Jet engine design drivers: past, present and future
The history of aero gas turbines is a history of innovation. Every challenge presented to the aero engine industry has been met with ingenuity, and today engineers continue to work on innovative concepts to meet the next set of challenges facing the aviation industry. This chapter will discuss past, present and future drivers for aero gas turbine development. It first looks at the motivation behind the development of the jet engine, and then outlines the technological and business drivers that have contributed to shape the first 50 years of aero gas turbine development.
New technologies that result in substantial leaps are often referred to as disruptive technologies. These are innovations that change the rules of the game, introducing a new value proposition. They do not have to outperform the technologies they seek to replace in every sense, but they must offer some new valuable feature that will justify their implementation, at least at a small scale. If enough momentum can be gathered, the new disruptive technology will evolve to replace its predecessor.
The jet engine is a prime example of a disruptive technology. Early jet engines performed poorly compared with the well-established piston engines of the time. However, they opened the door to faster air travel at higher altitudes (see Fig. 4.1), allowing passengers to fly above the weather and resulting in shorter, more comfortable trips. These advantages allowed the gas turbine to replace the piston engine as the prime mover of choice in civil air travel. The initial fuel consumption, reliability and cost issues were overcome, making air travel accessible to evergrowing numbers of passengers.
In the late 1920s, Frank Whittle – an RAF officer – became interested in high-altitude, high-speed flight. He saw it as the key that would enable aviation to grow further, providing safer, faster and more comfortable travel above the weather. Whittle concluded that incremental improvements in existing piston engine–propeller configurations were unlikely to enable flight at higher altitudes and speeds, and this conclusion led him to begin the development of what would eventually become the first jet engine.
For civil aircraft, the jet engine transformed the ‘earning capacity’ of aircraft, forming the basis of mass civil aviation in the following decades. The world’s three major gas turbine manufacturers, Rolls-Royce, Pratt and Whitney and General Electric, all started their gas turbine businesses based on Whittle’s W2/700 gas turbine. Nevertheless, much work would still be required to bring gas turbines to the standard we know today. As Whittle himself said, ‘inventing it was easy – making it work was the difficult bit!’ (Whittle, 2007).
Launching a large civil gas turbine requires anywhere between US $500 m and $2.5 bn (Singh, 2007). It typically takes between 15 and 25 years to break even after the initial investment (see Fig. 4.2). In essence, every time a gas turbine manufacturer launches a new gas turbine, it is ‘betting itself’ in the process. The risks are so great that many famous companies – such as Bristol, Armstrong-Siddeley and De Havilland in the UK – have disappeared or been absorbed by other companies along the way.
The timescales and capital investment required make it extremely difficult for new players to enter the aero gas turbine business. In fact, the long-term risks and exposure associated with the business are so great as to defy business logic, at least on the surface. What has driven large corporations to expose themselves repeatedly, putting their reputations and those of some of the world’s best engineers and managers on the line?
The reason lies in the strategic importance of gas turbine technology within the wider industrial context. The development and use of gas turbines requires a substantial upfront investment, but the resulting technology and infrastructure will benefit a country’s industrial base in the long term. The benefits are such that many governments are willing to heavily subsidise this industry. For this reason, some new companies have succeeded in breaking the Anglo-American dominance in aero gas turbines in certain sectors.
Once an aero gas turbine company has been in business for a number of years, it becomes increasingly possible for it to finance new developments by means of aftermarket revenues, which will be described later in this chapter. In addition, the technology developed for aero gas turbines can be transferred into other sectors with different exposure patterns to economic downturns, such as the marine propulsion industry or the power generation industry. This is the case, for example, of General Electric and Rolls-Royce, whose energy and marine propulsion revenues amounted to almost half the aero gas turbine turnover in 2008 (Rolls-Royce, 2009).
Another strategy adopted by gas turbine manufacturers that has helped the aerospace industry is the use of ‘common cores’. As civil aviation grew, an increasing number of aircraft types were launched, and engine companies were expected to offer engines with thermodynamic cycles precisely suited to the aircraft duty and operation. Instead of developing new gas turbines from scratch, existing core designs were adapted in order to minimise the time, cost and risk of development. Airframers welcomed this, as it improved the earning capacity of the aircraft they were offering to airlines.
The efforts of aero gas turbine companies have been partly responsible for the increase in aircraft productivity over the last five decades. Large aircraft today are eight times more productive in terms of seat miles per year than airlines in the 1950s. How was this accomplished, and what was the role of gas turbine technology? The next section will examine some of the most important challenges and drivers for jet engine development over the last 50 years.
The last 50 years have seen enormous changes within the field of aero gas turbines. The early 1950s saw the introduction of the first commercial gas turbine powered aircraft, the Viscount, powered by the Dart Turboprop gas turbine, followed by the early civil jet-powered aircraft, such as the Comet and the Boeing 707. The early 1970s saw the introduction of the supersonic civil airliner, the Concorde, powered by the Olympus 593 jet engine. The Anglo-French Concorde was an important example of international collaborative programmes, which were to become increasingly important for both military and civil gas turbines. The early 1970s also saw the launch of the wide-bodied aircraft: the Boeing 747, the DC10, and the Lockheed L1011. The Boeing 747, in particular, marked the start of the mass civil air transport market, with leisure travel becoming more important than business travel.
Throughout this period, aero gas turbine performance has never ceased to improve, and their commercial success has been mainly driven by technology. In fact, technology is so critical that a relatively small change in component efficiencies, cycle temperatures or pressure ratios can render an engine wholly uncompetitive.
