|
|
Infrared
Analysis as a Tool for Assessing Degradation in Used
Engine Lubricants |
Infrared analysis has become a
powerful, practical, analytical tool |
|
Introduction |
The use of infrared
spectroscopy for routine monitoring of oil-lubricated
components, breakdown products and contaminants has not
been widely used in the past, although infrared studies
of lubrication oils themselves have been performed for a
number of years. The reason for this is that older
dispersive infrared spectrometers would take several
minutes to generate a spectrum of the used oil and then
considerable additional time would be needed to reduce
and interpret spectral data.
New technology has yielded
the Fourier Transform Infrared (FTIR) Spectrometer and
the micro computer. This combination in modern equipment
produces infrared spectra in seconds and sub one minute
data reductions. Infrared analysis has become a powerful,
practical analytical tool for used oil analysis.
WearCheck purchased an FTIR Spectrometer and Oil Analysis
software from Perkin Elmer in September 1990.
Oil degradation processes |
To extract
information from infrared spectra of used oils, a basic
knowledge of the processes involved in oil degradation is
required.
The lubricant in a
combustion engine is operating in a very hostile
environment, temperatures are high, and the lubricant is
dispersed over a large surface area where it is exposed
to chemically reactive by-products of the combustion
process. In addition to this, the oil is exposed to
sources of internal and external contamination.
Oil degradation by
chemical change
The chemical degradation
of an engine lubricant may be defined by a number of
processes:
OXIDATION
- At elevated temperatures, oil exposed to oxygen from
the air, will oxidize (chemically combine with oxygen) to
form a variety of compounds. The majority of these are
carbonyl containing compounds (C=O) such as Esters,
Ketones and Carboxylic acids. Some of these compounds are
dissolved by the oil, or remain suspended owing to
dispersive additives in the oil. Carboxylic acids
contribute to the acidity of the oil and deplete its
basic reserve as neutralization takes place. The net
effect of prolonged oxidation is that chemically, the oil
becomes acidic causing corrosion, while physically an
increase in viscosity occurs. The increase in viscosity
however, may be masked by other factors such as fuel
dilution.
|
Infrared information is favoured
over conventional oil analyses as it is regarded as
having a greater diagnostic value |
|
NITRATION
- Nitration is another form of oxidation. Nitration
results from the reaction of oil components with nitrogen
oxides (NO, NO2 and N2O4)
that are produced from the oxidation of atmospheric
nitrogen during the combustion process. In addition to
causing oil thickening, nitration products are the major
cause of the build-up of varnish or lacquer. SULPHATE FORMATION
- The various oxides of sulfur and water, both of which
are combustion by-products, react together to form
sulphuric acid. This acid is neutralized by the basic
reserve in the additive package of the oil and normally
results in the formation of metallic sulfates. An
indication of the progress of neutralization is obtained
by monitoring the build-up of metallic sulfates in the
oil.
Oil degradation by
contamination
Detrimental contamination
may occur from both external and internal sources.
INTERNAL
CONTAMINATION - This mainly comprises soot and
metallic particulates (wear metals). Both these
contaminants occur on a continuous basis and will build
up to undesirable levels at which time the lubricant
should be discarded. The rate at which unacceptable
levels of contaminant are reached will depend on the
condition of the engine and its operating conditions.
EXTERNAL
CONTAMINATION - The main contaminants in this
category are unburnt fuel, coolant and dirt. These
contaminants result from malfunctions and failures in the
fuel, cooling and air intake systems respectively.
It is essential that these
contaminants are detected early if catastrophic failures
are to be avoided.
