发动机的组成（外文翻译及原文）-毕业作品网站

The engine is an important part of the automobile; it acts as the heart of a person, which provides the power that the cars need to run on the road. All the automotive engines today are the Internal Combustion Engines ( ICEs ) because the fuel is burnt inside their cylinders and the energy is provided.

ICEs are those heat engines that burn their fuel inside the engine cylinder. In ICEs, the chemical energy stored in their fuel converted into heat energy during the burning part of their operation. The heat energy is converted into mechanical energy by the expansion of gases against the piston attached to crankshaft that can rotate. The engines that burn petrol are known as petrol engine. Other types of ICEs burn heavier oils, of these types the diesel engine has come into the widest use.

Diesel and petrol engines have the same mechanical parts, except that diesel components are generally stronger and heavier. Both engines are internal combustion engines, but they have different fuel system and use different fuels. With a diesel, only air enters the cylinder during the intake stroke. A petrol engine takes in an air-fuel mixture. Following are some general comparisons between diesel and petrol engines:

In a diesel, the fuel is injected into the cylinder as a fine spray near the top of the compression stroke. With a petrol engine, the fuel is injected into the exhaust ports at the start of the induction stroke.

Small diesel engines, as well as petrol engines, are used in passenger cars and light commercial vehicles. Larger diesel engines are used in all heavy commercial vehicles, earthmoving equipment, and farm machinery.

The term engine configuration refers to the way that the cylinders of an engine are arranged. The cylinders can be in-line, or at an angle ( V-type ). Within these three basic arrangements, there are a number of variations.

With in-line engines, the cylinders are arranged in a straight line, one behind the other. Most in-line engines have their cylinders vertical, but some are slanted. That is, the engine is tilted at an angle to reduce the overall height. These engines are sometimes referred to as slanted engines.

Some in-line engines have their cylinders horizontal, so that the engine is more or less on its side. This reduces the overall height of the engine. This arrangement is

used mainly in larger commercial vehicles with the engine mounted under part of the cab. The mechanical arrangement of a four-cylinder in-line for a passenger car is shown in Fig.1-9. The cut-away view in Fig.1-10 enables the various parts to be identified.

This arrangement has its cylinders arranged in two flat blanks with the crankshaft between them. The engine shown has a short rigid crankshaft with five bearings. A horizontally opposed engine has even firing impulses and good balance. Movement of a piston in direction is opposed by movement of a piston in the opposite direction.

Horizontally opposed engines, with their flat design, give the engine a low height and also help to keep the center of gravity of the vehicle low. A low center of gravity gives the vehicle stability.

With V-type engines, the cylinders are arranged in two blanks at an angle. This reduces the length of the engine and makes it more compact. This also reduces the length of the crankshaft, which can be designed to be more rigid than a long shaft.

The complete engine assembly consists of the mechanical components that make up the engine itself and also a number of associated systems. These are the systems that are needed to start the engine and also to control it and keep it running. The mechanical parts of the engine assembly can be broken down into a number of sub-assemblies, or groups of associated components, although these are usually referred to merely as assemblies, for example, cylinder-head assembly and piston assembly. Some parts of an engine are internal, other parts are external, this illustration identifies a number of external parts.

The various components of an engine are assembled together during manufactured and most of these are secured by means of bolts, nuts and other types of fasteners. Some parts are internal and others are attached to the outside of the engine.

The cylinder block is the largest part of engine. The other parts are either fitted into the block or attached to it. As its name suggests, it is basically a block of cast metal, usually cast iron, but it can be aluminum alloy with cast iron or steel liners. Aluminum is used to reduce the weight.

The cylinder block has accurately bored cylinders to take the pistons. The lower part of the block is known as the crankcase and this has bearings that carry the crankshaft. The water-jackets that surround the cylinders are filled with coolant.

The top of the cylinder block has a machined surface for the cylinder head and the bottom of the cylinder block is machined to provide a mounting for the oil pan, or sump.

The crankshaft is mounted in bearings in the lower part of the cylinder block ( the crankcase ). The connecting rods connect the pistons to the crankshaft, which is rotated by the power strokes of the pistons when the engine is running. The up-and-down, or reciprocating, motion of the pistons is changed to rotary motion by the combination action of the connecting rods and the cranks of the crankshaft.

The term crankshaft comes from the word cranked, which also means bent. It is a shaft with a number of cranks or bends. With in-line engines, there a crank for each cylinder, but with some V-type engines, there is only one crank for each pair of cylinders.

