MIT OpenCourseWare
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2.61 Internal Combustion Engines
Spring 2008
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Engine Turbo/Super Charging
Super and Turbo-charging
Why super/ turbo-charging?
• Fuel burned per cycle in an IC engine is air limited
– (F/A)stoich = 1/14.6
Torq =
ηf m f QHV
2πnR
Power = Torq ⋅ 2πN
m f = F η V ρa,0 VD
A
( )
ηf,ηv– fuel conversion and volumetric
efficiencies
mf – fuel mass per cycle
QHV– fuel heating value
nR – 1 for 2-stroke, 2 for 4-stroke engine
N – revolution per second
VD – engine displacement
ρa,0 – air density
Super/turbo-charging: increase air density
Super- and Turbo- Charging
Purpose: To increase the charge density
• Supercharge: compressor powered by engine output
– No turbo-lag
– Does not impact exhaust treatment
– Fuel consumption penalty
• Turbo-charge: compressor powered by exhaust turbine
– Uses ‘wasted’ exhaust energy
– Turbo- lag problem
– Affects exhaust treatment
• Intercooler
– Increase charge density (hence output power) by cooling the
charge
– Lowers NOx emissions
Charge-air pressure regulation with
wastegate on exhaust gas end. 1.Engine,
2. Exhaust-gas turbochager, 3. Wastegate
Exhaust-gas turbocharger for trucks
1.Compressor housing, 2. Compressor
impeller, 3. Turbine housing, 4. Rotor, 5.
Bearing housing, 6. inflowing exhaust gas, 7.
Out-flowing exhaust gas, 8. Atmospheric fresh
air, 9. Pre-compressed fresh air, 10. Oil inlet,
11. Oil return
Images removed due to copyright restrictions. Please see illustrations of "Charge-air Pressure Regulation with Wastegate on Exhaust
Gas End", and "Exhaust-gas Turbocharger for Trucks." In the Bosch Automotive Handbook. London, England: John Wiley & Sons, 2004.
From Bosch Automotive Handbook
Compressor: basic thermodynamics
Compressor efficiency ηc
2
W
ideal
ηc =
W
W
m
actual
1
⎞
⎛T ′
2
⎜
c p T1
Wideal = m
− 1⎟
⎟
⎜ T1
⎝
⎠
T
P2
Ideal
process
2’
1
2
γ−1
T2′ ⎛ P2 ⎞ γ
= ⎜⎜ ⎟⎟
T1 ⎝ P1 ⎠
P1
Actual
process
s
γ−1
⎛
⎞
⎜
⎟
γ
⎛
⎞
P
1
2
c p T1⎜ ⎜ ⎟
W
m
− 1⎟
actual =
⎜
⎟
ηc
⎜⎜ ⎝ P1 ⎠
⎟⎟
⎝
⎠
W
T2 = T1 + actual
cp
m
Turbine: basic thermodynamics
Turbine efficiency ηt
4
W
ηt = actual
W
W
3
ideal
⎛ T ′⎞
⎜1− 4 ⎟
W
=
m
c
T
ideal
p 3⎜
T3 ⎟
⎝
⎠
m
T
P3
γ−1
⎛ P4 ⎞ γ
⎜ ⎟
T4′
=⎜ ⎟
T3 ⎝ P3 ⎠
Ideal
process
P4
3
4
4’
Actual
process
s
γ−1 ⎞
⎛
⎜ ⎛P ⎞ γ ⎟
c p T3 ⎜1− ⎜ 4 ⎟
⎟
Wactual = ηt m
⎜
⎟
⎟⎟
⎜⎜ ⎝ P3 ⎠
⎝
⎠
W
T4 = T3 − actual
cp
m
Properties of Turbochargers
• Power transfer between fluid and shaft ∝ RPM3
– Typically operate at ~ 60K to 120K RPM
• RPM limited by centrifugal stress: usually tip
velocity is approximately sonic
• Flow devices, sensitive to boundary layer (BL)
behavior
– Compressor: BL under unfavorable gradient
– Turbine: BL under favorable gradient
Typical super/turbo-charged engine parameters
• Peak compressor pressure ratio ≈ 3.5
• BMEP up to 22 bar
• Limits:
– compressor aerodynamics
– cylinder peak pressure
– NOx emissions
Compressor/Turbine Characteristics
• Delivered pressure P2
,RT ,P ,N,D,μ, γ, geometric ratios)
• P2 = f(m
1 1
• Dimensional analysis:
– 7 dimensional variables → (7-3) = 4 dimensionless parameters
(plus γ and geometric ratios)
⎛ P2 ⎞
N
m
⎜⎜ ⎟⎟ = f(
,
,Re, γ, geometric ratios)
γRT1 / D ⎛ P1 ⎞
⎝ P1 ⎠
⎜⎜
⎟⎟ RT1D 2
⎝ RT1 ⎠
Velocity
Density
Velocity
High Re number flow →weak Re dependence
For fixed geometry machinery and gas properties
T1 ⎞
⎛ P2 ⎞ ⎛⎜ N m
⎟
⎜⎜ ⎟⎟ = f
,
P1 ⎟
⎝ P1 ⎠ ⎜
⎝ T1
⎠
Compressor Map
3.4
7250
3.2
6960
3.0
72%
70%
2.6
2.0
65%
60%
it
2.4
2.2
6530
74%
Su
rg
el
im
Pressure ratio
2.8
75%
6070
1.8
5550
1.6
4840
1.4
4025 N/ T1
1.2
1.0
2650
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
.
"Corrected" Flow rate m T1/P1
Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering.
