Diesel Fuel

Diesel fuel is a liquid energy source formulated for use in diesel engines, a class of internal combustion engines in which fuel ignition occurs by compression rather than by an electric spark. Because combustion occurs when the compressed air in the cylinder reaches a temperature sufficient to ignite the injected fuel, diesel fuels must possess reliable compression ignition characteristics. Petroleum-derived diesel remains the most widely used form, though alternatives such as biodiesel, biomass-to-liquid (BTL) diesel and gas-to-liquid (GTL) diesel have gained prominence as cleaner or renewable substitutes. In many parts of the world, modern diesel fuel is subject to strict standardisation, ensuring consistent quality and low sulphur content.
Diesel is one of the highest-volume outputs of petroleum refineries. It is widely used in transportation, industry and marine applications because of its high energy density, efficient combustion and suitability for heavy-duty engines. Diesel exhaust, however, particularly from older engines without emission controls, has been associated with adverse health effects.

Names and Regional Terminology

Although commonly referred to simply as diesel, the fuel has several regional designations. In the United Kingdom, “white diesel” denotes road-legal diesel, distinguishing it from “red diesel”, a lower-tax, dyed variant for agricultural or off-road use. The formal term for road diesel is DERV (diesel-engine road vehicle). In Australia the term distillate is sometimes used, while in regions of the Middle East and Indonesia the fuel is widely known as Solar, a name historically associated with Pertamina, the Indonesian national petroleum company. The French term gazole (gas oil) is also used in several countries.

Historical Development

Diesel fuel emerged from the experiments of Rudolf Diesel, who invented the compression-ignition engine in the 1890s. Diesel initially theorised that his engine would run on virtually any combustible material, regardless of its physical state. Early tests involved crude oil, petrol, kerosene, lamp oils and ligroin, all of which proved viable to varying degrees. Crude oil, while functional, was overly viscous, leading Diesel to favour kerosene for most developmental work.
By the turn of the twentieth century, Diesel engines had demonstrated robust performance on various fuels including paraffin oil, gasoline, fuel oil and even illuminating gas. At exhibitions in 1900 and 1911, demonstration engines ran successfully on peanut oil without modification, showcasing the flexibility of the design. While Diesel considered the potential of coal-dust engines, suitable coal powders were not commercially available at the time, and early trials led to rapid mechanical failure.
Before fuel standardisation, many diesel engines operated on low-cost petroleum distillates or coal-tar derived oils; these were economical because they were lightly taxed or considered industrial by-products. Early vehicle engines, such as the Mercedes-Benz OM 138 of the 1930s, demanded higher-quality fuels with more predictable ignition behaviour. After the Second World War, the first modern diesel standards appeared (for example, DIN 51601 in Europe), culminating in today’s EN 590 specification used across the European Union.
Marine vessels, which adopted large diesel engines widely after the fuel crises of the 1970s, often continue to use heavy fuel oils (“bunker” fuels) instead of standard automotive diesel due to cost efficiency.

Types of Diesel Fuel

Petroleum DieselConventional diesel is obtained from crude oil via fractional distillation, typically in the boiling range corresponding to hydrocarbons with 9–25 carbon atoms. The distillate is subsequently treated by hydrodesulphurisation to reduce sulphur levels. Because straight-run diesel alone cannot meet demand or specifications, refineries blend in components produced by cracking and hydrocracking heavier fractions. Additional kerosene may be added to adjust viscosity, especially in colder climates.
The introduction of ultralow-sulphur diesel (ULSD), now standard in Europe and North America, has substantially reduced sulphur oxide emissions and allowed the adoption of modern exhaust-aftertreatment technologies.
Synthetic DieselSynthetic diesel can be produced from natural gas, coal or biomass via gasification to synthesis gas (syngas) followed by the Fischer–Tropsch process. The resulting fuel is dominated by paraffinic hydrocarbons and contains virtually no sulphur or aromatics, enabling clean combustion and blending compatibility with petroleum diesel.
BiodieselBiodiesel consists primarily of fatty acid methyl esters (FAME), produced through transesterification of vegetable oils or animal fats with methanol. Feedstocks may include soybean oil, palm oil, rapeseed oil, waste cooking oil or tallow. Biodiesel can be used in pure form (B100) or blended with petroleum diesel (commonly B5–B20). Its renewable origin and lower particulate emissions make it an important alternative, though cold-flow properties and oxidation stability require careful management.

Properties and Performance Requirements

Diesel fuels must possess appropriate ignition quality, often expressed as the cetane number, which reflects combustion readiness under compression. Additional performance criteria include viscosity, density, lubricity, volatility and low-temperature behaviour. Modern standards mandate minimal sulphur content to support catalytic aftertreatment systems.
Cold climates require diesel fuels formulated with anti-gel additives or kerosene blending to prevent wax formation. Lubricity agents are often added to ULSD to compensate for reduced natural lubrication caused by desulphurisation processes.

Applications

Diesel fuels power a wide array of engines and machinery including heavy trucks, buses, agricultural vehicles, construction equipment, power generators and marine vessels. High compression ratios and lean combustion enable diesel engines to achieve superior fuel efficiency compared with spark-ignition engines. Diesel can also serve as a combustion source in non-diesel devices, such as certain external-combustion engines and boilers.

Environmental and Health Considerations

While diesel combustion is efficient, exhaust emissions contain nitrogen oxides (NOₓ) and particulate matter, both associated with respiratory and cardiovascular health risks. Regulatory frameworks in many regions require particulate filters, selective catalytic reduction systems and low-sulphur fuels to mitigate these impacts. Continued development of synthetic fuels, biodiesel and advanced aftertreatment technologies contributes to reductions in environmental footprint.