The flywheel is a heavy cast iron wheel attached to the rear of the crankshaft. It reduces engine vibrations by smoothing out the power impulses of the pistons. The flywheel absorbs energy during the power stroke and gives up energy during the other strokes and this helps to keep the engine running smoothly. A ring gear fitted to the rim of the flywheel so that the engine can be rotated by the starter pinion when starting the engine. With automatic transmissions, a drive plate and torque converter take the place of the flywheel and perform the same functions as a flywheel, as far as the running of the engine is concerned.

The piston has grooves that carry the piston rings and these provide a seal between the piston and the cylinder wall. Compression rings are used as a gas seal and the oil ring is used to prevent excess oil from finding its way up past the piston into the combustion chamber.

The connecting rod has a removable cap and a split bearing at its lower end where it is connected to the crankshaft. Its upper has a piston pin that provides a wrist-type of action with the piston. Because of its action, the piston pin is sometimes referred to as a wrist pin.

The cylinder head is made of cast aluminum alloy. It is bolted to the top of the cylinder block so that it encloses the cylinders. It has combustion chambers above the cylinders in which the air-fuel mixture is burnt. Cylinder heads can be made of cast iron, which is more resistant to corrosion, but aluminum alloy is used for petrol engines because it has advantages of better heat transfer and lighter weight. The cylinder head has intake ports and exhaust ports. The intake valves open the intake ports to admit the fuel charge into the cylinder during the intake stroke. The exhaust valves open the exhaust ports to allow the burnt gases to leave the cylinder following the exhaust stroke.

The camshaft and the valve mechanism are used to open and close the valves at the correct time. The camshaft is driven from the crankshaft at half the crankshaft speed.

The valve cover, also called the cylinder-head cover, is fitted to the top of the cylinder head. This encloses the valve mechanism. An additional cover on top of the engine covers the ignition coils.

This cylinder-head arrangement, for a four-cylinder engine, has two camshafts and sixteen valves. Other four-cylinder engines can have two camshafts and twelve valves, or one camshaft and eight valves.

The timing belt and pulleys drive the camshaft at half the crankshaft speed ( engine speed ). A toothed drive belt is often used, but gears and chains are also used-in some instances a belt and chain are both used.

On some engines, the timing chain is used to drive the oil pump. On diesel engines, a timing chain or a timing gear is used to drive the injection pump as well as the camshaft.

The oil pan, or sump, holds the oil for the engine lubricating system. The oil pan is made of steel that has been pressed to shape. Other oil pans are made of aluminum alloy that has been cast to shape.

The oil pan is bolted to the underside of the engine so that it closes off the crankcase. Only the ends of the crankshaft that extend beyond the cylinder block are exposed.

Various forms of gaskets and seals are used between the surfaces where the parts are bolted together. Some of these are used to seal against oil, some against coolant and some against heat and pressure. Some provide a seal on flat surfaces, other seal against rotating shafts.

As well as the larger parts of an engine, there are numerous small parts. These include bolts, washers, retainers, spring and brackets which go to make up the mechanical components of the engine.

These are the systems for petrol engines. Diesel engines have similar systems except for the fuel and ignition systems. Some parts of the systems are built into the engine, some parts are attached to the engine and other parts are located on the body panels in the engine compartment.

　　The starter is used to rotate the engine during starting. It consists of an electric motor and a drive. The drive has a small pinion that meshes with the ring gear on the flywheel during starting. The battery supplies the electrical energy to operate the starter and rotate the engine until it fires and runs on its own.

　　These are four basic types of fuel systems: carburetor systems for petrol engines, fuel injection systems for petrol engines, gas fuel systems ( LPG or NGV ), and diesel injection systems. All these systems operate in different ways, but they all have somewhere to store fuel ( a fuel tank or a cylinder ) and a way of supplying the engine with the fuel. They also have a way of supplying air and fuel mixed in the correct proportion so that it can be effectively burn in the combustion chambers.

Petrol engines and engines operating on gas require an ignition system. This is needed to provide the sparks that fire the charges in the combustion chambers. For this reason, petrol engines are sometimes referred to as spark-ignition engines. This distinguishes them from them diesel engines that do not need a spark because they use compression ignition.

Combustion in a diesel occurs when the fuel is sprayed into the combustion chamber. The air in the cylinder is at high temperature from being compressed-high enough to ignite the fuel that is sprayed from the injector.

