KINETIC ENERGY RECOVERY SYSTEMS 2011

CHAPTER 1

INTRODUCTION TO HYBRID DRIVE TRAINS

Modern day road vehicles are all powered by a conventional internal combustion engine. A

hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The

term most commonly refers to hybrid electric vehicles (HEVs), which combine an internal

combustion engine and one or more electric motors.

Hybrids are classified by the division of power between sources; both sources may operate in

parallel to simultaneously provide acceleration, or they may operate in series with one source

exclusively providing the acceleration and the second being used to augment the first's power

reserve. The sources can also be used in both series and parallel as needed, the vehicle being

primarily driven by one source but the second capable of providing direct additional acceleration

if required.

Parallel hybrid

Parallel hybrid systems, which are most commonly produced at present, have both an internal

combustion engine and an electric motor connected to a mechanical transmission.

Series hybrid

Series hybrids have also been referred to as range-extended electric vehicles in order to

emphasize that they are electric vehicles with a combustion engine assist.

1Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

CHAPTER 2

HISTORY OF REGENRATIVE BRAKING

Vehicles driven by electric motors use the motor as a generator when using regenerative braking:

it is operated as a generator during braking and its output is supplied to an electrical load; the

transfer of energy to the load provides the braking effect.

Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy

lost during stopping. This energy is saved in a storage battery and used later to power the motor

whenever the car is in electric mode.

Early examples of this system were the front-wheel drive conversions of horse-drawn cabs by

Louis Antoine Krieger (1868–1951). The Krieger electric landaulet had a drive motor in each

front wheel with a second set of parallel windings (bifilar coil) for regenerative braking.

2Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

In England, the Raworth system of "regenerative control" was introduced by tramway operators

in the early 1900s, since it offered them economic and operational benefits as explained by A.

Raworth of Leeds in some detail. These included tramway systems at Devonport (1903),

Rawtenstall, Birmingham, Crystal Palace-Croydon (1906) and many others. Slowing down the

speed of the cars or keeping it in hand on descending gradients, the motors worked as generators

and braked the vehicles. The tram cars also had wheel brakes and track slipper brakes which

could stop the tram should the electric braking systems fail. In several cases the tram car motors

were shunt wound instead of series wound, and the systems on the Crystal Palace line utilized

series-parallel controllers. Following a serious accident at Rawtenstall, an embargo was placed

on this form of traction in 1911. Twenty years later, the regenerative braking system was

reintroduced.

Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-

Batumi railway (Transcaucasian railway or Georgian railway) started utilizing regenerative

braking in the early 1930s. This was especially effective on the steep and dangerous Surami

Pass. In Scandinavia the Kiruna to Narvik railway carries iron ore from the mines in Kiruna in

the north of Sweden down to the port of Narvik in Norway to this day. The rail cars are full of

thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts

of electricity by their regenerative braking. From Riksgränsen on the national border to the Port

of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is

sufficient to power the empty trains back up to the national border. Any excess energy from the

railway is pumped into the power grid to supply homes and businesses in the region, and the

railway is a net generator of electricity.

An Energy Regeneration Brake was developed in 1967 for the AMC Amitron. This was a

completely battery powered urban concept car whose batteries were recharged by regenerative

braking, thus increasing the range of the automobile.

Many modern hybrid and electric vehicles use this technique to extend the range of the battery

pack. Examples include the Toyota Prius, Honda Insight, the Vectrix electric maxi-scooter, and

the Chevrolet Volt.

3Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

CHAPTER 3

KINETIC ENERGY RECOVERY SYSTEMS

Kinetic Energy Recovery Systems (KERS) are automotive systems whereby the energy

generated under braking is stored in a reservoir (either a flywheel or a battery) for later use under

acceleration. KERS was used for the motor sport Formula One's 2009 season, and is under

development for road vehicles.

KERS Systems under development are classified based on the method used for energy storage:

1. Mechanical based energy storage: under this category, 2 main designs are under

development by Flybrid technologies and CPC – KERS.

