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2. Read the text and divide it into paragraphs according to the plan
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2. Read the text and divide it into paragraphs according to the plan
1. The purpose of an engine
2. Engines burning fuel inside them
3. Engines burning fuel outside them
4. Advantages of internal combustion over external combustion
5. Operation of two-stroke engines
6. Advantages and disadvantages of two-stroke engines
Almost all cars currently use what is called 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 purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine – combustion takes place internally. Two things to note: There are different kinds of internal combustion engines. Diesel engines are one form and gas turbine engines are another. Each has its own advantages and disadvantages. There is such a thing as an external combustion engine. A steam engine in old-fashioned trains and steam boats is the best example of an external combustion engine. The fuel (coal, wood, oil) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine. Internal combustion is a lot more efficient (takes less fuel per mile) than external combustion, plus an internal combustion engine is a lot smaller than an equivalent external combustion engine. This explains why we don’t see any cars from Ford and GM using steam engines. A two-stroke engine has no moving valves, and the spark plug fires each time the piston hits the top of its cycle. A hole in the lower part of the cylinder wall lets in gas and air. As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another hole in the cylinder. You have to mix oil into the gas in a two-stroke engine because the holes in the cylinder wall prevent the use of rings to seal the combustion chamber. Generally, a two-stroke engine produces a lot of power for its size because there are twice as many combustion cycles occurring per rotation. However, a two-stroke engine uses more gasoline and burns lots of oil, so it is far more polluting.
3. Before reading the text look at the words in bold and point out in which sentences the author 1) expresses contrast; 2) explains the reason; 3) shows the effect
Read the text, look at two plans and choose the one which corresponds to the content of the text
A B
| 1. Two ways of sending air into cylinder. 2. How the fuel is ignited. 3. Why part of fuel energy is lost. 4. Why diesels are more efficient 5. When Otto cycle engines are more efficient than diesels | 1. Operation of diesels 2. Advantages and disadvantages of diesels. 3. How fuel is burnt 4. Ways of increasing efficiency of diesels 5. Engines running at maximum power |
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Diesel Cycle
A diesel cycle engine draws air by itself, or with the air of a supercharger, into the engine and compresses it. When the piston reaches approximately top dead center, the fuel is injected directly into the cylinder with a high-pressure fuel injector. The air is extremely hot at this point, and so the fuel ignites immediately, however, diesel fuel has a higher molecular weight than gasoline, and so it vaporizes and burns more slowly. The piston is already moving down by the time the combustion starts, and combustion is usually not 100% finished when the piston reaches bottom dead center. Because of this incomplete combustion, diesel engines actually lose some of the potential energy of the fuel.
Diesel cycle engines are nevertheless more efficient than Otto cycle engines overall, but only when power needs to be scaled. Most land vehicle almost never use the maximum rated power of the engine. Unless the vehicle is at Wide Open Throttle (when the pedal is “floored”) the engine is only producing a fraction of its rated power. Since diesel engines use the heating effect of compressing the air to ignite the fuel, a diesel engine can inject as little or as much fuel as the situation demands. It is important to note that Otto cycle engines can be more efficient than Diesel cycle engines, but only when the engine is running at or near its maximum power. This is dependent on the (Otto cycle) engine’s compression ratio.
4. Read the text and 9 statements. Cross out the statements that do not correspond to the text content
Otto Cycle
In the Otto cycle the fuel and air are pre-mixed before being drawn into the cylinder. The mixture is then compressed and ignited at the right moment with a spark plug. The fuel-air charge is ignited just before the piston reaches top dead center. Gasoline burns more quickly than diesel fuel, and since the combustion cycle starts earlier (15 degrees before top dead center is common), the entire combustion process is usually complete by the time the piston reaches bottom dead center. It is critical for this type of engine to have a knock resistant fuel.
Otto cycle engines need a minimum amount of fuel to resist pre-combustion in order to run properly. When fuel is injected into this type of engine it has a cooling effect on the inside of the cylinder. The more fuel injected, the more it cools the cylinder, so that the fuel-air charge does not heat up too much. If too little fuel is injected along with the air, and then compressed, the fuel-air charge will overheat and pre-combust, making the engine knock. This is why gasoline engines have a minimum amount of fuel that must be injected. Efficiency decreases as a result, but only when the engine is not at wide open throttle.
1. Fuel and air are mixed before the charge enters the cylinder.
2. Fuel and air are mixed in a carburetor.
3. A spark plug is used to ignite the fuel-air mixture.
4. Combustion is completed before the piston reaches bottom dead center.
5. The fuel for Otto cycle engines must have one important property.
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6. There are many ways of avoiding pre-combustion.
7. When fuel charge is injected, it has a cooling effect on the cylinder.
8. Injection of too little fuel results in pre-combustion.
9. Efficiency of the engine does not depend on the amount of injected fuel.
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