Efficiency challenges for gas engines

As the global demand for inexpensive, sustainable and reliable power increases, natural gas is increasingly seen as a good option for power generation. And, as the use of gas grows, it is inevitable that the number of stationary engines - either involved in moving gas from wellhead to consumer, or in generating power - will also increase. A good understanding of OEMs’ requirements is essential to ensure lubricants meet the needs of this changing and challenging market.

With the continued growth in the use of natural gas as a fuel, one major issue that Felix Kiefer would like to see addressed is its variable quality. “As propane, cleaned biogas and hydrogen are increasingly fed into the supply chain, gas quality is becoming more of a problem,” he explains.

"Gas engine designs need to handle rapidly changing methane numbers and, because the compression ratio is set high to achieve maximum efficiency, robust knock (pre-ignition) control is also needed."

Not only does the industry have to deal with variable gas quality, but also with a number of different gas types. In Europe for example, there is continued growth in the number of engines running on biogas and landfill gas. Felix Kiefer explains that these so-called ‘sour gases’ need careful classification to avoid engine damage.

“We are working with three gas qualities: high, medium and low. For low quality gas we have defined maximum tolerable values for substances including sulphur, hydrogen sulphide, chlorine and ammonia, to ensure the engine and the plant are protected from damage. If the monitored substances are present at less than 20% of the maximum values the gas is rated as medium quality – typically this might be gas from a biogas plant with an active coal filter. Right now, our maintenance schedule and engine life are set at the same level for both medium and high gas qualities.”

Cost saving pressures

With fuel cost savings high on the agenda of gas engine operators we were keen to understand Felix Kiefer’s perspective on the use of lubricants claiming to offer fuel economy benefits. “Because gas engines have high cylinder pressures and high specific engine output, a minimum lubrication gap is needed to prevent mixed friction wear,” he explains.

“Even if short-term fuel economy benefits were visible, the use of SAE 30 or even SAE 20 oils could have negative impacts on the life of engine components. And, since we have not been able to show substantial fuel economy benefits with lower viscosity grade products, we are not focusing on them in our current developments. In my view, the development of new friction modifiers is a far more interesting approach.”

The desire to cut operating costs is also one of the key drivers for the introduction of new high efficiency engines. However, while these engines offer improved efficiency, their complexity and installation costs are rising significantly. As Felix Kiefer explains, it is important to provide the right engine for each market. “We must carefully balance engine performance, operating cost, initial installation cost, usability and serviceability to meet the requirements of each of our target markets.”

High efficiency engines

With emissions and cost reduction high on the agenda OEMs are working hard to introduce advanced hardware.

As we have mentioned previously in Insight, high efficiency engines are very different from conventional engines and will have significantly different lubrication requirements, which will present a new set of challenges.

“For the engine hardware, the challenge is offering the same or even better durability, which means longer service intervals as well as less downtime – all the time bearing in mind engine stress, and therefore wear, is significantly higher. As for the lubricant, it should still offer decent drain intervals, even though it is being stressed more and oil consumption or replenishment is lower. The reduction of crevice volumes, especially piston top land height, is particularly important when we consider that the gas engine oil is required to prevent deposits in ring grooves and to offer good engine cleanliness.”

Big topics for gas engines

One big topic currently being discussed is the use of gas engines for balanced energy and peak load operation. Felix Kiefer explains that gas engines are more flexible in load shifting than gas turbines or larger power plants, such as nuclear or coal.

“We think this might lead to a lot more start / stop events, which should be taken into account when formulating new gas engine oils. Multigrade properties are desirable for engines with no lubricant preheating and, with emissions legislation tightening, the introduction of catalysts seems inevitable. I think that the volume of lubricants with sulphated ash content greater than 0.6 w% will decline.”

Methane slip is another big topic of conversation. Compared with diesel engines, modern gas engines have relatively lower emissions of NOx and CO₂. However, emissions of unburnt methane, also known as ‘methane slip’, are gaining some attention – not only owing to their cost, but also because methane is a greenhouse gas.

“We are involved in several research projects regarding this topic,” confirms Felix Kiefer. “So far we have been focusing on combustion technologies to keep methane slip to a minimum, and research projects have shown that our engines deliver good results. New aftertreatment technologies will be evaluated in terms of conversion rate, cost and of course reliability.”

Current and future lubrication challenges

From a lubricant formulation perspective, we wanted to understand the challenges facing gas engines currently operating in the field. Felix Kiefer says he finds some current situations incomprehensible.

In the future he says that more engines are likely to be installed in areas with challenging environmental conditions, such as high humidity, heat and pollution. “With increasing complexity of these products, skilled service partners and personnel will become even more important. Being able to deal with varying gas qualities, changes in energy management and higher specific engine outputs - while still delivering good reliability - will be the main challenges in the coming years."

Regarding gas engine oils, we believe that good performance of base oil and detergent, in terms of deposit formation, will be increasingly important.

Felix Kiefer, Fluids Technical Expert at Caterpillar Energy Solutions GmbH

Speaking with Felix Kiefer it becomes very clear that he firmly believes engine protection is becoming even more dependent on the selection of a high performance lubricant and this added protection should not be sacrificed for extended oil drain interval.

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