Parallel and Series Hydraulic Hybrids
While all hydraulic hybrids use reservoirs, accumulators and pumps, there are two ways those components can be coupled with a vehicle. The first, a parallel hydraulic hybrid, simply connects the hybrid components to a conventional transmission and driveshaft. (To learn more about transmissions and driveshafts, read How Automatic Transmissions Work and How Manual Transmissions Work). While this allows the system to assist the gasoline engine in acceleration (when the gasoline engine is working its hardest), it doesn't allow the gasoline engine to shut off when the vehicle isn't in motion. That means the vehicle is always burning gas (unlike gas/electric hybrids, which have engines that shut off at slow speeds or during a stop).
Still, the parallel hydraulic system does have significant benefits, including a 40 percent increase in fuel economy [source: EPA]. Though not in widespread use yet, parallel hybrid systems could be added to regular vehicles. As it is now, however, most parallel hydraulic vehicles are built with the system in place and are used in heavy-duty delivery vehicles, not passenger cars.
Series hydraulic hybrid systems use the same process as parallel, but don't use a conventional transmission or driveshaft and transmit power almost directly to the wheels. As we discussed before, reducing the number of components energy has to pass through before hitting its target makes it more efficient. Because the hydraulic system itself is turning the wheels, not a regular transmission or driveshaft, the vehicle's gasoline engine can be shut off, resulting in even more fuel savings. With the added efficiency and the ability to shut off the gasoline engine, series hydraulic hybrids are estimated to improve fuel economy by 60 to 70 percent, and lessen emissions by as much [source: NextEnergy].
Series hydraulic hybrids are already on the road today. In 2005, the Environmental Protection Agency (EPA) announced that it had partnered with UPS and Eaton Corporation - Fluid Power to create a number of series hydraulic hybrid-powered trucks for UPS. The trucks look like regular UPS delivery vans, but have a series hydraulic hybrid propulsion system.
You may wonder why the EPA partnered with UPS and built a delivery van, instead of partnering with a carmaker to create a passenger car. But once you learn about the best uses of hydraulic hybrid, you'll see why the partnership makes sense. Keep reading to learn the best ways to maximize the fuel-saving potential of hydraulic hybrids.