A push for change puts focus on the powertrain.

As more than 35,000 engineers and executives gather in Detroit this week for the Society of Automotive Engineers World Congress, one question will dominate the convention: What will power the car of the future? The gas engine has dominated the U.S. auto market for decades, but gas-electric hybrids and diesel engines are gaining ground. Ethanol is being promoted as an alternative fuel, and hydrogen fuel-cell vehicles are being tested in real-world conditions.