exiting the turbine pass through the exhaust nozzle at high speed, which provides additional thrust. A turboprop is simpler in design though similar in concept to a turbofan, the primary difference being that a turboprop uses a propeller in free air to produce thrust rather than a fan in a nacelle.

For gas turbine engines, the primary engineering metrics are overall efficiency, weight, additional drag, and reliability. Overall efficiency here refers to the efficiency with which the engine converts the power in the fuel flow to propulsive power. It is the product of thermodynamic efficiency of the process that converts fuel flow power to shaft power (herein called motor thermodynamic efficiency) and propulsive efficiency (the conversion

of shaft power to propulsive power¹). The most efficient commercial aircraft gas turbines in service or entering service in this decade have takeoff thrusts of 20,000 lb and above. These turbines operate at cruise, with motor thermodynamic efficiencies of up to 55 percent and propulsive efficiencies of well over 70 percent, yielding an overall efficiency (the product of the two) of about 40 percent (see Figure 3.2). The total cost of ownership is also an important metric that influences design efficiency and weight. This cost includes manufacturing cost, maintenance cost (mainly overhaul), and fuel. A combination of these costs appropriate to each application is used to assess best value. Generally, at a given level of available technology, gas turbine weight can be traded for efficiency and maintenance cost. Thus, engines for longer range aircraft (now all twin aisle) optimize to higher efficiency levels since the weight and cost trades between the engine and the fuel weight favor increased efficiency as range increases.

Motor thermodynamic efficiency of commercial aircraft engines has improved from about 30 percent to over 50 percent over the past 50 years, as shown in Figure 3.3. Most commercial airline engines are designed to maximize efficiency at cruise, since that is where most fuel is burned. The ultimate cruise thermodynamic efficiency is constrained by thermodynamics to somewhat above 80 percent for an ideal cycle consisting of lossless components. Of course, this is not realizable in a practical sense since real components have losses. Where the practical limit lies given aviation’s important constraints of safety, weight, reliability, and cost is a matter of some speculation. However, several estimates have placed it at between 65 and 70 percent, given the development of new materials, architectures, and component technologies, as is discussed in the sections that follow.

Propulsive efficiency is defined here as the propulsive power delivered to the aircraft (which is equal to thrust times airspeed) divided by the shaft power input to the propulsor. For turbofan aircraft in service now, propulsive efficiency is 70-80 percent (Figure 3.4). Turboprops are about 10 percent more efficient at their current cruise Mach numbers. As noted in Chapter 2, as propulsors increase in size to increase propulsive efficiency, care needs be taken to distinguish and account for aircraft installation effects that may contribute to overall aircraft weight and increase drag but that are not normally attributed to engine efficiency.

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¹ Care must be taken since “efficiency” may be defined differently depending on the reference or organization. In this report the definition is chosen to allow consistent comparisons among alternative propulsion approaches.

Gas Turbine Characteristics

Gas turbine engines have several characteristics that distinguish them in important ways from other power plants such as internal combustion engines or electric drives. All engines produce waste heat that must be rejected. The penalties associated with such heat rejection increase with airspeed. One distinguishing characteristic of a gas turbine that is especially relevant to high-speed aircraft is that the heat from the fuel lost to inefficiency in gas turbines for the most part travels out as the exhaust and, indeed, produces positive thrust. This is in contrast to other power plants such as piston engines, Rankine and Sterling cycles, and electric drives. These power plants must explicitly reject waste heat, and their necessary cooling systems can add considerably to complexity, weight, and drag. Such penalties can be substantial. For example, the committee estimates that the drag increase (or net thrust decrease) to reject 10 percent of the propulsion power as heat may be on the order of 5 percent.

A second relevant characteristic is that at constant throttle settings, a modern turbofan engine’s thrust varies with speed and altitude in a way that matches the variation in thrust required by a commercial subsonic airliner. Specifically, current subsonic airliners require about three to five times more thrust to take off than they do to cruise, and the power produced by a high bypass ratio turbofan engine at constant throttle setting varies in much the same way. Thus, turbofan engines are well suited to current airliners. This is illustrated in Figure 3.5 for a

single-aisle, 150-180 passenger aircraft fueled for a 1,000 nautical mile (nm) mission. This means that compared to requirements at cruise, extra propulsion weight need not be carried nor drag incurred for an airliner to be able to take off. Also, since most gas turbine engines are already optimized for minimum fuel burn during cruise, there is little to be gained by better matching of the engine to the airplane characteristics. In addition, because of the combined effects of ram air compression at cruise speed and low ambient temperature at cruise altitude, a gas turbine’s motor efficiency is 6 to 8 percent greater at cruise than it is at takeoff.

