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Kai Ryssdal: Thirty-six billion gallons -- that's how much renewable fuel the government says the U.S. must produce by 2022. And here's an unexpected consequence of that ethanol mandate: Alcohol is murder on lawnmowers and small engines. Mechanics insist that as gasoline blended with ethanol takes over at gas stations, small engines across the country will start choking to death. Wyoming Public Radio's Peter O'Dowd reports.

Peter O'Dowd: At the WyoTech automotive school in Laramie, Wyo., Larry Wostenburg likes to conduct experiments with engines for his students. Today's test: how much ethanol a small engine can take before it breaks down.

Larry Wostenburg: We're going to put a little choke action on here and start this baby up.

Wostenburg pours alcohol into a lawnmower's fuel tank. His supervisor Jack Longress explains why using too much ethanol can destroy this kind of engine.

Jack Longress: It's a recipe for disaster because, eventually, when those pieces get brittle they're more susceptible to breaking.

Alcohol makes engines run dangerously hot. It melts rubber components. Longress says use anything higher than 10 percent ethanol on small engines long enough, and the insides will start to rot.

Longress: The corrosive properties, what you'd see is, much like what you see on the top of dirty battery terminals.

Drivers of flex-fuel cars don't have to worry much. Their on-board computers can regulate fuel mixtures. But small engines like WyoTech's lawnmower don't have those features. They're more likely to malfunction if they're filled with the wrong blend, and broken engines can mean injured operators. That's just one of the reasons why Kris Kiser is so worried. He's with AllSafe, an advocate group for small-engine manufacturers.

Kris Kiser: What were concerned about are mid-level blends entering into the marketplace in advance of consumers being educated about their use and what their affects will be.

Kiser says millions of chainsaws, lawnmowers and boats could be vulnerable to death by ethanol. This year the government ordered the production of 9 billion gallons of renewable fuel. A decade from now, that number will grow to 26 billion gallons. As the mandate expands, Kiser says higher blends of ethanol will be pumped from every gas station in America. And unless people know what they're doing, he says they could easily fill up with a blend far too potent for their machines.

Kiser: If they drive up to a pump and they see E-20, E-30, E-40, I don't think they know what that means. Even if they do know what it means -- that E-30 means 30 percent ethanol in the gallon they're producing -- if they are selling it at the pump, I think there is the assumption that it's OK, that it's going to work in whatever I put it in.

Ron Lamberty: That's kind of a moot point. We've already got those concerns.

Ron Lamberty works for the American Coalition of Ethanol. He points out that consumers are quite capable of telling the difference between diesel and regular fuel at the gas station. He says America's well on the road to using more renewable fuels like ethanol. Small engine manufacturers can either protest, he says, or start improving their products.

Lamberty: If we always listened to the naysayers, we would still be sitting here with leaded regular gasoline in the United States. We've got to move forward and the small engine guys have to come along.

Critics say they might come along more quickly if the science were more definitive. No one really knows exactly how sensitive small engines are to ethanol. The standard threshold for lawnmowers, for example, is 10 percent, but our experiment showed it could run on a much richer mixture.

The Department of Energy published a study on ethanol in small engines this fall. You can check just how deadly the fuel might be to your old John Deere.

In Laramie, Wyo., I'm Peter O'Dowd for Marketplace.