MAE 4261: AIR-BREATHING ENGINES
Air-Breathing Engine Performance Parameters
and Future Trends
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk
LECTURE OUTLINE
• Review
– General expression that relates the thrust of a propulsion system to the net
changes in momentum, pressure forces, etc.
• Efficiencies
– Goal: Look at how efficiently the propulsion system converts one form of
energy to another on its way to producing thrust
• Overall Efficiency, hoverall
• Thermal (Cycle) Efficiency, hthermal
• Propulsive Efficiency, hpropulsive
– Specific Impulse, Isp [s]
– (Thrust) Specific Fuel Consumption, (T)SFC [lbm/hr lbf] or [kg/s N]
• Implications of Propulsive Efficiency for Engine Design
• Trends in Thermal and Propulsive Efficiency
FLUID MECHANICS: DERIVATION OF THRUST EQUATION
Chemical
Energy
Thermal
Energy
F  m eVe  m oVo  Pe  Pa Ae
F  m Ve  Vo 
• Flow through engine is conventionally called THRUST
– Composed of net change in momentum of inlet and exit air
• Fluid that passes around engine is conventionally called DRAG
Kinetic
Energy
THERMODYANMICS: BRAYTON CYCLE MODEL
•
•
•
1-2: Inlet, Compressor and/or Fan: Adiabatic compression with spinning blade rows
2-3: Combustor: Constant pressure heat addition
3-4: Turbine and Nozzle: Adiabatic expansion
– Take work out of flow to drive compressor
– Remaining work to accelerate fluid for jet propulsion
•
Thermal efficiency of Brayton Cycle, hth=1-T1/T2
– Function of temperature or pressure ratio across inlet and compressor
P-V DIAGRAM REPRESENTATION
•
Thermal efficiency of Brayton Cycle, hth=1-T1/T3
– Function of temperature or pressure ratio across inlet and compressor
EXAMPLE OF LAND-BASED POWER TURBINE: GENERAL ELECTRIC LM5000
•
•
•
•
Modern land-based gas turbine used for electrical power production and mechanical drives
Length of 246 inches (6.2 m) and a weight of about 27,700 pounds (12,500 kg)
Maximum shaft power of 55.2 MW (74,000 hp) at 3,600 rpm with steam injection
This model shows a direct drive configuration where the LP turbine drives both the LP
compressor and the output shaft. Other models can be made with a power turbine.
BYPASS RATIO: TURBOFAN ENGINES
Bypass Air
Core Air
Bypass Ratio, B, a:
Ratio of bypass air flow rate to core flow rate
Example: Bypass ratio of 6:1 means that air volume flowing through fan and
bypassing core engine is six times air volume flowing through core
TRENDS TO HIGHER BYPASS RATIO
1958: Boeing 707, United States' first commercial jet airliner
Similar to PWJT4A: T=17,000 lbf, a ~ 1
1995: Boeing 777, FAA Certified
PW4000-112: T=100,000 lbf , a ~ 6
GE J85
•
•
•
•
•
•
•
•
•
J85-GE-1 - 2,600 lbf (11.6 kN) thrust
J85-GE-3 - 2,450 lbf (10.9 kN) thrust
J85-GE-4 - 2,950 lbf (13.1 kN) thrust
J85-GE-5 - 2,400 lbf (10.7 kN) thrust,
3,600 lbf (16 kN) afterburning thrust
J85-GE-5A - 3,850 lbf (17.1 kN)
afterburning thrust
J85-GE-13 - 4,080 lbf (18.1 kN), 4,850
lbf (21.6 kN) thrust
J85-GE-15 - 4,300 lbf (19 kN) thrust
J85-GE-17A - 2,850 lbf (12.7 kN) thrust
J85-GE-21 - 5,000 lbf (22 kN) thrust
TURBOJET / MODERATE BYPASS TURBOFAN
P&W F100 and 229
•
P&W 229 Overview
•
•
•
•
•
•
Type: Afterburning turbofan
Length: 191 in (4,851 mm)
Diameter: 46.5 in (1,181 mm)
Dry weight: 3,740 lb (1,696 kg)
Components
Compressor: Axial compressor with 3 fan and
10 compressor stages
Bypass ratio: 0.36:1
Turbine: 2 low-pressure and 2 high-pressure
stages
•
•
•
•
•
•
Maximum Thrust:
– 17,800 lbf (79.1 kN) military thrust
– 29,160 lbf (129.6 kN) with afterburner
Overall pressure ratio: 32:1
Specific fuel consumption:
– Military thrust: 0.76 lb/(lbf·h) (77.5
kg/(kN·h))
– Full afterburner: 1.94 lb/(lbf·h) (197.8
kg/(kN·h))
Thrust-to-weight ratio: 7.8:1 (76.0 N/kg)
ANTONOW AN 70
PROPELLER DETAIL
UNDUCTED FAN, a ~ 30
“HYBRID” DUCTED FAN + TURBOJET
EFFICIENCY SUMMARY
•
Overall Efficiency
– What you get / What you pay for
– Propulsive Power / Fuel Power
– Propulsive Power = TUo
– Fuel Power = (fuel mass flow rate) x
(fuel energy per unit mass)
•
Thermal Efficiency
– Rate of production of propulsive
kinetic energy / fuel power
– This is cycle efficiency
•
Propulsive Efficiency
– Propulsive Power / Rate of
production of propulsive kinetic
energy, or
– Power to airplane / Power in Jet
TU o
hoverall 
m f h
 m eU e2 m oU o2 



