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. 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. High-power turbofans generally have higher efficiency than turbofans designed for lower power, and all turbofans have higher efficiency than lower-power turboprops.
Systems of Commercial Turbofan Engines : An Introduction to Systems Functions
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. Also, the overall pressure ratio 2 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. Since the first aircraft gas turbines were built in the late s, overall efficiency—fuel flow to propulsive power—has improved from about 10 percent to its current value, approaching 40 percent see Figure 3.
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.
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 percent in overall efficiency compared with the best engines today may be achievable.
As shown in Figure 3. Some studies suggest that improvements in turbomachinery performance and reduction in cooling losses could improve thermodynamic efficiency by 19 percent and 6 percent, respectively. 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.
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. 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. 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 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 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 CO 2 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. Improving propulsive efficiency requires dropping the fan exhaust velocity by reducing the fan pressure ratio 6 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. 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. 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.
Engineering approaches that permit higher temperatures while reducing or eliminating cooling air are especially valuable. 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. 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. 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 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. 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 s and s 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 s 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. 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:.
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. 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, 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. Obvious areas of concern include sensitivity to geometry variations such as tip clear-. Epstein, , Aeropropulsion for commercial aircraft in the 21st century and research directions needed, AIAA Journal 52 5 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.
A modern engine uses 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. 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. Current combustion systems are better than 99 percent efficient in converting the chemical energy in fuel to heat. 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 CO 2 , 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 CO 2 emissions. 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.
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. Lefebvre, , Gas Turbine Combustion , second ed. 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 CO 2 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 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.
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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. 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.
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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. 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.
It is also necessary to understand the operation and the design of its auxiliary systems. This book fills that need by providing an introduction to the operating principles underlying systems of modern commercial turbofan engines and bringing readers up to date with the latest technology. It also offers a basic overview of the tubes, lines, and system components installed on a complex turbofan engine.
Readers can follow detailed examples that describe engines from different manufacturers. The text is recommended for aircraft engineers and mechanics, aeronautical engineering students, and pilots. In he earned his degree in aeronautical engineering at the Hamburg University of Applied Sciences. He has several years' experience as an instructor for engine and aircraft type training at Lufthansa Technical Training.
Table of Contents Introduction.
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