Chapter 2 Engine
Unit 1 Four-stage Engine Operation
There are various types of engines such as electric motors, stream engines and internal combustion engines. However, the internal combustion engine seems to be the most commonly used in the automotive field. According to the fuel energy used, internal combustion engines are further divided into gasoline engines and diesel engines.
The internal combustion engine, as its name indicates, burns fuel within the cylinders and converts the expanding force of the combustion into rotary force used to propel the vehicle. The actions taking place in the engine cylinder can be classified into four stages, or stroke. The stroke refers to piston movementa 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 the piston movement is called BDC bottom dead center. A stroke is the 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, as shown in Figure 2-1.
Figure 2-1 Four-stroke Engine
Almost all cars currently use the so-called four-stroke cycle engine to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nicolaus Otto, who invented it in 1867. The four strokes are intake stroke, compression stroke, power stroke and exhaust stroke, as shown in Figure 2-2.
Figure 2-2 The Operation of a Four-stroke Cycle Engine
Intake stroke. The piston is connected to the crankshaft by a connecting rod. On the intake stroke, the piston moves down from the TDC to the BDC as the crankshaft revolves. This downward movement of the piston creates a vacuum, a difference in pressure in the space above the piston. The intake valve opens automatically as or slightly before the piston starts down, therefore, the air fuel mixture pushed by the atmospheric pressure outside the engine, rushes through the intake manifold and into the engine cylinder. At the same time, the exhaust valve remains closed and prevents the entering air fuel charge from escaping through the exhaust 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 the BDC, the piston moves back up to compress this combustible mixture within the combustion chamber and the intake valve and the exhaust valve are all closed. Since both valves are closed, the piston compresses the air fuel mixture in the small space between the top of the piston and the cylinder head. When the mixture is compressed, not only does the pressure in the cylinder go up, but also the temperature increases. As the piston reaches the TDC again during its upward travel, the compression stroke of the piston is over. The air fuel charge is now under compression so that it will produce a great deal of power when the spark plug ignites it.
Power stroke. Just as or slightly before the piston reaches the TDC on the compression stroke with the air fuel mixture fully compressed, a timed electrical spark appears at the spark plug. This spark ignites the compressed air fuel mixture. The burning mixture begins to expand, and the cylinder pressure increase to as much as 3~5MPa or even more. This tremendous force pushed the piston downward on the power stroke, and a power impulse 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. In other words, the force resulting from the expansion of the burning air fuel mixture is turning the crankshaft.
Exhaust stroke. The exhaust valve opens, near the end of the downward movement of the piston on the power stroke, although much of the gas pressure has expended itself driving the piston downward, some pressure still remains when the exhaust valve opens. This remaining pressurized gas flows comparatively freely from the cylinder through the passage port opened by the exhaust valve. Then, as the piston again moves up in the cylinder, it drives any remaining gases out of the cylinder past the open exhaust valve. In other words, while the exhaust valve is open, the upward movement of the piston provides an effective method for discharging all waste gases from the engine cylinder and combustion chamber. As the crankshaft nears the end of its second complete revolution, the piston again approaches the TDC position. At this point the exhaust valve is closing and the intake valve starts to open. Both valves are open together for a short period of time in order to accelerate the fresh charge to flow into the cylinder.
As the piston travels through the TDC position and starts downward again in the cylinder, a new operating cycle begins. This fourstroke cycle of piston within the cylinder is repeated time and again to put the vehicle forward.
Words and Expressions
1. various [''v??ri?s] adj. 各种各样的
2. diesel [''di：z?l] n. 柴油机；内燃机
3. propel [pr?''pel] v. 推进；驱使
4. stage [steid?] n. 阶段；舞台；驿站 vt. 上演；实行，进行
5. approach [?''pr?ut?] n. 接近；途径；方法 v. 靠近，接近
6. crankshaft [''kr?k?ɑ：ft] n. 曲轴
7. vacuum [''vkju?m] n. 真空；空间；真空吸尘器 adj. 真空的
8. atmospheric [?tm?s''ferik] adj. 大气的, 大气层的
9. manifold [''mnif?uld] n. 多种；歧管 adj. 多种的，多方面的 v. 繁殖，增多
10. vaporize [''veip?raiz] v. 使蒸发
11. combustible [k?m''b?st?bl] adj. 易燃的，燃烧性的 n. 燃质物，可燃物
12. ignite [ig''nait] vi. 着火，发光 vt. 点燃，使燃烧，引发
13. slightly [''slaitly ] adv. 轻微地；微小地；稍微地；纤细地
14. transmit [trnz''mit] vt. 传输，传送；代代相传；传达
15. tremendous [tri''mend?s] adj. 巨大的；可怕的；非常的
16. comparatively [k?m''pr?tivli] adv. 比较地；相对地
17. gasoline engine 汽油机
18. diesel engine 柴油机
19. top dead center TDC 上止点；上死点
20. bottom dead center BDC 下止点；下死点
21. connecting rod 连杆
22. spark plug 火花塞
23. inlet intake valve 进气阀
24. exhaust valve 排气阀
25. swept volume 有效容积
Notes
1. The internal combustion engine, as its name indicates, burns fuel within the cylinders and converts the expanding force of the combustion into rotary force used to propel the vehicle.
