Ask someone to explain a hybrid, and odds are the answer will contain the terms “electric motor” and “battery.” But supplementing an internal-combustion engine with electric power is not the only way to boost fuel economy, and battery-powered hybrids are not without problems. Those batteries are heavy—the upcoming Chevy Volt’s 16-kilowatt-hour pack weighs 400 pounds—are very expensive, and can only use about half of their stored energy day to day without severely limiting their life spans.
A fresh alternative comes from, of all places, the government and improves fuel mileage by using high-pressure hydraulic fluid to capture and reuse energy. As with most new technologies, it’s not all that new. The Environmental Protection Agency (EPA) has been experimenting with hydraulic hybrids since the late 1980s and currently has a contingent of about 20 engineers here in Ann Arbor, Michigan, working on the technology. Charles Gray Jr., the intense yet soft-spoken director of the EPA’s Advanced Technology Division, believes it to be a better payoff than alternatives such as homogeneous charge compression ignition (HCCI) engines.
Ford was interested, too, showing a hydraulic parallel-hybrid powertrain in the F-350 Tonka concept at the 2002 Detroit auto show and nudging the technology toward the showroom. Then, cash became tight, and the program was abandoned.
The EPA’s latest hydraulic series-hybrid design was developed in partnership with Eaton, Navistar, and UPS with the specific goal of improving the fuel efficiency of UPS’s brown delivery trucks.
Here’s how it works: The 170-hp diesel engine is coupled to a hydraulic pump instead of a transmission. Therefore, the engine is no longer propelling the truck but is rather pressurizing hydraulic fluid, which is stored in an oblong accumulator—imagine an oversized, six-foot-long scuba tank made from carbon fiber. Inside the accumulator is a bladder filled with nitrogen; as it’s compressed by pressurized hydraulic fluid, energy is stored. At its maximum pressure of 5000 psi, the accumulator holds the equivalent of 0.6 kWh of energy. Unlike a battery, the accumulator can deliver all of this energy without compromising its longevity. To motivate the truck’s driving wheels, the high-pressure fluid powers two hydraulic motors, mounted end to end and integrated with the rear axle. The hydraulic motors are essentially hydraulic pumps run backward to generate torque when supplied with pressurized fluid. Under braking, however, the motors revert to their pump mode to both slow the truck and pressurize fluid, thereby capturing energy.
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