3. Chapter 3. Otto and Diesel cyclescontents.kocw.net/KOCW/document/2015/chungnam/parksuhan/... ·...

에너지변환특론Advanced Energy Conversion

Chapter 3. Otto cycle and Diesel cycle

교수 박 수 한

Fall semester, 2014

CNU Engine Research Lab.

참고.

- http://www.youtube.com/playlist?list=PLX2gX-ftPVXXMDW2aoPCk7nM-58n7nW5M

2


3

Operation of IC Engines

How to make the analysis of the engine cycle much more manageable?

Simplify!

- Actual Cycles:• Mechanical cycle• Thermodynamics point of view, non-cyclic, open

cycle, quasi steady-flow• Variable composition (combustion) with gas

mixtures (Fuel, CO2, H2O, O2, N2)


4

Simplify to the Air Standard Cycle

Mechanical Cycle Thermodynamic Cycle

Fuel, Air CO2, N2, H2O

Air

Qin Qout


5

Air Standard Cycle

- Air-Standard Cycle:• Simplified and more manageable

• Determines important design parameters

• Reasonable accuracy, particularly w.r.t. sensitivity to design parameters

• Error involves


6

Air Standard Assumptions

- Working fluid is Air

- Mass of air (gas phase) is constant (actually variation up to 7%)

- Closed cycle

• Recalculated Air

• Heat Exchangers for heat rejection and addition

- No Internal Combustion


7

Air Standard Assumptions

- Ideal Processes• Constant pressure exhaust at 1 atms.• N/A cycles have constant pressure intake at 1 atms• Turbo/Supercharged cycles have constant pressure >

1 atms.- Compression and Expansion are Isentropic with

constant specific heats- Heating is at constant volume (SI), constant pressure

(CI), or both (high speed CI)- Cooling Heat Rejection at constant volume- All processes are reversible


8

Thermodynamics - review

- Ideal Gas:

- The First Law of Thermodynamics

- The Second Law of Thermodynamics

dTcdudTcdhRTPmRTPVRTPv

vp

,,,

wduq

Tqds


9

Thermodynamics – Work and Heat

- For a closed system:

v

P

w

1

2

s

T

q

1

2

2

1Pdvw

2

1Tdsq


10

Thermodynamics - symbols

sound of speed workspcific

/

heats speific,energy internal specific

enthapy specificdensity

cylinderin gas of massetemperatur

air ofconstant gasgas of volumespecific

cylinderin volumecylinderin presure gas

cw

cck

ccuh

mTRvVP

vp

vp

efficiency combustionpower

cycle onefor work ration compressio

fuel of valueheatingratefer heat trans

cycle onefor fer heat transmassunit per ratefer heat trans

cycle onefor massunit per fer heat transrate flow mass

ratio fuelairAF

c

c

HV

W

Wr

QQ

Qqqm

gases all of mixtureexhaust ex fuel,air,

:

mfa

Subscripts


11

Polytropic Process

constnPv

1

2

1

1

1

2

1

2

2

1

1

2

nnn

n

vv

PP

TT

vv

PP

RTPv from

polytropic1const)( isochoricconst)( isentropic const)(T isothermal 1const)( isobaric 0

knvnskn

nPn


12

Polytropic Process

v

P(isobaric) 0n

l)(isotherma 1n

)(isochoric n

c)(isentropi kn

s

T

n

0n

kn

1n


13

Thermodynamics - review

- Isentropic process:

- Speed of sound

system closedfor )1(

)()1(

)(constant

constant constant

12112221

)1(

1

kTTR

kvPvP

w

TP

TvPv

kk

k

k

kRTc


14

Thermodynamic Properties - Air

- Engine operating conditions:

- Standard conditions:

K-kJ/kg 287.035.1/

R-BTU/lbm 196.0K-kJ/kg 821.0R-BTU/lbm 265.0K-kJ/kg 108.1

vp

vp

v

p

ccRcck

cc

K-kJ/kg 287.0

4.1/R-BTU/lbm 172.0K-kJ/kg 718.0

R-BTU/lbm 240.0K-kJ/kg 005.1

vp

vp

v

p

ccR

cckc

c


15

Otto Cycle

Real

Otto

스파크 점화 기관의 공기표준 사이클 정적 사이클 (constant volume cycle), 일정체적 하에서 연소 1876년 Nikolaus August Otto, 독일


참고. 상사점 (Top Dead Center, TDC):

