Natural gas engines (NGE) are commonly used to power natural gas compressors, standby electric generators, fire water and irrigation pumps and are increasingly being used to power primary cogeneration electrical power plants. The main advantages of a natural gas engine over a diesel engine are the lower exhaust emissions of nitrogen oxides (NOx), carbon monoxide (CO), particulates and in some cases, lower fuel costs.
These stationary gas engines are available in various configurations and sizes. The characteristics include:
The fuel typically used in these engines is sweet, dry natural gas (sales gas, greater than 85 percent methane). In some cases, in the gas production fields, raw sour natural gas containing hydrogen sulfide (H2S) (up to 8,000 ppm), carbon dioxide (CO2) and nitrogen (N) is used. There is also an increase in the use of digester gas collected from sewage systems and landfill gas that is being used as fuel. Both of these are much poorer quality fuels with lower methane content (50 percent) and may contain as much as 5 percent silicon-based compounds as well as fluorides, chlorides, copper, tin, iron, hydrogen sulfide and up to 50 percent CO2. Fuels with low energy content, high corrosive nature or abrasives (should be prefiltered to less than 0.5 micron) will affect engine performance.
Natural gas engine oils (NGEO) used in these engines have special formulations that differ from diesel and gasoline engine oil formulations. This is because natural gas engines perform the following:
NGEOs do not have American Petroleum Institute (API) minimum performance designations as are common with diesel and gasoline engine oils (for example, CI-4, SL). Most equipment manufacturers specify oils by the characteristics of the oil. Some equipment manufacturers refer to older API “CC” or “CD” diesel engine oil designations to try to establish a minimum performance level, but the use of diesel engine oil performance specifications to classify natural gas engine oils is questioned by many people. Several engine manufacturers have developed their own natural gas engine tests, but for the most part, performance is still measured by field performance. Dresser-Rand and Waukesha cogeneration applications have the only two published approval lists for brands of oils. There has been discussion about developing API NGEO performance designations since the late 1980s, but only limited progress has been made. Therefore, at this time, the user of natural gas engine oils must rely on the integrity of his oil supplier to provide a good quality, true natural gas engine oil and not a rebrand of a diesel engine oil.
Figure 1. V12 Waukesha engine
NGEOs are generally available in two viscosity grades, an SAE 30 (Society of Automotive Engineers) and an SAE 40, although historically most 30-weight oils were blended at the borderline between a 30- and a 40-grade. The monograde 40-weight oils are the most common. Multigrade NGEOs are available in a 15W-40 grade. These are receiving some acceptance in the market where frequent low-temperature start-ups are used or where sump heaters are unavailable or unreliable. Multigrades will provide better low-temperature starting capabilities and may offer reduced oil consumption (lower emissions) and improved fuel economy. However, they may be prone to increased nitration (degradation) of the oil and may not be recommended for use in the governor by the original equipment manufacturer (OEM).
Most equipment manufacturers specify engine oil based on its ash content and viscosity grade. The ash is the portion of the lubricant that is left behind as a deposit after complete burning of the oil. It is whitish-gray and comes from the metallic detergents (calcium and barium) and antiwear (zinc) additives. The ash content of NGEOs is available in four general levels: ashless (less than 0.1 percent sulphated ash), low ash (0.2 to 0.6 percent), medium ash (0.7 to 1.2 percent) and high ash (greater than 2.0 percent). The ash directly provides valve protection in four-stroke engines. The detergent additives (which are bases) neutralize acids. Therefore, the oil with the lowest ash content that will provide the necessary valve protection and acid neutralization is desired. The use of higher ash oils may cause more deposits to accumulate in the engine. Too much ash deposit may be caused by using an oil with too high of an ash content, overlubrication or many other mechanical factors. This may result in reduced heat transfer, preignition and/or detonation, ring sticking or breaking, plug fouling and valve burning.
Figure 2. Valve
Figure 3. Valve
Four-stroke engines typically require either a low or medium ash oil to provide the sacrificial protective layer of ash on the exhaust valves and seats to prevent valve recession. Older Waukesha four-stroke engines are the only engines that require high ash oils due to the high valve angles that were used. Higher ash oils, with higher base numbers are also used to neutralize highly corrosive fuels. Two-stroke engines do not have intake or exhaust valves and generally require an ashless or very low ash oil to minimize exhaust port blockage. These two-stroke engines typically have large bore cylinders and have oil injection ports feeding oil directly into each cylinder. The most universal oil is a low ash oil, making it the best choice in a situation where multiple types of engines are supplied from one lube oil tank.
Exhaust valve recession and torching (guttering) are major issues with engine operators. Valve recession is the pounding of the valve back into the cylinder head by the repeated closing action of the valve (Figure 3). The valve, as well as the insert seat in the head, is damaged. It can be caused by lubricant-related factors such as insufficient ash level of the oil and/or insufficient oil feed down the valve stem, and ash composition. Nonlubricant-related valve recession can be caused by many mechanical and operational factors. Guttering and torching valves (cutting of a channel across the valve face) can result from both lubricant (too much ash) and nonlubricant factors, as well as from valve recession itself (Figure 2). This is a complicated process as these factors are intertwined with each other, which makes the process of finding the root cause of a valve failure difficult.
Figure 4. Valve deck from a Caterpillar
3512TALE natural gas engine showing
very low to absent varnish and
sludge using Sentron LD 5000 (Petro-Canada).
Oxidation and nitration of the oil are both mechanisms which degrade NGEOs. Nitration is a degradation of the oil which occurs due to the reaction of the oil with gaseous NOx created during combustion. It is discussed here because unlike oxidation, nitration is somewhat unique to natural gas engine oils, especially stoichiometric engines, due to the combustion conditions within these engines.
