Hydraulic hybrids (like this UPS delivery truck) use energy efficiently and produce less pollution than conventional cars.

Image courtesy of UPS

Types of Hybrids

Gas/electric hybrid cars have become common, with many automakers offering hybrid models or versions of their models with hybrid drivetrains. Gas/electric hybrids use both gasoline engines and electric motors powered by lithium ion batteries to move the car. The batteries get charged though a process called regenerative braking. When a car brakes, two pads squeeze the wheel, which causes friction, slowing the wheels and stopping the car. But that friction is also energy -- energy that's lost when a conventional car brakes.

When a gas/electric hybrid brakes, that friction is captured and used to charge the battery. The battery then powers the electric motor. But the electric motor in most cars isn't very powerful and only moves the car at slow speeds. In most hybrid models, once the car hits 20 to 30 mph, the gas engine takes over and the car operates like any other.

Using an electric motor at slow speeds means gas/electric hybrids cut down on the amount of fuel they use and the emissions they release into the air. However, they aren't the only hybrid solution to the world's energy problems -- hydraulic hybrids can help too.

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. As pressure in the accumulator builds, it acts like a fully charged battery in a gas/electric hybrid, ready to power the electric motor.

But here's where hydraulic hybrids differ even more from gas/electric hybrids: Instead of sending power to the electric motor, which then sends it to the driveshaft (the part of the car which sets the wheels in motion), 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.

Are there different setups for hydraulic hybrids? Read on to find out.