New cylinder lube oils: more than just BN index

Managing acid neutralisation used to be the main issue for cylinder lube oils, but low-sulphur fuels are creating new challenges

The 2020 sulphur cap will impact a range of technologies and engineering practices, key among them being lubrication, especially cylinder lube oils. The cylinder lube oil’s essential function is to lubricate and help seal the combustion chamber by sealing the interface between the piston ring and the liner. Other functions include protecting surfaces against corrosion and cleaning the combustion chamber and components.

To date, the high sulphur content of most bunkers coming in has led to a focus on the corrosion-fighting capability of cylinder lube oils. Cylinder lube oils have been identified through their base number (BN), which indicates acid neutralisation potential. In general, cylinder lube oils with lower BN will be appropriate for low-sulphur fuels; too high alkalinity in the cylinder lube oil will increase deposit formation.

Many of the low BN lube oils designed for low-sulphur distillate fuels may not be appropriate for low-sulphur fuel oils, since these will be residual fuels or blends. Residuals are less pure than distillates and would therefore need better detergent capabilities in their cylinder lube oils. This would help ensure the cleanliness of the combustion chamber. Higher BN often brings with it better detergency but, with BN reducing, the additive package in the cylinder lube oil will have to handle the higher deposit-forming capability of residuals over distillates.

Cylinder lube oils will also have to keep impurities suspended in the oil so that they are transported to the piston underside and drained off; they should prevent agglomeration of impurities. This means better oxidative stability if their residence time becomes higher, taking advantage of less acid neutralisation requirements.

The risks that may arise from variations in bunkers remain unknown. For instance, according to LUKOIL, a major challenge will be the fuels’ capacity to dissolve asphaltenes which contribute to deposit formation and increase wear. Blends coming in may disturb the fuel’s asphaltene solvency and combustion quality: “Hence, the crucial properties of suitable cylinder oils are their detergency, asphaltene-handling properties and deposit control in order to ensure ring pack/engine cleanliness,” says the company. It continues: “The BN does not give any indication about the quality level of the lubricant, which makes it even more difficult for a customer to compare the different qualities that are on offer. Today’s standard 70 BN marine cylinder lubricants may not do the job, even existing 40 BN cylinder lubricants which were formulated for pre-2020 residual fuels may not.”

New products

In the run-up to the sulphur cap, nearly all lube oil vendors have introduced new products to cater to the incoming low-sulphur fuels. They incorporate elements such as the change in level of acid neutralisation requirement, enhanced wear protection due to the poorer lubricity of new fuels, handling the likely higher levels of insolubles in the fuel, as well as oil spreading qualities.

While Marpol Annex VI has reduced the sulphur cap worldwide to 0.5% and 0.1% for ECA, it has also spawned scrubbers, which allows the use of bunkers with higher sulphur than before. Lube oil companies have introduced cylinder lube oils with less BN for the new low-sulphur fuels, as well as much higher BN for those ships with scrubbers. The latter can use higher sulphur than the 3.5% permitted until 2019. Such ships may be fewer in number compared to those using low-sulphur fuels, which means cylinder lube oils with low BN will predominate as products for two-stroke, crosshead engines.

For crosshead engines, the system oil need not be changed but the correct cylinder lubricant is critical, according to Ian Thurloway, brand, marketing & business development manager at Chevron Marine Lubricants. For trunk piston engines the story is different. If the vessel is moving to a 0.5% low-sulphur fuel oil (LSFO) there is an opportunity to move to a lower BN lubricant, but monitoring the BN depletion is key to optimal operation. If the vessel is moving to a high-sulphur fuel oil (HSFO) with scrubber technology a higher BN lubricant should be used to cope with the increased sulphur level.

The lube oil tanks need not necessarily be cleaned to take in oil with a different BN, says Mr Thurloway, assuming they are in good condition. As the fuel is changed from high sulphur to low sulphur, the new lube oil can be mixed into the tank. “In the short term, using a lubricant with a higher BN than required on a low-sulphur fuel does not present too much of a problem, but should be minimised as much as possible,” explains Mr Thurloway.

Overall, the feed rate of cylinder lube oil may need to be adjusted depending on the operation. Feed rate optimisation is key to efficient operation and minimising cost. With feed rate and the correct BN being so important to engine operation, Mr Thurloway recommends that operators use the DOT.FAST drip oil test programme. This measures the total iron content in the product, giving the user the abrasive and corrosive wear potential and other properties to determine the optimal feed rate. DOT.FAST is used on board for immediate results on drip oil iron content and shore samples for a comprehensive overview and expert advice.

Mr Thurloway says one of the changes the 2020 cap will bring about that will have a bearing on the lube oil supply chain is that while the major bunkering centres like Rotterdam and Fujairah may be able to offer various fuel options, minor ports may be more limited in their fuel offering. Lube oil suppliers must be agile in supplying the requisite BN oil to cater to a potential shift in global demand, he says.

During the transition, LUKOIL advises that ships buy and store the new formulations in separate tanks. If mixing of lubricants is inevitable, LUKOIL says its recommended product for low sulphur fuel is miscible and compatible with other LUKOIL marine cylinder oils. In such cases, when topping up oil that is already in the tank with the newly ordered oil, LUKOIL recommends mixing ratios. The company recommends that the mixing of lubricants from different suppliers be avoided or kept to a minimum.

For trunk piston engine oils, LUKOIL recommends staying on the product currently in use and sending in a used oil sample for analysis after changing over to the new fuel. The new BN equilibrium will show whether a change to a lower BN product is advisable. If an engine prone to lacquer formation is changed over to a purely distillate fuel (0.1% sulphur), LUKOIL recommends changing the system lubricant.

Moving away from a high-sulphur level may well present opportunities in lubrication system design and functioning. With low-sulphur fuels offering a less corrosive environment for the liner, liner cooling may be optimised. The residence time of cylinder lubricants may be extended since acid neutralisation is not such a demanding requirement in low-sulphur oils. Additive requirements are fewer and so cylinder lube oils can focus more on higher oxidation stability, deposit control and lubricity.