Flywheels are used to store energy and even out fluctuations. In the case of a reciprocating engine it evens out the pushes from the pistons in order to deliver a constant source of energy.

A car  engine with more pistons needs a smaller flywheel.

The energy of a flywheel depends on the square of the rim velocity, so faster will store more energy.

Flywheel energy storage

A flywheel can be used to store energy. It has a similar storage capacity to a battery.

The rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor/electric generator.

Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in 2 hours.[26] Conversely, flywheels with magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.[27]


In the diagram there is an error. The energy of Hydrogen should be triple petrol/gasoline


Most modern high-speed flywheel energy storage systems consist of a massive rotating cylinder (a rim attached to a shaft) that is supported on a stator by magnetically levitated bearings. To maintain efficiency, the flywheel system is operated in a vacuum to reduce drag. The flywheel is connected to a motor/generator that interacts with the utility grid through advanced power electronics. Some of the key advantages of flywheel energy storage are low maintenance, long life (20 years or tens of thousands of deep cycles), and negligible environmental impact. Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics.
Typically, users of high-speed flywheels must choose between two types of rims: solid steel or carbon composite. The choice of rim material will determine the system cost, weight, size, and performance. Composite rims, while expensive, are both lighter and stronger than steel, which means that they can achieve much higher rotational speeds. The amount of energy that can be stored in a flywheel is a function of the square of the rpm making higher rotational speeds highly desirable.
Currently, high-power flywheels are used in many aerospace and UPS applications. Today 2 kW/6 kWh systems are being used in telecommunications applications. For utility-scale storage a ‘flywheel farm’ approach can be used to store megawatts of electricity for applications needing minutes of discharge duration. Currently several ‘flywheel farm’ facilities are in the planning or construction stages in order to sell regulation services into open ISO markets.
April 2010

Current Developers/Suppliers
Beacon Power
Active Power
Piller Gmb
Urenco Power Technologies

Electricity Storage Assn.

Flywheel farm

Beacon Power flywheel has 200 flywheels, each capable of generating or absorbing one megawatt of power for 15 minutes. Source


Porsche's hybrid flywheel

Porsche has developed a racing car with an electro-mechanical flywheel energy storage system.

Each of the front wheels of the 911 is connected to a 60 kilowatt (80 hp) electric motor/generator. Under braking, these motors produce electricity that accelerates a carbon-fiber flywheel.  

A flywheel can absorb energy much faster than a battery which would overheat and fail. So while hybrid vehicles are  limited to about 0.3 g's of deceleration from regen braking, flywheels can recover and store energy at deceleration of 1g or more.

A flywheel can store less energy than a battery, and it cannot store energy for as long, but it can release it more quickly. The Porsche 911 racing version can release 163 horsepower for up to six seconds.
An ultracapacitor would achieve the same result, but it would be heavier.
than a flywheel. 
The flywheel's high power and limited energy-storage capacity makes it less suitable for typical street driving applications, but it could be used for heavier vehicles that do a lot of stopping and starting. E.g. rubbish trucks, delivery vans, or buses.

Porsche 911 flywheel race car 

Wind turbine stores energy in flywheel

Sea Twirl

The sea Twirl vertical axis wind turbine stores excess energy in its water filled outer ring.