In order to understand the parameters that drive gas turbine development, it is useful to first define how aero gas turbines work. In its simplest form, an aero gas turbine is composed of an inlet, a compressor, a combustor, a turbine, and a nozzle. The air enters the engine through an inlet, and its total pressure is increased by the compressor before entering the combustion chamber. There, fuel is burned and mixed with the incoming air, increasing its total energy. The gases then pass through the turbine, which extracts the work necessary to drive the compressor. The exhaust gas is then expelled through the nozzle, ‘pushing’ against the atmospheric air to generate thrust.
Fuel consumption in aero gas turbines depends on two main parameters: thermal efficiency and propulsive efficiency. The former is a measure of how well the engine converts heat energy, and the latter describes how well the engine uses the kinetic energy of the jet to generate a thrust force. High thermal efficiency requires a high overall pressure ratio (OPR), high component efficiencies and a high turbine entry temperature (TET), whereas high propulsive efficiency results from low jet velocities.
The following section examines how the different design parameters have evolved over the years to meet different challenges in a quest for lower fuel consumption, weight, costs and environmental issues.
A gas turbine’s core specific power is the energy available to generate thrust (or drive the fan, in the case of turbofans). It is a crucial parameter to reduce the weight of the engine, and, if used in conjunction with a fan, it can lead to better propulsive efficiency. Over the past 50 years, core specific power has risen by a factor of five over the early Whittle engines. This improvement has resulted from increasingly high TET values. Investments in high-temperature technology have been directed at both the thermal capability of hot section materials (e.g. single crystal alloys and ceramic coatings) and turbine cooling (see Fig. 4.3). Whereas the TET of early engines only reached about 1000 K, today’s turbofans operate with TET values of about 1800 K at takeoff and OPR values of around 45. The limit for hydrocarbon fuels is the stoichiometric temperature of about 2600 K, but NOx emissions tend to rise with higher temperatures, and it might be necessary to limit TET to a value somewhere between 2000 and 2100 K for environmental reasons.
Considerable research has been undertaken to improve major components in the engine ‘gas path’. At first, the efficiency and weight of components were improved by means of experimentation, but high-performance computing marked the advent of computational fluid dynamics (CFD), which allowed detailed simulation and optimisation of components. Some of the resulting improvements have been 3D blade shapes, ‘wide chord’ fan blades, and hollow and composite blades. The cumulative effect of these polytropic efficiency improvements has been very significant, increasing thermal efficiency and specific power. The thermal efficiencies of current engines in service are approximately 45%, based on polytropic efficiencies of 88% at cruise conditions. During the early part of the twenty-first century, it is likely that engines will enter service with thermal efficiencies approaching 50%, based on polytropic efficiencies of 92% and increases in cycle pressure ratio and turbine entry temperature.
Perhaps one of the most significant improvements to the gas turbine has been the turbofan. By the 1960s, aviation traffic growth made it imperative that jet noise be reduced. It was recognised that jet noise increases with the seventh power of velocity, and a solution was sought to decrease jet speed to an acceptable level. By that time, core specific power had increased enough to allow the excess power to be used to drive a low-speed fan. This fan would be driven by an additional turbine, and produce a similar amount of thrust by moving large masses of cooler air at lower velocities.
Early turbojets achieved overall efficiencies of about 20%. Early turbofans increased this value to about 25%, and the current generation of high bypass ratio turbofans achieve overall efficiencies of around 35%. The result is more efficient engines (see Fig. 4.4) that lead to a higher aircraft earning capacity. This in turn allowed air travel to become accessible to an ever-increasing proportion of the population.
However, there is a limit to how much air can be diverted to the fan: as bypass ratios are increased (hence, more air ‘bypasses’ the core), the overall diameter of the engine increases, resulting in a drag and weight increase that eventually offsets the advantages offered by a higher propulsive efficiency.
In the early years, gas turbine maintenance was limited to run-to-failure strategies, visual inspections and oil monitoring. In the 1960s and 1970s, this strategy ceased to be acceptable, as growing numbers of passenger miles per year meant that failures were more likely to occur. The paradigm shifted accordingly to preventive maintenance: frequent overhauls were deemed the best solution, and engine manufacturers began generating substantial revenues from selling components to replace those deemed unfit during overhauls. In the 1980s and 1990s, advances in electronics and computing enabled the use of automatic engine health monitoring (EHM) techniques, revolutionising the way engine maintenance is carried out and increasing the safety and availability of aero gas turbines.
The early years of aero gas turbines were characterised by short mean times between failures (MTBF). Engine manufacturers began offering spares and overhaul services to aircraft operators, and planned maintenance was carried out at fixed intervals as recommended by the manufacturer. Despite the high safety margins, much of the maintenance was unplanned, and not much was done to improve the situation, since research emphasis was placed on achieving better performance.
The economic crisis brought about by the steep rise in oil prices in the early 1970s began to change the face of the aftermarket. Airlines, faced with lower profits and higher costs, demanded lower prices from engine manufacturers. As a result, the purchase prices of engines were driven down in a trend that has lasted to the present day. Faced with minute margins on engine sales, engine manufacturers found a means of survival in the aftermarket: spares were sold to airlines and third-party maintenance, repair and overhaul (MRO) outfits at increasingly high prices, recouping the losses made by offering low purchase prices.
The late 1980s and the 1990s were characterized by increased reliability and longer engine lives. Figure 4.5 illustrates the IFSD (in-flight shutdown) rate trend over the years for some of the main Rolls-Royce engines. As can be seen, the removal rate for new engines is less than a fifth of what it was in the 1970s.
