Capstone and an Inventor's Dream
by Harry Hutchinson, Executive Editor

Hydraulic hybrid drivetrains, in which energy is stored as hydraulic pressure to propel a vehicle, have been proposed for automobiles that make frequent starts and stops in their duty cycle—a delivery van, for instance, or a garbage truck. By using an engine to pressurize hydraulic fluid rather than to drive the wheels, and by recapturing energy lost in braking, vehicles can significantly improve their fuel economy.

So far, hydraulic drive systems have been proposed only for trucks. United Parcel Service has tested a hydraulic hybrid system in a delivery van, which is not in service now. According to a spokesperson, UPS has considered more vehicles of the sort, but has no immediate plans to order them.

The van was developed by Eaton Corp., International Truck and Engine Corp., and the U.S. Army National Automotive Center under the auspices of the U.S. Environmental Protection Agency.

Parker Hannifin Corp. is working on a hydraulic drive system of its own that it calls RunWise. Parker has proposed the system for a refuse truck, which is heavy and makes frequent accelerations to travel short distances. An executive at the company said RunWise is still in development. As in most hybrid designs, energy would be recaptured in braking.

An inventor in Virginia has come up with a design that he believes will make hydraulic drives practical for smaller vehicles in all manner of uses, even traveling down the highway. A research team at Virginia Tech plans to develop a working model as a proof of concept.

The inventor, Gary Greenwell, has devised a variable-stroke hydraulic pump/motor similar to a World War I era aircraft engine called the Gnome, in which the crankcase and cylinders rotate with the propeller. Greenwell pointed us to a useful animation and discussion of the Gnome online at www.keveney.com/gnome.html.

Greenwell’s pump/motor uses the same idea of an offset center of rotation. Unlike the Gnome, the cylinders are at the inside of the wheel. They drive pistons that turn the outer rim. His idea is to put a pump/motor on each wheel and attach it to a central accumulator for power. 

A non-working model illustrates Gary Greenwell’s idea for a hydraulic drive. Cylinders (red) drive pistons that turn the outer rim of the structure. The photo at left is a drive position; right is neutral.

An engine or motor would be controlled automatically to increase hydraulic accumulator pressure if it drops too low. When the brakes are applied, the wheel motor would become a pump to pressurize hydraulic fluid.

According to Greenwell, the engine would be in the range of 80 to 90 horsepower. “Maybe a larger engine for the guy who lives in the mountains,” he said. Greenwell expects that, under normal driving conditions, the engine would have to kick in only about 25 percent of the time.

“Many elements of transmission are no longer needed,” Greenwell said. “All you need is an accumulator and a drive motor in the wheel.”

The idea has caught the interest of Al Kornhauser, an associate professor in the mechanical engineering department at Virginia Polytechnic Institute and State University. “It seems suitable for a wheel motor,” he said.

Kornhauser will advise a team of eight students who will create a model of Greenwell’s design as their senior design project.

The model won’t operate at pressure suitable to drive an automobile, Kornhauser said, but will serve as a proof of concept. The model pump/motor will be attached to an electric motor. After the motor has charged the accumulator, the pump/motor will drive the electric motor in reverse as a generator. The students will be able to calculate the efficiency of the system by measuring the electrical output.

In its finished form, the variable stroke hydraulic pump/motor needs a control that changes the stroke in response to an accelerator pedal. The pedal would not control gas to the engine, but rather torque of the wheel motor.

Kornhauser expects that the students’ design won’t address that need, and will require the operator to stop the engine to adjust the stroke by hand.

Greenwell has teamed with Phillip Vera, president of Edge Office Solutions, a business consultant and Xerox reseller in Manassas, Va. Together they have formed a company, RIDE Inc., to market Greenwell’s inventions. The company name is an acronym for “rotational inertial dampening engine,” an engine design by Greenwell that stores energy in a flywheel.

The two-semester senior design project at Virginia Tech includes a requirement that the students build a piece of equipment. Kornhauser said the students will build a working model that RIDE Inc. can use to demonstrate the basic principle of Greenwell’s hydraulic pump/motor.