A Regenerative Braking Mechanism Using Kers

A Regenerative Braking System Using ‘KERS’ is a device used to recover otherwise wasted energy.

Abstract

Kinetic Energy Recovery System, or simply ‘KERS,’ is a system developed to recover a modest amount of energy during braking of an automobile or locomotive. This system harvests energy that would otherwise be wasted. KERS works on the principle of Regenerative Braking, which uses the braking energy to rotate a flywheel connected to the differential (in an automobile) through a gear mechanism. When actuated while braking, it uses the energy to spin the flywheel at more than 60,000 rpm.

There are various methods of achieving this. The most commonly used systems are Electrical KERS and Mechanical KERS. Electrical KERS uses a generator and a battery setup to store the energy while braking, whereas a Mechanical KERS system uses a flywheel to accomplish the same. Both systems are discussed in full length in this paper.

Either way, this stored energy is then utilized by the driver to achieve a ‘Boost’ or reduce their original energy demand. KERS is effectively applied in Formula 1, 24 Hours of Le Mans, and other prestigious races because of the energy boost it offers (around 60 kW in F1).

Kinetic Energy Recovery Systems (KERS) is a technology that aims to recover energy wasted during braking. Several companies, such as Volvo and Mercedes Benz, have implemented this technology to increase efficiency and decrease fuel consumption in their vehicles. Hybrid cars commonly use KERS, and it has also been successfully applied in tram cars and rail locomotives throughout Europe. Additionally, the Copenhagen Wheel bicycle utilizes KERS to reduce rider fatigue.

Salvaging every bit of wasted energy is a major step forward in the race to increase efficiency and reduce emissions for a better tomorrow.

Braking is used to slow down a vehicle or machine by absorbing kinetic energy and reducing motion. This kinetic energy is converted into other forms of energy, mainly heat, which is then dissipated. Kinetic Energy Recovery System (KERS) aims to reduce the generation of heat while maintaining the same level of retardation, and convert the kinetic energy into a more useful form. As we all know, work is a higher level of energy than heat, making it easier to recover and utilize. Physicist Richard Feynman first proposed this method in the 1950s; however, it took many years of development and testing to achieve an efficient practical application.

Early systems incorporated heavy flywheels and other equipment, which made them very inefficient and defeated their purpose. However, newer and more advanced versions have addressed this drawback by using composite materials and other technologies.

III. Why do we need KERS? No actual heat engine can be 100% efficient. When we use a naturally perishable matter to power our engines, every small drop of fuel counts. With ever-escalating fuel prices, it becomes economically feasible to improve the efficiency of our vehicles and machines.

We can only make an internal combustion engine efficient to a certain extent because it has various methods of losing heat, such as through the exhaust gases and cylinder walls. Unfortunately, these losses cannot be helped. However, one way to recover some of this wasted energy is through KERS which operates on the principle of Regenerative Braking. Depending on the application, every watt of power saved through this method counts as a watt used by the vehicle, resulting in fuel savings that would have been burned to generate that watt. Therefore, the system is slightly more efficient (see Fig. 1 IV).

There are several types of KERS, but two common methods are widely used. These are the two technologies applied in the most common KERS application:

1. Mechanical KERS
2. Electrical KERS
3. Pneumatic KERS

Mechanical KERS:

This type of system uses a flywheel to store energy. It has an axle coupling gear that meshes with a continuously variable transmission (CVT) to transmit power from the differential and absorb it in the flywheel.

This wheel absorbs kinetic energy from the differential through a mechanism, which retards the vehicle. This is an example of a flywheel KERS layout, displaying its basic components and their assembly.

Advantages:

• Specific power of flywheel is much higher than that of a battery.
• The setup cost is relatively less.
• The flywheel almost entirely returns power with high efficiency.
• Simpler and robust construction provides a long life almost equal to the vehicle’s life.

Disadvantages:

• Friction between bearings and gears may cause some decrease in efficiency. It is vulnerable to contaminants and corrosion.
• Bearings and seals may wear out over time.

B. Electrical KERS – A more technologically advanced method to implement KERS, it has three main components: Motor (or) Generator, batteries, and an electronic control unit (ECU). The motor or generator couples to the live axle using an electric clutch mechanism actuated by the ECU. While braking, kinetic energy gets absorbed by the generator which converts it into electric impulse; in this part of the cycle, motor/generator acts like a generator. This energy stores in batteries.

Li-ion batteries are the most preferred type for powering electric vehicles. When the driver commands it, the ECU directs power to be sent back to the differential, and the motor uses stored electric energy from the battery pack to give an additional power spike to the vehicle. The process is explained in a simple schematic diagram below.

Advantages:

• Capable of storing more energy

Disadvantages:

• This system is more complex and expensive
• A lot of heat is generated by batteries and other electric equipment, requiring additional cooling equipment
• The specific power of batteries is low, and they require a minimum charge-up time.

Pneumatic KERS systems store energy as compressed or pressurized air. During deceleration, the engine can act as a compressor and store the compressed air in a tank. This is known as compressor mode. During acceleration, the engine can use this stored air to run as an Air-Engine. This method is simpler and more feasible than previous methods, making it the cheapest option.

Advantages:

• Simpler construction
• Storing compressed air or gas requires only tanks
• Conventional engines can easily be modified to suit this system
• Cheaper to install and maintain
• Very feasible for heavy-duty applications

Fig. 2 Advantages:

• The construction is safe, without any moving parts for storage of energy.
• The efficiency is also higher.
• The system is more accurate and flexible in meeting demands of the driver.

Disadvantages:

• Storing a large quantity of compressed air poses its own risk.


• The power boost and supply are not as effective as previous systems.

