Petroleum products normally used as fuel for diesel engines are distillates composed of heavy hydrocarbons, with at least 12 to 16 carbon atoms per molecule. These heavier distillates are taken from crude oil after the more volatile portions used in gasoline are removed. The boiling points of these heavier distillates range from 177 to 343 °C (351 to 649 °F). Thus, their evaporation temperature is much higher than that of gasoline, which has fewer carbon atoms per molecule. In the United States, specifications for diesel fuels are published by the American Society of Testing and Materials (ASTM). ASTM D975 “Standard Specification for Diesel Fuel Oils” covers specifications for five grades of diesel fuel oils:

Water and sediment in fuels can be harmful to engine operation; clean fuel is essential to efficient injection systems. Fuels with a high carbon residue can be handled best by engines of low-speed rotation. The same applies to those with high ash and sulfur content. The cetane number, which defines the ignition quality of a fuel, is determined using ASTM D613 “Standard Test Method for Cetane Number of Diesel Fuel Oil.”