Although the request went largely unnoticed by the public, it got the attention of anyone who makes or sells gasoline engines, as well as some environmentalists and consumer advocates.  Approving E15 would have a huge impact on consumers, said Clarence Ditlow, executive director of the Center for Auto Safety, and could cause problems including the voiding of car warranties. “There’s a lot to worry about,” he said. “All a consumer has to do is look at the fuels section of the owner’s manual, which says that the use of fuel above 10 percent ethanol may result in denial of warranty claims.”  Nearly 250 million cars and light trucks are registered in the United States, according to Experian Automotive. But the impact would be even broader. Kris Kiser, executive vice president of the Outdoor Power Equipment Institute, a trade group, estimates that a change would affect 300 million engines in everything from chainsaws to weed trimmers.  The National Marine Manufacturers Association says 12 million boat engines would also be affected.  Growth Energy, whose co-chairman is Wesley K. Clark, the retired Army general and former Democratic presidential candidate, has told the E.P.A. that it has proof from several studies that E15 will not damage engines and will result in cleaner air while reducing the nation’s reliance on oil.  The studies were done by groups including the federal Energy Department, the State of Minnesota, the Renewable Fuels Association, the Rochester Institute of Technology, the Minnesota Center for Automotive Research and Stockholm University in Sweden.  Michael Harrigan, a former Ford Motor Company fuel-system engineer who is now a consultant to Growth Energy, said automakers had been doing enough testing that there should be no problems using E15.  And Tom Buis, the chief executive of Growth Energy, said, “We are confident in the science we prepared.”  But confident or not, Growth Energy has plenty of opposition from groups that say some of the studies it cites are inconclusive. The critics also say its assertions are unproved and in some cases misleading.  While automakers generally favor wider use of biofuels, Charles Territo, a spokesman for the Alliance of Automobile Manufacturers, a trade group representing 11 automakers, said Growth Energy had failed to prove that E15 would not damage vehicles engineered to run on a maximum of 10 percent ethanol. More testing is needed, he said.  “We are not asking for this to be delayed forever,” Mr. Territo said. “We are asking for this to be delayed until the testing is complete.”  Mr. Kiser, of the outdoor power equipment group, said some initial tests already indicated that E15 could cause serious problems — including safety issues — with some small engines.  At Honda, which makes a wide range of engines for products from minivans to power generators, the concern is that the effects of a big increase in an additive like ethanol are unknown, said Edward B. Cohen, vice president for government and industry relations at American Honda. “The impact can be on the emissions system, like the catalytic converter,” he said. “It can be on the various tubes or couplings that are part of the fuel system, and it could affect the performance of the vehicle, particularly cold starting.”  Honda can design engines to run well on new gasoline blends, Mr. Cohen said. The issue is the legacy fleet, whose engines were designed over two decades for varying requirements. There is no single answer, Mr. Cohen said, to the question of how E15 would affect them.The American Petroleum Institute is also concerned, said Robert Greco, the group director of downstream and industry operations. He said more research was needed — probably several years’ worth — before the institute would be convinced that E15 was safe for so many different kinds of engines.  “We think that the current waiver request is premature,” Mr. Greco said. “The science isn’t in yet.”  And Jeremy Martin, a senior scientist at the Union of Concerned Scientists, an environmental advocacy group based in Cambridge, Mass., said there was simply not enough solid information on which to make a decision that would have such a broad impact.  “We shouldn’t just look at a little data and extrapolate,” he said. “There are rules here, and there are procedures. And there is a proper engineering way to come to this determination. One can guess about the most likely outcomes, but that is not sufficient to put all the fleet at risk.”  Wendy Clark, group manager and principal researcher in the fuels performance group at the Energy Department’s National Renewable Energy Laboratory, said a lot of credible organizations were studying E15. But she said it was too early to know for sure how engines would be affected. One question is how many of the studies will be done before Dec. 1, the date by which the E.P.A. is required by law to make its decision.  Mr. Ditlow of the Center for Auto Safety said: “What the ethanol people are asking the consumer to do is bear the risk. If only 1 percent of the vehicles on the road today had E15-related problems, that would be about 2.5 million vehicles.”  Among those concerned about the proposed change are service station owners, many of whom fear that their pumps and fiberglass storage tanks would need to be replaced. They also fear legal problems including lawsuits from customers claiming their vehicles were damaged by the fuel.  “It is a horrible thing for our members,” said Carl Boyett, president of the Society of Independent Gas Marketers of America.In their March request to the E.P.A., proponents of the waiver said E15 would provide “increased energy security, enhanced economic development, creation of American jobs, reduced transportation costs and environmental benefits.” The ethanol manufacturers contend that the increase is necessary because of the Energy Independence and Security Act of 2007. That act includes a renewable fuels standard that requires a steady increase in the use of biofuels in the United States — to 36 billion gallons in 2022 from 11 billion gallons this year. To meet the goals, refiners must add biofuels to gasoline.  The industry has been meeting the requirements. In 2007 , it was required to use 4.7 billion gallons of ethanol and it actually used 6.85 billion, according to the petroleum institute. Last year, when the requirement was 9 billion gallons, the industry used 9.6 billion.  But Americans are now buying far less gasoline than was expected when the law passed. That decline has the industry worried that as early as 2011 or 2012 it will be impossible to meet the renewable fuels standard with a 10 percent limit, Mr. Greco said.  Mr. Buis of Growth Energy said: “We are up against a blend wall. That cap needs to be raised.”  While adding more ethanol would help refiners meet the law, it would not improve fuel economy. An October 2008 study for the Energy Department tested 16 late-model cars and found, on average, that mileage dropped 5 percent with E15 compared with gasoline that contained no ethanol.  In deciding whether to raise the cap, the E.P.A. says it must consider not just emissions, but also vehicles’ durability and drivability “over their useful lives.” The agency has acknowledged that E15 is a complex issue, given that engines vary widely in their age and sophistication. Some might run fine on E15 while others might be susceptible to problems.  The E.P.A. says one possibility is that it could approve the use of E15 for some vehicles or engines but not for others.  Mr. Martin of the Union of Concerned Scientists says tests may show that vehicles produced starting with 2004 models could run safely on E15. That year, more sophisticated engine controls were required, making it more likely their systems could detect and compensate for fuel variations. About 79 million cars and light trucks have been produced since the 2004 model year, Experian Automotive says.  Mr. Buis of Growth Energy said that the advantages and safety of E15 were clear and that allowing higher ethanol content would help to make the nation less dependent on petroleum. He said there was no reason to delay.  “You know, some people don’t want to do anything — they just want to test, test, test or study, study, study,” Mr. Buis said. “You know, this nation has been stalling for 30-some years from becoming energy independent.”
A push to get the E.P.A. to increase the maximum ethanol content in gasoline has drawn widespread opposition.