New Technologies - Engine Power

The aero engine industry is gearing up for a step change in technology that could reduce fuel burn and cut operating costs by another 15 to 20 per cent

Commercial aviation first took off during the 1930s. The favourite passenger aircraft of the time was the piston-engine, propeller-driven DC-3 Dakota. Power, however, was severely limited, since the propeller tips had to remain subsonic. When the first jet-powered airliners appeared during the early 1950s, the speed of the lumbering propeller planes doubled practically overnight. Soon the pace of innovation became fast and furious. No sooner a new jet engine entered service, an improved version delivering significant improvements in performance and efficiency was already in an advanced stage of development.

Some fundamentally new design ideas emerged to boost performance, including a change from radial to axial compressors which greatly reduced the engine diameter, while increasing thrust and invention of the turbofan which boosted thrust by over 50 per cent without increasing fuel consumption. By the early 1960s, the first modern airliners like the Boeing 707 and Douglas DC-8 entered service; thereafter, the pace of development in aero engines slowed down. During the rest of the 20th century, the focus shifted from larger, faster, further to fuel efficiency, driven primarily by the 1973 oil price shock. And around the turn of the century, this focus received a further fillip when ecological concerns came to the fore.

Perhaps more than any other component engines define an airliner. Weight for weight they are the most complex and technologically sophisticated part of an aircraft. That is why some of the best brains in the business toil night and day to coax every last bit of power and performance out of aero engines while being as stingy as possible with fuel. And the technologists get a pat on the back if they manage to enhance fuel efficiency by even one per cent a year.

Turbofans top the charts

Turbofans, first produced commercially in the 1960s, swiftly became the engines of choice. Nowadays, the main players in order of market share are General Electric, Rolls-Royce and Pratt & Whitney. Marketing alliances and joint ventures are common. Thus GE and SNECMA of France have a joint venture CFM International; Rolls-Royce and Pratt & Whitney have another, International Aero Engines and Pratt & Whitney and GE also have come together to form Engine Alliance.

In a typical turbofan design, a large fan powered by a turbine in the jet-exhaust stream, directs a small part of the incoming air into the combustion chamber, in order to burn fuel and produce power. However, the rest of the air flows around (or bypasses) the engine core and later mixes with the faster stream emerging from the core. This significantly muffles exhaust noise. The substantially slower bypass airflow produces thrust more efficiently than the high-speed air from the core, thus reducing the overall specific fuel consumption (SFC). Originally, the mass of air flowing around the combustor was about a third of that flowing through i.e. the bypass ratio was around 0.3:1. However, the engines currently powering most airliners have bypass ratios between 5:1 and 7:1; while ratios between 10:1 and 12:1 are also being flirted with.

The thermal efficiency and performance of a turbofan is largely defined by its compression ratio and combustion temperature. The higher they are, the more efficient the thermodynamic combustion process. These parameters are mainly limited by available material technology, since the turbine and engine casing must be able to withstand such demanding conditions. Better cooling devices also promote thermal efficiency. Just as important is the bypass ratio. By increasing the bypass ratio, thrust and propulsive efficiency are enhanced. However, the blade tips must rotate with subsonic velocities, so the fan diameter cannot be increased indefinitely. It is also more difficult to fit engines with large fans under wing, so fans seem to be reaching close to their maximum practicable size.

The Rolls-Royce Trent 1000 engine, available as an option for Boeing’s futuristic B787 Dreamliner, has a bypass ratio of 10:1 and a fan diameter of 112 inches, compared with the earlier Trent 700 which has a diameter of 97 inches and a bypass ratio of 5:1. The Trent 1000 also improves SFC by 14 per cent, compared with the Trent 700. Similarly, the other Dreamliner engine option, General Electric’s GEnx-1B, improves SFC by 15 per cent, compared with the CF6-80C2, an older GE product. The GEnx has a 111-inch fan diameter against 93 inches for the CF6. Its bypass ratio is around 9.5:1 compared with five for the CF6.