DieselShip

Liner

Cylinder liners are generally made from grey cast iron because it is easily cast and has self lubricating properties due to the graphite flakes for, some modern engines spheroidal graphite or nodular graphite is used. This has greater mechanical strength, but has the same self lubricating properties.

The critical part of any liner is the upper section were the temperature and pressure conditions are at their most difficult. Cooling is required to maintain strength and the temperature variations must be maintained within set limits in order to avoid cracking. Rapid change of temperature due to the rapid variation in cylinder condition or cooling water temperature can result in cracking.

Early engines e.g. Sulzer R's were lightly loaded and thin section liners could withstand the pressure , the thin sections avoided any problems of thermal stresses. Fire rings were often fitted to protect the inner face of the liner from impingement by the combustion flame.

With the advent of turbocharging e.g. Sulzer RD, it was necessary to provide strengthening in order to withstand mechanical stress increasing the wall thickness would have resulted in thermal stress.

Shrink rings or support rings were used to strengthen the upper section of the liner and the cooling space was provided , the support ring took about 50% of the load, between the liner and the strengthening ring.

For modern highly rated engines support or shrink rings are not suitable and thick section bore cooled liners are employed

,p> A typical cast iron as used in liner construction begins to lose its strength at a surface temperature of about 340^oC . A liner must therefore be either alloyed with expensive elements or cooled to about 80^oC below this temperature.

A typical cylinder lubricating oil forms a lacquer at about 220^oC . A liner must therefore be cooled to about 40^oC below this temperature in service, to reduce formation of carbon deposits.

A liner must therefore have a maximum temperature in the thickened region, of about 260^oC and a max. temperature in the thinned section of about 180 ^oC . This produces large temperature gradients axially in the liner and also across the walls of the liner. This could produce component failure due to high thermal stress if the material was too thick or failure by low metal strength if the material was too thin.

The design that has been adopted is to have the cooling surface around the combustion zone formed by a large number of hole drilled at an angle to the vertical axis of the liner. This produces a fully machined cooling water surface close to the combustion side of the liner, thus keeping thermal stresses low.

It is usual to allow the liner to expand freely in the axial direction away from the combustion zone. The cooling spaces may be sealed by neoprene rubber rings fitted in the grooves in the liner. The rings and grooves being closely matched to ensure a positive seal. Alternately copper rings may be fitted

Wear of cylinder liners

There are three main cause of damage to the liner material;

Corrosion-caused by the acidic products of combustion

Abrasion-caused by solid particles breaking through the lubricant film

Friction-Break down of the lubricating oil film leading to metal to metal contact

 Normal liner wear exists for the reasons given above. Wear rates are greatest towards the top of the stroke due to the high temperatures thinning out the oil film and high gas pressure behind the piston rings forces the land into contact with the liner wall. In addition, piston is moving slowly at the end of its stroke and a good oil wedge cannot be formed.

Wear rates reduce lower down the stroke because pressure and temperature conditions are less arduous and piston speed has increases. At the bottom end of the stroke wear rate increases again due to reduce piston speed, but also due to the scouring effect of the in coming scavenging air. The reduced temperature increases the viscosity of the oil so reducing its ability to spread evenly. Long stroke engines are sometimes provided with quills at the bottom of the stroke.

Cylinder Lubrication

Cylinder oil is injected by means of quills positioned in the liner, the number of which is governed by the diameter of the liner and ensures sufficient oil to be injected. The use of grooves in the liner helps spread and retain the oil film.

Vertical positioning of the quills is important and the oil should be injected so that it is spread upwards by the top two piston rings. If injected too early the top ring will scrape the oil upwards to be burnt. If too late the oil will be scraped off the liner by the next downstroke. Injection timing is therefore critical, too much so as experiments to inject the oil precisely have failed. The remedy has been to over supply the quantity of oil and provide extra quills at the bottom of the stroke

Cylinder Lubrication quill

Abnormal Liner Wear

a,Scuffing- This occurs if the cylinder lo quantity is insufficient. A complete oil film is not obtained and rings contact the liner surface. Local seizures takes place producing a hardened glassy surface on the rings and liner and as the rings rotate in their grooves scuffing speeds around the liner. If scuffing is extensive the only solution is to replace rings and liner. Minor scuffing may be corrected by replacing the rings and braking don the scuffing area on the liner with a rough stone to provide a key for the cylinder l.o.

It is necessary to determine the cause of scuffing and correct it. As stated the most likely cause is insufficient quantity of l.o.

b, Cloverleafing-if the cyl l.o. has inadequate acid neutralising properties for the fuel being burnt or if there is insufficient quantity of oil injected then cloveleafing can occur.. This is basically regions of corrosive wear midway between the quills and upwards towards the top of the liner. These areas may be visible due to the corrosive effect and they are cloverleaf shaped. Eventually the rings become unsupported in these areas, gas builds up on the front face and the ring is subject to collapse.

There are consequences of over lubrication, particularly with sticking rings and choked ports due to carbon build up. Excess unburnt oil can also accumulate in the scavenge space risking fire.

Ships which operate for long periods in the 'down by the stern' trim mat exhibit an increased wear in the for'd to aft direction over the athwartships direction. Athwartship wear is aggravated by the reaction forces from the piston and rotation of the crankshaft. Although the bulk of this is removed by the crosshead on slow speed engines, this resultant force still causes the athwartship direction to have the greatest wear rate.

Maximum allowable liner wear is determined by the manufacturer but generally is between 0.7 to 1.0%.