A considerable amount of heat is produced in an engine by the burning air-fuel mixture. Some of the heat is used to do useful work, some is transferred to other parts of the engine and some is carried away with exhaust gases. However, there is still enough heat to cause damage unless it is removed. This is still the function of the cooling system, which removes about one-third of the heat that is produced.

The cooling system does not just remove heat; it maintains the engine at a desirable operating temperature. In a liquid-cooled engine, this is done by circulating coolant through the water-jackets. In an air-cooled engine, cooling is by air over cooling fins.

　　The engine-lubricating system consists of an oil pump, a relief valve and a filter; also pipes, passages and drillings in various parts of engine through which the oil can flow. A quantity of oil is held in the oil pan. From this, oil is taken by the oil pump and circulated throughout the engine before returning to the oil pan. The oil lubricates all the moving parts and this is not only reduces friction, but it prevents wear and damage. The oil pump is driven directly by the crankshaft. This system has an oil cooler, located at the filter mounting under the filter.

In petrol engine with electronic fuel injection, the intake system includes the air cleaner, the throttle valve assembly and the intake manifold. The intake system provides clean air and carries it into the engine through the intake manifold. Nozzles of the fuel injectors spray fuel into the air passing from the intake manifold into the intake ports.

With carburetor fuel systems, a mixture of air and fuel is carried from the carburetor, through the intake manifold and into the engine through the intake ports.

　　The exhaust system carries the burnt gases away from the engine and also reduces noise. The system consists of the exhaust manifold, exhaust pipes, a catalytic converter and a muffler. Arrangements vary with different engines, there may be more than one muffler and more than one catalytic converter. Engines that run on Lead Replacement Petrol ( LRP ) do not have a catalytic converter, neither do diesel engines.

The battery supplies energy for the starter, the ignition system and electric fuel pump during the starting period ( petrol engine ), but once the engine is running, the alternator supplies all the electrical energy. It also recharges the battery to replace the energy used during starting.

The engine’s electronic control system consists of sensors, a control unit and actuators. The control unit receives signals from the sensors and then sends signals to various actuators.

There are sensors on the engine, in the intake system and in the exhaust system. The injectors are actuators that adjust the spray of fuel. There are also actuators in the ignition system that advance and retard the spark. These are just two examples of where electronic control is used.

During the downward motion of the piston, the air-fuel mixture is sucked from the carburetor into the cylinder. During the upward motion the mixture is compressed by the piston in the cylinder and ignited by an electric spark. When the mixture is burned in the cylinder, the resulting heat causes the gases to expand which exert pressure on the cylinder walls and on the piston. The piston, being movable, is pushed downward by this pressure to the full length of its stroke.

The pressure exerted on the piston is transmitted through the connecting rod to the crankshaft that is made to revolve. The crankshaft turns through one-half of a revolution as the piston moves downward. A flywheel attached to the crankshaft stores up energy. The momentum of the flywheel carries the piston through the balance of its motion until it receives another power impulse. The process is repeated over and over again, the crankshaft is turning continuously and the engine is running.

The events that are repeated make up the cycle of the engine. The number of stokes of the piston required to complete the cycle varies with the type of engine. In modern vehicles, the cycle is extended through four stroke of the piston or two revolutions of the crankshaft. This is called a four-stroke cycle engine. In two stroke cycle engines, the cycle is completed in two strokes of the piston or one revolution of the crankshaft.

In the four-stroke cycle engine, the four strokes are named suction, compression, power, and exhaust in accordance with the operation of the cycle which occur during each particular stroke.

Suction stroke: during suction stroke, the piston is moved downward by the crankshaft, which is revolved either by the momentum of the flywheel or by the power generated by the electric starting motor. The inlet valve remains open and the exhaust valve is closed during this stroke. The downward movement of the piston sucks air-fuel mixture in the cylinder from the carburetor through the open inlet valve. Here the fuel is petrol mixed with air, broken up into a mist, and partially vaporized in the carburetor.

Compression stroke: during compression stroke, the piston moves upward, thus compressing the charge. Ignition and much of the compression also take place during this stroke. The heat produced by the compression makes more homogeneous mixture of air and petrol inside the cylinder. The heat makes the petrol easier to burn, while the compression forces it into closer combination with the air. The mixture, under compression, is ignited by the spark produced by a spark plug, and the combustion is over half-completed when the piston is at Top Dead-Centre ( TDC ). Both the inlet and exhaust valves remain closed during the compression stroke.

Working, power or expansion stroke: the expansion of the gases due to the heat of combustion exerts a pressure on the cylinder and piston. Under this impulse the piston moves downward thus doing useful work. Both the valves remain closed during this stroke.