피스톤이 크랭크축으로부터 가장 먼 위치(TDC 이전 –BTDC, TDC 이후-ATDC)

하사점 (Bottom Dead Center, BDC):피스톤이 크랭크축으로부터 가장 가까운 위치(BDC 이전 –BTDC, BDC 이후-ATDC)

보어 (Bore): 실린더의 직경 또는 피스톤면의 직경

행정 (Stroke): 피스톤이 상사점에서 하사점 또는하사점에서 상사점으로 움직인 거리

간극체적 (Clearance Volume):피스톤이 상사점에 있을 때 연소실의 최소체적

배기량 (Displacement), 행정체적(Displacement Volume): 피스톤이 상사점과 하사점을 움직이면서 배제하는 체적

TDC

BDC

l

B

L

s

a

Vc

Vd

Vc : clearance volume

Vd : displaced or swept volume

B : cylinder bore

L : piston stroke

l : connecting rod length

a : crank radius

: crank angle


참고.

압축비 (compression ratio, rc)

C

CdC V

VVr

volumecylinderminimumvolumecylinderMaximum

SI engine : rc = 8 ~ 12 CI engine : rc = 12 ~ 24

보어 행정비 (bore to stroke ratio, Rbs)

LBR bs

Small & medium size engine = 0.8 ~ 1.2 Large slow speed CI engine = ~ 0.5 B L : under square engine, 저속 대형엔진 B = L : square engine, 소형·중형 승용엔진 B L : over square engine, 소형엔진

TDC

BDC

l

B

L

s

a

Vc

Vd


Otto Cycle 냉각손실 : 압축 후반, 연소 중, 팽창과정에서 연소실 내의작동유체와 연소실 벽면 등의 온도차로 인해 발생하는열손실

시간손실 : 이론 사이클에서는 상사점에서 순간적으로 연소가 일어나지만, 실제로는 그렇지 못하다. 연소에 필요한 시간은 대략크랭크 각도로 40~60CA가 필요하다. 이처럼 정적연소가아니라서 생기는 열손실을 말한다.

펌프손실 : 공기가 흡입되는 과정에서 이동통로의 조도, 공기청정기,인젝터, 스로틀밸브 등을 거치면서 손실되는 양.스로틀밸브에 의한 손실량이 가장 크다.

배기손실 : 배출가스를 통한 열손실


19

Air Standard Otto Cycle

1-2: Isentropic Compression2-3: Constant Volume Heat Addition3-4: Isentropic Expansion4-5: Constant Volume Heat Rejection5-6: Exhaust at 1 atm6-1: Intake at 1 atm


20

Otto Cycle: P-v & T-s Diagrams


21

Otto Cycle Thermodynamic Analysis at WOT

(6-1: Constant-pressure intake)

1-2: Isentropic compression

2-3: Constant volume heat addition

3-4: Isentropic expansion

4-5: Constant volume heat rejection

(5-6: Constant-pressure exhaust)


22

Isentropic Compression (1-2) Process

)()(

1

0

)(

)(

const.

21

21

11222

121

21

12

112

11

1

2

112

TTcuu

kvPvP

Pdvw

q

rPvv

PP

rTvvTT

Pv

v

kc

k

kc

k

k


23

Constant Volume Heat Addition (2-3) Process

)()(

0

2332

232332

32

23

TTcmQQTTcuuqq

wvv

vmin

vin


24

Isentropic Expansion (3-4) Process

)()(

1

0

1

1

const.

43

43

33444

343

43

34

334

1

3

1

4

334

TTcuu

kvPvP

Pdvw

qr

Pvv

PP

rT

vv

TT

Pv

v

k

c

k

k

c

k

k


25

Constant Volume Heat Rejection (4-1) Process

)()(

0

4154

4141

4554

1454

145

TTcmQQTTcuu

uuqqww

vvv

vmout

v

out


26

Thermodynamic Properties

crvv

vv

vvvv

3

4

2

1

3214 and

2

3

1

4

TT

TT

1-2 & 3-4 processes are isentropic


27

Indicated Thermal Efficiency of Otto Cycle

1

2

1

23

14

2

1

23

14

23

14

in

out

in

netOTTO

/11

11)/(1)/(1

)()(

1)()(

1

1

kc

v

v

t

r

TT

TTTT

TT

TTTT

TTcTTcqq

qw

• k = 1.3 ~ 1.4 and rc > 1 Thermal efficiency increases with compression ratio

• Higher compression ratio Higher Efficiency and Higher Power


28

Example Problem 3-1

- Given• 4-Cylinder, 2.5L, SI engine, WOT, 4-Stroke• Air standard Otto cycle, 3000 RPM• Compression ratio = 8.6:1, mech. eff. = 86 %• S/B = 1.025, AF = 15• iso-octane: HV = 44,300 kJ/kg, comb. eff. = 100 %• Initial conditions: P1 = 100 kPa, T1 = 60°C• Exhaust residual: 4%