4.5 Improvements in reliability. (Source: data from Howse, 2004.)
The application of techniques such as gas path analysis and vibration monitoring to engine diagnostics allowed operators to carry out predictive maintenance, resulting in increased reliability and engine lives. MRO shop utilization dropped, and airlines began to outsource maintenance.
Engine manufacturers have realised the importance of the aftermarket, and they are expected to capture an increasing share. Partnerships between engine manufacturers, airlines, and third-party MROs are set to become the norm in the industry. The concept of power by the hour, for example, allows airlines to pay a fixed fee for every hour the gas turbine is used, transferring the risks associated with reliability to the engine manufacturer, who also enjoys more predictable revenues. The trend seems to point towards asset management, where airlines could transfer ownership of the engine to the gas turbine manufacturer. In this case, however, manufacturers might have to finance their own engines upfront. This could lead to the emergence of an engine financing market similar to the one already existent for airframes.
There is a risk, as gas turbines become more and more reliable, that the aftermarket revenues will decrease sharply. If this happens, the business model will have to evolve again in order to ensure that the industry remains viable.
Whilst speed, range and comfort were the primary drivers in civil aviation in the early twentieth century, the picture has changed with the new millennium. Two concerns have drastically increased the need for fuel-efficient technology over the past decade: depletion of natural fuel reserves and global warming. Scientists fear that, with ongoing levels of current greenhouse gas emissions, global temperatures could increase by several degrees by the end of the twenty-first century. Solar radiation absorbed by the planet’s atmosphere and surface is converted into heat energy. The natural occurrence of greenhouse gases results in equilibrium conditions where terrestrial infrared radiation is offset by incoming solar radiation, and the energy fluxes of the ingoing and outgoing radiation are approximately equal on average, keeping the planet at an inhabitable temperature.
Pollutants of human origin, such as CO2, impose a perturbation on the equilibrium state by absorbing additional heat energy. The amount of energy absorbed is generally denoted by radiative forcing (RF), which is the amount of radiation forced to remain within the ‘Earth system’, i.e. the atmosphere, oceans and soil. As more and more heat is absorbed, the Earth’s temperature increases. A temperature rise will then occur, continuing until the outgoing and the ingoing radiation are in equilibrium again. The expected temperature change due to a particular pollutant depends on the strength of its radiative forcing. A positive radiative forcing will cause a temperature rise, whereas a negative radiative forcing will cause a temperature drop by shielding the Earth from incoming radiation. Furthermore, the temperature change is also determined by a pollutant-specific climate feedback parameter. It can be understood as a gain factor, accounting for three-dimensional atmosphere–ocean feedback mechanisms such as cloud or sea ice formation. The feedback parameter determines how effective the radiative forcing of the pollutant is and over what time period the temperature rise will take place.
Carbon dioxide is the dominant mode through which carbon is constantly transferred in the natural environment between a variety of carbon reservoirs (see Fig. 4.6). Burning fossil fuels, representing one reservoir, implies taking carbon from beneath the Earth’s surface, converting it to CO2 and emitting it into the atmosphere. Atmospheric carbon dioxide is absorbed by natural carbon dioxide sinks such as oceans, which take up carbon and facilitate the formation of carbonates and forests. The process of carbon transfer between carbon reservoirs is referred to as the ‘carbon cycle’. The consequences of additional CO2 in the atmosphere, causing a global temperature rise, are projected to be devastating: sea level rise, frequency and intensity of extreme weather events, changes in agricultural yields, and increases in the ranges of disease vectors, to name just a few. These consequences might have a catastrophic impact on our society, and current public consensus calls for an urgent reduction to counteract current trends. Although aviation only represents approximately 5% of man-made carbon emissions, projected passenger and cargo growth rates make it a potential major stakeholder in the drive towards lower emissions, and more environmentally friendly aircraft technology will be required.
However, CO2 is not the only pollutant resulting from air traffic, and the fact that pollutants are released in areas of the atmosphere where their impact is potentially worse than at sea level results in environmental technology trade-offs between different pollutants. For example, higher turbine entry temperatures in aircraft engines result in higher thermal efficiencies and less CO2 emissions, but also cause an increase in nitrogen oxide (NOx) emissions. These contribute to the formation of ozone in the upper troposphere and decrease the concentration of methane, another greenhouse gas, implying a cooling of the atmosphere. Both effects, tropospheric ozone formation and methane depletion, approximately cancel each other out. Nevertheless, stratospheric NOx emissions facilitate a reduction of ozone where it is needed to shield the earth from highly energetic cosmic radiation. When emitted at low altitudes, e.g. in the vicinity of airports, NOx exhibits additional environmental and health hazards. NOx can react to form nitric acid, which can penetrate into sensitive lung tissue and causes severe health damage and in extreme cases premature death. Inhalation of such particles may cause or worsen respiratory and heart diseases.
Water, which in its vapour phase is a relatively strong greenhouse gas, is also a combustion product. Water emitted by aircraft precipitates relatively shortly after emission, whereas stratospheric water can have longer residence times and hence cause radiative forcing, even though it is marginal compared with other aviation pollutants.
Elevated atmospheric aerosol concentrations caused by air traffic are relatively small compared with the contribution from surface sources. Soot emissions tend to warm the atmosphere, whereas sulphates have an opposite effect. Compared with other aircraft emissions, the direct radiative forcing of soot and aerosols is relatively small. Aerosols also play an important role in the formation of cirrus clouds that would not form in the absence of aviation, resulting in enhanced cloud formation and the modification of the radiative properties of natural cirrus clouds.
