John E. DecSandia National Laboratories

The Importance of Intermediate-Temperature Reactions in HCCI and HCCI-like Engines

MACCCR – 5th Annual Fuels Research ReviewSandia National Labs, CRF, Sept. 17 - 20, 2012

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

Program Manager: Gurpreet SinghChevron USA, Inc.

Program Manager: William Cannella

Acknowledge: Yi Yang and Nicolas Dronniou

Motivation Global demand for transportation fuels is increasing rapidly.

– New discoveries of petroleum are not keeping pace.

Global temperatures are rising.– CO2 from transportation is a major contributor to GHG.

Strong motivation to reduce petroleum consumption.

Engine-efficiency improvements offer a huge potential!– Could be implemented in

a relatively short time.

Biofuels or other renewable fuels also important part of solution.– Progress is being made.– Full discussion is beyond the

scope of this presentation.

Improving Vehicle-Fleet Efficiency Diesel engine is the most efficient transportation engine ever developed.

– Potential fuel savings of ~30% over spark-ignition (SI) engines.

Drawbacks to diesel engines:– Diesel emissions control is challenging expensive aftertreatment.– Cost of the engine alone can be significantly higher than SI.– Increased demand is driving up diesel fuel prices.

US, ~ equal taxes, diesel 8% higher. Spread will increase if diesel demand increases.

Total cost can be > gasoline.

For high-efficiency vehicles to have a large impact, it is important thatthe technology be economical. Particularly for the developing world.

For full utilization of crude oil stocks need a high-efficiency engine fueled with “light” distillates (e.g. gasoline), as well as diesel.

Looking to the future, the vast majority of new vehicles added will be in the developing world. 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

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5.5

Industrialized Developing World

Bill

ions

of V

ehic

les

20012050

Advanced engines using HCCI or partially stratified “HCCI-like” combustion can provide diesel-like or higher efficiencies & ultra-low emiss. NOX & soot.– Potential for significantly lower cost than an emissions-compliant diesel.– More-volatile, light-end fuels are well suited for HCCI.– HCCI can also work well with biofuels and biofuel/gasoline blends.

HCCI dilute premixed charge and compression ignition.– Volumetric flameless combustion.– Never truly homogeneous due to natural thermal stratification (TS).– Also advantages to adding controlled mixture stratification at some conditions

partial fuel stratification (PFS).

Advanced High-Efficiency Engines

Other low-T, HCCI-like concepts also show promise (PCCI or RCCI).

Challenges for HCCI:– Controlling start of combustion.– Extending operation to higher

loads.

Fuel autoignition chemistry is critical for both.

HCCI is the most fundamental low-temperature combustion process and the first to be widely investigated.– High Efficiencies Relatively high CR = 12 – 16, no throttle, and low

combustion temperatures less heat-transfer loss and higher .– Ultra-low NOx & soot Lean or dilute with EGR for low Tcombst , and Well mixed

HCCI is relatively well developed.– GM demonstrated prototype cars operating in HCCI mode from idle 70 mph,

using regular pump gasoline.– Strong potential to extend operating range substantially with intake boost & PFS.

Other low-T combustion modes rely on these same principles.– PCCI or PPCI developed to achieve HCCI-like combustion in diesel engines.

DI fueling ~60° bTDC for diesel-fuel vaporization and “premixed enough.”> Widely used with diesel fuel for low emissions at light load in production.> Also efforts to use gasoline for longer ignition delay more premixing for higher

loads, but difficulties with smoke & NOX low-octane gasoline, RON ~70.

– RCCI more recent concept introduced by Reitz et al. at UW shows promise, but requires two fuels (gasoline and diesel).

Low-Temperature Combustion Modes

Active research on these low-T modes & Autoig. chemistry important for all.

Sandia Dual-Engine HCCI Engine Laboratory

All-Metal Engine

Optical Engine

Optics Table

Dynamometer

Intake Plenum

Exhaust Plenum

Water & Oil Pumps & Heaters

Flame Arrestor

Matching all-metal & optical HCCI research engines.– Single-cylinder conversion from Cummins B-series diesel.– Open combustion chamber.

