LessononeTextHowdoesaMarineDieselengineWork.ppt

*The diesel engine is a type of internal combustion engine which ignites(点燃) the fuel by injecting(喷射) it into hot, high pressure air in a combustion chamber(燃烧室). The marine diesel engine is a type of diesel engine used on ships. The principle of its operation is as follows: When the air is compressed, its temperature rises so that it ignites the fine spray(喷雾、油雾) of fuel injected into the cylinder. In a four-stroke diesel engine, the cycle requires four separate strokes of the piston, i.e. suction(吸气), compression(压缩), expansion(膨胀) and exhaust(排气). The exhaust ports are uncovered also. Pressurized fresh air charges into the cylinder, blowing out any residual exhaust gases from the last stroke through the exhaust ports. The fuel injector injects a fine spray of fuel into the hot air and combustion take place, producing much higher pressure in the gases. At this low speed the engine requires no reduction gearbox between it and propeller. It is usual to draw such diagrams starting at TDC (firing), but the explanation will start at TDC (scavenge ). Top dead center is sometimes referred to as inner dead center (IDC). If the engine is naturally aspirated(自然吸气), or is a small high-speed type with a centrifugal turbocharger, the period of valve overlap(重叠), i.e. when both valves are open, will be short, and the exhaust valve will close some 10°after top dead center (ATDC). will be designed to allow a generous throughflow of scavenge air at this point in order to control the turbine blade temperature. To maximize this, balancing the reduced opening as the valve seats against the slight ram(进气阀座) or inertia effect of the incoming charge, the inlet (suction) valve will normally be held open until about 25- 35° ABDC (145-155°BTDC). the fuel burns over a period of 30-50°while the piston begins to descend on the expansion stroke, the piston movement usually helping to induce air movement to assist combustion. This is done: (a) to preserve(保留) as much energy as is pra