As odd as it sounds, running a car on air is a reality. Proof of concept and prototype compressed air vehicles - commonly referred to as "air cars" - have been running around for a number of years.
How could it be possible to run on air? Consider the physical work that compressed air already does to make our everyday lives easier. Mechanics rely on air-driven pneumatic tools every day to turn nuts and bolts with authority in garages around the world. Pneumatic tools are powerful, even at a relatively low pounds per-square-inch (psi) pressure setting. They can free rusted-on lug nuts and separate metal from metal through an air hammer or pneumatic chisel. Crank the pressure up and compressed air is a force to be reckoned with, providing enough power to even propel a wheel driven car.
Perhaps that was the inspiration that led former Renault F1 mechanic Guy Negre of Motor Development International (MDI) to pursue compressed air propulsion for the auto industry. And what could be more environmentally friendly than a car with atmospheric air as its only exhaust emission? There's no combustion whatsoever. Power comes from compressed air sourced from special high-pressure compressors run by electricity from the grid.
MDI's design uses a pair of air driven pistons, one large and one small, to turn a crankshaft that produces a rotational force. The technology can potentially be paired in two, four, or six cylinder engine configurations and the design is quite inventive. Since there is no combustion and the only engine heat comes from friction, the engine can be made primarily from lightweight aluminum.
For those who want the technical details, here's the scoop: In MDI's air engine, the small piston has a conventional connecting rod for turning the crankshaft, while its neighboring larger piston utilizes an innovative rocker arm configuration with the connecting rod. This design allows the large piston to pause at top-dead-center for 70 degrees of crankshaft rotation while metered air pressure builds in a prechamber as the small piston keeps the crank turning during its power stroke. The large piston then turns the crankshaft with greater power as the pair combine to produce power over 270 degrees of crankshaft rotation. Got that all?
Prototype air cars are minimalist transportation that typically exhibit a top speed of about 70 mph and a range of approximately 125 miles on flat roads before requiring a refill. Compressed air is stored at 300 bar (4351 psi) in carbon fiber tanks mounted longitudinally beneath the vehicle floor. Refilling can be accomplished in a matter of minutes at a special high-pressure pump or in about four hours via a home refueling appliance or even an on-board compressor.
In 2007, Tata Motors licensed the rights from MDI for $28 million to build and sell Tata-branded air cars in India. Tata has not confirmed if it will build one of the MDI prototype cars or, more likely, install the MDI technology in one of its existing cars like the light weight Nano (shown here). The Nano is Tata's $2,500 "scooter replacement" people's car that recently made headlines. While sought after in developing countries, this inexpensive car clearly won't meet federal emissions and safety requirements in the U.S. and other regulated markets around the world. Still, the addition of air power to an already inexpensive and efficient model would be quite appealing in the Indian market and others where fundamental transportation is in demand, and air pollution could be a serious challenge as exponentially greater numbers of vehicles make their way to the highway.
In the United States, a company called Zero Pollution Motors (ZPM) has licensed the rights to produce the MDI design in a U.S. factory. Based in New Platz, New York, ZPM has an ambitious goal of rolling out a North American compressed air vehicle for $18,000 by 2010. The company most recently unveiled MDI's newest car at the Automotive X-Prize exhibit at the New York Auto Show. ZPM and MDI will field two entries - the U.S. production six seat, four door prototype in the mainstream class, and the three seat, two door economy-utility model in the alternative class.
Air cars haven't escaped the attention of mainstream U.S. automakers, too. For example, Ford has worked with an engineering team at UCLA to develop an air hybrid. In this application, the air hybrid builds air pressure using the engine as a pump while shut down during deceleration, and then utilizes the recaptured energy to launch the vehicle from a stop. Special electrohydraulic actuators in the valvetrain make the transition possible.
Air powered cars are not a new idea, and in fact the concept actually predates a viable internal combustion engine. In his book, "Paris in the 21st Century," Jules Verne foresaw a transportation system utilizing compressed air. Now, modern visionaries are striving to make that dream come true with the air car.