PerkinElmer OilExpress System, Руководство пользователя

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72 . OilExpress System User's Guide
Figure 27 and Figure 28 illustrate the comparison of a used oil sample and a sample of the
same oil unused. From these spectra it would be fair to assume that minimal degradation has
occurred.
This situation changes dramatically when a differential spectrum is viewed.
Figure 29 shows the baseline corrected difference spectrum of the same two samples,
obtained by subtracting one spectrum from the other and then performing a baseline
correction.
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 500.0
-0.100
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
0.100
cm-1
A
Hydroxy
Oxidation
Nitration
Oxidation/
Sulfate
Antiwear Depletion
Fuel
Sulfate
Figure 29 Baseline corrected difference spectrum
This data can now be manipulated mathematically to provide information about the
differences between the new and used oil, and may be considered to be a spectrum of the
degradation products that exist in the used oil.
In both cases, the trends observed in the results from a series of samples taken at regular
time intervals from the same engine have a much higher diagnostic value than the results
from a lone sample.
Degradation Products
Oxidation
Oil exposed to oxygen from the air at elevated temperature will oxidize to a variety of
compounds, the majority of which are carbonyl compounds, including carboxylic acids.
Carboxylic acids contribute to the acidity of the oil, depleting the basic additives present in
the oil and contributing to corrosion. Oxidation can also increase the viscosity of the oil. The
degree of oxidation is a good indicator of oil degradation. A rapid increase in oxidation may
indicate an engine overheating or a depletion of the anti-oxidant additive due to an
over-extended oil change period.
Nitration
Nitrogen oxides, produced from the oxidation of atmospheric nitrogen during the combustion
process, react with the oil. Nitration increases the viscosity of the oil and is the major cause
of the build-up of varnish or lacquer.