飞轮

在看福特的书中，他提到，一缸的发动机不好造啊，需要的飞轮太大装不下，只能用两缸的。

感觉很有意思，原来以为多缸发动机好，怎么反而是单缸的难造呢？去查了一下，飞轮是储能设备，就是一个很重的盘，有巨大的惯性。

Wikipedia 上说的很清楚：

A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed. Energy is transferred to a flywheel by applying torque to it, thereby increasing its rotational speed, and hence its stored energy. Conversely, a flywheel releases stored energy by applying torque to a mechanical load, thereby decreasing the flywheel’s rotational speed.

四冲程单缸发动机，四冲程只有一个冲程做功，也就是输出的力是很不均衡的。如果想要设备的动力顺滑，就需要用飞轮来保持惯性。双缸发动机，整体来看，四个冲程里有两个冲程在做工，输出会平衡许多，也就不需要那么大的飞轮。

据说现在电力的峰谷调整也主要是用飞轮。

还有汽车轮胎也有飞轮效应。较大较重的轮胎惯性更大。启停都需要更大的马力去控制。