【Aviation English-26】Why Are Airplane Engines So Big? ( 為什麼飛機發動機這麼大？ )

Scroll down to view the Simplified Chinese version (向下滚动查看简体中文版本)  Why Are Airplane Engines So Big? Airplane jet engines have gotten bigger and bigger over time - and it’s not just because planes are getting bigger: the Airbus A350 is a Smaller plane than the Boeing 747 but has bigger engines.
The reason?
Bigger engines are simply more efficient, up to a point.
Modern jet engines actually consist of two propulsion systems working together: the jet core, which provides the power and a small amount of thrust, and the fan, which is just a big propellor driven by the jet core that provides most of the thrust.
Pure jet engines can be incredibly powerful (which is why they’re used for fighter jets), but they’re also horribly inefficient because they shoot out their exhaust at super-high speeds, which equates to huge amounts of kinetic energy since kinetic energy is proportional
to the speed squared, so accelerating exhaust to twice the speed takes 4 times the energy.
In exchange for power, they literally blast energy away like a bunch of hot air. 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.
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.
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.
But if you make the engine too big, then it starts to cause too much drag.
An ideal engine is somewhere in between – rough estimation puts the ideal engine somewhere around 4 meters in diameter, which is just slightly bigger than the current largest engines.
So you can expect jet engines to continue growing in size... but not forever. Simplified Chinese version 飞机发机为何如此庞大？ 随着时间的推移，飞机喷气发动机变得越来越大 - 这不仅仅是因为飞机越来越大：空客 A350 比波音 747 小，但发动机更大。 原因是什么？ 在一定程度上，更大的发动机效率更高。 现代喷气发动机实际上由两个共同工作的推进系统组成：喷气核心，提供动力和少量推力，风扇，只是一个由喷气核心驱动的大型螺旋桨，提供大部分推力。 纯喷气发动机可以非常强大（这就是它们用于战斗机的原因），但它们的效率也非常低，因为它们以超高速度喷出废气，这相当于巨大的动能，因为动能与速度的平方成正比，因此将废气加速到两倍的速度需要四倍的能量。 作为动力的交换，它们实际上像一堆热空气一样将能量吹走。 当然，你确实需要从发动机后部喷出一些空气来产生推力，但这并不需要很多能量。 你不必大量加速少量空气，而是可以通过少量加速大量空气来实现所需的推力 - 你可以获得相同的动量提升，但节省大量能量。 本质上，如果你制造的发动机太小，它必须加速空气，以至于浪费能量 - 有点像用机关枪推动你的汽车。 但是如果你把发动机做得太大，那么它就会开始造成太大的阻力。 理想的发动机介于两者之间 - 粗略估计理想发动机的直径约为 4 米，略大于目前最大的发动机。 因此，你可以预期喷气发动机的尺寸会继续增大......但不会永远增大。