Role of Engine Size

When considering the role of gas turbine size (thrust) on efficiency, one needs to distinguish between economic and physical factors. In general, current larger engines have better efficiency than smaller engines. Much of this difference is design intent. Large commercial engines are designed for long-range aircraft, for which fuel consumption is the overriding consideration. This is due to overall life-cycle cost considerations and reflects that the trade-off between engine weight and fuel consumption is more favorable the longer an aircraft’s range. That is, higher efficiency is advantageous, even if it comes at the cost of some increased engine weight, because more efficient engines allow aircraft to carry less fuel, and the reduced fuel loading becomes more and more significant for long-haul routes. Engine overhaul is another major operating cost for airlines. The number of on-off flight cycles is a major determinant of how frequently engines must be overhauled. Smaller engines designed for shorter-range commercial aircraft will have, on average, many more daily flight cycles than larger engines designed for large transports that are more likely to be flying long-haul routes. Therefore, it is particularly important for smaller engines to be able to execute a large number of flight cycles between overhauls. So, for the same level of technology, larger engines tend to be optimized for higher efficiency, while smaller engines tend to be optimized for lighter weight and more flight cycles between overhauls. (The even smaller engines designed for business and general aviation aircraft are principally constrained by purchase price, which is a much more important consideration for these relatively lightly used aircraft than is fuel or overhaul cost.) In other words, for economic reasons, small engines are not designed to the same efficiency as large engines.

Figure 3.6, which complements the historical evolution shown in Figure 3.3, shows the variation of motor thermodynamic efficiency with engine size in terms of takeoff power for existing turboprop and commercial turbofan engines. High-power turbofans generally have higher efficiency than turbofans designed for lower power, and all turbofans have higher efficiency than lower-power turboprops. The N + 3 region on the figure refers to NASA terminology for engines that may enter service beyond 2035. As discussed above, the differences between turboprops and commercial turbofans reflect market-driven design intent, different design operating altitudes and airspeeds, and the date of design and therefore the technology level of the engines (in general new commercial turbofans have entered the market more frequently than have new turboprops).

The efficiency of small gas turbines can be improved to the extent that high-efficiency technologies used in large engines can be incorporated in small engines, although that could result in prices that are too high for current small engine markets. Investment in technology specifically aimed at small engines is needed for engine cores having a small physical size to reach efficiency levels comparable to (or better than) large core engines. Physical limitations to such improvements have not been well established and could be an area of fruitful research. Such research addressing the specific needs of small engines intended for commercial transports could enable some distributed propulsion concepts. Perhaps most importantly, since as airplane and engine efficiency improves, less power is needed for flight, the engine size and power required at constant airplane capability will decrease in the

future. Also, the overall pressure ratio² of gas turbines has increased over time to improve thermodynamic efficiency. At the same time, however, the size of the high-pressure compressor, the combustor, and the turbine have decreased, exacerbating the challenges of smaller size.

Potential for Improvement

Since the first aircraft gas turbines were built in the late 1940s, overall efficiency—fuel flow to propulsive power—has improved from about 10 percent to its current value, approaching 40 percent (see Figure 3.2). It is likely that the rate of improvement of these engines can continue at about 7 percent per decade for the next several decades given sufficient investment in technology. The potential for overall improvement is best considered in terms of the constituent efficiencies: thermodynamic efficiency of the motor and propulsive efficiency of the propulsor.

As noted above, it is not clear how close to the theoretical limits it may be possible to come with a gas turbine for commercial aircraft given aviation’s important constraints of safety, weight, reliability, and cost. Several authors have considered the question of the practical limits for simple cycle gas turbines given the potential for new materials, engine architectures, and component technologies. Their estimates of the individual limits of thermodynamic and propulsive efficiency differ somewhat (and may divide losses differently between thermodynamic and propulsive efficiency), but they agree that an improvement of 30-35 percent in overall efficiency compared with the best engines today may be achievable. As shown in Figure 3.7, motor thermodynamic efficiencies of 65-70 percent and propulsive efficiencies of 90-95 percent may be possible.

