Chapter 9
GAS POWER CYCLES
(Part 1)
Chapter 9
GAS POWER CYCLES
(Part 1a)
Objectives
j
1. Evaluate the performance of gas power cycles.
2 Develop simplifying assumptions applicable to gas power cycles.
2.
cycles
3. Review the operation of reciprocating engines.
4 Analyze both closed and open gas power cycles.
4.
cycles
5. Solve problems based on the Otto and Diesel cycles.
6. Solve pproblems based on the Brayton
y cycle;
y ; Brayton
y cycle
y with regeneration;
g
;
and Brayton cycle with intercooling, reheating, and regeneration.
7. Identify simplifying assumptions and perform second-law analysis on gas
power cycles.
cycles
3
Basic Considerations In Power Cycles Analysis
Most power-producing devices operate on cycles.
Ideal cycle: A cycle that resembles the actual cycle
closely but is made up totally of internally reversible
processes is called an ideal cycle.
Recall: Thermal efficiencyy of heat engines
g
Reversible cycles such as Carnot cycle have the
highest thermal efficiency of all heat engines operating
between the same temperature levels.
Unlike ideal cycles, they are totally reversible, and
unsuitable as a realistic model.
The analysis of many complex
processes can be reduced to a
manageable level by utilizing
some idealizations.
4
Idealizations (simplifications) in the analysis of power cycles
On a T-s diagram, the ratio of the area
enclosed by the cyclic curve to the area
under the heat-addition process curve
represents the thermal efficiency of the
cycle.
1. The cycle does not involve any friction. Therefore,
the working fluid does not experience any pressure
drop as it flows in pipes or heat exchangers.
2. All expansion and compression processes take
place in a quasi-equilibrium manner.
3. The pipes connecting the various components of a
system are well insulated, so heat transfer
through them is negligible.
Care should be exercised in the
interpretation of the results from
ideal cycles.
On both P-v and T-s diagrams, the area enclosed by the
process curve represents the net work of the cycle.
5
Carnot Cycle - Its Value In Engineering
The Carnot cycle is composed of 4 totally reversible
processes: isothermal heat addition, isentropic expansion,
isothermal heat rejection, and isentropic compression.
For both ideal and actual cycles: Thermal
efficiency increases with an increase in the
average temperature at which heat is supplied
to the system or with a decrease in the
average temperature at which heat is rejected
from the system.
P-v and T-s diagrams of a Carnot cycle.
Example: A steady-flow Carnot engine.
6
Air-standard Assumptions
The combustion process is replaced by a
heat-addition process in ideal cycles.
1 Th
1.
The working
ki flfluid
id is
i air,
i which
hi h continuously
ti
l
circulates in a closed loop and always
behaves as an ideal gas.
2 All th
2.
the processes th
thatt make
k up th
the cycle
l
are internally reversible.
3. The combustion process is replaced by a
h t dditi process from
heat-addition
f
an external
t
l
source.
4. The exhaust process is replaced by a
h t j ti process that
heat-rejection
th t restores
t
the
th
working fluid to its initial state.
Cold-air-standard assumptions: When the working fluid is considered to be air
with constant specific heats at room temperature (25°C).
Air-standard
Air
standard cycle: A cycle for which the air-standard
air standard assumptions are
applicable.
7
Chapter 9
GAS POWER CYCLES
(Part 1b)
Overview of Reciprocating Engines
The reciprocating engine (basically a piston–cylinder device) is an invention that
has proved to be very versatile and has a wide range of applications.
Reciprocating engine is the
powerhouse
h
off the
th vastt majority
j it off
automobiles, trucks, light aircraft,
ships, electric power generators,
and many other devices.
devices
9
Basic Components
The piston
Th
i t reciprocates
i
t in
i the
th cylinder
li d between
b t
t fixed
two
fi d positions
iti
called
ll d the
th top
t dead
d d
centre (TDC) - the position that forms the smallest volume in the cylinder - and the bottom
dead centre (BDC) - position that forms the largest volume in the cylinder.
The distance between TDC and BDC is called the stroke of
the engine. The diameter of the piston is called the bore.
Compression ratio:
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Performance Characteristics
Net work output per cycle:
Mean effective pressure (MEP):
A fictitious pressure that, if it is acted on the piston
during the entire power stroke,
stroke would produce the
same amount of net work as that produced during the
actual cycle.