Figure 5. Cylinder head from a Caterpillar 3512TALE
natural gas engine showing very low deposits
after 8,824 hours (Petro-Canada).
To understand nitration, two processes need to be considered. First, the formation of NOx and second, the reaction of NOx with the oil (nitration).
NOx formation is related to the air-to-fuel ratio, peak combustion temperatures and engine RPM as well as engine spark timing, load, ambient air conditions and aftercooling.
These NOx gases may leave the combustion chamber with the exhaust or react with the oil film on the cylinder wall or with the oil in the crankcase sump to form various complex liquid nitration products. The reactions in the oil that cause nitration are different from those occurring in the oxidation process and are not as thoroughly understood.
Figure 6. Connecting rod
bearing shows very low
Nitration of the oil is affected by:
If the oil is exposed to severe nitration conditions, the nitration products in the oil will cause:
Nitration is a concern primarily in older, lower speed, naturally aspirated, four-stroke stoichiometric engines operating with cool oil sump temperatures. Two-stroke engines, generally do not have a problem with nitration due to the significant amount of oil that is removed from the crankcase and fed to the combustion cylinder oil injection system which scavenges most nitration products out the exhaust ports. Also, fresh make-up oil to the crankcase is increased.
Figure 7. Oil pan shows very low to absent
varnish and sludge (Petro-Canada).
Nitration is controlled by regulating the operational factors mentioned above. From an oil formulating perspective, nitration can be controlled only by the nitration-resistant components selected for use in the oil. There are no additives known at this time that can be added to an oil to inhibit the nitration process.
Oil change frequencies or drain intervals are recommended by each engine manufacturer and are typically in the range of 750 to 1,500 hours. The use of poorer quality fuels, as mentioned above, plus many other mechanical factors (for example, coolant leaks, poor filtration, high loads) can significantly reduce these drain intervals. They can also be extended, but require good maintenance and operational practices, and oil monitoring through used oil analysis. Additional oil testing, such as Fourier transform infrared spectroscopy (FTIR) to monitor oxidation and nitration, base number (BN) to monitor detergency additive depletion from acids neutralizing the reserve alkalinity, and acid number (AN) to monitor the buildup of organic acids that form should be used to monitor the oil when extending drain intervals.
Figure 8. Piston undercrown shows
extremely low varnish deposits
Standard spectrographic analysis is still required to monitor wear metals, and the standard tests for viscosity, water, glycol, etc. should be performed as well. Some two-stroke engines rarely, if ever, require an oil change because oil is taken from the engine sump and injected into the combustion chambers for lubrication and the high rate of make-up oil required in the sump virtually changes out the oil on a regular basis. Filtration on these units is therefore important. Overextended oil drains can cause head damage, varnish and sludging, ring wear and sticking, liner wear, higher maintenance costs and shorter filter life.
Figure 9. Piston ring zone from a
Caterpillar 3512TALE natural gas engine
after 8,843 hours shows extremely low
carbon deposits. Oil changeouts were
done at an initial 2,200-hour interval and then
at 3,000-hour intervals (Petro-Canada).
Figure 10. Piston liner shows very low polishing
The filters typically provided with a natural gas engine by the OEM have historically been large-depth sock filters containing cellulose. Centrifugal filters, installed on a side-stream of the circulating oil, are now being used in conjunction with the canister filter and are gaining significant acceptance. Inadequate filtration will cause varnish and sludge build-up, wear, shortened oil life and ring sticking.
Catalyst systems can reduce emissions even further and generally fall into three categories: nonselective catalytic reduction (NSCR) systems are the most common and are used on stoichiometric engines, selective catalyst reduction (SCR) systems which are used on lower air-to-fuel ratio lean burn engines, and oxidation catalyst systems which are used on lean burn engines to reduce hydrocarbon and carbon monoxide emissions. With most catalyst systems, catalyst element poisoning by phosphorous compounds (zinc antiwear, ZDDP) and ash is a concern and is related directly to the lube oil used. For this reason, the specific manufacturer of the catalyst may have limits on these compounds, such as 300 ppm phosphorous maximum, as well as oil consumption.
Cooling systems have a direct impact on the engine operating temperature and thus the engine oil. Systems need to be monitored and maintained. The water needs to be of a better quality than conventional drinking water because minerals (calcium and magnesium) in the water, as measured by water hardness, are less soluble in higher temperature water and will deposit on the hottest parts of the engine (cylinder head, etc.) and form an insulating layer. Many mechanical failures (valve sticking) which are initially attributed to the oil, have their root cause related to the glycol cooling system.
Cogeneration usually refers to generating energy from both the power at the crankshaft and heat recovery from the remainder of the engine. Cogeneration is becoming more common for electrical power generation. A generator is powered off of the crankshaft and heat from the exhaust, a glycol cooling system and the oil cooler are used to create steam or heat another process. Overall energy efficiency is increased from 33 percent to approximately 85 percent. The additional demands placed on the engine create higher operating temperatures which place higher demands on the engine oil and may contribute to valve recession and engine liner scuffing.
Natural gas engine oils have changed significantly in the past 15 years from a time when diesel engine oils were often used. The emphasis on emissions and the introduction of lean burn engines, catalytic exhaust systems, higher speeds and more turbocharging have changed the composition of the oils required. It is recommended to use the best fuel available, buy your oil from a reputable supplier, operate the engine according to the engine manufacturer guidelines, filter the oil as well as possible, maintain the coolant and implement a properly designed used oil analysis program.
This article originally appeared in the STLE Alberta Section, Basic Handbook of Lubrication, Second Edition, 2003.