Ethanol is an alcohol-based fuel derived primarily from plant biomass, such as corn, sugarcane, or cellulosic materials. It is widely used as an additive to gasoline to improve performance and reduce emissions. The octane rating measures a fuel’s ability to resist premature ignition, commonly called “knocking” or “pinging.” When a fuel-air mixture ignites from compression before the spark plug fires, it creates a pressure wave that can damage the engine. The octane number quantifies the fuel’s resistance to this uncontrolled combustion event.
Octane Rating of Pure Ethanol
Pure, denatured ethanol possesses an exceptionally high octane rating, far exceeding that of standard petroleum gasoline. This rating is expressed using two primary metrics: the Research Octane Number (RON) and the Motor Octane Number (MON). The RON measures knock resistance under mild engine conditions and is typically 113 to 116. The MON evaluates performance under severe, high-speed, and high-temperature conditions, and is lower, often falling between 90 and 100. In the United States, the Anti-Knock Index (AKI) is displayed on the fuel pump, calculated as the average of the RON and MON, or (R+M)/2. The AKI for pure ethanol is generally around 100.
How Ethanol Increases Fuel Octane
Ethanol significantly boosts the overall octane rating of gasoline through thermal and chemical effects. The primary mechanism is its high latent heat of vaporization. When liquid ethanol converts to a vapor inside the engine, it absorbs a substantial amount of heat from the surrounding air and fuel mixture. This absorption effectively cools the intake charge, resulting in a denser air-fuel mixture entering the combustion chamber.
The cooler, denser mixture is less susceptible to autoignition from compression, which increases knock resistance. The chemical structure of ethanol also contributes by acting as an oxygenate, modifying the combustion process and suppressing pre-ignition tendencies.
When mixed with lower-octane gasoline, ethanol’s ability to suppress knock is disproportionately high compared to its volume percentage. For instance, a small 10% addition of ethanol can raise the overall blend’s AKI by several points. This blending effect makes ethanol an efficient octane booster for refiners.
Practical Effects of Ethanol Fuel Blends
The majority of gasoline sold in the U.S. is a blend known as E10, containing 10% ethanol by volume. This blend provides a slight octane boost over pure gasoline while remaining compatible with nearly all modern vehicles. A specialized blend is E85, containing up to 85% ethanol, designed for use in Flex Fuel Vehicles (FFVs).
Ethanol blends result in a reduction in the fuel’s energy density compared to pure gasoline. Ethanol contains approximately 33% less energy per unit volume than gasoline, meaning that more fuel must be injected to achieve the same energy output. For E10, the impact on fuel economy is minimal, typically resulting in a decrease of only a few percent.
The high-octane nature of E85 allows FFVs and performance-tuned engines to take advantage of the fuel’s knock resistance. Engine control units (ECUs) in these vehicles can advance ignition timing and increase turbocharger boost pressure, which improves power and thermal efficiency. The cooling effect from ethanol’s high latent heat of vaporization aids performance by keeping the combustion chamber cooler under high-load conditions.
Compatibility concerns exist regarding ethanol with certain vehicle components, particularly in older models manufactured before the year 2000. Ethanol is hygroscopic, meaning it readily absorbs water, and the presence of water can lead to corrosion in steel fuel tanks and lines that were not designed for it. High ethanol concentrations can also degrade specific older materials, such as certain types of natural rubber seals and plastic components. Modern vehicles and all FFVs are built with ethanol-resistant materials to prevent these issues.