Lubricating Natural Gas Engines

"We are considering purchasing several natural gas engines and would like to know how they are different from gasoline and diesel engines?"

Unlike gasoline or diesel engines, natural gas engines require somewhat different oil formulations. These engines can reach up to 16,000 horsepower with up to 20 power cylinders and oil reservoir capacities of 1,585 gallons. Their speed can range from 300 to 2,000 rpm.

The quality of the natural gas fuel used can vary widely. It can range from sweet methane to raw sour natural gas containing hydrogen sulfide, carbon dioxide and nitrogen, or even poorer quality digester gas from landfills.

Most of these engines are turbocharged and may be of either two- or four-stroke design. Natural gas engines are most commonly used to operate natural gas compressors, electric generators or cogeneration power plants.

The engine and driven compressor units can be separately joined by a crankshaft or be of the integral type, where the engine and compressor have a common drive shaft.

Natural gas engines also have special requirements that differ from diesel and gasoline engines. This is because the natural gas engine combustion process is affected by the type of fuel.

Most of these engines operate indoors, particularly in warm climates such as the southern United States where higher ambient temperatures affect operation.

The combustion process burns much more cleanly, therefore soot levels are very low. Consequently, high detergent additive levels are not generally necessary.

The process burns methane type fuel, so fuel dilution is not a problem. However, because of higher combustion temperatures, oil viscosity increases can be a serious concern. These higher combustion temperatures cause chemical oxidation and nitration to occur within the oil.

Operating speed is generally constant; therefore, these engines are prone to deposit formation.

Exhaust valve recession and burning are other concerns when operating natural gas engines. Valve recession is the gradual wearing of the valve into the head. It is caused by insufficient lubrication or insufficient ash deposit cushioning the valve seat area.

Frequently, a valve may also suffer damage called guttering, which is a deep channel cutting across or into the valve seat area. The causes of these conditions are many and varied and can be quite different, depending upon the engine design.

Lubricant formulation (as it relates to ash deposits) and viscosity, operating temperatures, exhaust gas temperatures, natural gas fuel quality, engine design characteristics and air/fuel ratios should all be considered when investigating the root cause of an exhaust valve failure.

The issue of ash deposits in particular is much discussed among operators of natural gas engines. Ash deposits are the residue remaining after the oil is burned during operation. The ash residue is made up of metallic detergent additives, such as calcium, barium and magnesium compounds.

These ash deposits, if adequate, can prevent valve recession. However, if the ash content is too high, the result will be unwanted and harmful deposits. Consequently, lubricant manufacturers and blenders must take great care that the proper selection and quantities of anti-wear and detergent additives are applied when formulating natural gas engine oils. The selection and application also should always take the engine manufacturer's recommendations into account.