a new combustion technology based on the flexibly...

内燃机燃烧学国家重点实验室State Key Laboratory of Engines (SKLE)

A new combustion technology based on the flexibly

controllable kinetic pathway for IC engine

Mingfa Yao

State Key Lab Engines, Tianjin University

23 Aug 2018

International Summit on Breakout Technology of Engines and Fuels (ISEF2018)


Content

• Background & introduction

• Exergy Destructions of the Combustion Process

• The combustion technology based on controllable kinetic pathway

• Conclusions


The pathway to improve the TE

3

First-law of thermodynamics: Incomplete combustion

Second-law of thermodynamics:

There is more than 20% of the irreversible loss in the fuel

combustion process！ Includes: Exergy loss and incomplete combustion loss

• Method to improve TE： Decrease irreversible loss（chemical energy to thermal energy） Control heat release profile（thermal energy to mechanical energy）

CA50

C90-CA10H

eat

re

leas

e r

ate

From: K-Y Teh, Thermodynamic requirements for maximum internal combustion engine cycle efficiency Part 1 & Part2. International Journal of Engine Research, 449-480, 2008

• Improving the TE is the greatest challenge for IC Engine

𝜂𝑡𝑜𝑡𝑎𝑙 = 𝜂𝑐 • 𝜂𝑖 • 𝜂𝑒

𝜼𝒊is related to the heat release phase, heat

release position and combustion duration


• Exergy and incomplete combustion loss are related to the kinetic pathway.

• The kinetic pathway also affects heat release profile, then affects the indicate efficiency. It is also

limited by engine parameters, such as the highest pressure, pressure rise rate, exhaust temperature

etc.)

• To control the kinetic pathway for different operation mode could improve engine thermal efficiency

4

Break Thermal Efficiency

Control the kinetic pathway can improve the thermal efficiency

Work IrreversibilityHeat transfer Remaining exergy

Total fuel chemical energy /exergy


Combustion mode strongly depends on kinetic pathway

5

n-heptane initial high-temperature pyrolysis reaction pathway

From: D.R. Tree et al., Progress in Energy and Combustion Science, 2007, 33(3):272-309.

The soot generation processTo control the chemical kinetic pathway can reduce pollution emissions

𝑹𝑯

ሶ𝑹

𝑹 ሶ𝑶𝟐

𝑸𝑶𝑶𝑯

ሶ𝑶𝟐𝑸𝑶𝑶𝑯

𝑲𝒆𝒕𝒐𝒏𝒆.+ ሶ𝑶𝑯

𝑳𝑻 𝑩𝒓𝒂𝒏𝒄𝒉𝒊𝒏𝒈

𝑶𝟐

H𝐢𝐠𝐡 𝐭𝐞𝐦𝐩𝐞𝒓𝒂𝒕𝒖𝒓𝒆 𝜷pyrolysis

alkene+ ሶ𝑹′

𝑶𝟐

Low temperature

oxidation products

alkene+HO2

epoxide+OH

𝜷 pyrolysis

Traditional CI

combustion

Low-temperature

combustion


Purpose of this presentation

• Explore the exergy destructions of the combustion process, then propose the

combustion pathway with high efficiency and low emissions

• Introduce the Flexible Cylinder Engine (FCE) principle

• Present the preliminary test results for this combustion technology.





• Conclusions

Content


Exergy destruction of the n-heptane/air auto-ignition process

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4500

1500

2500

3500

4500

Time/τ [-]

Tem

pera

ture

[K

]

Tinit

=815.5 K

Tinit

=910.07 K

Tinit

=1046.4 K

1E-09

1E-07

1E-05

1E-03

1E-01

1E+01

T0 · d

Sd

t ·τ

/ E

xch init

[-]

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4500

1500

2500

3500

4500

Tem

pera

ture

[K

]

Time/τ [-]

Φ=0.5

Φ=1.0

Φ=2.0

1E-08

1E-06

1E-04

1E-02

1E+00

1E+02

T0 · d

Sd

t ·τ

/Ex

ch init

[-]

• Changing initial temperature (Tinit)：

Lower peak of entropy generation rate before ignition

exists due to the LTC pathway.

