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为什么飞机的引擎这么大?

Why Are Airplane Engines So Big?

随着时间的变化 飞机的引擎变得越来越大了 ——而这不仅是由于
Airplane jet engines have gotten bigger and bigger over time – and it’s not just because
飞机变大了:空客 A350比波音747要小
planes are getting bigger: the Airbus A350 is a smaller plane than the Boeing 747 but
但拥有更大的发动机 原因是什么?相对大的发动机在一定程度上显然更有效率
has bigger engines. The reason? Bigger engines are simply more efficient, up to a point.
现代喷气式发动机实际上由两套推进系统协同工作
Modern jet engines actually consist of two propulsion systems working together: the jet
射流核心 提供动力和小部分推力 ,桨扇
core, which provides the power and a small amount of thrust, and the fan, which is just
是被射流核心操纵的风扇 提供大多部分的推力
a big propellor driven by the jet core that provides most of the thrust.
纯喷气式发动机的动力令人难以置信(这就是为什么它们被用作歼击机)
Pure jet engines can be incredibly powerful (which is why they’re used for fighter jets),
但他们的低效也实在可怕由于它们以非常高的速度喷出风扇早先吸进来的气流
but they’re also horribly inefficient because they shoot out their exhaust at super high
其等于相当大数量的动能 因为动能与速度的平方成正比
speeds, which equates to huge amounts of kinetic energy, since kinetic energy is proportional
所以以提高一倍的速度喷气就需要四倍于原先的能量
to the speed squared, so accelerating exhaust to twice the speed takes 4 times the energy.
它们其实使一团热空气爆炸以获得能量
In exchange for power, they literally blast energy away as a bunch of hot air.
当然,飞机需要在发动机的后方喷气以得到推力
Of course, you do have to shoot some air out of the back of an engine to generate thrust,
但这不需要过多动能飞机可以通过在一定时间压缩更多的气体
but it doesn’t have to have lots of energy. Instead of accelerating a little bit of air
而不是少量的以获得所需的推力
a lot, you can achieve your desired thrust by accelerating a lot of air a little bit
——这样会得到了同样的爆炸动能同时节约了很多能量
– you get the same momentum boost but save a ton of energy.
实际上 如果将引擎制得很太小 飞机将需要加速压缩更多空气
Essentially, if you make an engine too small, it has to accelerate the air so much as to
从而浪费了许多能量——有点像用机关枪驱动你的车
be a waste of energy – kind of like using a machine gun to propel your car. But if you
如果将引擎制得过大 它将产生过多的阻力一个理想的发动机
make the engine too big, then it starts to cause too much drag. An ideal engine is somewhere
位于两者之中——粗略地估计理想引擎直径约4米
in between – rough estimation puts the ideal engine somewhere around 4 meters in diameter,
只比目前最大的引擎略大
which is just slightly bigger than the current largest engines.
所以我们可以期待喷气发动机将继续增大……但不会永远
So you can expect jet engines to continue growing in size… but not forever.
我想感谢Audible.com支持了此视频你可能听说过
I’d like to thank Audible.com for supporting this video. As you may have heard, Audible
Audible有各种类型的有声读物 包括小说 非小说 期刊
has audiobooks of all sorts, including fiction, non-fiction and periodicals. You can try Audible
你可以去audible.com/minutephysics免费试用30天 而且如果你这么做了
free for 30 days by going to audible.com/minutephysics, and if you do so I recommend the book Ender’s
我推荐Orson Scott Card的Ender’s Game这本书它是我少年时读过的最关键的科幻小说之一
Game by Orson Scott Card, it was one of the pivotal sci-fi novels I read as a teenager
而且我不能推荐它更多了记住 外星敌人的大门是向下的
and I can’t recommend it enough. Remember, the enemy’s gate is down. And you can try
而你可以在audible.com/minutephysics免费试用Audible30天
Audible free for 30-days by going to audible.com/minutephysics.

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