Storing, and Utilising, The Kinetic Energy of a Car
The law of energy conservation states that energy can only be converted into various forms. One area where this principle is used is in storing kinetic energy from cars. The car has a lot of energy in brakes, which is dissipated as heat and sound. Hybrids have a Kinetic Energy Recovery System to recover some of the energy and use it in the car as horsepower.
How Kinetic Energy Recovery Works
The KERS works in two ways. The KERS system uses electromagnets to convert kinetic energy into potential electric energy and finally into chemical energy, which is stored in batteries. The energy can then be sent to the drivetrain to power the motor. In another method, upon braking, energy is used to turn a flywheel that acts as a store of energy. When needed, the flywheel is connected to the car’s wheel where it can offer a power boost when needed. This mechanical implementation is usually more efficient compared to the electric conversion since fewer conversions are needed.
Using Energy Recovery Systems Commercially
Passenger carmakers already have energy recovery systems in place. Volvo is one of them. They built a prototype of the S60 that uses a mechanical implementation of energy recovery. It is lighter and smaller than the components needed for the gas-electric hybrid systems. Tests have shown fuel consumption is similar. Even bikes could one day use the system. MIT engineers have been working on a concept that is cheaper than what it costs to make an electric bike system. The result might be lower costs of power-assisted bikes in the future.
The Regenerative Braking System in the Toyota Prius
The Prius is amongst the most fuel-efficient cars in the world. Since its creation in 1997, the Prius has been an icon in terms of hybrid technology. One of the most efficient technologies in these cars is the regenerative braking system.
The system was first used in trolley cars. It uses an electric generator, which will recover kinetic energy lost when you step on the brakes. Instead of the normal brakes, this system places the engine in reverse, conserving energy whilst creating power. The power is stored in the batteries to run the electronic components.
With regenerative braking, the Prius has set the gold standard in fuel economy. The common models of the Prius come with a 1.8 litre Atkinson cycle four-cylinder engine and two electric motors that produce 121 horsepower. With the use of continuously variable transmission, the Prius can achieve 54 mpg in the city and 50 mpg on highways. With a lightweight design and better aerodynamics, the car achieves 58 mpg in the city and 53 mpg in the highway.
How Motor/Generator Systems Work
For the hybrid car, there has to be a mechanical connection between the Motor/Generator and the drivetrain. In an all-electric car, there could be an M/G at every wheel or just one central M/G connected to the drivetrain via a gearbox. In hybrids, it could also be a pancake M/G between the engine and transmission. There could also be various M/Gs in the transmission. In any setup, the M/G has to propel the vehicle and be driven by the vehicle.
Moving the Vehicle With A Motor/Generator System
Most hybrids have an electronic throttle control system. When the throttle pedal is pushed, a signal goes to the computer, which activates a relay system that sends current via the inverter/converter to the M/G, moving the car.
The more force that is applied on the pedal, the more current flows and the faster the car moves. In hybrids, this could activate the engine as well if there is not enough charge in the battery. When the foot is off the pedal, it will cause current to change the direction, turning the M/G into a generator.
Slowing the Vehicle Via Regenerative Power
This is the essence of regenerative braking. With the throttle closed, and the vehicle in motion, the kinetic energy is captured to slow the vehicle and charge the battery. The computer on board tells the generator to stop receiving power and start sending it.
In short, when the motor generator is supplied with electric power, it creates mechanical motion. When supplied with mechanical power it makes electric power. It slows down the car via magnetic friction. It's armature is slowed due to the force creating electricity in the windings as it moves over opposite poles in the stator. Each time it brakes due to a push or pull of the opposite polarities, some mechanical motion is lost to magnetic friction, which sucks out kinetic energy from the car, reducing it's speed.
The Practicality of Regenerative Braking
The regenerative braking is not enough to stop a car. It brings it to a lower speed, but a mechanical braking system is needed to ensure a complete stop. A technology often used is friction–based braking.
Current technology is not efficient enough for an emergency stop. However, vehicles like the Chevrolet Bolt have made good progress in stopping cars quickly on flat surfaces. This tech is called One Pedal Driving. A physical lock is still needed if the car is stopped on a hill.
Most cars with regenerative technology lack a drive motor on all wheels. The tech only works on wheels with motors. This is not as safe as braking on all wheels. For vehicles at high speed, this tech might not work well for bringing the car to a standstill quickly.
In early applications of the technology, the same position was used to apply power and braking. A switch made the change from motion to braking. This caused issues as some drivers forgot to apply the switch or applied it in the wrong way. Despite this, the technology has come a long way, and it can now be as safe as other braking system.
The energy recovery from braking systems could be an important part of a future free of carbon emissions. As demand for cars is expected to rise in the future, it makes sense that finding solutions to their emissions rather than trying to reduce the number of cars is more practical.