The hybrid drivetrain has been the great success story of the high-mileage movement. Cars like the Prius get up to twice the mileage of the industry average partially by recovering energy that's lost as heat during braking, storing that energy in a battery, and reusing it later on. Yet today's most efficient hybrids can recover only 30 percent of that energy for reuse.
One radical solution under development at the Environmental Protection Agency's Advanced Technology Division is to get rid of the heavy batteries altogether. The EPA has built a modified hybrid that uses a hydraulic system, not a battery, to store braking energy. When you press the brakes, the wheels drive a pump that compresses nitrogen gas, which is inexpensive and inert. When you accelerate again, that compressed gas runs the pump in reverse to help power the vehicle [see illustration, facing page].
The hydraulic-hybrid system, scheduled to begin testing in two UPS trucks this month, with another to follow next year, promises to return at least 70 percent of the braking energy back to the wheels, which would lead to a 60 to 70 percent jump in fuel economy and a 40 percent reduction in emissions. Perhaps that's why Charles Gray, the director of the Advanced Technology Division and one of the developers of the hydraulic hybrid, can't contain his excitement about its potential. "This is going to be the biggest revolution in automotive history," he declares. "Bigger than the assembly line."
That's yet to be seen, of course, but the hydraulic hybrid is also smaller and cheaper than conventional hybrids. "I can hold a 500-horsepower hydraulic pump motor in my hand, and I'm not a big guy," Gray says. Because the technology would eliminate the need for a transmission--the engine merely pressurizes the hydraulic system, while the hydraulic motors power the wheels--and several other parts, it could be installed in a small car for almost no additional cost. Ford, the U.S. Army and others are investigating the technology, yet UPS--with its fleet of vehicles that constantly suffer through stop-and-go driving--is its only committed customer so far.
Another potential player in the race to 100 mpg is the StarRotor, which began life as an air conditioner at Texas A&M University. Chemical-engineering professor Mark Holtzapple and his colleague Andrew Rabroker were attempting to build a better compressor for an air conditioner when they hit on the idea that became the StarRotor engine's basic architecture. Once they made the connection to car engines, "we quickly forgot about air conditioners," Rabroker says. They have since formed a business (also called StarRotor) to commercialize the technology.
The StarRotor uses the same thermodynamic process as jet engines to recuperate some of the heat normally lost to exhaust, something that the design of a piston engine doesn't allow. The exhaust heat warms the air that comes into the engine before the fuel is added [see illustration, above]. This hot air leads to more powerful combustion, which means the StarRotor can extract more energy from a given amount of fuel than a conventional engine could.