Английский язык: Сборник текстов и упражнений для студентов специальности "Эксплуатация судовых энергетических установок". Писарева Л.П. - 15 стр.

      4.   What is the action of the expanded gas?
      5.   Tell about the pressure and temperature during compression.
      6.   What takes place when the exhaust valve opens?
      7.   What happens after the exhaust valve closes?

     IX. Describe the four – cycle internal combustion engine and all the processes oc-
curring in it.




                                          Lesson 5
                               Text A. The Two – Cycle Engine

      In a two – stroke cycle compression occurs on the first or upstroke; combustion and ex-
pansion occur during the downstroke; exhaust, scavenging and recharging with air occur dur-
ing the latter part of the downstroke and the beginning of the next succeeding upstroke. This
sequence of events is made possible by substituting ports in the bottom of the cylinder wall for
one or more exhaust valves. There are two groups of these ports, one for the exhaust and the
other for scavenging air, usually on opposite sides of the cylinder, but in some designs both
groups are arranged on the same side. The exhaust ports connect with the exhaust manifold,
while the scavenging ports communicate with the scavenging air receiver.
      The exhaust ports are slightly higher than the scavenging ports, so that they are unco-
vered by the piston while the scavenging ports are still closed. This is necessary because the
cylinder gas is at a pressure of about 40 lb. per sq, in when exhaust begins, and, if the scaveng-
ing ports were open, the exhaust gas would blow into the scavenging air receiver. During the
short time before the piston uncovers the scavenging ports, the gas pressure falls to atmosphere
with the result that when the scavenging ports are uncovered, scavenging air rushes into the
cylinder and blows the reminder of the burned gas out. The scavenging ports are inclined up-
ward to direct the air toward the top of the cylinder and usually are positioned tangentially to
give the air whirling motion.
      This arrangement has the disadvantage that on the return stroke the piston covers the sca-
venging ports while the exhaust ports are still partislly open, so that when compression begins
the air pressure in the cylinders is about the same as that in the exhaust manifold. For this rea-
son some engine designs include valve - controlled scavenging ports. A two – cycle engine
must be provided with a scavenging compressor for supplying scavenging air.
      There are two main types of scavenging pumps, viz, the piston type and the Reotab-
lower type.


                            Text B. The Two-Cycle Diesel Engine

      Two-cycle, single-acting Diesel engines are also used for propulsion of ships. They pro-
vide one power-stroke per revolution, while a double-acting 2-stroke cycle engine develops
power on every stroke.
      A two-cycle engine develops twice as much power as a four-cycle engine with the same
size and number of cylinders. Since the four operations, i.e. suction, compression, firing (igni-
tion and expansion), and exhaust, have to be completed during two strokes of the piston, more
than one operation must be performed per stroke. This complicates the engine. The piston is
made to control the admission of air and release of the exhaust gases by covering and uncover-
ing ports or passages in the cylinder walls through which the air and gases pass. These ports,
scavenging and exhaust, are placed instead of air inlet valves and exhaust valves. The exhaust
ports are located in the bottom of the cylinder and scavenging ports may be placed at the same
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