Monitoring Oil Degradation in Gas Engines

Noria Corporation

"What are the acceptable limits of nitrogen oxides in the exhaust of gas engines? Recently, one of our customers was facing nitration issues in a gas engine. A nitrogen oxide test was recommended, and the result was 700 milligrams per cubic meter (mg/m3), which was adjusted to 300 mg/m3."

Oils can break down in a variety of ways. One of the most prevalent ways is the degradation of the base oil. Oxidation is perhaps the most widespread form of this degradation, but nitration is also common, especially in some engines.

Nitration is the breakdown of the base oil caused by the reaction of oil molecules with different compounds to form nitrous oxides and other nitrogen compounds. This can occur in a number of manners, but the most frequent is due to combustion issues.

As nitration progresses, more nitrous oxide compounds are formed, which can lead to an increase in the acidity of the oil, the likelihood of lubricant malfunction and subsequently the wear of internal engine surfaces.

Most oil analysis laboratories test for nitration through a test known as Fourier transform infrared (FTIR) spectroscopy. In this test, an infrared beam is passed through the oil and absorbed at different wavelengths, which correspond to different contaminants and constituents within the oil. This test provides reliable information on things such as soot, nitration, oxidation, fuel, glycol and water.

FTIR is most effective when a reference sample is used for comparison. The new oil should be tested, and the associated wave signature reported. This will serve as the reference signature against which all in-service oil samples will be compared. As a used oil sample is tested, the wave signature is overlaid on the new oil reference signature, and the difference of the spectrum signifies the contaminants or breakdown of the base oil.

When it comes to nitration, the typical absorbance length is approximately 1,630. This wavenumber has a few interferences such as viscosity index improvers and dispersants, but for the most part it is an adequate representation of the amount of nitration compounds found in the sample.

As for setting a limit for nitration, you must use the comparative sample. A cautionary limit is typically a 25-percent increase, while a critical limit would be about a 75-percent increase. This does require some consideration of the engine type and fuel type.

By monitoring the base oil health as well as additive health, you can ensure that you are changing the oil on time before the oil goes bad or excessive wear occurs inside the engine.

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