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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