6 Supercharging and Turbocharging
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66TOPICTOPIC
Supercharging Supercharging and and
turbochargingturbochargingChapter 6Chapter 6
Section 6.8Section 6.8
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• Term supercharging refers to increasing the air (or mixture) density by increasing its pressure prior to entering the engine cylinder
• Three basic methods of supercharging:– mechanical supercharging– turbocharging– pressure wave supercharging
• We consider operating characteristics of turbines and compressors
Methods of power boostingMethods of power boosting
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Mechanical superchargingMechanical supercharging
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TurbochargingTurbocharging
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Engine-driven compressor and Engine-driven compressor and turbochargerturbocharger
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Two-stage turbochargingTwo-stage turbocharging
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Turbocharging with turbocompoundingTurbocharging with turbocompounding
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Turbocharger with intercoolerTurbocharger with intercooler
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Centrifugal compressorCentrifugal compressor
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Basic relationshipsBasic relationships
Energy balance
Stagnation properties
Work transfer rate
2 2
2 2out in
C CQ W m h gz h gz
2 2 10
0 0 02 2, ,
p
TC Ch h T T p pc T
0 0, ,out inW m h h
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Compressor isentropic efficiencyCompressor isentropic efficiency
reversible power requirementactual power requirementC
Total-to-total isentropic efficiency
With constant cp
02 01
02 01
sCTT
h hh h
1
02 01
02 01
11CTT
p pT T
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Compressor isentropic efficiencyCompressor isentropic efficiency
Since kinetic energy of the gas leaving the compressor is not usually recovered, a more realistic definition of efficiency is based on exit static conditions
This is termed the total-to-static isentropic efficiency
1
0101
02 01
2
2 01
2
0
11C
sTS
pTT T T T
pT
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Power required to drive compressorPower required to drive compressorThermodynamic power requirement:
in terms of total-to-total isentropic efficiency
in terms of total-to-static isentropic efficiency
Power required to drive compressor
1
01 0202 01
01
1,,
i p iC i p i
CTT
m c T pW m c T Tp
1
0102 01
0
2
1
1,,
C
i p iC i i
Sp
T
m c TW m c T T
pp
,C
C Dm
WW
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Schematic of radial flow turbineSchematic of radial flow turbine
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Turbine isentropic efficiencyTurbine isentropic efficiency
Total-to-total isentropic efficiency
With constant cp
Note: cp for exhaust gas may vary significantly with temperature
actual power outputreversible power outputT
03 04
03 04TTT
s
h hh h
04 031
04 03
1
1TTT
T T
p p
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Turbine isentropic efficiencyTurbine isentropic efficiency
Since kinetic energy at the exit of the turbocharger is usually wasted, a more realistic definition of efficiency is based on exit static conditions
This is termed the total-to-static isentropic efficiency
04 0303 041
0303
44
1
1sTTS
T TT TT pT p
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Power delivered by the turbinePower delivered by the turbineIn terms of total-to-total isentropic efficiency
In terms of total-to-static isentropic efficiency
With a turbocharger, turbine is mechanically linked to compressor. Hence, at constant turbocharger speed
where m is the mechanical efficiency of turbocharger. It is usually combined with the turbine efficiency
1
03 04 031,
e
eT e p e TTTW m c T p p
1
0 0343 1,
e
eTTST e p eW m c T pp
C m TW W
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Dimensionless groups used to describe Dimensionless groups used to describe turbocharger performanceturbocharger performance
Most important dependent variables are mass flow rate , isentropic efficiency , and temperature difference across the machine ΔT0
Each of these are functions of independent variables
By dimensional analysis, these eight independent variables can be reduced to four dimensionless groups
m
0 0 0 0in out inm T f p p T N D R , , ,, , , , , , , , ,
0 002
0 0 00
, ,
, , ,,
, , , , ,in out
in in inin
m T pT Nfp T pT
R D mD DR
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Dimensionless groups used to describe Dimensionless groups used to describe turbocharger performanceturbocharger performance
Reynolds number has little effect on performance and is fixed by the gas, therefore they can be omitted
For a particular device, dimensions are fixed and value of R is fixed. So, it is convenient to plot
is corrected mass flow, is corrected speed
0 00
0 0 00
, ,
, , ,,
, , ,in out
in in inin
m T pT Nfp T pT
0,in
NT
0
0
,
,
in
in
m Tp
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Schematic of compressor operating mapSchematic of compressor operating map
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Centrifugal compressor operating mapCentrifugal compressor operating map
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Radial turbine performance mapRadial turbine performance map
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Radial turbine performance mapRadial turbine performance map
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Characteristics Characteristics of turbochargerof turbocharger
Steady-state turbocharger operating lines plotted as constant T03 /T01 lines on compressor map.
dash-dot-dash line is for p03 = p02
to the left p03 < p02
to the right p03 > p02
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That’s all for That’s all for todaytoday
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For those who For those who wants to know wants to know
more…more…
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Centrifugal compressorCentrifugal compressor
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Positive displacement compressorsPositive displacement compressors
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h-s diagram for h-s diagram for flow through flow through centrifugal centrifugal
compressorcompressor
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Velocity diagrams Velocity diagrams
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Schematic of radial flow turbineSchematic of radial flow turbine
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h-s and velocity h-s and velocity diagrams for radial diagrams for radial
turbineturbine