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9. Read another text devoted to the same issue and find information which differs
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9. Read another text devoted to the same issue and find information which differs
| A typical simple two-stroke engine contains a piston whose face is shaped, an exhaust port on one side of the cylinder, and an intake port on the other side. The downward movement of the piston first uncovers the exhaust port, allowing most of the exhaust gases to exit, and then it uncovers the intake port through which an air-fuel mixture (the fuel normally has some oil mixed in) is pulled into the cylinder. The piston then moves upwards, compressing the mixture which is ignited by a spark plug, driving the piston back down. |
10. Read about advantages and disadvantages of two-stroke engines. What type of engine (two- or four-stroke one) would you choose for a) your car, b) your scooter, c) truck? Explain your reasons
| advantages | disadvantages |
| It has no valves, so its mechanism is simple in construction | The lack of lubrication system that protects the engine parts from wear (изнашивание), as a result, the 2-stroke engines have shorter life. |
| For one complete revolution of the crankshaft, the engine accomplishes one cycle, while the 4-stroke engine accomplishes 1 cycle per 2 crankshaft revolutions. | They do not consume fuel efficiently |
| Less weight and easier to manufacture | They produce lots of pollution |
| High power-to-weight ratio | Sometimes part of the fuel leaks (протекать) to the exhaust with the exhaust gases. |
11. Look through the text and find information on the function of the piston in the two-stroke engine
Two stroke engines
Two-stroke engines are also used for propulsion of ships. These, if single-acting, provide one power-stroke per revolution, while a double-acting 2-stroke engine develops power one every stroke.
A two-stroke engine develops almost twice as much power as a four-stroke engine with the same size and number of cylinders. Since the four operations, i. e. suction, compression, firing and exhaust have to be completed during two strokes of the piston, more than one operation must be performed per stroke. This complicates the engine. The piston is made to control the admission of air and release of the exhaust gases by opening and closing ports or passages in the cylinder walls through which the air and gases pass. The fresh air-charge is pumped into the cylinder at low pressure by means of a scavenging pump which may be driven either by the engine itself or by a separate auxiliary engine or electric motor. This air not only provides the air charge necessary for the proper combustion of the fuel but assists in clearing the burnt gases rapidly out of the cylinder.
In a two-stroke cycle compression occurs on the first or upstroke; combustion and expansion occur during the down-stroke; exhaust, scavenging and recharging with air occur during the latter part of the down-stroke and the beginning of the upstroke. This sequence of events is made possible by substituting ports in the bottom of the cylinder wall for one or more exhaust valves. There are two groups of these ports, one for exhaust and the other for scavenging air, usually on opposite sides of the cylinder, but on some designs both groups are arranged on the same side. The exhaust ports connect with the exhaust manifold, while the scavenging ports communicate with the scavenging air receiver in which low-pressure air is stored.
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A two-cycle engine must be provided with the scavenging compressor for supplying scavenging air.
12. Read the text. Three parts of the sentences have been removed from it. Choose from sentences A-D the one which fits each gap (103). There one extra phrase which you do not need to use
Two-stroke Basics
This is what a two-stroke engine looks like:
You find two-stroke engines in such devices as chain saws (бензопила) and jet skis (гидроикл) because two-stroke engines have three important advantages over four-stroke engines:
· Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.
· Two-stroke engines fire once every revolution, 1___________________. This gives two-stroke engines a significant power boost.
· Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem 2_____________________. 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 combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio 3_____________________.
You don't normally see two-stroke engines in cars, however. That's because two-stroke engines have a couple of significant disadvantages that will make more sense once we look at how it operates.
A. compared to many four-stroke engine designs.
B. can add complexity to the engine.
C. usually on opposite sides of the cylinder.
D. while four-stroke engines fire once every other revolution.
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