Lightweighting is the No. 1 challenge facing automotive engineers today. Manufacturers are scrambling to build vehicles that contain a variety of weight-saving materials, such as aluminum, carbon-fiber composites, high-strength steel, magnesium and plastic.

Back in the day, engines were the exclusive domain of cast iron and steel. But, during the past decade, more lightweight materials, such as aluminum and hard thermoplastics, have been slowly creeping under the hood. The Holy Grail, an engine made almost entirely out of plastic, is finally close to reality.

DETROIT—General Motors uses seven joining methods—including spot welding, laser welding, self-piercing rivets and flow drill screws—to assemble the aluminum body of the new Cadillac CT6 sedan.

DEARBORN, MI—Roofs made of carbon fiber. Plastic windshields. Bumpers fashioned out of aluminum foam.What sounds like a science experiment could be your next car. While hybrids and electrics may grab the headlines, the real frontier in fuel economy is the switch to lighter materials.

DEARBORN, MI—Ford Motor Co. has introduced a new lightweight concept vehicle, built using advanced materials including new metals, alloys and composites.