• Changing equivalence ratio (Φ )：

The entropy generation decreases with increasing Φ.

Different behaviors of entropy generation after ignition

can be seen as the oxygen content decreases.

0.000 0.002 0.004 0.006 0.008600

800

1000

1200

1400

1600

Tem

pera

ture

[K

]

Time [s]

0E+00

1E+06

2E+06

3E+06

4E+06

Stage after ignition

Tem

pera

ture

ris

ing

rate

[K

/s]

Stage before ignition

0.0044 0.0045 0.00461000

1100

1200

1300

Tem

pera

ture

[K

]

Time [s]

1.6E+06

2.0E+06

2.4E+06

2.8E+06

3.2E+06

Tem

pera

ture

ris

ing

rate

[K

/s]• Staged combustion：

Splitting the auto-ignition process into two stages

with ignition delay time

• Definitions of ignition time： Time at ΔT=400K relative to Tinit (τ400)

Time at the maximum T_dot (τT_dot,max)

τ400< τT_dot,max



• The time at the maximum temperature rising

rate is preferred to define the ignition time

• The ELF at stage before ignition decreases

with increasing Φ, while the ELF at stage after

ignition shows less dependences on Φ

• The ELF at stage before ignition decreases

with increasing Tinit, while the ELF at stage

after ignition shows opposite dependences on

Tinit, with different Φ

Rich mixture Lean productsIgnition

Combustion process

Reduce exergy destruction

chinit

ELF 100%I

AExergy loss fraction：

10

14

18

22

26

Exer

gy

Loss

Fra

ctio

n (

%)

φ =0.5 φ =1.0 φ =2.0

(a) Totaln-hentane/air, p=50 bar

2

6

10

14(b) Stage before ignition

600 800 1000 1200 14002

6

10

14(c) Stage after ignition

T (K)

• Staged combustion：8

12

16

20

2410

14

18

22

26(a) Totaln-Heptane/Air, p=50 bar

(b) Stage before ignition

Φ=0.5 Φ=1.0 Φ=2.0

600 700 800 900 1000 1100 1200 13000

1

2

3

4(c) Stage after ignitionE

xerg

y L

oss

Fra

cti

on

[%

]

Tinit [K]

τ400 : Time at ΔT=400K relative to Tinit

τT_dot,max : Time at the maximum T_dot


* Reaction level: carbon number of the largest species participating in the reaction.

• The exergy destruction is mainly produced by the reactions of (C0-C3) and C7 species

• Reactions of (C0-C2) and C7 species are main exergy destruction sources of the stage before ignition

• The exergy destruction at the stage after ignition is mainly produced by the reactions of (C0-C2) species

C0 C1 C2 C3 C4 C5 C6 C70

2

4

6

8n-heptane/air, p=50 bar

Φ=0.5

Φ=1.0

Φ=2.0

Reaction Level

0

2

4

6

8

Tinit

=815.5 K

Tinit

=910.07 K

Tinit

=1046.4 K

Exe

rgy

Loss

Fra

ctio

n [

%]

n-heptane/air, p=50 bar

(a) Total

0

2

4

6

8

Tinit

=815.5 K

Tinit

=910.07 K

Tinit

=1046.4 K

(b) Stage before ignition

0.0

0.5

1.0

Tinit

=815.5 K

Tinit

=910.07 K

Tinit

=1046.4 K

(c) Stage after ignition

C0 C1 C2 C3 C4 C5 C6 C70

2

4

6

8

Φ=0.5

Φ=1.0

Φ=2.0

C0 C1 C2 C3 C4 C5 C6 C70.0

0.5

1.0

Φ=0.5

Φ=1.0

Φ=2.0



Reaction pathway analysis for exergy destructions

β-scission productscyclic ether+·OHolefin+HO2·olefin+R'·

O2

O2

products+·OH

KET+·OH·O2QOOH·QOOHRO

2·R·RH

600 800 1000 1200 14000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8(a)

Co

ntr

ibu

tio

n r

ati

o, α

T (K)

Red line φ=0.5

Green line φ=1.0

Blue line φ=2.0

n-heptane/air, p=50 atm

αLT α

H.Ab.

