In Europe, there is a growing demand for gas among large sites such as factories, schools and hospitals for use in heat and power generation. Similar market forces are at work in the United States, where the use of natural gas for powering pipeline distribution has risen by more than 25 percent in the past five years. The U.S. Energy Information Agencys 2017 Annual Energy Outlook shows that natural gas surpassed coal as the leading fuel for electricity generation in 2015, and the agency foresees domestic natural gas consumption growing almost 1 percent a year on average through 2050, led by de­mand from the industrial and power sectors.

This growth is attracting new market participants and encouraging existing original equipment manufacturers to increase their gas engine product portfolio. With an increase in the number of gas engines in use, there is clearly a corresponding global growth in the demand for gas engine oils.

Stationary gas engines differ greatly from conventional diesel or petrol engines. In addition to running on a different fuel, they can be very large, with some engines weighing more than 80 metric tons. Besides these fairly obvious differences, gas engines have higher combustion temperatures, higher engine loads and different operating design, all of which impact their lubrication requirements.

Gas engine oils have a number of important functions to perform. Not only must they protect the engine by preventing deposits, corrosion and wear, they also need to minimize engine downtime and continue to deliver protection over extended drain intervals. In short, imagine running a car engine flat out for 10 years at full load without changing the oil: That is equivalent to the demands put upon a gas engine oil.

A Complex Challenge

As the market continues to evolve, gas engine OEMs face several technical challenges. Legislated emissions reductions continue to tighten from country to country. In addition to high power output, todays end users look for low operating costs over the lifetime of the engine, expect their engine to stay reliably in service for long periods of time, and demand long oil drain intervals to reduce both engine downtime and maintenance costs.

These considerations are driving the development of new, more powerful, high efficiency engines that typically have tighter tolerances and reduced clearances for engine components, higher compression ratios, increased turbocharger pressures and more precise combustion and valve timing.

Performance improvements have been achieved through several modifications to engine design, including higher brake mean effective pressure; the adoption of Miller cycle timing; changes in piston bowl geometry, piston design and metallurgy; and movement to lean-burn combustion. However, these design modifications have also had the corresponding effect of creating a harsher environment for the engine lubricant.

Changes in piston design have put the lubricant closer to the combustion zone, which not only increases oil oxidation, but also impacts other used oil parameters. For example, base number can be rapidly depleted, which reduces the lubricants ability to neutralize combustion acids.

The increased variety of gas sources, including biogas and landfill gas, is adding to the lubricant challenge. While natural gas contains pure methane and clean-burning hydrocarbons, landfill gas and biogas contain significant levels of carbon dioxide, sulfur and chlorine, which can form acids during combustion. They may also create hard silica deposits on the cylinder heads and valves, potentially leading to damage.

The latest gas engines require more protection from less lubricant. Not only is exceptional hardware protection needed over longer oil drain intervals, but this must be achieved with reduced oil consumption. Since there will be less frequent top-up with fresh lubricant, the oil needs to work harder for longer.

The demands of modern gas engines raise questions about the ability of existing lubricants to provide the same level of engine protection that has been achieved up until now, throughout the expected oil drain duration.

This uncertainty may result in shorter oil drain intervals-a highly undesirable outcome for end users who are looking to improve the return on their investment by maximizing generating efficiency and uptime and reducing running costs. Clearly, taking the engine out of service for more frequent oil drains or incurring additional maintenance costs can negate any efficiency gains achieved by design changes.

Unlike engine oils formulated for heavy- and light-duty applications, gas engine oil approvals are not driven by industry specifications, but by field performance. Modifications in modern engines, taken alongside increased use of more severe gaseous fuels and consumer performance requirements, pose significant challenges to the gas engine oil formulator.

The answer to this set of challenges lies in higher quality lubricants, which can help maximize the performance of modern gas engines and allow them to be operated at higher temperatures and pressures for longer periods, without increasing the risk of engine damage.

Cleaner, Better, Longer

While the choice of base oil is certainly important in defining robust lubricating oils for future engine demands, prolonging lubricant life, and aiding in the protection of the engine to keep it running, a carefully formulated chemical additive package is crucial.

When formulated correctly, a tailored chemical additive package can help reduce the risk of ring sticking, bore polish, filter plugging and bearing damage in modern gas engines. In addition to this hardware protection, advanced lubricant technology can enable operators to keep their engines running efficiently for longer.

The biggest formulation challenge for the additive package is striking the correct sulfated ash balance in the finished oil. Ash, which is predominately derived from detergent and antiwear additives, is needed to form a protective layer over the valves.

However, too much ash can lead to valve torching or guttering-damage in which a channel is cut across the valve face-leading to loss of engine power. On the other hand, too little ash can cause valve recession, in which the sealing face of the valve is damaged, again resulting in loss of power. Both or either of these conditions lead to repair costs and engine downtime.

Detergents are key additive components that can help both to reduce deposit formation and to control acids. However, not all detergents are the same, and it is essential to choose the best detergent chemistry to deliver the best performance for any particular application.

As part of an ongoing gas engine oil development program, Infineum has been using a variety of tests to assess the effectiveness of various additive chemistries. While the correlation between bench and engine tests can be poor, the former can provide a useful way to probe the challenges presented by the latest engines. By understanding the tests and the chemistry at play, some bench tests can be modified to give a better correlation to the field.

Throughout this testing process, salicylate-based detergents have demonstrated superior cleanliness in the micro-coking test (GFC LU 27-A-13) compared to sulfonates or phenates. In addition, salicylate has showed the most robust oxidation and nitration performance as well as improved base number retention and acid control in fired engine testing. As such, salicylate detergents appear to provide superior cleanliness and offer an ideal choice for gas engine lubricants. As the project continues, Infineum will continue to monitor and report on the findings.

Developing lubricants for the latest high efficiency engines requires in-depth formulation knowledge to ensure the right additive chemistry combinations are carefully balanced to deliver the desired performance. The end product not only needs to deliver excellent deposit control, but also longer oil drain intervals, high-temperature stability, high-pressure stability, increased oxidation and nitration control, and improved total base number retention as more landfill and biogas is used.

The detergent, dispersant, antiwear and antioxidant components must be carefully selected based upon their individual performance attributes, and the formulator must also fully understand the positive and negative interactions that these components can have on each other. In addition, the impact of formulating in different base stocks must be taken into consideration, to ensure that the final lubricant maintains engine performance throughout its lifetime.

Ultimately, the challenges posed by next-generation gas engines mean that its now more important than ever for the industry to work collaboratively on hardware and lubricant development to meet current and future demands.

Jonathan Hughes, Ph.D., is a lubricants development technologist at Infineum U.K. Ltd. With a background in chemistry, he joined Infineum in 2014 and has since worked on the research, development and formulation of additive packages for lubrication of gas and marine engines. He can be reached at Jonathan.Hughes@
Infineum.com.