Environmental Engineering Reference
In-Depth Information
less net work than in the ideal cycle. Other mechanical power is required to operate the boiler fans
and condenser cooling water pumps, reducing the net power output. The boiler does not transfer
to the working fluid (water, steam) all of the fuel higher heating value, because the flue gases
exit from the boiler at higher than fuel input temperature and excess air, above that required for
stoichiometric combustion, is used. Even the best steam electric power plants seldom exceed 40%
thermal efficiency, based upon the ratio of the net mechanical power output divided by the heating
value of the fuel supply.
The steam turbine for an electric power plant experiences a large change in pressure between
entrance and exit, during which the steam density decreases greatly, requiring ever longer turbine
blades to extract power from the flow. Figure 3.4 shows the rotor of a 1500-MW steam turbine,
which is divided into three stages, from right to left in the photograph. In the high-, intermediate-,
and low-pressure stages the steam pressure is reduced from 71 to 10, 10 to 3, and 3 to 0.1 bar,
respectively.
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3.10.3
The Otto Cycle
The most ubiquitous fossil-fueled engine is that in the automobile. Unlike the steam plant, the auto-
mobile engine does not depend upon heat transfer to the working fluid from an external combustion
source. Instead, the fuel is burned adiabatically inside the engine, and the products of combustion
produce more work during the expansion stroke than must be invested in the compression stroke,
giving a net power output. The combustion products, which are exhausted to the atmosphere, are
replaced by a fresh air-fuel charge to begin the next combustion cycle. The working fluid flows
through the engine and is not recycled. This is termed an
open cycle
—in contrast to the steam cycle,
which is closed.
The thermodynamic heat engine closed cycle that replicates the pressure-volume character-
istics of the reciprocating internal combustion engine (ICE) is called the Otto cycle. As sketched
in the
T
-
s
plane of Figure 3.5, it consists of an isentropic compression from a volume
v
e
at the
T
Combustion products
3
4
v
c
2
w
Air
Spark ignition engine
4
1
v
e
1
Fuel
s
Figure 3.5
The Otto cycle comprises two isentropic compression and expansion strokes of a piston
in a cylinder
(
1
→
2
,
3
→
4
)
interspersed with two constant-volume heating and cooling processes
(
2
→
3
,
4
→
1
)
. It is a model for the spark ignition engine.
17
For a more detailed description of Rankine cycle power plants using fossil or nuclear fuels, see Chapters 5
and 6.