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What is the difference between system oil and cylinder oil?

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Cylinder oil and system oil are two types of lubricant used in marine engine applications. While both solutions are designed to ensure engines function at optimum performance, how they are used and what for differs. Read on to find out more.

Understanding marine engines

There are two different kinds of marine diesel engines. These are trunk piston and crosshead engines.

In a trunk piston marine diesel engine, the crankcase and cylinder are not separated. The pressure that pushes the piston down is imparted directly to the connecting rod, where it is then imparted to the crankcase, very similarly to conventional automotive engines.

In a crosshead marine diesel engine, the cylinder and the crankcase are both separated by a dedicated partition. The pressure that pushes the piston down is then transmitted to the crankshaft via the piston rod and the engine’s connecting rod.

Two oils for marine engines

Crosshead variants are found in multiple 2-cycle diesel engine with low RPMs. For increased stroke, it uses increased torque that enhances fuel efficiency by employing a low-grade burning oil

However, for the lubrication of each individual component within a crosshead 2-cycle type marine diesel engine where the crankcase and cylinder are separated, a lubricating oil must be separately fed into the cylinder liner that is in direct contact with the piston head within the crankcase and cylinder.

The lubricant that is fed into the engine’s cylinder liner is called cylinder oil and the other solution that is added to lubricate each component inside the crankcase is known as system oil.

Effective cylinder lubrication is necessary to lubricate the engine’s piston rings which reduces friction in between the liner and rings to create a seal between them, and also to cut down corrosion by neutralising acidity levels of combustion deposits.

Unlike cylinder oil, system oil is kept in the engine’s drain tank or in the lowest section of the crankcase. The system oil is extracted from the tank or crankcase via a strainer, pump and filter. The oil then passes through a cooler before it enters the engine and is distributed to the bypass pipes. The oil then feeds into lubricate each individual component in the engine system.

After lubricating the engine, the oil is then discharged to the engine sump or drain tank so it can be reused. Marine engines have a centrifuge that is engineered to clean in-use lubricating oil within the system so that refreshed oil can be supplied from the storage tank.

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