use of flywheel in automotive field

1. FLYWHEEL
A Flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a
significant moment of inertia and thus resist changes in rotational speed. The amount of energy stored in a
flywheel is proportional to the square of its rotational speed. Energy is transferred to a flywheel by
applying torque to it, thereby increasing its rotational speed, and hence its stored energy. Conversely, a
flywheel releases stored energy by applying torque to a mechanical load, thereby decreasing the flywheel's
rotational speed.
Common uses of a flywheel:
 Providing continuous energy when the energy source is discontinuous. For example, flywheels are
used in reciprocating engines because the energy source, torque fromthe engine, is intermittent.
 Delivering energy at rates beyond the ability of a continuous energy source. This is achieved by
collecting energy in the flywheel over time and then releasing the energy quickly, at rates that exceed
the abilities of the energy source.
 Controlling the orientation of a mechanical system. In such applications, the angular momentum of a
flywheel is purposely transferred to a load when energy is transferred to or from the 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 its mechanical loads when no piston is exerting torque on it. Other
examples of this are friction motors, which use flywheel energy to power devices such as toy cars.
Energy stored in a flywheel
Rotational Kinetic Energy, E = ½ Iω2
where,
I - moment of inertia of the flywheel (ability of an object to resist changes in its rotational
velocity)
ω - rotational velocity (Rad / sec)
The moment of inertia, I = kMr 2
where,
M - mass of the flywheel
r - radius of flywheel
k - inertial constant.
k depends on the shape of the rotating object. So for solid disk ; I= Mr 2 /2
Stresses in a flywheel rim

2. A flywheel consists of a rim at which the major portion of the mass or weight of flywheel is concentrated, a
boss or hub for fixing the flywheel on to shaft and a number of arms for supporting the rim on the hub.
The following stresses are induced in the rim.
 Tensile stress due to centrifugal force.
 Tensile bending stress caused by the restraint of the arms.
1. Tensile stress due to the centrifugal force.
The tensile stress in the rim due to the centrifugal force, assuming that the rim is unstrained by the
arms, is determined in the similar way as the thin cylinder subjected to internal pressure.
ft = ρ.R2.ω2 = ρ.v2 ( v = R.ω )
When ρ is in kg/m3, v is in m/sec, ft will be in N/m2
where ρ = density of the flywheel material
ω = angular speed of the flywheel
R = mean radius of the flywheel
v = linear velocity of the flywheel
2. Tensile bending stress causedby restraint of arms.
The tensile bending stress in the rim due to the restraint of arms is based on the assumption that each
portion of the rim between a pair of arms behaves like a beam fixed at both ends and uniformly loaded,
such that length between fixed ends,
L = π.D/n = 2.π.R / n
where n - number of arms
The max bending moment,
M = w.l2 /12 = b.t.ρ.ω2.R/12(2.π.R/n)
Section modulus, Z = 1/6 (b.t2)
So bending stress fb = M/Z = b.t.ρ.ω2.R/12 (2.π.R/n) *
6 / (b.t2)
Total stress in the rim
f = ft + fb
Stresses in flywheel arms
The following stresses are induced in the arms of the flywheel.
 Tensile stresses due to centrifugal force acting on the rim
 Bending stress due to the torque transmitted fromthe rim to the shaft or from the shaft to the rim.

3. Construction of Flywheel
 Flywheels are typically made of steel and rotate on conventional bearings; these are generally
limited to a revolution rate of a few thousand RPM
 The flywheel of smaller size( upto 600 mm dia)are casted in one piece. The rim and the hub are
joined together by means of web.
 If flywheel is of larger size (upto 2-5 meters diameter), then it is made of arms.
 The number of arms depends upon the size of the flywheel and its speed of rotation. But the
flywheels above 2-5 meters are usually casted in two pieces. Such a flywheel is known as “ split
flywheel “.
 A split flywheel has the advantage of relieving the shrinkage stresses in the arms due to unequal
rates of cooling of casting.
Modern 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.
For example, flywheels are used in riveting machines to store energy from the motor and release it during
the riveting operation. 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 would experience a
lateral momentum when passing the top of a hill or the bottom of a valley (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 wheels, a type of flywheel employed in satellites in which the flywheel is used to orient the
satellite's instruments without thruster rockets.
Flywheels have also been proposed as a power booster for electric vehicles. Speeds of 100,000 rpm have
been used to achieve very high power densities.Modern high energy flywheels use composite rotors made
with carbon-fibre materials. The rotors have a very high strength-to-density ratio, and rotate at speeds up to
100,000 rpm. in a vacuumchamber to minimize aerodynamic losses.
Benefits in Aerospace
Flywheels are preferred over conventional batteries in many aerospace applications because of the
following benefits:
 5 to 10+ times greater specific energy
 Lower mass / kW output
 Long life. Unaffected by number of charge / discharge cycles
 85-95% round trip efficiency
 Fewer regulators / controls needed

4.  Greater peak load capability
 Reduced maintenance / life cycle costs
Disadvantages
 There are safety concerns associated with flywheels due to their high speed rotor and the
possibility of it breaking loose & releasing all of it's energy in an uncontrolled manner.
 Its Bulkier, adds more weight to the vehicle
Conclusion
 Recent advance in the mechanical properties of composites has regained the interest in
using the inertia of a spinning wheel to store energy.
 Carbon-composite flywheel batteries have recently been manufactured and are proving to be viable
in real-world tests on mainstreamcars. Additionally, their disposal is more eco-friendly.