Why you can’t use E15 in your lawnmower

Mechanic Tim Brodd repairs small engines at his shop in Lincon, Neb. (Grant Gerlock/Harvest Public Media)
Mechanic Tim Brodd repairs small engines at his shop in Lincon, Neb. (Grant Gerlock/Harvest Public Media)

The future of E15, gasoline with 15 percent ethanol instead of the more common 10 percent, is a flash point between ethanol critics and supporters. But both groups agree on at least one thing: don’t put E15 in your lawn mower.

That’s a boon to much of the fuel industry, which is arrayed in opposition to the widespread adoption of the fuel.

Even biofuels groups will tell you running a small engine on E10 is fine, but using E15 actually breaks the law. The EPA approved E15 last summer, but only for cars and trucks made in 2001 or later.  

But why? It turns out that extra 5 percent ethanol makes a big difference when you’re talking about small engines, the kind find you find on many boats, motorcycles and outdoor implements.

Let’s start with E10. Check your warranty, but Tim Brodd of Brodd Small Engine Repair in Lincoln, Neb., said you’ll probably find using E10 is acceptable. Still, he said, there is some risk.

“Ethanol has alcohol in it. Alcohol absorbs water,” said Brodd, who repairs and sells mowers and other equipment at his shop. “And if that fuel is not rotated, that water just sitting there will cause corrosion issues over time.”

In that case, Brodd said, it’s really the water that causes the damage by corroding fuel lines or the carburetor, the part of the engine where fuel mixes with air to create the volatile mix that is compressed and exploded in the piston cylinder. So if you’re laid back about keeping the grass short or only pull the snow blower out a couple times per year, you could end up with a bad carburetor and a bill from your mechanic.

“Just in parts on some of these machines you could spend upwards of $150-200 just for the carburetor itself,” Brodd said.

If you’re careful and follow proper maintenance E10 works. Add another 5 percent of ethanol, though, and things change.

It goes back to the carburetor, which creates the mixture of fuel and oxygen from the air going into the engine. In most small engines the ratio of fuel to oxygen is fixed. If it is heavy on fuel it’s called “rich.” If it’s heavy on oxygen it’s called “lean.” Simply by using more ethanol, more oxygen is added to that mix.

“It’s as if you were pumping more and more air for the same amount of fuel, which is a lean condition,” said Scott Frazier, a professor of biosystems and agricultural engineering at Oklahoma State University.

An engine running lean runs faster and hotter.

“If it runs too hot you can get into mechanical issues,” Frazier said.

Cars with fuel injection don’t have that problem. Whereas small engines are generally resigned to a fixed ratio of fuel to air, Frazier said, fuel injection and automatic timing adjustments allow cars to compensate for higher amounts of ethanol in fuel and avoid mechanical breakdowns.

Small engine makers may eventually redesign engines to accommodate E15. Until then, their advice is to keep it away from your mower and that may lead some to second guess putting it in their car.

“The small engine argument is a pretty big one for people arguing against increasing ethanol blends,” said Frazier.

It’s tangible evidence that E15 just isn’t the same as E10.

With E15 still only available in pockets, it appears small engine manufacturers are satisfied to work against E15 and leave consumers to work around it.