Patent ID: 7611901

Claim:
A method for determining proportion of diesel fuel in a lubricating oil of a combustion engine, comprising: forming a mixture containing a sample of lubricating oil to be analyzed and a C 5 hydrocarbon or a C 5 alkane according to a predetermined proportion; injecting the mixture into an injector of a gas chromatograph; establishing a chromatogram of the sample to be analyzed; determining a first parameter M representative of the area of a peak of the chromatogram associated with the C 5 hydrocarbon or the C 5 alkane; determining a second parameter C representative of the area of at least one peak of the chromatogram associated with a hydrocarbon representative of the diesel fuel; and determining the proportion T of diesel fuel of the sample to be analyzed, by formula: T = C / ( M ) - b a in which a and b are constants defining equation y=ax+b of a calibration straight line of the ratio between the second and the first parameters as a function of the proportion of diesel fuel.