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A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels resist changes in rotational speed by their the moment of inertia. The amount of energy stored in a flywheel is proportional to the square of its rotational speed. The way to change a flywheel stored energy is by increasing or decreasing its rotational speed by the application of a torque at its axis of symmetry,

Common uses of a flywheel include:

Smoothing the power output of an energy source. For example flywheels are used in reciprocating engines because the energy source, torque from the engine, is intermittent.
Delivering energy at rates beyond the ability of an energy source. This is achieved by collecting energy in a flywheel over time and then releasing it quickly, at rates that exceed the abilities of the energy source.
Controlling the orientation of a mechanical system. See gyroscope

Flywheels are typically made of steel and rotate on conventional bearings; these are generally limited to a revolution rate of a few thousand RPM.^[1] Some modern flywheels are made of carbon fiber materials and employ magnetic bearings, enabling them to revolve at speeds up to 60,000 RPM (1 kHz).^[2]

Carbon-composite flywheel batteries have recently been manufactured and are proving to be viable in real-world tests on mainstream cars. Additionally, their disposal is more eco-friendly.^[3]

Applications

Flywheels are often used to provide continuous power output in systems where the energy source is not continuous. For example, a flywheel is used to smooth fast angular velocity fluctuations of the crankshaft in a reciprocating engine. In this case, a crankshaft flywheel stores energy when torque is exerted on it by a firing piston, and returns it to the piston to compress a fresh charge of air and fuel. Another example is the friction motor which powers devices such as toy cars. In unstressed and inexpensive cases to save on materials the bulk of the mass of the flywheel is toward the rim of the wheel. Pushing the mass away from the axis of rotation heightens rotational inertia for a given total mass.

A flywheel may also be used to supply intermittent pulses of energy at power levels that exceed the abilities of its energy source. This is achieved by accumulating energy in the flywheel over a period of time, at a rate that is compatible with the energy source, and then releasing energy at a much higher rate over a relatively short time when it is needed. For example flywheels are used in riveting machines. Flywheels can be used to control direction and oppose unwanted motions, see gyroscope. Flywheels in this context have a wide range of applications from gyroscopes for instrumentation to ship stability </ref>

The table below contains calculated values for materials and comments on their viability for flywheel applications. CFRP stands for carbon-fiber-reinforced polymer, and GFRP stands for glass-fiber reinforced polymer.

Material	Specific tensile strength $({\frac {kJ}{kg}})\$	Comments
Ceramics	200-2000 (compression only)	Brittle and weak in tension, therefore eliminate
Composites: CFRP	200-500	The best performance—a good choice
Composites: GFRP	100-400	Almost as good as CFRP and cheaper
Beryllium	300	The best metal, but expensive, difficult to work with, and toxic to machine
High strength steel	100-200	Cheaper than Mg and Ti alloys
High strength Al alloys	100-200	Cheaper than Mg and Ti alloys
High strength Mg alloys	100-200	About equal performance to steel and Al-alloys
Ti alloys	100-200	About equal performance to steel and Al-alloys
Lead alloys	3	Very low
Cast Iron	8-10	Very low^[4]

The table below shows calculated values for mass, radius, and angular velocity for storing 500 J. The carbon-fiber flywheel is by far the most efficient; however, it also has the largest radius. In applications (like in an automobile) where the volume is constrained, a carbon-fiber flywheel might not be the best option.

Material	Energy storage (J)	Mass (kg)	Radius (m)	Angular velocity (rpm)	Efficiency (J/kg)	Energy density (kWh/kg)
Cast Iron	500	0.0166	1.039	1465	30121	0.0084
Aluminum Alloy	500	0.0033	1.528	2406	151515	0.0421
Maraging steel	500	0.0044	1.444	2218	113636	0.0316
Composite: CFRP (40% epoxy)	500	0.001	1.964	3382	500000	0.1389
Composite: GFRP (40% epoxy	500	0.0038	1.491	2323	131579^[5]	0.0365

Table of energy storage traits

Flywheel purpose, type	Geometric shape factor (k) (unitless – varies with shape)	Mass (kg)	Diameter (cm)	Angular velocity (rpm)	Energy stored (MJ)	Energy stored (kWh)	Energy density (kWh/kg)
Small battery	0.5	100	60	20,000	9.8	2.7	0.027
Regenerative braking in trains	0.5	3000	50	8,000	33.0	9.1	0.003
Electric power backup^[6]	0.5	600	50	30,000	92.0	26.0	0.043^[7]^[8]^[9]^[10]

For comparison, the energy density of petrol (gasoline) is 44.4 MJ/kg or 12.3 kWh/kg.

