Historically, there's never been a lot of love lost between carmakers and the Environmental Protection Agency. Their relationship has long been a tenuous one, as the auto companies have resisted the agency's efforts to set higher standards for fuel economy and emissions for years -- and often publicly depicted those efforts as "government interference" in the marketplace, saying that stricter regulations would make cars too expensive for buyers.

Now the automakers are incensed about another issue: The ethanol content of gasoline.

The latest battle kicked off in late December 2010, when the Alliance of Automobile Manufacturers -- the trade association representing Detroit's Big Three carmakers, Toyota Motor Corp., and eight other car companies -- joined other trade groups in filing a legal petition that challenged a decision the EPA made in October. It granted a partial waiver that approved the sale of gasoline containing 15 percent ethanol (E15) for all 2007-model-year-and-newer passenger cars and light trucks.

Ethanol is added to gasoline to help oxygenate the fuel, allowing it to burn more completely, which ostensibly improves emissions. The current standard had limited the amount of ethanol blended with gasoline to 10 percent.

The EPA struck back in late January, expanding the ruling to also allow E15 be used in vehicles from model years 2001-2006. EPA Administrator Lisa P. Jackson made the decision after a review of the Department of Energy's testing and other available data on E15's effect on emissions.

Ethanol has been controversial for a variety of reasons. The powerful Midwest farm lobby supports ethanol becaue it is primarily produced using corn, though critics say that ethanol production wastes more gasoline to produce than it saves by being burned as fuel. Ethanol can have a detrimental effect on some materials commonly used in fuel systems in older vehicles. Gasoline blended with ethanol contains less energy than pure gasoline, as well.

The ethanol rulings have drawn parties other than car manufacturers into the fray. The legal petiton filed back in December was the result of a coalition that's calling itself the Engine Products Group, representing carmakers as well as manufacturers of other engine-powered products like boats, lawnmowers, and snowblowers that belong to the Association of International Automobile Manufacturers, the National Marine Manufacturers Association, and the Outdoor Power Equipment Institute. Oil companies and refineries are also parties to the suit, siding with the Engine Products Group.

Backing the EPA's decision is the Renewable Fuels Association, a group that has been advocating for increased use of ethanol.

The legal petition, filed with the U.S. Court of Appeals in Washington, D.C., asked that EPA's decision be remanded back to the EPA and requested judicial oversight and review of whether EPA's "partial waiver" approval for E15 fuels violates the federal Clean Air Act provisions, which limit the circumstances under which EPA can approve applications for new fuels and fuel additives. In 2009, Growth Energy, an ethanol industry trade group, petitioned the EPA to raise the limit on ethanol in gasoline from 10 to 15 percent.

The Engine Products Group has publicly prodded the EPA to be "thorough" in its testing, "to assure that E15 would not harm existing products or pose safety risks."

"By approving E15 use in a small subset of engines on the road, there is a high risk that consumers will unknowingly or mistakenly put E15 in products for which it has not been approved," said Kris Kiser, a spokesman for the Engine Products Group.

Kiser said that "all members of the Engine Products Group have supported, and continue to support, the development and use of safe and sustainable alternative fuels." But to permit E15 to be sold as a legal fuel, even if limited only to certain products, "would have adverse consequences for the environment and consumers," claimed Kiser. "A partial waiver, by its nature, necessarily will result in the mis-fueling of products not designed or tested for E-15 use."

The EPA points out that it did not approve the use of E15 for use in smaller engines, like the ones used in boats, snowmobiles and lawnmowers.

Last year, the EPA also proposed a new rule specifying that a new vehicle's window sticker would include clear information about what kinds of fuel consumers should or should not put into their tanks.

Meanwhile, the Renewable Fuels Association described the Engine Products Group's legal petition as "an attempt to block E15 from being sold as a legal fuel in the United States."

"We also believe EPA has failed to follow the science all the way through," said Matt Hartwig, the RFA's director of public affairs. "If it had, it could have avoided this market confusion. It is our reading of the available science that the EPA could have and should have approved E15 for use in all cars and light duty vehicles. Ethanol is a safe and effective fuel."

The RFA is still pushing the EPA to permit E15 in pre-2001 models, but the EPA said a decision on those older models will not be made any time soon, because not enough testing has been done on them to warrant an approval.

"The only way to meet the nation's energy, economic and environmental goals as put forth in the Renewable Fuels Standard is to increase ethanol consumption," Hartwig said. "Allowing for the use of E15 blends is a safe and appropriate step toward meeting these goals. The Renewable Fuels Association will continue to press for the safe and effective use of higher level ethanol blends in both conventional as well as flexible fuel vehicles."

For the most part, the EPA is not saying a great deal about the legal petition, since the agency's general policy is to not give detailed comments on ongoing legal matters or policy proposals that are still being evaluated. However, the EPA did issue a brief statement in response to the legal petition, saying that the agency "based its decision... on a comprehensive review of extensive testing data and on the law."

The Engine Products Group claims that the EPA's review of that testing was not comprehensive enough, and that more testing still needs to be done. "Recently-completed testing and data analysis show that E15 does not harm emissions control equipment in newer cars and light trucks," said EPA Administrator Lisa P. Jackson after the agency's January 21 decision to include model years '01-'06. "Wherever sound science and the law support steps to allow more home-grown fuels in America's vehicles, this administration takes those steps."