UNIT 3. COMPRESSION-IGNITION ENGINES

PRE-READING TASKS

1. Pay attention to the correct pronunciation of the following words.

noticeable ['noutisobl] inferior [nVfians] flywheel ['flaiwi:l]

initial [Ynifel] inadequate [m'asdikwit] edge [ed3]

rational [Ysefenl] comparatively [kam'paerativ] dead [ded]

primary ['praimari] widespread ['waidspred] incline [m'klam]

2. Try to understand the meaning of the following words used in the text Compression-Ignition Engines from the definitions given below.

noticeable a. discontinue v. flywheel n.

Incline V.

attracting attention; visible or seen easily to stop; to end

a heavy disk or wheel rotating on a shaft providing almost uniform rotation speed to the shaft and to all connected machinery

to deviate from a vertical or horizontal

manifold п. a chamber having several outlets through which a liquid

or gas is distributed or gathered terminate v. to finish; to end; to cease

at the expense (of) because of; as a result of

port/i. an opening through which a substance (fluid) can get/pass

into or out of some chamber (space) expel v. to force out; to withdraw; to remove

scavenging g. removing or expelling burnt gases from the cylinder

3. Translate the sentences paying attention to the participles and absolute participial constructions.

1. The cylinder is provided with exhaust ports connected to exhaust manifold and scavenging ports connected with the receiver.

2. The heavy liquid fuel introduced in an atomized state into the cylinder at the end of the compression stroke ignites spontaneously in the hot compressed air.

3. However, with a rationally designed scavenging system the discharge of the combustion products from two-stroke engine can be as efficient as in four-stroke engine.

4. The manufacture of air injection engines has been discontinued, but there are still many vessels in operation equipped with the engines of this type.

5. 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.

6. The entire working cycle of a four-stroke engine is completed during four strokes of the piston or two revolutions of the crankshaft, with only one stroke of the piston being a working one.

READING Compression-Ignition Engines

Text 1

Design and Operating Principles of Four-Stroke Diesel Engines

Compression-ignition or diesel engines are those in which heavy liquid fuel introduced in an atomized state into the cylinder at the end of the compression stroke ignites spontaneously in the hot compressed air. One specific feature of mixture formation in diesel engines built at the beginning of the last century is that fuel was introduced into the cylinder and atomized with the aid of compressed air and burned in it without any noticeable increase in pressure, i. е., at an approximately constant pressure. Such engines are called air injection engines and are provided with special compressors to obtain compressed air at the pressure of 50-60 atm. In airless injection engines the fuel is supplied to the cylinder and atomized by a pump under a high pressure. In this case part of the fuel burns at a constantly increasing pressure (theoretically at constant volume) and the rest without any noticeable increase in pressure (theoretically at constant pressure). The manufacture of air injection engines has been discontinued, but there are still quite many vessels in operation equipped with engines of this type.

If we assume that the initial position of the piston is at top dead center and that intake valve is open, the motion of the piston downward will be accompanied by admission of air into the cylinder. This stroke of the piston is known as the suction stroke. When the piston moves upward the intake valve is closed and the air is compressed in the cylinder. At the end of compression the pressure reaches 30-35 atm, and as a result the temperature of the air increases to above 500 °C. The compression stroke is the second one. The volume in the cylinder that corresponds to the position of the piston at the top dead centre is known as the compression volume.

When the piston approaches T.D.C. (the crank does not reach T.D.C. by about 20°) the nozzle will inject a portion of atomized fuel into the cylinder. The process of combustion of approximately 40 % of the fuel is close to a constant-volume process. The remaining 60 % of the fuel burns in conditions close to a constant-pressure process. After the combustion the products of combustion expand. Thus combustion and expansion take place during the third stroke of the piston which is called the power or working stroke since the engine performs useful work. After this stroke the exhaust valve opens, the piston moves upward, and the exhaust gases are discharged. Then the exhaust valve closes and the cycle is repeated again.

The entire working cycle of a four-stroke engine is completed during four strokes of the piston or two revolutions of the crankshaft, with only one stroke of the piston being a working one. The shaft rotates during the other strokes at the expense of the work performed in other cylinders or of the reserve energy stored by the flywheel during the power stroke of the piston. To make the engine rotate it must first be started by some outside source of power (compressed air or an electric motor) and only after the air has been compressed in the cylinder and the fuel has been supplied can a spark be obtained that will start the engine operating on its own.

DISCUSSION