Lesson one

Text How does a Marine Diesel engine Work?

•*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:

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.

•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.

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 ).

Exhaust valveInlet valve

Fuel injector

Cylinder

PistonPiston ring

Connecting rod

Four-stroke diesel engine

Combustion chamber

Piston pin

crankshaft

Crank web

Fuel injector Fuel injectorExhaust valve

扫气口Scavenge portExhaust

port

Two-strokeEngine

Cross scavenge system Uniflow scavenge system

Piston rod

Connecting rod

Piston pin

Crosshead

The operation between two fuel injections is called a working cycle, which consists of a fixed sequence of events.

This cycle may be achieved either in four strokes( 冲程、行程 ) or two.

•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 ).

If we combine the suction and exhaust operations with the compression and expansion strokes, the four-stroke engine will be turned into a two-stroke one, as is shown in Figures 1-a)—d).

Scavenge(a)

Compression(b)

Injection(c)

Exhaust(d)

Scavengeport

Exhaustport

Fuel injectorCylinderPiston

Connecting rod

Crank

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 sides of the cylinder being opened (Fig. 1-a)).

•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.

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)).

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.

•The fuel injector injects a fine spray of fuel into the hot air and combustion take place, producing much higher pressure in the gases.

The piston is forced(迫使 ) downward as the high pressure gases expand until it uncovers the exhaust ports.

The burning gases begin to exhaust (Fig. 1-d)) and the piston continues down until it opens the inlet ports. Then another cycle begins.

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.

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.

Inefficient scavenging and other losses, however, reduce the power advantage to about 1.8.

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 two-stroke cycle.

•At this low speed the engine requires no reduction gearbox between it and propeller.

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.

Reading Material Working cycles

A diesel engine may be designed to work on the two-stroke or on the four-stroke cycle: both of these are explained below.

The four-stroke cycle

Figure 2 shows diagrammatically the sequence of events throughout the typical four-stroke cycle of two revolutions.

•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).

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 ).)

•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).

TDC（ firing)

Fuel valve closed(full load)Injection commences≈10-20°BTDC

Exhaust valve opens≈120-150°ATDC

Exhaust valve closes ≈50-60°ATDC

Inlet valve open≈70-80 °BTDC

Inlet valve close≈146-155°BTDC

Fig 2

四冲程定时图

BDC

Propulsion engines and the vast majority of auxiliary generator engines running at speeds below 1000 rev/min will almost certainly be turbocharged and

•will be designed to allow a generous throughflow of scavenge air at this point in order to control the turbine blade temperature.

In this case the exhaust valve will remain open until exhaust valve closure (EVC) at 50-60°ATDC. As the piston descends to outer or bottom dead center (BDC) on the suction stroke, it will inhale(吸入 ) a fresh charge of air.

•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).

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 . ℃

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 ignites within 2-7°(depending on type again) and

•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.

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.

•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.

The rising piston expels( 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,

•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.

If the engine is naturally aspirated the IVO is about 10°BTDC. The cycle now repeats.

The two-stroke cycleFigure 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.

Exhaust closes≈110-150°BTDC

Exhaust opens≈110-120°ATDC

Inlet opens≈130-150°ATDC

Inlet closes≈130-150°BTDC

TDCFuel valve closes (full load )

Injection commences≈10-20°BTDC

Fig.3

二冲程定时图 The two-stroke cycle

•Two-stroke engines invariably have ports to admit air when uncovered by the descending piston.

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 ).

Starting at TDC combustion is already under the way and the exhaust opens (EO) at 110-120°ATDC to promote a rapid blow-down before the inlet opens (IO) about 20-30°later (130-150°ATDC).

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,

•because any unexpelled ( 未排出的） exhaust gases detracts( 减损，降低 ) from the weight of air entrained for the next stroke.

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.

It can be done in an engine with exhaust valves.

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.

Injection commences at about 10-20°BTDC depending on speed, and combustion lasts 30-50°as with the four-stroke.

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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6. from twenty to thirty / twenty to thirty / 从 20 到 307. some thirty feet / about thirty feet / thirty feet or so / 30 英尺左右 a hundred more or less / 100 上下 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) threetimes as compared with that of last year.

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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.

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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 asthe 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 .

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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。

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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 four-stroke 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;

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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。

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2 、主语如果是用成语连词 as well as 连接的两个名词或代词，谓语要与第一个名词或代词的人称和数一致。如： The young motorman as well as the others has learned to operate the oil separators.

这位年青机匠，和其他人一样，已经学会操作分油机。

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3 、主语如果是由一个名词或代词和介词 with 与名词、代词组成的介词短语所组成，谓语要与被介词短语所修饰的名词、代词的人称和数一致。例如： The main engine together with other driven machinery is running smoothly.

主机和其它被带动的机械（一起）正在平稳地运转。

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4 、主语如果是用连词 or , either…or, neither…nor, not only…but also 连接的两个名词或代词，谓语要与后一个名词或代词的人称和数一致。例如：Neither aluminum nor copper is easy to corrode.Not only the main engine but also the auxiliary machines have to be checked at intervals.

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5 、主语如果是集体名词或当作一个整体看待的表示时间、距离、重量的名词复数形式，谓语要用单数形式。例如：30 kilograms is the maximum weight for each passenger.每一位旅客携带物品的重量不得超过30 千克。

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6 、主语如果是成对使用的名词，谓语要用单数形式。例如：Trial and error is also a source of ourknowledge.试验和失败也是（我们获得）知识的源泉。

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