High-energy materials

For a given flywheel design, the kinetic energy is proportional to the ratio of the hoop stress to the material density and to the mass:

$E_{k}\varpropto {\frac {\sigma _{t}}{\rho }}m$

${\frac {\sigma _{t}}{\rho }}$ could be called the specific tensile strength. The flywheel material with the highest specific tensile strength will yield the highest energy storage per unit mass. This is one reason why carbon fiber is a material of interest.

For a given design the stored energy is proportional to the hoop stress and the volume:

$E_{k}\varpropto \sigma _{t}V$

Rimmed

A rimmed flywheel has a rim, a hub, and spokes.^[11] Calculation of the flywheel's moment of inertia can be more easily analysed by applying various simplifications. For example:

Assume the spokes, shaft and hub have zero moments of inertia, and the flywheel's moment of inertia is from the rim alone.
The lumped moments of inertia of spokes, hub and shaft may be estimated as a percentage of the flywheel's moment of inertia, with the majority from the rim, so that $I_{\mathrm {rim} }=KI_{\mathrm {flywheel} }$

For example, if the moments of inertia of hub, spokes and shaft are deemed negligible, and the rim's thickness is very small compared to its mean radius ( $R$ ), the radius of rotation of the rim is equal to its mean radius and thus:

$I_{\mathrm {rim} }=M_{\mathrm {rim} }R^{2}$

References

^ [1]; "Flywheels move from steam age technology to Formula 1"; Jon Stewart | 1 July 2012, retrieved 2012-07-03
^ [2], "Breakthrough in Ricardo Kinergy ‘second generation’ high-speed flywheel technology"; Press release date: 22 August 2011. retrieved 2012-07-03
^ http://www.popularmechanics.com/technology/engineering/news/10-tech-concepts-you-need-to-know-for-2012-2
^ Totten, George E.; Xie, Lin; Funatani, Kiyoshi (2004). Handbook of Mechanical Alloy Design. New York: Marcel Dekker. ISBN 0-8247-4308-3.
^ Kumar, Mouleeswaran Senthil; Kumar, Yogesh (2012). "Optimization of Flywheel Materials Using Genetic Algorithm" (PDF). Acta technica Corviniensis-Bulletin of Engineering. Retrieved 1 November 2015.
^ https://www.calnetix.com/vdc-kinetic-energy-storage-systems
^ "Flywheel Energy Calculator". Botlanta.org. 2004-01-07. Retrieved 2010-11-30.
^ "energy buffers". Home.hccnet.nl. Retrieved 2010-11-30.
^ "Message from the Chair | Department of Physics | University of Prince Edward Island". Upei.ca. Retrieved 2010-11-30.
^ "Density of Steel". Hypertextbook.com. 1998-01-20. Retrieved 2010-11-30.
^ Flywheel Rotor And Containment Technology Development, FY83. Livermore, Calif: Lawrence Livermore National Laboratory , 1983. pp. 1–2

External links

Flywheel batteries on Interesting Thing of the Day.
Flywheel-based microgrid stabilisation technology., ABB
PowerStore

[BBC-1] [1]; "Flywheels move from steam age technology to Formula 1"; Jon Stewart | 1 July 2012, retrieved 2012-07-03

[Kinergy-2] [2], "Breakthrough in Ricardo Kinergy ‘second generation’ high-speed flywheel technology"; Press release date: 22 August 2011. retrieved 2012-07-03

[3] ttp://www.popularmechanics.com/technology/engineering/news/10-tech-concepts-you-need-to-know-for-2012-2

[4] Totten, George E.; Xie, Lin; Funatani, Kiyoshi (2004). Handbook of Mechanical Alloy Design. New York: Marcel Dekker. ISBN 0-8247-4308-3.

[5] Kumar, Mouleeswaran Senthil; Kumar, Yogesh (2012). "Optimization of Flywheel Materials Using Genetic Algorithm" (PDF). Acta technica Corviniensis-Bulletin of Engineering. Retrieved 1 November 2015.

