Typical natural gas fired engines, such as those used in the petroleum industry to compress natural gas at well heads and along pipelines, have up to 16 cylinders, often generating between 500 to 3000 HP. These engines normally are run continuously near full load conditions with shut downs primarily being for maintenance such as for oil changes. The continuous operation near full load, of course, places severe demands on the engine lubricant. Indeed, because the lubricant is subjected to a high temperature environment oxidation processes occur which limit lubricant life. Also, natural gas engines emit nitrogen oxides (NOx) some of which comes into contact with the lubricant resulting in nitration processes that also will limit lubricant life. Typically these processes are accompanied by increases in oil viscosity. Thus it is desirable to extend the life of gas engine oils by enhancing the oil's resistance to oxidation and nitration and to reduce viscosity increases in the oil.
To extend lubricant life, base oils are formulated with various additives such as dispersants, detergents, antioxidants, viscosity index improvers and the like, to provide a lubricating oil composition. This art of lubricating oil formulation, however, has become increasingly complex with ever more stringent requirements of end-users. Indeed, experience has shown that incorporation of one type of additive in a lubricant composition can have a negative impact on the function of another type of additive. Consequently extensive research continues in the quest for lubricants of improved life and function. Additionally, gas engine oil lubricants are typically formulated with metal containing detergents and metal containing antiwear additives to provide lubricant ash that functions to protect the valve and seat interface of the engine. The ability to provide an ashless gas engine oil, i.e., one having an ash level as determined by ASTM test method D874 below 0.1 mass %, that has enhanced life is particularly difficult.
One object of the present invention is to provide an ashless gas engine lubricating composition that has enhanced resistance to oxidation and nitration.
Another object of the invention is to provide an ashless gas engine oil lubricating composition that has improved life as evidenced by reduction in viscosity increase.