Otto-Cycle Engines

1. The ordinary Otto-cycle engine is a four-stroke engine; that is, in a complete power cycle, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving, the cylinder ready to repeat the cycle.

2. The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent, in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy.

Diesel Engines

3. Theoretically, the diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most diesels are also four-stroke engines but they operate differently than the four-stroke Otto-cycle engines. The first, or suction, stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression, stroke the air is compressed to a small fraction of its former volume and is heated to approximately 440° C (approximately 820° F by this compression. At the end of the compression stroke, vaporized fuel is injected into the combustion chamber and burns instantly because of the high temperature of the air in the chamber. Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third, or power, stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.

4. The efficiency of the diesel engine, which is in general governed by the same factors that control the efficiency of Otto-cycle engines, is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesels are, in general, slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, have speeds up to 2000 rpm. Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built than Otto-cycle engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.

Exercise 2 Answer the following questions.

1. What kind of engine is the Otto-cycle engine? 2. Where is the mixture sucked? 3. When is the fuel mixture ignited? 4. What ignites the fuel mixture? 5. Which valve is opened during the final stroke? 6. What is the efficiency of the diesel engine? 7. Why is a diesel engine considered to be slow-speed? 8. Which engine, diesel or Otto-cycle, is more heavily built? 9. What oil and fuel is used in a diesel engine? 10. What disadvantage of a diesel engine is mentioned in the text?

Exercise 3 Translate and memorize the following expressions from the text.

A four-stroke engine, a complete power cycle, the intake valve, the motion of the piston, a quantity of a fuel and air mixture, expanding and exerting a pressure, the mixture is ignited by, the exhaust gases, to be limited by, increase of efficiency, at constant volume, combustion chamber, to be heated to, burns instantly, auxiliary electrical ignition systems, to control the efficiency, crankshaft speed, compression ratio, can be operated.

Exercise 4 Find synonyms among the following words.

Complete, counterbalance, permanent, achieve, mixture, approximately, in general, typical, efficiency, modern, increase, operate, repeat, power, motion, entire, energy, movement, reach, on the whole, present-day, growth, effectiveness, constant, act, nearly, normal, recur, neutralize, combination.

Exercise 5 Write a summary of the text, presenting the content of each paragraph in 2-3 sentences. Use the expressions:

The main idea of the text is ... The text deals with one of the most important (urgent) issues ... Much attention (consideration) is given to (classification, description)... It focuses on the matters of … The text gives an overview of... The text is mainly concerned with ... The aim of the survey is to show (demonstrate, find)... Particular emphasis is given to the analysis of... The text gives a detailed analysis of (reports on)... To sum up ... In conclusion ...

Индивидуальное задание для студентов заочного отделения к курсу

«Английский язык»

Вариант № 9/3c

Exercise 1 Read the text and translate it in written form.