Due to the fact that 100% efficiency cannot be achieved by any engine, products other than water and carbon dioxide will be formed during combustion, such as soot, which is formed as a result of incomplete combustion. Soot is chiefly made of carbon particles usually spherical in shape. When soot levels increase, the soot particles are clumped together and turn hazardous (Figure 1).
This increase in soot level and clumping together of particles continues until it attains a level that is high enough to precipitate out of the oil. This precipitation not only increases the viscosity of oil, but also gets attached to the engine surfaces, considerably increasing engine wear. It can also cause filter plugging. Carrying out regular soot checks can ensure cost savings by expanding the drain periods, minimizing the disposal of used oil, and increasing the service life of diesel engines.
Figure 1. Example of extreme soot build up
Methods for Testing Soot in Lubrication Oil
Thermal Gravimetric Analysis
Thermal gravimetric analysis (TGA) is the conventional test method for soot, as well as being an ASTM (D5967) compliant test method. The method involves using heat to initiate reactions as well as physical changes in the sample to provide a quantitative result of soot. In this method, weight of the sample placed in an oven is continuously monitored throughout the procedure by heating the oven until the entire organic material gets evaporated and only the insoluble particles are left out.
To prevent oxidation, the sample is placed under a blanket of nitrogen. Subsequently, the nitrogen is replaced by oxygen and the sample is heated until all the soot is burned off. At this point, the remnants are only the inorganic oxides. To determine the soot concentration, the weight difference of the sample before introduction of the oxygen is calculated and the sample weight after burning of the soot is subtracted. This method provides highly accurate results (within 0.1%) however, it is a laborious and time-consuming method. Figure 2 shows a Perkin Elmer thermogravimetric analyzer used to perform this method.
Figure 2. Perkin Elmer thermogravimetric analyzer.
Following are the advantages and disadvantages of using this method:
- Conventional method
- Highly accurate
- ASTM D5967 compliant
- Not designed for field-use
- Mandates trained operators
- Can be applied only for measuring soot
Fourier-Transform Infrared Spectrometer
Fourier-transform infrared (FTIR) spectroscopy is a fast and inexpensive ASTM (D7844) compliant field technology for soot level monitoring of diesel engine oils. FTIR spectrometers measure the entire spectrum of light transmitted by a sample in the mid-infrared (IR) region of the light spectrum (4000-400 cm-1), enabling the measurement of soot content in the oil, and also other properties such as oxidation, water, nitration, glycol content, and sulfation.
In this method, a clean oil sample is first placed in an IR cell and measured on the spectrometer to establish a baseline. It is then removed from the cell and a used oil sample is placed and measured on the spectrometer. As the method involves measuring the amount of IR light scattered and not the amount absorbed, the soot value is determined by using the difference in baseline between the clean oil and the used oil samples at 2000 cm-1.
Although this method is fast and provides highly repeatable results, it cannot ensure better accuracy always. Therefore, the FTIR method is best used only for trend analysis during in-field soot monitoring. Figure 3 depicts a Spectro Scientific Q400 FTIR spectrometer used to carry out this method.
Figure 3. Spectro Scientific Q400 FTIR spectrometer.
Following are the advantages and disadvantages of the FTIR method:
- Enables fast analysis
- Less expensive than TGA
- Can measure multiple parameters
- ASTM D7844 compliant
- Best suited for trend analysis
- Cleaning the IR cell can pose difficulties
- Works in limited range (up to 5%)
Grating IR Spectrometer
Similar to the FTIR spectrometers, grating spectrometers also measure the light transmitted by a sample across the mid- infrared region. Nevertheless, they include a diffraction grating for separating the light, providing a portable, rugged, and inexpensive system without any moving parts. As the grating spectrometers measure the same region of light similar to the FTIR spectrometers, they can be used to measure the same oil properties measured by FTIR spectrometers. Figure 4 shows a Spectro Scientific Fluidscan portable grating spectrometer used to perform this method.
Figure 4. Spectro Scientific Fluidscan portable grating spectrometer.
In compliance with the ASTM D7889 technique, the soot content in the diesel engine oil is determined with a four-point multivariate calibration. A weighted baseline is also used to reduce the errors caused due to improper cell background collection and other sample features such as high TBN and undissolved water.
After performing a background of the blank cell, the sample is then placed in the cell and measured. The instrument software calculates the soot concentration using the multivariate calibration. The time needed for a complete analysis is less than 3 minutes.
Following are the advantages and disadvantages of Grating IR spectrometer:
- Enables fast analysis
- Can measure multiple parameters
- ASTM D7889 compliant
- Multivariate calibration is sensitive to differences in base oil
- Limited range (up to 5%)
Fixed Filter IR Analyzer
Fixed filter IR soot analysis is an ASTM D7686 compliant test method that has recently gained popularity. The process of employing a fixed filter at a particular wavelength enables the analyzer to measure the percentage of soot in diesel engine oil up to 15% by weight. For this process, a horizontal attenuated total reflectance (HATR) crystal and a fixed filter at 3.9 µm wavelength are used for the measurement.
The HATR crystal enables easier cleaning between samples. For completing a measurement, a clean oilM sample is applied to the HATR crystal, and then a background measurement is carried out. Subsequently, after removing the clean oil sample from the crystal, the target sample is homogenized, used on the sample, and finally determined. As the soot tends to settle on the crystal and can generate artificially higher readings, it is advisable to measure the sample immediately. Figure 5 depicts a Spectro Scientific InfraCal soot meter used for this method.
Figure 5. Spectro Scientific InfraCal soot meter.
The absorbance of the sample calculated by the instrument is then converted into a concentration readingbased on the internal calibration. If this method is performed properly, it can provide highly accurate results. On the contrary, if the oil sample is not homogenized or if it is allowed to settle on the crystal, the results may be poor. Generally, the use of a HATR crystal and a fixed filter enables easy, quick, and inexpensive measurement in a laboratory or in the field.
Although multiple techniques exist for laboratory as well as in-field analysis of soot in diesel engine oil, the choice of a technique always depends on several factors like budget, location of testing, precision, time, etc. Hence, it is highly important to consistently monitor soot contamination of the diesel engine oil to prevent equipment breakdown and unexpected downtime.
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.