Energy storage
Applications include reciprocating engines, especially 1-cylinder four-stroke engines. These only have a power stroke every fourth half revolution, which transmits energy to the output via the crankshaft. In the remaining three strokes, they require energy to maintain rotary motion and compress the combustion air. The energy of the power stroke is temporarily stored in the flywheel and then continuously released again.
The simplest small toy vehicles do not require a spring mechanism and typically travel 3 m with flywheel drive alone. The flywheel consists of 1 to 3 punched circular discs with a diameter of 2 to 4 cm, made of 1 to 2 mm thick sheet iron, perforated and pressed onto a steel axle which is mounted in sheet metal or plastic.
Another application is flywheel storage power plants in modular design for balancing sudden additional demand in electricity grids. For storage, a flywheel is driven by means of an electric motor and the energy is held in reserve in the form of rotational energy for energy output in the minute range.
The ASDEX Upgrade fusion experiment uses a flywheel generator weighing 400 tonnes, which takes a few minutes to ramp up from 800 to 3000 revolutions, and then calls up the stored energy within fractions of a second to heat up the plasma.
On the Norwegian island of Utsira, a self-sufficient power grid was established in 2004, whereby short-term power fluctuations are compensated by a flywheel storage (5 kWh). The flywheel masses of all turbines and generators running in the grid power production (synchronously) have a significance for the stabilization of the grid frequency, i.e. energy storage for the period of the order of magnitude of a phase of 1/50 second. Similarly, the flywheel mass of electric motors means for smoothing the torque output. Flywheels in large stationary machines can only store local mechanical amounts of energy, which are used up in pressing, forging, punching, rolling and cutting processes in typically half a second, in order to be supplied again by electric motors in several seconds afterwards.
Position stabilization
Rotating flywheels do not allow any angular changes of their axis. This is used for stabilisation, e.g. in gyrocompasses for ships and the devices in satellites known as swirl wheels or inertial wheels.
Rotational irregularity, torsional vibrations
In many dynamic processes on machines, rotational irregularities (fluctuations in speed) occur. These are caused by periodically occurring torques and can lead to torsional vibrations (=torsional oscillations). Flywheels reduce the rotational irregularity through their mass inertia by absorbing energy during acceleration and releasing it again during deceleration. The rotational irregularity is thus reduced. The disadvantage is that a large mass has to be set in motion, which means additional weight for vehicles. For this reason, attempts are usually made to keep the non-uniformity low (for example, in the case of internal combustion engines by means of several cylinders) or to reduce the torsional vibration itself in other ways (vibration damping).
A vibration damper consists of a flywheel mass and a damping element (e.g. oil or rubber), which transmits the vibration-damping forces between the flywheel mass and the component to be damped. The oscillating part "supports" itself, so to speak, via a damping element on the smoother-running flywheel mass. The damping element thereby converts kinetic energy into heat and thus extracts the kinetic energy (vibration energy) from the vibrating component.
A flywheel was used for vibration damping (actually: amplitude reduction) for the first time in the engine of the BMW R 69 S motorcycle from the 1960 model year onwards, in order to prevent the previously frequent crankshaft fractures due to vibrations in the highly loaded engine. Here, a small flywheel on the side opposite the clutch at the front of the engine ensured less rotational irregularity of the oscillating crankshaft-flywheel-clutch system.
A similar special form of flywheel in passenger cars is the so-called dual-mass flywheel. Here, the use of a primary and a secondary flywheel mass with an elastic element in between greatly reduces the transmission of engine vibrations to the rest of the driveline (e.g. transmission idle rattle). The primary and secondary flywheel masses are separated from each other by a precisely matched spring/damper system. The flywheel mass on the transmission side (secondary flywheel mass) is heavier than the flywheel mass on the engine side (primary flywheel mass). This increases the mass moment of inertia of the gearbox, which greatly reduces the non-uniformity, especially at low speeds. The torsional vibration excitation acting on the drive train is greatly reduced.
The balance of the mechanical watch, in conjunction with the hairspring, represents a rotary oscillator, the period of which is highly constant (isochronism).
