Английский язык: Сборник текстов и упражнений для студентов специальности "Эксплуатация судовых энергетических установок". Писарева Л.П. - 12 стр.

are eliminated, and their place is taken by the scavenging and exhaust ports, with the engine
piston acting as a valve to control the opening and closing of these ports. A variation in this
arrangement is found in those two – cycle engines which use valve scavenging. The valves are
of the poppet type, operated by rocker arms and push rods which receive their motion from
cams on a camshaft driven by gears from the engine crankshaft.
      The timing of the various valves for four – cycle and two – cycle engines is shown by the
diagrams in Figs. 11 and 12 respectively. It will be noted that the inlet and exhaust – valve pe-
riods overlap so that for a period of about 35 degrees both valves are open. This is a means of
obtaining better filling of the cylinder. The exhaust gases that are pushed out through the ex-
haust valve by the rising piston have considerable inertia, which causes the gases to continue to
flow through the valve after the piston has approached the end of its travel and has very little
motion. This tends to create a partial vacuum in the cylinder; if the inlet valve is opened, the
partial vacuum tends to draw the air into the cylinder.
      In the two – cycle diagram all events occur during one revolution of the crank, instead of
requiring two revolutions as in the case of the four – cycle. When about 80 per cent of the
stroke has been completed, the piston uncovers the exhaust ports. After about 27 degrees, the
pressure in the cylinder has fallen to atmospheric, the piston uncovers the scavenging ports and
the remaining gases are blown out.

      Question to the text.

      1. What is the function of the cylinder – head valves?
      2. Of what importance is the timing of the valves?
      3. What is a means of obtaining better filling of the cylinder?
      4. When are both the inlet and exhaust valves open?




                                         Lesson 4
                               The Four – Cycle Diesel Engine

      Any internal combustion engine has a four – stroke cycle or a two – stroke cycle, accord-
ing to whether it receives a working impulse on the piston every other revolution or every revo-
lution. These two terms are commonly abbreviated to 4 – cycle and 2 – cycle, and engines of
either type may be single acting or double acting.
      In the four – stroke cycle, or, as it is commonly called, the four – cycle engine, four
strokes of the piston, or two revolutions of the shaft, are required to complete the cycle. As the
piston moves down on its suction or intake stroke, a charge of pure air containing the necessary
oxygen for combustion is drawn in through the inlet valve in the cylinder head. The inlet valve
was opened mechanically by a cam at the end of the previous upward stroke. After the piston
has completed the suction stroke, the inlet valve closes, and the piston starts on an upward
stroke, called compression stroke, compressing the charge of pure air into the inlet valve
closes, and the piston starts on an upward stroke, called compression stroke, compressing the
charge of pure air into the clearance volume to a pressure of between 350 and 450 lb. per sq. in.
About the time that the piston reaches the end of its upward stroke, the fuel injection valve
opens for a short period, and finely atomized fuel oil is sprayed in the combustion space under
high, pressure. Because of the heat compression, the fuel oil is ignited and burns generating
heat. The piston has now started downward on its third stroke, and the hot gases generated by
the combustion of the oil expand and force the piston downward on its working or power
stroke. Near the end of the working stroke the exhaust valve in the cylinder head opens, part of
the gases escape, and the pressure in the cylinder drops approximately to the atmospheric. The
piston now starts up on its exhaust stroke, and all the products of combustion except those con-
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