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Why does an automobile engine use a flywheel?

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A diagram displays six types of simple machines with labeled illustrations: inclined plane, wedge, screw, lever, wheel and axle, and pulley, each shown with real-world examples like ramps, axes, screws, seesaws, door handles, and pulleys lifting objects. — machine Six simple machines for transforming energy into work.

In an automobile engine, a flywheel is essential for maintaining smooth operation by moderating fluctuations in engine speed.12

Because internal combustion engines provide power in intermittent pulses rather than a continuous flow, the flywheel serves two primary functions:

Smoothing Power Delivery: It uses its inertia to dampen the speed variations caused by the uneven pulses of energy generated by combustion within the cylinders.12
Energy Storage: It stores excess kinetic energy during power strokes and releases that energy to assist the pistons during the compression stroke, ensuring the engine continues to rotate steadily.12

By utilizing high rotational inertia, the flywheel ensures that the engine's speed remains relatively constant despite the intermittent nature of the power supply and demand.1