Each engine is designed by the manufacturer to have specific clearances between the moving parts. If the engine is operated below the temperature range specified by the manufacturer, parts in the engine will fail to expand sufficiently to obtain the desired clearances. Excessive clearances between the reciprocating parts of the engine will cause pounding. Excessive clearances between the bearings and journals will allow the oil to escape before it can reach the areas that are distant from the oil inlet port.

On the other hand, if the engine is operated above the specified temperature range, the parts will expand and insufficient clearances between the moving parts will result. As the clearances are reduced, the required space for the oil film is also reduced. Once the oil film is reduced, inadequate lubrication of the moving parts will occur. As mentioned earlier, inadequate lubrication will cause metal-to-metal contact, accelerated wear, and seizures to occur in the engine.

RETAINING THE STRENGTH OF METALS

High temperatures change the strength and physical properties of the various metals used in an engine. For example, if a cylinder head is subjected to excessively high temperatures, the tensile strength of the metal is reduced. The probability of fracture or cracking is thereby increased. Such high temperatures also cause excessive expansion of the metal, which may result in shearing of the cylinder-head bolts. Overheating can cook the seal rings used on the water side of the cylinder liners, causing the sealing surface of the rings to harden. This condition will affect the ability of the material to prevent leakage of the coolant into the oil sump.

Consequently, by removing heat from the engine, the cooling system helps to prevent deterioration of the engine parts. If the parts, such as liners, pistons, valves, and bearings, are allowed to overheat, the tensile strength will be materially reduced, thereby accelerating wear and increasing the probability of failure. If over-heating is sufficiently severe, seizure of the engine will result.

TYPES OF COOLING SYSTEMS

The cooling system of a marine diesel engine functions to keep the engine parts and fluids at safe operating temperatures. In the open system, the engine is cooled directly by seawater. In the closed system, fresh water is circulated through the engine. The fresh (jacket) water is then cooled as it passes through a cooling device where heat is carried away by a constant flow of seawater or air. The closed cooling system is the design that is most commonly used on marine internal-combustion engines.