[6] ttps://www.calnetix.com/vdc-kinetic-energy-storage-systems

[7] "Flywheel Energy Calculator". Botlanta.org. 2004-01-07. Retrieved 2010-11-30.

[8] "energy buffers". Home.hccnet.nl. Retrieved 2010-11-30.

[9] "Message from the Chair | Department of Physics | University of Prince Edward Island". Upei.ca. Retrieved 2010-11-30.

[10] "Density of Steel". Hypertextbook.com. 1998-01-20. Retrieved 2010-11-30.

[11] Flywheel Rotor And Containment Technology Development, FY83. Livermore, Calif: Lawrence Livermore National Laboratory , 1983. pp. 1–2

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@@ Line 9: / Line 9: @@
 * Smoothing the power output of an energy source. For example flywheels are used in [[reciprocating engine]]s because the energy source, torque from the engine, is intermittent.
 * Delivering energy at rates beyond the ability of an energy source. This is achieved by collecting energy in a flywheel over time and then releasing it quickly, at rates that exceed the abilities of the energy source.
-* Controlling the orientation of a mechanical system. See [[gyroscope]] [[rotational energy]]
+* Controlling the orientation of a mechanical system. See [[gyroscope]]
 Flywheels are typically made of steel and rotate on conventional bearings; these are generally limited to a revolution rate of a few thousand RPM.<ref name="BBC">[http://www.bbc.com/future/story/20120629-reinventing-the-wheel]; "Flywheels move from steam age technology to Formula 1"; Jon Stewart | 1 July 2012, retrieved 2012-07-03</ref>  Some modern flywheels are made of carbon fiber materials and employ [[magnetic bearing]]s, enabling them to revolve at speeds up to 60,000 RPM (1 kHz).<ref name ="Kinergy">[http://www.ricardo.com/en-GB/News--Media/Press-releases/News-releases1/2011/Breakthrough-in-Ricardo-Kinergy-second-generation-high-speed-flywheel-technology/], "Breakthrough in Ricardo Kinergy ‘second generation’ high-speed flywheel technology"; Press release date: 22 August 2011. retrieved 2012-07-03</ref>
@@ Line 17: / Line 17: @@
 == Applications ==
 [[File:Landini VL30(Italien)2.JPG|thumb|A [[Landini (tractor)|Landini]] tractor with exposed flywheel]]
-Flywheels are often used to provide continuous energy in systems where the energy source is not continuous. In such cases, the flywheel stores energy when torque is applied by the energy source, and it releases stored energy when the energy source is not applying torque to it. For example, a flywheel is used to maintain constant angular velocity of the [[crankshaft]] in a reciprocating engine. In this case, the flywheel—which is mounted on the crankshaft—stores energy when torque is exerted on it by a firing [[piston]], and it releases energy to the crankshaft when a piston is in the process of compressing a fresh charge of air and fuel. Other examples of this are [[friction motor]]s, which use flywheel energy to power devices such as [[toy car]]s. In uses like this, the distribution of the mass of the flywheel toward the outside and away from the center is beneficial.  Pushing the mass away from the axis of rotation gives it greater [[rotational inertia]] without increasing its total mass. This increases the efficiency of the flywheel, since it does not have as much difficulty driving its own weight forward as well as that of the payload.
+Flywheels are often used to provide continuous power output in systems where the energy source is not continuous. For example, a flywheel is used to smooth fast angular velocity fluctuations of the [[crankshaft]] in a reciprocating engine. In this case, a crankshaft flywheel stores energy when torque is exerted on it by a firing [[piston]], and returns it to the piston to compress a fresh charge of air and fuel. Another example is the [[friction motor]] which powers devices such as [[toy car]]s. In unstressed and inexpensive cases to save on materials the bulk of the mass of the flywheel is toward the rim of the wheel.  Pushing the mass away from the axis of rotation heightens [[rotational inertia]] for a given total mass.
 [[File:P307.jpg|thumb|Modern automobile engine flywheel]]
-A flywheel may also be used to supply intermittent pulses of energy at transfer rates that exceed the abilities of its energy source, or when such pulses would disrupt the energy supply (e.g., public electric network). This is achieved by accumulating stored energy in the flywheel over a period of time, at a rate that is compatible with the energy source, and then releasing that energy at a much higher rate over a relatively short time when it is needed. For example, flywheels are used in [[riveting machines]] to store energy from the motor and release it during the riveting operation.
+A flywheel may also be used to supply intermittent pulses of energy at power levels that exceed the abilities of its energy source. This is achieved by accumulating energy in the flywheel over a period of time, at a rate that is compatible with the energy source, and then releasing energy at a much higher rate over a relatively short time when it is needed. For example flywheels are used in [[riveting machines]].
+Flywheels can be used to control direction and oppose unwanted motions, see [[gyroscope]]. Flywheels in this context have a wide range of applications from [[gyroscope]]s for instrumentation to [[ship stability]]
 <!--
 Please clarify for topic novices and uncomment:
 Flywheels can be used control the orientation of a mechanical system. In such cases, the angular momentum of the flywheel is transferred to the load during energy transfer. As angular momentum is a vector quantity, transfer of angular momentum helps orient the load to a particular axis.
--->
-The phenomenon of [[precession]] has to be considered when using flywheels in vehicles. A rotating flywheel responds to any momentum that tends to change the direction of its axis of rotation by a resulting precession rotation. A vehicle with a vertical-axis flywheel, that is rigidly attached to the vehicle, would experience a torque applied to the body of the vehicle that would rotate with as the flywheel precesses. This would produce an alternating rolling and pitching of the vehicle body as it moved up the incline. The descent of the hill would produce the opposite effect and so it would zero out the pitching and rolling ([[wikt:roll|roll]] momentum in response to a pitch change). Two counter-rotating flywheels may be needed to eliminate this effect. This effect is used in [[reaction wheel]]s, a type of flywheel employed in satellites in which the flywheel is used to orient the satellite's instruments without the use of thruster rockets. Alternatively, the flywheel would be mounted in two yokes, with axes at mutual right angles, and so allow limited changes to the orientation of the vehicle body thereby eliminating precession.
+The phenomenon of [[precession]] has to be considered when using flywheels in vehicles. A rotating flywheel responds to any momentum that tends to change the direction of its axis of rotation by a resulting precession rotation. A vehicle with a vertical-axis flywheel, that is rigidly attached to the vehicle, would experience a torque applied to the body of the vehicle that would rotate with as the flywheel precesses. This would produce an alternating rolling and pitching of the vehicle body as it moved up the incline. The descent of the hill would produce the opposite effect and so it would zero out the pitching and rolling ([[wikt:roll|roll]] momentum in response to a pitch change). Two counter-rotating flywheels may be needed to eliminate this effect. This effect is used in [[reaction wheel]]s, a type of flywheel employed in satellites in which the flywheel is used to orient the satellite's instruments without the use of thruster rockets. Alternatively, the flywheel would be mounted in two yokes, with axes at mutual right angles, and so allow limited changes to the orientation of the vehicle body thereby eliminating precession.>
 == History ==

