Don Gregory, managing director for Sustainable Maritime Solutions Ltd., which provides

vessel performance optimization for owners, operators and charterers, said during the webinar that piston liner lubrication is completely different in a 4-stroke engine, compared to in a 2-stroke diesel engine.

“Since the introduction of low-sulfur fuel oil in 2020, it has had really little impact on 4-stroke trunk piston engines as compared to the impact it has had on 2-stroke engines,” Gregory said Jan. 28 during a webinar, “Trunk Piston engine oils: transiting SECAs on 12-16 BN oil,” hosted by Riviera Maritime Media. “So the problems seen with 2-stroke diesels and with cylinder oil are not evident with 4-stroke engines.”

Sulfur Emission Control Areas are sea areas where stricter controls were put in place to minimize airborne emissions from ships, as defined by the Marpol Protocol. Marpol stands for International Convention for the Prevention of Pollution from Ships.

Gregory noted that what the switch to lower BN trunk piston oils does mean for operators both for auxiliary engines, and main propulsion for 4-stroke engines is that existing oil stocks can be used and then be replenished with the lower BN lubricant. “As with 2 stroke cylinder oil BN reduction, the base number coming down means that those performance enhancements provided by the base number have to be replaced by other components to retain the detergency,” he said.

As a result, he noted, other additives are being put in to the new trunk piston engine oils, which are ranging from about 9 base number up to 20 or 30. “But it’s important to stress that base number is not a big factor with IMO 2020 fuels in use,” he added.

In a trunk piston engine, the connecting road is directly connected to the piston by a piston pin. Trunk piston engines – which are medium speed marine diesel engines – differ from 2-stroke engines in terms of requirements, Gregory noted. “So some [trunk piston] engines may have large volumes of oil, and that effectively means oil is in there for life,” he explained. “For others, oil gets changed on a time basis or sometimes based on used oil analysis.” Different types of trunk piston engines may run on different fuels. “Auxiliary engines may be running on gas oil, and if the main propulsion is 4-stroke, it may be running on residual fuel – either high sulfur if it’s got a scrubber, or 0.5% or -0.1% [sulfur].”

Oil management systems for trunk piston engines varied, he said, ranging from centrifuges and large filter systems with back flush for systems where oils are pretty much permanently in the system throughout the engine’s life, to time-based where perhaps every thousand hours the oil and filters are changed. “There is a range of different requirements, depending on the size of the engine and type of installation,” he said.

“One thing that is pretty common with modern engines is they have a very low and controlled oil consumption,” Gregory noted. “That can be problematic, as we move to lower sulfur fuels. The base number recommended by the OEMs will go down to say 20 or 15 or some number like that, depending on type of fuel being used.”

He noted that some of that base number being removed has to be replaced with more expensive additives that provide the detergency, which is essential to achieving various performance characteristics required of trunk piston engine oil, such as keeping piston under crown clean, avoiding lacquers, keeping internal surfaces of the crank case and camshaft box clean. “But the other option you can also use is top-up with low base number replenished oil into an existing system of higher base number trunk piston engine oil,” he added.

One of the primary purposes of lube oil in marine applications is to neutralize the acid formed during combustion, said Michael Banning, technical consultant, fuel specialist and surveyor for Exponent International, a multi-disciplinary engineering and scientific consulting firm focused on investigations for the marine industry.

Higher base number oils are required for higher sulfur content, while lower BN levels are preferable for lower sulphur content, he noted. “Marine gas oil and higher sulfur ‘asphaltenic’ or ‘residual’ fuels – including [very low sulfur fuel oil] to a certain extent – face different operational difficulties and issues that the lube oil must deal with,” Banning explained. “Different lube oil packages must be incorporated to deal with the specific issues at hand.”

Banning explained that one of the main issues with operating trunk piston engines on marine gas oil is lacquering.  “The liner honing is filled with this lacquering, which can then be baked in place by the temperatures and pressures they are exposed to, disturbing the oil film,” he explained. “Excessive BN ash formation [results] in deposition on piston crown and exhaust valves. These deposits can be abrasive and lead to accelerated wear.”

Residual fuel operation deposits can play a bigger role in trunk piston engines. The deposits are typically seen on a piston but are more pronounced on the back side of piston rings, in piston ring grooves, reducing movement, he explained. “”This leads to the piston rings starting to ride out, resulting in excessive scuffing.”

Lube oil requirements based on fuel composition, he noted, not just depending on base number. “For long term operation on MGO, specific formulations are employed to handle lacquers,” he said. Overbased lube oil can be detrimental in terms of deposition, he noted. Banning emphasized it’s important to follow OEM guidelines.