αHT

αNTC

Contribution

ratio, α

RH: n-Heptane

600 800 1000 1200 14000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8n-heptane/air, φ=1.0

Co

ntr

ibu

tio

n r

ati

o, α

T (K)

Red line p=30 bar

Green line p=50 bar

Blue line p=80 barα

LT

αH.Ab.

αHT

αNTC

(b)

DLT

D

for LTchain branching pathways

for C7reactions

E

E

DNTC

D

for NTC pathways

for C7reactions

E

E

DHT

D

for HT -scission pathways

for C7reactions

E

E

DH.Ab.

D

for H-atom abstraction pathways

for C7reactions

E

E

• Hydrogen abstraction from the fuel.

• Low Tinit (<800K)，LTC pathway.

• Intermediate Tinit (800K-1050K), NTC

pathway.

• High Tinit (>1100K), HTC pathway.


• Low Tinit, the formation of KET is the dominant exergy destruction source

• With increasing Tinit, the Gibbs formation energy of the β-scission products from R· radical decreases

• High Tinit，HT pathway such as the β-scission of fuel radical is the dominant exergy destruction source

Chemical kinetic pathway analysis for exergy destructions

F+·OH→R·+H2O Δ -24.1kcal molG Δ -25.2kcal molG F+·OH→R·+H2O Δ -26.7kcal molG

100

120

140

160

180

200

220

·QOOH

(c)T=1300 K

NC7KETO

NC7KET

·P(OOH)2

C7H15·

·O2QOOH

C7H15O2·

R·+olefin

R1·+R2·

NC7H16

ER(OOH)

50

70

90

110

130

150

170

NC7KETO

NC7KET

ER(OOH)

·P(OOH)2

(b)T=925 K

·O2QOOH

·QOOH

R·+olefin

C7H15O2·

R1·+R2·

C7H15·

NC7H16

20

40

60

80

100

120

140

Gib

bs

Form

atio

n E

ner

gy

(kca

l·mo

l-1)

ER(OOH)

T=700 K(a)

R·+olefin

R1·+R2·

NC7KETO

NC7KET

·P(OOH)2

·O2QOOH

·QOOH

C7H15O2·

C7H15·

NC7H16

Fuel species decomposition

β-scission of fuel radical

LTC pathway

The detailed exergy destruction sources are significantly influenced by reaction pathways.

Reactions are largely driven by an increase in system entropy (ΔS), attributing to the Gibbsformation energy difference (ΔG) from products to reactants.


Exergy loss φ-T map and implications to IC engines

• Φ<2, 1000<T<2400, the exergy loss

region shows a peninsula shape.

• The ideally low exergy loss window

should be located in the region of Φ

<1.0 and T >2400 K.

• The exergy loss Φ-T map for n-

heptane has wide application.800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0

20.0%

17.5% 15.0%

12.5%

10.0%

Eq

iova

len

ce r

ati

o, φ

T (K)

7.5%(a)

Irreversibility

800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0

10%

20%

30%40%

50%

60%

70%

Eq

uiv

ale

nce

rati

o, φ

T (K)

80%

(b)

Incomplete combustion

800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0

p =100 bar

[O2] =21%

Eq

uiv

ale

nce

Rati

o

Temperature [K]

Residence time dependence

of 15% exergy loss.

τres

=1.0 ms

τres

=1.5 ms

τres

=2.0 ms

(a)

800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0

Pressure dependence

of 15% exergy loss.Eq

uiv

ale

nce

Rati

o

Temperature [K]

p=100 bar

p=70 bar

τres

=1.5 ms

[O2] =21%

(b)

800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0Oxygen concentration dependence

of 15% exergy loss.

