FUEL-CETANE-NUMBER ESTIMATION METHOD AND APPARATUS

An object is to provide a method and an apparatus which can be implemented with a simplified and inexpensive configuration without causing a misfire in a combustion state. In a fuel-Cetane-number estimation method of the present invention, a map which specifies a relationship between the Cetane number and a control parameter at which NOx concentration of exhaust gas reaches a predetermined value is prepared in advance. Then, the control parameter is controlled variably to detect transition of the NOx concentration of the exhaust gas of the internal combustion engine, and thereby the control parameter of the internal combustion engine at which the NOx concentration has reached the predetermined value is obtained, and the Cetane number which corresponds to the obtained control parameter is estimated on the basis of the map.