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1、Unit 23 Car EnginesTuning in Task 1 Discuss these questions in your group. (1) What is the purpose of a car engine? (2) How does a car engine work? Reading Reading diagram Task2 Study Figure 23-1 and see if you understand the principle on which internal combustion engines work. Then read the text an
2、d check your answers to Task 1. Text Internal Combustion Engines Why does a car need an engine? The purpose of it is to convert gasoline into motion.Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Thats why we say a car engine is an internal combust
3、ion enginecombustion takes place internally. Putting a drop of gasoline in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a potato 500 feet. In this case, the energy is translated into potato motion. Y
4、ou can also use it for more useful purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine! Almost all cars currently use what is calle
5、d a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 23-1. They are: 1. Intake stroke 2. Comprssion stroke 3. Combustion stroke 4. Exh
6、aust strokeFigure 23-1 Four strokes of a car engine A-Intake vale, rocker arm & spring B-Valve cover C-Intake port D-Head E-Coolant F-Engine Block G-Oil pan H-Oil sump I-Camshaft J-Exhaust vavle, rocker arm & spring K-Spark plug L-Exhaust port M-Piston N-Connecting rod O- Rod bearing P- Crankshaft H
7、eres what happens as the engine goes through its cycle: 1. The piston(M) starts at the top. At this time the intake port(C) opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed i
8、nto the air for this to work. (Figure a of Figure 23-1) 2. Then the piston moves back up to compress the gasoline and sir mixture, which makes the explosion more powerful. (Figure b of Figure 23-1)3. When the piston reaches the top of its stroke, the spark plug(K) emits a spark to ignite the gasolin
9、e. The gasoline charge in the cylinder explodes, driving the piston down. (Figur c of Figure 23-1) 4. Once the piston hits the bottom of its stroke, the exhaust vavle opens and the exhaust leaves the cylinder through the exhaust port (L) to get out of the tailpipe. (Figure d of Figure 23-1) Now the
10、engine is ready for the next cycle, so it intakes another charge of air and gasoline. Language study Cause and effects Study these pairs of action. What is the link between each pair? (1) The gas expands. (2) This drives the piston down (3) The piston ascends. (4) This forces the products of combust
11、ion out. There are two links between the actions: A. They happen at the same time. We can show this using As (see Unit 19): (1)+(2) As the gas expands, it drives the piston down. (3)+(4) As the piston ascends, it forces the products of combustion out. B. They are causes and effects: (1) Cause: The g
12、as expands. (2) Effect: This drives the piston down. (3) Cause: The piston ascends. (4) Effect: This forces the products of combustion out. Now were introducing a way to show both the time link and the cause and effect link like this: (1)+(2) The gas expands, driving the piston down. (3)+(4) The pis
13、ton ascends, forcing the products of combustion out. Can you link them in the way we learned in Unit 22? Can you find the differences between the pairs of cause and effect in the two units? Task 3 Try to link these actions.CauseEffect(1) The piston moves down the cylinder.This creates a partial vacu
14、um.(2) The piston creates a vacuum.This draws in fuel through the intake port.(3) The piston moves up the cylinderThis compresses the mixture.(4) The gas expands quickly.This pushes the piston down.(5) The piston moves up and down.This ratates the crankshaft.(6) The crankshaft spins round.This turns
15、 the wheels.(1) The piston moves down the cylinder, creating a partial vacuum. (2) The piston creates a vacuum, drawing in fuel through the intake port. (3) The piston moves up the cylinder, compressing the mixture. (4) The gas expands quickly, pushing the piston down. (5) The piston moves up and down, rotating the crankshaft. (6) The crankshaft spins round, turning the wheels. Speaking practice Task 4 Look at Figure 23-1 and try to describe the four strokes of a car engine. Each group selects one or two representatives to do the presentation before the class. Which group is the best?
