To keep the materials of the engine within the limits of their thermal and mechanical strengths it is necessary to regulate the heat build up in them. This is achieved by circulating coolants at the correct rate around and through the various components within the engine. Most of these systems can readily be traced out by the ship’s engineer, although the bore cooling passage ways may only become apparent when the engine is overhauled.
Water is used as the coolant, although it must be of the correct quality and treated in such a way that it does not, encourage corrosion. Hard waters, those that carry a high salt/lime content, should be avoided as they will only lead to scale deposition in the higher temperature zones with the consequent reduction in heat transfer.
It is usual to employ distilled water or some such equivalent. In any case the calcium oxide level should be very low as should the chloride content. To inhibit corrosion an additive such as soluble oil, chromate or nitrate borate can be used. Each of these have their own peculiar advantages and disadvantages. Soluble oil, for example, is safe to handle, reduces cavitation damage and is approved for use with fresh water generators. It is however prone to separation if the system is not clean or if water quality is incorrect, susceptible to foaming and may layer out on high temperature zones forming insulating layers. Its self-lubricating properties favour its use with telescopies used to supply piston cooling. On shut down, any oil based coolant must be circulated longer than water as there is a tendency for the oil to ‘bake’ out on hot spots, forming insulating layers that promote, when the engine is running, the burning away of piston material on the high temperature combustion side.
For many years chromate has been used as a very effective inhibitor, particularly as concentration levels are almost self-evident from its colour. Chromate too can be used in conjunction with zinc coatings etc. However it is harmful to the skin and must not be handled, and because it is poisonous cannot be used with fresh water generating systems run off the engine coolants.
Nitrate borate is probably the most universally used compound as it is an effective inhibitor, the level of which is easily determined aboard. It is also safe to handle and is approved for use with fresh water generator systems run off the engine cooling system. It does attack soft solders and cannot prevent corrosion of zinc.
The ship’s engineer generally has no choice but to use whatever inhibitor the engine builder recommends. To that end it is his responsibility to ensure that the coolant system is maintained clean and clear. He should also maintain regular checks upon concentrate level and avoid overdosing as a ‘safety margin’ as, for example, too much soluble oil causes deposition in high temperature zones. The engine operator should maintain the inhibitor, whatever the type, between the prescribed levels and take the usual precautions when handling the raw material.
It is worth bearing in mind that the return temperature of the circulated coolants is a ‘mean’ value of the coolant. There will be areas inside the cooling system where the local surface temperatures are very much higher than those indicated at the return. It is obviously very dangerous to run the returns at levels higher than those normally recommended.
Although the circulatory systems of large bore engines are generally considered to be coolants, these same fluids act as heating agents during the warming through period. As discussed in an earlier section, it is necessary to warm through the larger engines, not merely to limit thermal shocking that occurs during starting but also to important as much heat into the first compressions as possible. This helps to provide adequate temperature at the end of compression so that the fuel absorbing heat from the air will readily rise to its self-ignition temperature and combustion will be ensured. Once the engine has warmed up, the clearances will be taken up (piston to cylinder) and a more positive compression together with heat absorbed from the cylinder cover etc will provide adequate air temperatures for crisp ignition. The heating for the coolant can then be turned off and the normal cooling system reverted to.