Oil Requirements

Stationary gas engines are used in diverse applications, mainly for electricity generation. They are typically installed as stationary, continuous-generation units, but can also operate as peak-power generation plants or to smooth out fluctuations in local electricity demand. Also, they can produce electricity in parallel with the local grid, in whats known as island mode operation. Today, the demand for renewable and environmentally friendly technologies is growing, and these engines can produce energy close to where it is actually used, for example in remote areas.

Karin Baumann, head of European research, development and technical services at Lukoil Lubricants Austria, said that oil-cleansing methods and additive and base oil systems are vital in the formulation of modern gas engine oils. In addition, the approval process for these oils can be costly, time-consuming and fraught with uncertainty. Lukoil Lubricants Austria is a subsidiary of the Russian oil majors lubricant business operation in Europe.

OEMs mainly require oils that meet SAE 40 or API CD or CF specifications. The engine crankcase can hold from 150 to 800 liters, while oil change interval can vary from 300 to 4,000 hours, Baumann told the UNITI Mineral Oil Technology Conference, held in April in Stuttgart. She added that oil consumption can range between 0.1 and 1 gram per kilowatt-hour.

The approval procedure for stationary gas engine oils can take 2 to 5 years. One complicating factor is that laboratory and aging tests sometimes do not match field test results, for a variety of reasons. There is a need for about 4,000 to 10,000 hours of field testing to verify performance. Such rigorous testing is the only way to gain formal endorsement of the oil, she said.

Oil Demand

The European Unions annual demand for stationary gas engine oils stood at nearly 60,000 tons in 2011. In the same year, global demand reached 370,000 tons. General Electric Group and Caterpillar-MWM are the largest manufacturers of these engines. In 2011, the companies held the largest share of the market both in Europe and globally, Bauman said, citing a European Commission report.

In Europe, GE held 44.5 percent of the gas engine market, while Caterpillar-MWM held 31.4 percent of the market, followed by Tognum-Rolls Royce (3.8 percent), Guascor (1.6 percent) and Wrtsil (0.4 percent).

On the global level, GE Group and Caterpillar-MWM held 41.8 and 35.4 percent of stationary gas engine sales in 2011, respectively. The rest of the market was divided among Cummins (11 percent), Guascor (7.3 percent), Tognum-Rolls Royce (4.6 percent), Mitsubishi (1.3 percent) and Wrtsil (0.4 percent).

Lukoil found that to increase fuel economy, reduce running costs and reduce maintenance costs, OEMs have introduced high-efficiency engines with innovative materials and shorter piston designs. As the engines become more sophisticated, they put more stress on the oil, said Baumann. The oil has to have good neutralization capability and corrosion protection.

Other essential properties include good total base number retention, good acid neutralization, aging and oil thickening resistance (provided by thermal and oxidative stability) and wear protection over the entire oil drain interval. Finally, modern oils must have low sulfated ash content to prevent deposit formation in the combustion chamber, Baumann observed.

Gas engine oils must be capable of withstanding a number of harsh operating conditions. According to Lukoil, these include gas impurities such as acidic components, hydrogen sulfide and ammonia, which can reduce engine life. It depends on the type of gas used, Baumann explained. Natural gas has no impurities and a low methane number. But biogases or landfill, sewage and wood gasses have multiple impurities and higher methane numbers that have to be addressed by the oil formulation, she said.

Another factor is the engine operating regime, including varying powers and loads and high operating temperatures. Oil sump volume, oil consumption and the top-up interval also play roles in oil formulation.

Baumann added that the selection of additives and base oils is critical in formulating modern gas engine oils. Oxidation and nitration control are a must. Thats why we have seen the increased use of Group II and Group III base oils to provide this protection. The traditional base stock is Group I, with some minor use of Group IV and V. Brightstock is unpopular due to deposit formation, she related.

Another important consideration is oil formulation conflicts. For example, the selection and treat rate of detergents and dispersants are key factors controlling durability, wear and corrosion protection and deposit control.

Typically, engines operating at high temperatures require high levels of detergents and dispersants. However, this is complicated by the fact that most OEMs prefer low-ash and even ashless engine oils. Thus, detergent and dispersant levels must be carefully balanced to prevent combustion chamber deposits, exhaust system blockages and preignition problems, Baumann observed.

OEM approvals require a number of steps, including:

Product development and lab testing.

Field trial approval, including data approval and OEM in-house testing.

Field trial preparation, including identifying suitable test sites.

Field trial in which the OEM supplies the oil, prescribes the oil flushing method and provides compensation for downtime.

On site engine rating, with parts sent to the OEM.

Approval and listing of the product for specific applications or engines.

Baumann explained that the approval process is very costly and that successful completion cannot be predicted based solely on lab and oil aging results. There are no universally recognized laboratory tests for gas engine oils, and field performance remains the ultimate proof of performance to gain approvals, she said. Most OEMs specify physical and chemical properties for different application such as ash level, base number and viscosity, and field test procedures differ from OEM to OEM, she concluded.