Infrared analysis of used
oils |
Infrared analysis on
used engine oils can provide a great deal of information
about what happens to an engine in service within a
relatively short period of time. In a number of areas,
infrared information is favoured over conventional oil
analyses as it is regarded as having a greater diagnostic
value. One should, however, exercise caution when
interpreting data as engine design and operating
conditions play an important part. A series of results
from consecutive samples in which trends are evident has
far greater diagnostic value than results from a lone
sample. Important results that are available from
infrared measurements and their interpretation are
detailed below:
SOOT INDEX
- The soot index is a measure of the level of partially
burned fuel particles (soot) in the oil. The rate at
which soot is deposited in the oil is dependent on engine
design and operating conditions. An increase in the soot
index will indicate poor combustion or an oil and filter
change period that has been over-extended.
OXIDATION INDEX
- The oxidation index measures the degree to which the
oil has been oxidized and is a good indicator of oil
degradation. A rapid increase in oxidation may indicate
engine overheating or a depletion of the anti-oxidant
additive in the oil due to an over extended oil drain
period.
SULPHATE INDEX
- The sulfate index measures the extent to which
sulfur-based acids have entered the oil. A rapid increase
in the sulfate index could be due to depletion of oil
additives, poor combustion or over-cooling.
|
Infrared radiation is part of the
make-up of the electromagnetic spectrum |
|
NITRATE
INDEX - The nitrate index measures the build-up
of nitrogen compounds in the oil.. These compounds cause
oil thickening and deposits that interfere with
lubrication. Nitration is influenced by incorrect
fuel/air ratios, improper spark timing, high loads, low
operating temperatures and piston-ring blow-by. WATER AND GLYCOL -
Water and Glycol may be detected at relatively low levels
by FTIR. The presence of glycol and water or glycol alone
would indicate a coolant leak. Water alone does not
necessarily indicate a coolant problem, as traces of
water could result from condensation, if an oil sample
has been taken from a cold engine.
Background to infrared
analysis |
The
Electromagnetic Spectrum
Electromagnetic waves are
waves that have both an electric and magnetic component.
Well known examples of electromagnetic waves include
X-rays, visible light, microwaves and even radio waves.
These waves all travel at
the same velocity ("speed of light") but differ
in the wavelength and frequency bands used to describe
them. Figure 1 shows the whole range of electromagnetic
waves arranged in order of increasing wavelength and
depicts what is known as the electromagnetic spectrum.
Infrared radiation
Infrared radiation is part
of the make-up of the electromagnetic spectrum and covers
electromagnetic waves with wavelengths between 0.00008 cm
and 0.04 cm. Chemists have adopted a more convenient
method of describing infrared radiation in that it is
described in terms of the number of waves that occur per
centimeter. This number is called the WAVENUMBER and is
actually a measure of wave frequency. Wavenumbers are
calculated by dividing 1 by the wavelength expressed in
centimeters.
WAVENUMBER (cm-1)
= |
1
|
|
WAVELENGTH
(cm) |
Infrared
analysis only uses a portion of the IR spectrum known as
"Mid-range infrared". It is defined as infrared
waves having wavenumbers between 4000 and 400 cm-1.
The
Electromagnetic Spectrum. (Figure 1)
|
Different types of bonds within the
same molecule absorb different frequency bands |
|
Spectrum of 1-Octene. (Figure 2)
Molecular
vibrations and infrared absorption
The chemical bonds within
organic molecules are in a state of continual vibration,
with bonds stretching and contracting as well as bending
relative to one another. When an infrared beam falls on a
molecule, waves of specific frequencies (wavenumbers) are
absorbed from the beam by the molecule, and result in
changes to the molecular vibrations of the molecule. The
actual frequencies of the waves absorbed depends on the
types of bonds present in the molecule’s structure.
Different types of bonds within the same molecule would
absorb different frequency bands while several identical
bonds would all absorb the same frequency bands and give
rise to stronger absorptions.
(E.g. A C=O and a C-H bond
in the same molecule would be expected to yield at least
two different absorption bands while several C-H bonds in
the same molecule would all contribute to at least a
single relatively strong absorption band). Chemical bonds
within a molecule are therefore said to exhibit
"characteristic infrared absorptions". Details
of characteristic absorption for a number of common
chemical bonds are given in Table 1. Some of these
absorptions have been identified in the sample spectrum
of a relatively simple organic compound, 1-Octene, in
Figure 2.