Exhaust stroke: during this stroke, the inlet valve remains closed and the exhaust valve opens. The greater part of the burnt gases escapes because of their own expansion. The piston moves upward and pushes the remaining gases out of the open exhaust valve. Only a small quantity of exhaust gases remains in the clearance space which will dilute the fresh incoming charge.

Thus, in this type of engine, four strokes of the piston are required to complete the cycle, and the four strokes make two revolutions of the crankshaft. The operations are repeated over and over again in running the engine.

发动机是汽车的重要组成部分，它为汽车提供行驶的动力，对汽车而言它就像心脏对人一样重要。现今所有的汽车发动机都是内燃机（ICEs），这种发动机的燃料在气缸内燃烧，燃烧产生的能量直接提供给发动机。

内燃机是将燃料在发动机气缸里面燃烧的热机。在内燃机运转过程中，储存在燃料中的化学能通过燃烧转变为热能。内燃机的活塞通过连杆连接在可以旋转的曲轴上，热能使气体膨胀推动活塞转变为机械能。燃烧汽油的发动机叫汽油机。其它形式的内燃机燃烧重油，其中以柴油机的应用最为广泛。

柴油机与汽油机具有相似的构造，其区别在于柴油机比较笨重。这两种发动机都是内燃机，但是两者具有不同的燃烧系统，并且使用不同的燃料。对于柴油机，在进气行程中只有空气进入气缸。而对于汽油机，在进气行程中是吸入空气和燃料的混合物。通常，柴油机和汽油机具有以下区别：

在柴油机中，燃料是在压缩行程末期以很好的雾状形态喷入气缸。而在汽油机中，燃料在进气行程初期被吸入到排气孔附近。

小型柴油机和汽油机一样，都被应用于乘用车和轻型商用车上。大型柴油机被应用于所有的重型商用车、工程用车以及农用机械。

发动机的类型是指发动机气缸的排列形式。气缸可以排列在一条直线上，可以对置排列，还可以呈一定角度排列（V型）。以这三种排列形式为基础，又发展了多种排列形式。

顾名思义，直列式发动机中各气缸排成一直列，一个紧邻一个，在一条直线上。大多数直列式发动机其气缸都是垂直的，但也有一些是倾斜的。气缸倾斜设置可以降低发动机的整体高度，这类发动机有时叫做倾斜发动机。

有些直列式发动机的气缸是水平放置的，这类发动机的横向尺寸会比较大，不过这样可以降低发动机整高。这类发动机主要用于大型商用车上，其发动机架位于驾驶室底部。如图1-9显示的是轿车用直列四缸发动机的构造。图1-10是侧面剖视图，可以看到更多零部件，便于分别。

这种排列方式将气缸布置在曲轴两侧的平面上，曲轴是刚性较大的短轴，其上有五个轴承。水平对置式发动机有相当好的输出脉冲和平稳性。活塞的运动方向与对面的活塞运动方向相反。

在V型发动机中，气缸被布置在两个成一定角度的平面上。这样减小了发动机的长度，使其更紧凑。同时也减小了曲轴的长度，使之刚性增强。

发动机总成由许多构成它本身的零部件和相关系统组成。这些系统是发动机起动和维持运转所必需的。组成发动机的各个零部件可以分成一系列分总成或者相关的组件，通常也叫它们总成，例如缸盖总成、活塞总成。有些零部件在发动机里面，有些在外面，本文的插图展示了一些发动机外面的零部件。

发动机各种各样的零部件通过螺栓、螺母和其它的紧固件装配在一起。有些零部件安装在发动机里面，还有些固定在外面。

机体是发动机中最大的零件。其它零部件都是安装或连接在机体上的。顾名思义，机体是一块金属铸体，通常是铸铁，但也可以是铸铝合金或者铸钢。

机体中精确加工了气缸，可以使活塞在里面运动。机体的下部是曲轴箱，里面有安装曲轴的轴承。在气缸周围的水套里充满了冷却液。

曲轴安装在机体下部的轴承上（曲轴箱里面）。活塞和曲轴之间用连杆连接，发动机运转时曲轴靠活塞的做功行程旋转。活塞的往复运动通过连杆和曲轴上曲柄的共同作用转变为旋转运动。

曲轴这个词源于英文单词“cranked”，它是转动曲柄的意思，包含有弯曲的意思，通俗地讲，曲轴是一根包含若干曲柄的轴。在直列式发动机中，每个气缸对应一个曲柄，但是在某些V型发动机中，一对气缸会共用一个曲柄。

