Diesel fuel typically does not contain ethanol, marking a significant difference from the gasoline sold across many regions. While gasoline is frequently blended with up to 10% ethanol (E10) to meet renewable fuel standards and boost octane, the same approach is avoided with diesel. Ethanol is incompatible with the physical and chemical requirements of a diesel compression-ignition engine and its specialized fuel system. The inclusion of ethanol in commercially available diesel fuel is therefore intentionally restricted to prevent operational problems and component failure.
Why Ethanol and Diesel Don’t Mix
The primary reason ethanol is excluded from standard diesel fuel blends is a fundamental difference in their molecular structures. Diesel fuel is a hydrocarbon mixture composed of non-polar molecules, which means the electrical charges are evenly distributed across the molecule. Ethanol, conversely, is an alcohol with a polar hydroxyl group that gives the molecule a slight positive and negative charge at opposite ends.
This polarity difference makes ethanol and diesel fuel largely immiscible, similar to how oil and water do not mix. If ethanol is introduced into diesel, the blend is highly susceptible to phase separation, especially when temperature decreases or water is present. The ethanol will physically separate from the diesel base, sinking to the bottom of a fuel tank or storage container and creating a distinct, highly corrosive layer. This chemical instability is the root of the mechanical issues that arise when ethanol contaminates a diesel system.
Engine and System Damage Caused by Ethanol
The presence of ethanol in diesel fuel introduces several severe operational risks, the most immediate being a dramatic reduction in fuel lubricity. Diesel fuel is engineered to lubricate the moving parts of the fuel system, particularly the high-pressure injection pump and the injectors. Ethanol acts as a solvent and strips away the necessary lubricating film provided by the diesel, which can lead to premature wear.
The loss of lubricity causes rapid metal-on-metal contact within the precision-machined components of the injection system. This accelerated wear can result in scoring, seizing, and catastrophic failure of the high-pressure injection pump, leading to costly repairs and significant downtime. Modern high-pressure common rail (HPCR) systems, which operate at extreme pressures, are particularly vulnerable to this type of damage due to their tight tolerances.
Ethanol is also highly hygroscopic, meaning it readily absorbs and attracts water from the atmosphere. Even small amounts of water in a diesel-ethanol blend can trigger the phase separation process, causing the water-rich ethanol layer to drop out of the fuel. This separated water is highly corrosive and quickly leads to rust and pitting on metal fuel tank walls, lines, and internal pump components, compromising the integrity of the entire system.
The accumulation of water at the tank bottom also provides an environment for microbial growth, often referred to as “diesel bug.” These organisms thrive at the fuel-water interface, forming sludge and slime that rapidly clog fuel filters and lines, severely restricting engine performance and potentially causing complete shutdown.
Furthermore, ethanol’s solvent properties are aggressive toward the plastic, rubber, and composite materials used in fuel lines, gaskets, and seals. Exposure to ethanol can cause these components to swell, degrade, or crack, which compromises the integrity of the entire fuel delivery system.
Common Blends and Additives Used in Diesel Fuel
Since ethanol is incompatible with diesel fuel, the industry utilizes different, compatible oxygenated compounds and additives to enhance performance and meet renewable standards. The most common alternative is the use of biodiesel, which is an oily ester derived from vegetable oils or animal fats, rather than an alcohol. Biodiesel is frequently blended with petroleum diesel at levels like 5% (B5) or 20% (B20) to reduce reliance on petroleum sources and lower carbon emissions.
These biodiesel blends are compatible with most diesel engines and offer the added benefit of improved lubricity, offsetting the natural reduction in lubrication that occurred with the introduction of ultra-low sulfur diesel (ULSD). Other additives are routinely included to fine-tune the fuel’s performance characteristics. For instance, cetane improvers, typically alkyl nitrates, are added to reduce the ignition delay period, leading to smoother combustion and better cold-start performance.
Winterizing agents, or anti-gelling additives, are another common component, especially in cold climates. Diesel fuel contains paraffin wax molecules that can crystallize and cause the fuel to “gel” at low temperatures, clogging the fuel filter. These additives modify the structure of the wax crystals to prevent them from agglomerating, which maintains fuel flow during cold operation. Additional additives, such as detergents, corrosion inhibitors, and stabilizers, are used to keep the fuel system clean and protect against degradation during storage.