High-bypass engines of the future

Worldwide growth in air traffic has been on a continuous upward curve for decades. And it will continue to climb, with experts estimating an annual growth rate of between four and six percent. Another curve has a similar shape, the one showing how the bypass ratio (BPR) of engines has developed over time. The bypass ratio indicates how much air inside the nacelle flows past the combustor and how much passes through it. As a rule: the higher the ratio, the more efficient the engine. And more efficient engines mean lower fuel consumption and emissions.

Higher BPRs are therefore one of the most effective tools for engineers striving to make aviation more environmentally friendly. The BPR curve is an impressive record of the industry's success. Since the 1960s, the ratios have climbed from an initial value of 2:1, reaching 6:1 for the classic V2500 engine in the 1980s, before the Geared Turbofan™ (GTF) achieved the current record of 12:1. Dr. Edgar Merkl from MTU Aero Engines in Munich is now ensuring that the BPR curve continues to rise for the engines of tomorrow in his capacity as coordinator of the EU's ENOVAL project, which started four years ago.

Greater efficiency, less noise

In the ENgine mOdule VALidators (ENOVAL) project, 35 partners from ten different countries-including aviation companies, research institutes and universities-are developing engine technology for bypass ratios higher than 12:1. Experts refer to an ultra-high bypass ratio (UHBR). "In ENOVAL, we're working with a range between 14:1 and 16:1," says Merkl. By virtue of the higher BPR, there will not only be an increase in thrust efficiency, with corresponding reductions in fuel consumption and emissions, explains the ENOVAL Coordinator, "but the new engines that result will be even quieter than the Geared Turbofan™ is already."

This is achieved by slower flow speeds in the exhaust jet and lower rotational speeds in the fan. As a result, the blade tips no longer enter the supersonic range, which is one of the main sources of noise for engines that-unlike the GTF and engines with ENOVAL technologies-do not use a gear system to allow the fan and the turbine to rotate separately in their own optimum speed range. Overall, the new UHBR engines will be 1.3 decibels quieter and will emit up to five percent less CO₂. For medium-haul jets such as the Airbus A320, this saves 1,200 tons of CO₂ per year, which is equivalent to the carbon emissions produced in serving the annual electricity needs of 325 average households.

Focus on the low-pressure system

"If we also factor in technologies from predecessor projects such as LEMCOTEC and E-BREAK, then we're actually nine decibels quieter than an engine from the year 2000-and we've reduced CO₂ emissions by around 28 percent," says ENOVAL Chief Engineer Dr. Jörg Sieber. Overall, this already comfortably fulfills the ACARE goals for 2020. Whereas LEMCOTEC and E-BREAK were about developing technologies that increase the overall pressure ratio and therefore thermal efficiency, while also adapting materials and subsystems to the rising pressures and temperatures in the future, ENOVAL focuses on development of the low-pressure system for UHBR engines.

MTU's role in the project is predominantly concerned with the high-speed low-pressure turbine, with other ENOVAL partners responsible for the fan, gear and low-pressure compressor systems. Always keeping the improvement of the overall system in sight is a major challenge. "Just optimizing each component in the engine separately and then fitting them all together-that alone won't give you an optimum UHBR engine," says Sieber.

Another challenge is that UHBR engines will be larger and heavier in the first instance. "To be able to move more air mass, the fan needs to be larger," says Sieber, and this increases air resistance in turn. These negative factors will have to be counterbalanced by lighter and more efficient low-pressure modules, explains the MTU engineer. However, the new UHBR engines will definitely need more space beneath the wing: they will be between 20 and 35 percent larger than a year-2000 engine, depending on whether they are optimized for short- or long-haul aircraft.

Larger engine diameters to influence aircraft design

Rolls-Royce's UltraFan will be just as large, if not larger. For the engine, which is set to have a BPR of over 15:1 and is slated for release in 2025, the British company is inserting a gearbox between the fan and turbine for the first time. "Because of the advantages that a gear configuration yields in terms of overall efficiency, there is a clear trend among all competitors in favor of this design," says Merkl. With the UltraFan, Rolls-Royce is targeting the successors of the large passenger jets, such as the Boeing 747 and the Airbus A380. Engines with ENOVAL technology also still fit beneath the wings of aircraft with classic design configurations.

However, when engine diameters get even larger, you quickly reach the point at which you have to consider other aircraft configurations, says Dr. Jochen Kaiser, Head of Visionary Aircraft Concepts at the Bauhaus Luftfahrt research institution in Munich. "For instance, you could place the wings higher up on the fuselage, or build larger high-wing aircraft," says Kaiser. This is the name given to aircraft such as the Airbus A400M, whose wings are mounted flush with the upper edge of the fuselage.

Such a configuration may even make open-rotor concepts a viable possibility. These engines, in which one or two rotors are spun by a core engine, have BPRs in excess of 30, but their open designs without casing create noise problems. "For large aircraft, we think there's no real question of using them," says Merkl. "But for smaller aircraft, the open rotor might become attractive at some point." Possibly, these engines could be placed on the wings, which would shield the noise. However, this would require new aircraft designs.

"For the classic aircraft configuration, it will be possible to increase the BPR further up to 20:1-but then we have reached the maximum," says Merkl. Achieving this, however, will require new materials and improved sealing systems, which allow the pressure ratio in the core engine to be increased further. No progress can be expected on this front before 2035. The ENOVAL generation of UHBR engines will be ready long before then: "We expect entry into service to begin from 2025," says the ENOVAL Coordinator.

By then, work will probably be underway on the following generation of UHBR engines. "After all, we want to maintain our edge," says Merkl.