τres

=1.5 ms

p =100 bar

Eq

uiv

ale

nce

Rati

o

Temperature [K]

[O2] =21%

[O2] =10%

(c)


800 1200 1600 2000 2400 2800

1.0

2.0

3.0

4.0

Exergy destructionHCCI

EGR diluted

Fast mixing

15%16%

18%

20%

21%

500 ppm

Soot

10%

Equ

ival

ence

rat

io

Temperature (K)

Ignition zone

[O2] 21%

0.25

1%

5%

15%

20%

5000 ppmNOx

Exergy loss φ-T map and implications to IC engines

CDC regime: avoids high exergy destruction region produced by chemical reactions, but it crosses both the soot and NOx formation regions

HCCI regime: avoids soot and NOxformation regions, but it locates in the high exergy destruction region

Split LTR/HTR combustion: Reforming the rich fuel/air mixtures (LTR) before ignition followed by the HTR combustion of lean reformed products

How to realize the combustion process: LTR-Rich

fuel/air mixture, HTR-Lean LTR species/air mixture





• Conclusions

Content


The Concept of Flexible Cylinder Engine (FCE)

Flexible Cylinder Engine (FCE)

Operation principle:

Flexible cylinder＋Work cylinder

Fuel is reformed by the flexible cylinder (rich

mixture), the reformed species mix with the air,

then the mixture is rebreathed into the work

cylinders (lean mixture).

Controllable reformed species (Controlled

kinetic pathway of LTR)Multi-combustion mode can be achieved

Split HCCI combustion (LTR and HTR in different cylinder)

Single fuel RCCI combustion: reformed products＋DI micro-fuel(reformed species control mixture reactivity + DI fuel control mixture stratification)

LTC : Low temperature combustion controlled by EGR


The Concept of Flexible Cylinder Engine (FCE)

Advantages:

Split kinetic process: LTR+HTR

Controlled kinetic pathway : LTR of rich

mixture + HTR of lean mixture to decrease

Exergy loss

Controlled heat release phase to improve

thermal efficiency (Mixture reactivity)

Decrease emissions

H2, CO

CO2

CH2O &

aldehyde

𝑹𝑯

ሶ𝑹

𝑹 ሶ𝑶𝟐

𝑸𝑶𝑶𝑯

ሶ𝑶𝟐𝑸𝑶𝑶𝑯

𝑲𝒆𝒕𝒐𝒏𝒆.+ ሶ𝑶𝑯

𝑳𝑻 𝑩𝒓𝒂𝒏𝒄𝒉𝒊𝒏𝒈

𝑶𝟐

𝜷−scission

olefin+ ሶ𝑹′

𝑶𝟐

Products of LTR

olefin+HO2

peroxides+OH

𝜷 − scission species

By adjusting the operation parameters of both the

reforming cylinder and the work cylinders, the

combustion reaction pathways can be flexibly controlled

to reduce the pollutant emissions and the exergy loss

and thus to improve the fuel efficiency.


Reforming boundary in flexible cylinder

Larger low-temperature reaction region outlines obtained as initial temperature, pressure, and ϕ decreased.

As the ϕ increases, the starting reaction line begins to move toward higher initial temperature regions

As the reformed products thicken, the reformed area being narrow.

The fuel undergoes low-temperature reformed rather than rapid oxidation or no reaction under the ϕ being 1.0-2.0.


