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英文文摘
Four-stage-engine and Two-stage-engine Operation
The actions taking place in the engine cylinder can be divided into four stages, or strokes.“Stroke”refers to piston movement；a stroke occurs when the piston moves from one limiting position to the other. The upper limit of piston movement is called TDC（top dead center）. The lower limit of piston movement is called BDC (bottom dead center). A stroke is piston movement from TDC to BDC or from BDC to TDC. In other words, the piston completes a stroke each time it changes its direction of motion.
Where the entire cycle or events in the cylinder requires four strokes (or two crankshaft revolutions), the engine is called a four-stroke-cycle engine, or a four-cycle engine. The four piston strokes are intake, compression, power, and exhaust.
Intake stroke. On the intake stroke, the intake valve has opened, the piston is moving downward, and a mixture of air and vaporized gasoline is entering the cylinder through the valve port. The mixture of air and vaporized gasoline is delivered to the cylinder by the fuel system and carburetor.
Compression stroke. After the piston reaches BDC, or the lower limit of its travel, it begins to move upward. As this happens, the intake valve closes. The exhaust valve is also closed, so that the cylinder is sealed. The exhaust valve is also closed, so that the cylinder is sealed. As the piston moves upward(pushed now by the revolving crankshaft and connecting rod), the air-fuel mixture is compressed. By the time the piston reaches TDC, the mixture has been compressed to as little as one –tenth of its original volume, or even less. This compression of the air-fuel mixture increases the pressured, not only does the pressure in the cylinder go up, but the temperature of the mixture also increases.
Power stroke. As the piston reaches TDC on the compression stroke, an electric spark is produced at the spark plug. The ignition system delivers a high-voltage surge of electricity to the spark plug to produce the spark. The spark ignites, or sets fire to, the air-fuel mixture. It now begins to burn very rapidly, and the cylinder pressure increases to as much as 3-5 MPa or even more. This terrific push against the piston forces it downward, and a power impulse is transmitted through the connecting rod to the crankpin on the crankshaft. The crankshaft is rotated as the piston is pushed down by the pressure above it.
Exhaust stroke. As the piston reaches BDC again, the exhaust valve opens. Now, as the piston moves up on the exhaust stroke, it forces the burned gases ort of the cylinder through the exhaust-valve port. Then, when the piston reaches TDC, the exhaust valve closes and the intake valve opens. Now, a fresh charge of air-fuel mixture will be drawn into the cylinder as the piston moves down again to-ward BDC, The above four strokes are continuously repeated.
In the four-stroke-cycle engine, already discussed in Lesson 1、2, the complete cycle of events requires four piston strokes (intake, compression, power, and exhaust). In the two-stroke-cycle, or two-cycle, engine, the intake and compression strokes and the power and exhaust strokes are in a sense combined. This permits the engine to produce a power stroke every two piston strokes, or every crankshaft rotation.
In the two-cycle engine, the piston acts as a valve, clearing valve ports in the cylinder wall as it nears BDC.
A fresh air-fuel charge enters through the intake port, and the burned gases exit through the exit through the exhaust port. The complete cycle of operation is as follows: As the piston nears TDC, ignition takes place. The high combustion pressures drive the piston down, and the thrust through the connecting rod turns the crankshaft. As the piston nears BDC, it passes the intake and exhaust ports in the cylinder wall. Burned gases, still under some pressure, begin to stream ort through the exhaust port. At the same time, the intake port, now cleared by the piston, begins to deliver air-fuel mixture, under pressure, to the cylinder. The top of the piston is shaped to give the incoming mixture an upward movement. This helps to sweep the burned gases ahead and out through the exhaust port.
After the piston has passed through BDC and starts up again, it passes both ports, thus sealing them off. Now the fresh air-fuel charge above the piston is compressed and ignited. The same series of events takes place again and continue as long as the engine runs.
We mentioned that the air-fuel mixture is delivered to the cylinder under pressure. In most engines, this pressure is put on the mixture in the crankcase. The crankcase is sealed except for a leaf, or reed, valve at the bottom. The reed valve is a flexible, flat metal plate that rests snugly against the floor of the crankcase. There are holes under the reed valve that connect to the engine carburetor. When the piston is moving up, a partial vacuum is produced in the sealed crankcase. Atmospheric pressure lifts the reed valve off the holes, and air-fuel mixture enters the crankcase. After the piston passes TDC and starts down again, pressure begins to build up in the crankcase. This pressure closes the reed valve so that further downward movement of the piston compresses the trapped air-fuel mixture in the crankcase. The pressure which is built up on the air-fuel mixture then causes it to flow up through the intake port into the engine cylinder when the piston moves down far enough to clear the intake port.
The two-stroke engine is not only very simple but gives nearly twice the power of a four stroke engine from a cylinder of given size, but it is wasteful of gasoline, as some mixture inevitably finds its way into the exhaust system on the combines intake/exhaust stroke, and there are always some combustion products left in the cylinder which reduce the rapid burning of the fuel. This kind of engine is always used in motorcycles.
