Английский язык: Сборник текстов и упражнений для студентов специальности "Эксплуатация судовых энергетических установок". Писарева Л.П. - 32 стр.

                                     Double-acting Engine

      The double-acting engine has a head on each end of the cylinder, with fuel injection
valves in each, a piston rod attached to the piston and extending through a stuffing box in the
lower head to connect to the crosshead, and ports in the centre of the cylinder wall for exhaust
and scavenging. Double-acting engines are operated on the two-cycle system.
      Although the double-acting engine has been widely used for marine propulsion, it has not
supplanted the single-acting engine. It finds its best application where a high concentration of
power on a single shaft is desired or where fore and aft room is restricted. Except in the largest
ships, the possibilities of dividing up the power among several shafts or utilizing multiple en-
gines on a single-acting engine available for the majority of vessels.

                                    Opposed-piston Engine

      In the opposed-piston engine the cylinder has a length equivalent to two ordinary cylind-
ers and is open at both ends. Two pistons are provided for each cylinder, the lower piston being
connected to the crankshaft in the usual way. The upper piston is attached by means of a piston
rod to a yoke above the upper end of the cylinder. The ends of this yoke are connected by side
rods to connecting rods extending downward to the cranks, one on each side of the crank that is
driven by the lower piston. The two side cranks are set at 180º to the centre crank. The stroke
of the upper cylinder is sometimes less than that of the lower cylinder.
      This crank arrangement causes the pistons to move in opposite directions. On the com-
pression stroke the two pistons move inward, compressing the air between them, and the space
between the pistons forms a combustion chamber into which the fuel is injected. The pressure
then forces the pistons apart and as they travel in opposite directions to the ends of the cylinder
each acts on its own connection to the crankshaft, the centre crank pushing downward and the
two side cranks pulling upward. At the end of the strokes the upper piston uncovers the exhaust
ports and the lower piston the scavenging ports, and the gases are blown out of the cylinder.
This arrangement has proved highly successful in practice. An important feature is the good
scavenging obtained by admitting scavenging air at one end of the cylinder, blowing it straight
through without change of direction, and forcing the exhaust gases out at the other end.


                                            Exercises

      1. Speak on the single-acting engine.
      2. Speak on the double-acting engine.
      3. Speak on the opposed-piston engine.




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