Contrails are artificial clouds composed of ice crystals. They form in the aircraft wake if water saturation occurs during the process of mixing the engine’s exhaust gases with ambient air. An analogy can be found by comparing the formation of contrails to human breath in a cold winter day, which can become visible in the form of steam. Contrails can persist in the atmosphere if it is ice-supersaturated. Due to the shape and size of contrail ice crystals, contrails reflect incoming solar radiation to a lesser extent than terrestrial infrared radiation. As a result, heat energy is trapped in the atmosphere below the contrail. Although radiative properties of contrails vary depending on the ambient conditions, occasionally even leading to local cooling of the atmosphere below the contrail, the average effect of global contrail occurrences is a net contribution to global warming.
Moreover, dependent on ambient conditions, line-shaped persistent contrails can spread to form large-scale cirrus clouds, so-called contrail cirrus. Since they cover much larger areas than line-shaped contrails, and thus trap more heat in the atmosphere, it is estimated that their environmental impact is larger than that of line-shaped contrails. Furthermore, locally elevated atmospheric soot and aerosol concentrations due to air traffic can initiate water nucleation, leading to the formation of so-called secondary cirrus clouds. These clouds, with radiative properties similar to those of contrails, also contribute to global warming. It is important to note that they would not form in the absence of air traffic.
Recent climate assessments have emphasised the environmental impact from persistent contrails and contrail cirrus. Their radiative forcing might already exceed the radiative forcing from all other air traffic pollutants combined. This concern has initiated the search for strategies and technologies to avoid their formation.
Apart from environmental concerns, there is another driver away from the fossil fuel-based economy. The decrease of natural fuel reserves and its political implications demand alternatives. As hydrocarbon supplies diminish, fuel prices will increase and higher fuel prices are most likely to lead to increased alternative, renewable energy supplies. Renewable energy sources are currently more expensive than conventional fossil fuels sources, but may become economically viable in the near future. While there are enough coal reserves to make it a viable energy source well into the next century, associated environmental implications would create a need for drastic environmental protection measures such as CO2 sequestration.
The challenge of the environment has spurred the aero gas turbine industry into undertaking research programmes to achieve more environmentally friendly engines in the short to medium term. Figure 4.7 offers a view of the evolution in gas turbine fuel consumption from 1960 to the present day. It is possible to see a very large improvement in the early years, partly due to the adoption of the turbofan, and a constant reduction in fuel consumption ever since. Research and development programmes today are looking at concepts that could lead to step fuel efficiency improvements. In the short to medium term open rotor configurations might be seen to emerge, and recuperated cycles might become viable in some applications. However, long-term solutions could require more radical innovations. The following section will consider the prospects in the near term, and the drivers and technologies that might affect aero gas turbines in the long term.
4.7 Aero gas turbine fuel consumption evolution. (Source: Green, 2005.)
Research oriented towards near- to medium-term improvements is mainly directed at increasing thermal and propulsive efficiency of conventional aero engines. Aside from research into conventional supporting technologies, it is worth mentioning two promising lines of research: intercooling and recuperation to increase thermal efficiency, and open rotor technology to improve propulsive efficiency.
As discussed above, core power is principally a function of TET, which also affects thermal efficiency. Fig. 4.8 shows the specific fuel consumption of current engines as it relates to thermal and propulsive efficiency. Whereas current high-bypass ratio engines exhibit thermal efficiencies in the region of 0.45, it might be possible to reach values close to 0.6 by developing materials technologies that allow further increases in TET, along with higher component efficiencies and pressure ratios. Nevertheless, given that NOx tends to rise with higher combustion temperatures, further developments in low-NOx combustors would be necessary to reach this value in practice.
Recuperation techniques use heat exchangers to scavenge heat that would otherwise be lost to the atmosphere from the back of the gas turbine, using it to increase the temperature of the air before it enters the combustion chamber, and thus reducing the amount of fuel that must be burned to reach a given value of TET. Intercoolers reduce the temperature of the air between successive compressor stages, increasing the core specific power. A cycle making use of these techniques could in theory outperform current gas turbines. In practice, neither technology is used in aero gas turbines today because of the weight and volume associated with the heat exchangers they would both require. Nevertheless, much research is being carried out to manufacture lighter, more compact heat exchangers, and they should not be ruled out in the medium to long term, especially for power-producing applications such as helicopter engines.
Propulsive efficiency depends on jet velocity (see Fig. 4.9), and can be improved by increasing the gas turbine’s bypass ratio (BPR). However, higher BPR values mean that the linear velocity of the blades could be high enough at the tip to encounter supersonic flow, and generate shockwaves. This is not an insurmountable problem for current engines (with BPR values of about 10), since the inlet diffusers reduce the speed of the incoming air to values close to Mach 0.4. If attempts are made to increase BPR further, however, the weight and drag associated with the fan cowling become excessive. Without the help of the inlet diffuser, it is difficult to avoid shockwaves (in fact, this phenomenon has traditionally limited the maximum Mach number at which propeller-driven aircraft can fly).
Given the drive for lower fuel consumption, different ways of achieving higher BPR values are being explored by major manufacturers. Open rotors could potentially be the solution. Open rotor configurations eliminate the fan cowling and use advanced aerodynamic design techniques to reduce shock waves at the fan blade tips, allowing large fans to be used in free stream conditions and resulting in bypass ratios in excess of 30. However, open rotors have a significant drawback: noise. Without a cowling to absorb the noise generated by the fan blades, open rotor configurations can be unpleasant for both passengers and airport environments. Open rotors were already developed in the 1980s in the USA and Russia, but the resulting cabin noise prevented them from being adopted at the time. With a far stronger drive for low fuel consumption, open rotors might have a better chance this time. Noise reduction techniques are currently being studied, and alternative engine placement arrangements could allow better noise shielding.
