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 TM 9-8000
(2) The two-stroke cycle diesel uses either two
measured. Section I demonstrated that each piston
or four exhaust valves per cylinder, which complicates
stroke is one-half of a crankshaft revolution. This is
the valve mechanism.
expressed as 180 degrees of crank-shaft rotation. We
also can recall that there are two complete crankshaft
(3) As with the two-stroke cycle gasoline engine,
revolutions for every four-stroke operating cycle. This is
the diesel counterpart will not produce twice as much
expressed as 720 degrees of crankshaft rotation.
power as a four-stroke cycle engine even though it
b. Power Overlap. In a simple four-stroke cycle
produces twice as many power strokes. By studying
engine, the power stroke produces a driving force that
only a portion of the downstroke of the piston in a two-
rotates the crankshaft. This means that out of a 720-
stroke cycle diesel. In a four-stroke cycle diesel, the
degree operating cycle, there are only 180 degrees when
power stroke lasts from top dead center to bottom dead
the crankshaft actually receives any driving force. In
center.
reality, the power stroke is actually even shorter. This is
due to the fact that engineers have found that an engine
e. Usage. The two-stroke cycle diesel is used in
will run better if the exhaust valve is set to begin opening
most of the same applications as the four-stroke cycle
approximately four-fifths of the way through the power
diesel.
stroke. This reduces the power stroke still further, to
approximately 145 degrees (para 2-7). When the engine
runs, it has to rely on power that is stored in the flywheel
2-15. Multicylinder Engine Versus Single-
from the power stroke to push it through the 575 degrees
Cylinder Engine.
remaining in the operating cycle. A much smoother
running engine can be made by making it a multicylinder
a. General. The rotation of a crankshaft is
engine. A multicylinder engine
measured by breaking up one revolution into 360 equal
parts. Each part is called a degree. The standard
starting point is with the piston at top dead center. Figure
Figure 2-23 . Comparison of Two-and Four-Stroke cycle Diesel Power Stroke Lengths.
2-17
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