(Photograph by Getty Images) It’s hard to look at a car’s tailpipe and not be depressed. After all, even the most efficient internal-combustion engines use only 30 percent of the fuel’s energy to propel the vehicle. Much of the rest exits out the rear as waste heat. Now, researchers at General Motors are working on an energy-scavenging device that could convert that exhaust heat into electricity.The key is the use of a shape-memory alloy (SMA), explains Jan Aase, director of GM’s Vehicle Development Research Lab. “When you heat it up, it shrinks to its original length and gets stiffer,” he says. “When you cool it, you can stretch it out. So if you wrap shape-memory-alloy wire around two pulleys—one hot, one cold—the material will actually cycle the pulleys.”Imagine a pack of cigars wrapped around the exhaust pipe, and you have a good idea of what the proposed generator will look like. The “cigars” are actually tubular pulleys arranged in two sets. The hot set is next to the pipe, while the cold one is offset and cooled by fresh air. The SMA wire coils around the pulleys. As the material expands and contracts, it causes the pulleys to spin, which drives a generator. GM is working with California-based Dynalloy, a company that recently developed a process to produce a nickel-titanium SMA capable of repeating millions of heat/cool cycles. “Now we can get the material in large quantities and with predictable shape-changing characteristics,” Aase says.The researchers hope that the unit will produce enough juice to power all of a car’s electrical accessories—including electric power-steering pumps—allowing the engine to burn less fuel. GM R&D last year received a $2.7 million government grant to pursue the technology, which could potentially harness energy from factory smokestacks and house furnaces, as well as from automotive tailpipes. GM hopes to have a prototype ready by late 2010.