Lesson 2.   The Four-stroke Cycle

         四冲程循环

Anyone familiar with the working of the ordinary petrol engine is aware that the motive power is produced by igniting above the piston in each cylinder a mixture of petrol vapour and air, the resultant combustion of which causes an expansion of gases exerting pressure on the top of the piston.

By this means the heat released by the fuel is transformed into work, the reciprocating movement of the pistons being converted into rotary movement of a crankshaft by means of the connecting rods

翻译：熟悉普通汽油发动机原理的人知道，通过点燃每个汽缸里活塞上面汽油蒸气和空气混合气体，混合气体燃烧的结果使气体膨胀，给活塞顶部施加压力而产生动力。通过这种方式，燃料释放的热量被转换成功。活塞的往复运动通过连杆被转换成曲轴的旋转运动。

The four-stroke cycle is the operating sequence more commonly employed.

It requires that each cylinder be provided with an inlet valve to admit a combustible mixture of petrol and air drawn by the suction of the engine from a mixing device known as the carburettor into the space above the piston, while there must also be an exhaust valve to control the outlet through which the residual products of combustion are ejected after they have done their work.

四冲程循环是被普遍采用的工作顺序。四冲程循环要求装配一个进气阀，允许把发动机从混合装置也就是化油器上吸入的可燃的汽油和空气混合气体进入活塞上面的空间，同时，也必须有一个排气阀，控制排出气体，把燃烧的残余物质在做完功后喷射出去。

 To describe the complete cycle as it occurs in any one cylinder it may be assumed that the piston is at the top of the stroke and that both the inlet and the exhaust valves are closed.

As the piston descends the inlet opens so that charge of mixture can flow into the cylinder. This is called the suction(or induction)stroke, the charge flowing under the influence of external atmospheric pressure.

Immediately after the piston has reached its lowest position and begins to rise again the inlet valve is closed so that the mixture can be compressed by the rising of the piston on the compression stroke As the piston once again approaches its topmost position an electric spark is caused to pass between the points of a sparking plug located in the top of the cylinder in a zone called the combustion chamber.

The spark ignites the mixture and as both valves are closed during its combustion, there is a considerable pressure rise.

As a result, the piston is forced back again down the cylinder, this movement being termed the expansion or power stroke

翻译：

为了描述在任何一个汽缸里发生的完整的工作循环，可以假设活塞处于冲程的顶部，进气阀和排气阀关闭。

随着活塞下降，进气阀打开，以便混合气体燃料能流进汽缸。这过程被称为进气冲程（或感应冲程），燃料受外部大气压力的影响流动。

在活塞已经到达最地位置后，活塞立即再次上升，进气阀关闭，以便混合气体被上升的活塞压缩，称为压缩冲程。随着活塞上升到接近最高位置，一个电火花被打着，从火花塞的顶尖穿过，火花塞位于被称为燃烧室的区域的汽缸的顶部。

火花点燃混合气体，随着两个气阀在燃烧时关闭，有巨大的压力上升。

结果，活塞再次被压回汽缸，此运动被称为膨胀冲程或动力冲程。

Before the piston reaches its lowest point, the exhaust valve is opened, pressure is released, and as the piston again rises it sweeps the residual gas through the exhaust outlet, whence it is led through a silencer to the atmosphere; this is the exhaust stroke which completes the continuously repeated cycle.

Thus it will be seen that there is one power stroke for every four strokes of the piston or two revolutions of the crankshaft

在活塞达到最低点时，排气阀打开，压力释放，随着活塞再次上升，活塞把残余气体压向排气出口，这里，排出气体经过一个消声器被导进大气；这是排气冲程，完成不断连续重复的循环。

因而，可以看出活塞的每4个冲程或曲轴的两转有一个动力冲程。

It should be noted that the size of the combustion chamber above the piston (known as the clearance space )is considerably smaller than that part of the cylinder in which the piston travels.

Thus at the end of the compression stroke the explosive mixture is compressed to a pressure varying from about 5. 5 to 7kg/cm2(80 to 100 lbf/in2).

By such compression the mixture is heated to some extent, resulting in better vaporisation of the fuel, and more intimate mixing of the vapor and air; it also enables the engine to work with higher thermal efficiency, to which further reference is made later.

