The Effects of EGR and Its Constituents on the

The Effects of EGR and Its Constituents on the Autoignition of Single- and Two-Stage Fuels

John Dec, Magnus Sjöberg, and Wontae Hwang Sandia National Laboratories

13th Diesel Engine-Efficiency and Emissions Research Conference August 13 – 16, 2007, Detroit, MI

Sponsors: U.S. DOE, Office of FreedomCAR & Vehicle Technologies

Program Manager: Gurpreet Singh

Saudi Aramco Oil Company Program Manager: Ahmar Ghauri

Introduction

z Advanced LTC diesel and HCCI engines can provide both high efficiencies and very low emissions of NOX and PM.

z Nearly all LTC and HCCI strategies use high levels of EGR/residuals. 1.Reduce peak combustion temperatures to control NOX.

2.Delay the onset of ignition. ⇒Controlling combustion phasing. ⇒For HCCI-like diesel, it allows more time for premixing before ignition.

3. For gasoline HCCI, hot residuals are used to promote autoignition, and combined with cooled EGR, to control combustion phasing and NOX.

z Although EGR is widely used, its thermodynamic and chemical effects on autoignition are only understood in general terms.

z Objective: Provide a fundamental understanding of how EGR and its constituents affect the autoignition of single- and two-stage ignition fuels. – Conduct well-controlled experiments in an HCCI engine.

HCCI Engine and Subsystems

Only premixed fueling is used for this study.

Cummins B 0.98 liter / cyl.

CR = 14 piston

EGR Systems z Real EGR z Simulated EGR &

EGR constituents

Baseline Points40

330 335 340 345 350 355 360 365 370 Crank Angle [°CA]

iso-Octane Gasoline PRF80 PRF60, 29% CSP

CA50 = 368°CA

PRF60, 29% CSP PRF80 Gasoline iso-Octane

Baseline Points

EGR

C/F = 37.8 (φ = 0.40) CA50 = 368°CA

Single-stage Ignition Two-stage

Ignition (LTHR)

No EGR ≈29% CSP

Gasoline

TDC

z Establish baseline operating points for all four fuels.Î CA50 = 368°CA (8° aTDC)

35

30

25

20

15z Study the retarding effects of EGR 10

and its constituents. 1200

1100

z Iso-octane and gasoline exhibit single-stage autoig. at these conds.

1000

900

800

z PRF80 and PRF60 have two-stage 700

autoignition with LTHR. – Similar to diesel fuel (CN = 21 & 30).

180

z Baseline conditions have no EGR except for PRF60 ⇒ 29% CSP.

Operating Conditions Pin = 100 kPa (naturally asp.)

1200 rpm. C/F-mass ratio = 37.8, (φ = 0.40 without EGR.)

160

140

120

100

80

60

40

Pres

sure

[bar

]M

ass-

Ave

rage

d Te

mp.

[K]

Req

uire

d In

take

Tem

p. [°

C]

50 60 70 80 90 100PRF Number, Anti-knock Index

Composition of EGR (CSP) z Consider complete combustion of iso-octane in "air".

(Argon and atmospheric CO2 lumped with N2.)

z C8H18 + 12.5 (O2 + 3.773 N2) ⇒ 8 CO2 + 9 H2O + 47.16 N2.

z For complete combustion with φ = 1, the gas composition (excl. fuel) changes from air: 21% O2 & 79% N2 (mole basis).

z to: 12.5% CO2, 14.0% H2O and 73.5% N2 for wet exhaust (CSP).

z to: 14.5% CO2 and 85.5% N2 for dry exhaust (Dry CSP).

z These gas compositions will be referred to as: Complete Stoichiometric Products Î CSP & Dry CSP.

Thermodynamic Effects of EGR (CSP)

z EGR influences the compressed-gas temperature. – Heat-capacities of CSP & individual components are different from that of air.

z Start with motored operation. ⇒ Examine effects of CSP & its components on the mass-averaged temperature near TDC.

z Displace air while maintaining Pin = 100 kPa. 900

890

z Trends can be explained by 880

changes in specific heat (Cv). Te

mpe

ratu

re @

350

°CA

[K]

870

– CO2 has highest Cv (mole bs.). 860

– H2O high Cv. 850

– N2 ⇒ Cv slightly less than air. 840

– CSP between air and H2O. 830

– Dry CSP slightly lower Cv 820

than CSP. 1112131415161718192021

Intake O2 Mole Fraction [%]

N2 Dry CSP CSP H20 CO2

Motored Data

Increasing Diluent

900

Autoignition Retard for iso-Octane

z Investigate the effect of these gases on combustion phasing.

z The autoignition retard is highly dependent on the type of the diluent.

z The retarding effect of the various gases is ordered consistently with their "cooling capacity" - thermodynamic effect.

z However, N2 addition increases the compression temperature, but the phasing is retarded, so there must be a weak chemical [O2] effect.

