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Airplane jet engines have gotten bigger and bigger over time - and it's not just because planes are getting bigger:
随着时间的推移，飞机的喷气发动机变得越来越大，这不仅仅是因为飞机变得越来越大：
the Airbus A350 is a smaller plane than the Boeing 747 but has bigger engines.
空客A350比波音747小，但拥有更大的发动机。
The reason?
原因？
Bigger engines are simply more efficient, up to a point.
在某种程度上，更大的发动机效率更高。
Modern jet engines actually consist of two propulsion systems working together:
现代喷气发动机实际上由两个协同工作的推进系统组成：
the jet core, which provides the power and a small amount of thrust,
提供动力和少量推力的喷气核心，
and the fan, which is just a big propellor driven by the jet core that provides most of the thrust.
和提供大部分推力的风扇，后者就是由喷气核心驱动的一个大螺旋桨。
Pure jet engines can be incredibly powerful (which is why they're used for fighter jets), but they're also horribly inefficient
纯喷气式发动机可以非常强大（这就是为什么它们被用于战斗机），但它们的效率也非常低，
because they shoot out their exhaust at super-high speeds, which equates to huge amounts of kinetic energy,
因为它们以超高速喷射废气，这相当于巨大的动能，
since kinetic energy is proportional to the speed squared, so accelerating exhaust to twice the speed takes 4 times the energy.
因为动能与速度的平方成正比，所以加速排气到两倍的速度需要4倍的能量。
In exchange for power, they literally blast energy away as a bunch of hot air.
为了交换能量，它们实际上把能量当作一堆热空气吹走。
Of course, you do have to shoot some air out of the back of an engine to generate thrust, but it doesn't have to have lots of energy.
当然，你确实需要从引擎后部喷出一些空气来产生推力，但那不需要很多能量。
Instead of accelerating a little bit of air a lot, you can achieve your desired thrust by accelerating a lot of air a little bit - you get the same momentum boost but save a ton of energy.
代替大量加速少量空气，你可以通过少量加速大量空气来获得你想要的推力——你可以得到同样的动量提升，但节省大量的能量。
Essentially, if you make an engine too small, it has to accelerate the air so much as to be a waste of energy – kind of like using a machine gun to propel your car.
从本质上讲，如果你把发动机做得太小，它就得极大地加速空气从而造成能量浪费——有点像用机关枪来驱动你的汽车。
But if you make the engine too big, then it starts to cause too much drag.
但如果发动机太大，就会产生太大的阻力。
An ideal engine is somewhere in between – a rough estimation puts the ideal engine somewhere around 4 meters in diameter, which is just slightly bigger than the current largest engines.
一个理想的发动机是介于两者之间的——粗略估计，理想的发动机直径大约在4米左右，只比目前最大的发动机稍大一点。
So you can expect jet engines to continue growing in size... but not forever.
所以你可以预计喷气发动机的尺寸还会增大...但不会一直增加下去。
