Why Are Biofuels Less Efficient?

Biofuels are often less efficient because they pack less energy per liter than fossil fuels, engines and supply chains aren’t always optimized to use them, and producing them can consume considerable energy—so vehicles typically travel fewer miles per gallon and the overall “well-to-wheel” conversion of sunlight or biomass into motion is relatively low. In practice, gasoline blended with ethanol yields 3–4% lower fuel economy at E10 and roughly 25–30% lower at E85, while biodiesel can reduce diesel mileage by about 5–10%, though there are notable exceptions and use cases where biofuels perform well.

What “efficiency” means in this debate

Efficiency can refer to several stages: tank-to-wheel (how far a vehicle goes on a liter), well-to-wheel (total energy from feedstock to motion), and land-use/solar-to-wheels (how effectively sunlight captured by plants ultimately turns into vehicle movement). Biofuels often lag on all three, mainly due to lower energy density, engine calibration limits in typical vehicles, and energy-intensive production and logistics. Still, specific technologies and use cases can narrow or reverse the gap.

Physical and chemical reasons

Lower energy density and oxygen content

Most common biofuels contain chemically bound oxygen, which reduces their lower heating value (LHV) compared with hydrocarbon-only fossil fuels. On a volumetric basis—the metric that matters to tanks and mileage—typical values are:

Below is a list that compares the volumetric energy content of common fuels, helping explain mileage differences in conventional engines.

• Gasoline: ~32 MJ/L (≈114,000 BTU/gal)
• Ethanol: ~21 MJ/L (≈76,000 BTU/gal)
• E85 (85% ethanol, 15% gasoline): ~23 MJ/L on average
• Petro-diesel: ~36 MJ/L (≈129,500 BTU/gal)
• Biodiesel (FAME): ~33 MJ/L (≈118,000 BTU/gal)
• Renewable diesel (HVO): ~34–35 MJ/L, close to fossil diesel

Because a liter of ethanol or FAME biodiesel carries less energy, you need more of it to do the same work, which typically shows up as lower miles per gallon unless an engine is redesigned to exploit specific properties like ethanol’s high octane.

Stoichiometry and engine calibration

Gasoline engines are calibrated around a stoichiometric air–fuel ratio near 14.7:1 by mass; ethanol’s is about 9:1. Blending ethanol requires more fuel mass per unit air to maintain proper combustion. Unmodified vehicles can’t fully leverage ethanol’s very high octane rating without higher compression or boost, so the volumetric energy penalty dominates. Similarly, biodiesel’s oxygen content lowers its heating value; while its high cetane improves ignition quality, typical fleet engines see modestly lower fuel economy.

Vaporization, cold starts, and water affinity

Ethanol vaporizes and ignites differently from gasoline and is hygroscopic (absorbs water), which can complicate cold starts and storage. These factors prompt conservative engine maps and distribution practices that, in aggregate, can erode real-world efficiency in legacy fleets. Biodiesel can have higher viscosity and poorer cold-flow characteristics unless carefully formulated, affecting atomization and combustion in some conditions.

Cetane, octane, and the important exceptions

Not all comparisons go against biofuels. Ethanol’s research octane number (RON) is typically above 108, enabling higher compression ratios and advanced turbocharging strategies; in engines designed for it, thermal efficiency can match or exceed gasoline despite lower energy per liter. Renewable diesel (HVO) closely mimics fossil diesel’s energy density and often improves combustion and emissions, narrowing efficiency gaps in modern compression-ignition engines.

System-level efficiency and energy return

Beyond what happens inside the cylinder, biofuels face energy and material losses across their supply chains: farming, harvesting, transport, fermentation or hydrotreating, distillation or upgrading, drying coproducts, and distribution (often via trucks due to ethanol’s water affinity). A common yardstick is EROI—energy return on energy invested.

The following list summarizes typical EROI ranges reported in recent literature and industry assessments, illustrating why system-level efficiency can be modest for some pathways.

