1. Look through the following statements and decide which of them requires correction:

a) In early diesel engines the fuel was introduced into the cylinder and atomized with the aid of compressed air.

b) In airless injection engines special compressors are used to supply and atomize the fuel.

c) In a four-stroke engine the air comes into the cylinder through the intake valve during the piston motion upward.

d) During the second stroke the air in the cylinder is compressed and as a result the temperature of the air increases to above 500 °C.

e) The compression volume of the cylinder is known as the volume that corresponds to the position of the piston at the bottom dead centre.

f) The process of combustion of all the fuel is close to a constant-volume process.

g) The stroke during which combustion and expansion occur is called the power or working stroke.

h) The entire working cycle of a four-stroke engine is completed during four strokes of the piston and four revolutions of the crankshaft.

i) To make the engine rotate it must first be started by some outside source of power.

2. Give definitions or explanations of the following words and phrases:

an air injection valve; an airless injection engine; a compression volume; a suction stroke; a power stroke; an intake valve.

3. Scan the text Design and Operating Principles of Four-Stroke Diesel Engines again to answer the questions given below.

a) What does the name 'compression-ignition engine' imply?

b) Where does the difference between air and airless injection engines lie?

c) How does the process of burning fuel differ in air injection and airless injection engines?

d) Why is one of the compression-ignition engines called 'a four-stroke engine'?

e) What is a stroke?

f) When is the air introduced into the cylinder?

g) What happens inside the cylinder before the fuel is injected into the cylinder?

h) Why is the third stroke called the power or working stroke and what processes take place during this stroke?

i) What does a complete working cycle consist of? j) How does the engine start to rotate?

4. Using the information of the text Design and Operating Principles of Four-Stroke Diesel Engines describe:

• The construction of a four-stroke engine.

• The operating cycle of a four-stroke engine.

Text 2

Design and Operating Principles of Two-Stroke Diesel Engines

The working cycle of a diesel engine can also be completed in two strokes of the piston. The design of a two-stroke engine is the following.

The lid of the cylinder contains nozzle /. Instead of intake and exhaust valves, as in a four-stroke engine, the cylinder in a two-stroke engine is provided with exhaust ports 2 connected to exhaust manifold 3 and scavenging ports 4 connected with receiver 5. A special scavenging pump forces air into the receiver at a pressure of 1.15-1.25 atm. The scavenging ports are inclined to improve scavenging of the upper part of the cylinder.

Let us assume that at the initial moment the piston is at its bottom dead centre and the ports are completely open. Here scavenging takes place that continues until the piston moves upward and closes the scavenging ports. When the piston continues its motion it will close the exhaust ports and expel part of the fresh air charge from the cylinder. After the piston has closed the exhaust ports the air remaining in the cylinder is compressed. When the piston approaches top dead centre the pump begins to inject fuel into the cylinder through the nozzle. Part of the fuel burns at a constant volume and the remaining fuel - at a constant pressure. After this the products of combustion expand. The expansion terminates when the edge of the piston opens the exhaust ports. From this moment discharging of the exhaust gases begins and the pressure of the gases in the cylinder drops.

Then the piston opens the scavenging ports and scavenging of the cylinder begins, upon completion of which it is filled with fresh air.

Thus the working cycle of a two-stroke engine is completed in two strokes of the piston, i. e. during one revolution of the shaft. Theoretically, with the same size of the cylinder and an equal speed a two-stroke engine can develop the power twice as high as its four-stroke counterpart. In practice, all other conditions being equal, the power of a two-stroke engine is only 1.7 to 1.8 times greater than that of a four-stroke engine, since part of the piston stroke is spent on discharge and scavenging. Besides, about 6-8 % of the engine power goes to drive the scavenging pump.

Two-stroke engines are inferior to four-stroke ones in performance, which is due mainly to inadequate evacuation of the combustion products from the cylinder during scavenging.

However, with a rationally designed scavenging system the discharge of the combustion products from two-stroke engines can be as efficient as in four-stroke engines. Two-stroke diesel engines find a widespread use on vessels mainly as main high-power engines (over 1000 hp) when reduced weight and size are of primary importance.

Today the diesel engine, or compression-ignition, engine is the main type of internal-combustion engine used on sea-going vessels. This is due to their highly reliable operation, high performance and comparatively light weight.

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