As seen in Fig. 4.8, open rotors and high-temperature technologies could bring about a 30% improvement in fuel consumption. Considering intercooled–recuperated cycles, there is still plenty of scope for improving the conventional propulsion configuration. By combining current developments in aero gas turbines, airframes, and air traffic management, the environmental impact of aviation will be lowered significantly within the next two decades.
It would nevertheless be unwise to set our long-term goal for aero gas turbine fuel consumption reduction at 30%. Even if passenger growth rates only averaged 4% per annum, it would only take seven years for an aviation industry based on a ‘greener’ gas turbine to produce as much CO2 as today.
Despite increasing environmental pressures, aero engine development cycles mean that alternative propulsion systems are unlikely to become available in the medium term. This means that gas turbine technology will continue to be the norm for decades to come. Once the predicted improvements in core and propulsive efficiency have materialised, there might still be room to increase efficiency further. This could be done by seeking synergies between the airframe and the propulsion system, and using alternative propulsion system architectures.
One such architecture could be distributed propulsion, which consists of spreading the propulsive force along the wings or airframe. A small gas turbine distributed propulsion (SGTDP) system could be composed of a large number of small gas turbines spread along the wings of the aircraft. Since the engines are now very small, they could be better shielded to reduce noise. Mass manufacturing might become a real possibility for large numbers of engines, and studies on land turbines indicate that manufacturing costs could be reduced by up to 90% through the implementation of appropriate large-scale manufacturing techniques. Safety could be enhanced by higher propulsion system redundancy, and this would also allow the redesign of aircraft around the propulsion system: less stringent airframe requirements could result in lighter, more efficient, and cheaper aircraft.
However, SGTDP is not feasible at the time of writing. A recent study at Cranfield University (Ameyugo and Singh, 2007) indicated that small gas turbine fuel consumption represents the single greatest hurdle to this concept. A solution could be found in heat exchangers, which improve small gas turbine efficiency to a larger extent than their large counterparts, and could thus become an enabling technology. Lighter, less bulky heat exchangers would allow the implementation of more efficient recuperated cycles.
In practice, the advantages of the resulting recuperated engine system might not be substantial enough to justify the large shift in industry structure and operations that would be required to bring about a change of configuration. Nevertheless, in a scenario where environmental concerns had driven the development of cheap near-zero emissions fuels for the land transport and power generation industries, aviation could benefit from the resulting shift in relative importance of fuel costs to operations by adopting a distributed propulsion configuration. In such a scenario, small gas turbine configuration distributed propulsion could provide a cheaper, more efficient, and safer way of travel.
Improvements in engine efficiency are ultimately constrained by physical laws. Assuming stoichiometric turbine inlet temperature and open rotor propulsive efficiency, the theoretically achievable maximum overall engine efficiency is about 55%. This fact represents the dilemma of the aerospace industry today: every improvement in engine efficiency is a step toward the maximum achievable overall engine efficiency. As this value is approached, the law of diminishing returns sets in: improvements become more and more difficult to achieve, and take place at subsystem level with larger time spans between equivalent amounts of efficiency gains. Larger research and development spending is required for ever-smaller improvements, reducing profit margins in an effort to remain competitive in the eyes of the airlines. Medium-term solutions may exhibit a more disruptive character than short-term solutions, as more time for development is available.
Alternative fuels could constitute an interim solution to reduce the environmental impact of aviation in the medium term. Two main avenues of research are currently open: Fischer-Tropsch fuel synthesis and biofuels. The Fischer-Tropsch (FT) process is a chemical process that transforms coal into hydrocarbon fuels. However, FT processes require high temperatures and pressures, and release CO2 into the atmosphere during the transformation process. Although CO2 sequestration could be an option, biofuels might offer an interesting alternative.
Biofuels rely on using biological matter – such as plants – to produce hydrocarbon-based fuels, creating an additional loop in the carbon cycle. The biomass used to generate fuel would absorb CO2 from the atmosphere while it is growing, before it is harvested. For this reason, the use of biofuels as a substitute for kerosene has been discussed many times over the past few years.
Hydrotreated renewable jet fuel (HRJ) is an option that could be implemented within the next few years. This process removes oxygen and other contaminants from vegetable oil, and generates a fuel with performance properties exceeding current jet fuel requirements.
Another option to generate biofuels is to convert lignocellulosic material, such as forest and agricultural waste products, into a biofuel cocktail. This process would be very attractive from an environmental point of view, and it is already being used to generate ethanol for ground transportation. Research and development efforts are progressing rapidly to make this process economical.
Although biofuels remain a long-term possibility, there are several hurdles to their application in aerospace propulsion systems. The use of sugarcane-derived ethanol for road transportation has been successfully proven over the last two decades in Brazil. However, it has been necessary to devote a significant part of the country’s arable land to provide enough sugarcane for the fuel used for road transportation. Ethanol itself is not well suited for aviation use, as it contains a significant amount of oxygen, decreasing its energy density and resulting in bulkier, heavier fuel tanks. Other alternatives are possible, but the amount of land required to provide enough biofuels for even 15% of the US commercial fleet would be equivalent to the size of Florida. Microalgae are being studied as a promising source for vegetable oils suitable for biofuel applications. However, current production systems are small, and production costs would still have to be reduced anywhere from 10–100-fold (Dagget, 2009) for algae to be cost-competitive with predicted oil prices for the next decades.
