A new celebrity with a lusciously curved body is turning heads on France’s Côte d’Azur. No, not that kind of body. This one belongs to the AirPod, a 220-kilogram car with a sculpted composite shell and a back-to-the-future energy supply: 80 kg of air compressed to 350 times sea-level atmospheric pressure, roughly 350 bars. The engine of this tiny three-seater converts that air into mechanical energy, just as a pneumatic jackhammer does to blast apart concrete.
The AirPod won’t exactly tear up the road, though: The current version tops out at 45 kilometers per hour (28 miles per hour). And yet there’s definitely something addictive in its joystick steering and featherlike suspension. With expanding air pumping its pistons, the exhaust is literally a superchilled breeze. Grab the stick, step on the accelerator, and any guilt you may be harboring from driving an ordinary smog-producing carbon spewer falls away. Wouldn’t life be great if everybody got around town in these clean little machines?
This rosy vision of future urban transport is the product of Motor Development International (MDI), a company registered in Luxembourg whose tech-chic atelier lies in Carros, a palmy industrial suburb of Nice, France. Guy and Cyril Nègre [above], the father-and-son team behind MDI, predict their technology will find mass appeal in the emerging city-car category, an automotive segment of small, efficient cars well suited for crowded European and Asian cities and not meant for long-haul trips.
Most carmakers think that battery power is the future for this category, but the Nègres beg to differ. The AirPod, they promise, can tank up in just 2 or 3 minutes using no more than 1.5 euros’ worth of France’s nuclear-heavy, low-CO2 electricity to provide some 220 km (137 miles) of city driving. It has no batteries to wear out and replace—and so will cause no worries about its power source ever erupting in flames. And the AirPod will cost a mere €6000, the Nègres say (less than US $9000).
Storing energy in a long-lasting pressure tank made of carbon fiber rather than in batteries, posit the Nègres, makes the AirPod cheaper, more practical, and cleaner than a comparable electric vehicle (EV), once you take manufacturing and disposal into account. It’s a bold assertion, and one that remains to be proved, which won’t be possible until these cute little cars actually hit the road. And when that will happen is anyone’s guess.
Though the AirPod is supposed to go on sale in a couple of months, the Nègres have been struggling to commercialize pneumatic vehicles for more than a decade, incurring a reputation for unfulfilled promises. No independent testing laboratory has assessed the AirPod’s performance. And while deals over the past three years with India’s Tata Motors and Paris-based Air France have bolstered MDI’s credibility, it remains tough to find an automotive engineer who buys into the company’s vision. The fundamental problem, they say, is the laws of thermodynamics, which make compressed air an impractical power source for vehicles. The AirPod’s 200-liter tank is roughly the size of a common 55-gallon drum, but it carries the energy of little more than a liter of gasoline. And its air-powered engine makes inefficient use of it. MDI counters that the ultralight, low-speed AirPod needs very little to get around. Yet skeptics abound.
“I don’t know how they can deliver what they claim,” says Denis Clodic, a mechanical engineer and thermodynamics expert at France’s prestigious École des Mines de Paris. “It’s not a solution for the sort of vehicle we expect today,” says Pascal Higelin, professor and director of the Mechanics and Energy Laboratory at the University of Orléans, in France.
And yet Higelin and Clodic count themselves among the growing number of propulsion authorities who say that vehicles combining compressed air and fuel combustion could overcome the primary drawbacks of both, providing an economical alternative to today’s gasoline-electric hybrids. The greatest impediment to realizing such pneumatic hybrids, according to these two experts, is that the failure of MDI’s air car could cast doubt on the whole idea.
So if the AirPod does whoosh onto French streets within months as promised, there will be quite a bit more riding on the quirky little runabout than MDI’s fate. It could finally prove the viability of compressed-air transport—or doom it for the foreseeable future.
Pneumatic propulsion was high tech in the late 19th century, when compressed-air engines and equipment became commonplace in Europe and North America. Networks of compressed-air piping vied with then-nascent electrical grids to power machine tools, railway hoists, and switchyards, among other heavy gear. Meanwhile, the first jackhammers were revolutionizing mining and tunnel construction. Propulsion uses included pneumatic torpedoes, locomotives, and streetcars. Addison C. Rand, founder of Rand Drill Co., lauded pneumatic streetcars in his 1894 guide The Uses of Compressed Air, noting that they had neither the “distressing, jerky motion” of cable cars nor the capital costs of electric railways.
Combustion-powered automobiles and buses ultimately prevailed, as we all know. But a vestige of air propulsion survives in today’s Formula 1 racing pits, where blasts of air crank the big engines to life, and it is from this world that Guy Nègre emerged. The self-taught mechanic studied philosophy and worked in French carmaker Renault’s advertising department in the 1960s before setting up his first engine-design shop. There Nègre developed an unusual valveless engine for light aircraft, a design that was never commercialized. Nègre’s second shop extended the valveless concept to powerful Formula 1 race-car engines. In 1990, a racing club installed Nègre’s engine for the storied Le Mans 24-hour endurance race. But the engine refused to start, let alone endure for 24 hours. This firm, like the one before it, slid into obscurity.
In 1991, Nègre made the intellectual 360 that led to the AirPod. Together with his son Cyril, then an engineer for Bugatti Automobiles, Nègre formed MDI to develop low-emissions engines, and by 1996 they had locked onto air propulsion. While both father and son’s names are listed on the firm’s patent filings, Cyril is officially the R&D director, and Guy is the president, responsible for selling their vision.
The “thermo” of thermodynamics—the unstoppable flow of heat—makes pneumatic propulsion a considerable engineering challenge. The molecules of oxygen, nitrogen, and other gases in air give off heat when compressed, representing a loss of energy up front. Do the compression quickly, before the heat can dissipate into the surroundings, and the losses rise further. And the trouble only mounts when all this compressed gas is later released from the tank. The same molecules cool when they expand, hence the chill on your hand when you empty a spray can. Expand the gas slowly, and the pneumatic equipment can stay warm by reabsorbing energy from the atmosphere. But power-hungry vehicles must expand the gas quickly, so they are subject to extreme cooling, which hampers the engine or, at worst, freezes its air-feed lines.