Diesel engines vary widely in their cetane requirements, and there is no commonly recognized way to measure this value. In general, the lower an engine s operating speed, the lower the CN of the fuel it can use. Large marine engines can tolerate fuels with CNs as low as 20, while some manufacturers of high-speed passenger car diesel engines specify 55 CN fuel. Canadian railroads now purchase fuel to a 37 CN minimum specification. Most diesel gen-set manufacturers call for a 40 or 45 minimum.
Viscosity influences the spray pattern when the fuel is injected into the cylinder.
Minimum viscosity limits are imposed to prevent the fuel from causing wear in the fuel injection pump. Unlike gasolines, which have freezing points well below even the most severe winter ambients, diesel fuels have pour points and cloud points well within the range of temperatures at which they might be used. This presents no problem on ships or even in railroad applications where heated storage can be arranged. This is of particular concern when storing fuel above ground. Fuel delivered during the summer season, if unused, should be tested prior to the onset of cold weather.