Why Gasoline Has Ethanol, And How It Affects Your Engine

Folks have been experimenting with ethanol in modern combustion engines even when they weren't so modern. Nicolaus Otto, the inventor of the traditional four-cycle engine, used it to power one of his first motors all the way back in 1876. It was also a popular performance-boosting additive to gasoline in the 1920s and 1930s. However, the current interest in ethanol dates to the 1970s, when gas shortages and environmental concerns had people looking for ways to address both.

At that point, ethanol was being touted as a cleaner-burning fuel than gasoline, made from renewable sources like corn that was grown right here in the United States, so we didn't have to rely on foreign countries for our energy needs. Those claims have become a lot more contentious nowadays, though, with a lot of people now believing ethanol fuel is a total fraud. We won't necessarily go that far, but it does seem like you can't mix gasoline and ethanol without also mixing facts and feelings.

Let's look a little more carefully at the key reasons ethanol is added to gasoline, starting with its use as a way to prevent early combustion, e.g., engine knocking. Right off the bat, most U.S. gasoline is a mixture of 90% gas and 10% ethanol to help with that issue, taking advantage of ethanol's higher octane rating. It's true that ethanol actually contains less energy per gallon than gasoline, so the more you add the less efficient your engine runs, but that's usually countered by ethanol's lower cost at the pump.

Ethanol is also added to gasoline for environmental reasons. For instance, ethanol can burn cleaner and more completely than gasoline, leaving behind fewer greenhouse gases to contribute to air pollution. Still, things get a little murky when you try to look at the big picture. Some scientists say that if you also take into account emissions produced by transforming corn into ethanol, the fuel ends up making more emissions than gasoline. Others – albeit those with a stake in the matter — say the opposite.

Plus, most ethanol is derived from corn, which is relatively easy to regrow and renew. You can't farm a crop of gasoline by planting old dinosaurs, although you can't feed hungry people with corn turned into ethanol, either.

Whatever the effects of ethanol on the world around us, its effects on your motor are a little more clear. Most modern cars built from the 2000s onward are specifically engineered to run on fuel that's 90/10 gas/ethanol. After all, that's what you'll find at the pumps.

Older engines can face problems based on ethanol's chemistry. One consideration is that ethanol is a solvent for untreated rubber and plastic, which is often used for hoses and seals in older engines. This puts parts at risk for failure. In addition, ethanol has a tendency to absorb water right out of the air, introducing a key rust and corrosion promoter directly into the engine and fuel system. 

Needless to say, using E85 fuel — a mix of 85% ethanol and 15% gasoline — ups the ante all around. The higher levels of ethanol emphasize its positive effects but require higher levels of protection against its negative effects. In fact, you're not supposed to use E85 in anything but a flex fuel vehicle (FFV), although with the right modifications you actually can get some vintage cars to run on E85 fuel.

FFVs were pretty popular in the early to mid 2000s, with about 20 million still on the road today. Yet only 10 new models are certified as flex-fuel-capable for 2025, ranging from the underrated Buick Encore GX small crossover to large Ford Super Duty F-450.