内燃发动机，正如其名字所示，在汽缸内燃烧燃料并转换燃烧的膨胀力为回转力来推动车辆运动。
2. The intake valve opens automatically at or slightly before the piston starts down, therefore, the air fuel mixture pushed by the atmospheric pressure outside the engine, rushes through the intake manifold and into the engine cylinder.
进气门自动打开或者稍微提前于活塞开始向下运动时打开，因此，空燃混合气在发动机外部大气压力的推动下，急速通过进气歧管并进入发动机汽缸。
3. When the mixture is compressed, not only does the pressure in the cylinder go up, but also the temperature increases.
当混合物被压缩时，不仅汽缸里的压力会上升，而且温度也会上升。
4. This tremendous force pushed the piston downward on the power stroke, and a power impulse transmitted through the connecting rod to the crankpin on the crankshaft.
在做功行程中，这样巨大推力作用于活塞使它向下运动，并且脉冲推力通过连杆传到曲轴轴颈上。
5. This remaining pressurized gas flows comparatively freely from the cylinder through the passage port opened by the exhaust valve.
气体的余压使废气能相对自由地从汽缸通过排气门的排气口。
Exercises
A. Vocabulary
I. Translate the following expressions into Chinese.
1. power stoke
2. clearance volume
3. compression ratio
4. revolution
5. intake stroke
6. compression stroke
7. exhaust stroke
8. engine block
9. bore
10. engine capacity
II. Identify the English names of the four-stroke engine according to the picture.
1． 2．
3． 4．
5． 6．
7． 8．
9． 10．
B. Comprehension
I. Discuss the following questions in groups and write down your answers.
1. Would you please tell different types of engines?
2. What does internal combustion engine mean?
3. How does the engine produce power for the automobile?
4. What is the function of each stroke?
II. Read the following passage carefully and fill in the blanks with the proper forms of the given words.
expand appear rotate reach transmit
compress tremendous crankshaft push increase
Just as or slightly before the piston TDC on the compression stroke with the air fuel mixture fully , a timed electrical spark at the spark plug. This spark ignites the compressed air fuel mixture. The burning mixture begins to , and the cylinder pressure
to as much as 3~5MPa or even more. This force pushed the piston downward on the power stroke, and a power impulse through the connecting rod to the crankpin on the . The crankshaft is as the piston is down by the pressure above it.
C. Translation
I. Translate the following sentences into Chinese.
1. The mixture of air and vaporized gasoline is delivered to the cylinder by the fuel system and carburetor.
2. The air fuel charge is now under compression so that it will produce a great deal of power when the spark plug ignites it.
3. Both valves are open together for a short period of time in order to accelerate the fresh charge to flow into the cylinder.
II. Translate the following sentences into English.
1. 冲程，是指活塞运动，一个冲程发生时，活塞从一个极限位置运动到另外一个极限位置。
2. 活塞每完成一个冲程，活塞将改变其运动方向一次。
3. 与此同时，排气门仍然关闭，阻止进入的可燃混合气通过排气口逸出。
Reading Material
Read the following passages and answer the questions according to the information given in the passages.
Passage One Two-stroke Engine
We are familiar with two types of engines found in nearly every car and truck on the road today. They are gasoline and diesel engines. Both are classified as four-stroke reciprocating internal combustion engines and two-stroke engines. The two-stroke engines are commonly found in lower power applications such as jet skis. The two-stroke engines have three important advantages over four-stroke engines as follows：
* Two-stroke engines do not have valve, which simplifies their construction and lowers their weight.
* Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a signification power boost.
* Two-stroke engines can work in any orientation. A standard four-stroke engine may have problem with oil flow unless it is upright, and solving this problem can add complexity to the engine.
These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combustion of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs.
Start with the point where the spark plugs fire. Fuel and air in the cylinder have been compressed when the spark plugs fire the mixture. The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder. As the piston finally bottoms out, the intake port is uncovered. The pistons movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel. Note that in many two-stroke engines which use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesnt simply flow right over the top of the piston and out the exhaust port. Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air-fuel mixture from the carburetor.
Once the piston moves up to the end of the compression stroke, the spark plug fires again to repeat the cycle. Its called a two-stroke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust strokes. You can see that the piston is really doing the following different things in a two-stroke engine：
1. On one side of the piston is the combustion chamber, where the piston is compressing the air fuel mixture and capturing the energy released by the ignition of the fuel.
