Five years ago, General Motors Corp. gave the world the Hummer H2, a vehicle so fuel-thirsty that GM took advantage of a federal loophole that allowed the company not to publish its estimated mileage. Today, the No. 1 U.S. auto maker by sales, usually the most conservative of Detroit’s Big Three, has assigned hundreds of engineers and millions of dollars to an effort to become the greenest company in the auto industry. Engineering teams at GM’s technical center in Warren, Mich., are scrambling to turn a recently unveiled electric concept car into a production vehicle within three to four years. This month, GM kicked off a drive to hire 400 technical experts to work on fuel-saving technology and other innovations, and became the first auto maker to sign up for a cap-and-trade system for carbon emissions, which are blamed for global warming. This year, GM’s research labs are scheduled to turn its hydrogen fuel-cell technology over to an engineering group that prepares new powertrains for commercial launch, a sign of increased determination to put hydrogen-powered vehicles on the road.