Environmental Engineering Reference
In-Depth Information
heat loss to the cylinder during the power strokes all combine to reduce the power output com-
pared to the ideal cycle. For four stroke cycle gasoline engines, the low intake pressure experi-
enced at part load is an additional loss that has no counterpart in diesel engines. The best thermal
efficiencies for automotive engines are about 28% and 39% for the gasoline and diesel engine,
respectively.
20
The thermodynamic analysis of the Otto cycle does not explain the most salient feature of the
reciprocating ICE—that is, that it can be constructed in useful sizes between about 1 kilowatt and
10 megawatts. This is in marked contrast to the steam power plant which, for the generation of
electric power, is usually built in units of 100 to 1000 MW. These differences are a consequence
of mechanical factors related to the limiting speed of pistons versus turbine blades and other factors
unrelated to the thermodynamics of the cycles.
21
3.10.4
The Brayton Cycle
Since the middle of the twentieth century the gas turbine has become the dominant propulsive
engine for large aircraft because of its suitability to high subsonic speed propulsion, light weight,
fuel economy, and reliability. But it has made inroads into other uses such as naval vessel propulsion,
high-speed locomotives, and, more recently, electric power production. In the latter case, the gas
turbine is often used with a Rankine cycle steam plant that is heated by the gas turbine exhaust, the
coupled plants being termed a
combined cycle
.
In its simplest form the gas turbine plant consists of a compressor and turbine in tandem, both
attached to the same shaft that delivers mechanical power. Situated between the compressor and
turbine is a combustion chamber within which injected fuel burns at constant pressure, raising the
temperature of the compressed air leaving the compressor to a higher level prior to its entering
the turbine. In passing through the turbine, the hot combustion gas is reduced in pressure and
temperature, generating more turbine power than is consumed in compressing the air entering the
compressor and making available a net mechanical power output from the shaft. The compression,
combustion, and expansion processes are adiabatic; and like the reciprocating engine cycles, the
gas turbine is an open cycle.
The ideal thermodynamic cycle that models the thermal history of the air and combustion
gas flow through a gas turbine power plant is called the
Brayton cycle.
Illustrated in Figure 3.6,
it consists of an isentropic compression of air in the compressor from the intake pressure
p
i
to
the compressor outlet pressure
p
c
(1
→
2 in Figure 3.6), followed by a constant-pressure heating
→
(2
3) that raises the gas temperature to the value
T
3
at the turbine inlet. The combustion gas
expands isentropically while flowing through the turbine, its pressure being reduced from
p
c
to
p
i
(3
4).
In the Brayton cycle, the steady flow work produced by the turbine and that absorbed by the
compressor are each equal to the change in enthalpy of the fluid flowing through them. Per unit
mass of fluid, the net work
→
w
of the gas turbine plant is the difference between the turbine work
20
Under average operating conditions, the thermal efficiency is less than these maximum values because the
engine operating conditions are selected to optimize vehicle performance rather than efficiency.
21
For a detailed description of the Otto cycle, see Chapter 8.
