Page 263 - The Mechatronics Handbook

0066_frame_C12 Page 28 Wednesday, January 9, 2002 4:22 PM

of Table 12.4 apply to cold air-standard analyses, as does the following expression for the turbine power
obtained from Table 12.1 (Eq. (10c′′)):

[
W t = m ˙ ------------ 1 ( p 4 p 3 ) ( k – 1) k ] (12.30)
kRT 3
˙
–
–
k 1
An expression similar in form can be written for the power required by the compressor.
Otto, Diesel, and Dual Cycles
Although most gas turbines are also internal combustion engines, the name is usually reserved to
reciprocating internal combustion engines of the type commonly used in automobiles, trucks, and buses.
Two principal types of reciprocating internal combustion engines are the spark-ignition engine and the
compression-ignition engine. In a spark-ignition engine a mixture of fuel and air is ignited by a spark
plug. In a compression ignition engine air is compressed to a high-enough pressure and temperature that
combustion occurs spontaneously when fuel is injected.
In a four-stroke internal combustion engine, a piston executes four distinct strokes within a cylinder
for every two revolutions of the crankshaft. Figure 12.7 gives a pressure-displacement diagram as it might
be displayed electronically. With the intake valve open, the piston makes an intake stroke to draw a fresh
charge into the cylinder. Next, with both valves closed, the piston undergoes a compression stroke raising
the temperature and pressure of the charge. A combustion process is then initiated, resulting in a high-
pressure, high-temperature gas mixture. A power stroke follows the compression stroke, during which
the gas mixture expands and work is done on the piston. The piston then executes an exhaust stroke in
which the burned gases are purged from the cylinder through the open exhaust valve. Smaller engines
operate on two-stroke cycles. In two-stroke engines, the intake, compression, expansion, and exhaust
operations are accomplished in one revolution of the crankshaft. Although internal combustion engines
undergo mechanical cycles, the cylinder contents do not execute a thermodynamic cycle, since matter is
introduced with one composition and is later discharged at a different composition.
A parameter used to describe the performance of reciprocating piston engines is the mean effective
pressure, or mep. The mean effective pressure is the theoretical constant pressure that, if it acted on the

FIGURE 12.7 Pressure-displacement diagram for a reciprocating internal combustion engine.

©2002 CRC Press LLC