Some studies suggest that improvements in turbomachinery performance and reduction in cooling losses could improve thermodynamic efficiency by 19 percent and 6 percent, respectively.³ This magnitude of gain is not achieved by simply inserting new technology in existing engines. Rather it requires optimization of the cycle given specific levels of component performance characteristics, temperature capability, and cooling. Practical intercooled or recuperated cycles could increase efficiency by another 4.⁴ Improved fans and propellers could also increase propulsive efficiency by 10 percent.⁵ Of course, the practical limits to propulsive efficiency cannot be addressed at the engine level alone without reference to airplane configuration and propulsion integration, as discussed in Chapter 2.

To summarize, aircraft gas turbine engines have considerable room for improvement, with a potential to improve overall efficiencies by 30 percent or more over the best engines in service today, with the potential for improvement of propulsive efficiency being about twice that of thermodynamic efficiency. This level of performance will require many technology improvements and come in the form of a number of relatively small increments, a few percent or less, rather than through a single breakthrough technology. The following section discusses many of these technologies.

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² The overall pressure ratio is the ratio of the compressor outlet pressure to the compressor inlet pressure.

³ D.K. Hall, 2011, “Performance Limits of Axial Turbomachine Stages,” M.S. thesis, Massachusetts Institute of Technology, Cambridge, Mass.

⁴ J. Whurr, 2013, “Future Civil Aeroengine Architectures and Technologies,” presented at the 10th European Turbomachinery Conference, http://www.etc10.eu/mat/Whurr.pdf.

⁵ D. Carlson, 2009, “A Propulsion Renaissance: New Cycles, New Architectures and the Opportunity for Workforce Development,” presented at the 19th International Society for Air Breathing Engines ISABE Conference, Montreal, Canada.

OPPORTUNITIES FOR REDUCING CARBON DIOXIDE

Improving aircraft fuel efficiency can be considered in two parts. The first is increasing propulsive efficiency. Work in this area is important no matter the choice of motor to power the propulsor. The second part is improving the motor thermodynamic efficiency of an aircraft gas turbine engine. The following sections discuss areas of technology investments that could yield substantial gains in aircraft fuel burn. The general categories listed are not new; the same list would have been appropriate for the past several decades. What are new are many of the particulars of specific investment opportunities. Each advanced technology might offer only a percent or so in improvement, or even less. In aircraft engine development, progress has been made through the development of many relatively small technology steps that together amount to steady improvement.

The relative value of a new technology may very much depend on engine architecture. In other words, a new technology might be very valuable for a particular engine design approach but be much less so for others. Furthermore, newly designed engines are highly optimized at the system level to realize the benefits the incorporated technologies provide. Therefore, a new technology might offer less benefit when applied to an existing engine design than it would when applied to a new design.

Improving Propulsive Efficiency

Independent of the source of shaft power, aircraft are dependent on propulsors (that is, either fans or propellers) to convert the shaft power to thrust. With very few exceptions, large commercial aircraft use turbofan engines. Some regional commercial aircraft with a capacity of fewer than 80 passengers are powered by turboprop engines.

Propellers

Propellers can offer superior efficiency to fans at lower flight Mach numbers at the cost of noise. Such lower speeds are not economically significant at relatively short stage lengths such as 300 nm. Propellers optimized for higher Mach numbers than are currently being flown by propeller aircraft have been demonstrated in flight. At the current state of the art, high flight-speed, unducted propulsors, such as open rotors, face significant noise, mechanical complexity, and installation safety concerns that need to be overcome before they can be considered attractive alternatives to ducted fans, and the committee concluded that they should not be pursued as a high priority for the purpose of reducing CO₂ emissions from large commercial aircraft. Therefore, the discussion of propulsors in the rest of this chapter will focus on the performance of ducted fans used in the turbofan engines of large commercial aircraft.