2. Electrical based energy storage: Magneti Marelli Motorsport and Robert BOSCH GmbH.

3.1 Mechanical Based Energy storage Technology:

3.1.1 Flybrid Systems:

Flybrid Systems is an award winning, innovative engineering company at the forefront of hybrid

vehicle technology.

In 2007 the company developed an entirely mechanical high-speed flywheel based energy

storage and recovery system to meet the 2009 Formula One regulations. This Flybrid®

technology is now being applied to a range of applications outside motorsport and is well suited

to use in road, rail and off highway vehicles.

High-speed flywheel based energy storage systems using Flybrid technology are powerful, small

and light giving a better power to weight ratio than existing automotive hybrid technologies. This

higher power makes it possible to store more energy during short braking periods dramatically

increasing system effectiveness. The systems are also very efficient with up to 70% of braking

energy being returned to the wheels to drive the vehicle back up to speed. The devices are readily

4Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

recycled and relatively inexpensive to make as they can be made entirely from conventional

materials.

Vehicles equipped with this type of hybrid system promise to deliver low CO2 emissions at an

attractive price and this combination of characteristics has already attracted vehicle development

programmes with several major car makers.

3.1.2 Parts of the Flybrid KERS System:

As said before Flybrid systems is a fully mechanical KERS system. It consists of the following

main parts:

1. A High Strength Flywheel: A flywheel is a mechanical device with a significant moment

of inertia used as a storage device for rotational energy. KERS flywheels operate at very

high RPM’s and as a result the hoop’s stress induced onto the flywheel is large. Thus

KERS flywheels are made of a composite material which includes steel and carbon fiber.

A KERS Flywheel used by Flybrid systems

5Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

2. A Continuously Varying Transmission: A continuously variable transmission (CVT) is a

transmission that can change steplessly through an infinite number of effective gear ratios

between maximum and minimum values. This contrasts with other mechanical

transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the

driving shaft to maintain a constant angular velocity over a range of output velocities.

This can provide better fuel economy than other transmissions by enabling the engine to

run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.

Alternatively it can be used to maximize the performance of a vehicle by allowing the

engine to turn at the RPM at which it produces peak power. This is typically higher than

the RPM that achieves peak efficiency.

3.1.3 Working of the Flybrid KERS system:

Flybrid’s mechanical hybrid uses a lightweight, high speed flywheel connected via a

continuously variable transmission (CVT) to an existing powertrain. The CVT comprises a full-

toroidal traction drive, or variator, and an epicyclic gear train. Together, these elements

accommodate the large speed variations between the flywheel and the driveline while permitting

the exchange of mechanical energy in either direction.

Drive comes into the device’s continuously variable transmission which provides a seamlessly

changing ratio between the inputs and the flywheel. Control pistons manage the ratio within the

CVT. It contains a clutch, an epicyclic gearbox and a flywheel. The flywheel spins much faster

than the input drive – it’s a 5:1 ratio. Controlling the position of the levers manages the torque

6Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

transfer within the CVT, and therefore how much energy is stored or released. The energy is

stored in the flywheel which spins at 64,000 rpm.

The toroidal drive works as follows:

The components within each variator include an input disc and an opposing output disc. Each

disc is formed so that the gap created between the discs is ‘doughnut’ shaped; that is, the toroidal

surfaces on each disc form the toroidal cavity.

Two or three rollers are located inside each toroidal cavity and are positioned so that the outer

edge of each roller is in contact with the toroidal surfaces of the input disc and output disc.

As the input disc rotates, power is transferred via the rollers to the output disc, which rotates in

the opposite direction to the input disc.

The angle of the roller determines the ratio of the Variator and therefore a change in the angle of

the roller results in a change in the ratio. So, with the roller at a small radius (near the center) on

the input disc and at a large radius (near the edge) on the output disc the Variator produces a

“low” ratio. Moving the roller across the discs to a large radius at the input disc and

corresponding low radius at the output produces the “high” ratio and provides the full ratio

sweep in a smooth, continuous manner.

The transfer of power through the contacting surfaces of the discs and rollers takes place via a

microscopic film of specially developed long-molecule traction fluid. This fluid separates the

rolling surfaces of the discs and rollers at their contact points.

