Jimei university
Lesson one
Text How does a Marine Diesel engine Work?
2001-03-20
Jimei university
*The diesel engine is a type of internal combustion engine which ignites(点燃 the fuel 点燃) 点燃 by injecting(喷射 it into hot, 喷射) 喷射 high pressure air in a combustion chamber(燃烧室 燃烧室). 燃烧室
2001-03-20
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The marine diesel engine is a type of diesel engine used on ships. The principle of its operation is as follows:
2001-03-20
Jimei university
A charge of fresh air is drawn or pumped into the engine cylinder( 气 缸 ) and then compressed by the moving piston( 活 塞 ) to very high pressure.
2001-03-20
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When the air is compressed, its temperature rises so that it ignites the fine spray(喷雾、 喷雾、 喷雾 油雾) 油雾 of fuel injected into the cylinder.
2001-03-20
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The burning of the fuel adds more heat to the air charge, causing it to expand and force the engine piston to do work on the crankshaft which in turn drives the ship’s propeller(螺旋桨 螺旋桨). 螺旋桨
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Fuel injector Exhaust valve Inlet valve Combustion chamber Piston ring Piston pin Piston Cylinder crankshaft Crank web Connecting rod
Four-stroke diesel engine
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Fuel injector Fuel injector 扫气口 Scavenge port Piston pin Piston rod Crosshead Exhaust valve
Exhaust port
Two-stroke Engine
Cross scavenge system
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Connecting rod
Uniflow scavenge system
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The operation between two fuel injections is called a working cycle, which consists of a fixed sequence of events.
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This cycle may be achieved either in four strokes(冲程 、 冲程、 冲程 行程) 行程 or two.
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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 排气). 排气
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If we combine the suction and exhaust operations with the compression and expansion strokes, the four-stroke engine will be turned into a twostroke one, as is shown in Figures 1-a)?d).
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Fuel injector Cylinder Piston
Exhaust port
Scavenge port
Connecting rod Crank
Scavenge (a)
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Compression (b)
Injection (c)
Exhaust (d)
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The two-stroke cycle begins with the piston coming up from the bottom of its stroke, i.e. bottom dead center (BDC), with the air inlet ports or scavenge ports(扫气口 in the 扫气口) 扫气口 sides of the cylinder being opened (Fig. 1-a)).
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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.
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As the piston moves about one fifth of the way up, it closes the inlet ports and the exhaust ports. The air is then compressed as the piston moves up. (Fig. 1-b)).
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When the piston reaches the top of its stroke, i.e. the top dead center (TDC), both the pressure and the temperature of the air rise to very high values.
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The fuel injector injects a fine spray of fuel into the hot air and combustion take place, producing much higher pressure in the gases.
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The piston is forced( 迫 使 ) downward as the high pressure gases expand until it uncovers the exhaust ports.
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The burning gases begin to exhaust (Fig. 1-d)) and the piston continues down until it opens the inlet ports. Then another cycle begins.
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In the two-stroke engine, each revolution of the crankshaft( 曲 轴 ) makes one power or working stroke, while in the four-stroke engine, it takes two revolutions to make one power stroke.
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That is why a two-stroke cycle engine will theoretically develop twice (two times) the power of a four-stroke engine of the same size.
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Inefficient scavenging and other losses, however, reduce the power advantage to about
  1.
  8.
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Each type of engine has its application on board ship. The slow speed (i.e. 90 to 120 rev/min) main propulsion diesel operates on the twostroke cycle.
2001-03-20
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At this low speed the engine requires no reduction gearbox between it and propeller.
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The four-stroke engine (usually rotating at medium speed, between 250 to 750 rev/min) is used for alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 90 to 120 rev/min.
2001-03-20
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Reading Material
Working cycles A diesel engine may be designed to work on the twostroke or on the four-stroke cycle: both of these are explained below.
2001-03-20
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The four-stroke cycle Figure 2 shows diagrammatically the sequence of events throughout the typical four-stroke cycle of two revolutions.
2001-03-20
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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).
2001-03-20
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Proceeding clockwise round the diagram, both inlet (or suction) and exhaust valves are initially open. (All modern four-stroke engines have poppet valves (提升阀 提升阀).) 提升阀
2001-03-20
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If the engine is naturally aspirated(自然吸气 or is a small 自然吸气), 自然吸气 high-speed type with a centrifugal turbocharger, the period of valve overlap(重叠 i.e. when both valves 重叠), 重叠 are open, will be short, and the exhaust valve will close some 10°after top dead center (ATDC). °
2001-03-20
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四冲程定时图
TDC(firing) ( Injection commences Fuel valve closed(full load) ≈10-20°BTDC ° Exhaust valve closes ≈5060°ATDC °
Inlet valve open ≈70-80 °BTDC
Inlet valve close ≈146-155°BTDC °
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Exhaust valve opens ≈120-150°ATDC °
Fig 2
BDC
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Propulsion engines and the vast majority of auxiliary generator engines running at speeds below 1000 rev/min will almost certainly be turbocharged and
2001-03-20
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will be designed to allow a generous throughflow of scavenge air at this point in order to control the turbine blade temperature.
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In this case the exhaust valve will remain open until exhaust valve closure (EVC) at 5060°ATDC. As the piston ° descends to outer or bottom dead center (BDC) on the suction stroke, it will inhale(吸 吸 入) a fresh charge of air.
2001-03-20
Jimei university
To maximize this, balancing the reduced opening as the valve seats against the slight ram(进气 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). °
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This event is called inlet valve closure (IVC). The charge is then compressed by the rising piston until it has attained a temperature of some 550℃. ℃
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At about 10-20°BTDC(firing), ° depending on the type and speed of the engine, the injector admits finely atomized(雾化的 fuel which 雾化的) 雾化的 ignites within 2-7°(depending on type again) and
2001-03-20
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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.
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At about 120-150°ATDC the ° exhaust valve opens (EVO), the timing being chosen to promote( 促 进 ) a very rapid blow-down( 排 出 ) of the cylinder gases to exhaust.
2001-03-20
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This is done: (a) to preserve( 保 留 ) as much energy as is practicable to drive the turbocharger, and (b) to reduce the cylinder pressure to a minimum by BDC to reduce pumping work on the “exhaust” stroke.
2001-03-20
Jimei university
The rising piston expels(排出 排出) 排出 the remaining exhaust gas and at about 70-80° BTDC the ° inlet valve opens (IVO) so that the inertia of the outflowing gas,
2001-03-20
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plus the positive pressure difference, which usually exists across the cylinder by now, produces a through flow of air to the exhaust to ‘scavenge’ the cylinder.
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If the engine is naturally aspirated the IVO is about 10°BTDC. The cycle now ° repeats.
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The two-stroke cycle Figure 3 shows the sequence of events in a typical two-stroke cycle, which, as the name implies, is accomplished in one complete revolution of the crank.
2001-03-20
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二冲程定时图The two-stroke cycle
Injection commences TDC ≈10-20°BTDC ° Fuel valve closes (full load )
Exhaust closes ≈110-150°BTDC ° Inlet closes ≈130-150°BTDC °
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Exhaust opens ≈110-120°ATDC ° Inlet opens ≈130-150°ATDC °
Fig.3
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Two-stroke engines invariably have ports to admit air when uncovered by the descending piston.
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The exhaust may be via ports adjacent to the air ports and controlled by same piston (loop scavenge 回 流 扫 气 ) or via poppet exhaust valves at the other end of the cylinder (uniflow scavenge直流扫气 直流扫气). 直流扫气
2001-03-20
Jimei university
Starting at TDC combustion is already under the way and the exhaust opens (EO) at 110120° 120°ATDC to promote a rapid blow-down before the inlet opens (IO) about 2030°later (130-150°ATDC). ° °
2001-03-20
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In this way the inertia of the exhaust gases?moving at about the speed of sound?is contrived( 设 计 、 设 法 ) to encourage the incoming air to flow quickly through the cylinder with a minimum of mixing,
2001-03-20
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because any unexpelled (未排 未排 gases 出 的 ) exhaust detracts(减损,降低 from the 减损, 减损 降低) weight of air entrained for the next stroke.
2001-03-20
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The exhaust should close before the inlet on the compression stroke to maximize the charge, but the geometry of the engine may prevent this if the two events are piston controlled.
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It can be done in an engine with exhaust valves.
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At all events the inlet ports will be closed as many degrees ABDC as opened before it (i.e. again 130~150° BTDC) and ° the exhaust in the same region.
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Injection commences at about 10-20°BTDC depending on ° speed, and combustion lasts 30-50°as with the four-stroke. °
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Jimei university Jimei university

