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Shifting Gears

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That jet engines have evolved over the past few decades will be apparent to any seasoned air traveller. Early jet engines had narrow inlets and were very noisy, but as the diameter of the fans at the front increased, the engines became quieter. Compared with a rowdy 1960s jet, a modern turbofan is some 80% quieter and burns as little as half as much fuel-thus producing fewer greenhouse gases. But with regulation of aviation emissions likely in the coming years in order to combat climate change, jet engines must become cleaner and more frugal still.rnWhen a jet engine is running, a fan at the front draws in air, which is then compressed and burned with fuel to produce hot, rapidly expanding gases which roar out of the back. On the way out, these gases drive a turbine which is connected to the fan via a shaft, thus drawing in more air and keeping the process going. But not all the air passing through an engine needs to go through the turbine. Over the years the fans have grown larger and the fraction of the air that passes through the turbine has fallen. The latest "high bypass" turbofans, such as the Rolls-Royce Trent, push around nine times more air around the core of the engine than through it.
机译：对于任何经验丰富的航空旅行者来说，喷气发动机在过去几十年中的发展都是显而易见的。早期的喷气发动机进气口狭窄且噪音很大，但是随着前风扇直径的增加，发动机变得越来越安静。与1960年代吵闹的喷气式飞机相比，现代涡扇发动机的噪音要低80％，燃烧的燃料少一半，从而减少温室气体的排放。但是为了应对气候变化，未来几年可能会调节航空排放，喷气发动机必须变得更加清洁和节俭.rn当喷气发动机运行时，前端的风扇会吸入空气，然后将其压缩并燃烧。用燃料产生热的，迅速膨胀的气体，这些气体从后部呼啸而出。在排出气体时，这些气体驱动涡轮机，该涡轮机通过轴连接到风扇，从而吸入更多的空气并使过程继续进行。但是并非所有通过发动机的空气都需要通过涡轮。多年来，风扇变得越来越大，通过涡轮的空气比例也有所下降。劳斯莱斯T达（Rolls-Royce Trent）等最新的“高旁通”涡轮风扇在引擎核心周围推动的空气比通过引擎周围推动的空气多9倍。

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    《The economist》 |2009年第8621期|iiiiv|共2页
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