发动机工作原理

大多数汽车的发动机是内燃机，往复四冲程汽油机，但是也有使用其它类型的发动机，包括柴油机，转子发动机，二冲程发动机和分程燃烧发动机。

往复的意思就是上下运动或前后运动，在往复发动机中，气缸中活塞的上下运动产生发动机的动力，这种类型几乎所有的发动机都是依赖气缸体即发动机缸体，缸体是铸铁或铸铝制的，它包括发动机气缸和冷却液循环用的水套。缸体的顶部是气缸盖，它组成了燃烧室，缸体底部是油底壳。

气缸内活塞的直线运动产生动力，然而，必须将直线运动转化成旋转运动，使汽车车轮转动，活塞销将活塞连接在连杆顶部，连杆底部与曲轴连接，使汽车车轮转动，活塞销将活塞连杆顶部，连杆底剖与曲轴连接，连杆将活塞的往复运动传递给曲轴，曲轴将其转化为旋转运动，连杆是用连杆曲轴安装在曲轴上的，用类似的轴承即主轴承将曲轴固定在缸体内。

气缸的直径称为发动机的内径，排量和压缩比是两个常用的发动机参数，排量是指发动机的大小，压缩比是气缸总容积与燃烧室压缩容积之比。

术语: 冲程是用来说明活塞在气缸内的运动，也就是活塞行程的距离根据发动机类型的需要二冲程或四冲程来完成一个工作循环四冲程发动机也叫做奥托发动机，为了纪念德国工程师奥托，他是在1876年第一个应用该原理的，在四冲程发动机中，要求气缸活塞四冲程来完成一个完整的工作循环，每个冲程根据其行为命名分别为: 进气冲程，压缩冲程，作功冲程和排气冲程。

1、        进气冲程

当活塞下移时，雾化后的可燃混合气通过打开的进气门进入气缸，为了达到最大的进气量，进气门在活塞到达上止点前10°打开，使进、排气门有20°打开重叠角，进气门一直打开到活塞到达下止点充分进入混合气之后50°左右。

2、        压缩冲程

活塞开始向上移动时，进气门关闭，混合气在燃烧室中压缩，根据不同因素包括压缩比，节气门开度，发动机转速压力上升到约1兆帕，接近冲程顶部时，火花塞产生的电火花击穿点火间隙点燃可燃混合气。

3、        作功冲程

燃烧膨胀的气体产生的压力上升到3.5个兆帕时，推动活塞下移，接近气缸底时，排气门打开。

4、        排气冲程

随着排气门开启约下止点前50°，活塞回升，使气缸内压力下降在排气冲程，减少对活塞回压，派出废气，为下一个进气冲程作准备，通常情况下，进气门在排气冲程完成前打开。

只要发动机保持运转，每个气缸内四个冲程循环连续不断地重复下去。

两冲程发动机也同样通过四行程来完成，一个工作循环但是进气冲程，压缩冲程合为一个冲程，作功冲程形成另一个冲程，术语两行程循环和两行程就是所谓的术语双循环但实际上并不太准确。

在所用的汽车发动机中，所有的活塞都是固定在一个曲轴上的，气缸中发动机越多，每转为发动机的作功冲程产生越多的动力，这就意味着八缸发动机运转的越平顺，因为发动机在作功冲程中运转时间和旋转角度紧密。

多气缸发动机有三种排列形式，任其一种

1、        直列式发动机用一个气缸体，大多数四缸发动机和一些六缸发动机都采用这种型式，这种气缸不必垂直分布，它们可以向任一方向倾斜。

2、        V-型发动机用两排同样的气缸，通常夹角为60°或90°，大多数有六缸或八缸，尽管四缸和十二缸也有采用V型的。

3、        卧式或者对置式发动机有两排互为180°的气缸，这些节省窨发动机设计通常采用风冷式，在雪弗兰、富士、大众车采用这种型式，富士汽车采用水冷式，在顶置式风冷发动机中，大众的新型厢式汽车采用水冷式。

 

Engine Operating Principles

Most automobile dngines are internal combustion, reciprocating 4-stroke gasoline engines, but other types have been used, including the diesel, the rotary ( Wankel ) , the 2-srtoke, and stratified charge.