1112131415161718192021 Intake O2 Mole Fraction [%]


Motored Data

10%

Bur

ned

[°C

A]

373

372

371

370

369

368

367

1919.2 19.4 19.6 19.8 2020.2 20.4 20.6 20.8 21 Intake O2 Mole Fraction [%]

CO2 H2O CSP Dry CSP N2

Tin = 175°C

Tem

pera

ture

@ 3

50°C

A [K

] 890

880

870

860

366

365 820

830

840

850

365

366

367

368

369

370

371

372

373

16.51717.51818.51919.52020.521Intake O2 Mole Fraction [%]

10%

Bur

ned

[°C

A]

CO2H2OCSPDry CSPN2

iso-OctaneTin = 175°C

373

Temperature Traces for iso-Octane

1050

Air, Baseline N2 CO2 CA10

≈ 370.5°CA

CA10 ≈ 365.7°CA

CO2

N2

Air

z Retarding effect of lower [O2] becomes more significant at higher levels of N2 addition.– Clearly, reduction of [O2] has to

lead to misfire at some point.

z Temperature traces illustrate

1040

1030

1020

1010

1000 990 980 970 960

10%

Bur

ned

[°C

A]

Mas

s-A

vera

ged

Tem

p. [K

]

different mechanisms for retard. 950 350 355 360 365 370 375

Crank Angle [°CA]

16.5 1717.5 1818.5 1919.5 2020.5 21 Intake O2 Mole Fraction [%]


iso-Octane Tin = 175°C

373

372

371

370

369

368

372



Tin = 175°C

10%

Bur

ned

[°C

A]

371

370

369

367

366

365 365

366

367

368

900

Autoignition Retard for PRF80 (2-Stage Ignition)

z For PRF80, the retarding effects are not consistent with the "cooling capacity" of the added gases (except for CO2).

z N2 addition (reduced [O2]) strongly retards combustion phasing.

10%

Bur

ned

[°C

A]

Tem

p. R

ise

335

- 352

°CA

[K] 147

146

145

144

143

142

z PRF80's high sensitivity to [O2] occurs for two reasons:– First, a reduced [O2] reduces the

LTHR.

CO2 H2O N2 Dry CSP CSP

Tin = 72°C

PRF80, N2 2nd Order Curve Fit

890



Motored Data

373

374

Tem

pera

ture

@ 3

50°C

A [K

]

880 372

371870

860 370

369

368

367

366 21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19

820


830

840

850

900

Autoignition Retard for PRF80 - [O2]z Second, a reduced [O2] also affects hot ignition.

1060

1040

1020

1000

980

960

940

920

900

880

Tem

pera

ture

@ 3

50°C

A [K

]M

ass-

Ave

rage

d Te

mp.

[K]

N2CO2

[O2] = 20.7%

[O2] = 19.3%

Post-LTHR temperature is the same.

10%

Bur

ned

[°C

A]

Tem

p. R

ise

335

- 352

°CA

[K] 147

146

145

144

143

142


Tin = 72°C


350 355 360 365 370 375Crank Angle [°CA]

890



Motored Data

373

374

880 372

371870

860 370

369

368

367

366

21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19820


830

840

850

820

830

840

850

860

870

880

890

900

1112131415161718192021Intake O2 Mole Fraction [%]

Tem

pera

ture

@ 3

50°C

A[K

]

N2Dry CSPCSPH20CO2

Motored Data

400

500

600

700

800

900

1000

260 280 300 320 340 360 380Crank Angle [°CA]

Mas

s-A

vera

ged

Tem

p.[K

]

PRF80, N2

PRF80, H2O

880

900

920

940

960

980

1000

350 355 360 365 370 375Crank Angle [°CA]

Mas

s-A

vera

ged

Tem

p.[K

]