• Corn ethanol (U.S.): ~1.2–1.6 (improves toward upper end with efficient plants and renewable process heat)
• Sugarcane ethanol (Brazil): ~3–8 (benefits from bagasse for process energy and high yields)
• Soy biodiesel (FAME): ~2–3
• Renewable diesel (HVO) from waste oils: ~2–5 (highly dependent on hydrogen source and plant efficiency)
• Cellulosic ethanol: ~2–3 when lignin supplies heat/power; commercialization remains limited
• Petroleum fuels (conventional): historically ~10–30; lower for oil sands and heavy oil

Lower EROI doesn’t automatically negate climate benefits, but it does mean more input energy is required to deliver a unit of fuel, which affects costs, emissions, and net energy available to society.

Land-use and photosynthetic conversion efficiency

Plants convert sunlight to chemical energy via photosynthesis with modest field-average efficiency—typically around 0.5–1% annually in temperate crops and up to ~1–2% for high-performing tropical crops like sugarcane. After harvesting and refining, and then burning the fuel in an engine at ~20–40% efficiency, the overall sunlight-to-wheels efficiency is low. In contrast, photovoltaics convert sunlight to electricity at ~20% module efficiency; when combined with power electronics, batteries, and efficient electric drivetrains, sunlight-to-wheels can reach ~10–15% in practice.

The list below offers indicative sunlight-to-wheels comparisons to illustrate land-use efficiency, recognizing wide variability by location and technology.

• Corn ethanol in a typical gasoline car: ~0.1–0.3% sunlight-to-wheels
• Sugarcane ethanol in an optimized spark-ignited engine: ~0.3–0.6%
• Cellulosic ethanol (future best cases): ~0.5–1%
• PV electricity in a battery-electric vehicle: ~8–15%

These orders of magnitude mean PV-to-EV pathways can be an order of magnitude or more (roughly 10–40×) more land-efficient than crop-based biofuels, a key reason biofuels are considered “less efficient” at converting natural resources into transportation work.

Variations across biofuels

“Biofuel” spans diverse chemistries and performance profiles. Some are closer to fossil fuels in energy content and compatibility, while others differ substantially.

The following list highlights how common biofuel types compare in efficiency-related characteristics.

• Ethanol: Low volumetric energy, very high octane; best when engines are designed for high compression/boost (e.g., dedicated E85 or flex-fuel performance calibrations)
• Biodiesel (FAME): Lower energy density, high cetane; minor fuel economy penalty but improved lubricity; cold-flow management needed
• Renewable diesel (HVO): “Drop-in” with energy content close to diesel; minimal efficiency gap in modern diesel engines
• Biomethane (upgraded biogas): Similar engine efficiency to fossil CNG; efficiency depends on upgrading and compression energy
• Sustainable aviation fuel (SAF): Engine efficiency similar to Jet A once in-spec; production energy and feedstock sourcing drive system-level efficiency

These differences matter: while the broad statement “biofuels are less efficient” is often true in conventional vehicles and systems, certain modern pathways and engine designs narrow the gap substantially.

Where biofuels can still make sense

Even with efficiency disadvantages, biofuels are valuable where batteries are heavy or charging is constrained, and where low-carbon drop-in liquids are essential.

The following list outlines situations where biofuels can be strategically efficient or practical.

• Aviation and long-haul marine: Liquid energy density and existing engines/infrastructure favor SAF and certain renewable diesel/marine fuels
• Heavy-duty on-road: HVO and high-biodiesel blends can decarbonize legacy fleets with minimal efficiency penalty
• Waste-to-fuels: Using residues (used cooking oil, tallow, agricultural and forestry wastes) improves EROI and carbon intensity
• Engine-optimized ethanol: High-compression, turbocharged, spark-ignited engines can exploit ethanol’s octane to recover efficiency
• Hybridization: Pairing biofuels with hybrid drivetrains reduces the volumetric energy penalty’s impact on real-world consumption

When targeted to hard-to-electrify uses and waste feedstocks, biofuels can complement electrification while moderating efficiency drawbacks.

Key numbers you can use

These benchmark figures help set expectations for efficiency outcomes in typical conditions.