Optical EngineAll-Metal Engine

Bore x Stroke = 102 x 120 mm 0.98 liters, CR=14

PTDC-motored = 30 – 100 bar

Thermal-stratification image at TDC

Side View

Nature of HCCI Combustion Ignition timing is mainly controlled by fuel autoignition chemistry.

– Heat release prior to hot autoignition is kinetically controlled (LTHR, and ITHR).– Adjustment of operating parameters required to control ignition timing.

High-T combustion kinetics are very rapid. Main combustion HRR is controlled mostly by thermal stratification (TS).

– TS occurs naturally due to heat transfer and imperfect mixing with hot residuals.– TS causes sequential autoignition & combustion of the in-cylinder charge.

> Crucial for controlling HCCI HRR for fully premixed combustion.

-50

50

50

50

335 345 355 365 375 385 395Crank Angle

Hea

t Rel

ease

Rat

e

AUTOIGNITION

Fuel autoignition kinetics control ignition timing, and pre-igntion heat release, i.e. LTHR & ITHR.

Thermal stratification usually controls HRR of main combustion.

KINETICALLY CONTROLLED THERMALLY CONTROLLED

LTHR ITHR

(Dronniou and Dec, SAE 2012-01-1111)

Thermal-stratification image at TDC

Side View

Chemiluminescence of HCCI (iso-octane, = 0.24, fully premixed)

Nature of HCCI Combustion Ignition timing is mainly controlled by fuel autoignition chemistry.

– Heat release prior to hot autoignition is kinetically controlled (LTHR, and ITHR).– Adjustment of operating parameters required to control ignition timing.

High-T combustion kinetics are very rapid. Main combustion HRR is controlled mostly by thermal stratification (TS).

– TS occurs naturally due to heat transfer and imperfect mixing with hot residuals.– TS causes sequential autoignition & combustion of the in-cylinder charge.

> Crucial for controlling HCCI HRR for fully premixed combustion.

-50

50

50

50

335 345 355 365 375 385 395Crank Angle

Hea

t Rel

ease

Rat

e

AUTOIGNITION

Fuel autoignition kinetics control ignition timing, and pre-igntion heat release, i.e. LTHR & ITHR.

Thermal stratification usually controls HRR of main combustion.

KINETICALLY CONTROLLED THERMALLY CONTROLLED

LTHR ITHR

(Dec et al., SAE 2006-01-1518)

Ignition Chemistry Changes with Fuel-Type

CA50 = TDC, Premixed, Pin = 1 bar, 1200 rpm Intake temperature (Tin) required for a given combustion phasing varies substantially with fuel type.– Representative real-fuel

constituents and gasoline.

Hot-ignition temperature also varies, but proportionally less than Tin.

One main reason is that low-temp. heat release (LTHR, or “cool flame”) raises temperature for PRF80.– 80% iso-octane + 20% n-heptane

However, for single-stage ignition fuels, early reactions also important. – “Intermediate temp. heat release”

(ITHR) reactions.– Raise temperature to thermal

runaway (hot-ignition point CA10).

600

800

1000

1200

1400

1600

1800

300 320 340 360 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineMCH-methylcyclohexaneDIB-diisobutlyenePRF80

CA

10

020406080

100120140160180200

Toluene Iso-Oct. Gasoline DIB MCH PRF80

Inta

ke T

empe

ratu

re [°

C]

50°C

107°C

(Hwang et al., C&F 154(3), 2008)

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

Motored

iso-Octane

CA

10

-2000

200400600800

10001200140016001800

335 340 345 350 355 360 365 370 375 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

04080120160200240280320360400

Hea

t-Rel

ease

Rat

e [J

/°CA

]Tolueneiso-OctaneMCHDIBPRF80

Intermediate Temp. Heat Release (ITHR)

For all fuels, slower ITHR reactions precede hot ignition ( CA10).– 900 < T < 1050 K

For many single-stage fuels, trends are self-similar, Tig. varies with Tin. – Tig varies significantly with fuel-type.

However, DIB (diisobutylene) has more intense ITHR reactions.

Also, for two-stage PRF80, ITHR reactions more intense after LTHR.– Note that HRR never goes to zero

after LTHR ends.

