Main engine lubrication system

Purpose and application

Moving surfaces in bearings and between piston rings and cylinder liners carry substantial loads. An oil film reduces friction and wear; circulating oil also carries heat and contamination away from lubricated components.

In a crosshead engine, the piston rod connects the piston to a crosshead — the articulated connection between the rod and connecting rod. The crankcase is separated from the piston underside space. Bearing lubrication and lubrication of the liner running surface therefore use two separate systems.

Main components

Circulating system

Main bearings support the crankshaft. Crankpin bearings work at the connecting rod–crankshaft connection, while crosshead bearings work at the upper connecting rod–crosshead connection. Dedicated passages supply oil to each of these components.

• Drain tank — receives oil returning from the engine and provides the pumps with their working inventory. In this arrangement it is below the engine; the crankcase sump and the separate drain tank are different components.
• Duty and standby oil pumps — provide continuous delivery. External pumps allow circulation before engine starting. Pump type and standby logic are installation-specific.
• Oil cooler and regulating valve — transfer heat to cooling water and maintain the required oil temperature. Regulation may divert part of the flow through a bypass, a line around the cooler.
• Full-flow filter — retains particles before oil enters the engine. Differential pressure indicates flow resistance; automatic cleaning and standby sections depend on filter design.
• Engine oil passages — distribute oil to bearings and other consumers. Modern arrangements also use system oil for piston cooling; the internal route depends on the engine.
• Separation circuit — treats oil taken from the drain tank to remove separable water and contamination. Its separator has a dedicated supply and specified heating; it is not in series with every bearing.
• Instruments and protection — monitor level, pressure, temperature and filter differential pressure. A separate detector monitors crankcase oil mist: normal oil pressure does not rule out local overheating.

Cylinder system

• Cylinder-oil storage and service tanks — hold the selected lubricant and supply the lubricators. Separate stocks of different oil specifications may be provided.
• Lubricators — meter oil for each cylinder. Pumping elements, drive and feed control differ between mechanical and electronic arrangements.
• Supply tubes and liner injection points — deliver oil doses to the liner running surface. Piston rings and the specified liner geometry distribute the oil across the contact area.
• Piston-underside and stuffing-box drains — collect residual oil and contamination. Sampling points support condition monitoring of individual cylinders.

The piston rod stuffing box separates the spaces

The piston rod passes through a stuffing box. Its sealing and scraper rings restrict scavenge-air passage into the crankcase and keep dirty cylinder oil away from the bearing-oil space.

Dirty drains lead to the designated collection system. In some designs, clean system oil scraped from the lower rod returns to the crankcase. Do not combine these routes: identify each outlet from the stuffing-box drawing.

How the oil moves

System oil: a closed circulating path

1. Before starting, establish the specified pre-lubrication circulation. Check tank level, standby readiness and pressure at the engine reference point, not just pump discharge pressure.
2. The pump draws oil from the tank and delivers it to the pressure line. Regulating devices maintain supply conditions; the pump needs an adequate inlet supply.
3. The cooler transfers heat to water, commonly the central-cooling low-temperature freshwater circuit, LT. Oil and water pass on separate sides of the exchanger and do not mix in a sound unit.
4. Filtered oil reaches the consumers. It supplies bearings and carries heat away; in the modern arrangements described, separate internal branches also deliver oil to oil-cooled pistons.
5. Oil drains into the sump and flows through return pipes to the drain tank, ready to be drawn by the pump again. The oil path does not end at a bearing.
6. In parallel with the main circuit, the separator treats a portion of tank oil and returns cleaned oil to the designated point. Water and sludge leave separately. Not all oil passes the separator on every circuit cycle.

Pump pressure is necessary to deliver oil. Film load capacity also depends on surface movement, load, viscosity and bearing design. A normal gauge reading alone therefore cannot establish that every bearing is healthy.

Cylinder oil: metered total-loss delivery

1. The selected cylinder oil flows from its service tank to the lubricators. Supply conditions and any heating follow the system documentation.
2. Each lubricator meters doses for its cylinder. Electronic systems control timing and quantity using maker-specific algorithms; mechanical systems meter oil through their own drive arrangement.
3. Oil passes through supply tubes and injection points onto the liner. Piston rings distribute it to maintain a film across the contact area.
4. Oil works at high temperature in contact with combustion products. Part is consumed in the cylinder; residues enter piston-underside and stuffing-box drains. There is no normal return to the lubricator.

What to monitor

SAE describes oil viscosity grade; BN describes alkaline reserve. They are not interchangeable. A thicker oil or a higher BN is not automatically better. Use a lubricant approved for the relevant circuit and operating conditions.

There is no single safe pressure, temperature or feed-rate setting for every low-speed engine. Do not transfer MAN Alpha settings to WinGD or another lubricator arrangement without checking applicability.

Operation and maintenance

Before starting and during the watch

• Check the two oil inventories separately, pump readiness, valve alignment against the working diagram and absence of leakage.
• Confirm the required circulation and cylinder-lubrication readiness before starting is permitted. Test alarms and standby functions using the approved procedure.
• Record pressure, temperature, filter differential pressure and cylinder-oil consumption together with engine load. Comparing equivalent conditions is more useful than a single reading.
• Monitor separation and unobstructed drainage. Oil leakage onto hot surfaces requires an immediate safe response.

Planned work through PMS

• Include readings and trends in daily checks. Define monthly and annual tasks from engine, pump, filter and separator manuals and the ship PMS — planned maintenance system.
• Sample system oil and cylinder drains on the established schedule. Identify sampling point, cylinder, load and lubricant; do not mix different drains into one sample.
• Assess ring and liner condition through scavenge ports using the maker procedure. Inspect only after stopping, isolation and authorisation for work.
• Service filters, lubricators, stuffing boxes and coolers with safe isolation, depressurisation and protection against unintended starting. Do not change operating settings on the strength of one questionable analysis.

After stopping

A stopped engine retains heat. Maintain oil circulation and cooling for the duration and in the mode required by its manual. Stopping every auxiliary at the same time as the engine is not a universal procedure.

Common faults

These are investigation directions, not permission to run in an unsafe condition. When emergency limits are exceeded, first apply the required load reduction or stopping procedure and secure personnel safety.

Questions and answers

The essential distinction

The circulating system repeatedly supplies conditioned oil to bearings and returns it to the tank. Cylinder lubrication meters a separate oil onto the liner without a normal return circuit. Reliability depends on delivery, oil quality, component condition and timely response to changes, not one pressure gauge.

Related material