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Could the future of automobiles be compressed air engines rather than ones fueled by batteries or hydrogen fuel cells? On May 1, 2015, “Shark Tank" investor and motorsports lover Robert Herjavec pledged $5 million to Zero Pollution Motors for their AIRPod. He purchased a 50% share of their not-yet-built Hawaii plant contingent on the company's ability to acquire the whole US market. ZPM currently is limited to only building and selling in Hawaii.
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Motor Development International of France designed the AIRPod. It's a small three-wheeled city car that seats a driver in the front and two rear-facing passengers in the back. It has no pedals or steering wheel. Rather, a joystick controls this car. Range is about 100 miles with a top speed of 50 mph. Built mostly from resin and fiberglass, the AIRPod weighs a mere 600 pounds. ZPM hopes to sell them buy the end of 2015 for around $10,000
Plugged into a wall, the onboard compressor refills the tank in four hours. A scuba-grade compressor can refuel the car in a few minutes. The tank is built from carbon fiber and designed to crack open in case of a collision. The air will be released safely all at once. A small puncture will not turn the tank into a projectile.
It looks like nothing else on the road, which is not always a good thing for sales. It's tough to tell which end is the front. The vehicle looks more like a teardrop trailer than a car. The round windows are reminiscent of an aquarium. Will car buyers, Americans in particular, be inspired to purchase a small, slow, weird, short-ranged green car even if it's cheap? Just ask the Mitsubishi i-MiEV. Even if the AIRPod flops, the technology may stick.
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How the Engine Works
Any liquid or gas under pressure, when released, will rush to find equilibrium like the air in a popped balloon. A compressed air engine works the same way. The AIRPod tank is filled to 3600 psi, which is about 100 times the pressure of a car tire or 248 times atmospheric pressure at sea level. When this pressure is released, it has great power.
The accelerator is linked to a valve that incrementally releases air. In the MDI design, air is fed into a two-cylinder engine. Indian mechanical engineering professors Bharat Raj Singh and Onkar Singh are developing a rotary air engine that can operate with as little as 60 psi. They plan to retrofit motor scooters with these engines, but the range and power are currently poor.
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Gas is Still Needed
Topping off the tank of your air car at the gas station is a romantic pastoral idea, but it has its problems. First of all, gas station compressors are built to fill rubber tires, not carbon fiber tanks at a hundred times the pressure. Station compressors would need to be upgraded. Would gas companies make the move ahead of an air car revolution, or would a plethora of air cars need to be on the road first?
Adding a sufficient compressor and clean power generation to your home could cost as much as an AIRPod, but you could then do the job at home.
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Compressed air engines are not a new idea. In 1863, the Plongeur, the first non-human powered submarine, used compressed air for power and to clear its ballast tanks. From the 1880s to the 1930s, air powered locomotives were common. They were popular in mining because they created no dangerous heat or sparks to react to the potentially explosive gasses in mines. In the 1940s, the gas engine improved and the oil industry gained power. Other forms of powering cars faded away.
Inventors throughout the 20th century attempted to build air cars, but experienced freezing up when the compressed air rapidly cooled upon release. In 1979, Terry Miller built the Air Car One for $1500 with off-the-shelf parts. Its four-stage engine recompressed the same air repeatedly. The engine ran at a fairly slow speed to prevent freezing up. He published complete instructions on how to build his engine just before his death in 1997.
In 2010, the Cadillac Aera won the LA Auto Show Design Challenge. The Aera is a 1000 pound four seat car built from advanced flexible polymers. It is powered by a pneumatic drive fed by a 10,000 psi tank and has a range of 1000 miles.
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The Future: Hybridization
Like a hybrid automobile that combines gas and electric, an air-gas hybrid can create a balanced machine. Compressed air does not store as much energy as gasoline. Internal combustion engines create a lot of waste heat. When the heat from ICE exhaust is recompressed, it eliminates freeze-up and increases the power of the engine. A gasoline-powered compressor, similar to a range-extending engine, can refill the tank of an air car while it drives.
Robert C. Burt was a GE scientist who patented an air car in 1932. It used a four-cylinder engine to power his compressor. The design lay stagnant for decades before receiving a little interest in 1980. The machine never made production.
Guy Negre of MDI continues to improve his air vehicles.
CJ Marquand, from the University of Westminster, is developing an engine that efficiently recaptures and reincorporates engine heat. He is also using regenerative braking to improve efficiency.
UCLA's Tsu-Chin Tsao is developing a gas-air hybrid engine that does not idle and uses a compressor to refill the air tank from braking energy.
Angelo DiPietro is perfecting a rotary air engine.
Will a tank of compressed air power your car to the future? It's not that simple. There may be too many barriers for a car run only on compressed air. Compressed air does not have the power to produce the speed needed and filling the tank may use more energy that it's worth. But the concept has great potential. When combined with electric and gas technology, compressed air power could greatly reduce the environmental impact of daily transportation.