N2H2O

CA10 ≈ 371°CA

CA

10

All Constituents for PRF80 - H2O

z Thermodynamic "cooling" should add to [O2] effect for all constituents.

z Why is the retarding effect of H2O equal to N2?

z H2O enhances the intermediate chemistry, so thermal run-away occurs at lower temperature. – Almost perfectly counteracts the cooling effect of H2O!

z Also explains why CSP is less retarded than dry CSP.

z H2O does not enhance the intermediate chemistry for iso-Octane. 374

373

372

371

370

369

368

367

366

10%

Bur

ned

[°C

A]

880

900

920

940

960

980

1000

350 355 360 365 370 375 Crank Angle [°CA]

Mas

s-A

vera

ged

Tem

p. [K

]

iso-Octane, CO2 iso-Octane, H2O PRF80, N2 PRF80, H2O

PRF80 CA10 ≈ 371°CA

iso-Octane CA10 ≈ 370.4°CA



Tin = 72°C

366

367

368

369

370

371

372

373


10%

Bur

ned

[°C

A]

N2 addition

366

367

368

369

370

371

372

373


10%

Bur

ned

[°C

A]

N2 addition

366

367

368

369

370

371

372

373


10%

Bur

ned

[°C

A]

N2 addition

Comparing [O2] Sensitivities z Have now identified three mechanisms by which EGR affects phasing. 1. Cv effect (thermodynamic – retarding). 2. O2-concentration effect (chemical – retarding). 3. H2O effect (chemical – enhancing).

z Compare side by side for all fuels.

z N2 addition gives most reduction in [O2] for least change in Cv.

373

372 z The two-stage ignition fuels

PRF80 and PRF60 are 371

much more sensitive. 370

369 z Lower [O2] both reduces

LTHR and counteracts 368

the hot ignition. 367

366 z Gasoline is initially very

insensitive to [O2].

10%

Bur

ned

[°C

A]

15.5 1616.5 1717.5 1818.5 1919.5 2020.5 21 Intake O2 Mole Fraction [%]

PRF60 PRF80 iso-Octane Gasoline Gas., 3rd order

N2 addition

Comparing Thermal Sensitivities

1. Cv effect (thermodynamic – retarding). 2. O2-concentration effect (chemical – retarding).

3. H2O effect (chemical – enhancing).

z CO2 addition gives most thermodynamic cooling, with 373

least change in [O2].


Gasoline iso-Octane PRF80 PRF60

CO2 addition 372

371

10%

Bur

ned

[°C

A]

z The single-stage ignition fuels,iso-octane and gasoline, aremuch more sensitive to thecooling effect of CO2.

370

369

368

367z The reason for this explained in:

Proceedings of the Combustion 366

Institute, Vol. 31, pp. 2895–2902, 2007.

Comparing CSP Effects

1. Cv effect (thermodynamic – retarding).

2. O2-concentration effect (chemical – retarding).

3. H2O effect (chemical – enhancing).

z The overall effect of CSP results from the combination of these three mechanisms.

374 z CSP gives similar amounts of 373

retard for all four fuels. 372

z But underlying mechanisms 37110

% B

urn

Poin

t [°C

A]

are different. 370

369

z Trace species in real EGR 368 add to these effects. 367

366 19.2 19.4 19.6 19.8 2020.2 20.4 20.6 20.8 21


iso-Octane PRF60, Shifted 6.1% PRF80 Gasoline

CSP addition

Real EGR Effects

z Real EGR contains trace species 373 372

that also affect the autoignition. 371

– Unburned fuel, partially oxidized 370 369

fuel, formaldehyde, CO, and other 368

species. 367 366

z For iso-Octane and Gasoline, 365

372 real EGR retards less than CSP. 371 370

z For PRF80, real EGR has a 369

retards slightly more than CSP. 368 367 366

z The net effect of these species is 365

to advance the ignition for iso- 372 371

octane and gasoline... 370 369

z ...but to retard it for PRF80. 368 367 366

10%

Bur

n Po

int [

°CA

]CSP Real EGR

PRF80 Tin = 72°C

CSP Real EGR

Gasoline Tin = 149°C

CSP Real EGR


21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19 Intake O2 Mole Fraction [%]

Iso-Octane Gasoline PRF80 PRF60

± Effect: Single-stage Two-stage with LTHR

Cv effect (thermodynamic) Strong Weak Retarding

[O2] effect (chemical) Weak Strong

Enhancing H2O effect (chemical) None to weak Strong

Trace Species Moderate enhancing Moderate retarding



Cv effect (thermodynamic) Strong Weak Retarding




Summary / Conclusions z EGR is very effective for suppressing autoignition reactivity.

