Today, the turbofan engine has found a home on practically all
jet-propelled aircraft. However, the ambitious emission goals of ACARE 2020
cannot be fully met with the current turbofan concepts and industry majors need
to look elsewhere to find a viable solution.
A high
bypass engine is the key to reducing both fuel consumption and noise and
developments are underfoot aimed to raise the bypass ratio above ten and optimize
individual components for better aerodynamic efficiency.
A new development that offers a great potential is a concept called the Geared Turbofan. Unlike conventional turbofans, where fan and low-pressure turbine rotate on a common shaft and at the same speed, the two components in this design are decoupled by a gearbox arranged between them. Accordingly, the large fan operates at a slower and the low-pressure turbine at a faster speed, which improves their respective efficiencies, lowers the noise level and about halves the number of stages in the turbine. Bypass ratios of 12 and beyond have become a possibility. In parallel, alternatives are being investigated, such as the counter-rotating integrated shrouded prop-fan, or Crisp for short. In this derivative of the geared turbofan, two counter-rotating fan rotors are arranged one behind the other. This arrangement boosts the propulsive efficiency of the engine, with the fan diameter remaining unchanged, and permits bypass ratios as high as 20 to 25 to be achieved.
The GTF, also called Pure power
engine, is designed to reduce specific fuel consumption 12% over current
turbofans by increasing bypass ratio to 10-12. Reducing fan pressure ratio requires
a large diameter, and a geared fan allows tip speed to be reduced for lower
noise. The design is also likely to
reduce carbon emissions by well over 20 % compared to today’s turbofan engines.
The reduction gearbox also allows the LP compressor and turbine speed to be
increased, to extract more work from fewer stages and aerosols, reducing
weight.
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