2. On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air fuel from the carburetor through the reed valve and then pressurize the crankcase so that air fuel is forced into the combustion chamber.
3. Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the sides of the cylinder wall.
You can now see that two-stroke engines have two important advantages over four-stroke engines： they are simpler and lighter, and they produce about twice as much power as four-stroke engines. But why do cars and trucks use four-stroke engines? There are four main reasons as follows：
1. Two-stroke engines dont last nearly so long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear a lot faster.
2. Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You would burn about a gallon of oil every 1,000 miles if you use a two-stroke engine in a car.
3. Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon.
4. Two-stroke engines produce a lot of pollution. The pollution comes from two sources. The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke. The second is that each time a new charge of air fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. Thats why you see a sheet of oil around any two-stroke boat motor. The combustion of the leaking hydrocarbons from the fresh fuel and the leaking oil is real mess for the environment.
These disadvantages mean that two-stroke engines are used only in applications where the motor is not used very often and a fantastic power-to-weight ratio is important.
Questions
1. What are the shortcomings of two-stroke engines?
2. Please describe different things that a piston can do in a two-stroke engine.
3. Why do cars and trucks use four-stroke engines?
Passage Two From Carburetors to Fuel Injection
Why did the car makers change from carburetors to fuel injection? The standard reply to this question is that fuel injection provides a better way to meet government fuel economy and emission standards, which is true. But equally important is the fact that fuel injection is an all-round better fuel delivery system.
Fuel injection has no choke, but sprays atomized fuel directly into the engine. This eliminates most of the cold start problems associated with carburetors. Electronic fuel injection also integrates more easily with computerized engine control systems because the injectors are more easily controlled than a mechanical carburetor with electronic add-ons. Multiport fuel injection where each cylinder has its own injector delivers a more evenly distributed mixture of air and fuel to each of the engines cylinders, which improves power and performance. Sequential fuel injection where the firing of each individual injector is controlled separately by the computer and timed to the engines firing sequence improves power and reduces emissions. So there are some valid engineering reasons as well for using fuel injection.
Types of Fuel Injection
The earliest fuel injection systems were mechanical and were more complex than carburetors. Consequently, they were expensive and their use was limited. Chevrolet introduced a Rochester mechanical fuel injection system back in 1957, and it became the hot setup on Corvettes up through 1967.
The Europeans, however, were the real leaders in fuel injection technology. Bosch offered an early electronic system on Volkswagen Squarebacks in the late 1960s and early 1970s. By the early 1980s, almost all of the European auto makers were using some type of Bosch multiport fuel injection system.
In the mid-1980s, the domestic auto makers first turned to throttle body injection as a stop-gap system as they made the transition from electromechanical carburetors to fuel injection.
Throttle Body Injection TBI
Throttle body injection is much like a carburetor except that theres no fuel bowl, float, needle valve, venturi, fuel jets, accelerator pump or choke. Thats because throttle body injection does not depend on engine vacuum or venturi vacuum for fuel metering. Fuel is sprayed directly into the intake manifold instead of being siphoned in by intake vacuum.
A TBI fuel delivery system consists of a throttle body with one or two injectors and a pressure regulator. Fuel pressure is provided by an electric pump. Its a relatively simple setup and causes few problemsbut doesnt provide all of the advantages of a multiport or sequential fuel injection system.
Multiport Injection
The next step up from TBI was multiport injection. Engines with multiport injection have a separate fuel injector for each cylinder, mounted in the intake manifold or head just above the intake port. Thus, a four cylinder engine would have four injectors, a V6 would have six injectors and a V8 would have eight injectors.
Multiport injection systems are more expensive because of the added number of injectors. But having a separate injector for each cylinder makes a big difference in performance. The same engine with multiport injection will typically produce 10~40 more horsepower than one with TBI because of better cylinder-to-cylinder fuel distribution. Injecting fuel directly into the intake ports also eliminates the need to preheat the intake manifold since only air flows through the manifold. This, in turn, provides more freedom for tuning the intake plumbing to produce maximum torque. It also eliminates the need to preheat the incoming air by forcing it to pass through a stove around the exhaust manifold.
There are other differences between multiport injection systems. One is the way in which the injectors are pulsed. On some systems, all the injectors are wired together and pulse simultaneously once every revolution of the crankshaft. On the others, the injectors are wired separately and are pulsed sequentially one after the other in their respective firing order. The latter approach is more complicated and requires more expensive electronic controls, but provides better performance and throttle response by allowing more rapid changes in the fuel mixture.
Questions
1. Why did the car makers change from carburetors to fuel injection?
2. What fuel injection engine types are there in the world now ?
3. What are the differences between two-stroke engines and four-stroke engines?
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