Mixing ethanol-blended gasoline, typically E10 (10% ethanol), with non-ethanol gasoline is a common question for small engine owners, especially for lawn mowers. Ethanol is an alcohol blended into fuel as an octane enhancer and oxygenate to reduce emissions. Non-ethanol gasoline, often called pure gas, contains no alcohol additive and is favored for its stability in small engines. Understanding how these two fuels interact is important for maintaining engine health and performance.
Understanding Ethanol’s Impact on Small Engines
Ethanol’s chemical properties introduce several challenges for small engine fuel systems. A primary concern is its hygroscopic nature, meaning ethanol readily absorbs moisture from the air. This water absorption can lead to rust and corrosion within metal components like the fuel tank and carburetor.
When the fuel reaches a saturation point, phase separation occurs. The water and ethanol separate from the gasoline and sink to the bottom of the tank. This water-ethanol mixture is then drawn into the engine, causing poor combustion, difficult starting, or engine failure.
Ethanol also acts as a solvent, which can degrade materials like rubber, plastic seals, and gaskets in older small engines. Furthermore, the lower energy content of ethanol compared to gasoline can cause the engine to run leaner, potentially resulting in higher operating temperatures. While modern small engines operate with up to E10 fuel, these properties still pose a risk during periods of infrequent use.
Effects of Combining Ethanol and Non-Ethanol Fuels
Mixing ethanol-blended fuel, like E10, with pure non-ethanol gasoline is chemically possible and will not cause immediate engine damage. The primary result is the dilution of the ethanol content in the fuel supply. For example, combining equal parts of E10 and non-ethanol gasoline results in a blended fuel that is approximately E5 (5% ethanol).
This dilution is beneficial because it mitigates risks associated with higher ethanol concentrations. A lower ethanol percentage reduces the fuel’s capacity to attract moisture, slowing phase separation and corrosion. Adding non-ethanol fuel to an E10-fueled engine is a practical way to lower the overall alcohol content without draining the tank first.
However, the risk of water absorption and component damage is reduced but not eliminated, as ethanol can still attract moisture over time. Mixing the fuels creates a compromise, not a solution for long-term storage. Pure non-ethanol fuel remains the superior choice for equipment that sits idle for extended periods.
Fuel Handling and Storage Best Practices
Preventative maintenance and proper fuel management are the most effective ways to protect a lawn mower engine, regardless of the fuel blend used. Since gasoline begins to degrade in as little as 30 days, using fresh fuel is recommended. Purchase only the amount of fuel that can be used within a 30-day period.
For any fuel stored beyond this timeframe, even pure gasoline, a quality fuel stabilizer should be added according to instructions. Stabilizers help prevent the formation of gum and varnish deposits that can clog the carburetor and fuel lines. For seasonal storage, never leave fuel in the mower’s tank over the winter.
Before storing the equipment for more than 90 days, the fuel system should be completely drained, or the engine should be run until it exhausts all the fuel in the tank and carburetor bowl. This prevents volatile components from evaporating and leaving behind sticky residues that cause starting problems. Storing fuel in an approved, sealed container away from direct sunlight and heat also helps maintain its integrity.