v t e Internal combustion engine
Part of the Automobile series
Engine block and rotating assembly	Balance shaft Block heater Bore Connecting rod Crankcase Crankcase ventilation system (PCV valve) Crankpin Crankshaft Core plug (freeze plug) Cylinder (bank, layout) Displacement Flywheel Firing order Stroke Main bearing Piston Piston ring Starter ring gear
Valvetrain and Cylinder head	Flathead layout Overhead camshaft layout Overhead valve (pushrod) layout Tappet / lifter Camshaft Chest Combustion chamber Compression ratio Head gasket Rocker arm Timing belt Valve
Forced induction	Blowoff valve Boost controller Intercooler Supercharger Turbocharger
Fuel system	Diesel engine Petrol engine Carburetor Fuel filter Fuel injection Fuel pump Fuel tank
Ignition	Magneto Compression ignition Coil-on-plug Distributor Glow plug Ignition coil Spark plug Spark plug wires
Engine management	Engine control unit (ECU)
Electrical system	Alternator Battery Dynamo Starter motor
Intake system	Airbox Air filter Idle air control actuator Inlet manifold MAP sensor MAF sensor Throttle Throttle position sensor
Exhaust system	Catalytic converter Diesel particulate filter EGT sensor Exhaust manifold Muffler Oxygen sensor
Cooling system	Air cooling Water cooling Electric fan Radiator Thermostat Viscous fan (fan clutch)
Lubrication	Oil Oil filter Oil pump Sump (Wet sump, Dry sump)
Other	Knocking / pinging Power band Redline Stratified charge Top dead centre
Portal Category