BMW aims to put rocket science in your car

By Christiaan Hetzner

GENEVA (Reuters) - Germany's BMW wants to use NASA technology to boost the fuel efficiency of its luxury cars and lower emissions of harmful greenhouse gases.

Engineers at the BMW's high tech experimental lab in Palo Alto, California are attempting to adapt the thermoelectric system for cars that NASA scientists employ to help power their deep space probes navigating the far reaches of the solar system, BMW's director of vehicle integration told Reuters.

The NASA probes use so-called radioisotope thermoelectric generators (RTGs) which transform heat emitted from naturally decaying plutonium into electricity. BMW wants to bring this concept back to earth by reclaiming the thermal energy released in engine combustion.

"I firmly believe that when you look at this technology in 20 years, this will be the key issue that will replace everything else," Hans Rathgeber told Reuters in Geneva.

Rathgeber said simple physics prevents a conventional combustion engine from ever exceeding one-third efficiency. A motor with an output of 200 kilowatts (kw) generally creates 400 kw thermal energy as a waste by-product.

Recycling this abundant energy source would require replacing the plutonium in a RTG with a non-hazardous material that conducts electricity while resisting heat, not an easy task since most metals become hot when a current is applied to them.

BMW settled on a semiconductor element called Bismuth Telluride as a link between the car's engine coolant and its exhaust gas manifold to fuel the thermoelectric cycle.

Simply put, a charge is induced when you heat and cool two separate ends of a closed circuit. The amount of electricity generated is proportionate to the difference in temperature at the two points -- a property known as the "Seebeck Effect."

According to BMW's Rathgeber, batteries are better suited to storing this energy since it is constant and predictable.

Hybrids like the Toyota Prius by comparison have difficulty transforming quickly and effectively the rapid changes in a moving car's kinetic energy that is harnessed as electricity when the momentum slows or comes to a stop.

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"When you have brake energy recovery (as with hybrids), you produce and trap electricity in a battery every time you take your foot off the pedal. With the thermoelectric generator, electricity is produced every time you step on the gas so it's an ideal supplement," he said.

A full-size BMW sedan like the 530i could reduce its fuel consumption by as much as 13 percent, or 1 liter less petrol per 100 km, simply by equipping it with a thermoelectric generator.

Not only would drivers save at the pump, BMW can lower the carbon footprint of its luxury cars just when automakers are forced to cut overall European average fleet emissions to 120 grams of CO2 per kilometerstarting 2012 or face heavy fines.

BMW's cars in Europe still spew out some 156 g/km of CO2 across its entire model range, for example.