Oxidation of gas engine oils
Oil aging or oil oxidation plays a decisive role in assessing how long a gas engine oil can still be used or whether an oil change is necessary. Longer oil operating times, higher operating temperatures or rising contamination increase oil oxidation and thus the formation of deposits and acid reaction products. Oils from gas engines that are operated with biogas, landfill gas, sewage gas or even wood gas often still have to cope with stronger acidic components from the gases. Since the composition of these gases often varies greatly, even an oil change at fixed intervals can become a risk, in contrast to the relatively clean natural gas. In accordance with many manufacturers' regulations, oils from biogas, sewage and landfill gas engines in particular must therefore be monitored using ongoing lubricant analyses.
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A DIN standard that is no longer effective
For a long time infrared spectroscopy was the only method for measuring oil oxidation. In the case of mineral oils, which were mainly in use about 25 years ago, the spectrum showed oxidation so clearly that the values determined with the IR according to DIN 51453 were specified by gas engine manufacturers. This takes advantage of the fact that during oil aging, oxygen accumulates on the molecular chains of oils consisting of hydrocarbons and new molecular chains are formed. When they are illuminated with infrared light, they absorb it differently than fresh oil. A clear peak is visible in the IR spectrum of an oxidized mineral oil at a wave number of about 1,710 cm-1. After the fresh oil spectrum is subtracted from the used oil spectrum in this region, the oxidation is stated as the absorption of IR raditation referenced to a one-centimeter oil layer thickness (A/cm).
However, this logical procedure does not work with some modern gas engine oils. In the region of wave number 1,710 cm-1, the IR spectrum also indicates the double bonds typical of synthetic oils containing esters or some oil additives. Oxidation products and esters cause peaks in the same region, with a distinct overlapping of the ester peaks and the oxidation peak.
For gas engine oils based on petroleum, IR spectroscopy still provides reliable information on oil ageing and the limits specified by the DIN standard are still valid. But since many modern gas engine oils contain synthetic group I or group II base oils and most of them also contain low-ash additives, IR spectroscopy in accordance with the DIN standard cannot provide any useful oil oxidation values for these oils. Base oils and additives may contain thermally stable ester-based synthetic oils that distinctly overlap the oxidation peak at the specified wave number of 1,710 cm-1. If such a peak is already present in the fresh oil, subtraction often does not provide a meaningful oxidation value. Even heavily oxidised oils will yield a value of 1 A/cm. The value at 1,710 cm-1 often lies on a shoulder of the peak, giving rise to what appears to be strongly varying oxidation values. For example, a value of 1 A/ cm, 18 A/ cm or even 27 A/ cm might be calculated for oils with the same degree of oxidation using the standard-compliant method.
Why does IR spectroscopy not always work?
For the main part of mineral oil-based gas engine oils (Group I-III oils), IR spectroscopy still provides reliable information on oil aging and the limit values specified according to DIN are applicable. For some Group III, and many synthetic Group IV gas engine oils, mostly "low-ash" oils, IR spectroscopy according to DIN cannot provide usable values for oil oxidation. These more temperature-stable synthetic oils with ester components produce a prominent "ester peak" in the IR spectrum, which clearly overlaps the emerging "oxidation peak" at wavenumber 1710 cm-1. If such a peak is already present in the fresh oil, subtraction often does not provide a meaningful oxidation value. Even strongly oxidized oils do not give a usable result, since not infrequently the "oxidation peak" at 1710 cm-1 lies on the "shoulder" of the "ester peak" giving rise to what appear to be strongly varying oxidation values. Thus, values from 1 A/cm to > 30 A/cm could be calculated for oils with the same degree of oxidation if the procedure is in accordance with the specifications.
Time for a new, uniform rule
Engine manufacturers and insurance companies are still specifying that the oxidation value has to be determined using IR spectroscopy in accordance with DIN 51453, and most of them set a limit of 20 A/cm. This is counter-productive for assessing some modern oils. It is time to define a standard test method for determining oxidation that is also valid for modern gas engine oils. It should be defined such that it can be implemented worldwide in any laboratory for used oil analysis.
Until then, we at OELCHECK wish to assure our customers that when assessing the ageing of modern engine oils we do not rely solely on oxidation as determined in accordance with DIN 51453, which is partly not usable in this situation. Instead, we presently use trend analysis of a combination of several analysis values as a reliable way to evaluate aging. Along with the engine type, gas type and engine oil used, the findings from IR spectroscopy are augmented by viscosity, AN, BN, and ipH data. In making this assessment, OELCHECK tribologists benefit from their extensive experience and our comprehensive database. Our customers can therefore rely on accurate oil aging information in the laboratory reports. However, a standardized redefinition of the test method is urgently needed.
P.S.: Determining oxidation by subtracting peaks at a specific wavenumber is not only problematic for modern gas engine oils. New synthetic gear oils, such as those used in wind energy gears, also show a similarly problematic oxidation determination, as they also frequently contain synthetic base oils and additives with ester-containing components.
OELCHECKER Frühling 2013, page 5