Classifications of IC Engines:
1. Spark-ignition (SI) or Petrol engines
2. Compression-ignition (CI) or Diesel
engines
11
Otto Cycle: Ideal Spark-Ignition Engines Cycle
The piston executes four complete strokes within the cylinder.
cylinder The crankshaft
completes two revolutions for each thermodynamic cycle.
These engines are called four-stroke IC engines.
12
Actual and ideal cycles in spark-ignition engines on a P-v diagram.
T-s Diagram of Ideal Otto Cycle
IC Engines Classifications:
Four-stroke
F
t k cycle
l
1 cycle = 4 stroke = 2 revolutions of crankshaft
Two-stroke cycle
1 cycle = 2 stroke = 1 revolution of crankshaft
Sequence of processes:
13
Two-Stroke IC Engines
IIn two-stroke
t
t k engines,
i
allll four
f
f ti
functions
d
described
ib d earlier
li are executed
t d in
i two
t
strokes: the power and compression stroke.
Generally less efficient, but are relatively simple and inexpensive. They have high
power-to-weight
t
i ht andd power-to-volume
t
l
ratios.
ti
14
Thermal Efficiency of Otto Cycle
The heat supplied to the working fluid during
constant-volume heating (combustion),
The heat rejected from the working fluid during
constant-volume cooling (exhaust),
Temperature-volume
p
relation,,
Thermal efficiency,
Cold-air standard assumption.
Compression ratio,
15
Engine Knock (Autoignition)
Premature ignition of the fuel produces audible noise called engine knock. It hurts
performance and causes engine damage.
Autoignition places upper limit on compression ratios that can be used in SI engines.
S ifi heat
Specific
h t ratio,
ti k affects
ff t the
th thermal
th
l efficiency
ffi i
off the
th Otto
Ott cycle.
l
16
Chapter 9
GAS POWER CYCLES
(Part 1c)
Diesel Cycle: Ideal Cycle for CI Engines
In diesel engines, only air is compressed during the compression stroke, eliminating
the possibility of autoignition. These engines can be designed to operate at higher
compression ratios, typically between 12 and 24.
Fuels that are less refined (thus less expensive) can be used in diesel engines.
The combustion
Th
b ti process takes
t k place
l
over a
longer interval - fuel injection starts when
the piston approaches TDC and continues
duringg the first ppart of ppower stroke.
Hence, combustion process in the ideal
Diesel cycle is approximated as a constantpressure heat-addition process.
18
Sequence
Seque
ce of
o processes:
p ocesses
1-2 Isentropic compression
2-3 Constant-pressure heat addition
3 4 Isentropic
3-4
I t i expansion
i
4-1 Constant-volume heat rejection.
Note:
Petrol and diesel engines differ only in the
manner the heat addition (or combustion)
process takes
t k place.
l
It is approximated as a constant volume
process in the petrol engine cycle and as a
constant pressure process in the Diesel
engine cycle.
19
Thermal Efficiency of Diesel Cycle
Heat supplied to the working fluid during the
constant-pressure heating (combustion),
Heat rejected from the working fluid during the
constant-volume
t t l
cooling
li ((exhaust),
h t)
Th
Thermal
l efficiency
ffi i
off Di
Diesell cycle
l ((general),
l)
- constant specific heats
Cutoff ratio,
20
For the same compression ratio, thermal efficiency of Otto cycle is greater than that
of the Diesel cycle.
cycle
As the cutoff ratio decreases, the thermal
efficiency of the Diesel cycle increases.
increases
Thermal efficiencies of large diesel engines
range from about 35 to 40 percent.
When rc =1, the efficiencies of the Otto
and Diesel cycles are identical.
Higher efficiency and lower fuel costs
make diesel engines attractive in
applications such as in locomotive engines,
engines
emergency power generation units, large
ships, and heavy trucks.
21
Dual Cycle: Realistic Ideal Cycle for CI Engines
Approximating the combustion process as
a constant-volume or a constant-pressure
heat addition process is overly simplistic
heat-addition
and not quite realistic.
A better approach would be to model the
combustion process in both SI and CI
engines as a combination of two heattransfer processes, one at constant volume
and the other at constant pressure.
pressure
The ideal cycle based on this concept is
called the dual cycle.
Note: Both the Otto and the Diesel cycles can be obtained
as special cases of the dual cycle.
22