Additives in lubricant oils need to be fine-tuned to optimise the operations of two-stroke engines burning very low-sulphur fuel oil

Lubricants prevent friction and engine wear, clean piston rings and crowns of debris and neutralise acidity in the fuel mix. These beneficial properties come from additives in the lube oil, which are especially important in the era of ultra-low sulphur fuel, changing fuel availability and requirements for maintaining engine operation.

MAN Energy Solutions director of new technologies and two-stroke promotion department Kjeld Aabo has been a senior engineer with the MAN B&W and now MAN Energy Solutions since graduating as a mechanical engineer in 1983. He has been the head of research and development and a customer-facing engineer for decades and has vast experience with marine two-stroke propulsion units.

Mr Aabo said that although MAN ES has no experience in producing additives, as a major OEM the company has a vast experience of managing cylinder condition and testing lube oil. “We work very closely with additives and lube oil companies in order to have the best solutions,” he said. The main functions of cylinder oil (SL2020-894 cylinder oil) are: lubricate the piston ring and liner; reduce the friction; introduce wear protection; minimise the risk of seizures; neutralise acids and oxidation products; and keep the piston, piston rings, ringlands and liner clean.

Lubricants help to maintain engine performance to prevent unplanned machinery downtime, lubricating the piston and liner, reducing friction, and introducing wear protection to prevent seizures. “Cleanliness of the piston rings and crowns is important to secure an acceptable time between overhaul of the cylinder units,” said Mr Aabo.

The current and foreseeable scenario is for operation on very low-sulphur fuel oil (VLSFO), which will be predominant in the coming years; additives in lubricant oils will need to be adjusted accordingly. “This is such a big change from before, when most engines were operating on heavy fuel oil with a much higher sulphur content,” he said. “Highly fuel-efficient engines with higher pressure and higher temperature require lubricants with matching performance,” Mr Aabo said.

MAN ES gives letters of no objection to the cylinder oils from different manufacturers and splits them into two different categories. Category 1 is for the MAN ES engines of Mark 8 and below. Mark 9 and higher require category 2 lube oils. At the moment, there is no 40 BN lube oil: “We have set a target for the industry to produce a Category 2 lube oil of 40 BN with a good performance. We have also removed the 15-25 BN columns from our guidance. They may be re-instated if a lube oil can produce a good performance in this range,” said Mr Aabo.

Referring to the Category 2 cylinder oils, Mr Aabo was generally pleased with the performance. The BN 100 and BN 140 cylinder oils have been tested and are now applicable and recommended for MAN ES two-stroke engines of Mark 9 and higher when operating with high-sulphur fuels with scrubbers. The guidelines from MAN ES are to: focus on monitoring cylinder condition; ensure the ring pack is clean and moving freely; use a cylinder oil with a higher BN for a short time as a cleaning regime to remove deposits; and monitor scavenger port condition and act quickly.

“This is a lot of work onboard and not the best solution,” said Mr Aabo. “The solution is to produce a BN 40 cylinder oil with the needed properties.”

Lubrizol technical manager for marine diesel engine oils Ian Bown is familiar with the challenges facing additive manufacturers in producing the cylinder oil specification outlined by Mr Aabo. Mr Bown joined Lubrizol in 1988 and has held the position of technical manager for marine diesel engine oils since 2013.

Mr Bown said there were additive design challenges because of the different engines, fuels, and operational requirements. Challenges include the temperature these oils work in, properties of new fuels and maintenance requirements of engines.

“We have to consider how we decouple acid neutralisation from cleanliness,” he said. “The focus has shifted more toward cylinder and ring pack cleanliness, but we have to maintain acid neutralisation as we go through that process.”.

The majority of constituents of lubricants are base oils, but the cleaning and acid neutralisation comes from the performance package, said Mr Bown.

With VLSFO, there are different requirements from these packages and challenges.

“With different chemistry, modifications and additives we could overcome these issues,” he said. “The additive content of the cylinder is going to change and maybe increase.” Mr Bown said new bench tests of lubricant oils with engines are required because of higher VLSFO consumption in shipping.

Lubrizol has developed a hot tube test method which incorporates VLSFO and shows better correlation to the engine test results.

This led to the development of 40 BN additive with novel dispersant.

Gulf Oil Marine technical manager for Europe and Africa Paul Elliott has been with Gulf Oil Marine for five years and was previously the marine fuels consultant for BP Marine. He noted that IMO 2020 was not the first instance of low sulphur fuels bringing about change in the marine lubricant market. There were the ECAs and SECAs that demanded low sulphur fuel oil, and Gulf Marine had provided products, such as GulfSea Cylcare 5040H. These were “legacy lubricants” said Mr Elliott.

The situation is being made worse, said Mr Elliott, by Covid-19 and the difficulty in accessing test ships.

Mr Elliott said further challenges will be coming to lubricant oil development and testing as shipping begins to use different fuels to lower carbon emissions. He noted: “Our 70 BN oil represented 60-70% of our portfolio. That dropped to 10% in January (2020). Customers stressed the supply chain, but we managed to deliver,” he said.

“Some open-loop scrubber customers found they needed two oils on board where vessels were in ports where they were unable to operate the scrubbers in the open-loop format,” he said.

These fuels could involve dual-fuel engines to burn LNG, ammonia, methanol, LPG, and bio-based distillates. “Future oils will depend on the choice of fuel,” he said. “We will have an appropriate oil for the fuel. I confirm it will need additives of some description. Additives will be an essential part of the mix.”

Adapted from the Riviera Maritime Media webinar ‘Assessing the added benefits of marine additives and lubricants,’ sponsored by Lubrizol.