Examine the effects of ITHR in more detail for iso-octane (single-stage) and PRF80 (two-stage)

(Hwang et al., C&F 154(3), 2008)

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

Motored

iso-Octane

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCHPRF80

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Toluene

iso-Octane

Gasoline

MCH

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCH

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCHPRF80

CA

10

-2000

200400600800

10001200140016001800

335 340 345 350 355 360 365 370 375 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

04080120160200240280320360400

Hea

t-Rel

ease

Rat

e [J

/°CA

]Tolueneiso-OctaneMCHDIBPRF80

Intermediate Temp. Heat Release (ITHR)

For all fuels, slower ITHR reactions precede hot ignition ( CA10).– 900 < T < 1050 K

For many single-stage fuels, trends are self-similar, Tig. varies with Tin. – Tig varies significantly with fuel-type.

However, DIB (diisobutylene) has more intense ITHR reactions.

Also, for two-stage PRF80, ITHR reactions more intense after LTHR.– Note that HRR never goes to zero

after LTHR ends.

Examine the effects of ITHR in more detail for iso-octane (single-stage) and PRF80 (two-stage)

(Hwang et al., C&F 154(3), 2008)

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

Motored

iso-Octane

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCHPRF80

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Toluene

iso-Octane

Gasoline

MCH

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCH

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

] Tolueneiso-OctaneGasolineDIBMCHPRF80

CA

10

800

900

1000

1100

1200

1300

348 350 352 354 356 358 360 362Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

iso-Octane

PRF80

CA

10

-2000

200400600800

10001200140016001800

335 340 345 350 355 360 365 370 375 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

04080120160200240280320360400

Hea

t-Rel

ease

Rat

e [J

/°CA

]Tolueneiso-OctaneMCHDIBPRF80

-2000

200400600800

10001200140016001800

335 340 345 350 355 360 365 370 375 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re [K

]

04080120160200240280320360400

Hea

t-Rel

ease

Rat

e [J

/°CA

]iso-Octane

PRF80

Intermediate Temp. Heat Release (ITHR)

For all fuels, slower ITHR reactions precede hot ignition ( CA10).– 900 < T < 1050 K

For many single-stage fuels, trends are self-similar, Tig. varies with Tin. – Tig varies significantly with fuel-type.

However, DIB (diisobutylene) has more intense ITHR reactions.

Also, for two-stage PRF80, ITHR reactions more intense after LTHR.– Note that HRR never goes to zero

after LTHR ends.

Examine the effects of ITHR in more detail for iso-octane (single-stage) and PRF80 (two-stage)

(Hwang et al., C&F 154(3), 2008)

Why does timing retard slow PRR?

10

20

30

40

50

60

70

80

90

350 355 360 365 370 375 380Crank Angle [° aTDC Intake]

Cyl

inde

r Pre

ssur

e [b

ar]

Experiment, Phi=0.3, 360°CAExperiment, 365°CAExperiment, 369°CA

0

2

4

6

8

10

12

14

358 360 362 364 366 368 370 372 374 376 37850% Burn Point [°CA aTDC]

Rin

ging

Inte

nsity

[MW

/m2]

Phi = 0.30

Ringing limit

0

2

4

6

8

10

12

14

358 360 362 364 366 368 370 372 374 376 37850% Burn Point [°CA aTDC]

Rin

ging

Inte

nsity

[MW

/m2] Phi = 0.30

Phi = 0.34Phi = 0.38Phi = 0.40Phi = 0.42

Ringing limit

Nat. TS in the bulk gas significantly slows the combustion rate.

High-load op. still limited by knock. Retarding combustion phasing

reduces PRR & ringing intensity. Allows higher fueling stab. limit

Retarded Combustion Allows Higher Loads

0

10

20

30

40

50

60

70

80

345 350 355 360 365 370 375Crank Angle [°] aTDC

Cyl

inde

r Pre

ssur

e [b

ar]

Phi = 0.3

Phi = 0.26

Phi = 0.22

Phi = 0.18

= 0.3

Decrease Tin

0

10

20

30

40

50

60

70

80

345 350 355 360 365 370 375Crank Angle [°] aTDC

Cyl

inde

r Pre

ssur

e [b

ar]

Phi = 0.3Phi = 0.26Phi = 0.22Phi = 0.18CHEMKIN, Phi = 0.3CR = 18

Const. CA50 = 360°CA (TDC)

Timing retard amplifies the benefit of a given thermal stratification.

