Jake Brake

Ever wondered why it is that big rigs sometimes make a lawnmower like sound as they slow down? That is the Jacobs Brake, named after its inventor, often called a Jake Brake. It is a system that uses the engine to brake the vehicle instead of using friction at the wheels. This reverses the usual role of the engine, which is to accelerate or maintain constant speed. The Jake Brake works by using the cylinders in the engine to compress air. This compressed air is then vented to the atmosphere. There is no power stroke, and since it takes energy to compress air, this energy is bled out of the kinetic energy of the vehicle. The rotating drive train and wheels are the source of the energy which is used to compress that air. The distinctive sound is the result of venting that compressed air to the outside atmosphere.

Regenerative Braking Video

In this video a Toyota representative explains how the regenerative braking system in a modern Camry sedan works. The brake system uses both electric and mechanical parts. They say that 80% of the braking is done by the recoverable electric system, with the last 20% provided by a standard mechanical pressure based system. Although when he says that the regenerative braking works by converting heat in the brakes to stored energy, I am almost certain he is oversimplifying. Normally regenerative braking systems work by using the rotational kinetic energy stored in the drivetrain to run a generator, which then charges the battery.

Regenerative Braking

Motion requires energy. To overcome inertia and add speed to a body, energy must be added. The energy a body has by virtue of its motion is called kinetic energy. The formula giving the amount of kinetic energy K that a body of mass m will have at a speed v is K = ½mv². Note the occurrence of the square of the speed. That means doubling the speed does not double the energy needed but rather quadruples it. Now energy cannot be created or destroyed. So to get your car up to speed the required kinetic energy has to come from somewhere. In a standard internal combustion engine driven vehicle, this energy comes from the chemical energy that was stored in the fuel, in the gasoline or diesel.

When you want to slow down or stop the kinetic energy will be lower at the new lower speed. Since it cannot just be destroyed, this energy must go somewhere. In a normal vehicle it is converted to frictional heat in the brakes. This heat then escapes and the energy is lost forever. Every time you slow down, you are throwing away all of the chemical energy that came from the fuel. Your brakes slow you down and in the process turn gasoline into heat.

What if it were possible capture the kinetic energy that is lost as you slow down and store it for reuse later? Obviously it cannot be converted back into chemical energy in the fuel. Internal combustion engines and standard brakes have no way of capturing the kinetic energy and it must be lost as heat. This is the reason that the Golden Rule of Hypermiling is "Maintain Momentum". Every time you slow down and speed up again, you have turned some fuel into heat.

Electric engines, such as used in hybrid vehicles or NEVs can capture some energy while slowing down. This is because electric engines run off of batteries, which can both provide and store electric energy. Compare that to liquid fuel, which can put energy out but it is impossible to pump energy into. Also an electric engine run in reverse is in fact an electric generator. Depending on which direction you run it, an electric engine can draw stored energy out of a battery to provide work or it can accept outside work, generate electricity and store it in the battery. Regenerative braking basically switches the electric engine to run in reverse, in generator mode. It makes the spinning wheels and drivetrain do work on it. As the drivetrain torques against the electric engine, it slows down, thus slowing your vehicle down. The drivetrain torquing the engine allows it to generate electricity, which is then stored in the battery.

Although regenerative braking is not capable of capturing all of the kinetic energy due to inevitable losses, any percentage it does recover is available to accelerate you back up to speed without needing to put in fresh energy. Unlike normal internal combustion engines and standard braking systems, not all of your motion energy is lost when you slow. And that means you save on energy.

Update your Driving Habits

I am afraid that high and higher gas prices might be here to stay. The big gas price spike last year is past (thank goodness). Right now we still have $2.50 gas all around us, which is not like it used to be. So maybe it is time to think about our driving habits. We learned to drive in a time when gas prices were low enough that you really didn't think about it much. Now we have to relearn some basic habits so we can save on gas while driving.

How you brake can make a big difference in saving or wasting gas. Cars use more gas speeding up (accelerating) then when cruising at constant speed. When you are cruising steadily, the engine only has to burn enough gasoline to combat friction. But when you speed up, the engine has to both combat friction and generate enough extra power to up the speed. So to get better gas mileage, you have to keep the car going at a smooth, steady pace.

A lot of drivers alternately mash the accelerator pedal and the brake pedal. In a line of traffic, they move up to the car ahead, then brake and fall back. In reality, this is not necessary. Most of the time you can keep your position using only the accelerator. Just open up a little more space between you and the car ahead. Then keep watch not only the car right in front of you, but the action happening up front several cars away. Now you can see slowdowns coming, and instead of hitting the brake, just ease up a bit on the gas. I bet you can get 10% better gas mileage this way.

Try it and leave us a comment here on Save on gas telling how it works out for you!