Diesel fuels are classified 1D, 2D, and 4D. Low speed, stationary units use 4D fuels. 4D fuel is not appropriate for most mobile equipment. 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 1D fuel is designed for cold weather operation; thus, it is less viscous and has a lower pour point. 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. Because 2D fuels contain more Btu's (British thermal units - the amount of heat necessary to raise one (1) pound of water one (1) degree fahrenheit) per gallon, they are able to deliver more power per gallon. This is critical to diesel engine fuel economy. The higher the Btu rating a diesel fuel has, the greater power yield per gallon; thus, higher mpg will result. Cetane rating is the diesel equivalent to gasoline's octane rating. Unlike an octane rating, which rates gasoline's resistance to spontaneous ignition, the cetane rating number (usually 40 to 55 for medium to high speed engines) notes the relative ease with which diesel fuel ignites. The higher the cetane number, the easier the fuel ignites; the higher the octane number, the more resistant the fuel is to ignition. Each manufacturer usually specifies a minimum or maximum cetane rating and the suggested operating temperature for 1D and 2D fuels. A given fuel may meet 1D or 2D specifications, but if the Btu rating is too low, then decreased fuel mpg will result