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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 ηf,ηv– fuel conversion and volumetric

efficienciesηfm f QHV mf – fuel mass per cycle Torq = QHV– fuel heating value 2πnR nR – 1 for 2-stroke, 2 for 4-stroke engine

N – revolution per second

Power = Torq ⋅ 2πN VD – engine displacement ρa,0 – air density

Fmf = ( A)ηVρa,0VD

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

Exhaust-gas turbocharger for trucks 1.Compressor housing, 2. Compressor impeller, 3. Turbine housing, 4. Rotor, 5. Bearing housing, 6. inflowing exhaust gas, 7.

Charge-air pressure regulation with Out-flowing exhaust gas, 8. Atmospheric fresh wastegate on exhaust gas end. 1.Engine, air, 9. Pre-compressed fresh air, 10. Oil inlet, 2. Exhaust-gas turbochager, 3. Wastegate 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

W� η = W� ideal

c �Wactual

⎛ ′ ⎞

T W� ideal = m� cpT1⎜⎜

TT2

1 − 1⎟

⎟⎝ ⎠

γ−1′ T2 = ⎜⎜

⎛ P2 ⎟⎟⎞ γ

T1 ⎝ P1 ⎠

⎛ γ−1 ⎞ 1 ⎜⎛ P2 ⎞ γ ⎟

W� actual = m� cpT1⎜⎜⎜ ⎟⎟ − 1⎟ ηc ⎜⎜⎝ P1 ⎠ ⎟⎟

⎝ ⎠

W� actuals T2 = T1 + m� cp

1

2

m�

P1

P2

1

2’ 2

Ideal process

Actual process

Turbine: basic thermodynamicsTurbine efficiency ηt4

W� ηt = W�� actual

Wideal3

m� W� ideal = m� cpT3

⎛⎜⎜1−

TT4

3

′ ⎞⎟⎟

T ⎝ ⎠ γ−1

′ T4 = ⎜⎜⎛ P4

⎟⎟⎞ γ

T3 ⎝ P3 ⎠

⎛ γ−1 ⎞⎜ ⎛ P ⎞ γ ⎟

W� actual = ηtm� cpT3 ⎜⎜⎜1−

⎝⎜⎜ P3

4

⎠⎟⎟

⎟⎟⎟

⎝ ⎠

W� actual s T4 = T3 −

m� cp

P4

P3

4’

3

4

Ideal process

Actual process

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

• P2 = f(m� ,RT1,P1,N,D,μ, γ, geometric ratios) • Dimensional analysis:

– 7 dimensional variables → (7-3) = 4 dimensionless parameters (plus γ and geometric ratios)

⎛ P ⎞ N m� ⎜⎜ 2

⎟⎟ = f( , ,Re,γ, geometric ratios)⎝ P1 ⎠ γRT1 /D

⎜⎜⎛ P1

⎟⎟⎞

RT1D2

⎝ RT1 ⎠Velocity Velocity

Density High Re number flow → weak Re dependence For fixed geometry machinery and gas properties

⎛ P2 ⎞ f⎜⎛ N m� T1 ⎟

⎞⎜⎜ ⎟⎟ =

⎜⎝ P1 ⎠ ⎝ T1,

P1 ⎟⎠

Compressor Map

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)

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.00 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Pres

sure

ratio

"Corrected" Flow rate m .

T1/P1

T1

Surge l

imit

7250

4840

2650

4025 N/

6960

72%70%

65%

74%

75%

5550

60%

6070

6530

Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.

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 stall¾ reverse flow and violent flow rate surges

Turbine Map

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 andWatson)

oitar erusserP

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.00.5 1.0 1.5 2.0 2.5 3.0

.Flow rate m T03/P03

4000

3500

30002500

1500500

NT03

07. = STtπ

5.6

0.6

5.5

0.4

.50

Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.

Compressor Turbine Matching Exercise

• For simplicity, take away intercooler and wastegate

• Given engine brake power output ( ) and RPM, compressor map, turbine map, and engine map

WE �

1 4

• Find operating point, i.e. air flow ( ), fuel flow rate ( ) turbo-shaft revolution per second (N), compressor and turbine pressure ratios (πc and πt) etc.

mf �ma

�

mf �

WE �

Engine

C T

QL �

2 3

Compressor/ turbine/engine matching

solution

Compressor

Turbine

Pres

sure

ratio

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.00.5 1.0 1.5 2.0 2.5 3.0

2.8

2.6

Flow rate m .

T03/P03

4000

3500

30002500

1500500

NT03

.40

.50

.55

.60

.65

ηt TS

= .7

0

Procedure : 1. Guess πc; can get engine inlet conditions :

⎡ γ−1 ⎤

P2 = πcP1 T2 = T1 ⎢

⎢( )πc γ

−1⎥⎥

ηc ⎢ ⎥ ⎢ ⎥⎣ ⎦

2. Then engine volumetric efficiency calibration will give the air flow m� a that can be 'swallowed'

3. From m� a and πc , the compressor speed N can be obtained from the compressor map

4. The fuel flow rate m� f may be obtained from the engine map :

W� E = m� f LHV ηf (RPM,W� E,A/F) 5.Engine exhaust temperature T3 may be obtained from

energy balance (with known engine mech. eff. ηM)

(m� a + m� f )cpT3 = m� acpT2 + m� fLHV − W� E − Q� LηM 6. Guess πt , then get turbine speed Nt from turbine map 7.Determine turbine power from turbine efficiency on map

⎡ γ−1 ⎤ ⎢ ⎛ 1 ⎞ γ ⎥

W� t = ηt ⎢1− ⎜⎜ π ⎟⎟ ⎥

⎢ ⎝ t ⎠ ⎥ ⎣ ⎦

8.Iterate on the values of πc and πt until W� t = W� c and Nt = Nc

Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.00 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Pres

sure

ratio

"Corrected" Flow rate m .

T1/P1

T1

Surge l

imit

7250

4840

2650

4025 N/

6960

72%70%

65%

74%

75%

5550

60%

6070

6530

Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principlesand Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.

Compressor/ Engine/ Turbine Matching

Compressor characteristics, with airflow requirements of a four-stroke truck engine superimposed. (From “Principles and Performance in Diesel Engineering,” Ed. by Haddad and Watson)

• 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

C T

Inter-Cooler Wastegate

Engine

Surg

e lim

it

Constant lo

adCo

nstan

t spe

ed

0.70

0.72

0.67

0.65

0.60

0.55

01.0

1.5

2.0

2.5

3.0

1 2 3 4

P p/P

1

mp1

T1

Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Principles and Performance in Diesel Engineering. Chichester, England: Ellis Horwood, 1984.

Advanced turbocharger development

Electric assisted turbo-charging

• Concept– Put motor/ generator on

turbo-charger– reduce wastegate function

• Benefit – increase air flow at low

engine speed Engine– auxiliary electrical output

at part load

Motor/ Generator

Inter-Cooler

C T

Wastegate

Battery

Advanced turbocharger development

Electrical turbo-charger • 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

Generator

Inter-Cooler

Engine

C TMotor

Battery

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