The Basics
Diesel engines, just like many other lubricated machines, are susceptible to wear from silt particles in the 1-10 µm range. While in hydraulic systems and other components, optical particle counting has become the norm to determine levels of fluid cleanliness; particle counting is rarely performed on diesel engine oils since these oils are not transparent enough to transmit light because of high concentrations of soot particles.
In some cases, sample dilution can make it possible to run optical particle count analysis on dark oil samples, however, for diesel engine oils, such large dilution factors are required that the fraction of sample passing through the cell after dilution may not be representative of the neat sample (unmixed) and hence the data may be less than accurate.
However, by using proper solvent blends to achieve the required properties of solvency and viscosity, it is possible to get an isotropic sample with a convenient particle concentration to obtain a representative and natural size distribution and therefore, accurate particle count data using an optical particle counter. In this article, the procedure used to prepare diesel engine oils for optical particle counting is outlined.
Optical Particle Counting
In the field of used oil analysis, optical, automatic particle counters are the most widely used due to their high sensitivity and precision. Moreover, in the author’s experience, automatic particle oil analyzers are faster, more accurate and more reliable than any other method in determining fluid cleanliness levels.
One of the main limitations of this method is that the particle counter’s upper limit is typically around 20,000 particles per milliliter. Used engine oils normally contain higher concentrations than this, requiring heavy dilution. If not performed properly, this dilution can adversely affect the accuracy of this method. Similarly, because optical particle counting relies on either light blockage or light scattering, the results can be affected by the color of the fluid, particularly very dark samples such as used diesel engine oils.
Pore Block Particle Counters
Pore block particle counters work by correlating the number of particles passing through a filter membrane, with the resultant increase in either the differential pressure across the membrane or the flow decay through the membrane. This approach has an advantage in that air bubbles and water droplets are not detected. In addition, because the measurement does not depend on the color of the fluid, it is possible to perform particle count analysis on diesel engine oils and other dark fluids using pore block particle counters.
The biggest drawback with pore block particle counters is that because the membrane pore size is fixed (typically 10 µm) for a particular measurement, determining the particle size distribution in multiple size ranges, as per ISO 4406-99 requires an interpolation of the data based on the pressure drop or flow decay characteristics. This results in a fairly narrow dynamic range of sensitivity, and potential inaccuracy in correctly determining particle concentrations in different size ranges. This is illustrated in Table 1.
Optical Particle Counting on Dark Fluids
In order to allow the use of the more accurate optical particle count method on used diesel engine oils and other dark oil samples, Ingelube S.A. has developed a special sample preparation procedure that allows for appropriate dilution of the sample, without affecting the integrity of the data.
The technique relies on diluting the sample with a blend of high viscosity white oils, chosen to maintain the sample oil viscosity, adjusted with aliphatic organic solvents, such as hexane and cyclohexane to maintain the same solvency, keeping soot and other particles in suspension in the sample. The key success factor was to find a solvent mix that allows dilution of the particle concentration without affecting the viscosity or solvency of the sample.
The degree of dilution used is dependent on the number of particles contained in the sample. Successive dilutions can be made to ensure a measured concentration in the diluted sample of less than the 20,000 particles per ml sensitivity of the particle counter. The actual particle concentration of the sample can then be back calculated based on the dilution factors used.
After dilution, particle counts were made using a standard laser particle counter. By correlating the result obtained from many different diesel engine oil samples with data from other tests, Ingelube was able to relate increases in the concentration of particles in different size ranges with certain problem conditions.
The results of Ingelube’s work shows that automatic laser particle counters can be used in the analysis of diesel engine oils. The benefit of incorporating this technique, as discussed in the introduction, is that automatic particle counters are faster, more accurate and more reliable, compared to alternate methods such as pore block particle counting.
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