TABLE
1: CHARACTERISTIC INFRARED ABSORPTION FREQUENCIESa
|
BOND |
COMPOUND
TYPE |
FREQ
RANGE (CM-1) |
|
|
|
C-H |
Alkanes |
2850-2960 |
|
|
1350-1470 |
C-H |
Alkenes |
3020-3080
(m) |
|
|
675-1000 |
C-H |
Aromatic
Rings |
3000-3100
(m) |
|
|
675-870 |
C-H |
Alkynes |
3300 |
C=C |
Alkenes |
1640-1680
(v) |
C=C |
Alkynes |
2100-2260
(v) |
C...C |
Aromatic
Rings |
1500,
1600 (v) |
C-O |
Alcohols,
ethers, carboxylic acids, esters |
1080-1300 |
C=O |
Aldehydes,
ketones, carboxylic acids, esters |
1690-1760 |
O-H |
Monomeric
alcohols, phenols |
3610-3640
(v) |
|
Hydrogen-bonded
alcohols, phenols |
3200-3600
(broad) |
|
Carboxylic
acids |
2500-3000
(broad) |
N-H |
Amines |
3300-3500
(m) |
C-N |
Amines |
1180-1360 |
C=N |
Nitriles |
2210-2260
(v) |
NO2 |
Nitro
componds |
1515-1560 |
|
|
1345-1385 |
a All bands strong
unless marked: m, moderate; w, weak; v, variable. |
|
The spectrum of a used oil must be
compared against that of an unused oil to be of
analytical value |
|
Infrared
spectra and spectrometers A record of the frequencies at
which infrared absorption takes place for an organic
compound is a highly characteristic property of the
compound and is called its INFRARED SPECTRUM.
An infrared spectrum of a
compound will reveal information about molecular
structure as the existence of specific groups of atoms
may be confirmed from the presence of their
characteristic absorptions. The instrument used to record
infrared spectra is called an INFRARED SPECTROMETER.
Modern FTIR spectrometers scan frequencies in an infrared
beam and measure the radiant powers [P] of frequencies
after the beam has passed through, and interacted with a
sample in a sample cell. These values are compared
against stored values of radiant powers [Po], obtained
with an empty sample cell, and absorbance values
calculated for output in an infrared spectrum. The output
of spectra from a spectrometer is generally in the form
of a plot of Absorbance vs. Wavenumber, however most
instruments will also output spectra in the form of
Percent Transmittance vs. Wavenumber. Absorbance is the
preferred output for quantitative analysis as it is
directly proportional to the concentration of the
absorbing species.
For interest, the
mathematical relationship between absorbance,
transmittance and concentration are detailed in Figure 3.
Analysis of used oil
spectra |
Used oil samples are
complex mixtures of a large number of different chemical
compounds and include compounds derived from the original
formulation of base oil and its additives, oil
degradation products and oil contaminants.
As a result of this a used
oil spectrum is complex and essentially the net sum of
the spectra of all the individual compounds making up the
sample.
In fact, because of this
complexity, the spectrum of a used oil alone is of
limited value, and it must be compared against the
spectrum of the unused oil to be of significant
analytical value.
The
relationship between absorbance, transmittance, and
concentration. (Figure 3)
|
The difference spectrum may be
regarded as a spectrum of degradation products |
|
Comparison of used and unused diesel
lubricant. (Figure 4)
Figure 4 shows
transmittance spectra from two oil samples that are
superimposed on a common spectral grid. Spectrum A is
that of a new oil (original fill) and Spectrum B is that
of the same oil, degraded by a period of usage in a
diesel engine. Apart from the displacement of
transmittance values, caused by the presence of soot in
sample B, there appears to be little difference between
the two samples and it would be reasonable to expect the
assumption that minimal degradation has occured.
This situation changes
dramatically when a differential spectrum is viewed.
