5 Ways to Power the Green Cars of the Future
Hydraulic hybrid UPS truck. Photo: UPS.
HowStuffWorks, also owned by the Discovery Channel, has a good description of how hydraulic hybrids work:
Hydraulic hybrids use three main components to power a vehicle at slow speeds and to augment the gasoline engine. Fluid is stored in a low-pressure reservoir. A pump moves the fluid from the reservoir to a high-pressure accumulator. The accumulator holds not only the fluid brought over by the pump, but also pressurized nitrogen gas.
These three components work together, but to get things started, they need energy. Like gas/electric hybrids, that energy is gathered through regenerative braking. Kinetic energy from the brakes powers the pump. As the vehicle slows, the pump is activated, and moves fluid from the reservoir to the accumulator. [...]
[...] the accumulator sends its energy (in the form of nitrogen gas) directly to the driveshaft. As that happens, the vehicle accelerates, and the pump moves the fluid back to the reservoir, ready to charge the accumulator again.
The process is very simple and efficient. You know what happens when you shake a bottle of soda: Pressure builds until the energy is released. It's the same thing with a hydraulic hybrid, except instead of releasing the energy by spraying soda everywhere, it channels the energy to the drivetrain (the parts that connect the transmission with the driving axels), making the car go. Bypassing an electric motor helps it stay efficient, too.
These types of hybrids work particularly well with trucks like the UPS one pictured above; lots of low-speed city driving, lots of stopping and starting.
Compressed Air/Pneumatic Hybrids
So called "air cars" and pneumatic hybrids works by storing energy in the form of compressed air, which can then be used to power a vehicle alone or in combination with another source (for example, see this pneumatic-gasoline hybrid system). The main benefit of using compressed air is that there are no tailpipe emissions. That doesn't mean that there are no emissions at all, though. You need energy to compress that air, and this usually comes from the electrical grid (so how clean it is depends on the source). The main problem is getting enough range out of it and safety. The higher the pressure in the compressed air tank, the more you have to be careful about failure (during accident, fire, etc).
Right now pure compressed air cars don't look like they'll be mainstream anytime soon, but maybe pneumatic hybrids like this one could help make internal combustion engines more efficient.
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