Maintenance of MAN B&W and Yanmar Marine Diesels on Low-Sulfur Fuel

The transition to low-sulfur fuels (VLSFO <0.5% S and LSMGO <0.1% S) has profoundly altered the tribological environment within the cylinders of marine diesel engines. While the threat of cold sulfuric acid corrosion has diminished, new challenges such as abrasive wear from catalytic fines and ring zone deposits have emerged. This guide outlines the specific maintenance strategies required for two-stroke (MAN B&W, WinGD) and four-stroke (Yanmar) engines in this new era.

Cylinder Lubrication Strategy (Two-Stroke Engines)

For large low-speed engines like MAN B&W and WinGD, the cylinder oil Base Number (BN) and feed rate must be precisely matched to the fuel sulfur content to maintain engine cleanliness without causing excessive ash deposits.

• MAN B&W Guidelines: For operation on VLSFO, MAN typically recommends a Category II BN 40 cylinder oil. These oils possess enhanced detergency properties compared to older BN 40 oils, keeping the piston ring grooves clean without the need to alternate with high-BN oils. The feed rate must be optimized based on regular scavenge port inspections.
• WinGD Approach: WinGD validates specific lubricants for their engines. While low-BN oils are used, the emphasis is heavily placed on monitoring the residual BN and iron (Fe) content in the piston underside drain oil to dynamically adjust the feed rate.

Combating Catalytic Fines (Cat Fines)

VLSFO often contains high levels of aluminum and silicon particles (cat fines) from the refining process. These act as an abrasive paste, capable of destroying cylinder liners and fuel pump plungers within hours.

The primary defense is meticulous fuel treatment. Purifiers (like Alfa Laval ALCAP) must be operated at optimal efficiency (maximum 98°C, minimum throughput). Regular laboratory analysis of the bunker is crucial, but more importantly, onboard testing of the fuel after the purifiers is necessary to confirm cat fine removal.

Four-Stroke Engines: Yanmar Specifics

Medium-speed trunk piston engines like Yanmar rely on a single system oil for both bearing lubrication and cylinder cooling/lubrication. Operating these engines on low-sulfur fuel requires careful selection of the Trunk Piston Engine Oil (TPEO).

Using an oil with an excessively high TBN (Total Base Number) when burning LSMGO can lead to unburned calcium ash deposits on the piston crown and valves. Regular analysis of the system oil is mandatory to monitor viscosity, TBN depletion, and the buildup of insolubles. Furthermore, the lubricity of LSMGO is naturally lower than HFO, requiring close monitoring of the fuel injection pump plungers for signs of wear or scuffing.

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Diagnostic Tools

Modern engine maintenance relies heavily on condition monitoring rather than purely running hours:

1. Scavenge Port Inspections: Visual checks of the piston rings for free movement, carbon deposits, and the liner surface for honing marks or scuffing.
2. Drain Oil Analysis: Measuring magnetic iron, total iron, and residual BN in the scavenge drain oil provides an early warning of corrosive or abrasive wear.
3. Indicator Diagrams: Electronic indicator systems (e.g., PMI) are vital for analyzing combustion quality, identifying faulty fuel injectors, and ensuring balanced power distribution across all cylinders.