Using Photometry to Analyze Engine Oil Soot

Oil analysts have trusted the blotter spot method to identify excessive engine soot, to evaluate the dispersancy performance of lubricants and to detect the presence of glycol and other contaminants (Figure 1). Though this method has proven effective, individual subjectivity in readings sometimes affects the reliability of results. To increase the accuracy and repeatability of measurements, a Belgian company has refined the blotter spot test using a photometric technique. With new regulations on emissions looming, this technology could play a major role in the management of elevated soot levels. Moreover, it provides information about the oil’s dispersancy performance, a parameter for which no oil analysis test exists except for the blotter spot test.

Instrumentation Scientifique de Laboratoire (ISL), a PAC company, has enhanced the traditional blotter spot method by incorporating a Charged Coupled Device (CCD) camera to measure light absorption across the spot. This eliminates subjectivity and reduces arbitrary aspects of the manual method, producing unbiased results that are highly accurate. ISL’s VPH 5G CCD Photometer is a small bench-top tool for determining the soot contamination and dispersancy performance in used automotive and marine diesel lubricants. Blotter spots are compared to reference standards to produce quantifiable results about soot load and dispersancy - two properties of heightened interest to engine and fleet owners.

Diesel Engine Car
IC: 3.0 MD: 75 DP: 75
Very high quantity of soot due to a long draining period
Diesel Engine Car
IC: 0.5 MD: 28 DP: 36
Poor dispersancy due to a coolant leak
Hydraulic Shovel
IC: 0.3 MD: 60 DP: 12
Feeble dispersancy; poor engine cooling
IC: 1.1 MD: 85 DP: 16
The combustion is correct but the disper-
sancy is slightly degrading due to oil oxidation as the result of a tiring engine
Gas Engine
IC: 0.2 MD: 65 DP: 7
Poor dispersancy due to an extended drain interval
Marine Diesel Engine
IC: 0.2 MD: 59 DP: 8
High lubricant contamination with
heavy fuel
Worksite Truck
IC: 0.8 MD: 95 DP: 4
Good combustion and good

To conduct the test, a small quantity of the homogenized sample is heated to 240°C (464°F) for five minutes. The purpose of this short period of intense heating is to stress any oil that is close to thermal or oxidative failure, so that the blotter spot shows a positive response. Any oil that is still in good shape will not be affected by such a short heating period, which will be reflected in the dispersion pattern of the blotter spot. Once the sample cools, a approximately 2 ml aliquot is dropped onto chromatography paper and allowed to spread, or wick, for one hour in an oven set at 80°C (176°F). The VPH 5G then optically measures the opacity of each spot, calculating a contamination index (IC) and residual dispersancy (MD) by measuring both the light absorbance in the central zone, which provides a measurement of sooty insoluble material present in the sample, and the radial distribution of absorbance, which relates to the oil’s dispersancy. The microprocessor-controlled photometer calculates both values to produce trendable indices.

The Index of Contamination (IC), which represents the concentration of insoluble matter present in oil, is affected by various factors, including machine application, hours on the lubricant and operating environment. This measurement is reported on a 0.1 to 4 percent scale. High IC often signifies inefficient combustion, which can be caused by a fuel injector problem, low compression, ineffective engine respiration, excessive idling or lugging. It is necessary to establish baselines and statistical limits to assess different engine types.

Residual Dispersancy (MD) reflects the oil’s ability to homogenously wick across the blotter and depends upon a lubricant’s dispersive qualities. Dispersancy performance is affected by a number of factors, including oil quality and condition, and engine fuel injection technology, combustion chamber environment. A number of contaminants (including water, coolant, fuel and combustion soot) can also lower the MD ratings. These contaminants degrade the lubricant’s residual dispersion ability, leading to combustion soot build-up, which could possibly block the flow of oil to engine components or could interfere with additive functions.

In addition to IC and MD indices, the instrument also generates a “Pondered Demerit,” which is a combination of inseparable measurements that detect any degradation of IC, MD or both parameters. The Pondered Demerit rating helps the user make an informed decision when the data may not necessarily be clear-cut. For example, if the IC and MD values are reportable but not critical, the Pondered Demerit will determine if there is a problem based on a weighting of the two measurements. The VPH 5G was designed as an effective screening tool for soot loading and dispersancy problems. Whenever a problem with either property is observed, further diagnosis, including more stringent analytical tests, may be required to determine the root cause of the problem.

Figure 2. Analyzing Oil Spots withthe VPH 5G CCD Photomter.

Further Reading
Troyer, Drew. (1999). Practicing Oil Analysis magazine (July - August). “Get Ready for More Soot: Is Your Soot Monitoring Program Up to the EPA’s New Challenge?” p. 17-22.

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