Many of the world’s spec diesel and pieces of heavy equipment run on diesel engines. These engines, and the fuel that powers them, need to adhere to specific specifications. Spec diesel refers to the properties of a petroleum-derived diesel fuel that has been engineered for use in diesel engines. These are properties related to air pollution regulations and other performance related requirements demanded of the fuel for diesel engines. The fuel is also subject to composition specifications.

Diesel is a complex mixture of hydrocarbons, including paraffins, naphthenes, aromatics and in small concentrations, olefins. The complexity of the hydrocarbons results in a broad range of property limits. However, the limits do not determine a precise distribution of hydrocarbons by class and size. The most important characteristics of a diesel fuel are its vapor point (temperature at which it turns from a liquid to a gas), flash point (temperature at which it will catch fire) and water content (how much microscopic entrained water it has).

Decoding Specialized Diesel: Navigating the World of Spec Diesel Engines

The fuel’s chemical properties are specified in ASTM D975-22: Standard Specification for Diesel Fuel Oil. The physical characteristics of diesel fuel are described in BS 2869:2010 Classes A2 and EN 590. The fuel is referred to as red diesel in the United Kingdom and other parts of Europe. It is dyed red for identification, and its use in a taxed vehicle or for nontaxed purposes such as home heating, agriculture or refrigeration on vans/trucks carrying perishable goods can result in fines.