飞轮是一个笨重的盘形铸铁件，它安装在曲轴后端。它通可以使活塞输出的动力脉冲更平滑，从而减少发动机的振动。它在做功行程吸收能量，在其它行程释放能量，这样可以保持发动机平稳地运转。飞轮的边缘装有一圈齿圈，在发动机起动时，起动机上的小齿轮通过齿圈带动飞轮旋转。对于自动变速器，在发动机起动时，是用传动盘和变矩器代替飞轮实现发动机的起动。

活塞和气缸壁之间靠活塞环来密封，活塞环装在活塞上的凹槽中。气环用来密封气体，油环用来阻止多余的机油从活塞上进入燃烧室。

连杆上有一个可以拆掉的连杆盖，其下端通过一个剖分的连杆轴承与曲轴连接。连杆上端通过与活塞销组成转动副而与活塞相连。因为可以转动，所以有时候也称活塞销为肘节销。

气缸盖通常是铝合金铸件，通过螺栓将它固定在缸体顶部并且与气缸相吻合。在气缸的上部是燃烧空气与燃料混合物的燃烧室。气缸盖也可以是铸铁，这样具有更强的抗腐蚀性，但是由于铝合金有较好的热传导性和较轻的重量，因而用于汽油机上。气缸盖上有进气孔和排气孔。在进气行程，进气门打开，燃料从进气孔进入气缸。在排气行程，排气门打开，燃烧后的废气从排气孔排出。

凸轮轴和配气机构使气门在正确的时刻打开和关闭。凸轮轴由来自曲轴的动力驱动，其转速是曲轴的一半。

四气缸发动机的气缸盖包含两根凸轮轴和十六个气门。在另一种四气门发动机上，含有两根凸轮轴和十二个气门。还有一种发动机只有一根凸轮轴和八个气门。

皮带轮通过正时皮带以一半的曲轴转速（曲轴转速即发动机转速）驱动凸轮轴。常常将有齿轮的皮带用在这里，但是齿轮和链条也会用于某些发动机，甚至同时使用皮带和链条。

在某些发动机上，会用正时链条来驱动油泵。在柴油机中，喷油泵和凸轮轴都是用正时链条或正时齿轮来驱动。

油底壳或者机油箱用来盛放发动机润滑系统的机油。油底壳用薄钢板压制成型，有些油底壳用铝合金精密铸造成型。

用螺栓连接的零件贴合表面常常要用到各种各样的垫圈或密封条。有些地方是为了防止泄漏机油，有些地方是为了防止泄漏冷却液，还有些地方是为了绝热和保持压力。有些密封条用于平整表面，还有些用于转轴上。

发动机除了这些大部件，还有许许多多小零件。包括螺栓、垫圈、止动垫圈、弹簧，还有组成发动机部件的支架。

这是汽油机的组成系统。除了燃油系统和点火系统，柴油机与汽油机具有相似的系统。这些系统中，有些部件是和发动机一体的，有些是附加上去的，还有些是安装在发动机舱里的。

在发动机起动时需要用起动机带动。起动机由电动机和驱动器组成。在起动时，驱动器上的小齿轮与飞轮上的齿圈相啮合。蓄电池给起动机供电带动发动机转动，当发动机着火并自行运转时就算起动成功。

燃油系统有四种类型：汽油机的化油器式系统，汽油机的燃油喷射系统，气体燃料系统（液化石油气和天然气），柴油机燃油喷射系统。所有这些系统的工作方式都不同，但都有储存燃料的容器（燃油箱或气罐）和向发动机供燃料的输油管。它们都能将空气和燃料按合适的比例提供给发动机，使之可以在燃烧室里高效地燃烧。

汽油机和气体发动机都需要点火系统，因为需要提供火花才能将燃烧室里的燃料引燃，所以，有时也把汽油机叫做点燃式发动机。这个特性使汽油机与柴油机有明显区别，柴油机不是靠火花点火而是靠压缩点火。

在柴油机中，燃料一喷入燃烧室就立即着火。气缸里的空气由于压缩而产生高温，使从喷嘴里喷出来的燃料达到足够着火的温度。

发动机里的空气燃料混合物燃烧时会产生大量热量。部分热量用来做有用功，部分热量会转移到发动机的零部件上去，还有部分热量会随废气排出。无论怎样，如果不及时带走发动机零部件上的热量，就会对发动机造成损害。冷却系统有带走有害热量的功能，它能带走发动机三分之一的热量。

