The oil drain interval of stationary gas engines differs from engine to engine. By analysing oil samples you can determine the oil drain interval. Oxidation rating is one of the elements that influence oil drain interval. In this article we describe the different methods to test the oxidation rating of used gas engine oil samples.
For stationary gas engine operations the oil drain interval is dependent on gas quality, engine operations and also the lubricant.
Oil analysis, such as QUOTA (Q8 Used Oil Test Analysis) analysis help to determine the oil drain interval.
The two main properties used to determine the oil drain interval are:
Total Base Number (TBN) decrease
Oxidation rating increase
Different oxidation rating test methods
Most test procedures, such as for TBN testing, are prescribed in ASTM-, DIN- or ISO standards. For the determination of the oxidation rating, multiple test methods are used. All of these methods rely on infra-red spectroscopy, but the oxidation rating results may vary due to the difference in calculation methods.
Figure 1 shows the infra-red spectra of a used and fresh Q8 Mahler HA gas engine oil. Although the spectra itself will be equivalent regardless of the used test method, the calculated oxidation rating can differ.
Figure 1: infra-red absorbance spectra of a used and fresh Q8 Mahler HA
To determine the oxidation rating of the oil, gas engine OEMs used to refer only to infra-red spectroscopy in general, instead of specifying the actually used test method. Today they increasingly refer to DIN 51453. At Q8 Research also in-house methods (like KPI 99) have been developed to determine the oxidation value.
DIN 51453 oxidation test method
DIN 51453 uses the distance in the differential absorbance IR spectrum (IR-spectrumused oil – IR-spectrumfresh oil) between the peak height at 1710 cm-1 and the base line as a measure for the oxidation rating. The base line is a straight line in the differential spectrum between the points at wavenumbers 580 cm-1 and 1970 cm-1.
Figure 2: oxidation rating DIN 51453 procedure
Q8 Research in-house oxidation test method
The in-house method KPI 99 uses the highest peak between wavenumbers 1670 cm-1 and 1760 cm-1 (instead of 1710 cm-1) to determine the oxidation rating. The difference in the absorbance IR spectra between the peak height and the base line is a measure for the oxidation rating. In the QUOTA procedure the base line is a horizontal line crossing the fresh oil IR spectrum at 1960 cm-1.
Figure 3 is an overview of this general principle in the KPI 99 procedure.
Figure 3: oxidation rating KPI 99 procedure
Difference between DIN 51453 and KPI 99
For the IR spectra of Q8 Mahler HA gas engine oil as shown in figure 1 the results are:
- Oxidation rating acc. to DIN 51453 = 18.9 ABS/cm
- Oxidation rating acc. to KPI 99 = 19.4 ABS/cm
In this example the difference is 0.5 ABS/cm. Although in other cases the difference can be bigger.
Why you need a reference spectrum?
To guarantee a correct calculation of the oxidation rate, a reference spectrum of the fresh oil should always be available.
If the oxidation value is calculated using the wrong fresh reference spectrum, it could wrongly be reported as much too high (or much too low). This principle is clarified in Figure 4.
Figure 4: oxidation rating if the incorrect reference spectrum is used.
DIN 51453 is becoming the industry standard
The DIN 51453 is becoming more and more the industry standard to determine the oxidation value of used gas engine oil.
When using other outside labs, the test method may differ. We recommend to request a copy of the test method to better understand the test result.
For gas engine oils the oxidation rating is one of the properties that is used to determine the oil drain interval and therefore also the quality of the oil.
To determine the oxidation value, you should take into account the following aspects:
- There are different calculation methods available which can result in different oxidation values for the same used oil.
- To guarantee reliable oxidation values, you must use the correct fresh oil spectrum.