Funcionamiento de las sesiones

Because of the stress reversal mentioned above, bottom end bolts have a limited life. This varies from engine to engine, but is generally around 12-15000 hours. If a bottom end bolt was to fail in operation, then the results would be disastrous.

Bottom end bolts should be treated with care when removed from the engine during overhauls. They should be inspected for any damage to the surface from which a crack could start. This damage could be due to corrosion (water in LO) or because of incorrect handling.

The Cylinder Liner

The cylinder liner is cast separately from the main cylinder frame for the same reasons as given for the 2 stroke engine which are:

- 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 nodular 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.

Modern liners employ bore cooling at the top of the liner where the pressure stress is high and therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits so that there is not a breakdown in lubrication or excessive thermal stressing. Although the liner is splash lubricated from the revolving crankshaft, cylinder lubricators may be provided on the larger engines.

On the example shown opposite, the lubricator drillings are bored from the bottom of the liner circumferentially around the liner wall. Another set of holes are drilled to meet up with these vertically bored holes at the point where the oil is required at the liner surface.

Sulzer ZA40 Liner (vee engine; The straight engine is similar)

Other engines may utilise axial drillings as in a two stroke engine.

Where the cooling water space is formed between the engine frame and the jacket, there is a danger that water could leak down and contaminate the crankcase if the sealing O rings were to fail. As a warning, "tell tale" holes are led from between the O rings to the outside of the engine.

Modern engines tend not to use this space for cooling water. Instead a separate water jacket is mounted above the cylinder frame. This stops any risk of leakage of water from the cooling space into the

crankcase (or oil into the cooling water space), and provides the cooling at the hottest part of the cylinder liner.

Note that the liner opposite is fitted with a fireband. This is sometimes known as an antipolishing ring. It is slightly smaller in diameter than the liner, and its purpose is to remove the carbon which builds up on the piston above the top ring. If this carbon is allowed to build up it will eventually rub against the liner wall, polishing it and destroying its oil retention properties.

The liner must be gauged regularly to establish the wear rate and check that it is within manufacturers tolerances. The wear rate for a medium speed liner should be below 0.015mm/1000hrs. Excessive wear is caused by lack of lubrication, impurities in fuel air or Lubricating oil, bad combustion and acid attack.

The Piston

Pistons for medium speed trunk piston engines which burn residual fuel are composite pistons; i.e the crown and the skirt are made of different materials.

The crown is a heat resisting steel forging which may be alloyed with chromium, molybdenum and nickel to maintain strength at high temperatures and resist corrosion. It is dished to form a combustion chamber with cutouts to allow for the valves opening. The topland (the space between the top ring and the top of the piston) may be tapered to allow for expansion being greater where the piston is hottest.

The skirt can either be a nodular cast iron or forged or cast silicon aluminium alloy. Aluminium has the advantage of being light, with low inertia, reducing bearing loading. However because aluminium has a higher coefficient of expansion than steel, increased clearances must be allowed for during manufacture. This means that the piston skirt clearance in the liner is greater than that for cast iron when running at low loads. The skirt transmits the side thrust, caused by the varying angularity of the con rod, to the liner. Too big a clearance will cause the piston to tilt.

The piston pin for the con rod small end bearing is located in the piston skirt. The piston pin floats in the piston skirt and is located in place by circlips. Depending on the material used for the skirt (esp. cast aluminium), a bushing may be used for the pin.

The piston rings may be located in the crown or in both crown and skirt. Normally, the rings are chrome plated or plasma coated to resist wear. Because the liner is splash lubricated, an oil scraper (oil control) ring is fitted to the piston skirt.

The piston is oil cooled. This is achieved by various means; The simplest is for a jet of oil to be directed upwards from a hole in the top of the con rod onto the underside of the crown. A more efficient method is to use an oil catcher as shown in the picture above. This directs oil into the cooling spaces on the underside of the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is unusual for the oil return temperature to be monitored (unlike the 2 stroke slow speed crosshead engine, where both temperature and quantity are monitored).

Some engines are fitted with one piece pistons manufactured from either cast iron or silicon alloy aluminium . These cannot be used with residual fuel, because the higher temperatures causes burning of the piston crown. Aluminium also suffers from carbon build up above 300º C. Ring grooves in aluminium pistons usually take the form of a chrome plated cast iron insert.