How Hydraulic Hybrids Work

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 gas/electric hybrids all they're cracked up to be? There's no question that gas/electric hybrids can use less fuel and pollute less than conventional cars, but they aren't necessarily the silver bullet. For one, their hybrid components (extra batteries and electric motor) add a lot of weight to the car, decreasing efficiency. Their batteries (like all batteries) contain toxic parts that are difficult to dispose of.

Many communities have also been too enthusiastic when embracing hybrids, opening up High Occupancy Vehicle (HOV) lanes to single hybrid drivers. When the hybrids zip through those lanes, they aren't using their electric motors and are emitting just as much pollution as a regular car. You could even argue that the pollution is worse since regular cars in HOV lanes are carpooling -- removing cars from the road -- while hybrid drivers are allowed to use the lanes by themselves.