Turbofan Propulsor

Here, “turbofan” refers to the entire internal flow path of the fan stream, comprising inlet, fan, fan duct, and fan exhaust nozzle, which together comprise the propulsor of a turbofan engine. Improving propulsive efficiency requires dropping the fan exhaust velocity by reducing the fan pressure ratio⁶ as well as the pressure losses along the internal flow path. The fan rotor adds energy to the flow. Some of this energy is then lost to drag along the inlet and duct walls, the fan stators, and imperfect fan nozzle expansion. Thus technology will need to be developed to reduce pressure loss within the fan stream flow path taking into account overall system weight and noise. (Unlike early jet aircraft, for which exhaust jet noise dominated, the noise of most modern large commercial aircraft is dominated by fan noise. Fan duct walls include acoustic treatment, which attenuates this noise but adds weight and pressure loss.) Thus, significant payoffs can arise from advances in technologies such as high efficiency, low noise, low fan pressure ratio (1.35:1 and below), fan turbomachinery with improved acoustic, aeromechanical and stability behavior, fan duct acoustic liners with improved acoustic damping and pressure loss characteristics, as well as lighter blading and containment systems. Advancements in exhaust nozzles, fixed and variable, also fall under this topic. For boundary layer ingestion to become a viable aircraft design approach (see Chapter 2), propulsor-duct solutions must be found that are acoustically and aeromechanically acceptable and in which the losses due to distortion are small compared to the gains from wake cancellation.

Improving Thermodynamic Efficiency

There is a vast literature on aircraft gas turbine engines and the improvements needed to reduce fuel burn. The specifics of which approaches offer the most promise evolve as progress is made and new engine designs are developed. The thermodynamic constraints and current mechanical limitations on improving efficiency are very well understood. Simply put, increasing efficiency requires increasing compressor exit and turbine inlet temperatures while concomitantly reducing aerodynamic losses and structural weight.⁷ Large aircraft gas turbines are now constrained as much by limitations on compressor temperatures as by turbine temperatures. Engineering approaches that permit higher temperatures while reducing or eliminating cooling air are especially valuable. Technologies

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⁶ Fan pressure ratio is the ratio of the pressure at the fan exit to that at the fan inlet.

⁷ Higher temperatures will be accompanied by higher pressures, but accommodating higher pressures is primarily an engineering design task. Developing the ability to accommodate higher temperatures is a much more difficult challenge that can only be overcome through a program of research and technology development.

that allow engines to retain “like new” efficiency would also reduce fuel burn. Now engines lose several percent in efficiency as they age between overhauls, and they do not recover their original performance after overhauls.

Improved aircraft efficiency means that engine cores will shrink since less power will be needed for the same mission. This implies that reduced engine core size will challenge engine efficiency for single-aisle aircraft.⁸ One element of increasing engine thermal efficiency is increasing the overall pressure ratio, which increases the density of the air in the core. The combination of increased thermal efficiency and reduced airplane power requirements means that core size (usually measured in terms of compressor exit area) shrinks. For the same mission aircraft, it has shrunk by a factor of 10 since 1972 and will continue to do so in the future. Also, as discussed above, gas turbine engines for smaller aircraft are less efficient than engines for larger aircraft.

Materials and Manufacturing

The history of the aircraft gas turbine engines is the history of advanced material development specifically aimed at improving gas turbines; some highly successful examples include forged titanium alloys (now widely used in aircraft structure as well), several nickel superalloys, single-crystal turbine airfoils,⁹ forged high-temperature powder metal alloys, coatings for environmental protection and for thermal barriers, and, most recently, titanium aluminides. There are few applications other than gas turbines that can justify the cost of developing these specialty materials, which tend to be expensive to use as well as develop and require decades to move from lab bench to commercial service. Nevertheless, advanced materials have been a particularly fruitful investment area because a successful material can often be used to improve existing engines as well as enable new concepts. There is no reason to believe that this cannot continue to be the case. The system-level benefits from new materials come from reduced weight, higher temperature capability, or reduced cooling, each of which increase efficiency. Even though an aircraft engine application may justify material costs of hundreds or even thousands of dollars per kilogram, cost-benefit is still a major consideration. For example, a large national investment in metal-matrix composites in the 1980s and 1990s resulted in both a technically viable manufacturing process and several successful demonstrations of metal matrix components in engines. Nevertheless, when projected to wide-scale adoption, the parts appeared to be too expensive to be viable.

Even at a conceptual level, it is often difficult to distinguish between materials development and the manufacturing technology required to fabricate parts from that material. This is especially true for many high-temperature materials (such as single-crystal turbine airfoils, powder metal disks, and high-temperature coatings) as well as some polymer composites. This is not the case for materials adopted from other applications such as steel, aluminum, and some nickel alloys, where the material manufacturing is distinct from the part fabrication. New manufacturing methods such as the additive manufacture of high-temperature materials like titanium and nickel superalloys can be considered either an innovation or a confluence of the additive manufacture of plastics (in use since the early 1990s) with the powder metal processing long used for disks. In either case, it represents an alternative path to the realization of complex parts and new materials. It offers intriguing possibilities to realize structures or properties that would otherwise be prohibitively expensive. This technology is in its infancy in terms of dimensional control, surface finish, and material properties, so significant progress should be possible. Manufacturing technology advances such as this may be a significant contributor to improving engine performance, weight, and perhaps cost.

