Thursday, May 15, 2008

BMW looks at exhaust-gas and solar energy for efficiency gains

As manufacturers seek ways to create more efficient cars without compromising power or driveability, eyes are increasingly turning to new technologies and ideas. BMW is looking to electricity generated by exhaust gas heat and solar power to take the electrical load off the engine and thereby improve efficiency and reduce emissions.

In fact, BMW has found that re-purposing the otherwise wasted exhaust heat to power a thermoelectric generator generating up to 1kW could be used to reduce real-world fuel consumption by as much as 5%. The benefit comes from storing the electricity and using it to pre-heat the engine or power the air-conditioning systems. The illustration below shows the approximate location of the proposed system.

Honda’s similar work on the Rankine Cycle, which uses exhaust gas to heat water, creating steam that spins a turbine to generate electricity, has found as much as 32kW can be generated by the method, though the weight penalty for the device reduces fuel efficiency benefits to 3.8% at a 100km/h (62mph) cruise in a 2.0L direct-injection petrol four-cylinder.

BMW is also working on employing solar panels in the sun/moon roof sections of some of its cars. Prototypes have achieved 200W output levels, enough to improve efficiency by about 1%. Expanding solar panel coverage to the entire roof surface could generate up to 1kW, the same output as the exhaust-powered thermoelectric generator, and could benefit efficiency by the same amount, around 5%.

Other future technologies BMW is working on include regenerative braking, stop-start operation, variable frontal aerodynamics, electric power steering and a satellite-aided traffic management system that helps improve efficiency by anticipating when acceleration or braking will be necessary, and smoothing out the transitions between the two, resulting in better fuel economy. In Germany alone, BMW estimates that up to 12 billion liters (3.17 billion gallons) of fuel are wasted annually in traffic jams and other road hindrances.

The individual technologies are part of the overarching EfficientDynamics campaign to create more environmentally-aware cars while maintaining the driving dynamics that have made BMW famous.


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