Reformed Products Reactivity

19

Mole fraction

n-heptane PRF90

n-heptane 1.69E-04 1.44E-04

iso-octane 0 6.53E-03

H2O 6.18E-02 3.25E-02

CO 1.31E-02 2.10E-02

CH2O 7.11E-05 5.89E-04

H2O2 2.88E-03 3.96E-04

C3KET13 3.33E-04 4.05E-06

H2 1.45E-06 4.03E-05

CH4 5.64E-06 2.69E-04

C2H2 5.55E-05 1.19E-06

C2H4 3.21E-06 1.19E-06

C2H6 0 2.06E-05

C3H6 0 5.11E-06

CH3CHO 0 3.21E-06

CH3O2H 0 4.03E-03

CH3COCH3 0 3.06E-05

IC4H8 0 6.32E-05

SpeciesFuel

Aldehydes, ketones, and peroxides would be produced by both

high/low reactivity fuels The low-temperature reformed intermediates of PRF90 are more

than n-heptane, and the key species of CH3O2H and CH3COCH3

could be produced in this reforming process

The concentration of ketones from PRF90 ia lower than that of

produced from n-heptane

Low-reactivity fuel could generate more high-reactivity reformed

speciess

The effect of operation conditions on reforming process


Experimental setup

The FCE test engine is being modified. A reformer was used to substitute the reforming cylinder. The

preliminary experiment was carried on an optical engine.

Schematic diagram of optical diagnostic systemSchematic diagram of LTR system for the optical engine


Effects of temperature on reformed species

• If the temperature is below 650K，the reformed species include fuel, aldehyde (CH3CHO), and some short chain alkanes, olefin and alkyne. If the temperature is above 650K, more and more CO and CO2 are generated, the fuel is almost completely converted to reformed species.

21

PRF50


• High temperature would shorten the pathway for reformed species. If the temperature is at 750K,

CO mainly generates from formaldehyde (CH2O).

22

Effects of temperature on reformed species


Split LTR/HTR HCCI combustion: reformed species mix with air, then

breath into the cylinder.

The critical temperature of reformed species reactivity of n-heptane,

PRF50 and PRF90 is 550K, 650K and 650K

When the reforming temperature was less than the critical temperature

point, the reformed species could advance the combustion.

The PRF90 reformed species could result in misfire when the reforming

temperature was higher than 600K

23

Effects of Reformed Temperature on Reactivity

Advance

Advance

Advance

Misfire

Postpone

Postpone


Reformer (case2-5) can improve combustion efficiency (95.05%)， However, the combustion stability becomes worse if the temperature is too high

High reactivity can reduce CO and HC emissions

Reformer (case2-5) can improve engine efficiency (39%-44%)24

Effects of reformed temperature on performance and emissions


Single fuel RCCI

• Reforming can improve mixture

reactivity or decrease mixture reactivity

• For high reactivity fuel, the reforming

species could be reduces by

controlling the reforming condition

Fuel:

direct injection

Reformates:

port injection

Physical process:

mixture stratification

controlling

Chemical process: controllable

reaction pathway, reforming

and reactivity controlling

Fuel

Main-fuel reformed ( LTR)

+DI micro-fuel

Single-fuel RCCI

• The experimental study was carried out on an

optical single cylinder engine

Parameter Value

Engine speed 1200 r/min

Intake pressure/temperature 0.1 MPa/398 K

Injection pressure/timing 600 bar/-15 ºCA ATDC

Injected n-heptane mass per cycle 8 mg

Reformed n-heptane mass per cycle 9 mg

Reforming temperature 423 K, 523 K, 623 K

Reformed equivalence ratio 8

Overall equivalence ratio 0.52


• Reforming products

rebreathing the cylinder don’t

conduct the further low

temperature reactions prior to

in-cylinder fuel injection due to

the decreased reactivity.

CH2O-PLIF images

• Reforming retards and slows

down low temperature heat

release.

Low temperature heat release


High temperature heat release

Natural flame

luminosity

Chemiluminescence

Soot luminosity (higher)

Red component of spatially

integrated flame luminosity (SIFL)

Soot emission

Typical single-shot CH2O-PLIF images

• Reforming decreases the combustion

rate.

• Reforming reduces the soot formation.


