6 Supercharging and Turbocharging

66TOPICTOPIC

Supercharging Supercharging and and

turbochargingturbochargingChapter 6Chapter 6

Section 6.8Section 6.8

• 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

Mechanical superchargingMechanical supercharging

TurbochargingTurbocharging

Engine-driven compressor and Engine-driven compressor and turbochargerturbocharger

Two-stage turbochargingTwo-stage turbocharging

Turbocharging with turbocompoundingTurbocharging with turbocompounding

Turbocharger with intercoolerTurbocharger with intercooler

Centrifugal compressorCentrifugal compressor

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

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

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

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

Schematic of radial flow turbineSchematic of radial flow turbine

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

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

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

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

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

Schematic of compressor operating mapSchematic of compressor operating map

Centrifugal compressor operating mapCentrifugal compressor operating map

Radial turbine performance mapRadial turbine performance map

Radial turbine performance mapRadial turbine performance map

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

That’s all for That’s all for todaytoday

For those who For those who wants to know wants to know

more…more…

Centrifugal compressorCentrifugal compressor

Positive displacement compressorsPositive displacement compressors

h-s diagram for h-s diagram for flow through flow through centrifugal centrifugal

compressorcompressor

Velocity diagrams Velocity diagrams

Schematic of radial flow turbineSchematic of radial flow turbine

h-s and velocity h-s and velocity diagrams for radial diagrams for radial

turbineturbine