(Sjöberg et al., SAE 2004-01-2994, and Dec et al., SAE 2006-01-1518)

700

750

800

850

900

950

1000

1050

1100

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

Mas

s-A

vera

ged

Tem

p. [K

] ISO-OCTANEPRF80 TDC

700

750

800

850

900

950

1000

1050

1100

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

Mas

s-A

vera

ged

Tem

p. [K

] ISO-OCTANEPRF80

LTHR

TDC

Strong ITHR causes high TRR

850

900

950

1000

1050

1100

1150

345 350 355 360 365 370 375 380Crank Angle [°CA]

Mas

s-A

vg. T

empe

ratu

re, O

ffset

(to m

atch

T34

5 of

the

160k

Pa

case

)

Pin = 100 kPa

Pin = 100 kPa, motored

900

950

1000

1050

1100

1150

1200

1250

1300

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

Tem

pera

ture

[K]

T350 = 987.5 K + 1.9 KT350 = 987.5 KT350 = 987.5 K - 1.9 KRun-Away TemperatureT350 = 974.3 K + 1.9 KT350 = 974.3 KT350 = 974.3 K - 1.9 K

Model, = 0.38

-0.005

0

0.005

0.01

0.015

0.02

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

HR

R /

tot H

R [J

/°CA

]

PRF80

ISOOCTANE

Pre-Ignition Heat Release

Allowable CA50 retard limited by combustion stability.– Must maintain positive temperature

rise rate (TRR) before hot ignition.– Lack of sufficient TRR causes

combustion to become unstable and eventually misfire.

Fuels with stronger ITHR, e.g. PRF80, produce higher TRR.– Can sustain more CA50 retard.

Combustion Retard Relies on ITHR

(Sjöberg and Dec, Proc. Combust. Inst. 2007)

Strong ITHRHigh TRRMore retarded CA50 with Good

StabilityReduced HRRHigher loads without knock

900

950

1000

1050

1100

1150

1200

1250

1300

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

Tem

pera

ture

[K]

T350 = 987.5 K + 1.9 KT350 = 987.5 KT350 = 987.5 K - 1.9 KRun-Away TemperatureT350 = 974.3 K + 1.9 KT350 = 974.3 KT350 = 974.3 K - 1.9 K

Model, = 0.38

700

750

800

850

900

950

1000

1050

1100

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

Mas

s-A

vera

ged

Tem

p. [K

] ISO-OCTANEPRF80 TDC

700

750

800

850

900

950

1000

1050

1100

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

Mas

s-A

vera

ged

Tem

p. [K

] ISO-OCTANEPRF80

LTHR

TDC

Strong ITHR causes high TRR

-0.005

0

0.005

0.01

0.015

0.02

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

HR

R /

tot H

R [J

/°CA

]

PRF80

ISOOCTANE

Pre-Ignition Heat Release

Combustion Retard Relies on ITHR

(Sjöberg and Dec, Proc. Combust. Inst. 2007)

0

2

4

6

8

10

12

365 367 369 371 373 375 37750% Burned (CA50) [°CA]

Std.

Dev

. of I

MEP

g [%

] iso-OctanePRF80Acceptable Limit

0

2

4

6

8

10

12

14

366 368 370 372 374 376 378 380

CA50 [°CA]

IMEP

g [b

ar]

Pin = 200 kPa

Pin = 100 kPa

1.6 bar6ºCA

Intake Boosting Enhances ITHR of Gasoline For dilute combst. like HCCI, intake-

press. boost required for high loads. Enhances ITHR of gasoline.– Remains as single-stage ignition.– Allows considerable CA50 retard

with good stability.

For Pin = 200 kPa, CA50 retard can be > 379°CA. – 6°CA more than for Pin = 100 kPa.– Allows significantly higher IMEPg.

As a result, very high loads can be reached with boosted gasoline HCCI.– IMEPg = 16.3 bar at Pin = 3.25 bar.– E10, IMEPg = 18.1 bar at Pin = 3.4 bar.– Approaching loads of conventional

diesel engines.

Enhanced ITHR with boost is the key to high-load operation with premixed fueling, using gasoline.

