Air hybrid cars could bring big fuel savings for city drivers, according to a recent study released by UCLA engineers. Experiments based on modeling and simulations showed that the air hybrid engine improved fuel efficiency by 64 percent in city driving and 12 percent in highway driving. The study also suggested that by adopting the air hybrid approach, car-makers could avoid some of the manufacturing costs associated with the more common electric hybrid design.

The air hybrid engine design concept.

Tsu-Chin Tsao, professor of mechanical and aerospace engineering at the UCLA Henry Samueli School of Engineering and Applied Science, and graduate student Chun Tai have been collaborating with engineers at Ford Motor Company and consultant Michael M. Schechter for more than a year on an air hybrid vehicle design that uses a camless valvetrain. Tai presented the team's findings at the Society of Automotive Engineers World Congress in March.

Like its cousin the electric hybrid, air hybrid vehicles are being explored as a more fuel-efficient means of traveling the nation's roads, especially in urban areas, where stop-and-go traffic leads to a wasteful use of gas. During a typical day of city driving, fuel energy used to accelerate the vehicle is partially wasted during deceleration, when kinetic energy is converted into heat in the friction brakes.

Fuel economy could be greatly improved, say researchers, if that braking energy could be captured, stored and later used to help the vehicle speed up, for instance.

To make the air hybrid design work, Tsao introduced a few clever modifications to a traditional 2.5 liter V6 engine, including a valve design that allows the engine to not only burn fuel more efficiently, but to compress and expand air captured during braking as well. When it is compressed, air can store energy that is neither toxic nor explosive. Once the air is expanded, the burst of energy that is released can be used to help accelerate the car.

The concept is closely tied to that of electric hybrid vehicles, which are becoming an increasingly well-known alternative to traditional automobiles and have already proven capable of reusing braking energy. While still fueled by gasoline, the electric hybrid vehicle's engine and transmission combination is augmented by an energy conversion and storage system housed in a black box under the car's hood. This collection of sophisticated electronic components captures brake energy, stores it as electricity and then releases it when it is needed.

The additional hardware required to make it work includes a battery and a supplemental electric motor, which adds weight to the car and drives up costs. Manufacturers are forced to reduce weight in other ways.

"Automobile manufacturers are turning to more expensive lightweight materials like aluminum to compensate for the added weight involved with the electric hybrid approach," said Tsao. "With an air hybrid you don't have to worry about that."

Thanks to Tsao's innovative valve design, the air hybrid can achieve similar fuel efficiencies but needs only an air storage unit weighing no more than 30 kilograms.

"The air hybrid does not require a second propulsion system," said Tsao. "This approach allows for significant improvements in fuel economy without the added complexity of the electric hybrid model."

The UCLA researchers avoid the need for an additional motor by introducing greater functionality into the engine's valve system. During conventional combustion engine operation, the camshaft causes the intake and exhaust valves to open and close in a synchronized fashion to let in air and fuel and to let out exhaust. The camshaft is designed to perform in a predictable and fixed way. The same operation occurs over and over -- nothing more.

Tsao's industrial collaborators designed an electrohydraulic camless valvetrain system that allows for more variable valve operation, with greater control over when a valve opens and for how long. Tsao developed methods to precisely control the valve operation over a wide temperature range. This, in turn, makes it possible for the traditional engine to do more than just burn fuel.

"We wanted the engine to compress air and charge the compressed air back to the engine," said Tsao. "So we replaced the cam shaft with an electronically-controlled valve system."

Tsao's proposed valve system allows the engine to operate in four different modes. When a vehicle decelerates, the engine is used as an air compressor to absorb the braking energy and store it into the air tank. Whenever the vehicle stops, at a red light for example, the engine is shut down. Once the light turns green and the driver touches the accelerator pedal, the engine is started by compressed air. As the car speeds up, the engine is used as an air motor to drive the vehicle until the compressed air is depleted, at which point the engine is switched to conventional combustion mode and begins burning fuel.

The concept of driving a vehicle with compressed air is not new. In fact, a compact version of an air-powered car was introduced at the Paris Auto Show in late 2002. That car had a four-cylinder piston engine powered only by compressed air that is stored in an on-board air tank.

Road tests are needed to prove Tsao's concept, and other challenges need to be addressed before air hybrid vehicles become widely accepted. "We want to optimize the size of the air storage tank, and begin testing the air hybrid operation using a diesel engine," said Tsao.

As consumer demand grows for more environmentally friendly road vehicles, drivers may one day find themselves riding on air.

-Chris Sutton
07/21/03