"What are the main causes of nitration in gas engines — operating load, air/fuel ratio, gas quality or oil temperature? What else needs to be checked?"
Engine combustion chambers provide one of the few environments where there is sufficient heat and pressure to break down the atmospheric nitrogen molecule into two atoms that can react with oxygen to form nitrous oxides (NOx). Nitration is a degradation of the oil that results from a reaction of the oil with gaseous NOx created during combustion. Nitration is somewhat distinctive to natural gas engine oils, especially stoichiometric engines due to their combustion conditions.
Nitration is a concern primarily in older, lower speed four-stroke stoichiometric engines operating with cool oil sump temperatures. Two-stroke engines generally do not have problems with nitration because of the significant amount of oil that is removed from the crankcase and fed to the combustion cylinder’s oil injection system. This scavenges most nitration products out of the exhaust ports. Fresh makeup oil to the crankcase is also increased.
Nitration is an undesirable condition that indicates the oil is becoming saturated with soluble and/or insoluble nitrogen-oxide compounds. The reaction of nitrogen with the base oil forms two kinds of nitrogen: organic nitrates, which are the result of a reaction in the cylinder walls, and nitro compounds, which are caused by a blow-by process where the gas reacts with the oil in the sump. They are independent of the oxy-products that lead to oil oxidation, which is another form of oil degradation.
To identify the source of excessive nitration, consider the following factors, which affect the nitration rate:
Better exhaust gas scavenging (removal of exhaust gases from the combustion chamber), as would occur in turbocharged units, reduces the interaction of NOx with the oil and decreases nitration.
The reaction between NOx and the oil, which causes nitration of the oil, occurs partially on the thin layer of oil on the cylinder wall. The liquid nitration products that form in the oil are apparently destroyed at cylinder wall temperatures greater than 150 degrees C (300 degrees F). Therefore, lower cylinder wall temperatures are needed for oil to nitrate. These lower temperatures are more common in lower speed and naturally aspirated four-stroke engines.
Reduced blow-by lessens NOx contact with the oil in the sump. Blow-by of combustion gases into the crankcase adds to a buildup of nitro compounds in the oil. When ring sealing is poor, more highly nitrated oil will migrate back into the crankcase instead of out through the exhaust port.
Better crankcase ventilation has the same effect as improved scavenging by reducing the contact between NOx and the oil.
Nitration of the oil from NOx in the blow-by gases that enter the sump increases at lower oil sump temperatures, i.e., below 80 degrees C (175 degrees F), and increases as sump temperatures approach 70 degrees C (160 degrees F). This is different from oxidation, which increases with temperature and becomes significant at oil temperatures greater than 90 degrees C (190 degrees F).
Certain base oil types are more susceptible to nitration than others. Base oils with saturated hydrocarbon structures, such as polyalphaolefin (PAO) synthetics and hydrotreated paraffinic oils, seem to be less prone to nitration. Lower viscosity base oils and viscosity index improver additives used in some multigrade oils may be more likely to experience nitration.
Oil makeup does not reduce nitration, but it does dilute the nitrated oil with fresh lubricant. The higher the oil makeup rate in a given engine, the slower the rate of oil deterioration detected in the lubricant.
While evidence exists indicating that spark timing influences nitration, there are no defined studies on its impact.
Low rates of nitration may be maintained if the oxygen level in the exhaust is outside the range of 0.5 to 4.5 percent, with nitration reaching a peak at nearly 3.3 percent oxygen.
Field tests have shown that nitration increases when ambient air temperatures rise and/or engine loads are higher.
Using these factors, you can prepare a troubleshooting guide and inspection/verification list to help control nitration in your engines.