-0.002

0

0.002

0.004

0.006

0.008

0.01

-35 -30 -25 -20 -15 -10 -5 0 5 10Crank Angle relative to Peak HRR [°CA]

HR

R /

tota

l HR

[1/°C

A]

Pin = 200 kPa, Tin = 60°CPin = 160 kPa, Tin = 74°CPin = 130 kPa, Tin = 105°C Pin = 100 kPa, Tin = 130°C

TDC Range

(Dec and Yang, SAE 2010-01-1086)

0

2

4

6

8

10

12

14

366 368 370 372 374 376 378 380

CA50 [°CA]

IMEP

g [b

ar]

Pin = 200 kPa

Pin = 100 kPa

1.6 bar6ºCA

Intake Boosting Enhances ITHR of Gasoline For dilute combst. like HCCI, intake-

press. boost required for high loads. Enhances ITHR of gasoline.– Remains as single-stage ignition.– Allows considerable CA50 retard

with good stability.

For Pin = 200 kPa, CA50 retard can be > 379°CA. – 6°CA more than for Pin = 100 kPa.– Allows significantly higher IMEPg.

As a result, very high loads can be reached with boosted gasoline HCCI.– IMEPg = 16.3 bar at Pin = 3.25 bar.– E10, IMEPg = 18.1 bar at Pin = 3.4 bar.– Approaching loads of conventional

diesel engines.

Enhanced ITHR with boost is the key to high-load operation with premixed fueling, using gasoline.

-0.002

0

0.002

0.004

0.006

0.008

0.01

-35 -30 -25 -20 -15 -10 -5 0 5 10Crank Angle relative to Peak HRR [°CA]

HR

R /

tota

l HR

[1/°C

A]

Pin = 200 kPa, Tin = 60°CPin = 160 kPa, Tin = 74°CPin = 130 kPa, Tin = 105°C Pin = 100 kPa, Tin = 130°C

TDC Range

(Dec and Yang, SAE 2012-01-01107)

2

4

6

8

10

12

14

16

18

20

0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6Intake Pressure [bar]

Max

imum

IMEP

g [b

ar]

Gasoline, PM

Gasoline, PM, no EGR

2

4

6

8

10

12

14

16

18

20

0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6Intake Pressure [bar]

Max

imum

IMEP

g [b

ar]

Gasoline, PM

Gasoline, PM, no EGR

E10, PM

-0.002

0

0.002

0.004

0.006

0.008

0.01

-30 -25 -20 -15 -10 -5 0Crank Angle offset by CA10 [°CA]

HR

R /

tot H

R [1

/°CA

]

Gasoline, Pin = 200 kPa, 31% EGRiso-Pentanol, Pin = 200 kPa, 37% EGRGasoline, Pin = 100 kPa, Tin = 141°CIso-Pentanol, Pin = 100 kPa, Tin = 132°C

TDC

TDC

Can Biofuels Give Similar Performance?

Iso-pentanol is a next-generation biofuel process developed by JBEI.

Iso-pentanol shows gasoline-like ITHR, naturally aspirated.– Greater than ethanol.

ITHR of iso-pentanol is significantly enhanced by boost, similar to gasoline.– Provides good stability with significant

timing retard.

Allows a substantial increase in load with boost like gasoline.

Ethanol shows no enhancement in ITHR with boost.– Not as good for high-load HCCI.– Likely more knock resistance for

boosted SI operation.

-0.04

0

0.04

0.08

0.12

0.16

-20 -15 -10 -5 0 5 10 15 20 25Crank Angle relative to CA10 [°CA]

HR

R /

tota

l HR

[1/°C

A]

Iso-PentanolGasolineEthanol

-0.002

0

0.002

0.004

0.006

0.008

0.01

-30 -25 -20 -15 -10 -5 0Crank Angle relative to CA10 [°CA]

HR

R /

tota

l HR

[1/°C

A]

Iso-PentanolGasolineEthanol

TDC

Naturally Aspirated

Engine compatibility must be considered in developing new biofuels/blends.

Iso-Pentanol and Ethanol Isopentanol

Iso-pentanol closely matches gasoline performance good compatibility.

