Operational Information

The Two Stroke Crosshead Engine

The Cylinder Liner
 

The cylinder liner forms the cylindrical space in which the piston reciprocates. The reasons for manufacturing the liner separately from the cylinder block (jacket) in which it is located are as follows;

  • The liner can be manufactured using a superior material to the cylinder block. While the cylinder block is made from a grey cast iron, the liner is manufactured from a cast iron alloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)

  • The cylinder liner will wear with use, and therefore may have to be replaced. The cylinder jacket lasts the life of the engine.

  • At working temperature, the liner is a lot hotter than the jacket. The liner will expand more and is free to expand diametrically and lengthwise. If they were cast as one piece, then unacceptable thermal stresses would be set up, causing fracture of the material.

  • Less risk of defects. The more complex the casting, the more difficult to produce a homogenous casting with low residual stresses.

The Liner will get tend to get very hot during engine operation as the heat energy from the burning fuel is transferred to the cylinder wall. So that the temperature can be kept within acceptable limits the liner is cooled.

Cylinder liners from older lower powered engines had a uniform wall thickness and the cooling was achieved by circulating cooling water through a space formed between liner and jacket. The cooling water space was sealed from the scavenge space using 'O' rings and a telltale passage between the 'O' rings led to the outside of the cylinder block to show a leakage.

To increase the power of the engine for a given number of cylinders, either the efficiency of the engine must be increased or more fuel must be burnt per cycle. To burn more fuel, the volume of the combustion space must be increased, and the mass of air for combustion must be increased. Because of the resulting higher pressures in the cylinder from the combustion of this greater mass of fuel, and the larger diameters, the liner must be made thicker at the top to accommodate the higher hoop stresses, and prevent cracking of the material. 

If the thickness of the material is increased, then it stands to reason that the working surface of the liner is going to increase in temperature because the cooling water is now further away. Increased surface temperature means that the material strength is reduced, and the oil film burnt away, resulting in excessive wear and increased thermal stressing.

The solution is to bring the cooling water closer to the liner wall, and one method of doing this without compromising the strength of the liner is to use tangential bore cooling.

Holes are bored from the underside of the flange formed by the increase in liner diameter. The holes are bored upwards and at an angle so that they  approach the internal surface of the liner at  a tangent. Holes are then bored radially around the top of the liner so that they join with the tangentially bored holes.

On some large bore, long stroke engines it was found that the undercooling further down the liner was taking place. Why is this a problem? The hydrogen in the fuel combines with the oxygen and burns to form water. Normally this is in the form of steam, but if it is cooled it will condense on the liner surface and wash away the lube oil film. Fuels also contain sulphur. This burns in the oxygen and the products combine with the water to form sulphuric acid. If this condenses on the liner surface, then corrosion can take place. Once the oil film has been destroyed then wear will take place at an alarming rate. One solution is to insulate the outside of the liner so that there was a reduction in the cooling effect. On the latest engines, the liner is only cooled by water at the very top, relying on the air in the scavenge space to cool the lower part of the liner.

 

The photo shows a cylinder liner with the upper and mid insulation bands known as "Haramaki"

Although Haramaki is a type of Japanese armour, the word also means literally " Stomach or Body Warmer". i.e an insulator.

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.

It has been known for ships to go for scrap after 20 + years of operation with some of the original liners in the engine.

Gauging a Liner

As well as corrosive attack, wear is caused by abrasive particles in the cylinder (from bad filtration/purification of fuel or from particles in the air), and scuffing (also known as micro seizure or adhesive wear). Scuffing is due to a breakdown in lubrication which results in localised  welding between points on the rings and liner surface with subsequent tearing of microscopic particles . This is a very severe form of wear.

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