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Battery size matters.

Cars are becoming more electric every year. While the impetus for hybrid electric, plug-in hybrid, and full battery electric vehicles may be energy savings and emissions reductions, a recent study by a mechanical engineer at Carnegie Mellon University in Pittsburgh suggests that not all electrified vehicles do the same amount of good. Somewhat counter-intuitively, it turns out the smaller the battery pack, the better the results.

Automakers have been working toward vehicles with battery packs large enough to enable motorists to drive 30 miles or more on electric power alone. Plug-in hybrid electric vehicles, such as the Chevrolet Volt, have recently entered the mainstream car market to much acclaim.

But according to an analysis of life-cycle air emissions and oil consumption from conventional vehicles, hybrid-electric vehicles, plug-in hybrid-electric vehicles, and battery electric vehicles conducted by CMU professor Jeremy Michalek and his colleagues, the push toward PHEVs may not be the optimal public policy.

"Although large battery packs allow vehicles to travel longer distances using electricity instead of gasoline," the engineers reported in the Proceedings of the National Academies of Science, "large packs are more expensive, heavier, and more emissions intensive to produce, with lower utilization factors, greater charging infrastructure requirements, and life-cycle implications that are more sensitive to uncertain, time-sensitive, and location-specific factors."

In addition, because larger battery packs are so much more expensive than smaller ones, the same amount of money puts fewer long-range plug-in hybrids on the road compared to standard hybrids or plug-ins with smaller batteries.

Current policies that subsidize the purchase of hybrid vehicles offer more support for the more expensive plug-in hybrid, the engineers reported in their study. A more cost-effective policy would be one that encourages standard hybrid vehicles, since that would provide more emissions reduction and fuel efficiency bang for the buck.

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Title Annotation:NEWS & NOTES
Publication:Mechanical Engineering-CIME
Date:Nov 1, 2011
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