planes are getting bigger: the Airbus A350 is a smaller plane than the Boeing 747 but

飛機變大了：空中巴士A350體積比波音747小，

has bigger engines. The reason? Bigger engines are simply more efficient, up to a point.

引擎卻更大。 為何？更大的引擎在一定範圍內更有效率。

Modern jet engines actually consist of two propulsion systems working together: the jet

現代噴射引擎包含了兩組動力系統：

core, which provides the power and a small amount of thrust, and the fan, which is just

核心，提供動力和少量推力；與風扇，其實只是個

a big propellor driven by the jet core that provides most of the thrust.

被核心帶動的大型螺旋槳，提供大部分動力。

Pure jet engines can be incredibly powerful (which is why they’re used for fighter jets),

純引擎核心力量極大(所以用在戰鬥機上)，

but they’re also horribly inefficient because they shoot out their exhaust at super high

但它們效率極低，因為會將廢氣以超高速噴出，

speeds, which equates to huge amounts of kinetic energy, since kinetic energy is proportional

製造大量動能，由於動能與速度平方成正比，

to the speed squared, so accelerating exhaust to twice the speed takes 4 times the energy.

因此廢氣噴速加倍時，需要消耗四倍能量。

In exchange for power, they literally blast energy away as a bunch of hot air.

為了提供動力，這類引擎根本是用噴射熱氣燒能源。

Of course, you do have to shoot some air out of the back of an engine to generate thrust,

當然，你的引擎一定要往後噴氣才能產生推力，

but it doesn’t have to have lots of energy. Instead of accelerating a little bit of air

但它不需要燒那麼多能量。與其猛烈加速一小撮空氣

a lot, you can achieve your desired thrust by accelerating a lot of air a little bit

你可以稍微加速大量空氣達到同樣速度─

- you get the same momentum boost but save a ton of energy.

單位時間內推力相同但省下一堆能源。

Essentially, if you make an engine too small, it has to accelerate the air so much as to

然而，若你把引擎做太小，它得把空氣加到極高速

be a waste of energy – kind of like using a machine gun to propel your car. But if you

而顯得浪費能量─好比用機關槍讓車前進。但若你

make the engine too big, then it starts to cause too much drag. An ideal engine is somewhere

把引擎造太大，又會增加太多阻力。理想的引擎大小

in between – rough estimation puts the ideal engine somewhere around 4 meters in diameter,

介於中間─大致估計直徑在四公尺上下，

which is just slightly bigger than the current largest engines.

大約只比現行的最大型引擎大一點。

So you can expect jet engines to continue growing in size... but not forever.

因此你能想見噴射引擎會繼續變大......但非永遠。

I’d like to thank Audible.com for supporting this video. As you may have heard, Audible

感謝 Audible.com支持本片。你也許聽過，

has audiobooks of all sorts, including fiction, non-fiction and periodicals. You can try Audible

Audible擁有各式有聲書，包含故事、非故事和期刊。 你可以免費試用Audible

free for 30 days by going to audible.com/minutephysics, and if you do so I recommend the book Ender’s

30天，只要前往audible.com/minutephysics，我推薦

Game by Orson Scott Card, it was one of the pivotal sci-fi novels I read as a teenager

Orson Scott Card寫的Ender’s Game，這是我青少年時

and I can’t recommend it enough. Remember, the enemy’s gate is down. And you can try

讀過最重要的科幻小說，誠心推薦。 記住，敵人的大門已經關上。

Audible free for 30-days by going to audible.com/minutephysics.

你可以上audible.com/minutephysics 免費試用Audible30天。