Auxiliary engines (4-stroke) lubrication explained…

This is another on-demand post and its purpose is to explain the 4-stroke lubrication system from operational point of view.

All running gear of the 4-stroke engine is forced lubricated by an engine-driven gear type pump which takes the oil from the engine sump tank. The same oil from engine sump tank is also provided to the engine pistons as a cooling medium.
An electrically-driven pre-lubricating oil pump is fitted to feed oil to the bearings and other operating gear before the engine starts, as this decreases wear on the engine in the period between the engine starting and the engine-driven pump building up lubricating oil pressure. After the engine has started and the engine-driven lube oil pump is supplying oil at the correct pressure, the pre-lubrication pump is stopped. The LO pre-lubricating pump is normally operated from its local control panel and will run constantly whilst the engine is on automatic standby; the pump switch should be set to AUTO, however it can be changed to the MANUAL position for manual control.

Example of pre-lubricating LO pump

The engine-driven pump and the electrically-driven LO pre-lubricating pump both take suction from the engine sump then discharge through a duplex filter to the engine oil supply rail. A pressure control valve at the oil entry maintains a steady pressure independent of the engine rpm and oil temperature, same as the electrically-driven LO pump features a relief valve back to the pump suction.

The temperature of the lubricating oil is maintained at 65°C by the LO cooler, which has a three-way temperature controlled bypass valve.
The LO cooler is a plate heat exchanger with oil circulating through the flow channels and water circulating through parallel channels in a counter-flow design from the central fresh water cooling system.

Auxiliary engine LO cooler opened for cleaning and maintenance

The flow of oil through the cooler is regulated by the three-way valve which allow some or all of the circulating lubrication oil to by-pass the cooler in order to maintain the correct LO temperature.

A centrifugal bypass filter, especially on big engines, mounted on the engine base frame supplements the main LO filter. A portion of the LO supplied by the engine-driven LO pump enters the centrifugal filter and returns to the oil sump in the base frame during operation. The filter uses centrifugal force to remove high-density sub-micron particles and it is powered by the oil supply.

Centrifugal oil filter arrangement on auxiliary engine

The engine main bearings, crankpin bearings, top end bearings, camshaft system, valve rocker units and turbocharger bearings are lubricating from the system. Normally, the turbocharger receives lubrication oil from the main circuit via a branch pipe. A flow of cooling oil is also directed to the piston and cylinder liner running surface lubrication is primarily supplied by splash oil and oil vapour from the crankcase.

Basic principle of wet sump lubrication. Source and credit: Thomas Schwenke


The entire lubrication system is part of the engine construction and there are no valves which need to be operated within the engine lubricating system.

On some of engine types lubrication of the piston rings is from below, through bores in the lower part of the cylinder liner. This separate cylinder lubrication is supplied by a separate lubrication system via a cylinder lubrication pump which takes suction from the main LO supply to the engine bearings. Lubricating oil is taken from the crankcase system and supplied to each cylinder liner via a block-type distributor which is controlled by pulses from the engine monitoring system. Oil is directed into the cylinder liner through a number of radial holes located around the liner, and opposite the piston rings when the piston is at bottom dead canter. This ensures that there is a small quantity of oil between the piston rings, and the piston rings spread this oil over the liner surface when the piston moves up and down the cylinder. Usually, the cylinder lubrication pump may be selected for OFF or AUTO (ON) at its local control panel. When selected for AUTO, the cylinder lubrication pump operates in accordance with the preset mode which is either as follows:

      • Manual/OFF
      • Automatic (normal setting, the cylinder lubrication operates when the engine load is higher than 50%)
      • Run-in (cylinder lubrication operates continuously irrespective of the load when the engine is started.

Only lubricating oil of an approved grade and quality must be used in the generator engine system. The engine sump tank is replenished with fresh oil from on of vessel’s LO storage tanks and LO in the engine sump must be cleaned in a LO separator on a regular basis, and ideally, the
oil should be continuously centrifuged when the engine is running. Usually, a LO separator is kept running on the generator engine that is in use. If more than one generator engine is running, the separator is to be run on each diesel generator sump on a daily basis. It is most important that the correct valves are lined-up for separation to prevent the pumping out of the sump on one running engine to another running engine.

The generator engine LO separators operate on the same principle as the main engine LO separator. LO from the generator engine sump flows from the sump to the suction side of the relevant separator feed pump, via a suction filter. The LO feed pump circulates the LO through the heater and then supplies it to the separator. From the separator, the cleaned oil returns to the generator engine sump. Sludge from the generator engine LO separators flows to the LO sludge tank.

Generator engine LO should be sampled and tested at intervals recommended by the LO supplier. The results of the testing provide an indication as to engine operation and the need for LO replacement.

Please follow the link if you are interested on learning and understand more about Maritime Bearings and Lubrication. The courses are provide by Alison, are free and you need only to subscribe to their platform via this link.

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