2
2 

hthermal 
m f h
h propulsive 
TU o
2

 m eU e2 m oU o2  1  U e



Uo
2 
 2
hoverall  hthermalh propulsive
PROPULSIVE EFFICIENCY AND SPECIFIC THRUST AS A
FUNCTION OF EXHAUST VELOCITY
Ue
T

1
m U o U o
Conflict
2
h propulsive 
Ue
1
Uo
COMMERCIAL AND MILITARY ENGINES
(APPROX. SAME THRUST, APPROX. CORRECT RELATIVE SIZES)
GE CFM56 for Boeing 737 T~30,000 lbf, a ~ 5
•
•
•
•
•
•
•
•
•
•
Demand high T/W
Fly at high speed
Engine has small inlet area
(low drag, low radar crosssection)
Engine has high specific
thrust
Ue/Uo ↑ and hprop ↓
Demand higher efficiency
Fly at lower speed (subsonic, M∞ ~ 0.85)
Engine has large inlet area
Engine has lower specific thrust
Ue/Uo → 1 and hprop ↑
P&W 119 for F- 22, T~35,000 lbf, a ~ 0.3
EXAMPLE: SPECIFIC IMPULSE
SSME
PW4000 Turbofan
• Airbus A310-300, A300-600, Boeing
747-400, 767-200/300, MD-11
• T ~ 250,000 N
• TSFC ~ 17 g/kN s ~ 1.7x10-5 kg/Ns
• Fuel mass flow ~ 4.25 kg/s
• Isp ~ 6,000 seconds
•
•
•
•
•
Space Shuttle Main Engine
T ~ 2,100,000 N (vacuum)
LH2 flow rate ~ 70 kg/s
LOX flow rate ~ 425 kg/s
Isp ~ 430 seconds
PROPULSIVE EFFICIENCY FOR DIFFERENT ENGINE
TYPES [Rolls Royce]
OVERALL PROPULSION SYSTEM EFFICIENCY
• Trends in thermal efficiency are driven by increasing compression ratios and
corresponding increases in turbine inlet temperature
• Trends in propulsive efficiency are due to generally higher bypass ratio
FUEL CONSUMPTION TREND
•
U.S. airlines, hammered by soaring oil prices, will spend a
staggering $5 billion more on fuel in 2007 or even a greater
sum, draining already thin cash reserves
•
Airlines are among the industries hardest hit by high oil prices
•
“Airline stocks fell at the open of trading Tuesday as a spike in
crude-oil futures weighed on the sector”
Fuel Burn
JT8D
PW4084
JT9D
Future
Turbofan
PW4052
NOTE: No Numbers
1950
1960
1970
1980
1990
Year
2000
2010
2020
CRUISE FUEL CONSUMPTION vs. BYPASS RATIO
SUBSONIC ENGINE SFC TRENDS
(35,000 ft. 0.8 Mach Number, Standard Day [Wisler])
AEROENGINE CORE POWER EVOLUTION:
DEPENDENCE ON TURBINE ENTRY TEMPERATURE [Meece/Koff]
PRESSURE RATIO TRENDS (Jane’s 1999)
AIR-BREATHING PROPULSION SYSTEMS
RAMJETS
TURBOJETS
TURBOFANS
Daniel R. Kirk
Assistant Professor
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
RAMJETS


