Abstract 
 

This dissertation presents a contribution to the study of the lubricating oils degradation in reciprocating internal combustion engines (ICE) used in the automotive industry and fuelled with Compressed Natural Gas (CNG) or conventional Diesel fuel, considering that in both cases working conditions are quite hostile to the lubricants. 
 
Attending substantial behavioural differences observed in the oil analysis monitoring program applied in a urban transport fleet with respect to the oil drain period related in these two types of engines, was raised to conduct a preliminary study in order to compare the degradation ratio suffered by these lubricating oils under real operating conditions and using a very complete monitoring program, including different analytical techniques such as: Fourier transform infrared spectroscopy (FT-IR), linear sweep voltammetry technique, potentiometric and properties measurements such as viscosity. This initial study, presents higher demands on the lubricant used in CNG engines versus diesel, mainly related to higher thermal stress. 
 
These results raised the need to conduct a second study in which it was possible to analyze the behaviour of different formulations, with different quality levels, but in all the cases accepted by the CNG engines manufacturers; in order to assess the effects on the degradation rate, in terms of oxidation, nitration and additive depletion and to establish correlations between these variables. Also, the study allowed an evaluation, from an economic point of view, of the effects of using higher quality formulations and therefore better performance in this type of engine applications. 
 
Finally, these studies prompted the development of a specific methodology for measuring oxidation level, and oil degradation in general term, for the specific case of ICE oils by FT-IR spectrometry, and analyze the effects of those variables that have a greater influence on the measurements such as temperature and potential fuel dilution problems.