The cetane rating is somewhat like the octane rating. It measures engine roughness and the higher the rating the smoother the engine will run. Most fuels have cetane ratings of 40 while some premium diesels may have cetane ratings of 50 or more.
Typically engines are designed to use fuels with Cetane Numbers of 40-55, because below 38 a more rapid increase in ignition delay. The significance of the cetane number increases with the speed of the engine, and large, low speed diesel engines often only specify viscosity, combustion and contaminant levels, as Cetane Number requirement of the engine is met by most distillate and residual fuels that have the appropriate properties.
High speed diesel engines ( as in cars and trucks ) virtually all are designed to accept fuels around 50 Cetane Numbers, with higher numbers being a waste.
However, Cetane Number is only one important property of diesel fuels, with three of the others being also very important.
Firstly, the viscosity is important because many injection systems rely on the lubricity of the fuel for lubrication.
Secondly, the cold weather properties are important, remember that normal alkanes are desirable, but the desirable diesel fraction alkanes have melting points above 0C temperature, so special flow-enhancing additives and changes to the hydrocarbon profiles occur seasonally. That's why it's never a good idea to store diesel from summer for winter use.
Thirdly, diesel in many countries has a legal minimum flash point ( the minimum temperature it must attain to produce sufficient vapors to ignite when a flame is applied. In all cases it's usually well above ambient ( 60C+, kerosene is 37C+, whereas gasoline is typically below -30C ), and anybody mixing a lower flash point fraction with diesel will usually void all insurance and warranties on the vehicle. The recent increase in blending fuels has resulted in significantly more frequent analyses of fuel tank contents from diesel vehicle fires.