T
 M 0  b 1
m 0 a0
Cycle analysis employing general form
of mass, momentum and energy
h overall
TU 0

m f h
Energy (1st Law) balance across burner
• Thrust performance depends solely on total temperature rise across burner
• Relies completely on “ram” compression of air (slowing down high speed flow)
• Ramjet develops no static thrust
TURBOJET SUMMARY
 t
T
2
 o c t  1

m o ao
 1
  o c
T
2  
1

 t 1 
m o a o
  1    o c
h overall

  M o



   0  c  1  M o


 T
M 0   1
 m 0 a 0

 t   c 0 



Cycle analysis employing general form
of mass, momentum and energy
Turbine power = compressor power
How do we tie in fuel flow, fuel energy?
Energy (1st Law) balance across burner
TURBOJET TRENDS: IN-CLASS EXAMPLE
Plot of Non-Dimensional Thrust and Specific Impulse for Maximum Thrust Condition
7
Heating Value of Fuel = 4.3x10 J/kg, Specific Heat Ratio = 1.4, T0=200K
10000
5
Max Non-Dim Thrust: Theta_t=6
Max Non-Dim Thrust: Theta_t=9
Max Thrust Isp: Theta_t=6
Max Thrust Isp: Theta_t=9
Maximum Specific Thrust
4
9000
8000
3.5
7000
3
6000
2.5
5000
2
4000
1.5
3000
1
2000
0.5
1000
0
0
0
0.5
1
1.5
Flight Mach Number
2
2.5
3
Specific Impulse, Maximum
Thrust, s
4.5
TURBOJET TRENDS: IN-CLASS EXAMPLE
(SEE INLET SLIDES FOR MORE DETAILS)
Plot of Thrust Normalized by Compressor Inlet Area and Ambient Pressure
vs. Flight Mach Number for Compressor Inlet Mach Number, M 2=0.5
30
Thrust Normalized by A2 and P0
Theta_t=6
Theta_t=9
25
20
15
10
5
0
0
0.5
1
1.5
Flight Mach Number
2
2.5
3
TURBOJET TRENDS: HOMEWORK #3, PART 1
Tt4 = 1600 K, pc = 25, T0 = 220 K
5.00
120%
4.50
100%
4.00
80%
3.00
2.50
60%
2.00
40%
1.50
Specific Thrust
Propulsive Efficiency
Thermal Efficiency
Overall Efficiency
1.00
0.50
20%
0.00
0%
0
0.5
1
1.5
Mach Number
2
2.5
3
Efficiency
Specific Thrust
3.50
TURBOJET TRENDS: HOMEWORK #3, PART 2a
Tt4 = 1400 K, T0 = 220 K, M0 = 0.85 and 1.2
3.00
90%
80%
2.50
2.00
60%
50%
1.50
40%
1.00
30%
Specific Thrust, M=0.85
Specific Thrust, M=1.2
Propulsive Efficiency, M=0.85
Thermal Efficiency, M=0.85
Overall Efficiency, M=0.85
Propulsive Efficiency, M=1.2
Thermal Efficiency, M=1.2
Overall Efficiency, M=1.2
0.50
20%
10%
0.00
0%
0
10
20
30
Compressor Pressure Ratio
40
50
Efficiency
Specific Thrust
70%
Specific Thrust
TURBOJET TRENDS: HOMEWORK #3, PART 2b
Tt4 = 1400 K and 1800 K, T0 = 220 K, M0 = 0.85
4
80%
3.5
70%
3
60%
2.5
50%
2
40%
1.5
30%
1
20%
Specific Thrust, Tt4=1400K
Specific Thrust, Tt4=1800 K
Propulsive Efficiency, Tt4=1400 K
Thermal Efficiency, Tt4=1400 K
Overall Efficiency, Tt4=1400 K
Propulsive Efficiency, Tt4=1800 K
Thermal Efficiency, Tt4=1800 K
Overall Efficiency, Tt4=1800 K
0.5
10%
0
0%
0
10
20
30
Compressor Pressure Ratio
40
50
TURBOFAN SUMMARY

