Diesel fuel is a mixture of hydrocarbons obtained by distillation of crude oil. The important properties which are used to characterize diesel fuel include cetane number (or cetane index), fuel volatility, density, viscosity, cold behavior, and sulfur content. Diesel fuel specifications differ for various fuel grades and in different countries. Diesel, can be any liquid fuel used in diesel engines. The most common form of diesel is a specific fractional distillate of petroleum fuel oil.