Automotive

Published on July 23rd, 2013 | by Daniel Sherman Fernandez

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Think Blue. When Energy Is Used To Save Fuel

Energy cannot be created, nor can it be destroyed. It exists in multiple forms, whether in chemical form as fuel, or kinetic form as momentum and speed, or even in the form of heat energy. While we talk about modern engines being more fuel efficient, the reality is that they are still only capable of roughly 35% energy efficiency as most of the energy from the burnt petrol becomes heat and sound energy.

When a car travels at speed, it possesses a certain amount of kinetic energy. The energy is dependent on the weight of the car and the speed of the car, but the fact remains that there is some energy that the car has simply due to its motion. When you apply the brakes to a moving car, that kinetic energy slowly changes into heat energy, as evidenced by your brakes getting hotter (to the point of brake fade on occasion). Remember that energy cannot be created, nor destroyed; it can only change between forms. In this case, it is between kinetic energy (the speed of the car) and heat energy (the temperature of the brakes).

A couple of years ago when hybrid cars started to become more popular, so too did the concept of regenerative braking. The idea was simple and achievable in many different ways, but the gist of it is to convert the kinetic energy lost during braking into something more useable than merely heat energy. Some of the more complex methods are employed in Formula 1 or European GT racing, employing the use of a flywheel that spins up to store this kinetic energy during braking.

But for a road car, a flywheel spinning at ludicrous speed isn’t very practical or safe. Granted, while a certain Swedish brand has started development on a road-going version of the system, the majority of cars that employ regenerative braking simply prefer to convert it into electrical energy, and store it as chemical energy in a battery.

Hybrid cars in particular make use of something called electromagnetic braking. Returning to your high school physics class, the idea is that a ferromagnetic  object (steel, iron, anything that would be attracted to a magnet) spinning in an applied magnetic field will generate a drag force that in turn converts the kinetic energy into electrical energy. The system is only active during braking so as to not slow the car down when it’s cruising along, but the downside to such a system is also reduced braking feel.

Volkswagen chose to go with a much simpler route. A car’s alternator is critical in recharging the battery, by using kinetic energy from the engine and converting it into electrical energy. In doing so, the alternator also creates a minor load (resistance) to the engine. It’s not enough to cause any particular problems, but in the pursuit of efficiency this system can also be refined and improved.

The answer was simple enough. By relying more on battery power during starting up and moving off, the engine would be less affected by alternator load and could therefore be slightly more efficient. On the other end, the alternator load could be increased during braking. In order to assist with the braking of the car as well as charge the battery at a more rapid rate.

With more robust components in place, this method of regenerative braking allows for systems like the automatic start-stop, which kills the engine when at a standstill to reduce fuel consumption but fires it back up when moving off again. Without the improvements for regenerative braking, a regular car battery would likely run out of power quite quickly given the constant starting and stopping.


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