- Calculate parameters for one cylinder- Do a complete thermodynamic analysis


29

Example Problem 3-1


30

Real Air-Fuel Engine Cycles

1. Real engines operate on an open cycle• Changing gas composition via combustion• Changing mass for CI cycle via fuel addition

2. Properties differ from air• Fuel & combustion products• Specific heat varies by up to 30 % (300 K to 3000 K)

3. Heat losses during the cycle (up to 12 %)


31


4. Combustion requires finite time (~ 6 %)• 30 to 60 degrees of crank rotation• More compression work, Less expansion work

Finite time combustion losses


32


5. Blowdown process requires a finite time (~2 %)• Exhaust valve opens bBDC• Work loss at the end of power stroke

Early Exhaust Valve Opening Loss


33


6. Intake valve closes aBDC• Improves volumetric efficiency• Momentum of entering air continues flow through

intake valve after piston starts up• Reduces effective compression ratio• Reduces T and P due to compression

7. Finite valve opening and closing times• To assure the fully opened valves at TDC• Valve overlap at TDC

8. Error in LHV (less LHV at higher temperatures)


34

Real Air-Fuel Cycle vs. Ideal Cycle

- Errors due to the differences between real air-fuel cycles and ideal air standard cycles

- Some errors tend to cancel, e.g., specific heats- The efficiency of real cycle efficiency is less than

that of air standard cycle

OTTOactual 85.0 tt


35

SI Engine Cycle at Part Throttle

Negative pump work

– P1 is lower than Po

?


36

SI Engine Cycle with T/C or S/C

Positive pump work

– P1 is higher than Po

?


37

Part Throttle, T/C or S/C

– Part throttled: Negative– T/C or S/C: Positive

dexipump VPPW )(net


38

Exhaust Process

Exhaust stroke

Blowdown

EVO


39

Real Exhaust Blowdown P-v

KEhh so


40

Real Exhaust Blowdown T-s

KEhh so


41

Real Exhaust Blowdown Equations

oex

kk

okk

ex

PPP

PP

TPP

TT

7

)1(

44

)1(

447

where,

-Approximated by Isentropic Process


42

Exhaust Residual

- Residual exhaust gas in clearance volume, Vc, at TDC starting intake stroke

m

exr mm

x

Mass of exhaust gas carried into the next cycle

Mass of gas mixture within the cylinder for the entire cycle


43

Calculating Exhaust Residual

oex

k

oex

k

PP

PP

vv

PP

PP

PP

vv

PP

33

3

7

7

3

44

4

7

7

4

7

2

7

6

7

1

7

557

vV

vV

m

vV

vV

vV

m

ex

ex


44


4

4

7

2

7

7

7

2

1

/

PP

TT

rx

VV

vV

vV

mm

x

ex

excr

m

exr

7

2

7

6

7

7

2

2

1

1

andvV

vV

m

vV

vV

vV

m

ex

m


45


arexrm TxTxT )1()( 1

mmaaexex hmhmhm

aex TTT 7 where,


46

Exhaust Residuals in Real Engines

- Exhaust residual amount• SI engine at WOT: 3~7 %• SI engine at part throttle: up to 20 %• CI engine: Generally less than for SI engine

- The effect of exhaust residual• Less heat addition • Dilution Max. temperature decreases• Volumetric efficiency decreases


47

Diesel Cycle

Real

Diesel


48

Air Standard Diesel Cycle

1-2: Isentropic Compression2-3: Constant Pressure Heat Addition3-4: Isentropic Expansion4-5: Constant Volume Heat Rejection5-6: Exhaust at 1 atm6-1: Intake at 1 atm


49

Diesel Cycle: P-v & T-s Diagrams


50

Diesel Cycle Thermodynamic Analysis



2-3: Constant pressure heat addition





51

Isentropic Compression (1-2) Process

)()(

1

0

)(

)(

const.