A more practical limitation lies in the freezing point of biofuels: whereas conventional fuels can be used in aircraft without complications, biofuels will freeze at the operational temperatures experienced by aircraft in cruise. Possible solutions could include the use of insulating materials, anti-freeze agents in the fuel, or even heating the fuel with the excess heat from the engines.
Current jet engines are based on the Brayton cycle, where the working fluid successively experiences isentropic compression, isobaric increase in temperature, and isentropic expansion. Deviations from the ideal cycle in real engines occur through component inefficiencies. The maximum achievable thermodynamic efficiency of the Brayton cycle increases hand-in-hand with its peak cycle temperature. Since the peak cycle temperature is limited by material properties of the turbine, the maximum cycle efficiency of current jet engines is limited by the laws of thermodynamics. Hence, efficiency improvements of jet engines beyond what is possible with conventional designs are only feasible through novel thermodynamic cycles that incorporate features such as constant volume combustion.
Technologies enabling constant volume combustion for aircraft propulsion include wave rotors, pulse detonators and piston engines. Wave rotors are non-steady flow pressure exchangers based on wave processes; they act as a compressor and turbine without blades (see Fig. 4.10). Two scenarios for wave rotor applications are conceivable. The wave rotor could take the place of a set of high-pressure compressor stages, for example to make the engine more compact. Alternatively, a wave rotor can enable higher turbine delivery pressure without the need to increase the turbine entry temperature.
4.10 Practical limits to engine efficiency. (Source: Green, 2005.)
In its most common form, the wave rotor is an array of cylindrically arranged tubes in the form of a drum. In the four-port configuration, the wave rotor is located between the compressor exit, the inlet as well as the outlet of a combustion unit, and the turbine inlet. The rotation of the drum causes a periodical opening and closing of both sides of the tubes in such a way that the working fluid can either enter or exit the tubes.
The wave rotor works in four steps. In the first step, low-pressure air from the engine compressor enters the tubes of the wave rotor. Through the rotation of the drum, the ports of the tubes become connected with the inlet and outlet of the combustor containing high-pressure gas. This causes compression of the low-pressure air via shock waves. In effect, the high-pressure air enters the combustor at its inlet. That leaves the tube with compressed gas at an elevated temperature. Through further rotation, the tube is now connected with the turbine and the gas leaves the tube to drive the turbine.
The general advantage of wave rotor topping is an increase of the overall pressure ratio achieved in the engine, increasing the output work and therefore thermal efficiency. The weight of a particular engine is believed to increase unless the wave rotor is used as a substitute for the final compressor stages. Therefore, weight penalties associated with the wave rotor would have to be taken into account to assess the installed performance of a wave rotor-topped engine. The performance enhancement via wave rotors is particularly effective if turbomachinery component losses are high. Generally, component efficiencies decrease with decreasing engine mass flow, which can be found in engines with low power output. Therefore, the implementation of wave rotor-supported performance enhancements is probably more effective in low-power engines.
Pulse detonation is a non-steady flow quasi-constant volume combustion process. As opposed to deflagrations, detonations are supersonic combustion waves. Across a detonation front, the pressure increases while the specific volume decreases. A detonation taking place in a tube that is closed at one end leaves hot, pressurized gas behind. In its simplest form, a pulse detonation engine is a tube where one end is repeatedly opened and closed while a combustible gas is ignited while the tube is closed. Hot, pressurized gas exits the tube at its closed end shortly after ignition takes place.
A wide variety of applications have been proposed for pulse detonation engines. Due to the high specific impulse of the exhaust gas, thrust-generating pulse detonation engines exhibit much higher fuel consumption than turbojets in the low Mach number regime. However, pulse detonation engines would outperform turbojets above Mach numbers of about 3.5. An application of pulse detonation engines for subsonic, commercial airliners is hence less attractive in the light of the current environmental debate. Pulse detonation engines can be used in hybrid mode to increase the performance of conventional aero engines. In the ultimate embodiment of this concept, the pulse detonation engine takes the place of the combustor and, depending on the overall design, a set of high-pressure compressor stages. The air leaving the high-pressure compressor stage enters a pulse detonation tube, which delivers hot gases at increased pressure to a turbine. Combining pulse detonation with conventional aero engine technology would hence enable higher turbine delivery pressure without the need to increase the turbine entry temperature.
Lastly, piston engines, generally based on the Otto cycle featuring constant volume combustion, could replace the combustor of a jet engine. This would enable higher combustor delivery pressure, leading to improved thermal efficiency. The idea is not new: the Napier Nomad engine, which was in commercial production in the early 1950s, had a piston engine instead of a combustor in its core.
When in 1937 one of the first jet engine prototypes was tested in a German laboratory it ran with pressurised hydrogen. However, hydrocarbon-based fuels being more abundant, more accessible and easier to store, kerosene eventually succeeded as aviation fuel and the first jet aircraft were fuelled exclusively with kerosene. Subsequently, kerosene became the common aviation fuel in both civil and military aviation. Some research programmes were undertaken to investigate hydrogen as fuel for military aircraft. For example, in the late 1950s a B57 was modified to fly with hydrogen and tests were successful; but as of today, with the exception of some research aircraft, the global fleet flies with kerosene.
Hydrogen is the first element in the periodic table. With just one electron in its shell it is also the lightest element. Hydrogen is very reactive and very rare in its natural, diatomic form (H2) on Earth. It is a constituent of many known materials, gases and fluids e.g. water (H2O). Hydrogen is highly flammable, reacting exothermically with oxygen to form water. It burns in air with concentrations between 4 and 75% by volume.