It is worth noting that other car companies, such as Volvo, Mercedes Benz, and Toyota, have taken a keen interest in this technology and are trying to incorporate it into their road cars. Volkswagen AG has also attempted to integrate KERS into their BlueMotion series of diesel cars in Europe. Currently, these engines are being tested to solve all the initial issues. Although new to conventional IC engine cars, KERS is not unfamiliar with hybrid and alternate fuel cars worldwide as they go hand-in-hand with the motive of a hybrid car.

KERS has been applied to various tram cars and locomotives in Europe, including the Skoda Astra tram and some Bombardier Locomotives. These vehicles use a technology similar to electric KERS, but they don’t always store the electric energy. Instead, they give it back to the main supply. One innovative application of KERS is the Copenhagen Wheel, a bicycle developed by students from M.I.T in Boston, MA. The Copenhagen Wheel utilizes a mechanical KERS system with a flywheel that is fitted and aligned with the sprocket on the rear wheel.

A small continuously variable transmission (CVT) is employed to make or break the connection between the flywheel and the wheel. When braking, the CVT engages the flywheel to absorb kinetic energy from the wheel. When not braking, this flywheel gives its energy directly to the wheel and is eventually disconnected by the CVT. This cycle is designed in such a way that it only takes energy while braking and does not spin under load. Additionally, the flywheel was made of cheap composite materials to keep weight and cost low.

The results of this experiment have been astonishing. The cycle was efficient and could reduce rider fatigue by 20%, making it easier to cycle for long distances.

One small disadvantage of using a rotating flywheel beside the wheel is that it can add a minute gyroscopic couple while turning. However, the effect of this is negligible (see Fig. 3).

KERS has been widely encouraged in motor sports, particularly by FIA which conducts the Formula One races every season. Formula One has received criticism from environmental protection agencies for being ignorant about fuel usage and greenhouse gas emissions. In a single race, an average F1 car burns around 500 liters of fuel. To combat this image, FIA president Max Mosley took an initiative to use ethanol and smaller engines among other steps. The development and application of KERS was a major step taken in this regard.

KERS was first developed to achieve more speed that could be used to boost performance. Almost all F1 teams have employed electrical KERS as they are more feasible and weigh only 25 Kg. KERS gives an average F1 car about an 80 hp boost, making it a significant advantage on the track.

Mclaren became the first team to win a championship using KERS.

Audi’s Motorsport division has attempted to implement KERS technology in its multiple championship-winning R8 for the 24 Hours of Le Mans race. This allowed Audi to race the car for a longer duration without stopping for refueling as frequently as their competitors. A typical F1 steering wheel with a KERS activation button is shown in Fig. 5, while an exploded view of wheel components of the Copenhagen Wheel” is shown in Fig. 4.

Design and packaging of KERS units are surprisingly compact and space-efficient despite performing complex functions.

Crediting its size to advanced electronics and smart materials, even newer and lighter mechanical components, a typical KERS unit may weigh from 40 to 50 kg, even for a heavy application.

VI. How and when to use it? – With an automated system, there always exists a risk of an unwanted happening or an accident, especially with a device that gives back power to the driving wheels more than what the engine would normally deliver. This can shock a driver and make them lose control of the vehicle. To counter this fact, most KERS are activated by the driver when and how they wish to use the extra power. In Formula One as mentioned earlier, energy is stored as electrical charge in a battery which can be activated by pressing a button on the steering wheel. However, some mechanical KERS systems use an intelligent system called KERS Control Unit (KCU) which analyzes various parameters such as throttle position and brake position to automatically control the boost.

Fig. 6 A KERS unit of a city bus manufactured by Volvo.

VIII. Limitations of KERS – Although KERS is an innovative new technology it is not free from flaws and requires further development to answer many of its currently faced problems.

Many engineers from Formula One and other motorsports believe that Kinetic Energy Recovery Systems (KERS) have a long way to go before they can be easily and effectively employed in everyday road cars. This is mainly due to the high cost of development and other parameters. On average, the KERS unit used in F1 cars costs upwards of $100,000 and weighs only 25kg. If electrical KERS were to be employed, the battery systems would require more storage capacity. Otherwise, increasing the weight of the car excessively would have an adverse impact on fuel consumption, defeating the purpose of installing such a system.

The specific power-to-weight ratio of these units must improve drastically. While carbon fiber does the job, it is a very expensive material and not accessible to everybody. Many Formula 1 drivers have complained about ineffective and spongy braking in their cars fitted with KERS. As a result, many teams decided not to use KERS from 2011 as their drivers crashed out or reported damage to the car.

References:

• 2012 FIA Rules and Regulations
• Volkswagen AG, Germany
• BMW F1 Team (Fig. 5)
• MIT Journal – “The Copenhagen Wheel” (Fig. 4)
• Volvo Trucks, Sweden (Fig. 6)
• Autocar Magazine Online Edition (Fig. 3) – All photographs are property of their respective owners.

In conclusion, as mechanical engineers we must strive to make our machines as efficient as possible by effectively employing new ideas and innovations.

KERS is an idea that should be taken seriously. It utilizes wasted work, which if converted to heat cannot be utilized as effectively. KERS coupled with DRS(1)* is used in many races to achieve extra performance. Car companies like Volvo have managed to achieve better efficiency by using KERS in their road cars. Although not free from its flaws, it can prove to be an innovative technology that can improve our machines. From rough estimates, if a power of up to 60kW can be salvaged from a road-going car, it can lead to fuel savings of around 35%.

1)*DRS is a drag reduction system that employs a spoiler or wing to reduce air drag and boost aerodynamic performance.

References:

1. http://www.flybridsystems.com/Technology.html
2. Top Gear Magazine, August 2010
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5. ASME Online Journal January 2012, “Stopping Power”
6. http://www.formula1.com/inside_f1/rules_an