A test engine is used to measure the cetane number of diesel fuel. When using other mehods of measurig the cetane number, the result will depend on the diesel fuel composition. The motor method provides the exact results for the fuels with any fractional composition. The various express analyzers require the recalibration when changing from one type of oil product to another, which is very difficult and time consuming.
In general, three approaches are used to identify an accurate value of the cetane number: (1) flare coincidence; (2) ignition lag; and (3) the critical rate of compression. The ‘method of flare coincidence has gained a widespread acceptance. It is realized by such types of equipment as IT9-3, It9-3M, or ITD-69 which have single-cycle four stroke engines. When testing, the shaft speed should be maintained at the level of 900±10 revolutions per minute (rpm).
The cetane number of diesel fuel is largely determined by the hydrocarbon composition. The cetane number of the benzene hydrocarbons is the lowest while that of the paraffin hydrocarbons is the highest.