Reciprocating means up and down or banck and forth, It is the up and down action of a piston in the cylinder blick, or engine block. The blick is an iron or aluminum casting that contains engine cylinders and passges called water jackets for coolant circulation. The top of the block is covered with the cylinder head. Which forms the combustion chanber. The bottom of the block is covered with an oil pan or oil sump.

Power is produced by the linear motion of a piston in a cylinder. However, this linear motion must be changed into rotary motion to turn the wheels of cars of trucks. The piston is attached to the top of a connecting rod by a pin, called a piston pin or wrist pin. The bottom of the connecting rod is attached to the crankshaft. The connecting rod transmits the up-and-down motion of the piston to the crankshaft, which changes it into rotary motion.

The connecting rod is mounted on the crankshaft with large beaings called rod bearings. Similar bearings, called main bearings, are used to mount the crankshaft in the block. Shown in Fig. 1-1

The diameter of the cylinder is called the engine bore. Displacement and compression ratio are two frequently used engine specifications. Displacement indicates engine size, and compression ratio compares the total cylinder volume to compression chamber volume.

The term stroke is used to describe the movement of the iston within the cylinder, as well as the distance of piston travel. Depending on the type of engine the operating cycle may require either two or four strokes to complete. The 4-stroke engine is also called Otto cycle engine, in honor of the German engineer, Dr. Nikolaus Otto, who first applied the principle in 1876. In the 4-stroke engine, four strokes of the piston in the cylinder are required to complete one full operating cycle. Each stroke is named after the action it performs intake, compression, power, and exhaust in that order, shown in Fig1-2.

1、        Intake stroke

As the piston moves down, the vaporized mixture of fuel and air enters the cylinder through open intake valve. To obtain the maximum filling of the cylinder the intake valve opens about 10°before t.b.c., giving 20°overlap. The inlet valve remains open until some 50°after b.d.c. to take advantage of incoming mixture.

2、        Compression stroke

The piston turns up, the intake valve closes, the mixture is compressed within the combustion chamber, while the pressure rise to about 1Mpa, depending on various factors including the compression ratio, throttle opening and engine speed. Near the top of the stroke the mixture is ignited by a spark which bridges the gap of the spark plug.

3、        Power stroke

The expanding gases of combustion produces a rise in pressure of the gas to some 3.5Mpa, and the piston is forced down in the cylinder. The exhaust valve opens near the bottom of the stroke.

4、        Exhust stroke

The piston moves back up with the exhaust valve open some 50°before b.d.d., allowing the pressure within the cylinder to fall and to reduce ‘back’pressure on the piston during the exhaust stroke, and the burned gases are pushed out to prepare for the next intake stroke.The intake valve usually opens just before the exhaust stroke.

This 4-stroke cycle is continuously repeared in every as long as the engineremains running.

A 2-stroke engine also goes through four actions to complete one operating cycle.

However, the intake and the compression actions are combined in one seroke, and the power and exhaust actions are combined in the other stroke. The term2-stroke cycle or 2-stroke is preferred to the term 2-cycle, which is really not accurate.

In automobile engines, all pistons are attached to a single crankshaft. The more cylinders an engine has, the more power strokes produced for cach revolution. This means that an 8-cylinder engine runs more smoothly bdcause the power atrokes are closer together in time and in degrees of engine rotation.

The cylinders of multi-cylinder automotive engines arranged in one of three ways.

1、        Inline engines use a single block of cylinder.Most 4-cylinder and any 6-cylinder engines are of this design. The cylinders do not have to be vertical. They can be inclined either side.

2、        V-type engines use two equal bands of cylinders, usually inclined 60degrees or 90degrees from the cach other. Most V-type engines have 6 or 8 cylinders, although V-4 and V-12 engines have been built.

3、        Horizontally opposed or pancake engines have two equal banks of cylinders 180degreeas apart. These space saving engine designs are often air-cooled, and are found in the Chevrolet Carvair, Porsches, Subaus, and Volkswagens. Subaus design is liquid cooled.

Late-model Volkswagen vans use a liquid-cooled version of the air cooled VWhorizontally opposed engine.