(Yang et al., SAE 2010-01-2164)

-0.002

0

0.002

0.004

0.006

0.008

0.01

-30 -25 -20 -15 -10 -5 0Crank Angle offset by CA10 [°CA]

HR

R /

tot H

R [1

/°CA

]

Gasoline, Pin = 200 kPa, 31% EGRiso-Pentanol, Pin = 200 kPa, 37% EGRGasoline, Pin = 100 kPa, Tin = 141°CIso-Pentanol, Pin = 100 kPa, Tin = 132°C

TDC

TDC

-0.002

0

0.002

0.004

0.006

0.008

0.01

-30 -25 -20 -15 -10 -5 0 5 10Crank Angle relative to CA10 [°CA]

Hea

t Rel

ease

Rat

e / T

otal

HR

[1/°C

A]

Pin = 247 kPa, Tin = 94°C, Phi=0.355Pin = 181 kPa, Tin = 108°CPin = 146 kPa, Tin = 119°CPin = 124 kPa, Tin = 129°CPin = 100 kPa, Tin = 141°C

Ethanol

Can Biofuels Give Similar Performance?

Iso-pentanol is a next-generation biofuel process developed by JBEI.

Iso-pentanol shows gasoline-like ITHR, naturally aspirated.– Greater than ethanol.

ITHR of iso-pentanol is significantly enhanced by boost, similar to gasoline.– Provides good stability with significant

timing retard.

Allows a substantial increase in load with boost like gasoline.

Ethanol shows no enhancement in ITHR with boost.– Not as good for high-load HCCI.– Likely more knock resistance for

boosted SI operation.

-0.04

0

0.04

0.08

0.12

0.16

-20 -15 -10 -5 0 5 10 15 20 25Crank Angle relative to CA10 [°CA]

HR

R /

tota

l HR

[1/°C

A]

Iso-PentanolGasolineEthanol

-0.002

0

0.002

0.004

0.006

0.008

0.01

-30 -25 -20 -15 -10 -5 0Crank Angle relative to CA10 [°CA]

HR

R /

tota

l HR

[1/°C

A]

Iso-PentanolGasolineEthanol

TDC

Naturally Aspirated

Engine compatibility must be considered in developing new biofuels/blends.

Iso-Pentanol and Ethanol Isopentanol

Iso-pentanol closely matches gasoline performance good compatibility.

(Dec and Yang, SAE 2010-01-1086)

363

364

365

366

367

368

369

370

371

0.3 0.32 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5Charge Mass Equivalence Ratio [m]

CA

10 [°

CA

]

368

369

370

371

372

373

374

375

376

CA

10 [°

CA

]

PRF73, Pin = 1 bar, base-phi = 0.42

Gasoline, Pin = 1 bar, base-phi = 0.42

Pin varies

363

364

365

366

367

368

369

370

371

0.3 0.32 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5Charge Mass Equivalence Ratio [m]

CA

10 [°

CA

]

368

369

370

371

372

373

374

375

376

CA

10 [°

CA

]

PRF73, Pin = 1 bar, base-phi = 0.42

Gasoline, Pin = 1 bar, base-phi = 0.42

Gasoline, Pin = 2 bar, base-phi = 0.42

Pin varies

-Sensitivity of Gasoline and ITHR

363

364

365

366

367

368

369

370

371

0.3 0.32 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5Charge Mass Equivalence Ratio [m]

CA

10 [°

CA

]

368

369

370

371

372

373

374

375

376

CA

10 [°

CA

]

PRF73, Pin = 1 bar, base-phi = 0.42Gasoline, Pin = 1 bar, base-phi = 0.42Gasoline, Pin = 1.6 bar, base-phi = 0.367Gasoline, Pin = 2 bar, base-phi = 0.345Gasoline, Pin = 2 bar, base-phi = 0.42

Pin varies

High -sensitivity of gasoline with boost offers the possibility of using mixture stratification to reduce the HRR and allow higher loads.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Cycle #

Pres

sure

= variable = 0.42Fire 19/1

Acquired cycle ITHR correlates with -sensitivity over

a range of fuels & operating conditions.(Dec et al., SAE 2011-01-0897)

Isolate fuel-chemistry effects from thermal effects, use Fire19/1 technique.– Dec & Sjöberg SAE 2004-01-0557.

