THE FOUR-STROKE CYCLE

To complete the firing cycle the piston must move along the cylinder four times. These movements called strokes.

The suction (drawing in, intake) stroke.

In this stroke the piston is moving down the cylinder. The inlet valve is open. A mixture of air and petrol is drawn into the cylinder above the piston.

The compression (pressing together) stroke.

In this stroke the piston is moving up the cylinder. Both valves are closed. The piston moves up as far as it can. It pushes the mixture of air and petrol in front of it. It compresses it into the space above the cylinder.

The explosion (firing, power) stroke.

In this stroke, the spark fires the mixture and it explodes. The energy released by the burning mixture forces the piston down the cylinder.

The exhaust (emptying out) stroke.

In this stroke the piston is moving up the cylinder. The exhaust valve is open. Smoke and gases remain after the explosion. The piston pushes them in front of it. They are pushed through the exhaust valve opening. They leave the cylinder through the pipe. Now the cycle begins again.

a) Complete the sentences with the active form of the verb in brackets:

1. The company CAV ……… (never/to manufacture) 5 million engines a year.

2. She ……… (to be) to London four times. We ……… (to go) there by plain.

3. Our company ……… (to employ) over 1000,000, ……… (to operate) in many overseas markets and ……… (to offer) a wide range of hi-tech products for the 21^st century.

4. I’m afraid Mr Bansall can’t see you right now. He’s ……… (to interview) someone.

5. A: Is John feeling OK? He ……… (to look) very red in the face.

6. B: Yes, I know. I ……… (to look for) the doctor’s phone number now.

7. A: ……… (you/to have) a car?

8. B: Yes, but I …..... (to have) some problems with it, so it …..(to be) at the garage now.

Text 9

THE CARBURETOR

The carburetor has two main parts. There is a container (the chamber) which controls the flow of petrol. The pump forces the petrol through a pipe into the chamber. If the chamber is full, the float rises. This causes the needle valve to close. Petrol cannot enter. As the chamber empties, the float sinks. The valve opens. More petrol enters the chamber. There is also a choke tube. This tube is open at the top so that air can be sucked in. Pipes lead from the bottom of the tube to the inlet valves of the cylinders. The air and petrol mixture is sucked into the cylinder. This flow of mixture is controlled by a throttle valve. This is a round piece of metal like a large coin. It is moved by the accelerator pedal.

A small pipe leads from the chamber to the choke tube. The petrol flows through this. There are very small holes (jets) at the end of the pipe. As the induction stroke sucks air down the tube, it also sucks petrol through these jets. The petrol is broken up into thousands of very small drops to form a vapor. It mixes with the air. The mixture is sucked into the cylinders.

When the accelerator pedal is pushed down, the throttle is opened. More air is sucked into the carburetor. More petrol and air mixture goes to the cylinders. The explosions in the cylinders are stronger. The car goes more quickly. When the pedal is let out, the throttle closes. There is less mixture. The explosions are weaker. The car goes more slowly.

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