Liquid cooled engines and rotating equipment use glycol-based coolants because of their superior heat transfer characteristics. However, glycol-based coolants are not necessary at all in lubrication oil itself. Coolant ingression to the lubricant is a mainly nasty contaminant for the range of damage it can cause.
Glycol coolants disintegrate in the high temperature engine environment, causing the formation of glycolic acids. These acids attack nonferrous bearing surfaces and form metal salts. The acids also react with the oil anti wear and anti-oxidant additives and, together with water, form sludges that plug filters and cause the oil to lose its lubricity properties, thereby increasing abrasive wear. Glycol contamination in engines and transmissions is thought to be a more severe contaminant than just water (up to 10 times more damaging). Based on the oil temperature, the glycol coolant may break down quickly or over time. This unpredictability is a huge challenge for establishing the true glycol content in the oil at a particular time, and is the key reason why field and lab tests frequently do not agree with each other.
Lab tests by Cummins revealed that, “an oil containing four percent coolant will retain only 10 percent of the glycol originally present upon heating at 200 °F (93 °C) for eight hours.” However, other telltale signs of glycol contamination do stay in the oil.
Methods for Measuring Glycol Contamination
Infrared Spectroscopy is typically used in the lab or in the field to detect a series of molecular contaminants and lubricant chemistry degradation parameters. Infrared spectroscopy uses an Infrared radiation source, a detector, a sample holder and a computer to examine the interaction of matter and light. Molecules from a range of compounds vibrate in characteristic and reproducible frequencies so that a spectrum scan in the area of interest will specify the nature and concentration of the contaminant of interest. Glycol has a robust absorbing band in the region around 3450 cm-1 equivalent to the O-H functional group, as well as a more unique band for ethylene glycol at 1070-1030 cm-1 equivalent to the C-O functional group. Interferences from oil and water additives can create errors in measurement, so a significant quantity of signal processing is needed to guarantee reliable and consistent results.
There are test techniques for laboratory grade FTIR measurement as well as for portable field testing. ASTM E2412 defines the standard practice for FTIR measurement of common engine oil degradation products and contaminants including glycol. In this test, a 15 ml oil sample is pumped through an automated system to a sampling cell where the measurement is done. The system is then flushed with heptane to prepare it for the ensuing oil sample.
Typical used oil FTIR spectrum.
For checking oil chemistry in the field, ASTM D7889 uses a grating infrared spectrometer like the FluidScan which is easy to run and does not require a skilled technician. The portable FluidScan depends upon a patented flip top optical cell which only needs a few drops of oil to perform the analysis. Solvents are not needed to clean the flip top cell. It can be wiped down using a disposable wipe or a clean cloth.
- Easy to use
- Sensitive to glycol
- Field tool accurately detects liquid glycol and data can be saved to monitor trends
- Other parameters can be measured simultaneously (soot, water, etc.)
- Liquid glycol in oil can be difficult to detect
- Requires spectrum processing to determine interferences
- Limit of detection not low enough for particular equipment
Atomic Emission Spectroscopy
Oil sample being analyzed by Rotating Disc Electrode spectrometer.
Elemental analysis by arc-spark Rotating Disc Electrode (RDE) emission spectroscopy or by Inductively Coupled Plasma (ICP) emission spectroscopy has been the basic practice for oil analysis testing labs for years, as mentioned in ASTM D6595 and ASTM D5185. Elemental analysis is a very reliable technique to confirm that glycol ingression has taken place, as traces of the coolant stay in the oil, specifically the metallo-organic corrosion inhibitors that exist in high concentrations in the glycol coolant, but not native to the oil formulation. Sodium, potassium, boron and silicon are typically added to coolant for corrosion inhibition.
For RDE, the oil sample is poured directly into a cap and a new graphite disk electrode and pencil electrode launched to the chamber and energized. In roughly a minute, the oil analysis will expose typically 24 to 30 elements, and the additive values compared to a sample of new oil. One leading engine manufacturer recommends that an increase in sodium in the oil by as little as 50 ppm can mean as much as one gallon of coolant has leaked into a 10 gallon (38 L) lube oil system.
- Easy to use (RDE)
- Most reliable indicator that coolant contamination occurred
- Multi-elemental, 60 second test
- Molecular glycol is not measured (liquid glycol in oil) – when it is most destructive
- Workshop or lab environment – not ideal for field test
- Requires knowledge of coolant chemistry and additives for diagnosis
A sheet of common blotter paper is positioned on a flat surface and a dipstick is used to place some drops of oil in the middle of the sheet and left for an hour. If the oil drop absorbs outward and there are clear “soot rings” with a yellow/brown center, that is qualitative indication that glycol is present in the oil. It is a simple go/no go test, a typical consequence of glycol contamination. A black sticky paste with a well-defined (sharp edge) periphery is cause for serious worry. Very frequently a soot ring forms around a yellow/brown center when glycol is present.
Examples of blotter tests from different oils
- Verifies presence
- Not quantitative
- Cannot be used for trending data
- Water contamination can obstruct
Schiff’s Reagent Method (Gly-Tek Kit)
The ideal quick test for glycol is perhaps the Schiff’s reagent method (ASTM D2982). This is a colorimetric technique for detecting trace quantities of glycol in lubricating oils. In this technique, a sample of oil is pipetted into a solution of hydrochloric (HCl) and iodic (HIO3) acid to oxidize any glycol that may exist. The solution is then manually pipetted up a glass tube for superior viewing. The reaction yields an aldehyde, which in turn reacts with the Schiff’s reagent, producing a positive color change from colorless to pink/purple – the darker the color, the more glycol is present.
Color chart used to determine concentration of glycol in oil
- Glycol specific
- Color test provides some quantitative result
- 15 minute test minimum
- Additive or refining residuals interference with certain new oils
- Difficult to establish pink color with sooty/dark oils, particularly transmissions oils (red to start)
- Reaction proceeds after test is read-results subjective
- Uses hazardous acids
The most typically used GC procedure for glycol analysis is ASTM 4291, “Standard Test Method for Trace Ethylene Glycol in Used Engine Oil.” The process involves first extracting the glycol from the oil using water followed by centrifugation. The aqueous extract is injected into the column, and the eluting compounds are detected by a flame ionization detector (FID) and reported out. Gas chromatography (GC) can also be used for analysis, but the ethylene glycol is hard to detect and quantify because of its low volatility, low molecular weight and high polarity. Ethylene glycol chromatographic peak shape is frequently difficult to control and carryover can be an issue.
Chromatogram of a Glycol peak in engine oil by headspace GC.
Perkin Elmer Clarus 500 GC with headsampler.
- Low limit of detection
- Easy to automate for high volume labs
- Headspace alternate techniques enhance accuracy
- Glycol specific
- Impractical for field use
- Labor intensive preparation and unforgiving chromatography technique (30 minutes per test)
- False positives due to oil additives or manufacturing byproducts
- Requires well trained personnel
- Glycol may be already decomposed by time it arrives a lab with a GC
Glycol is a powerful contaminant that should be monitored whenever it is used for heat transfer duties in rotating equipment and engines. There are several direct and indirect techniques for glycol detection in oil, and more than one may be necessary to understand if an issue is occurring.
Determination of Ethylene Glycol in Used Engine Oil by Headspace-Gas Chromatography, Ruppel, T, Hall G Perkin Elmer
This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
For more information on this source, please visit AMETEK Spectro Scientific.