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1.1 The Kinetic Energy Recovery System

Another consideration of scientists is the high percentage of CO2 emissions coming from the

area of transportation. The transportation sector is responsible for approximately 21% of CO2

emissions and thus plays a very significant role in Global Warming. Cars are the major polluters

and more specifically in the UK they account for 44% of total transportation CO2 emissions.

Throughout the years a lot of progress has been done in the area of eco-friendly cars. There are

electric cars, hybrid cars or even solar powered cars. The drawbacks of these vehicles repel the

majority of the population to purchase them. They have a high purchase cost and expensive short life

electric batteries. On top of that electric cars can only travel for limited distances due to need for

recharge. Another way to approach the problem is to develop new techniques in order to reduce the

emissions of the widespread internal combustion engine cars.

In order to accelerate, a car burns gasoline or diesel providing motion to the wheels. After

accelerating and cruising at a certain speed the driver decides to stop and thus hits the brakes. All that

kinetic energy the car gained will be wasted and converted into heat by the brakes (friction). One of

the goals of modern vehicle design is the recycling of this wasted energy, in order to increase the fuel

economy of vehicles.

Electric Vehicles and Hybrid Electric Vehicles (HEV) make use of this concept, using

Regenerative Braking. A hybrid car is one in which there are two means of providing motion, i.e. in

HEVs there is a conventional combustion engine and an electric propulsion system. The electric engine

consists of batteries that store energy and are connected to

an electric motor-generator. When the car accelerates the

batteries power the motor which in turn accelerates the car.

When the brakes are hit, the system is reversed and the

motor acts as a generator. The generator will absorb

kinetic energy from the car, the car will decelerate and the

batteries will be charged. The energy efficiency of a

conventional car is roughly 20%, meaning that a lot of

energy is wasted. The advantage of regenerative braking is that it can capture up to half of that energy

wasted and recycle it. This way HEVs are able to increase fuel economy and achieve greater miles per

gallon (mpg) ratings. Typical HEVs use 10%-20% less fuel than conventional cars, but researchers

believe that 70% fuel economy is approachable.

An application of the regenerative braking system is the so called KERS (Kinetic Energy

Recovery System). KERS was introduced during last years F1 championship. The teams were

allowed to develop a system that recovers some of the kinetic energy of the car during braking and

store it, in order to access it later on and get a speed boost for a limited amount of time. This was

Fig 2.6.1: Regenerative Braking in a HEV

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introduced to the sport in order to show that the sport is concerned about Global Warming and that it is

getting greener by trying to improve performance without damaging the environment.

A typical F1 car is responsible for about 50 tons of CO2 per year. Although it is not a big

number itself, the actual CO2 emissions in F1 sport are greater. In addition this would provide more

thrill to the spectators by increasing the chances of overtaking. Since in F1 a huge amount of money is

spent this was considered as an investment to develop a new technology that can be used in our

everyday vehicles. KERS that would fit in cars is currently under research and development.

There are two different kinds of KERS: electrical and mechanical, depending on how the energy is

stored during braking. Electrical systems make use of batteries or supercapacitors and mechanical

systems use a flywheel. In electrical KERS the

battery pack (or supercapacitors) is connected to

the wheels through a motor-generator unit (MGU)

placed on the crankshaft. This acts as a generator

when kinetic energy is recovered, i.e. the kinetic

energy of the wheels will run the generator and

recharge the battery unit. When the KERS button is

pressed the battery provides energy to the MGU

which will now act as a motor and contribute to the

motion of the car, boosting thus its performance.

Current system designs can store up to 400kJ of energy which can be

dissipated during a period of 7 seconds, providing a power increase of

= / = 400/7 = . This corresponds to an extra 80hp available

during that period. If now this system is configured to work in a normal car an

increased fuel economy can be achieved. By using KERS there is more power

available to the car than what the engine can provide. Thus smaller engines can be used with less

horsepower and lower emissions but can be helped by the addition of KERS when the power is

needed.

The recent Toyota Prius model has a 59kW electric motor. When it accelerates from 0 to

30mph, only the electric motor works. Hence this extra power provided by the KERS can play the role

of the electric motor in HEVs. When the equipped with KERS conventional car accelerates KERS can

be activated.

The most widely used cars are those with a 1.6l-1.8l engines and a horsepower in the range of

120-175bhp. Typical cars of that strength have an average of 155g/km CO2 emissions.

= 24,000 155 = .

Electronics-

(Battery or

supercapacitors)

MGU

P: Power (W)

E: Energy stored (J)

t: Time (s)

\

Fig 2.6.2: The KERS in a F1 racing car

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If KERS is installed then the engine can be replaced by a

smaller one which will only produce 105-155bhp which have on

average 140g/km CO2 emissions. The driver can compensate for

the weaker engine by activating the KERS during times that

acceleration is needed (Advanced electronics may be used in

order to distribute the power accordingly).

= , = .

In addition to this, high performance cars are responsible for a significantly higher amount of

emissions than normal cars. Their operation is quite inefficient especially during acceleration.

This is where KERS would be ideal increasing their fuel economy.

Some engineers say that KERS is a very promising solution since every car can benefit

from it while others say that there is no point to improve this technology since there will be a

transition to HEVs where regenerative braking is already being implemented. Time will show but

for sure vehicles will get greener.

The energy efficiency

of a conventional car is

roughly 20%.

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2 REFERENCES

The Kinetic Energy Recovery System

1. Low Carbon News,[Online] Available from:

http://www.lowcarboneconomy.com/community_content/_low_carbon_news/6578 [Accessed 7 December 2009]

2. Act On CO2, 2009. Compare emissions. Available from: http://actonco2.direct.gov.uk/actonco2/home/what-you-can-

do/Compare-CO2-emissions.html [Accessed 7 December 2009]

3. E&T magazine, June 2009.On the Grid for Green article.

4. Wikipedia, Regenerative Braking 2009,[Online] Available form: http://en.wikipedia.org/wiki/Regenerative_brake[Accessed 7

December 2009]

5. Formula1, Kinetic Energy Recovery Systems, 2009 [Online] Available from:

http://www.formula1.com/inside_f1/understanding_the_sport/8763.html[Accessed 7 December 2009]

6. Cnet,F1's KERS system to be used on mainstream vehicles,2009, [Online] Available from:

http://asia.cnet.com/crave/2009/08/11/f1-s-kers-system-to-be-used-on-mainstream-vehicles-/[Accessed 7 December 2009]

7. Wayne,2008.[Online]http://webpages.eng.wayne.edu/~az0131/Quiz%20%231_files/image001.gif[Accessed 19 December

2009]

8. Howstuffworks website,Regenerative braking.[Online] Available from:http://auto.howstuffworks.com/auto-

parts/brakes/brake-types/regenerative-braking5.htm [Accessed 19 December 2009]

9. Racecar Engineering, BMW KERS sinflow.[Online] Available from:http://www.racecar-

engineering.com/imageBank/b/BMWKERSinflow.jpg [Accessed 19 December 2009]

10. The motor report,Ferrari Confirms Hybrid Development,[Online] Available

from:http://www.themotorreport.com.au/22609/ferrari-confirms-hybrid-development-first-model-due-in-2015/ [Accessed 19

December 2009]

11. Regenerative braking ups fuel economy by 70 percent,[Online] Available

from:http://www.lasvegascitylife.com/articles/2009/03/02/ride/shift/iq_27114029.txt [Accessed 19 December 2009]