Why Is Ethanol Added to Gasoline?

The Short Answer

Ethanol is added to gasoline in the US for three main reasons: it raises the fuel's octane rating, it adds oxygen that helps the fuel burn more completely, and federal law requires refiners to blend a growing volume of renewable fuel into the transportation supply. Most regular, midgrade and premium gasoline sold at US pumps contains some ethanol, typically labeled as containing up to 10 percent, a blend known as E10.

One qualification matters: the answer differs by blend. E10 is designed to work in essentially all gasoline vehicles on the road. Higher blends such as E15 and E85 are approved only for specific vehicles, so whether ethanol content is a non-issue or a real concern depends on which fuel you are buying and what you drive.

What Ethanol Is

Ethanol is an alcohol, the same basic compound found in alcoholic drinks, but fuel ethanol is denatured so it cannot be consumed. In the US it is produced mostly by fermenting corn, although other plant materials can be used. Because it comes from crops rather than crude oil, it is classed as a renewable fuel.

How Ethanol Raises Octane

Octane rating measures a fuel's resistance to knock, which is when the air-fuel mixture ignites on its own from heat and pressure before the spark plug fires at the intended moment. Knock can reduce performance and, if severe and sustained, damage an engine. US pumps display octane as the Anti-Knock Index, the number shown on the yellow label, which is the average of two laboratory test methods.

Pure ethanol has a much higher octane rating than ordinary gasoline. Blending it in therefore lifts the octane of the finished fuel. In practice, many US refiners produce a base gasoline with an octane rating below the pump grade it will eventually be sold as, then rely on the ethanol added at the distribution terminal to bring it up to the advertised rating. That means ethanol is often part of how your 87, 89 or premium fuel reaches its octane number, not just an additive on top of it.

Ethanol also has a cooling effect as it evaporates in the intake or cylinder, which further helps resist knock. Engine designers and tuners can take advantage of this, which is one reason high-ethanol fuels are popular in some performance applications.

Ethanol as an Oxygenate

Ethanol molecules contain oxygen, so blending ethanol into gasoline makes it an oxygenated fuel. The extra oxygen helps the fuel burn more completely, which tends to reduce carbon monoxide in the exhaust. This is why some US regions with air quality problems have required oxygenated gasoline, particularly in winter when carbon monoxide levels are a greater concern.

Before ethanol became the dominant oxygenate, many refiners used a compound called MTBE. MTBE was phased out in the US after it was found contaminating groundwater, and ethanol largely took its place.

The Policy Reason: The Renewable Fuel Standard

Beyond its chemistry, ethanol is in gasoline because federal policy requires it. The Renewable Fuel Standard, administered by the US Environmental Protection Agency, sets annual volumes of renewable fuel that must be blended into the nation's transportation fuel. Corn ethanol blended into gasoline is the main way those requirements are met. Supporters cite reduced reliance on imported oil and support for domestic agriculture as goals of the program.

Whether ethanol delivers a meaningful reduction in greenhouse gas emissions over its full lifecycle, from farming through production and combustion, is debated. Results depend on assumptions about land use, farming practices and production energy, and no specific figures were available for this article.

Common Ethanol Blends in the US

Blend | Ethanol content | Which vehicles can use it; E10 | Up to 10 percent | Practically all gasoline cars and light trucks; the standard US pump fuel; E15 | Up to 15 percent | Approved by the EPA for many newer light-duty vehicles; not approved for older vehicles, motorcycles, boats or small engines; E85 | High-level blend, up to 85 percent with seasonal variation | Flex-fuel vehicles only

What Ethanol Means for Your Vehicle

Ethanol contains less energy per gallon than gasoline. As a result, fuels with more ethanol generally deliver somewhat lower fuel economy. With E10 the difference is small enough that many drivers do not notice it; with E85 in a flex-fuel vehicle it is more pronounced. The exact change varies by vehicle, engine and driving conditions, and no measured figures were supplied here.

Ethanol also attracts and absorbs water. In fuel that sits for long periods, this can lead to phase separation, where a water-and-ethanol layer settles at the bottom of the tank. This is mainly a concern for seasonal equipment, boats, lawn equipment and stored vehicles rather than cars driven regularly.

Some older vehicles and small engines were built with fuel system materials, such as certain rubbers and plastics, that may not tolerate ethanol well. Modern vehicles are designed for at least E10.

Choosing the Right Fuel

Use the octane grade your owner's manual specifies; ethanol content does not change that recommendation. Treat E10 as the default safe choice for virtually any gasoline vehicle. Use E15 only if your vehicle is approved for it; check the owner's manual and the pump label. Use E85 only in a flex-fuel vehicle, which is typically identified in the manual, on the fuel door or by a badge. For small engines, marine engines and vehicles that will sit unused, follow the manufacturer's fuel guidance, which may favor ethanol-free fuel where it is available.

Bottom Line

Ethanol is in your gasoline because it is an effective, renewable way to raise octane, it adds oxygen that can reduce carbon monoxide emissions, and federal rules require renewable fuel to be blended into the supply. For most drivers using E10 in a modern car, it is a routine part of the fuel. The trade-offs, slightly lower energy content and sensitivity to water in stored fuel, matter most with higher blends, older equipment and engines that sit unused.