Kinetic Energy Recovery System

By- Ankit Singh

Contents Abstract Introduction Mechanism

• The flywheel• The flywheel vacuum chamber• Magnetic bearing• The Continuously variable transmission (CVT) unit• Step-up gearing and clutch• The clutch

Advantages and Disadvantages Conclusion

Abstract

• Today, many hybrid electric vehicles have been developed in order to reduce the consumption of fossil fuels; unfortunately these vehicles require electrochemical batteries to store energy, with high costs as well as poor conversion efficiencies.

• By integrating flywheel hybrid systems, these drawbacks can be overcome and can potentially replace battery powered hybrid vehicles cost effectively.

• The advantages of this technology over the electric hybrids will be elucidated carefully. The latest advancements in the field, the potential future and scope of the flywheel hybrid will be assessed.

Introduction• The Kinetic Energy Recovery System

(KERS) is a type of regenerative braking system which has the capability to store and reuse the lost energy.

• Unfortunately the poor conversion efficiencies cancel out most of the advantages these battery powered hybrid vehicles bring with them. The flywheel-based kinetic energy recovery system is a possible solution which could potentially replace the electric hybrids.

• The KERS was first designed for formula one racing cars.

Drawbacks of Electric Hybrid• The flywheel energy storage is based on the principle of regenerative

braking. Regenerative braking is a mechanism which reduces the vehicle speed, converting the kinetic energy into another useful form of energy mechanical energy, electrical energy or the like.

• When the vehicle brakes, the motor rotates in the opposite direction, acting as a generator and in the process slows down the car. The electricity produced is then sent to the battery which stores the energy as chemical energy.

• When desired, the battery supplies the stored energy back to the wheels, giving an additional “boost” of power. Unfortunately the many energy conversions- from mechanical to electrical energy and from electrical to chemical energy, reduces the overall efficiency to the system to about 30%. The mechanical hybrid overcomes this drawback.

Mechanism• The flywheel hybrid primarily consists of a rotating flywheel, a

continuously variable transmission system (CVT), a step up gearing (along with a clutch) between the flywheel and the CVT and clutch which connects this system to the primary shaft of the transmission.

• When the brakes are applied or the vehicle decelerates, the clutch connecting the flywheel system to the driveline/ transmission is engaged, causing energy to be transferred to the flywheel via the CVT.

• The flywheel stores this energy as rotational energy and can rotate up to a maximum speed of 60000 rpm. When the vehicle stops, or the flywheel reaches its maximum speed, the clutch disengages the flywheel unit from the transmission allowing the flywheel to rotate independently.

• Whenever this stored energy is required, the clutch is engaged and the flywheel transmits this energy back to the wheels, via the CVT. Generally the flywheel can deliver up to 60 kW of power or about 80 HP.

Volvo’s flywheel KERS system Layout

The Flywheel• The flywheel is the component which harvests kinetic energy, when the vehicle brakes, by

increasing its rotational speed. The ability of flywheels to store energy is explained by the relation between the flywheel’s inertia, angular velocity and kinetic energy.

• The equation for the inertia of a flywheel is:

• Therefore energy stored in the flywheel is given by:

• A flywheel's energy is proportional to its mass, and proportional to the square of its rotational speed or angular velocity. In other words, by doubling the mass, the energy stored is also doubled, and by doubling the speed, the energy stored is quadrupled. Thus by increasing the speed of the flywheel it will be possible to reduce the mass and size of it, to a level where its weight is insignificant while analyzing fuel efficiency.

• The weight of the flywheel is a very important factor in determining the efficiency of the system.

Flywheel System

The Flywheel Vacuum Chamber• The vacuum chamber is another very essential part of the flywheel hybrid

system. • The major function of the vacuum chamber is to minimize the air

resistance as the flywheel rotates. Without the vacuum chamber, the friction caused by air resistance is enough to cause significant energy losses and heat the carbon fiber rim to its glass transition temperature.

• . Vacuum chambers for KERS systems are frequently made of metals like aluminum, stainless steel, or the like because these metals can provide adequate strength to withstand differential pressure between an evacuated interior and the surrounding atmosphere, as well as to provide a barrier to the passage of atmospheric gases through the chamber wall by diffusion or flow through structural defects.

Magnetic Bearings• Magnetic bearings have replaced

mechanical bearings as they greatly reduce losses due to friction.

• The magnetic bearings support the flywheel by the principle of magnetic levitation. It is a method by which an object is suspended with no support other than magnetic fields. A permanent or electro permanent magnetic bearing system is utilized.

• The best performing bearing is the high-temperature super-conducting (HTS) magnetic bearing, which can situate the flywheel automatically without need of electricity or positioning control system.

Waukesha magnetic bearing

Continuously Variable Transmission (CVT)

• The most important interface that connects the flywheel to the transmission system is the CVT.

• The speed ratio between the vehicle and the flywheel constantly changes between acceleration and braking. The reason why a stepped drive unit is not preferred in this system is because it has only a fixed number of gear ratios as opposed to the CVT’s which have an infinite number of gear ratios between the maximum and the minimum value which allows a seamless transfer of energy without any loss of power.

• The surface contact area is the factor which determines which shaft rotates faster. When both the rollers have equal contact on the toroidal surfaces of the input and output shafts, the gear ratio is 1:1. Any change in the CVT ratio can be viewed as the transfer of kinetic energy between the flywheel inertia and vehicle.

Step-up Gearing & Clutch

• A step up gearing system consisting of Epicyclic gears is connected between the CVT and the flywheel unit.

• The reason why this gearing system is used is because the high speed at which the flywheel rotates (60000 RPM) needs to be reduced to a manageable speed outside the vacuum chamber, in order for the energy to be smoothly transferred back to the CVT.

• The clutch disconnects the CVT from the flywheel when it is not transferring power to reduce free running losses.

The Clutch

• The clutch is used to couple the flywheel hybrid system to the transmission.

• It engages the system while the flywheel is accelerating from rest and disengaging while the flywheel is rotating and the vehicle is at rest.

• Torque is transferred through clutch between the flywheel and vehicle. Hence, the power transmitted in the flywheel system can be controlled by a clutch that could continuously manipulate the torque.

Advantages

• High Efficiency.• Low fuel consumption.• Low cost compared to electric hybrid.• Tests have proven that flywheel-based KERS

can recover and store over 70% of the vehicles energy.

Conclusion• Cars with a flywheel based energy recovery system, though

significantly more expensive than cars without this system, have more power and better fuel efficiency.

• According to www.thegreencarwebsite.co, “the system could reduce fuel consumption by as much as 20% and give a four-cylinder engine acceleration like a six-cylinder unit.” This effectively means that cars with the Flywheel KERS system have better fuel efficiency and more power than the cars without the KERS system.

• Better fuel efficiency directly translates to a cleaner, greener environment. It reduces the negative impact on the environment by decreasing harmful CO2 emissions.

The End

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