– Can be used beneficially for controlling combustion phasing across load and speed ranges.



Retarding Cv effect (thermodynamic) Strong Weak




z The net result of the different thermal, O2, and H2O sensitivities is a fairly similar effect of CSP for all fuels.

z Trace species (unburned and partially oxidized fuel, and CO) influence the effect of real EGR.

Details in: SAE 2007-01-0207

End Presentation

900

Autoignition Retard for PRF80 - [O2] z Lower [O2] also affects hot ignition, in addition to reducing the LTHR.

Mas

s-A

vera

ged

Tem

p. [K

]

N2

CO2

[O2] = 20.7%

[O2] = 19.3%

1040

1000

960

920

880

840

800

10%

Bur

ned

[°C

A]

Tem

p. R

ise

335

- 352

°CA

[K] 147

146

145

144

143

142


Tin = 72°C


760

335 340 345 350 355 360 365 370 375Crank Angle [°CA]

890



Motored Data

373

374

Tem

pera

ture

@ 3

50°C

A [K

]

880 372

371870

860 370

369

368

367

366

21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19820


830

840

850

820

830

840

850

860

870

880

890

900


Tem

pera

ture

@ 3

50°C

A[K

]

N2Dry CSPCSPH20CO2

Motored Data

Autoignition Retard for Gasoline

z N2 addition shows that the chemical [O2] effect is very weak.

z Retarding effect of H2O is weaker than expected based on its high Cv.

z CSP (with water) has a slightly weaker retarding effect than dry CSP.

z Temperature traces confirm that H2O has an enhancing effect on autoignition for gasoline.

z Effect is stronger than for iso-octane, but much weaker than for PRF80.

350 355 360 365 370 375 Crank Angle [°CA]

CO2 H2O

Gasoline CA10 ≈ 370.3°CA

373 1020

CO2 H2O Dry CSP CSP N2


Mas

s-A

vera

ged

Tem

p. [K

] 372

371

370

1000

10%

Bur

ned

[°C

A]

980

369960

368

367

366

365

21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19 Intake O2 Mole Fraction [%]

900

920

940

Real EGR Effects (2)z Intake concentrations increase with

EGR level and retard. 373 372

z These effects of Real EGR are also 371 370 important for explaining the effects of 369

retained residuals. 368 367

z Discussed in Proceedings of the 366 Combustion Institute, Vol. 31, pp. 365

CSP Real EGR

PRF80 Tin = 72°C

CSP Real EGR


CSP Real EGR


2895–2902, 2007.

10%

Bur

n Po

int [

°CA

] 372

371

370

369

368

367

366

PRF80 iso-Octane Gasoline

Intake CO

iso-Octane PRF80 Gasoline

Intake HC

300

250

200

150

100

Inta

ke C

O -

[ppm

]

50 365 0 372

1200

1000

Inta

ke H

C -

[ppm

] 371

370

369

368

400 367 200 366

21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19 21 20.8 20.6 20.4 20.2 20 19.8 19.6 19.4 19.2 19 Intake O2 Mole Fraction [%]


0

600

800

Compare with NIST Database z All air displacement except N2 reduce the compression temperature. z Can be explained by changes of the specific heat capacity. z CO2 has highest specific heat capacity on a mole basis. z H2O has high specific heat capacity. z N2 has slightly lower specific heat capacity compared to air. z CSP falls between Air and H2O. z Dry CSP has slightly lower Cv compared to CSP.

Tem

pera

ture

@ 3

50°C

A [K

]

15

20

25

30

35

40

45

50

Hea

t Cap

acity

- C

v [J

/K*m

ol]

CO2 H2O CSP Dry CSP Air N2

NIST Chemistry WebBook

900

890

880

870

860

850

840

Motored Data

N2 Dry CSP CSP H20 CO2 830

820 300 400 500 600 700 800 900 1000 1100 1200 21 20 19 18 17 16 15 14 13 12 11

Temperature [K] Intake O2 Mole Fraction [%]

Documents

The Effects of EGR and Its Constituents on the