冷却系统不仅仅是带走热量，它还能将发动机的工作温度维持在合适的范围内。水冷发动机中，由循环在水套中的冷却液来冷却。风冷发动机中，靠吹过散热片的空气来冷却。

发动机润滑系统由机油泵、限压阀、滤清器组成，还有输油管、回油管以及流动在机油里的发动机零件碎屑。油底壳中储存有大量机油，机油泵把机油从油底壳输送到发动机各个地方，然后返回油底壳。用机油润滑发动机所有活动部件，不仅仅是为了减小摩擦力，还可以减少零部件的损耗。机油泵由曲轴直接驱动。该系统在滤清器附近还安装有一个机油冷却器。

在电子控制燃油喷射系统汽油机中，进气系统包含了空气滤清器、节气门总成和进气歧管。进气系统通过进歧管向发动机提洁净空气。燃油喷射器将燃料喷射在进气歧管或进气道内。

排气系统将燃烧后的废气从发动机中排出去，并且还能减小噪声。排气系统由排气歧管、排气管、催化转化器和消声器组成。不同的发动机有不同的配置，有的发动机不只一个消声器和催化转化器。使用含铅汽油的发动机和柴油机都没有催化转化器。

起动机、点火系统和汽油机起动时的电动燃油泵都是使用蓄电池的电能，但是发动机起动以后，就由发电机提供所有的电能了。同时发电机也会向蓄电池充电以补充在起动时消耗的电能。

发动机的电控系统由传感器、控制单元和执行器组成。控制单元接收从传感器来的信号后，就会向不同的执行器发出相应的信号。

发动机的进气系统和排气系统中都有传感器。喷油嘴是喷射燃油的执行器，在点火系统中有提前和推迟点火的执行器。以上仅仅是例举了两个用到电子控制的地方。

在活塞向下移动时，化油器中的空气燃料混合物被吸入气缸。气缸里的混合物在活塞上移时被压缩，并由电火花点燃。混合物燃烧时会产生大量热量，使气体膨胀产生高压，并作用于气缸壁和活塞上。活塞在高压的作用下会向下移动一个行程。

作用于活塞上的压力通过连杆传给曲轴，并使曲轴旋转。活塞向下移动一个行程，曲轴便旋转半周，同时安装在曲轴上的飞轮储存能量。在下一个做功行程开始前，飞轮的转动惯量会带动活塞越过它的运动平衡点。不断地重复这个过程，曲轴便会不断地旋转，发动机也就运转了。

重复以上相同的运动就形成了发动机工作循环。对于不同类型的发动机，一个循环所需的活塞行程数不同。在现代交通工具中，活塞运动四个行程即曲轴转两周才构成一个循环，这就是四冲程发动机。在二冲程发动机中，一个循环是由两个活塞行程即曲轴转一周构成。

在四冲程发动机中，这四个冲程分别是：吸气、压缩、做功和排气。这四个冲程依次出现每个工作循环中。

吸气冲程：在吸气冲程中，活塞由曲轴带动向下运动，曲轴由飞轮或起动机驱动。吸气时，进气门打开，排气门关闭。活塞向下运动时，进气门保持打开，从化油器来的空气燃料混合物被吸入气缸。这里用汽油作为燃料，汽油会被分散为雾状油滴，也有一部分汽油会在化油器中蒸发。

压缩行程：在压缩行程中，活塞向上运动，压缩气缸里的混合物。大部分压缩过程和点火都是发生在这个行程。压缩产生高温，使气缸里的空气和汽油混合更均匀。高温使汽油更易着火，高压使汽油分子和空气分子靠得更近。处于压缩状态的混合物由火花塞产生的电火花点燃。当活塞移动到上止点（TDC）时，燃烧完成一半。在该行程中，进气门和排气门都处于关闭状态。

做功或膨胀行程：燃烧产生的高温使气体膨胀，然后对气缸和活塞产生作用。在膨胀作用下，活塞向下移动做有用功。该行程中，进气门和排气门都处于关闭状态。

排气行程：在排气行程中，进气门关闭，排气门打开。大部分燃烧后的废气在自身的膨胀作用下排出气缸。活塞向上移动将剩余的废气经排气道排出。少部分废气会留在余隙空间，将与进入气缸的新鲜空气混合。

因此，在这类发动机中，一个完整的工作循环由活塞的四个行程组成，并且这四个行程使曲轴转两周。发动机运转时，就是重复着这个循环。