中文翻譯
四行程發(fā)動機和二行程發(fā)動機的工作過程
發(fā)動機氣缸內(nèi)的工作過程，可以分為四個階段，或行程。行程涉及塞的運動；活塞從某一限定位置到另一限定位置的動動稱為一行程。活塞運動的上限稱為TDC（上止點），下限稱為BDC（下止點）。一個行程就是活塞從上止點到下止點，或下止點到上止點的運動。換句話說，活塞每完成一個行程，就改變一次其運動的方向。
發(fā)動機氣缸中的全部工作過程分為四個行程的（或嗜曲軸旋轉(zhuǎn)兩周的），叫做四行程循環(huán)發(fā)動機。發(fā)動機的四個活塞行程是進氣、壓縮、做功、排氣。
進氣行程：在進氣行程中，進氣門打開，活塞向下移動，可燃混合氣通過進氣門進入氣缸。適當(dāng)濃度可燃混合氣是由燃料系統(tǒng)和化油器提供的。
壓縮行程：在活塞到達下止點時，或者是活塞下限時，活塞開始向上運動。同時，進氣門關(guān)閉，排氣門也關(guān)閉，所以這時的氣缸是封閉的。當(dāng)活塞向上運動時（這時是由轉(zhuǎn)動的曲軸和連桿推動活塞），可燃混合氣被壓縮。當(dāng)活塞到達上止點時，可燃混合氣被壓縮到只有原體積的十分之一，甚至更少。當(dāng)油氣混合燃料被壓縮時，不僅氣缸里的壓力上升，可燃混合氣的溫度也隨之增加了。
做功行程：當(dāng)活塞到達壓縮行程到上止點時，火花塞產(chǎn)生電火花。電火花是由點火系統(tǒng)向火花塞提供高壓電脈沖而產(chǎn)生的。電火花點燃可燃混合氣。可燃混合氣開始發(fā)生劇烈燃燒，氣缸內(nèi)壓力達到3~5兆帕，甚至更高。作用于活塞上強大的推動力推動活塞下運動，并將這一推力通過連桿傳到曲軸上的連桿軸頸上。因此，當(dāng)活塞受壓向下動動時，推動曲軸轉(zhuǎn)動。排氣行程：當(dāng)活塞受壓向下運動時，推動曲軸轉(zhuǎn)動。
排氣行程：當(dāng)活塞再一次到達下止點時，排氣門打開。同時，活塞向上移動，把廢氣經(jīng)排氣門排出氣缸。隨后活塞達到上止點，排氣門關(guān)閉，進氣門打開。當(dāng)活塞又一次向下移動到達下止點時，新鮮可燃混合氣被吸入氣缸，上述的四個行程又繼續(xù)重復(fù)。
四行程發(fā)動機整個工作循環(huán)需要活塞完成進氣、壓縮、作功、排氣四個行程。在二行程發(fā)動機中，進氣和壓縮行以及作功和排氣行分別合并在一起的，這使得二行程發(fā)動機每兩個行程，即曲軸旋轉(zhuǎn)一周，就完成一次工作循環(huán)。
在二行程發(fā)動機中，活塞起到閥門的作用，當(dāng)活塞接近下止點時，使氣缸壁上的進排氣口打開。新的可燃混合氣通過進氣口進入，燃燒過的廢氣從排氣口排出。整個工作過程如下：當(dāng)活塞接近上止點時，發(fā)動機點火。燃燒產(chǎn)生的高壓驅(qū)動活塞向下運動，通過連桿，推動曲軸轉(zhuǎn)動。當(dāng)活塞接近下止點時，以過氣缸壁上的進排氣口，燃燒過的氣體，仍有剩余壓力，開始從排氣口噴出。同時，活塞使進氣口露出，受壓縮的可燃混合氣開始進氣缸?；钊敳康男螤钍惯M入氣缸的可燃混合氣向上運動，這樣有助于掃氣，引導(dǎo)燃燒過的廢氣向上運動并從排氣口排出。
在活塞通過下止點，并開始上升時，活塞經(jīng)過進氣口和排氣口，將其堵住。這時活塞上方新的可燃混合氣受壓縮后點火。只要發(fā)動機運轉(zhuǎn)，同樣的動作就會反復(fù)不斷地發(fā)生。
我們曾提到可燃混合氣是被壓進氣缸的。在大多數(shù)發(fā)動機中，可燃混合氣是在曲軸箱中受壓的。除了底部有一葉片閥（片簧閥）之外，曲軸箱是密封的。片簧閥是一片柔韌、平整的金屬片。金屬片緊貼在曲軸箱的底部。在片簧閥的下面有孔產(chǎn)生部分真空。大氣的壓力將片簧閥提起。打開孔道，可燃混合氣進入曲軸箱。當(dāng)活塞經(jīng)過上止點，又開始向下運動時，壓力就開始在曲軸箱中產(chǎn)生，壓力使片簧閥關(guān)閉，這樣，活塞進一步的向下運動壓縮留在曲軸箱中的可燃混合氣。當(dāng)活塞向下運動直到露出進氣口，可燃混合氣所具有的壓力使其上升，通過進氣口進入氣缸。
二行程發(fā)動機不僅結(jié)構(gòu)簡單，而且其功率相當(dāng)于具有相同氣缸容積的四行程發(fā)動機的二倍。但是，在進氣和排氣行程中，一些可燃混合氣不可避免地進入排氣系統(tǒng)，并且總會有一些燃燒物滯留在氣缸中，致使燃料不能充分燃燒。所以二行程發(fā)動機耗油較大。通常使用在摩托車上。