Figure 5 shows the Difference Absorbance Spectrum of the
same two samples in which very obvious differences are
apparent. A DIFFERENTIAL or DIFFERENCE SPECTRUM is
obtained by subtracting the absorbance spectrum of one
sample form that of the other. This process is carried
out by the spectrometer’s internal microprocessor.
Data for each sample is collected and converted into a
numerical format which is subsequently subtracted to
yield the difference data.
Difference
Spectrum including soot. (Figure 5)
|
Oil analysis software calculates
severity indices for various degradation processes |
|
Difference
spectrum excluding soot showing important spectral
regions. (Figure 6)
Difference data may be
used for further calculations or be converted back to a
graphical representation. Once in numerical form spectral
data may be manipulated mathematically to yield vast
amounts of information in a short time period.
Figure 6 shows the same
difference spectrum that has been further enhanced by
"soot correction". The soot loading is
estimated from absorbance values determined at two
specified wavenumbers and the values applied to a
mathematical model. The mathematical model determines the
shape of the "soot curve" to be subtracted.
The data in this corrected
form now contains all the information about the
"differences" that exist between the new and
the used oil, and may be considered as being a spectrum
of the degradation products that exist in the used oil.
To convert this data into a meaningful form, the
numerical data of the corrected difference spectrum is
examined in various spectral regions by software routines
that calculate numbers representative of the degree to
which types of degradation and contamination has
occurred.
Typical spectral regions
of interest and the degradation processes they represent
are detailed in table 2 and are represented graphically
in Fig 6.
At WearCheck, used engine
oil samples are run on the Perkin Elmer FTIR and the
resulting spectrum matched and compared to a new oil
spectrum contained in a "new oil" library. The
difference spectrum generated from this match is
processed by the oil analysis software and "SEVERITY
INDICES" are calculated for the various degradation
processes. Severity indices are reported rather than
concentrations owing to the complex and variable nature
of the compounds that are being measured in each process.
In the case of the simpler contaminants, such as Water
and Glycol, concentration may be expressed directly. The
software in use at WearCheck calculates and reports
indices for SOOT, OXIDATION, SULPHATES and NITRATES.
Water and Glycol are also reported but are quantified
where necessary by additional tests. Fuel dilution may
also be measured but owing to the complexity of local
supplies of fuel, the method is unreliable.
|
The diagnositc department at
WearCheck has considerable experience in interpreting
infrared results |
|
TABLE 2
|
CHARACTERISTIC INFRARED
ABSORPTION BANDS THAT ARE USEFUL IN MONITORING
OIL DEGRADATION PROCESSES
|
Degradation
Process |
Spectral
Region Centre (cm-1) |
OXIDATION
(CARBONYL) |
1720 |
NITRATION |
1630, 1553 |
SOOT CONTAMINATION |
3800, 1980 |
WATYER
CONTAMINATION |
3450, 1640, 770 |
SULPHATE FORMATION |
1160, 606 |
GLYCOL |
3370, 1087, 1043 |
FUEL(AROMATIC) |
3052, 1605, 874,
811, 748 |
Where regular
sampling is undertaken severity indices are particularly
useful in monitoring engine trends, determining oil
change periods and determining the onset of potential
problems. If oil is used over a prolonged period with
minimal top-up, the severity indices would be expected to
increase as concentrations of degradation products build
up to unacceptable levels. The interpretation of these
values however, requires considerable skill as numerous
considerations such as engine type, engine conditions and
operating environment must be taken into account.
The diagnostic department
at WearCheck has considerable experience in interpreting
infrared results and successfully identifying many
potential problems in engines that would otherwise
require extensive testing to detect.
At WearCheck, Infrared
analysis is a valued technique in used oil analysis.
References |
- Instrument Methods
of Analysis - Willard, Merrilt, Dean and
Settle.
- Evaluation of used
crankcase oils using computerized Infrared
Spectroscopy - JOAP - TSC report 84-01.
- Organic Chemistry
(3rd edition) 1975 -
Morrison and Boyd.
|