On-highway and mobile equipment use 1D and 2D fuels. High speed diesel engines use either 1D or 2D fuels. Important characteristics of diesel fuels are its viscosity, pour point, and cetane number. The primary differences between 1D and 2D fuel are the pour point and the viscosity. Pour point is the lowest temperature at which a liquid will flow.
Viscosity is the resistance of a liquid to flow.
A 2D fuel is used in warmer weather because it has a higher viscosity and pour point. The higher viscosity provides better lubrication qualities for the moving parts of the fuel injection system. This is critical to diesel engine fuel economy. Cetane rating is the diesel equivalent to gasoline's octane rating. Each manufacturer usually specifies a minimum or maximum cetane rating and the suggested operating temperature for 1D and 2D fuels.