Transcript chapter 8 Gas Power Cycle - China University of Mining and

chapter 8 Gas Power Cycle
8-1 The Analysis of a Cycle
8-1-1 The average temperature of a process
T
We define:

T 
T
2
1
Tds
s2  s1
2
That is:
1
T ( s2  s1 )   Tds
2
1
s1
s2
s
8-1-2 The Analysis of a Cycle
As to a cycle:
wcycle 
T
 Tds   Tds
1a 2
wcycle  T2 ( s2  s1 )  T2 ( s2  s1 )
a
T2
T2 ( s2  s1 )  T1 ( s2  s1 )
cycle 
T2 ( s2  s1 )
2
T1
1
b
s1
1b 2
s2
s
T2  T1
T1
cycle 
 1
T2
T2
Nicolaus
AugustCycle
Otto the inventor of the
8-2 Otto
four-stroke cycle was born on 14th June
8-2-1 N. A. Otto 1831 in Germany. In 1862 he began first
experiments with four-strokes engines.
The first four-stroke engines is shown.
they correspond to the today's engines.
He died on 26th January 1891 in Cologne
8-2-2 The Cycle - The Four Strokes
Intake stroke:
The piston moves down the cylinder and the pressure will drop
(negative pressure). The intake valve is opend. Because of the low
pressure the air/fuel mixtures is sucked into the cylinder.
Compression stroke:
At Bottom Dead Center (BDC) the cylinder is at its maximum volume
and the intake valve is closed. Now the piston moves backward the
Top Dead Center (TDC) and compresses the air/fuel mixtures.
Near the end of the compression stroke, the ignition starts the
combustion and the mixture burns very rapidly. The expanding
gas creates a high pressures against the top of the piston.
Power stroke
The force drives the piston downward to crank shaft (the valves
are closed). The volume is increased and the pressure is decreased.
No more energy is added and because of this the internal energy of
the gas is decreased as so as the temperature.
Exhaust stroke
At BDC the exhaust valve is opened and the piston moves up the
cylinder. The pressure drops near the pressure outside the cylinder
because of the opened exhaust valve. Exhaust gas leaves the
cylinder. The volume is decreased.
8-2-3 The Cycle - The Four Strokes
The theory cycle
p
3
Adiabatic process
2
4
5
1
v
Theory efficiency of Otto cycle
Cv (T4  T1 )
  1
Cv (T3  T2 )
T4
1
T1 T1
 1 
T2 T3  1
T2
V1 V4
 
V2 V3
T2  V1 
  
T1  V2 
( k 1)
T3  V4 
  
T4  V3 
( k 1)
Then：
T4 T3

T1 T2
1
  1 
T2
T1
1
 1
( k 1)
 p2  k
 
 p1 
1
 1

( k 1)
k
ε compression ratio
8-3 Diesel Cycle
8-2-1 Rudolf Diesel
Rudolf Diesel (1858 – 1913) was born in
Paris in 1858. After graduation he was
employed as a refrigerator engineer. However,
his true love was in engine design. In 1893,
he published a paper describing an engine
with combustion within a cylinder, the
internal combustion engine. In 1894, he filed
for a patent for his new invention, the diesel
engine. Diesel was almost killed by his engine
when it exploded - however, his engine was
the first that proved that fuel could be ignited
without a spark. He operated his first
successful engine in 1897.
8-3-2 The Diesel Cycle
8-3-3 The Efficiency of Diesel Cycle
The theory cycle
p
2
3
4
5
1
v
Theory efficiency of Diesel cycle
Cv T4  T1 
  1
C p T3  T2 
 T4 
T1   1
1  T1 
 1 
k T3  T2
P1 V1k  P2 V2k
P4 V4k  P3 V3k
p1  V2 

  
p 4  V3 
V1  V4
P2  P3
We define
V3

V2
p1  1 
  
p4   
k
k
Since process 1-4 has a constant volume
T1
p1  1 

  
T4
p4   
k
 T4

T1   1
T1
1


  1 
k T3  T2
1  k 1
  1 
k T3 T2

T1 T1
1  k 1
  1 
k T3 T2 T2

T2 T1 T1
1
 k 1
  1 
k T3 k 1 k 1
 
T2
1
 k 1
  1   k 1 k 1
k   
To increase efficiency:
p
2
The
compression
pressure
should be
higher
3
The volume increase
should be smaller
4
5
1
v
Other internal combustion engine
8-4 Brayton Cycle
8-4-1 The Equipments of Brayton Cycle
Advantages
Gas turbine engines have a great power-to-weight
ratio compared to reciprocating engines. That is, the
amount of power you get out of the engine compared to
the weight of the engine itself is very good.
Gas turbine engines are smaller than their
reciprocating counterparts of the same power
8-4-2 Brayton Cycle
p
2
T
Constant
pressure
adiabatic
3
4
2
1
4
v
3
1
s
8-4-3 Efficiency of Brayton Cycle
  1
C p T4  T1 
C p T3  T2 
T4 T1  p1 
    
T3 T2  p2 
k 1
k
T4  T1  p1 

  
T3  T2  p2 
k 1
k
1
  1

 k 1 


 k 
8-4-3 The Optimum Compression Ratio
T
3’
Tmax
3
4
2
If T3 is limited:
the compression ratio
will be increased to
get high efficiency
But the power ratio
will decrease
4’
1
s
We have to compromise between high efficiency
and high power ratio.
Usually in aerospace field the power ratio is more
important
T
Tmax
Obviously there must
be an optimum
compression ratio
which makes the
cycle has maximum
power ratio
T0
s
εmax
This ratio is denoted as:
 max
 T3 
  
 T1 
1
  1

 1
k
2  k 1
 k 1 


 k 
T1
T3
The efficiency depends on T3 basically
8-4-4 The methods to increase the efficiency
(1) Regenerative Brayton Cycle
T
T2
T2
T1
T1
s
Engine Characteristic
Type
Twin-Spool, Augmented Turbofan
Application
F-22 Advanced Tactical Fighter
Thrust
35,000 Pound Thrust Class
Engine Control
Full-Authority Digital Electronic Control
Compression System
Twin Spool/Counter Rotating/Axial Flow/
Low-Aspect Ratio
Three-Stage Fan
Six-Stage Compressor
Combustor
Annular
Turbine
Axial Flow/Counter Rotating
• One-Stage, High-Pressure Turbine
• One-Stage, Low-Pressure Turbine
Nozzle
Two-dimensional Vectoring Convergent/Divergent
oil
Combustion
chamber
regenerator
Air in
compressor
gas turbine
(2) Isothermal compression and regenerative cycle
T
s
8-5 Jet Engine
Engine Characteristic
Type
Twin-Spool, Augmented Turbofan
Application
F-22 Advanced Tactical Fighter
Thrust
35,000 Pound Thrust Class
Engine Control
Full-Authority Digital Electronic Control
Compression System
Twin Spool/Counter Rotating/Axial Flow/
Low-Aspect Ratio
Three-Stage Fan
Six-Stage Compressor
Combustor
Annular
Turbine
Axial Flow/Counter Rotating
• One-Stage, High-Pressure Turbine
• One-Stage, Low-Pressure Turbine
Nozzle
Two-dimensional Vectoring Convergent/Divergent
2
1
3
4
5
6
4
T
5
6
3
2
1
s
The methods to increase the power ratio of jet engine
(1) After burning
After burner
6
4
T
5
7
3
2
1
s
(2) Increase T4
4’
T
5’
4
6’
6
3
2
1
s
8-5 The Stirling Cycle
p
T
3
3
2
1
4
v
2
4
1
s
The End Of This Chapter
Thank You