A Comparison of Combustion and Emissions of Diesel Fuels and Oxygenated Fuels in a

Modern DI Diesel Engine

Eric Kurtz, Douglas Kuhel, James E. Anderson, Sherry A. Mueller

This work was supported by the U.S. Department of Energy, under special project number DE-FC26-07NT43278.

Acknowledgement: Dennis Miller, Lars Peereboom, and Carl Lira at Michigan State University for supplying the fuels used in this study.

Outline

• Background • Experimental Method • Experimental Results • Summary & Conclusions

This work has been published in SAE 2012-01-1695

Outline

• Background • Experimental Method • Experimental Results • Summary & Conclusions

MSU Fuels Investigations • Canola-based FAMEs (CME) has relatively good cold

flow properties & oxidative stability.

• DBS further improves the cold flow properties of CME.

• To stay within the 40 CN U.S requirement, DBS content in CME must be limited to 40%.

Tem

pera

ture

(oC

)

Dibutyl succinate (DBS)

Single-Cylinder Study Objectives

Study the influence of selected oxygenated fuels on combustion and emissions in a modern diesel engine

– Conventional Combustion – Low Temperature Combustion (LTC)

Outline

• Background • Experimental Method • Experimental Results • Summary & Conclusions

Fuels Tested Mineral Diesel Fuels

(Control group) Oxygenated Fuels

720 727 668* CME 60-40

Cetane No. 45.6 41.8 56.5 50.8 40.8

NHV (MJ/kg) 42.9 42.4 43.2 37.4 33.2

H:C ratio 1.81 1.775 1.952 1.88 1.86

O:C ratio 0 0 0 0.11 0.19

Aromatics 28.6% 32.4% <5% 0% 0%

*A low aromatic diesel fuel included in the study (97.7% saturates)

Test Conditions

CC1 CC2

CC3 CC4

CC5

CF1

CF2 CF3

A100 C100

25-4

• A single-cylinder version of the production 6.7L V8 PowerStroke®. • Evaluated over the entire engine map.

Test Procedure Testing attempted to mimic diesel engine controls

• Calibration settings are based on engine speed and fuel quantity – Fuel pressure – Pilot fuel quantity

Test Procedure • Established base calibration settings using 720 fuel (46 CN) • Identical settings for all fuels:

• Select conditions also tested with constant injected fuel energy by adjusting the quantity of each fuel pulse (adjusted for fuel NHV).

– Main SOI – Pilot SOI

– EGR rate – Boost pressure

Rail pressure

Main quantity

Pilot quantity

Pilot SOI

Main timing

EGR rate

Conventional 720 calibration setting 720 SOI Sweep LTC 720 cal. settings No pilot 720 SOC Sweep

Outline

• Background • Experimental Method • Experimental Results

– Emissions – Combustion noise (not presented) – BSFC & Efficiency (not presented)

• Summary & Conclusions

NOx and Oxygenated Fuels

• NOx emissions appear to be primarily a function of intake oxygen concentration for both fuels (independent of fuel oxygen content)

• At the same intake O2, no statistical difference in NOx was observed with oxygenated fuels

• EGR is typically controlled based on a EGR rate or air mass flow

• Fuel O increases the total intake O2 for a given EGR rate – NOx increases

14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0Intake Oxygen Concentration

24 26 28 30 32 34 36 38 40 42 44 46 48 50EGR Rate

V8_p

rj.TP

.BSN

OX

[g/k

Wh]

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

2.4

Increasing EGR Increasing EGR

Proj

ecte

d BS

NO

X [g

/kW

h]

Proj

ecte

d BS

NO

X [g

/kW

h]

720 diesel fuel100% CME

1200 1500 1800 2100 2400 2700 3000 3300 3600 3900 4200 4500Engine_Speed [rpm]

BMEP

[bar

]

0

2

4

6

8

10

12

14

16

18

20

5

10

15

2025

30

3545

Decreasing EGR

1200 1500 1800 2100 2400 2700 3000 3300 3600 3900 4200 4500Engine_Speed [rpm]

BMEP

[bar

]

0

2

4

6

8

10

12

14

16

18

20

5

10

15

2025

30

3545

Decreasing EGR

BM

EP [b

ar]

Engine Speed [rpm]

24 26 28 30 32 34 36 38 40 42 44 46 48 50EGR Rate

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

2.4

Increasing EGR

Proj

ecte

d BS

NO

X [g

/kW

h]

Example Control Scenario • Commanded fuel quantity will increase

to adjust for lower fuel energy.

• As commanded fuel quantity increases, typically EGR rate decreases, boost and injection pressure increase.

• Leads to a further increase in NOx emissions.

1

2

1 – higher intake O2

2 – lower EGR rate, higher boost pressure, higher injection pressure

720 diesel fuel100% CME

Fuel

Qua

ntity

Particulate Emissions

80.0

82.5

85.0

87.5

90.0

92.5

95.0

97.5

100.0

CME 60-40Perc

ent P

M R

educ

tion

vs. 7

20 F

uel

CC2CC3CC4CC5A100C100LTC_CC2LTC_CC3LTC25-4

• Large PM reductions with both oxygenated fuels • Mechanism #1: PM reduction is due to displacement of aromatic

− A relatively small PM reduction with low aromatic fuel (668) − PM reduction with 668 was not statistically significant

• Mechanism #2: PM reduction is the result of fuel oxygen − PM reduction is consistent with fuel oxygenation − Consistent with estimated oxygen equivalence ratio at the lift-off length

CME 60-40

~97%

~91%

%PM

redu

ctio

n vs

. 720

[g

/kW

h]

CC2CC3CC4CC5A100C100LTC_CC2LTC_CC3LTC25-4

Hydrocarbon Emissions

-18 -12 -6 0 6 12 18 24 30 36CA [deg]

Cyl1.

