The acid number (AN) is considered an important indicator of oil condition, especially in terms of defining oil oxidation and the extent of acidification of used oils collected from two engines, each running with a different biogas. The results of chemometrical models comprising principal component analysis (PCA) for qualitative evaluation and partial least squares regression (PLS) for quantitative evaluation of used gas engine oils, respectively, are discussed. The investigations are based on infrared spectrometry and acid number measurements of monograde mineral-oil-based gas engine oils SAE 40. It is reported how IR spectrometry and chemometrics can be used to reveal the influence of the gas fuel type on oil aging (PCA) and for the indirect determination of the acid number (PLS). In contrast to the conventional time-consuming AN determination according to ASTM D 664, the joint use of IR spectrometry and chemometrics offers results within a few minutes. The chemometrical “measurement error” is in the range of precision and bias of the standard method.

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