Nitration refers to an unfavourable condition in which the oil in gas engines is saturated with insoluble or soluble nitrogen oxide compounds. The combustion chamber of a natural gas engine is one of the few environment that contains sufficient heat and pressure to react nitrogen molecules with oxygen to form nitrous oxides (NOx).There are two types of nitrogen compounds formed when nitrogen reacts with hydrocarbons during combustion: organic nitrates or nitro compounds. While often considered similar to oxidation, nitration is a separate and distinct lubricant degradation mechanism that must be corrected using different methods.
Nitration is typically not observed in modern natural gas-fuelled engines; it is more common in older four-stroke stoichiometric engines that are slower and have lower oil sump operating temperatures. Two-stroke engines are generally not affected by nitration due to the large amount of oil removed from the crankcase that is fed to the oil injection system of the combustion cylinder. This removes most of the nitration products from the exhaust ports. The crankcase also receives more makeup oil.
Organic nitrates – most common form
The most common nitrogen compounds found in gas engine oils are the organic nitrates. As NOx compounds are formed they deposit on the cylinder wall but are wiped into the crankcase by the piston rings. In the crankcase they play an important role in the formation of sludge and/or varnish. These compounds are oil-=soluble but only to a point. When the saturation level is reached they fall out of solution and form light amber-to-maroon deposits around rocker arms, valve assemblies and on the piston skirts. These gummy deposits can also cause oil rings to stick thereby increasing oil consumption as the clearance between the piston ring and liner increases. Like other varnish particles they also reduce filter life.
Nitro compounds – many species
There are many causes for the formation of nitro compounds: piston blow-by due to stuck, worn or broken compression rings; worn-out or scored liners; or exhaust gases leaking into oil from poor valve seating or valve guide wear. Leakage of turbocharger seals, critical engine ignition patterns and combustion patterns, or excessive oil service time are all possible causes.
An oil with a higher concentration of nitro compounds than expected means there is unreacted nitrogen oxide gas. These compounds will cause oil to thicken abnormally and premature dropout of varnish or sludge. This is evident by reddish piston skirt varnish, and sludge in lower compression ring grooves.