cylinder liner & its lubrication::marine engine

The lip of the upper part of cylinder liner sits on a machined face at the top of the cylinder block and liner is held in place by cylinder cover. This arrangement allows the liner to expand freely downwards when it is heated during running of the engine.

Liner is manufactured from good quality cast iron alloy .To improve the strength cast iron is alloyed with inclusion of small quantities of nickel, chromium, molybdenum, vanadium, copper, etc. It must satisfy the contradictory requirements of being thick and strong enough to withstand high temperature and pressures that occur during combustion and thin at the same time to allow heat transfer.

This is achieved by bore cooling. Boring is carried out in upper part of liner at an angle to the longitudinal axis by which the cooling water is led close to the combustion space area. By using this technique of bore cooling good heat transfer and high overall strength is achieved.

A cooling water space is provided between cylinder block and liner and this space is sealed at its lowest part by means of silicon rubber ‘O’ rings. Sealing is provided by two rings, upper one for sealing cooling water space and lower one to seal scavenge air. A telltale hole is provided between two rings to indicate leakage of cooling water or air.

Scavenge air ports are cut in lower section of liner and these ports are machined at an oblique angle to the axis of cylinder in order to impart a rotary motion to scavenge air.

Oil is injected through a number of holes drilled in the liner, usually 6 or 8, located circumferentially around the liner. Lubricator quills are connected to the oil holes and each contains a non return valve to prevent blowback of gases.

 

Cylinder lubrication: Because the cylinder is separate from the crankcase there is no splash lubrication as on a trunk piston engine. Oil is supplied through drillings in the liner. Grooves machined in the liner from the injection points spread the oil circumferentially around the liner and the piston rings assist in spreading the oil up and down the length of the liner. The oil is of a high alkalinity which combats the acid attack from the sulphur in the fuel. The latest engines time the injection of oil using a computer which has inputs from the crankshaft position, engine load and engine speed. The correct quantity of oil can be injected by opening  valves from a pressurized system, just as the piston ring pack is passing the injection point.

                                           As mentioned earlier, cylinder liners will wear in service. Correct operation of the engine (not overloading, maintaining correct operating temperatures) and using the correct grade and quantity of cylinder oil will all help to extend the life of a cylinder liner. Wear rates vary, but as a general rule, for a large bore engine a wear rate of 0.05mm/1000 hours is acceptable. The liner should be replaced as the wear approaches 0.8 - 1% of liner diameter. The liner is gauged at regular intervals to ascertain the wear rate.