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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越来越大：
the Airbus A350 is a smaller plane than the Boeing 747 but has bigger engines. The reason?
空客A350比波音747小，但它的发动机却更大。原因呢？
Bigger engines are simply more efficient, up to a point.
在某种程度上，发动机越大，效率越高。
Modern jet engines actually consist of two propulsion systems working together:
现代喷气发动机实际上由两个推进系统共同组成：
the jet core, which provides the power and a small amount of thrust, and the fan,
射流核，提供能量和少量的推力，还有风扇，
which is just a big propellor driven by the jet core that provides most of the thrust.
这是由喷气核心驱动的大螺旋桨，提供大部分的推力。
Pure jet engines can be incredibly powerful (which is why they're used for fighter jets),
纯喷气式发动机可以非常强大（这就是它们被用于战斗机的原因），
but they're also horribly inefficient because they shoot out their exhaust at super high speeds,
但它们的效率也很低，因为它们以极高的速度排出废气，
which equates to huge amounts of kinetic energy, since kinetic energy is proportional to the speed squared,
这就等于有大量的动能，由于动能与速度的平方成正比，
so accelerating exhaust to twice the speed takes 4 times the energy.
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.
作为能量的交换，它们像一团热空气一样把能量吹走。
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，你必须从发动机的后部放出一些空气来产生推力，但它不需要有很多能量。
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
相比大量加速一点空气，你可以通过一点一点地加速空气来获取你所需的推力，
you get the same momentum boost but save a ton of energy.
你得到了同样的动力，但节省了大量的能量。
Essentially, if you make an engine too small, it has to accelerate the air so much as to be a waste of energy –
从本质上讲，如果你把发动机做得太小，它就得加速空气，这样就浪费了能量
kind of like using a machine gun to propel your car.
有点像是用机枪来驱动你的车。
But if you make the engine too big, then it starts to cause too much drag.
但是如果引擎太大，就会产生太大的阻力。
An ideal engine is somewhere in between – rough estimation puts the ideal engine somewhere around 4 meters in diameter,
理想的发动机是介于两者之间——粗略估计，理想发动机的直径大约是4米，
which is just slightly bigger than the current largest engines.
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. I'd like to thank Audible.com for supporting this video.
所以你可以期待喷气发动机的尺寸继续增长，但不会永远增长。感谢Audible.com对本期视频的支持。
As you may have heard, Audible has audiobooks of all sorts, including fiction, non-fiction and periodicals.
大家可能听说过，Audible有各种各样的有声读物，包括小说、非小说和期刊。
You can try Audible free for 30 days by going to audible.com/minutephysics,
登录audible.com/minutephysics获得30天免费权限，
and if you do so I recommend the book Ender's Game by Orson Scott Card,
如果大家要去，我推荐Orson Scott Card的书籍《Ender's Game》，
it was one of the pivotal sci-fi novels I read as a teenager and I can't recommend it enough.
这是我十几岁时读过的最重要的科幻小说之一，我怎么推荐都不为过。
Remember, the enemy's gate is down. And you can try Audible free for 30-days by going to audible.com/minutephysics.
记住敌人的大门已经关闭。你可以登录audible.com/minutephysics获取Audible 30天免费试用。