Effects of reforming species on ignition

Effect Species (Advancing ignition) Species (Retarding ignition)

Thermal effect ---- All GC-identified species

Chemical effect Acetylene (C2H2), acetaldehyde (CH3CHO), acrolein

(C2H3CHO), n-butyraldehyde (NC3H7CHO), ethene

(C2H4), propene (C3H6), 1-butene (1-C4H8), butadiene

(C4H6), 1-pentene (1-C5H10), 1-hexene (1-C6H12), 1-

heptane (1-C7H14), 2-heptane (2-C7H14)

Carbon monoxide (CO), Hydrogen (H2), methanol (CH3OH),

formaldehyde (CH2O), propionaldehyde (C2H5CHO), acetone

(CH3COCH3), butanone (C2H5COCH3), methane (CH4), ethane

(C2H6), propane (C3H8), 1,3-pentadiene (1,3-C5H8), 3-heptane

(3-C7H14)

Overall effect Acetylene (C2H2), acetaldehyde (CH3CHO), acrolein

(C2H3CHO), n-butyraldehyde (NC3H7CHO), propene

(C3H6)

Hydrogen (H2), carbon monoxide (CO), methanol (CH3OH),

formaldehyde (CH2O), propionaldehyde (C2H5CHO), acetone

(CH3COCH3), butanone (C2H5COCH3), methane (CH4), ethene

(C2H4), ethane (C2H6), propane (C3H8), butadiene (C4H6), 1,3-

pentadiene (1,3-C5H8), 1-pentene (1-C5H10), 1-heptane (1-

C7H14), 2-heptane (2-C7H14), 3-heptane (3-C7H14)


Combined impact of reforming species on ignition

Combined impact of reforming species on

ignition through chemical calculation

• Combined

effect of

reforming

species on

main reactions:

Effect of reforming species on the rate of the

predominant reactions at the LTHR peak

C2H2 + O2 = HCCO + OH

C2H4 + OH = C2H4OH

C2H4 + OH = C2H3 + H2O

C2H4 + H (+M) = C2H5 (+M)

C7H14 + OH = C7H13 + H2O

• Combined effect of reforming

species (e.g. C2H2 , C2H4 , C7H14) on

active radicals (e.g. OH, H):

+

Ignition is delayed due to the combined effect of reforming species.


On-line identified species at reforming temperatures of 523 K and 623 K

CA10, CA50 and fuel conversion versus reforming temperature

• There is a wide variety of reforming species.

• There is an increase in the mole fraction of reforming products

with the increasing reforming temperature.

• Ignition timing is delayed due to reforming. The overall indicated

thermal efficiency can be improved.

• The combustion phase is further retarded with increasing reforming

temperature due to larger fuel conversion.

Engine performance

Average cylinder pressure and apparent heat release rate





• Conclusions

Content


Conclusions

• A new combustion technology based on the flexibly controllable chemical kinetic pathway for IC engine

is proposed. Multi-combustion mode can be achieved, such as split LTR/HTR HCCI, single-fuel RCCI

and LTC.

• The detailed exergy destruction sources are significantly influenced by the reaction pathways and species

Gibbs formation energy difference (ΔG). HCCI regime avoids the NOx and soot formation, but it locates

in the high exergy destruction region.

• The combustion pathway that fuel reforming before ignition followed by the low temperature combustion

of lean reformed products offers the potential to simultaneously reduce exergy destruction and avoid soot

and NOx formation.

• Reformed temperature affects the mixture reactivity, if the temperature is too high, the reactivity

decreases. Split LTR/HTR HCCI can decrease the exergy loss, improve combustion efficiency, thus

increase the indicate efficiency, and decrease HC and CO emissions.

• Single-fuel RCCI is achieved through rebreathing reforming products and injecting fuel into cylinder

directly. The overall indicated thermal efficiency could be improved.


Thanks for your attention!

Acknowledgements

• The project was supported by the National Science Foundation of China

(Grant No.91541205)

• This work was finished by Ph. D student Daojian Liu, Yang Wang, Chao Gen.

Dr. Haifeng Liu and Dr. Hu Wang also contributed to this work.

a new combustion technology based on the flexibly...

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