Standard diesel fuel does not contain ethanol. This confusion often arises because ethanol is a widely used additive in gasoline, blended to create fuels like E10 or E85. The fundamental differences between diesel and gasoline engines make ethanol unsuitable for the diesel fuel supply chain. Ethanol’s chemical properties are incompatible with the requirements of a diesel engine’s operation and its sensitive fuel system components.
Standard Diesel and Ethanol: The Chemical Incompatibility
The primary reason ethanol is excluded from diesel is the difference in how the two engine types ignite fuel. Diesel engines are compression-ignition engines. They compress only air, which heats up significantly, and then inject the fuel. The fuel must auto-ignite immediately upon contact with the hot air.
Diesel fuel quality is measured by its cetane number, which indicates the fuel’s ignition delay time. Standard diesel fuel typically has a cetane number near 50. Ethanol, however, has an extremely low cetane number, often ranging from 5 to 15. Adding ethanol significantly lowers the overall cetane rating, causing a longer ignition delay, poor combustion, and potential engine damage.
Another technical barrier is ethanol’s hygroscopic nature; it readily absorbs and holds water. Diesel fuel systems are not designed to handle significant amounts of water. Ethanol can cause phase separation, where the water-ethanol mixture separates from the diesel fuel. This separated layer leads to corrosion and microbial growth in the fuel tank, potentially drawing water into the engine’s sensitive high-pressure components.
Clarifying the Confusion: Ethanol vs. Biodiesel
The misconception that diesel contains ethanol often stems from the widespread use of another biofuel: biodiesel. Ethanol is a simple alcohol produced primarily through the fermentation of sugars or starches. Biodiesel, however, is a chemically distinct compound known as Fatty Acid Methyl Ester (FAME).
Biodiesel is manufactured through transesterification, reacting vegetable oils, animal fats, or recycled cooking grease with an alcohol and a catalyst. The resulting fuel is chemically similar to petroleum diesel and is fully compatible with modern diesel engines. Biodiesel is commonly blended into petroleum diesel to meet renewable fuel standards.
These blends are designated with a “B” number, such as B5 (up to 5% biodiesel) or B20 (up to 20% biodiesel). Biodiesel is valued for its renewable source and its ability to improve the lubricity of ultra-low sulfur diesel fuel. The presence of this common biofuel additive, FAME, often leads consumers to mistakenly believe that ethanol is also present in their diesel.
The Dangers of Ethanol in a Diesel System
Introducing ethanol into a diesel fuel system can lead to severe operational and mechanical problems. Diesel fuel is formulated to provide necessary lubrication for the high-precision components of the fuel system, particularly the fuel pump and injectors. Ethanol is a poor lubricant and significantly lowers the fuel’s viscosity, which can cause premature wear and failure of these expensive parts.
The chemical properties of ethanol also pose a threat to the system’s non-metallic components. Ethanol acts as a solvent that can degrade certain seals, gaskets, and plastic materials used in older or non-compatible diesel fuel lines and pumps. This degradation can lead to leaks and the introduction of debris into the fuel stream, causing further damage.
The poor combustion caused by low cetane results in a loss of power, excessive engine noise, and potentially catastrophic engine damage over time. The presence of water, which ethanol attracts, also accelerates corrosion and can lead to fuel filter icing in cold weather.