Electric Air Apparent

MDI Air Car is the 'whackiest' hybrid yet, writes Motor Trend's Frank Markus.

Published: 15-Jul-2007

Say you're a developing country with a billion or so poor, upwardly mobile, would-be drivers pining for personal transportation. You could tool up for a billion cheap two-stroke Tuk-tuks and hope your citizenry evolves a genetic ability to breathe blue air. Or you could hope to attract a billion tech-support phone-bank jobs so they can afford Priuses. Or maybe you could engineer a $9000 car that runs on air.

All three solutions sound equally unlikely, but indeed French R&D firm MDI has licensed technology for an air engine to India's largest car company, Tata. And indeed the engine runs on compressed air-and electricity and the combustible liquid fuel of your choice. This system omits the priciest bits of a typical gas/electric hybrid vehicle-the battery and power controller-in favor of a composite compressed-air tank and some valves. The air is metered into an ingenious piston engine using matched pairs of small and large pistons. A small one works with a conventional connecting rod, while the large one employs an intermediate rocker arm that allows it to pause at top-dead-center for 70 degrees of crankshaft rotation, building pressure during the small piston's power stroke. This allows each pair of cylinders (two-, four-, and six-cylinder variants are envisioned) to produce power over 270 degrees of crankshaft rotation, while sharing a common intake and exhaust valve. CLICK HERE for a video demonstration of how the engine works.

In a small-city car weighing 1100 pounds, two 100-liter (26.4-gallon) cylindrical tanks pressurized to 4350 psi provide 60 to 90 miles of city driving range. Refilling would take about two minutes at a compressed-air station. Plugging into a 220-volt electrical outlet at home or work, the tank fills in about four hours, while an on-board motor spins the engine as a compressor. This same starter/alternator is used to maneuver the car in reverse and to recharge the ordinary 12-volt battery that runs the lights and accessories when slowing or stopping the car. Running the engine as a compressor during braking is technically feasible, but expensive and trickier to integrate seamlessly than this electrical regeneration is. Reversing on electricity simplifies the design of the automated manual transmission, too.

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