Environmental groups sued the U.S. government yesterday for violating a 1992 law requiring federal agencies to buy vehicles powered by alternative fuels. The suit, filed by the Sierra Club, the Center for Biological Diversity, and the Bluewater Network, charges 18 agencies for failing — in some cases miserably — to meet the terms of the Energy Policy Act, a Gulf War-era law designed to reduce U.S. dependence on oil. The law requires 75 percent of all agency-owned vehicles to use alternative fuels; alleged violators include the U.S. EPA, the Department of Energy (which is supposed to enforce the law), and the departments of Justice, Commerce, Transportation, Defense, Agriculture, and Interior.