While advanced materials can reduce fuel burn by reducing weight, they can be especially valuable when they improve temperature capability and reduce cooling requirements. This is true for compressor materials to

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⁸ Current engines for twin-engine, twin-aisle aircraft have twice the core size of engines for single-aisle aircraft, so thrust requirements of twin-aisle aircraft would need to drop by more than a factor of two before core size would become an issue for them.

⁹ “Airfoil” refers to the stationary vanes, or stators, in a turbine and the rotating blades.

enable higher compression ratios needed to improve engine thermal efficiency (a capability of 1300°F to 1500°F is desired in the near term) as well as for combustors and turbines to improve engine power-to-weight ratios (where long lives at material temperatures of 2200°F to 3000°F are needed). Materials can also improve part durability to retain rather than increase fuel burn as an engine ages.

The most fruitful areas of materials research at this time appear to be in advanced high-temperature metals, ceramics, and coatings:

• High-temperature ceramics. This is an area that may see considerable progress over the next decades. This includes ceramic matrix composites (CMCs) as well as monolithic ceramics. Some CMCs are already entering commercial service. Additional CMCs and monolithics may enter commercial service in the next few years, and, should they prove viable and cost effective at large scale, will see widespread use. The advantage of these materials is their high-temperature capability and low density. Challenges include low fracture toughness, low thermal conductivity, and manufacturing cost. The materials, which could enter service in the next few years, are capable of service at 2200°F -2400°F. Of particular research interest are less developed high-temperature materials, ones with capability up to about 2700°F, which would dramatically reduce or eliminate cooling in many parts of an engine and thus boost efficiency and lower weight.
• High-temperature metallic alloys. Advances in these alloys will arise from further development of nickel-based alloys as well as new materials classes such as niobium and molybdenum. Nickel-based materials can be improved by moving to disks constructed from dual or graded alloys or even single crystals. While denser than the ceramics, niobium and molybdenum have temperature capability approaching that of CMCs and much higher fracture toughness and thermal conductivity. This combination of properties makes them potentially attractive for static, internally cooled parts such as turbine vanes or combustors. Work is needed on fabrication technologies and coatings for environmental protection.
• Coatings. Coatings can add value to many engine parts. They are required at high temperature for environmental protection. For cooled parts, thermal barrier coating can significantly increase the temperature capability and reduce cooling requirements. Erosion coating can extend part life and retain performance. Ice-phobic coating can reduce the threats posed by ice formation. Further progress in coatings of all types can be expected given sufficient investment.

Turbomachinery

The state of the art in compressor and turbine turbomachinery efficiency is about 90 percent, while studies suggest that efficiencies of better than 95 percent may be possible.¹⁰ Thus, there is considerable room for improvement. Applications of interest include aerodynamics, aeromechanics, and the mechanical arrangements of complete components, especially those that enable higher compressor discharge temperatures. Improved analysis tools and emerging manufacturing technologies may open new approaches or make old ideas feasible. Historically, turbomachinery efficiency improved as machine size increased, all else remaining equal. As engine and airplane efficiency improves, less thrust is needed for a given mission, so the size of engine turbomachinery shrinks. Also, as the overall pressure ratios (OPRs) of engines have been increased to improve thermodynamic efficiency, the flow areas and thus the dimensions of airfoils in the core, especially at the rear of the compressor and in the high-pressure turbine, have shrunk dramatically. Indeed, the newest engines entering service at the 30,000 lb thrust level have the same core diameter as older designs that are still in production and deliver only one-fifth the thrust. Current turbomachinery design trades between size and efficiency are based on empirical practice rather than first principles limitations.¹¹ This implies that research to realize higher efficiency at small sizes could reduce the fuel burn of advanced aircraft. Obvious areas of concern include sensitivity to geometry variations such as tip clear-

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¹⁰ D.K. Hall, 2011, “Performance Limits of Axial Turbomachine Stages,” M.S. thesis, Massachusetts Institute of Technology, Cambridge, Mass.

¹¹ A.H. Epstein, 2014, Aeropropulsion for commercial aircraft in the 21st century and research directions needed, AIAA Journal 52(5):901-911.

ance and airfoil shape, which become more challenging as size is reduced. Manufacturing technology investments could assist here.

