船舶轮机英语lesson 4.ppt

The high temperature of the working cycle would quickly heat up the metal forming the cylinders, cylinder covers and pistons if steps were not taken to prevent this happening. It is necessary to keep the piston rings at temperatures sufficiently low for the lubricating oil surrounding them to survive, and mechanisms such as fuel valves, exhaust valves and starting air valves must be kept at temperatures at which they will work satisfactorily and remain lubricated. Also the metal of parts forming the combustion spaces must be kept at a temperature low enough to avoid losing mechanical strength(强度). The usual used coolant is fresh water. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problem. As a result of its low specific heat (比热) however about twice the quantity of oil compared to water would be required. The heat picked up by these cooling fluids is transferred to seawater in heat exchangers and thus rejected to the sea, the cooling fluids becoming cooled and ready to circulate the engine again. Seawater cannot be used directly in the cooling spaces of modern engine, as it is both corrosive and liable to(易于…的) leave deposits(沉淀物). There are two kinds of fresh water system being used on board ship, closed systems and open systems. In a closed fresh water system, the engine jackets, the heat exchanger and the circulating pumps form a continuous circuit which is not open to the atmosphere. The cylinder jacket cooling water after leaving the engine passes to a sea water circulated cooler—heat exchanger—and then into the jacket water circulating pumps. It is then pumped around the cylinder jackets, cylinder heads, exhaust valves and turbo-blowers. However, provision has to be made for the expansion of the water due to the increase in temperature and due to small amounts of air which may enter the system and become entrained(以气泡的状态存在) and for water make-up (补偿) due to leakages. These are met by a small head ta