• E10 vs gasoline: ~3–4% lower mpg; E85: ~25–30% lower mpg in unmodified vehicles
• B20 biodiesel blend: ~1–2% lower mpg; B100: ~5–10% lower mpg, engine-dependent
• Energy content (LHV): gasoline ~32 MJ/L; ethanol ~21 MJ/L; diesel ~36 MJ/L; biodiesel ~33 MJ/L; HVO ~34–35 MJ/L
• Typical engine efficiencies: spark-ignition ~25–40% peak (lower in real-world average); compression-ignition ~35–45% peak
• EROI ranges: corn ethanol ~1.2–1.6; sugarcane ethanol ~3–8; soy biodiesel ~2–3; HVO (waste oils) ~2–5; conventional petroleum ~10–30

Actual values vary with vehicle calibration, climate, driving cycle, blend quality, plant technology, and feedstock logistics, but these ranges capture mainstream experience.

Summary

Biofuels are generally less efficient because they carry less energy per liter, are often used in engines not optimized for their properties, and require substantial energy to produce and distribute. From a system perspective, photosynthesis-to-wheels conversion is inherently modest, making land-use efficiency lower than electricity from solar powering EVs. Yet “less efficient” does not mean “not useful”: renewable diesel, optimized ethanol engines, waste-based pathways, and applications like aviation and heavy-duty transport can harness biofuels’ strengths while mitigating their weaknesses.

What are five disadvantages of biofuels?

Disadvantages of biofuels

• Impact on drive units.
• Less energy efficiency.
• Increase in food prices.
• Risk to biodiversity.
• Water demand.
• Degradation of natural habitats.
• Technical problems.

Why are biofuels inefficient?

Biofuels allow us to turn sunlight into a liquid fuel, but the process is inefficient. Sunlight is a very diffuse resource; during photosynthesis plants only capture a small percentage of the sun’s energy as biomass.

Why are biofuels less energy efficient?

Biofuels could be C12H24O2 which is already partially oxidised due to the presence of the oxygen atom unlike a fossil fuel such as C12H26. Due to the biofuel being partially oxidised, it has less potential to further oxidise and will thus will generate a lower amount of energy per gram than a fossil fuel.

What is the main problem with biofuels?

A significant problem with biofuels is the competition for land and water resources with food production, which can lead to increased food prices, food insecurity, and deforestation to create new farmland for biofuel crops. Furthermore, biofuel production and burning can release air pollutants like ozone and nitrogen dioxide, posing risks to public health. 
Competition for Resources

• Food Prices and Security: Opens in new tabGrowing crops for biofuels requires vast amounts of land, water, and fertilizers, diverting these resources from food production. This reduced supply of food crops can drive up prices, threatening food security, especially in lower-income countries. 
• Deforestation and Habitat Loss: Opens in new tabTo expand biofuel crop cultivation, natural habitats like forests and grasslands are often cleared. This deforestation leads to habitat destruction, loss of biodiversity, and the release of stored carbon from trees into the atmosphere. 
• Water Strain: Opens in new tabThe large volumes of water needed for growing biofuel crops and refining them can deplete underground aquifers, which are also used by farmers for irrigation. 

Health and Environmental Impacts 

• Air Pollution: Opens in new tabThe burning of biofuels and emissions from biofuel refineries can release toxic particles, ozone, and nitrogen dioxide into the air. These pollutants can trigger respiratory illnesses like asthma and are associated with heart attacks, cancer, and premature death, particularly affecting vulnerable groups like the elderly and children. 
• Soil Degradation and Water Contamination: Opens in new tabThe intensive farming practices often used for biofuel crops, such as monoculture, can lead to soil degradation. Runoff from fertilizers and pesticides used to grow these crops can also pollute soil and water sources. 
• Greenhouse Gas Emissions: Opens in new tabWhile often presented as a carbon-neutral alternative, the process of producing and refining biofuels can result in significant greenhouse gas emissions, sometimes comparable to or even exceeding those of fossil fuels, especially when deforestation and land-use changes are involved.