Work on analytical tools can help progress in this area. Significant investments over 40 years have yielded complex computer simulations that analyze turbomachinery aerodynamics at the design point. These tools are inadequate at important operating conditions away from the design point, such as idle. Mechanical analysis tools suffer from inadequate models of nonlinear mechanical interactions such as friction, sliding interactions, and plastic deformation. Aeromechanics is another turbomachinery discipline in which physics-based simulations are not yet capable of adequately predicting engine behavior over the entire operating regime. Overall, the advancement in the accuracy and speed of simulation tools so that they can be better used to optimize the overall engine system in a timely manner during design may add several percentage points of improvement in fuel burn and certainly reduce development cost and time.

In conclusion, although there have been substantial investments in turbomachinery over many decades, efficiency, weight, and cost could still be improved significantly.

Cooling and Secondary Flow Reduction

A modern engine uses 20-30 percent of the compressor core flow for hot section cooling and purging. This is a direct debit to engine efficiency since the work that must be done to compress this air is only partially recovered as thrust. Turbine cooling is another area that has received considerable attention over decades. Improved methods have reduced the amount of cooling air required and enabled longer engine life even at higher temperatures. Manufacturing technologies to realize sophisticated cooling schemes have been one area of progress, but more can be done here, especially for nonmetallic materials. Another constraint on cooling is the clogging of small passages and holes over time by dirt ingested by the engine.¹² Currently, cooling hole sizes are dictated by clogging concerns rather than by cooling efficiency—that is, the holes are oversized to keep them from clogging. Thus, technologies that improve dirt separation and rejection could contribute to a reduction in fuel burn. These challenges are exacerbated as engine size is reduced.

Combustion Systems

Current combustion systems are better than 99 percent efficient in converting the chemical energy in fuel to heat.¹³ The design challenges are mainly ones of retaining this level of performance and the reliability needed for commercial airline service while reducing regulated emissions. Both lean burn and rich burn approaches have proven competitive to date. Continued emissions work will be needed given the expected tightening of emissions requirements coupled with the increase in engine pressure ratio that will be needed to further reduce fuel burn. As engine overall pressure ratios are increased to improve thermodynamic efficiency and reduce CO₂, combustor design will be further challenged to meet both emissions and mechanical integrity goals. Areas that may be helpful include new design concepts and improved modeling tools, especially physics-based approaches capable of accurate prediction of regulated emissions. Alternative fuels to date are compatible with existing combustor technology. New approaches to combustor design may be able to significantly shorten combustor length, thus reducing engine weight and CO₂ emissions.

Controls, Accessories, and Mechanical Components

Overcoming the limitations and constraints of existing engine controls and accessories such as generators, pumps, and heat exchangers offers the potential to improve fuel consumption, reduce weight, and reduce cost. This

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¹² Fuel consumption degrades as an engine is operated because deposits (a.k.a. dirt) accumulate on airfoils and reduce their aerodynamic efficiency, as evidenced by the fact that semiannual engine washing can improve fuel burn by about 1 percent. Dirt can also cause erosion that increases tip clearance, which increases fuel burn, and dirt can clog cooling holes in the turbine. These effects are much worse in places with poor air quality.

¹³ Arthur H. Lefebvre, 1998, Gas Turbine Combustion, second ed., CRC Press, Boca Raton, Fla.

is an area in which there has been little research over the past few decades. While many advanced engine control architectures have been proposed and analyzed, the lack of enabling hardware, including processors, sensors, and actuators with the needed temperature capabilities, has inhibited practical application. As aircraft subsystems become more electrical and as fan pressure ratios drop to improve propulsive efficiency, this challenge will be exacerbated. The inefficiency of current fuel pumps consumes much of the heat capacity of the fuel flow that would otherwise be available for the cooling needed by other aircraft heat sources. Therefore, improving fuel pump efficiency, especially at low fuel flows, would reduce the size and pressure drops associated with other engine and aircraft cooling requirements. Heat exchangers, which are addressed in more detail below, are far from their theoretical maximum performance.

Taken together, engine accessories occupy a significant portion of the propulsion system volume, especially on smaller engines; this problem becomes more challenging as fan pressure ratio is lowered to improve propulsive efficiency. Reducing the volume of these accessories could lead to lower fan pressure ratios by enabling better nacelle designs. Overall, improving the performance, efficiency, and size of external components such as pumps, heat exchangers, and controls would help to reduce CO₂ emissions.