Chichester, England: Ellis Horwood, 1984.
T1= inlet temperature (K); P1= inlet pressure (bar); N = rev. per min.; m = mass flow rate (kg/s)
(From “Principles and Performance in Diesel Engineering,” Ed. by Haddad and Watson)
Compressor stall and surge
• Stall
– Happens when incident flow angle is too large
(large Vθ/Vx)
– Stall causes flow blockage
• Surge
– Flow inertia/resistance, and compression system
internal volume comprise a LRC resonance system
– Oscillatory flow behave when flow blockage occurs
because of compressor stall
¾ reverse flow and violent flow rate surges
Turbine Map
2.8
2.6
π tTS = .70
2.4
2.0
1.8
.65
Pressure ratio
2.2
1.6
.6
0
4000
3000
2500
.40
1.2
1.0
3500
5
N
T03
1500
500
0.5
1.0
.5
1.4
.50
1.5
2.0
.
Flow rate m T03/P03
2.5
3.0
Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering.
Chichester, England: Ellis Horwood, 1984.
T03=Turbine inlet temperature(K); P03 = Turbine inlet pressure(bar); P4= Turbine outlet
pressure(bar); N = rev. per min.; m = mass flow rate (kg/s)
(From “Principles and Performance in Diesel Engineering,” Ed. by Haddad and
Watson)
Compressor Turbine Matching Exercise
• For simplicity, take away
intercooler and wastegate
• Given engine brake power
) and RPM,
output (W
E
compressor map, turbine map,
and engine map
• Find operating point, i.e. air
flow ( m a ), fuel flow rate ( m f )
turbo-shaft revolution per
second (N), compressor and
turbine pressure ratios (πc and
πt) etc.
1
4
T
C
2
m f
3
Engine
W
E
Q
L
Compressor/
turbine/engine matching
solution
Procedure :
1. Guess πc ; can get engine inlet conditions :
γ−1
⎡
⎤
γ
⎥
T1 ⎢
P2 = πc P1
T2 =
− 1⎥
⎢(πc )
ηc ⎢
⎥
⎢⎣
⎥⎦
2. Then engine volumetric efficiency calibration
a that can be ' swallowed'
will give the air flow m
Compressor
3.4
7250
3.2
6960
3.0
72%
70%
2.6
6530
74%
2.4
it
65%
60%
rg
e
lim
2.2
2.0
75%
Su
Pressure ratio
2.8
a and πc , the compressor speed N can be
3. From m
6070
1.8
5550
1.6
obtained from the compressor map
f may be obtained from the
4. The fuel flow rate m
4840
1.4
4025 N/ T1
1.2
1.0
2650
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
.
"Corrected" Flow rate m T1/P1
Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles
and Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.
2.8
2.6
2.4
η tTS = .70
6. Guess π t , then get turbine speed Nt from turbine map
7. Determine turbine power from turbine efficiency on map
⎡
= η ⎢1− ⎛⎜ 1
W
t
t⎢
⎜
⎢ ⎝ πt
⎣
2.0
.65
1.8
4000
.60
1.0
3000
1500
500
0.5
1.0
⎟
⎟
⎠
⎥
⎥
⎦
=W
and N = N
8.Iterate on the values of πc and π t until W
t
c
t
c
3500
2500
1.2
γ−1 ⎤
⎞ γ ⎥
.5
5
N
T03
.4 0
Pressure ratio
2.2
1.4
5. Engine exhaust temperature T3 may be obtained from
energy balance (with known engine mech. eff. ηM )
a +m
f )c p T3 = m
a c p T2 + m
f LHV − WE − Q
(m
L
ηM
Turbine
1.6
engine map :
=m
,A/F)
f LHV ηf (RPM,W
W
E
E
.50
1.5
2.0
.
Flow rate m T03/P03
2.5
3.0
Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering.
Chichester, England: Ellis Horwood, 1984.
Compressor/ Engine/ Turbine Matching
0.65
3.0
0.67
2.5
0.60
0.70
0.55
ns
ad
Co
nst
ant
Co
t lo
tan
sp e
ed
it
el
im
2.0
Su
rg
Pp/P1
0.72
• Mass flows through compressor, engine,
turbine and wastegate have to be
consistent
• Turbine inlet temperature consistent with
fuel flow and engine power output
• Turbine supplies compressor work
• Turbine and compressor at same speed
1.5
T
C
1.0
0
1
2
3
4
m T1
p1
Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David,
and Watson, N. Principles and Performance in Diesel Engineering.
Chichester, England: Ellis Horwood, 1984.
Compressor characteristics, with airflow
requirements of a four-stroke truck engine
superimposed.
(From “Principles and Performance in Diesel
Engineering,” Ed. by Haddad and Watson)
Inter-
Cooler
Wastegate
Engine
Advanced turbocharger development
Electric assisted turbo-charging
• Concept
– Put motor/ generator on turbo-charger
– reduce wastegate function
• Benefit
InterCooler
– increase air flow at low engine speed
– auxiliary electrical output at part load
Motor/
Generator
T
C
Wastegate
Engine
Battery
Advanced turbocharger development
Electrical turbo-charger
Battery
• Concept
– turbine drives generator;
compressor driven by motor
• Benefit
– decoupling of turbine and
compressor map, hence much more
freedom in performance optimization
– Auxiliary power output
– do not need wastegate; no turbo-lag
C
Motor
InterCooler
Engine
T
Generator
Advanced turbocharger development
Challenges
• Interaction of turbo-charging system with
exhaust treatment and emissions
– Especially severe in light-duty diesel market
because of low exhaust temperature
• Cost