The Downside of Ethanol in Gasoline

Ethanol blends can lower fuel economy, complicate storage, and cause material and water-related issues—especially in older vehicles, small engines, and marine equipment—while offering mixed environmental and economic trade-offs. Ethanol’s benefits (higher octane, some tailpipe pollutant reductions) are real, but its drawbacks include reduced miles per gallon, hygroscopicity leading to phase separation, potential corrosion and seal degradation in incompatible systems, seasonal volatility challenges, and ongoing debates over lifecycle emissions and land use. Below is a detailed look at what those downsides mean in practice.

Engine and Fuel-System Impacts

Ethanol changes how fuel behaves in tanks, lines, and combustion, which can introduce reliability issues if equipment isn’t designed or maintained for alcohol-blended fuels. The following points outline the main technical concerns seen across vehicles, small engines, and marine applications.

• Lower energy content: Ethanol has about 76,000 BTU/gal vs. ~114,000 BTU/gal for pure gasoline. Typical penalties are roughly 3–4% lower MPG with E10, ~5% with E15, and 25–30% with E85 (vehicle-dependent).
• Hygroscopicity and phase separation: Ethanol absorbs water from air and storage systems. At sufficient water uptake (around 0.5% by volume for E10, temperature-dependent), water and ethanol can separate from gasoline, causing poor running, stalling, or no-start—especially after long storage or in humid/marine environments.
• Material compatibility: Ethanol can swell, harden, or crack certain elastomers and plastics (older nitrile rubber hoses, gaskets) and can accelerate corrosion in metals in the presence of water (e.g., zinc, brass, some aluminum alloys). Modern vehicles are built for E10; older vehicles, boats, and many small engines may not be.
• Solvent effect and debris mobilization: Ethanol can loosen varnish, rust, and deposits, clogging filters and injectors after switching fuels or in neglected systems.
• Cold-start challenges: Ethanol’s higher heat of vaporization and different volatility can hinder cold starts, most notably with high-ethanol fuels like E85 in cold climates (modern closed-loop controls mitigate this in flex-fuel vehicles).
• Evaporative emissions and volatility: Adding ethanol can raise fuel vapor pressure at low blend levels unless the base gasoline is adjusted, increasing evaporative emissions if not properly managed. Regulations and fuel formulations aim to control this, but seasonal variability remains a concern.
• Misfueling risks: E15 is approved by the U.S. EPA for 2001+ light-duty vehicles but is not approved for many small engines, motorcycles, and marine engines. Using higher-ethanol fuel in non-approved equipment can void warranties and cause damage.

Taken together, these engineering factors explain why ethanol-blended gasoline works well in most modern road vehicles yet continues to cause reliability and maintenance issues in legacy systems, small engines, and boats without ethanol-compatible components and storage practices.

Storage and Seasonal Use

Storage stability is a common pain point. Ethanol blends are more sensitive to moisture and time, which affects users who store fuel or equipment seasonally.

• Shorter shelf life: Unstabilized E10 can degrade noticeably in 30–90 days, especially with heat and humidity. Oxidation and water uptake increase gum formation and phase-separation risk.
• Marine/small-engine sensitivity: Ventilated tanks and humid environments accelerate moisture ingress. Phase-separated fuel can cause lean running, detonation, or corrosion.
• Seasonal volatility rules: In summer, volatility limits tighten to control smog-forming emissions, which can constrain sales of certain blends (e.g., E15) absent regulatory waivers or region-specific rules. Recent years have seen temporary waivers and state-level actions, but availability still varies by season and location.

For users with seasonal equipment or stored fuel, ethanol’s moisture affinity and shorter stability window can translate into hard starts, fuel-system cleaning needs, and more frequent maintenance.

Environmental and Health Trade-offs

Ethanol’s environmental profile is complex: it can reduce some pollutants but introduce others, and its overall climate impact depends on feedstocks, farming practices, and land use. The following points summarize key concerns often cited by researchers and regulators.

• Lifecycle greenhouse gases: Corn ethanol’s net GHG emissions vary widely. Gains from displacement of gasoline can be offset by fertilizer use, process energy, and land-use change. Some analyses find modest GHG benefits; others show parity or higher emissions versus gasoline when land conversion is included.
• Air toxics and ozone formation: Ethanol blending generally lowers carbon monoxide and some aromatics (e.g., benzene), but increases emissions of acetaldehyde and can influence ozone formation depending on volatility management and local atmospheric chemistry.
• Water quality and land impacts: Intensive corn production is linked to nutrient runoff (nitrogen, phosphorus), contributing to algal blooms and hypoxia in waterways, plus soil erosion and biodiversity loss where land is converted to monoculture.
• Evaporative emissions: Without proper control of fuel vapor pressure and vehicle evaporative systems, ethanol blends can increase evaporative hydrocarbon emissions, which contribute to smog.

While advanced, waste-based, or cellulosic ethanols can improve the environmental picture, today’s large-scale corn ethanol presents mixed outcomes that hinge on agricultural practices and regional policies.

Economic and Policy Considerations

Ethanol’s market presence reflects policy mandates and credits, which affect prices at the pump and total cost per mile. These points explain the budget and policy angles consumers encounter.

• Cost per mile: Ethanol-blended fuels are often cheaper per gallon but can cost more per mile due to lower energy content—particularly true for E85 unless priced with a steep discount.
• Mandates and credits: In the U.S., the Renewable Fuel Standard (RFS) requires blending renewable fuels. Additional incentives and low-carbon fuel programs in some regions further shape supply, demand, and pricing.
• Fuel availability and logistics: Infrastructure for blending, transporting, and dispensing higher-ethanol fuels is uneven, affecting price stability and access, especially outside the Midwest.

