Higher cetane rating means faster ignition and propogation of the flame front across the combustion chamber due to the generation of ultra short existing free radials that are extremely reactive.
The paraffins are high in cetane value, while the aromatics are low in cetane value. Which is why straight cut diesel fuel #2, with allot of paraffins is great fuel. However, it is also why gasoline, with allot of aromatics, is low in cetane value and bad for diesel engines.
40 cetane is the lowest, anything below 40 diesel engines will severely knock.
45 cetane is pretty good and will work pretty good in all climates.
50 cetane is real good and is premium diesel fuel.