Gas turbine mechanical components such as bearings and seals offer many opportunities for improvement. Bearings and their need for cooling and lubrication add considerable complexity to an engine. The bearings in a midsized gas turbine dissipate about 100 kW into the oil, heat that must be rejected to the fuel or the environment. The oil system of a modern gas turbine is exceedingly complex. One reason is that bearings are located where the ambient temperatures exceed the autoignition temperature of the oils. Thus the bearing compartments must be cooled with seals to inhibit oil leakage. Efforts to replace oil-lubricated, rolling-element bearings have not been successful to date, but the combination of smaller engine cores, advanced analytical techniques, and new materials may permit the use of either air bearings or magnetic bearings on smaller commercial aircraft. Air bearings have been used for decades on aircraft environmental control systems and some auxiliary power units, so safe, long-term service has already been demonstrated, albeit in less thermally demanding environments. Modeling and materials work could help here. Industrial magnetic bearings are used on some ground-based power turbines and on industrial pumps and compressors. In addition to elimination of oil and the oil system, they offer the potential advantage of active control of rotor dynamics, a serious issue for aircraft engines. Challenges in the past include the weight and volume of the power electronics needed, as well as high-temperature capabilities of the magnets themselves. There has been much progress here in the past two decades, especially in power electronics, so this may be another area that could contribute significantly to improving aircraft engines.¹⁴

Alternative Thermodynamic Cycles

Engines in commercial service today use simple Brayton cycles. There are many variations of the Brayton cycle that could theoretically offer improvement. Regenerative cycles capture heat from the exhaust and move it to the compressor to improve engine performance when operating off the design point. Intercooled cycles cool the air during compression to improve compressor efficiency while reducing compressor discharge temperature. Combined cycles capture some of the exhaust heat, which is then routed to a Rankine cycle to produce additional power for a given fuel burn. These cycles all require large (relative to the motor) heat exchangers, which add considerable weight, volume, cost, and maintenance burdens. While prevalent in ground power plants, to date they have not been used in aircraft engine applications because these cycles have not appeared attractive given the current state of the art of components. (Intercooled and combined cycle gas turbines are extensively used in ground-based power generation, where size, weight, and on-off cycling are lesser issues.) Significant improvements in heat exchanger technology would be required to make such approaches viable for low-carbon propulsion of commercial transport aircraft. These advanced engine cycle concepts are constrained by the capabilities of current heat exchanger technology.

Intermittent combustion approaches and those that use shock waves have been studied for many decades and in some cases have been brought to the point of laboratory demonstration. For example, the Humphrey cycle uses

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¹⁴ Power electronics are discussed in detail in Chapter 4.

unsteady processes to realize a pressure gain in the combustor rather than the pressure drop of a Brayton cycle, but it does so at a loss in combustion efficiency. The Humphrey cycle poses several engineering challenges, including the mechanical integrity of the system with large pressure pulses. The potential value of various hybrid cycles to commercial aircraft propulsion for fuel burn reduction has yet to be clearly established. The committee determined that hybrid cycles should not currently be considered a high-priority research area for subsonic commercial aircraft compared to other investment opportunities.

Heat Exchangers

Heat exchangers are an important part of any propulsion system, air-breathing or electric. Their temperature capability, life, volume, and weight are limiting in many applications. Current turbofan engines use heat exchangers to cool engine oil, generator coolant, and bleed air to the aircraft. In the near future, some engines will soon use heat exchanges to produce cooling air for the turbines. As cores get smaller and electrical demands grow, more heat must be rejected to the fan stream. At the same time, as fan pressure ratios drop, this heat rejection becomes increasingly expensive in terms of fuel burn, weight, and volume. Some advanced cycle concepts are even more dependent on heat exchanger technology. Indeed, the viability of airborne intercooled and regenerative cycles is constrained by heat exchanger penalties. This may be an even larger constraint on electric and hybrid-electric approaches in which the heat is of low quality, exacerbating heat rejection penalties. Airborne heat exchangers have not seen much progress over many decades. Heat exchangers used on ground-based engines are often the largest and most expensive component and the one requiring the most maintenance. Airborne concepts are needed that reduce pressure drop, weight, and volume per unit heat transferred; work at high temperatures; and have longer life and lower cost. New manufacturing technology, such as additive manufacturing, may enable new concepts.

