Recovering energy from the drive

-May 17, 2013

As conventional engine cars strive for better gas mileage and performance in the wake of hybrid and full EV vehicles entering the market, the auto industry is looking at both old and new ideas for areas of improvement. One method starting to make inroads to "everyday" vehicles, rather than just high-end vehicles, is the use of regenerative braking systems.

The laws of physics dictate that the energy that is stored in the car while in motion must go somewhere and become something when the car brakes and slows down. The long running solution was that this energy was just thrown away in the form of heat from friction in the mechanical components of the brake mechanism itself. Today, many vehicles are now reclaiming this kinetic energy, in the form of electrical energy for use in the car to improve range, and MPGs.

The obvious first place to see these systems are in hybrid and full Electric Vehicles (EV). In these systems, the battery that is used to store the "operating charge" energy for the vehicle is incrementally recharged with this braking system. The regenerative brakes have been used in trolleys since the early 1930s and is not a new idea. These electric vehicles, slow down by having the the electric motor controlling the wheels spin backwards, reducing the speed of the vehicle and acting as a generator that produces electricity. This electricity is stored in batteries to operate other electrical functions in the vehicle.

In hybrid and full EVs, these systems work the same way. The amount of energy returned is dependent on the speed and other load factors of the vehicle when it needs to slow down. In most cases, for hard fast stops, the system is supplemented with traditional friction based braking systems as a backup. In these vehicles the primary motor is the main electrical drive for the wheels, so individual response from each drive wheel is possible in this system. Care must be taken to not "spot" overload the local electrical grid in the vehicle due to the high level of current generated from the wheel. A typical system is shown in the first figure.

This configuration shows a conventional engine adjacent to the regeneration electrical motor that feeds the storage system. In most vehicles, there are only one to two wheels that are used for this regeneration function, as there are only one to two wheels driven by the internal combustion engine. These systems have been in vehicles like the Toyota Prius, and Honda hybrids since the early 2000s.

New systems require new control electronics and are appearing in many vehicle without dominant battery powered drive. These systems have a new engine design, such as the one from Mazda, that incorporate directly into the engine and integrated alternator and a new element to store the power (see illustration below, courtesy of Mazda). The new element is a supercap that can be retrofitted for placement in most existing model year vehicles. As there are no large batteries in the system, the cost and weight reduction impact on the vehicle while the miles per gallon goes up.

The cars use high-speed switching regulators to be able to steer the energy to be stored in a high capacity supercap. This supercap is smaller, lighter and easier to keep on board than a large battery bank. The use of this new technology will allow conventional car engines to achieve more MPGs due to the addition of the brakes' interface to the electronics, and it should become a universal support structure like antilock brakes and air bags.

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