The inside of the George R. Brown Convention Center had all the trappings of an advanced shop class on Friday, as teams of students sawed, hammered and even taped the final touches on their eco-friendly race cars.

But this isn't your average race, it's the Shell Eco-marathon, and the goal isn't a car that moves around the track the quickest — it's to get it across the finish line using as little energy as possible.

The cars feature everything from traditional (if retooled) fuel injection engines to biodiesel, solar and hydrogen cell energy sources. Though some are large and boxy, most aim for an aerodynamic shape, with a few deviating more towards a Jetsons-like egg shape or a long, skinny coffin.

But don't let the amateur engineers give you the wrong impression — these cars are serious, with price tags of up to $100,000.

Last year the team from Laval University in Montreal won the competition with an average of 2,487.5 miles per gallon, their second consecutive win. This year the team still has some work to do. They designed a new car since then, but it wasn't quite ready for competition, so they are putting together last year's model with the same engine and carbon fiber frame and a new electrical system.

"You never know what's going to happen," Laval's Anthony Bernier said. "Last year we had a flat tire and our motor didn't work on the first day. We worked all night and were the first run of the day on Sunday, and it was the best run until we went again in the afternoon. That was the one that won it for us."

This year Laval's greatest challenge, shared with other teams from the northern part of the country, was tweaking the engine from the cold weather conditions it was built under to work at maximum efficiency in Houston's hot, humid climate.

"We're mainly just adjusting the engine," University of Illinois at Urbana-Champaign's Eric Wright said. "Since last year we cut the weight of the car from 170 pounds to 95 pounds, and we're using an aluminum honeycomb body instead of steel."

But while several cars look like mini versions of street-ready vehicles, others didn't limit themselves to traditional materials when working to increase lightness or visibility. The Lamar University team chucked their Mylar roof (which was hard for the driver to see through) for one made of shower curtains, with a heavier carbon fiber top as a back up.

Though it's mostly colleges competing, a few high schools have entered the fray, including first-timers St. Paul's School from Covington, La. The St. Paul's team built two cars — one one conventional in shape and engine type, and one biodiesel powered with a steel frame (made from recycled bicycle parts) and a clear core plastic.

"We've learned a lot this year," coach Mike Cobb said. "I learned not to special order anything, because if it breaks you can't replace it on site — that's why we bought this Home Depot motor to take apart and see if we can use any of it."

One car might be out of commission, but the other is looking good. With the other biodiesel competitors withdrawn, the car only has to successfully make it around the track to win the division.

One of these prototypes could be the next Tesla or Volt, and the best place to check them out is on the road. The Shell Eco-marathon takes place at Discovery Green throughout the day on Saturday and Sunday, with awards presented Sunday at 6 p.m.

Hear the NPR report on the Shell Eco-marathon:

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