RoHR

[J/de

g]

-40

-20

0

20

40

60

80

100

120

Inj.AS

E-OL

[volt

s]

-10

0

Pilot injection command

Main injection command

Rat

e of

Hea

t Rel

ease

(J/d

eg)

Inje

ctio

nRed – 720 (46 cetane)Green – 60-40 (41 cetane)Black – CME (51 cetane)

-18 -12 -6 0 6 12 18 24 30 36CA [deg]

Cyl1.

RoHR

[J/de

g]

-40

-20

0

20

40

60

80

100

120

Inj.AS

E-OL

[volt

s]

-10

0

Pilot injection command

Main injection command

Rat

e of

Hea

t Rel

ease

(J/d

eg)

Inje

ctio

nRed – 720 (46 cetane)Green – 60-40 (41 cetane)Black – CME (51 cetane)

Conventional Combustion • Higher HC emissions observed

with the 60-40 blend. • Pilot heat release was weak with

the 60-40 blend due to low energy content and low cetane number.

LTC • Results track with cetane

number rather than oxygenation − 668 & CME: low HC − 727 & 60-40: high HC

• True also of combustion noise (not shown)

∆H

C v

s. 7

20 F

uel [

g/kW

h]

High cetane

Low cetane

HC Emissions with Compensation

-1

0

1

2

3

CC3 CC4 A100 LTC_CC2 LTC_CC3

BSHC

Diff

eren

ce R

elat

ive

to 7

20 F

uel [

g/kW

-h]

CMECME w/comp60-4060-40 w/comp

• Equivalent HC emissions with the 60-40 blend vs. the base fuel once injected quantity was adjusted for fuel energy content

• Adjusting quantity reduced HC in LTC for both oxygenated fuels − Increased load − Shorter ignition dwell

CME

CME w/comp60-4060-40 w/comp

∆H

C v

s. 7

20 F

uel [

g/kW

h]

Outline

• Background • Experimental Method • Experimental Results • Summary & Conclusions

Effect of Oxygenation - Summary

Conventional Combustion

Low temperature Combustion

NOx Same as diesel fuel1

PM Decreased significantly w/ fuel oxygen HC Same as diesel2 Function of cetane3

Noise Same as diesel2 Function of cetane

Thermal Efficiency Same as diesel2

Fuel Consumption Degraded due to lower NHV4

1 Maintaining calibration settings, including intake O2. 2 Adjusting injected fuel quantity for fuel energy content. 3 Lower HC when injected fuel quantity adjusted for fuel energy content. 4 A function of fuel energy density.

Conventional Combustion

Low Temperature Combustion

NOx Oxygenation had no effect1

PM Decreased significantly w/ fuel oxygen HC Same as diesel2 Function of cetane3

Noise Same as diesel2 Function of cetane

Thermal Efficiency Oxygenation had no effect2

Thank you!

Engine Description

Type Single-cylinder Cycle 4-stroke Valves per cylinder 4 Bore 99 mm Stroke 108 mm Displacement 0.83 L Compression Ratio 16.2:1 Maximum Rail Pressure 2000 bar Combustion system design Chamfered * Engine & combustion system specifications matched the production 6.7L PowerStroke®

Combustion Noise

• Differences in combustion noise correlate with cetane number of the test fuel in both conventional combustion and LTC.

• Compensation for NHV reduces slightly difference from 720 fuel.

High Cetane Low Cetane 668 CME 727 60-40

CDC Lower Similar LTC Similar Lower

∆N

oise

vs.

720

[g/k

Wh]

BSFC & Thermal Efficiency • Higher BSFC with

oxygenated fuels − Lower NHV − Lower BMEP

• Thermal efficiency of CME was comparable to the diesel fuels

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

Ther

mal

Effi

cien

cy d

iffer

ence

from

720

Fue

l

727668CME60-40

CC1 CC2 CC3 25-4CC1 CC2 CC3 CC4 CC5 A100 C100 LTC LTC LTC LTC

Chassis Cert Part Load Dyno Cert LTC

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

Ther

mal

Effi

cien

cy d

iffer

ence

from

720

Fue

l

727668CME60-40

CC1 CC2 CC3 25-4CC1 CC2 CC3 CC4 CC5 A100 C100 LTC LTC LTC LTC

Chassis Cert Part Load Dyno Cert LTC

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

CC3 CC4 A100 LTC_CC2 LTC_CC3

Proj

ecte

d Br

ake

Ther

mal

Effi

cien

cy D

iffer

ence

Rel

ativ

e to

72

0 [a

bs %

]

CMECME w/comp60-4060-40 w/comp

• Lower thermal efficiency with the 60-40 blend without fuel quantity adjustment – later combustion phasing

• Similar thermal efficiency for all fuels when injection quantity was adjusted for energy content

CME

CME w/comp60-4060-40 w/comp

∆B

TE v

s. 7

20 [%

] ∆

BTE

vs.

720

[%]

Additional Conclusions It is speculated that NOx increase found in the literature may be due to

– An increase in intake O2 with fuel oxygen content when EGR rate or air mass flow are controlled

– Reduced EGR, increased boost and increased injection pressure when the commanded fueling injection is increased to meet torque demand with oxygenated fuels (lower energy content)

– When the intake O2 and engine calibration are the controlled to the same value, oxygenated fuels do not appear to have a negative impact on NOx emissions in a modern diesel engine