Torsional vibration occurs when the __________ speeds up and slows down.

Torsional vibration specifically refers to the oscillations that occur in the rotational axis of a shaft, which is primarily related to the crankshaft in an engine. When the crankshaft speeds up and slows down during operation—such as during acceleration and deceleration of the engine—these fluctuations can lead to variations in torque, resulting in torsional vibration.

The crankshaft's role in converting the linear motion of pistons into rotational motion makes it a critical component where torsional vibrations can be most pronounced. These vibrations can impact engine performance and longevity, making it essential for technicians to understand and address these conditions during maintenance and repairs.

In contrast, while the camshaft, flywheel, and transmission are important components of an engine and drivetrain, they do not primarily contribute to torsional vibration in the same way the crankshaft does. The camshaft primarily controls valve timing, the flywheel helps to smooth out engine operation and store rotational energy, and the transmission focuses on gear ratios, thus managing power transfer rather than contributing significantly to torsional vibration effects.