PRF73, ↑ ITHR strong -sensitivity– Autoignition timing significantly

advanced by higher .– Chemical reaction rates promoted by

higher fuel concentration.

Gas. Pin = 1 bar, ↓ ITHR not -sensitive– Retards due to cooling effect of .

ITHR of gasoline increases greatly from Pin = 1 to 2 bar.

Corresponding incr. in -sensitivity.

Fully Premixed

Equivalence ratio

Prob

abili

ty

Fully Stratified

Premixed

DI

Partially Stratified

356

357

358

359

360

361

362

363

364

365

0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32Equivalence Ratio [ ]

50%

Bur

n Po

int [

°CA

aTD

C]

PRF80iso-Octane

Fire F19/1-ExperimentConst. Wall and Residual Temp.

356

357

358

359

360

361

362

363

364

365

0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32Equivalence Ratio [ ]

50%

Bur

n Po

int [

°CA

aTD

C]

PRF80iso-Octane

Fire F19/1-ExperimentConst. Wall and Residual Temp.

Controlling the HRR with Mixture Stratification Potential to reduce HRR beyond TS Higher loads & Higher efficiency.

Two requirements must be met for mixture stratification to be effective:1) -Sensitive Fuel 2) Appropriate -Distribution

Dec and Sjöberg, SAE 2004-01-0557

PRF80 is strongly -sensitive (high ITHR), iso-octane is not (weak ITHR)

Partial fuel stratification (PFS) most fuel premixed, up to 20% late DI.1. Provides sufficient stratification.2. Good air utilization with leanest regions burning hot enough for good comb.

Sjöberg and Dec, SAE 2006-01-0629

5

6

7

8

9

10

11

12

13

14

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6Charge-Mass Equiv. Ratio [m]

IMEP

g [b

ar]

42

43

44

45

46

47

48

49

50

51

Indi

cate

d Th

erm

al E

ff. [%

]

IMEPg, PFSIMEPg, PreMixedT-E, PFST-E, PreMixed

Advantages of PFS for Boosted Gasoline, Pin = 2 bar

Increase PFS by increasing the DI%.– Greatly reduces HRR, PRR and ringing.

PFS allows significantly higher loads.– Premixed IMEPg = 11.7 bar, m = 0.47– PFS IMEPg = 13.0 bar, m = 0.54– Approaching O2 availability limit (0.9%).

Thermal Eff. is higher for the same load.– More advanced CA50.

Ultra-low NOX and soot.

Using PFS to exploit -sensitivity provides substantial advantages.

Important to understand the I-T chemistry that allows this operation.

50

60

70

80

90

100

110

340 350 360 370 380 390Crank Angle [°CA]

Pres

sure

[bar

]

3% DI @ 300°CA6% DI @ 300°CA9% DI @ 300°CA13% DI @ 300°CA17% DI @ 300°CA20% DI @ 300°CA

-1000

100200300400500600700

340 350 360 370 380 390Crank Angle [°CA]

HR

R [J

/°CA

]

3% DI @ 300°CA6% DI @ 300°CA9% DI @ 300°CA13% DI @ 300°CA17% DI @ 300°CA20% DI @ 300°CA

CA50 = 374°CA, m = 0.44

(Dec et al., SAE 2011-01-0897)

Summary and Conclusions Advanced low-temperature compression-ignition engines such as HCCI

offer substantial advantages for high-efficiency, low emiss. & moderate cost.

Early autoignition chemistry is critical for the operation of these engines. – LTHR 760 < T < 870 K– ITHR 900 < T < 1050 K

ITHR varies significantly with fuel type even when no LTHR is present.

Magnitude of the ITHR is critical for allowing sufficient combustion-phasing retard to prevent engine knock.– ITHR must be sufficient to keep a positive TRR during the early expansion.

Intake boosting greatly increases the ITHR of gasoline allowing very high loads without knock and with good stability up to IMEPg = 16.3 & 18.1 bar– Enhanced ITHR also found with the biofuel iso-pentanol, but not with ethanol.

Amount of ITHR also correlates with the -sensitivity of the fuel. – -Sensitivity allows the use of PFS to significantly reduce the HRR & PRR.– Provides higher loads and higher efficiencies.

A more complete understanding of I-T chemistry at engine conds. is needed.