21

21

11222

121

21

12

112

11

1

2

112

TTcuu

kvPvP

Pdvw

q

rPvv

PP

rTvvTT

Pv

v

kc

k

kc

k

k


52

Constant Pressure Heat Addition (2-3) Process

)()(

)(

)(1)AF(

)(

232233232

232332

23

2332

vvPuuqw

hhTTcqq

TTcQ

TTcmQmQQ

pin

pcHV

pmcHVfin

– Cutoff ratio (Load ratio)

2

3

2

3

2

3

TT

vv

VV

• Heat addition period

cr 1


53

Cut-off Ratio for Indicated Thermal Efficiency


54

Isentropic Expansion (3-4) Process

)()(

1

0

1

1

const.

43

43

33444

343

43

34

334

1

3

1

4

334

TTcuu

kvPvPPdvw

qr

Pvv

PP

rT

vv

TT

Pv

v

k

c

k

k

c

k

k

2

1

3

4:Notevv

vv


55

Constant Volume Heat Rejection (4-1) Process

)()(

0

4154

4141

4554

1454

145

TTcmQQTTcuu

uuqqww

vvv

vmout

v

out


56

)1()1(11

)()(11

1

DIESEL

23

14

in

out

in

netDIESEL

kr

TTcTTc

qq

qw

kk

ct

p

vt

Indicated Thermal Efficiency of Diesel Cycle

• β 1, then (βk -1) 0

• β rc, then state 3 state 4

OTTODIESEL tt

luelowest va the todecreases DIESELt


57

Cut-off Ratios and Thermal Efficiencies


58

Dual Cycle

Real

Dual

Pre-mixed CombustionNon-premixed Combustion


59

Air Standard Dual Cycle

1-2: Isentropic Compression2-x: Constant Volume Heat Additionx-3: Constant Pressure Heat Addition3-4: Isentropic Expansion4-5: Constant Volume Heat Rejection5-6: Exhaust at 1 atm6-1: Intake at 1 atm


60

Dual Cycle: P-v & T-s Diagrams


61

Dual Cycle Thermodynamic Analysis



2-x: Constant volume heat addition

x-3: Constant pressure heat addition





62

Constant Volume Heat Addition (2-x) Process

222

23232

2

2

)(

)()()(0

uuTTcq

TTcmmTTcmQw

vvv

xxvx

vfavmx

x

TDCx

– Pressure ratio

1

3

22

3

2

1PP

rTT

PP

PP

k

c

xx


63

Constant Pressure Heat Addition (x-3) Process

)()(

)(

)(1)AF(

)(

232233232

232332

23

2332

vvPuuqw

hhTTcqq

TTcQ

TTcmQmQQ

pin

pcHV

pmcHVfin

– Cutoff ratio (Load ratio)

xx TT

VV

vv

vv 3

2

3

2

33

cr 1Note:


64

Indicated Thermal Efficiency of Dual Cycle

1)1()1(11

)()()(11

1

DUAL

32

14

in

out

in

netDUAL

kr

TTcTTcTTc

qq

qw

kk

ct

xpxv

vt

− Heat in)()( 3232 xxxxin hhuuqqq

− Thermal efficiency


65

Comparison of Otto, Diesel, and Dual Cycles

α 1:dual cycle Diesel cycle

β 1: dual cycle Otto cycle

1)1()1(11

1

DUAL

kr

kk

ct


66

Comparison of Cycles – Fixed rc

DIESELDUALOTTO ttt


67

Comparison of Cycles – Fixed Pmax

OTTODUALDIESEL ttt


68

Atkinson Cycle


69

Miller Cycle

N/A T/C

IVC


70

Comparison of Miller Cycle and Otto Cycle


71

New Design and New Product

- What is all about the design in mechanical engineering?• Optimization with constraints including

performance, cost, endurance, operation etc.

- How can the new technology/product be accepted by customers?• No sacrifice on old functions• Additional functions, convenience, price

cuts etc.


72

New Design and New Product

- How can it be achieved?• Advances in many fields such as

manufacturing, new technology, electronics, control, materials etc.

- New constraints• Environmental• Economy• Life style . . .

3. Chapter 3. Otto and Diesel cyclescontents.kocw.net/KOCW/document/2015/chungnam/parksuhan/... ·...

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Transcript of 3. Chapter 3. Otto and Diesel cyclescontents.kocw.net/KOCW/document/2015/chungnam/parksuhan/... ·...