Hydrogen is not a natural resource and can be seen as an intermediate energy carrier. Hydrogen would have to be produced by consuming energy before it could be utilised to provide energy, e.g. to power aircraft. The probably most viable approach for large-scale hydrogen production is electrolysis, where water is broken down into its constituents – oxygen and hydrogen – by means of electrical energy. An alternative to electrolysis of water is provided through gasification of biomass. When hydrogen is burned with pure oxygen the only combustion product is water. Hydrogen combustion with air yields secondary combustion products, such as NOx, forming in the presence of high temperatures.
With hydrogen being used as an energy carrier, other, more abundant energy sources than crude oil could be used to power aircraft. Biofuels, nuclear power or renewables such as solar or wind power could provide electrical energy to produce hydrogen. If the production of hydrogen itself were not a source of greenhouse gases, hydrogen would certainly be a more environmental aviation fuel than kerosene.
Hydrogen being a very light gas, it contains 2.8 times more energy than kerosene in terms of weight. In terms of energy per volume, kerosene is more advantageous: liquefied hydrogen stored at −253 °C still requires four times more volume than kerosene for the same amount of energy. Although hydrogen exhibits large weight advantages, its volume requirements place technical challenges around storage and fuel systems, especially because liquid hydrogen would have to be stored at very low temperatures. Hence, if hydrogen were to be used as an aviation fuel, large fuel tanks would add to the drag and weight of the airframe, and the complexity of the fuel systems would increase. The gross aircraft weight may be reduced due to the low energy specific weight of hydrogen. Hence, the trade-off is an increase in wetted area adding to drag against aircraft weight. This trade-off depends on size and configuration of the aircraft. Revolutionary aircraft configurations, such as the blended wing body, or engine technologies such as the fuel cell, may even be more suitable for hydrogen as aviation fuel.
While hydrogen could imply less energy-efficient aircraft, this would not necessarily involve an increase in aviation’s environmental impact or economical viability if hydrogen could be produced and distributed without greenhouse gas emissions and in an economical way. The most important consideration in this case would probably be economical viability.
The absence of infrastructure for hydrogen distribution would require enormous investments to achieve a network coverage that would enable refuelling of aircraft at any airport. Hydrogen could therefore be distributed in gaseous form to airports and then liquefied via cooling, a process that would require extra energy. Alternatively, hydrogen could be directly produced at the airport via electrolysis or gasification. Establishing the hydrogen network would most likely require more time than designing the first hydrogen-powered, commercial airliner.
Hydrogen could be used to power conventional turbofan engines. Major modifications to the power plant would be required in the fuel supply and combustion section. Liquid hydrogen would have to be heated before injection into the combustion chamber, requiring additional heat exchangers. The pumps delivering the hydrogen to the engine would have to be modified to operate at very low temperatures. Low NOx emissions combustors compatible with hydrogen would be required to keep NOx emissions at a minimum.
Alternative solutions may be found in fuel cell technology. Hydrogen would be a suitable candidate to power low-temperature fuel cells. With higher efficiencies than gas turbines and the potential of being more economical, fuel cells may be used to produce electricity to power electrically driven fans. Whilst the practical feasibility is currently being investigated, fuel cells may become substitutes for gas turbine-based auxiliary power units in the shorter term.
In burning hydrogen with air, the only combustion products would be water and NOx. Since NOx occurs also during the combustion of kerosene, there would not be an additional impact through NOx emissions. With water having a far lower atmospheric residence time compared with CO2, this would have a positive impact in terms of environmental compatibility. Although emissions from hydrogen-powered aircraft would not include CO2, they could contribute to additional contrails. The additional amount of water in the engine exhaust would facilitate contrail formation over a wider range of atmospheric conditions and altitudes. Studies suggest that the global annual mean contrail coverage from hydrogen-fuelled aircraft could increase by as much as 1.56 times. Due to the lack of soot particles in the exhaust of hydrogen-powered aircraft, contrails would consist of fewer but larger particles, having an impact on the optical properties of contrails, reducing their radiative forcing and hence impact on the environment. Even though more contrails would appear in the sky, the overall effect may be a reduction in the contribution of contrails to global warming.
The sustainability of aviation in the long term is restricted by two factors: environmental compatibility and limited natural fuel resources. Both factors can impede the growth in passenger kilometres, but continuous growth is vital for the development of sustainable technology in the aviation industry sector. The importance of the above-mentioned factors has changed over time, with environmental concerns dominating the current debate. Fuel availability, pricing and price stability will also become important in the coming decades. Considering the forecast growth rates in passenger kilometres, sustainable aviation is believed to be achievable only if certain criteria regarding fuel economy are met. The changes that may prove necessary to meet the challenge of the environment imply the introduction of disruptive technologies and levels of sustained investment and risk large even in comparison with the norm for this industry. Though it is not straightforward to foresee the future of aviation beyond the next three decades, it is possible to begin to form an image of what future airframes and propulsion systems might look like by predicting the drivers of future technological development.
The main drivers for change in the aero gas turbine industry in the long term are likely to be the environment, profitability, and the emergence of disruptive technologies. The importance of achieving environmental sustainability for continued growth of the industry has been discussed above. Airline operating costs are likely to be increasingly affected by environmental regulations in the future, and the emergence of competition from developing countries will increase the pressure to reduce costs. Increased competitive pressure could also drive the industry towards more risky, innovative concepts, and might eventually lead to the introduction of new disruptive technologies in an effort to achieve step improvements in environmental and cost performance. In the coming decades, the latter could include nano-technology, micro-electromechanical systems, and superconductivity.