应该注意活塞上面燃烧室的大小（被人称为间隙空间）比汽缸里活塞行程的那部分相对要小一些。因而，在压缩冲程结束时，混合可燃气体被压缩到大约5.5--7 kg/cm2 (80 - 100 lbf/in2)的压力。

通过这种压缩，混合气体被加热到一定程度，使燃料汽化更好， 并使空气和燃料汽的混合更精细，这也使发动机以更高的热效率做功，后面文章会对热效率作进一步的解释。

Incidentally, the ratio of the total volume of the cylinder above the piston when it is in its lowest position to that of the combustion or clearance space when it is at its highest position is known as the compression ratio and in modern commercial vehicle petrol engines varies from about 6:1  to 8:1

顺便提一下，活塞最低位置时汽缸的总容量和活塞最高位置汽缸燃烧时的容量或间隙空间的比率被称为压缩比。现代商用车的汽油发动机的压缩比范围从大约6:1到8:1。

The operating cycle of the four-stroke diesel is identical in so far as the sequence of strokes is concerned but there is a major difference in principle.

On the induction stroke only pure air is drawn into the cylinder through the inlet valve port.

Then, because the clearance space is so much smaller than that in a corresponding petrol engine, the compression ratio is considerably higher, being not less than 12:1 and even in some engines as high as 21:1.

The final pressure of the air charge at the end of the compression stroke may be 35kgf/cm2(500 1bf/in2)or more, with a corresponding temperature rise to not less than 1000°F(538℃), which is sufficient to ignite finely atomized but non-volatile fuel oil without the assistance of an electric spark.

4冲程柴油机的工作循环就冲程顺序而言是相同的。但是原理上有一个重要的差别。在感应冲程，只有纯净的空气通过进气阀口吸进汽缸。那么，因为间隙空间比相应的汽油发动机的要小的多， 其压缩比相当高，不低于12:1，甚至有些发动机达到21:1。压缩冲程结束时冲气的最终压力可能为35kgf/cm2(500 1bf/in2)或更高， 随着相应的温度上升到至少1000°F(538℃)，足以点燃精细雾化的但尚未挥发的燃油，无须电子火花塞的辅助。

Just before the piston reaches the end of the compression stroke the fuel oil is injected into the combustion chamber through an injection nozzle, or sprayer,  mounted on the cylinder head in much the same position as the sparking plug in a petrol engine.

The pressure at which the fuel is injected must necessarily, of course, be higher than the combustion pressure , usually not less than 140 kgf/cm2(2000 1bf/in2).

During injection the fuel is split up into finely divided particles, and the mixture of these with the air forms an explosive charge that is ignited by the heat of compression.

在活塞到达压缩冲程结束前，燃油通过一个喷射喷嘴或喷雾器被喷射入燃烧室，喷嘴或喷雾器安装在缸盖上，和汽油发动机上火花塞的位置完全相同。

燃油喷射的压力当然且必须比燃烧压力高，通常至少达到140 公斤力/平方厘米kgf/cm2 (2000 磅力/平方英寸 1bf/in2)

喷射时，燃油被碎成微小颗粒，这些颗粒和空气的混合体形成爆炸（可燃）气体，被压缩的热量点燃。

Injection is continued for a short period, during which the piston passes its highest position and begins to descend on the power stroke.

The expansion of combustion products only begins to have effect when the piston has passed the top of its stroke, and during the injection period the burning fuel maintains a more or less constant pressure somewhat above that of compression; When the fuel is cut off expansion of the gases continues, the pressure is falling as the piston descends.

The overall effect on the piston is a more sustained pressure than that associated with the combustion process in a petrol engine.

The effective part of the power stroke is concluded before the piston descends to its lowest point, and the cycle is completed when the exhausted gases are expelled by the rising piston.

喷射持续很短的时间，此时活塞经过最高位置，开始下降到动力冲程。

当活塞已经通过压缩冲程的顶部时，燃烧物的膨胀才开始起作用，喷射期间，燃烧的燃油保持较稳定的压力，稍微高于压缩压力；当燃油被切断，气体继续膨胀，压力随活塞的下降而下降。

对活塞的总的作用是与汽油发动机里和燃烧过程相关的压力相比具有一个更稳定的压力

动力冲程的有效部分在活塞下降到最低点前结束，当废气被上升的活塞排出时，循环完成。

To inject the fuel a special type of pump driven by the engine is employed, and this is a distinguishing feature of the diesel engine, just as the carburettor and the magneto or coil and distributor ignition apparatus distinguish the normal petrol engine.

燃油的喷射靠使用一个由发动机带动的专用型式的泵来实现的。这是柴油发动机一个明显的特征。而化油器、磁发电机或线圈和分配器点燃装置显示出正常的汽油发动机的特点。