  1. More than 50 / above 50 / over 50 50多
  2. Less than 50 / under 50 / below 50 不到50 或50 以下 The temperature is below 20℃.
  3. 50 more 再加50 ten more days 再10天 Please add 50 more grams of salt to the water.
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  4. a kilometer or more 一公里或一公里多些 a long kilometer 一公里多 a long hour 足足一小时 a dozen 一打,dozens 几十、几打
  5. 数字+ odd twenty odd / twenty-odd / twenty and odd / 20多
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Jimei university Jimei university

  6. from twenty to thirty / twenty to thirty / 从20 到30
  7. some thirty feet / about thirty feet / thirty feet or so / 30英尺左右 a hundred more or less / 100上下 100 more or less twenty pages / 20页左右
  8. n time(s) / 增加到n倍,n倍于 The output has increased (by) three times as compared with that of last year.
2001-03-20
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Water conducts heat about 20 times better than air does.
  8. 增加多少量, 常用by引出。 The coke used has been increased by 18% as compared with last year. This pipe is 2 inches longer than that one.
2001-03-20
Jimei university Jimei university

  9. 降低多少倍:减少n times,译为 降低到几分之几,或降低了几分之 几。 The new motor is ten times as light as the old one. 新马达的重量是旧马达的十分之一. The switching time of the new-type transistor is reduced 3 times. 新晶体管的开关时间减缩了三分之 二.
2001-03-20
Jimei university Jimei university

  10. 减少多少量,常以by引出, 为 绝对数量。 The prime cost decreased by 40%. 主要成本下降了百分之四十。 This pipe is shorter than that one by 3 meters. 这根管子比那根短3米。
2001-03-20
Jimei university Jimei university

  1、主语如果是用连词and, both…and连接的并 列主语,谓语要用复数形式。但是,如果两 个词是指同一个人或同一概念时,谓语要用 单数形式。 如课文:Both of these (two-stroke and fourstroke cycle) are explained; Both the m/v “Zheng Zhou” and the m/v “Chang Feng” are 25 000t Ocean-going cargo ships designed and built entirely by China;
2001-03-20
Jimei university Jimei university
The iron and steel industry is a basic industries. 钢铁工业是基础工业。 The steward and commissar has/have come. 服务员兼政委 / 服务员和政 委已经来了。
2001-03-20
Jimei university Jimei university

  2、主语如果是用成语连词 as well as 连
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