These market dynamics mean drivers may see attractive pump prices for ethanol blends but should weigh energy-adjusted costs and local availability, which can change seasonally and by region.

When Ethanol Works Well—and How to Mitigate Downsides

Ethanol’s drawbacks are context-dependent. Modern vehicles handle E10 reliably, and drivers can minimize issues with a few practical steps.

• Use the right blend for the equipment: Follow the owner’s manual. Avoid E15/E85 in non-approved engines. For small engines and boats, consider ethanol-free gasoline where available.
• Manage storage: Buy fresh fuel, use airtight containers, add fuel stabilizer for storage beyond 30–60 days, and keep tanks near full to reduce moisture ingress.
• Maintain fuel systems: Replace older rubber lines and gaskets with ethanol-rated materials; change filters after switching fuels to catch loosened deposits.
• Consider cost per mile: Compare MPG and price to decide if E10, E15 (where approved), or E85 in a flex-fuel vehicle is economical in your area.

Applied consistently, these practices preserve reliability while letting many drivers benefit from ethanol’s high octane and widespread availability.

Summary

Ethanol in gasoline lowers MPG, can absorb water and separate, and may stress incompatible materials—issues most visible in older vehicles, small engines, and marine settings. Environmental and economic impacts are mixed, varying with blend level, local regulations, and agricultural practices. For most modern cars, E10 is routine and reliable, but equipment-specific compatibility, storage habits, and cost-per-mile calculations are key to avoiding ethanol’s downsides.

What cars should not use ethanol gas?

The EPA says E15 can be used in flexible-fuel vehicles, as well as 2001 and newer cars, light-duty trucks, and medium-duty SUVs. Don’t use E15 in motorcycles or other small engines, heavy-duty trucks, or nonroad vehicles such as boats and snowmobiles.

What is the main disadvantage of using ethanol as a fuel?

One of the primary disadvantages of ethanol is that it is not as energy-dense as gasoline. This means that it has a lower energy content and is less efficient than gasoline. As a result, vehicles that use ethanol as a fuel source may not get as many miles per gallon as those that use gasoline.

Is gasoline with ethanol bad for your car?

Yes, ethanol-blended petrol (especially at higher concentrations like E20) can be harmful to some vehicles, particularly older ones, due to its corrosive nature, its ability to absorb moisture and cause rust, and its tendency to degrade rubber and plastic components. While modern vehicles are designed for these blends, older or non-compatible vehicles may experience fuel system damage, leaks, and clogs, requiring component replacement or resulting in performance issues. 
Why Ethanol is Harmful to Some Vehicles

• Corrosion and Component Degradation: Ethanol is a solvent and can corrode or degrade various plastic and rubber components commonly found in older fuel systems, such as seals, hoses, gaskets, and fuel lines. 
• Moisture Absorption: Ethanol is hygroscopic, meaning it readily absorbs water from the air. This moisture can lead to rust in fuel tanks and lines, and cause “phase separation,” where the water-ethanol mixture separates from the gasoline. 
• Fuel System Clogs: The loosened deposits from ethanol’s solvent action and the presence of moisture/rust can clog fuel filters and injectors, reducing engine performance. 
• Cold Start Issues: Ethanol has a higher heat of vaporization, which can make cold starts more difficult, especially in older, carbureted engines, leading to longer cranking times or the need for increased choke use. 

Older vs. Modern Vehicles

• Older Vehicles (Pre-E20 Capable): Opens in new tabThese vehicles are more vulnerable, as their fuel system components may not be compatible with the corrosive nature of ethanol blends. Prolonged use can lead to accelerated wear and require replacement of parts like rubber seals and gaskets. 
• Modern Vehicles: Opens in new tabVehicles specifically designed and calibrated for higher ethanol blends (like E20) generally perform well, though a slight reduction in fuel mileage may occur due to ethanol’s lower energy density. 

What You Can Do

1. Check Your Owner’s Manual: Refer to your vehicle’s owner’s manual to confirm if it’s designed to handle ethanol-blended fuel. 
2. Use Recommended Fuel: Stick to the fuel type recommended by the manufacturer to avoid potential damage and costly repairs. 
3. For Older Vehicles: If you own an older vehicle not designed for high ethanol blends, consider using ethanol-free petrol if available or be prepared for possible component wear and replacements during routine servicing. 

Is it better to use gasoline with or without ethanol?

Regular gasoline typically contains ethanol, usually as an E10 blend (10% ethanol, 90% gasoline), though some stations may offer ethanol-free gas or E15 blends. Ethanol is a plant-based additive that helps oxygenate the fuel and reduce air pollution, but it can also cause damage to older or small engines, such as lawnmowers and chainsaws. 
Why ethanol is in gas:

• Environmental benefit: Ethanol is a renewable fuel made from biomass, like corn, and it helps to oxygenate the fuel, leading to cleaner burning and reduced air pollution. 
• Octane booster: Ethanol can also be used to increase the octane rating of gasoline. 

Why ethanol might be avoided:

• Engine damage: Opens in new tabEthanol is corrosive and can degrade certain fuel system components in older cars and small engines. 
• Fuel system issues: Opens in new tabEthanol can absorb water, leading to issues with fuel lines and carburetors, especially in small engines that are not frequently used. 
• Lower fuel economy: Opens in new tabBecause ethanol has a lower energy content than gasoline, using blends can lead to slightly lower gas mileage. 

How to know what you’re getting:

• Check the pump: Look for “E” numbers, like “E10” or “E15,” which indicate the percentage of ethanol in the blend. 
• Use the owner’s manual: Always check your car’s or equipment’s owner’s manual for fuel requirements and recommendations. 
• Find ethanol-free gas: Resources like pure-gas.org can help you locate stations that sell ethanol-free fuel, which is often recommended for small engines and older vehicles.