Hybrid Alternatives Explored

Even as hybrid-electric vehicles expand their market share, researchers are working on other hybrid systems using compressed air or hydraulics.

At the IFP International Conference on Advances in Hybrid Powertrains in Paris, university professors and entrepreneurs explain their ideas for lower-cost ways to improve the fuel economy of internal-combustion engines.

Hydraulic hybrid systems are advanced, but developers of pneumatic hybrids believe their idea is simpler. Researchers at the University of Orleans in France are working with an unnamed auto maker to develop an engine that is hybridized, using compressed air. The concept adds a new valve to the cylinder head.

When braking or coasting downhill, the engine turns without burning fuel. Air is introduced into the cylinder; the piston compresses it and sends the compressed air into a tank. Later, when acceleration is needed, the compressed air is released into the cylinder either to turn the crank directly or to supercharge an injection of fuel.

Researchers have calculated a 15% gain in fuel consumption. Instead of 32.7 mpg (7.2 L/100 km), the average now for the European mixed driving cycle, an engine would achieve 38.6 mpg (6.1 L/100 km). Carbon-dioxide emissions would fall from 173 g/km to 146 g/km.

So far, the researchers have developed only a single-cylinder test engine using gasoline, compressing air to 145 psi (10 bar). Their calculations are based on a supercharged, 1.6L engine.

The university has been working on the idea for several years, expecting it will become a less costly way to improve fuel consumption. The costly part of their proposal is the charge valve that would lead to the compressed air chamber. The valve must move quickly, be totally variable and open against the pressure of the fully charged air tank, which would hold up to 1.8 cu.-ft. (50 L) of air compressed to 290-319 psi (20-22 bar).

Air pressure is used for two purposes in the system: to drive the engine, and thus the vehicle, for a few seconds at launch and to eliminate turbo lag by introducing compressed air directly into the cylinder before the turbo has a chance to spin up.

Meanwhile, hydraulic hybrids are on the road. The U.S. Environmental Protection Agency has used a hydraulic system on a Ford Expedition, United Parcel Service has had it on 20 delivery trucks and Texas has a fleet of garbage trucks.

Ford Motor Co., Eaton Corp., Rexton Bosch and Parker Hannefin Corp. all are developing hydraulic hybrid drivetrains, says Zoran Filipi, director of the Center for Automotive Research in Ann Arbor, MI.

Ford presented the system on its concept F-350 Tonka at the 2002 Detroit auto show, and the auto maker widely has been rumored to have developed a system for the '09 F-150.

Two approaches have been developed. A Hydraulic Launch Assist mainly would gather energy from braking and use it to launch the vehicle when it starts. It would not have much more than several seconds of power.

Enrico Chiappini, of the University of l'Aguila in Italy, says that on the European ECE 15 driving cycle, a hybrid launch assist would improve fuel economy 15%. The ECE 15 cycle is a series of starts and stops, with accelerations to 9, 21 and 31 mph (15, 33 and 50 km/h). Chapin's research is aimed at bringing the benefits of hydraulic launch assist to small cars.

An alternative is a hydraulic series hybrid, in which all power to the wheels would come from hydraulic motors, with an internal-combustion engine powering the hydraulics. In the U.K., the Energy Saving Trust subsidized a project by Artemis Intelligent Power that transformed a BMW 530i into a hydraulic series hybrid that doubled the car's fuel efficiency in city driving and improved overall economy 30%.