Engine compression ratio is varied to obtain start of combustion at top dead centre i.e. ignition delay is maintained equal to 13° CA for the test fuel and  two blends of reference fuels that bracket the compression ratio obtained with the test fuel. The reference fuel blends should not be more than 5 CN units apart. The cetane number of the test fuel is determined by interpolation from the compression ratio values and the corresponding cetane numbers of the reference fuel blends.

Correlations of ignition quality with the physical properties of the diesel fuels have also been developed for ease of quality control during refining. These correlations are applicable only to the neat petroleum derived diesel fuels when no additives are used to improve ignition quality. The Calculated Cetane Index (CCI) determined by ASTM D 976 or ASTM D 4737 methods are more commonly used as an alternative to CN for the neat diesel fuels. The calculated cetane index is not a substitute for ASTM cetane number. It is only a supplementary parameter for predicting cetane number when used keeping in view its limitations.  The CCI calculation methods are not suitable for pure hydrocarbons, or non-petroleum based fuels derived from coal.
ASTM D 976 uses a two variable equation to determine CCI from the mid-boiling point and density of the diesel fuel as below,

where:
D = density at 15°C (g/ml) determined by Test Method ASTM D 1298.
B =  50% evaporation (mid-boiling)  temperature (°C) determined by Test Method ASTM D 86 and corrected to standard barometric pressure.

ASTM D 4737 is a newer more exhaustive method in which CCI is correlated to four variables; the density and temperatures for evaporation of 10% (T₁₀), 50% (T₅₀) and 90% (T₉₀). Details are available in the relevant standards and other texts.
CCI values are quite close to the cetane number.  Several national fuel standards also specify cetane index values in addition to cetane number.
Cetane or ignition improvers are used to improve ignition quality of the diesel fuels. Nitrates like isopropyl nitrate, cyclo-hexyl nitrates, ethyl-hexyl nitrate (EHN) and peroxides like di-tertiary-butyl peroxide are used as cetane improvers. These compounds readily decompose at the engine compression temperatures and produce free radicals that accelerate precombustion reactions and thereby reduce ignition delay. The cetane improvers are used typically in dosages of around 500 to 2000 ppm by volume.

With high cetane fuels cold starting is easier and engine warm up is faster. Therefore, use of high CN fuels results in lower HC emissions during engine warm-up phase and the reduction in HC is more significant at low ambient temperatures.   High cetane number fuels give shorter ignition delay and thus, reduce the amount of fuel burned in premixed phase which results in lower peak combustion pressures and temperatures. Therefore, high CN fuels are expected to give lower NOx emissions. Typical effect of CN on NO_x emissions is shown on Fig. 8.5. Ratio of NO_x emissions obtained for 58 CN fuels relative to that obtained with 50 CN fuel are shown at different engine loads. Depending upon the engine load, up to 6 - 8% lower NO_x may result with increase of CN from 50 to 58. At full engine load however, a slight increase in NO_x with increase in CN was seen.