The cars were designed by students at the top engineering schools in the country, who are competing to see who can come up with the most super fuel-efficient car.
But there's a catch.
Not only do the cars have to be fuel efficient - some of them get up to 80 miles per gallon - but they also must be affordable.
"If not, people aren't going to buy it," said Ronald Beeber, communications director for the U.S. Council for Automotive Research, a consortium of the big three automakers.
The council and the Energy Department formed a partnership in 1993 that sponsors the contest.
Each team was given a brand new mid-size American car and some seed money.
The team from Lawrence Technological University in Southfield, Mich., came up with the solar power-assisted air conditioning system.
They needed it on the two-day, 600-mile drive from Detroit.
The car is powered by a combination diesel and electric engine. A computer determines when the electric engine isn't powerful enough and turns on the diesel engine.
Estimated gas mileage: 60 miles per gallon.
Like a NASCAR race, the drivers had a 30-minute lunch pit stop at the Shell station on Sharpsburg Pike in Hagerstown
They used the time not only to eat, but to tweak the car engines.
Aaron Sullivan, who just got a master's degree in mechanical engineering from the University of Wisconsin at Madison, reached into the engine to make an adjustment and burned his arm for the second time in two days.
"Yeow. That's hot," he said.
The Wisconsin team's car, dubbed "FutureCow" because of the state's high dairy production, recaptures the heat a normal car loses during a stop and uses it to accelerate.
The team also converted to aluminum the Dodge Intrepid's engine cradle, which holds up the engine, making it about 25 pounds lighter.
The gas mileage is consistently close to 60 miles per gallon, they said.
Not all of the cars were performing well on the trip, their first on-road test. All but three were being pulled by trailers on Tuesday morning.
Endurance is only worth 60 out of a possible 1,000 points in the contest, Beeber said.
Members of the West Virginia University team hauled their car over mountainous terrain in the morning, but took it off the trailer for a more impressive ride into Washington.
Their car is powered by a combination of electricity and natural gas, which their state produces in abundance.
Graduate student William Kellermeyer estimated the car's gas mileage at 35 to 40 miles per gallon.
There is a good chance that the technology used by the students will lead to significant energy, environmental and economic benefits, Beeber said.
"Students have fresh ideas," he said.