Headquartered in the south of France, near Nice, MDI has been promoting compressed-air engine technology for over a decade.The small, innovative company employs about 50 people, and is managed by its founder, Guy Nègre. In 2007, against all odds, the company signed its first partnership agreement with a major car maker, Indian conglomerate Tata. This partnership will allow MDI to complete the development and optimization of its technology for the Indian market. Tata Motors is the company that acquired Jaguar and Land Rover from Ford early in 2008, and is the maker of the $2,500 car. In its homeland, this large industrial group is the number one commercial vehicle manufacturer and the number two for private vehicles. There are over four million licensed Tatas on the road today.
With this agreement, MDI will be able to thumb its nose at European automakers who've ridiculed their engine's highly unusual features. Motors running on compressed air are the norm for industrial power tools, but there are few who believe that such a system could be an effective power source for a car. Another stumbling block is the fact that air-powered cars, unlike air-powered tools, must carry along their own supply of compressed air, which has resulted in numerous complications.
Changing minds through innovation
MDI managed to get around the limitations of ordinary air motors with an exclusive technology; their system limits the compressed air supplied from the vehicle's tank with each cycle for increased vehicle range. Two major features differentiate MDI's engine from any other air-powered engine. First, the piston engine draws ambient air to fill its cylinders and injects a tiny amount of compressed air at the end of the compression cycle to increase total pressure within the engine's cylinder. After "inhalation," the air is warmed up. This further increases the pressure within the cylinder, boosting power and performance. MDI claims that its prototypes can travel 200 km on a tank of compressed air, which rivals that of a typical electric car.
Second, MDI's air engine features a unique connecting rod system which improves the engine's efficiency. Unlike the pistons of a regular gasoline-powered engine, which are linked to the crankshaft by rigid connecting rods, the compressed-air engine's pistons are connected by a hinged mechanical link that MDI calls a "crank rod". This patented system stops the piston for a few milliseconds at the top dead center position - the limit of its travel - without interrupting the crankshaft's rotation. This pause allows more time for pressure to increase in the cylinder, even when the car is accelerating. The result is a very stable torque curve at any speed. Although air-powered engines are typically small and weak, MDI's engine offers decent performance as its peak torque is generated at low engine speeds.
Compressed air: Source of energy or storage system?
MDI is marketing its air-powered engine as an alternative to electric-powered cars rather than fossil-fuel powered vehicles. Although they may seem totally different (air vs. electricity) these two "green" technologies share many things in common.
First, supporters of both engines claim the enhanced green factor, arguing that the driving force of their vehicle does not pollute and could (one day) be developed from the wind or the sun. However, unlike hydrocarbon, neither compressed air nor a battery is a source of energy in itself. They are energy storage strategies and the energy they store comes from alternative sources, usually coal or oil-fuelled powerplants which are not necessarily environmentally-friendly. The car may not emit any tailpipe emissions, but the generation of electricity needed to run the car might not be as kind to the environment.
Second, this energy to power the car needs to be stored somewhere: One system requires large, heavy and expensive batteries and the other needs huge air tanks, limiting its application to small, lightweight vehicles. The range of either vehicle type is seriously compromised as soon as the greater demands are put on the vehicle (for performance, or range); fitting larger batteries or bigger air tanks could be a potential solution, but it would seriously reduce passenger and cargo-carrying abilities.
Finally, there is the number one question that MDI's detractors love to ask: "What is the point of using electricity to compress air (a process known to be ineffective) instead of using electricity to charge the batteries directly?"
A comparison of kilowatts and horsepower gives weight to this energy-efficiency argument. Compressed-air engine supporters argue that even if the technology seems less efficient on paper, the weight of the batteries required by electric cars limits their performance and results in higher energy consumption for similar abilities.
The compressed-air car has a few advantages over the electric car. It takes less than three minutes to fill an air tank versus six to eight hours to charge the batteries of an electric car. The compressed-air car also comes equipped with its own electrical compressor to recharge the vehicle in a few hours only, simply by plugging it in. And it wins hands down over the electric car for air conditioning as the electric car uses a large quantity of its precious energy to drive an air conditioning compressor the car while MDI's car reuses frigid air expelled from the exhaust. MDI says that if this cold, decompressed air - which is clean and safe for passengers to breathe - was sent directly into the car, its occupants would freeze in less than 20 minutes!
However, there are some down sides. The current compressed air engine is about as noisy as one of the early diesels and its numerous loose parts do not bode well for reliability, bringing back fears of the piston engine. There is no comparison with the quietness and excellent reliability of electric engines.
Adding fuel to the equation
Aside from the compressed-air engine - described by MDI as a "mono-energy engine" - the company is refining the idea of a second generation, "dual-energy" engine. The engine would be identical to the mono-energy version, powered by compressed air only when the vehicle runs at low speed, but it would also have a combustion chamber to warm up the air before its injection, which would significantly increase the car's power. 
Since the fuel - or the energy adjuvant, as MDI calls it - would only be used to heat up the air, several other solutions which wouldn't require any huge modification to the engine's basic mechanics could be considered, from bio-diesel to hydrogen. With this system, MDI expects speeds up to 90 km/h with a fuel consumption of about 1.7 L/100km. This engine could even recharge its compressed-air supply while on the road!
For the time being, MDI is planning the introduction of the AirPod, a vehicle whose size is about as small as its price (less than $10,000), or its operating cost. This four-seater will not exceed 45 km/h and the design, according to MDI, is playful and futuristic. The AirPod won't pretend to replace a regular automobile, but it will provide an alternative to the problems of traffic jams and urban travel. This joystick-controlled vehicle will have its rear passengers facing backward. Large lift gates will act as doors for the rear passengers, and will double up as a door and windshield for front passengers.
Production of the AirPod should begin midway through 2009.More images:
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