Diesel fuel oil is essentially the same as furnace fuel oil, but the proportion of cracked gas oil is usually less since the high aromatic content of the cracked gas oil reduces the cetane value of the diesel fuel.
The allowable sulfur content for ultra-low sulfur kerosene and ultra-low sulfur diesel (15 ppm) is much lower than the previous US on-highway standard for low sulfur diesel (500 ppm), which not only reduces emissions of sulfur compounds (a cause of acid rain) but also allows advanced emission control systems to be fitted that would otherwise be poisoned by these compounds. These systems can greatly reduce emissions of oxides of nitrogen and particulate matter.
Diesel fuels were originally straight-run products obtained from the distillation of crude oil. However, with the use of various cracking processes to produce diesel constituents, diesel fuels also may contain varying amounts of selected cracked distillates to increase the volume available for meeting the growing demand. Care is taken to select the cracked stocks in such a manner that specifications are met.
Under the broad definition and properties of diesel fuel (Table 3.3), many possible combinations of characteristics (such as volatility, ignition quality, viscosity, gravity, stability, and other properties) exist. To characterize diesel fuels and thereby establish a framework of definition and reference, various classifications are used in different countries.