T
2
 o c t  1  t

m ao
 1
  o c



2

  M o   





1

M
0
  1 0 f



 2

T
 0 f  1  M 0 
 1   

ma o
  1

 T

 m ao


Two streams:
Core and Fan Flow
Turbine power = compressor + fan power
Exhaust streams have same velocity: U6=U8


  1 2


2
t

  1   
  0  1  M 0 
  1  1 


 max




Maximum power, c selected
to maximize f
TURBOFAN TRENDS: IN-CLASS EXAMPLE
Non-Dimensional Thrust vs. Flight Mach Number
t=6, To=200 K (PW4000 Series,  ~ 5-6)
Higher  of interest in range of Mo < 1 and lower  of interest for supersonic transport
16
Bypass Ratio = 1
Bypass Ratio = 5
Bypass Ratio = 10
Bypass Ratio = 20
Non-Dimensional Thrust
14
12
10
8
6
4
2
0
0
0.5
1
1.5
2
Flight Mach Number, M0
2.5
3
TURBOFAN TRENDS: IN-CLASS EXAMPLE
Non-Dimensional Thrust vs. Flight Mach Number
t=6, To=200 K (PW4000 Series,  ~ 5-6)
Higher  of interest in range of Mo < 1 and lower  of interest for supersonic transport
16
Plot of Non-Dimensional Thrust and Specific Impulse for Maximum Thrust Condition
7
Heating Value of Fuel = 4.3x10 J/kg, Specific Heat Ratio = 1.4, T0=200K
10000
5
4.5
Maximum Specific Thrust
Non-Dimensional Thrust
4
12
10
9000
8000
3.5
7000
3
6000
2.5
5000
2
4000
1.5
3000
1
2000
0.5
1000
Bypass Ratio = 1
Bypass Ratio = 5
Bypass Ratio = 10
Bypass Ratio = 20
0
0
8
Specific Impulse, Maximum
Thrust, s
14
Max Non-Dim Thrust: Theta_t=6
Max Non-Dim Thrust: Theta_t=9
Max Thrust Isp: Theta_t=6
Max Thrust Isp: Theta_t=9
0
0.5
1
1.5
2
2.5
3
Flight Mach Number
Improvement over turbojet:
4 – 2.4 → 66% at Mach 1
8 – 3.3 → 142% at Mach 0
6
4
2
0
0
0.5
1
1.5
2
Flight Mach Number, M0
2.5
3
TURBOFAN TRENDS: IN-CLASS EXAMPLE
Propulsive Efficiency vs. Flight Mach Number
t=6, To=200 K
1
0.9
Propulsive Efficiency
0.8
0.7
0.6
0.5
0.4
0.3
Bypass Ratio = 1
Bypass Ratio = 5
Bypass Ratio = 10
Bypass Ratio = 20
0.2
0.1
0
0
0.5
1
1.5
2
Flight Mach Number, M0
2.5
3