The input and output discs are clamped together within each variator unit. The traction fluid in

the contact points between the discs and rollers become highly viscous under this clamping

pressure, increasing its ‘stickiness’ and creating an efficient mechanism for transferring power

between the rotating discs and rollers.

1. During breaking the toroidal drive sets the gear ratio to low and drives the flywheel as

shown in the fig below

7Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

2. Then during acceleration the toroidal drive sets the gear ratio to high which enables the

flywheel to discharge to the differential. The duration of discharge can be controlled

either by the driver or can be controlled by the car’s ECU.

3.1.4 CPC-KERS:

The Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System is a patented

mechanical regenerative braking system that fits between the driveshaft and road wheel of a car,

truck or other wheeled vehicle.

8Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

Very simple design – a simple differential gear system distributes torque between engine,

flywheel, and road wheel- easy to manufacture with standard transmission components – much

cheaper than any existing design. It is of similar complexity to existing hybrid power split

devices (without the generators, motors & batteries).

Retrofit on existing designs. Fits into the wheel hub – positioned and looking like a drum brake

Very efficient – differential system transmits nearly 100% of vehicles kinetic energy to flywheel.

Light & safe – flywheel needs only store enough kinetic energy for one acceleration cycle.

3.1.5 BRIEF DESCRIPTION OF THE DRAWINGS:

001 Drive Shaft. Attached to “motor” (typically via gearbox).

002 Ring Gear

9Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

005 Gear face of Ring Gear. Can be two co-axial gears allowing different sized side gears.

(301/302) to allow different distribution of torque, power and angular velocity of Axle shaft and

flywheel.

201 Rotating Differential Case

202 Spider Gear (typically multiple units – single unit shown here for simplicity).

203 Flange (optional)

301 Side Gear (1)

302 Side Gear (2)

401 Axle shaft (attached to “wheel”)

501 Flywheel. Typically energy storage would be enough to accelerate the vehicle to cruising

speed.

502 Flywheel backplate. Locked to or geared to Side Gear (2). Gearing would serve to ensure

that there is adequate stored energy in 501 for a given rotational velocity of the axle shaft 401.

601 Rotation of drive shaft (can be either way). This is where the vehicle’s input energy is put

into the system.

605 Initially this causes rotation of differential case through the crown wheel 005

602-604 Drive shaft rotation is split between output (axle shaft 401) i.e. 604 and flywheel

603, through the flywheel backplate 602 which my contain reduction or more commonly step-

up gearing or may be a direct drive.

701, 702 The same methods may be used to control torque/power distribution between the

flywheel and output (i.e. the ratio of power stored to used for tractive purposes) as is used in

automobile differential units and limited slip differential units. These methods can also be used

to determine how much of the power comes from the flywheel v.s. the main power supply.

10Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

 3.1.6 Working of CPC –KERS:

Ring gear 002 is driven by the “motor” by the pinion wheel. If the side gear 302 locked relative

to the differential case all the torque is transferred through side gear 301 and transmitted to the

“wheel” on shaft 401.

 During braking or descent, torque from 401 can be transmitted through the differential case

gearing system 201 to the large flywheel 501. The vehicle’s kinetic or gravitational potential

energy is thus transferred to the kinetic energy of the flywheel.

 This energy is available to be returned through the differential case gearing system 201 from the

large flywheel 501 to the wheel 401 again using – for example a friction brake to distribute

torque.

 The flywheel may be shape to encompass the rest of the system to allow the system to fit within

a standard road wheel like a drum brake. Alternatively the whole system may be mounted

inboard on the half shafts or main driveshaft or anywhere on the transmission between the

“wheels” and “motor”

 The gear face may be placed on the inside or outside edge of the ring gear to allow the input

shaft 001 to be parallel or co-axial to the output shaft 401 with or without additional gearing.

11Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

CHAPTER 4

ADVANTAGES and BENEFITS:

4.1 General Advantages over conventional drive trains:

•    High power capability

•    Light weight and small size

•    Long system life

At high depths of discharge

Over a wide temperature range

On severe stop start duty cycles

•    Rugged and reliable

Fully supported bearing design resists processional torque

Bearings outside the vacuum can be cooled and lubricated

•    Completely safe

Patented containment technology

No retained charge in the workshop

Safe for emergency service workers after an accident

•    A truly green solution

High efficiency storage and recovery

Low parasitic losses

Low embedded carbon content

12Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

4.2 Advantages Of CPC – KERS over conventional hybrids:

1. Hybrids require exotic and often imported materials for batteries and motors: Resulting

issues include: Environmental impact of hybrid car battery, Raw materials shortage e.g.

Dysprosium. CPC-KETS uses the same materials currently used in the vehicles

differential gear with a steel or steel carbon fibre flywheel.

2. Hybrids are relatively complex, heavy and costly limiting them to high end vehicles.

CPC-KETS systems have a higher power density, lower unit cost (approx. 30% of the

unit cost) and are significantly less complex.

3. Because of the electrical generation, electrical storage and electrical propulsion stages

hybrids have significantly lower efficiency than the CPC-KETS which disposes of these

stages. The CPC-KETS is of similar weight and efficiency to the planetary gear set in the

Prius Power Split Device without the electric motor, generator and battery (but with a

5kg flywheel)

4.3 Vehicle benefits:

1. The primary purpose is to recapture significant amounts of energy normally wasted

during braking etc. (regenerative braking) This is a mechanism that reduces vehicle speed

by converting some of its kinetic or gravitational potential energy into angular

momentum and back. Storage is typically enough for one acceleration to cruising speed.

2. The peak power of the vehicle can be significantly increased using the flywheel and high

efficiency power transfer system, which is not dependent on electric motor sizes or fluid

coupling limits. This results in a smaller gasoline or diesel engine sized more for average

usage rather than peak power usage.

3. The flywheel may be used in stop –start urban traffic so that the main motor does not

have to run in heavy traffic. This depends on flywheel size.

4.4 Manufacturing benefits:

1. Uses existing material and plant

2. Very simple. Development cycle similar to normal transmission components.

13Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

3. Retrofit – can be fitted to existing models to extend model life

4.5 Commercial benefits:

1. Stimulates new vehicle purchase during an economic downturn as fuel costs and cost of

ownership reduced significantly by new CPC-KERS models

2. Gives a decided USP and competitive advantage during recession. “Why buy a thirsty

slow non-regenerative vehicle when the cheaper to run, more powerful CPC-KERS

models available”

3. No new vehicle development needed.

4.6 Environmental benefits:

1. Better fuel mileage. Lower ownership costs.

2. Reduced carbon footprint owing to regeneration & smaller engines (in more powerful

vehicles.

3. Lower emissions especially in urban cycle.

4. Fewer environmentally unfriendly imported exotic materials.

5. No hard to recycle, poisonous waste products from batteries.

14Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

Conclusion:

Almost all modern day vehicle manufactures are experimenting with hybrid technology so as

gain market share in future. KERS technology provides these OEM’s with an alternative to

conventional hybrid technologies. The advantages which KERS provides is extensive and is been

listed above.

KERS is being used in the Federation Internationale de l’Automobile sanctioned Formula 1 race

series. Tests carried out by the major formula teams indicate the following results.

Power storage capacity of KERS = 400kJ.

Output power during discharge is = 60kW (82hp).

KERS systems have been tested to have an overall efficiency in energy conversion rate of up to

90%

But it is to be noted that the formula 1 cars have a restriction on the usage of KERS. This

restriction states that each car is allowed only 6.2 seconds of KERS usage per lap.

This restriction need not be applied to modern day vehicles hence proving that KERS usage can

not only increase the efficiency of the vehicle but also prolong the engine life.

15Department of Mechanical Engineering, SIR MVIT

KINETIC ENERGY RECOVERY SYSTEMS 2011

Chapter 6:

References

1. http://en.wikipedia.org/wiki/Kinetic_Energy_Recovery_Systems

2. http://www.gizmag.com/torotrak-mechanical-kers-system-for-buses/13023/

3. http://www.flybridsystems.com/flywheeltech.html

4. http://www.bhr-technology.com/CPC-KERS.pps

16Department of Mechanical Engineering, SIR MVIT