The technology is more than just cool. In our car-filled world, plug-in vehicles could reduce the amount of gas we use, which keeps rising in cost now and then. Besides, driving around in these vehicles may even help the environment. Gas-burning cars produce a lot of greenhouse gas, which causes global warming.

　　The first company-produced plug-in vehicles could hit the roads by 2020. But engineers still have a lot of work to do to make the technology practical and inexpensive.

　　Batteries are the biggest challenge. In the plug-in-vehicle world，Li-ion(锂离子)batteries are getting the most attentionThese batteries can store a large amount of energy in a small package, and they last a longer time between charges. Li-ion batteries can fit laptops, cell phones, heart instruments and other similar pocket ones.

　　But because cars are so big and heavy, it would still require a suitcase-sized Li-ion batteries to power about 12km of driving. What's more, the batteries are much expensive.

　　“A car filled with batteries could go a long distance,” says Ted Bohn, an electrical engineer in Chicago. “But it couldn't pull any people, and it would cost $100,000.” So researchers need to work out how to make batteries smaller and cheaper, among other questions.

　　“The answers don't exist yet, ” Bohn says, “As a kid, I thought someone someplace knows the answer to everything. All of these questions haven't been decided. That's what engineering is about-making a guess, running tests and getting fine results.”

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