Diesel fuel

(materials) Fuel used for internal combustion in diesel engines; usually that fraction of crude oil that distills after kerosine.

A broad mixture of hydrocarbons produced as distillates, as residual materials, or as blends of the two during the refining of crude petroleum. Diesel fuel usually has a distillation range of 390–715°F (200–380°C) and a specific gravity range of 0.760–0.935 [equivalent to 61.2–19.8° on the American Petroleum Institute (API) scale]. In addition to these properties, diesel fuel must have <1 wt % sulfur, <0.1 wt % ash, <0.5 vol % water and sediment, and a high flash point (greater than 131°F or 55°C).

Diesel fuel quality is defined by the cetane number, which usually falls into the range 30–60. A high cetane number indicates the potential for easy starting and smooth operation of the engine. The cetane number is the analog of the automobile engine octane number, with cetane (n-hexadecane, C₁₆H₃₄) having the arbitrarily assigned number of 100. At the other end of the scale, heptamethylnonane, an isomer of cetane, has the assigned cetane number of 0. The cetane number of a diesel fuel is determined by comparison with blends of cetane and heptamethylnonane. It corresponds to the number of parts by volume of cetane in a cetane-heptamethylnonane blend which has the same ignition quality as the fuel. See also Cetane number.

The American Society for Testing and Materials (ASTM) has categorized diesel fuels into three general groups. The need to categorize these fuels results from the varied uses of diesel engines, which are designed to operate efficiently on one of the standard diesel fuels. See also Diesel engine.

No. 1-D is a light distillate, similar to kerosine, for engines where frequent load changes and speed changes (truck, tractor engines) are essential. This fuel has a flash point greater than 100°F (38°C), with a minimum cetane number of 40. This fuel is believed to be particularly suitable for cold-weather operation. See also Kerosine.

No. 2-D is a medium distillate fuel with a lower volatility and higher density than No. 1-D. This fuel finds use in heavier-duty engines, for example, railroad engines, which operate at uniform speeds but with heavier loads than encountered during the use of No. 1-D. The flash point is greater than 125°F (52°C) and the minimum cetane number is 40.

No. 3-D is a heavy distillate fuel with the highest density and lowest volatility of the three diesel fuels. It finds use in low- and medium-speed engines such as marine engines and electric power generation engines, which operate under sustained loads. The flash point is greater than 130°F (54°C) and the minimum cetane rating is 30.