RATIONALE FOR GAS TURBINE ENGINE RESEARCH

The overall efficiency of commercial aircraft engines has been improving at a rate of about 7 percent per decade since 1970 (see Figures 3.3 and 3.4). Today, the overall efficiency of commercial aircraft propulsion is approaching 40 percent. Aircraft engines are not mature: Given sufficient investment, there is a potential to continue this rate of improvement for the next several decades. Additional benefit may be realized by innovative propulsion–airframe integration technologies, discussed in Chapter 2.

CHALLENGES

Aircraft gas turbine challenges were discussed above to elucidate some of the many opportunities available to improve engine performance. These opportunities are often presented in a traditional, disciplinary sense:

• Materials and manufacturing,
• Turbomachinery—aerodynamics and structural concepts,
• Heat exchangers,
• Low-emissions combustion systems operating at very high pressure ratios,
• Controls and accessories,

• Manufacturing, and
• Improved simulation capability.

To focus on improving efficiency and CO₂ as fast as possible at given levels of investment, it is useful to consider the challenges and research opportunities by topical area. Overcoming the challenges will require a mix of disciplines to become an engineering reality and will involve work on both scientific advances and design concepts. Balanced investments in simulation and experimental capabilities are needed. In each area, research is needed not only for the advancement of methods and materials, but also to provide explicit resources for the exploration of new concepts. As discussed above, many gas turbine propulsion technologies could be advanced to reduce aviation’s CO₂ emissions. The areas whose promise for reducing CO₂ emissions over the next three decades justifies the most investment are summarized in the following challenges:

Technical Challenges

Propulsive Efficiency

Low fan pressure ratios are needed to reduce exhaust velocities and thereby improve propulsive efficiency, regardless of whether the fan is driven by a gas turbine or an electrical motor. For a constant level of thrust, this requires that the effective fan area increase so as to avoid commensurate increases in weight, drag, and integration losses.¹⁵

Thermodynamic Efficiency

Enabling higher operating temperatures is a prerequisite to achieving significant improvement in gas turbine engine thermodynamic efficiency, and a major impediment to achieving higher operating temperatures is the difficulty of developing advanced materials and coatings that can withstand higher engine operating temperatures.

Small Engine Cores

Activities being pursued to either improve the thermodynamic efficiency of gas turbine cores or improve overall aircraft efficiency result in smaller core sizes. For single-aisle aircraft, this tendency to core size reduction creates multiple challenges for maintaining and improving efficiencies of the overall engine and engine-aircraft integration.

Improvements in overall aircraft efficiency from better airframe and engines design will reduce the engine power needed and thus the physical size of the engine core for a given aircraft. This trend to smaller cores will be exacerbated by the need to increase engine overall pressure ratios to improve thermodynamic efficiency. Efficient small cores can also be an enabling factor for distributed propulsion architectures with gas turbine engines.

RECOMMENDED HIGH-PRIORITY RESEARCH PROJECTS

Low-Pressure-Ratio Fan Propulsors

Develop low-pressure-ratio fan propulsors to improve turbofan propulsive efficiency.

Key research topics for this project are turbomachinery design, duct losses, acoustics, aeromechanics, nacelle aerodynamics and weight, manufacturing, and aircraft integration. The degree to which propulsive efficiency can be improved will reflect design optimizations for all of these factors. A less certain investment would be in research

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¹⁵ This challenge, which also appears as a challenge for aircraft–propulsion integration, is listed as a challenge for gas turbine research because it is a prerequisite for achieving significant improvement in gas turbine engine propulsive efficiency.

aimed at both loss and noise reduction for fans in the presence of the distorted inflow characteristic of BLI-wake cancellation schemes, which are attractive only if the losses and noise incurred by the distorted propulsor are relatively small. This research project is closely related to the aircraft–propulsion integration research project on nacelles for ultrahigh-bypass-ratio gas turbines, and work on these two projects should be closely coordinated.

Engine Materials and Coatings

Develop materials and coatings that will enable higher engine operating temperatures.

Key research topics for this project are advanced materials that could lead to the reduction or elimination of turbine film cooling as well as to compatible coatings for environmental protection, erosion prevention, ice rejection, and thermal barriers.

Small Engine Cores

Develop technologies to improve the efficiency of engines with small cores so as to reach efficiency levels comparable to or better than engines with large cores.

Key research topics for this project are turbomachinery aerodynamic performance, manufacturing, tip clearance control, secondary flow losses, thermal management, combustion, and the life span of turbine airfoils.