The Battle Against Nitration of Natural Gas Engine Oils

Nitration of natural gas engine oil (GEO) remains a leading concern for operations. Why? Because of its potential to cause serious problems for today’s engines, including shortened filter life, varnish and sludge deposits and sticking piston rings.

This article explores:

  • Causes of nitration
  • How to combat nitration

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Nitration of natural GEOs remains a leading concern for operations. Why? Because of its potential to cause serious problems for today’s engines, including:


  • shortened filter life
  • varnish and sludge deposits
  • sticking piston rings

Although the issue is especially prevalent for low-speed (less than 700-800 rpm) 4-stroke stoichiometric and lean engines that operate at various loads, nitration can also occur in engines running at higher speeds.


Knowing the causes of nitration and how to correct them is vital to maintaining reliable and efficient operations, including the ability to effectively extend oil drain intervals.

Nitration of the lubricant occurs when there is enough heat, pressure and even oxygen inside the combustion chamber to break down atmospheric nitrogen from its stable N2 form into individual nitrogen atoms. These reactive atoms can then combine with free oxygen to form nitrous oxides, commonly referred to as NOx (i.e., NO and NO2).


From here, 2 types of nitrogen compounds can be formed:


  • Organic Nitrates on the cylinder walls, piston lands, internal side of rings and valves
  • Nitro Compounds in the oil sump, oil galleries, oil coolers, etc.

Organic nitrates are typically the leading form of nitration observed in natural GEOs.


As these nitrates are formed they are wiped into the crankcase by the piston rings. Though they are soluble in the engine oil initially, when an excess level is reached these nitrates can drop out of the lubricant to form harmful varnish and deposits in an engine’s rocker and valve assembly. These deposits can also cause piston ring sticking and plugged oil filters.

A number of factors increase the occurrence of nitration. Some of the most prominent are:


  • Exhaust Gas Scavenging, the removal of exhaust gases in the combustion chamber. This is improved in turbocharged units, which then reduces the opportunity for interaction between the NOx and the engine oil. Crankcase ventilation has a similar effect.

  • Cool Cylinder Wall Temperatures promote nitration as organic nitrates decompose at cylinder wall temps above 150°C (300°F).

  • Blow-By increases the amount of combustion gases entering the crankcase and reacting with the oil, making good piston ring sealing a necessity for high-risk engines.

  • Low Oil Sump Temperatures, below 80°C (165°F), increase the occurrence of nitration. This is the opposite of oxidation of the engine oil, which increases above temperatures of 90°C (190°F). As a general rule, the oxidation rates of oils double for every 18°C (64°F) increase in temperature above 70°C (158°F).

  • Base Oil Type used in the formulation of the gas engine oil has a large effect on the susceptibility of the oil to nitrate. High-quality Group II/II+ and Group IV (PAO synthetic) oils are less susceptible to nitration because of their saturated hydrocarbon nature, which makes them less reactive.

  • Air/Fuel Ratio, the highest rates of nitration occur when oxygen levels measured in the exhaust gas are between 0.5 to 4.5 percent with nitration rates peaking around 3 to 4 percent excess oxygen. Therefore, measuring air/fuel ratio is very important in determining nitration rates.

  • Air Temperatures and Loads. Petro-Canada field tests and experience have shown that nitration also increases when ambient air temperatures rise and engine loads increase in many stoichiometric engine types due to some of the above effects.

The first step is to correct the causes of nitration in the first place; however, diligent monitoring is also key. This includes:


  • regular visual inspection for deposits in the rocker arm and valve assembly
  • close monitoring of higher oil consumption and filter plugging

Operators should also consider monitoring engine oil nitration through known techniques such as Fourier Transform Infrared spectroscopy (FTIR). Oil analysis laboratories will determine the extent of nitration by comparing the used engine oil sample to the reference spectrum of the same oil in its fresh state.


Selecting the right product for the application is, of course, also critical.


Petro-Canada Lubricants’ SENTRON™ engine oils naturally offer excellent nitration resistance due to the use of high-quality, 99.9% pure base oils.


In addition, premium gas engine oil products such as SENTRON LD 8000 and SENTRON 590, formulated with selected additives, provide enhanced nitration control for less downtime and better engine efficiency.

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