Air hybrid cars could bring big fuel savings for citydrivers, according to a recent study released by UCLA engineers. Experimentsbased on modeling and simulations showed that the air hybrid engine improvedfuel efficiency by 64 percent in city driving and 12 percent in highwaydriving. The study also suggested that by adopting the air hybrid approach,carmakers could avoid some of the manufacturing costs associated with the morecommon electric hybrid design.
Tsu-Chin Tsao, professor of mechanical and aerospaceengineering at the UCLA Henry Samueli School of Engineering and AppliedScience, and graduate student Chun Tai have been collaborating with engineersat Ford Motor Co. and consultant Michael M. Schechter for more than a year onan air hybrid vehicle design that uses a camless valvetrain. Tai presented theteam's findings at the Society of Automotive Engineers World Congress in March.
Like its cousin the electric hybrid, air hybrid vehicles arebeing 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 ofgas. During a typical day of city driving, fuel energy used to accelerate thevehicle is partially wasted during deceleration, when kinetic energy is convertedinto heat in the friction brakes.
Fuel economy could be greatly improved, say researchers, ifthat braking energy could be captured, stored and later used to help thevehicle speed up, for instance.
To make the air hybrid design work, Tsao introduced a fewclever modifications to a traditional 2.5 liter V6 engine, including a valvedesign that allows the engine to not only burn fuel more efficiently, but tocompress 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 isexpanded, the burst of energy that is released can be used to help acceleratethe car.
The concept is closely tied to that of electric hybridvehicles, which are becoming an increasingly well-known alternative totraditional automobiles and have already proven capable of reusing brakingenergy. While still fueled by gasoline, the electric hybrid vehicle's engineand transmission combination is augmented by an energy conversion and storage systemhoused in a black box under the car's hood. This collection of sophisticatedelectronic components captures brake energy, stores it as electricity and thenreleases it when it is needed.
The additional hardware required to make it work includes abattery and a supplemental electric motor, which adds weight to the car anddrives up costs. Manufacturers are forced to reduce weight in other ways.
"Automobile manufacturers are turning to more expensivelightweight materials like aluminum to compensate for the added weight involvedwith the electric hybrid approach," Tsao said. "With an air hybrid you don'thave to worry about that."
Thanks to Tsao's innovative valve design, the air hybrid canachieve similar fuel efficiencies but needs only an air storage unit weighingno more than 30 kilograms.
"The air hybrid does not require a second propulsionsystem," Tsao said. "This approach allows for significant improvements in fueleconomy without the added complexity of the electric hybrid model."
The UCLA researchers avoid the need for an additional motorby introducing greater functionality into the engine's valve system. Duringconventional combustion engine operation, the camshaft causes the intake andexhaust valves to open and close in a synchronized fashion to let in air andfuel and to let out exhaust. The camshaft is designed to perform in apredictable and fixed way. The same operation occurs over and over — nothingmore.
Tsao's industrial collaborators designed an electrohydrauliccamless valvetrain system that allows for more variable valve operation, withgreater control over when a valve opens and for how long. Tsao developedmethods to precisely control the valve operation over a wide temperature range.This, in turn, makes it possible for the traditional engine to do more thanjust burn fuel.
Tsao's proposed valve system allows the engine to operate infour different modes. When a vehicle decelerates, the engine is used as an aircompressor 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 shutdown. 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 isused 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 andbegins burning fuel.
Road tests are needed to prove Tsao's concept, and otherchallenges need to be addressed before air hybrid vehicles become widelyaccepted. "We want to optimize the size of the air storage tank, and begintesting the air hybrid operation using a diesel engine," Tsao said.