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Electric cars are becoming more and more popular, so today we're going to talk about regenerative braking systems for electric cars--and of course there are other ways you can stop a car.

Why are brake lights installed on the rear of the car? They are designed so that when a car brakes, the driver behind it can detect it and slow down in time. But electric cars can brake in a way that doesn't light up the brake lights! I didn't know about this method until I saw a video on the Internet about the "kinetic energy recovery" driving mode of electric vehicles. This mode allows the driver to control the speed of the vehicle by relying solely on the accelerator pedal. When the pressure on the accelerator pedal decreases, the electric motor of the electric vehicle switches to regenerative braking mode and charges the battery. This means that the car is slowing down, but the brake lights are not on.

This article explains everything you need to know about regenerative braking and discusses the various ways you can stop a car and their energy conversion.

Force, Energy and Motion Suppose a ship sails through space without air, without gravity and, of course, without friction. If the spacecraft starts its engines, it will accelerate. But what happens when the engines shut down and there are no external forces acting on the ship? You might say that the ship will slow down gradually. It won't, however. The ship will move in a straight line at a constant speed.

This is a direct consequence of Newton's second law. Newton's second law states that the resultant force acting on a body is equal to its mass multiplied by its acceleration. The acceleration must be zero if the resultant force is zero. Acceleration tells us the rate of change of velocity-so zero acceleration means no change in velocity.

How does the rocket stop? Stop means speed changes from a certain value to zero. Yes, which means the rocket has to accelerate. The term acceleration does not mean simply "increasing speed," but rather changing speed, either from higher to lower speeds or down to zero speed. For the rocket here, you need a force to generate acceleration, and that force has to push the ship in the opposite direction to the velocity. This is how you slow something down: it takes a push backwards.

source| RHETT ALLAIN Let us now consider energy. If that rocket is moving through the universe, it has energy because of its motion. We call this energy kinetic energy, which depends on the speed and mass of the rocket. As the rocket slows down, the decrease in its speed also results in a decrease in kinetic energy. But energy doesn't disappear. If the ship's energy decreases, then something else must increase its energy. In this example, if the spacecraft is slowed by firing rocket engines, the velocity of the gases ejected by the engines will increase. This means an increase in the kinetic energy of the gas. Conservation of energy means that the total energy before and after a physical process (e.g. gas ejected from a rocket) occurs is equal.

Now we can use this knowledge of physics to understand the methods commonly used to slow cars on the ground.

For an external force to stop a car, some form of force must be applied to the car in the opposite direction of motion, and this applies to all the braking methods we will discuss. In some cases, this backward force comes from the car itself, but it doesn't have to be. Have you ever seen barrels lined up on a highway? They're called crash barrels. These buckets are actually buckets filled with water or sand so that cars can slow down by colliding with these buckets. (Note: Do not use force to slow down, unless you really have no choice!)

These barrels provide the backward force that slows the car, but they do so in a clever way. Buckets filled with water or sand are soft and do not exert a tremendous force on cars like tree trunks or concrete barricades. With less force from the barrel, the car takes longer to slow down, and the occupants are safer. But as the kinetic energy of the car goes down, there must be some other form of energy going up, right?

If you've ever seen a video of a car hitting one of these buffer buckets, you'll notice sand or water being thrown into the air. Yes, that's where the kinetic energy of the car goes.

Wheel Brakes and Friction We all know that the correct way to stop is to apply the brakes. But how exactly does this stop the car? The answer is friction. We can divide friction into two categories. The first is static friction, which occurs when two surfaces are stationary relative to each other. The other is sliding friction, which occurs when two surfaces slide relative to each other.

Let's consider a car whose tires slide on the ground to stop (this is not recommended). In this case, we can plot the following forces:

source| RHETT ALLAIN Sliding friction pushes the car in the opposite direction to its speed and slows it down. But when the car stops, where does the kinetic energy go?

The following animation shows a car with locked rear wheels sliding and slowing down on the ground until it stops. This is an infrared camera shot, so the brighter (more orange) parts of the image represent hotter objects.

source| RHETT ALLAIN Note that a tire is slipping, leaving a hot bright track on the ground, and the tire is getting hot. This is exactly where kinetic energy goes: resulting in an increase in thermal energy.

But what if, like a normal driver, you stop instead of locking the brakes? Because the tire does not slip, what actually works is static friction. It turns out that if the interaction is static friction rather than dynamic friction, greater frictional forces can be obtained between the two surfaces. Therefore, almost every car has an anti-lock braking system (ABS) to prevent wheel slip when braking, so as to achieve better braking distance.

In both cases, there is another thing to consider: if the car stops because of friction between the wheels and the road, what makes the wheels stop first? This is what brakes are designed for. Most cars have a disc (called a brake rotor) attached to the wheel. For each rotor, there are two brake pads that block rotation of the rotor to slow it down. Yeah, that's friction again. Below is an infrared image of the wheel stopped:

source| RHETT ALLAIN's brighter (more orange) rotor indicates it's really hot. So when a car stops, a decrease in kinetic energy means an increase in thermal energy from the ground, tires, and rotors. In fact, under extreme braking conditions, such as when a Boeing 747 comes to a stop with only the brake pads, the brake rotors can become so hot that they glow red.

Air drag What happens if you drive on a level road at a constant speed and then turn off the engine? Unlike rockets in the universe, your car obviously doesn't go on forever; it will eventually slow down and stop.

