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TM 9-8000
exhaust event. Figure 10-3 illustrates the comparison of
events between the four-stroke cycle piston-type engine
10-4. Cycle Characteristics and Variations. The four-
and the gas turbine.
stroke cycle piston-type engine is designed to perform
four events: intake, compression, power, and exhaust.
10-5. Otto Cycle Versus Brayton Cycle.
One cycle (four events) is completed as the crankshaft
rotates twice for a total of 720 degrees (fig. 10-3). Each
a. Otto Cycle. Both the piston engine and the gas
event is completed within 180 degrees of crank- shaft
turbine operate on cycles that can be represented
rotation and is called a stroke.  Gas turbine engine
graphically. Figure 10-4 illustrates the pressure volume
operation consists of four events that are essentially the
relationship for both the Otto-cycle engine and the
same as the reciprocating engine.  Air is first drawn
Brayton-cycle engine. The Otto-cycle engine pressure/
through the air inlet section that relates to the intake
volume relationship is as follows: The intake stroke is
event. It then passes through the compressor section,
represented by line 1-2. As this event occurs, the piston
relating to the compression event. The air then enters
is moving downward, increasing the volume in the
the combustor, mixes with fuel, and is ignited. As the air-
cylinder.  At the close of the intake valve, a slight
fuel mixture burns, the pressure increase is directed
pressure differential occurs, due to pumping losses.
through the turbines that extract work from the flowing
This results in a small vacuum in the cylinder at the start
gases which relates to the power event. Passing through
of the compression stroke.  Line 2-3 illustrates the
the turbines, the used gases are exhausted to the
Figure 10-3. Comparison of Piston Engine to Turbine Engine


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