Breakthroughs in nanotechnology are leading to lighter materials with ever-increasing strengths (potentially over 30 times stronger than steel with much lower material densities). The application of carbon nanotubes in aero gas turbines is still some years away, but promises to drive their weight down considerably. Carbon nanotubes have also been proposed as a means of safely storing liquid hydrogen with minimum losses, and might therefore play a part in alternative fuel development.
A second result of future developments in nanotechnology might be low-cost micro-electro-mechanical systems (MEMS). By using MEMS pressure sensors with smart materials or even MEMS actuators to control boundary layer flow, it might be possible to reduce drag and therefore achieve considerably higher aerodynamic efficiency for propulsion system turbomachinery. MEMS technology could also be used to control shockwave formation on blade tips, helping, for example, to lower open rotor noise during taxiing, take-off and landing.
Another disruptive technology is that of superconductivity: by minimising electrical losses, this technology could result in more efficient electrical systems. Electric motors could one day provide a more efficient way of transferring fuel energy to the propulsive elements than conventional drives. By making use of this technology, more environmentally friendly aircraft without the need for alternative fuels could be envisaged.
Long-term propulsion systems might be electrically driven. A first step towards this more efficient aircraft could be a change towards alternative airframes. Of the airframes proposed so far, the blended wing body (BWB) seems the most promising for future applications. The BWB concept is discussed elsewhere in this book, but, apart from its inherent benefits to aerodynamic efficiency, BWB configurations could enable a higher degree of integration between the airframe and the propulsion system. This could lead to lower drag, and lighter, more efficient propulsion configurations.
Electrical propulsion systems would be too heavy to power a commercial airliner today. A possible configuration could use a gas turbine core to generate enough power to drive an electrical generator (see Fig. 4.11). Power could be routed through a set of transmission lines, and transferred to a number of low-velocity electrical fans. If the propulsion system is integrated within, say, a BWB airframe, it might be possible to increase the thermal efficiency of the core to a similar level to industrial gas turbines (e.g. by using recuperated cycles). Such systems would offer unparalleled flexibility to achieve unprecedented levels of overall efficiency, and could eventually afford reductions in fuel consumption over conventional gas turbine technology of up to 40% (Ameyugo, 2007), allowing emissions to be minimised. However, in order to achieve this goal, a fundamental disruptive technology would be needed: superconductivity.
4.11 Distributed electrical fans concept. (Source: Ameyugo and Noppel, 2007.)
Superconductive motors have recently come under the spotlight as the aerospace industry turns towards the search for an emissionless aircraft. Although some high-temperature superconducting materials have been known for some time (Edmonds et al., 1992), most operate at very low temperatures. A recent superconductive motor concept for application to light aircraft, for example, had an operating temperature of 30 K (Masson and Luongo, 2005).
The implication of the low temperature required to operate superconductive components is that, in general, a cryogenic source would be required onboard. Hydrogen fuel could provide the solution, but the date at which distributed driven fans might become feasible could be set back several decades. Nevertheless, it might not be necessary to run the main propulsion system on hydrogen fuel: if auxiliary power units (APUs) are replaced with fuel cells in the medium term, it might make sense to operate them with hydrogen. This would eliminate the need for a reformer, making them lighter, and the only emission from the APU would be water. Although hydrogen should enter the fuel cell in gaseous form, the most efficient way of storing it is in liquid form. It is not difficult to envisage a liquid-H2 tank for the APU, with fuel lines that cool the generators and main power transmission lines, at the very least, enabling superconductive elements to be used in those components and reducing the weight and losses of the propulsion system.
In this chapter have been seen the factors that influenced the development of gas turbines. The main motivation behind aero gas turbines was flying faster and above the weather. Subsequent development efforts helped reduce the cost of flying, allowing ever-increasing numbers of people to enjoy the benefits of air travel, and turning aviation into one of the main driving forces of economic progress and globalisation.
The challenge currently facing the aero engine industry is that of supporting a new generation of environmentally friendly aircraft. Making aviation sustainable is a critical task, and the industry’s research and development efforts are opening up a number of avenues to achieve this.
While evolutionary improvements to current gas turbines, biofuels, and alternative engine and aircraft configurations are set to lead the way into a new, cleaner paradigm for air travel, evolutionary improvements cannot sustain aviation indefinitely, and we must not lose sight of our long-term aim. Fig. 4.12 shows alternative fuel consumption scenarios for different performance improvements over the next century. At current passenger growth rates, a reduction of even 80% in block fuel consumption would still lead to a 20-fold increase in aviation fuel consumption by the end of the century. It is therefore clear that alternative fuels such as hydrogen could one day play a central role in achieving sustainability. A land-based hydrogen economy might very well be a prerequisite to develop the necessary infrastructures for generating, distributing and storing hydrogen.
4.12 Fuel consumption evolution. (Source: Singh, 2007.)
The road towards sustainable aviation was, is, and will continue to be supported by innovations in aero engines. Fig. 4.13 shows a (literal) road map that highlights some of the main avenues that might affect aero engine development over the next few decades. There is plenty of scope for synergies and creativity: the coming years will certainly be an exciting period for aero gas turbine engineers.
Edmonds, J.S., Sharma, D.K., Jordan, H.E., Edick, J.D., Schiferl, R.F. Application of high temperature superconductivity to electric motor design. IEEE Transactions on Energy Conversion. 1992; 7(2):322–329.