But doesn't slowing down require a force on an object that opposes its velocity? Yes, it is. In this example, the force in the opposite direction is air resistance. As the car moves, there are lots of tiny collisions between the car and molecules in the air. These collisions give the car a push to slow down (in a moving car, you know air resistance exists when you stick your hand out the window and feel the air pushing your hand backward).

Modern cars have shapes designed to reduce air drag and improve fuel efficiency. However, if you do want to slow a moving car down with air, it is possible to increase air drag significantly. You just need to make your car have a larger windward area. Racing cars use this method when they deploy drag parachutes (small parachutes that extend from the back of the car). (This isn't a practical way to park, since it can only be used once, after which you have to repackage the umbrella, but it's still important.)

Where did the energy go? When cars interact with air, the air is pushed so molecules move faster and temperatures rise. This energy change occurs over a large area of air, and it is difficult to measure this energy, but this is where the kinetic energy of the car goes.

Gravity. You can actually stop a car with gravity. You may have seen this method on cart bumpers on hilly roads. These bumpers are the branches of the road toward the hill. If a vehicle-usually a heavy semitrailer truck-fails to brake, it can run up a bumper slope. Yes, there's a backward thrust, and that force is gravity. The schematic diagram is as follows:

source| RHETT ALLAIN Because the car is going up the slope and gravity is going down vertically, gravity has a component that slows the car down in the opposite direction of speed. As the car moves up an incline, gravitational potential increases. The higher the car goes, the greater the gravitational potential energy.

Of course, the reverse is the same. If you move an object down an inclined plane, the gravitational potential energy decreases resulting in an increase in kinetic energy. So you still need brakes or some type of friction to stop the car from eventually sliding backwards. Most of these bumpers are constructed of soft gravel to create high friction so that trucks can stay stationary on them.

Downshifting cars into manual gear is not as common as automatic gear, but they still exist. In a manual car, the driver must manually shift into gear to accelerate, and the same process can be used to decelerate.

Suppose the car is traveling in fourth gear at 70 kilometers per hour. If the driver shifts down to third gear and releases the accelerator pedal, the car will slow down. The driver does not need to touch the brake pedal, which means the car brake lights do not light up, even if the car is slowing down. Of course, if you need to stop for a short distance, this downshift method is not enough, and the driver has to use traditional brakes.

How does downshifting achieve deceleration? Only a superficial description of the internal combustion engine will be given here, but this is enough to understand downshifting. The engine provides power by adding gasoline to the compression space inside the cylinder. When the fuel is ignited, the gas expands and pushes the piston. The piston moves up and down to rotate the crankshaft, which rotates the wheel after passing through a plurality of transmission parts. So your car moved. To make it all work, you need gasoline, a spark to ignite the gasoline, and compression of the gas.

What happens if you remove the spark and the fuel? If the wheel and engine are coupled through the transmission, the gas in the cylinder will still be compressed. The process of compressing the gas adds resistance to the rotation of the engine and can be used to slow down the car. (Of course, you still need friction between the tires and the road.)

From an energy point of view, we still need some increase in energy for a decrease in kinetic energy. Not surprisingly, heat increases. As the gas is compressed, it gets hotter, and this heat comes from the kinetic energy of the car.

Regenerative braking What if there was a way to slow a car down, reduce its kinetic energy, and store that energy? Regenerative braking is exactly that.

It all starts with the electric motor, which is essentially just a coil mounted on a rotating shaft near a magnet. When current flows through the coil, there is an interaction between the current and the magnet, which causes the coil to rotate on the shaft. It can also work in reverse. If you move a wire in a magnetic field, an electric current will flow in the wire. This means that the motor and generator are the same machine. As an electric motor, you give it an electric current and it moves objects. As a generator, you rotate the shaft and you get an electric current.

This means that if you have an electric motor in your car, it will act as a generator to charge the car battery. When the car slows down, kinetic energy is converted into electrical energy and stored in batteries. Of course, at least some of the energy is stored-there is always some loss, because it is not a 100% efficient process. Objects are always heated, at least to a small extent.

So what about brake lights and kinetic energy recovery? Whether electric or gasoline-powered, brake lights light up whenever the driver presses the brake pedal. But now we see electric car drivers slowing down simply by releasing the accelerator pedal-no brake pedal required. In this case, the on-board computer switches between the drive mode and the generator mode of the motor--and it's the same computer that decides whether or not to turn on the brake lights. The brake lights may not light up at this time.

(We know that computers may one day take over the world.) They start with the brake lights. Weak human beings can only accept the possibility that we can no longer make any decisions.)

It's legal. Current U.S. motor vehicle safety standards state: "The brake lights on each vehicle shall be activated during service braking. The high-mounted brake lights on each vehicle are activated only when the service brakes are depressed. "(China has similar provisions)

Should this provision be amended? Assuming that I can decide-and I certainly can't-I will make a rule that the brake lights should come on when the deceleration acceleration of an electric vehicle is greater than a certain value, such as 1m/s. The driver then signals to the rear car,"I'm going to stop, so maybe you should do the same. "Isn't that the whole reason brake lights exist?

Written by Rhett Allain

Translation: There are benefits.

Reviser: Xiao Cong

All the Ways to Slow a car

This article comes from Weixin Official Accounts: Institute of Physics, Chinese Academy of Sciences (ID: cas-iop), Author: RHETT ALLAIN

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