PengYe Dissertation 2011

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The Pennsylvania State University

The Graduate School

John and Willie Leone Family Department of Energy and Mineral Engineering

INVESTIGATION OF IMPACT OF FUEL INJECTION STRATEGY AND

BIODIESEL FUELING ON ENGINE EMISSIONS AND PERFORMANCE

A Dissertation in

Energy and Mineral Engineeringby

Peng Ye

2011 Peng Ye

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

December 2011


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Abstract

Both biodiesel fueling and changes of fuel injection pressure have significant

impacts on diesel engine emissions. The investigations of their impacts on engine exhaust

NOx and particulate matter emissions were conducted with an 8-cylinder common-rail

turbocharged direct injection diesel engine using ultra low sulfur diesel fuel and soybean

methyl ester (SME) based biodiesel blends. The engine was running at moderate speed

and different loads. Three fuel injection parameters: start of injection, fuel injection

pressure and fuel injection duration were investigated to investigate their impact on

engine emissions. With the control of fuel injection strategy, it is shown in this work that

the biodiesel engine NOx emission penalty can be eliminated.

A fuel spray, mixture stoichiometry field and lift-off length model was employed

to explain the variations of NOx emission from biodiesel fueling and change of fuel

injection strategy. Linear correlations between the average oxygen equivalence ratio of

the fuel-air mixture at the autoignition zone near the lift-off length and brake specific

NOx emissions were observed for all load conditions, regardless of fuel type. This

confirms that the dominant factor that determines NOx emissions is the ignition event

controlled by the oxygen equivalence ratio at the autoignition zone.

The impact of late in-cylinder (post) injection combustion with biodiesel on

lubricating oil dilution was investigated in this work. It is shown that this injection

strategy could effectively decrease engine NOx emissions, while increase the CO and

unburned hydrocarbon emissions. The lubricating oil dilution depends on the post

injection timing: an increase in the lubricating oil dilution can be only observed if the

post injection timing is later than 45 after top dead center.


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The impacts of fuel injection pressure on diesel and biodiesel soot morphology

and oxidative reactivity were investigated. It is shown that compared with engine

condition and fuel injection pressure, biodiesel has much less significant impact on soot

morphology. For soot oxidative reactivity, it is found that both diesel and biodiesel soot

from higher fuel injection pressure have higher reactivity, and biodiesel soot has higher

reactivity than diesel soot when both of them are obtained from the same injection

pressure.

The optimized apparent heat release pattern for improved engine thermal

efficiency was investigated with a zero-dimensional engine thermodynamic simulation.The results suggest that the optimized apparent heat release is a wide and low peak.

The reason for this kind of heat release is that it can decrease the in-cylinder temperature

and consequently decrease heat loss.


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Table of Contents

List of Figures ................................................................................................................... vii

List of Tables. .................................................................................................................. xiv

Nomenclature .................................................................................................................... xv

Acknowledgements ........................................................................................................ xviii

Chapter I. Introduction ................................................................................................... 1

Chapter II. Literature Review.......................................................................................... 7

2.1 Biodiesel background ............................................................................................ 7

2.2 Impact of biodiesel fueling on engine emissions ................................................ 13

2.2.1 Nitric oxides (NOx) ....................................................................................... 15

2.2.2 Particulate Matter (PM) ................................................................................. 19

2.3 Origin of biodiesel emission effect ..................................................................... 22

2.3.1 Biodiesel NOx effect...................................................................................... 22

2.3.2 Biodiesel particulate matter effect ................................................................. 27

2.4 Fuel Injection strategy with biodiesel fueling ..................................................... 29

2.5 Post injection strategy ......................................................................................... 33

2.6 Biodiesel and fuel injection pressure on PM morphology and soot oxidative

reactivity .............................................................................................................. 362.6.1 PM morphology ............................................................................................. 36

2.6.2 Soot oxidative reactivity ................................................................................ 40

2.7 Impact of biodiesel fueling, injection strategy and heat release pattern on Engineefficiency ............................................................................................................. 42

Chapter III. Technical Approach .................................................................................... 49

3.1 Experiment apparatus.......................................................................................... 49

3.2 Engine emission bench: ...................................................................................... 50

3.3 Test fuel .............................................................................................................. 513.4 Engine test conditions ......................................................................................... 53

3.5 Biodiesel lubricating oil dilution ........................................................................ 57

3.6 Soot characterization ........................................................................................... 60

Chapter IV. Investigation of injection strategy and biodiesel fueling on diesel engineemissions and performance ........................................................................ 64


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4.1 Impact of injection timing and fuel injection pressure on NOx emissions ......... 64

4.2 Impact of injection timing and fuel injection pressure on particulate matter (PM)emissions ............................................................................................................. 68

4.3 NOx-PM trade-off ............................................................................................... 72

4.4 Brake thermal efficiency ..................................................................................... 784.5 Apparent heat release analysis ............................................................................ 83

Chapter V. Origin of biodiesel NOx effect ................................................................... 92

Chapter VI. Investigation of late in-cylinder injection strategy on engine emissions andlubricant fuel dilution ............................................................................... 108

6.1 Exhaust Emissions ............................................................................................ 108

6.2 Exhaust Temperature and BSFC ....................................................................... 110

6.3 Heat Release Analysis....................................................................................... 112

6.4 Oil Dilution Level, Total Acid Number (TAN) and mfMEP ........................... 120

6.5 Analysis of Hydrocarbons from Lubricant and Exhaust................................... 123

Chapter VII. Impact of fuel injection pressure on soot morphology and oxidativereactivity ................................................................................................... 129

7.1 Morphology analysis ......................................................................................... 129

7.2 Analysis of soot oxidative reactivity................................................................. 139

Chapter VIII.Optimized heat release pattern to achieve high engine efficiency ............ 149

8.1 Model formulation ............................................................................................ 149

8.2 Computational procedure .................................................................................. 156

8.3 Optimized heat release pattern probing ............................................................ 163

Chapter IX. Conclusions and recommendations for future work ................................. 193

References. .................................................................................................................. 199

Appendix A: Matlab code ............................................................................................... 229

Appendix B: Repeatability of thermogravimetric analysis ............................................. 236

Appendix C: Repeatability of Raman analysis .............................................................. 239

Appendix D: Repeatability of Morphology analysis ...................................................... 240

Appendix E: Repeatability of particle size distribution (SMPS) result .......................... 242


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List of Figures

Figure II-1: A diagram representing the transesterification process, replicated from [6].The general scheme of the process is to modify a triglyceride (a fat or oil

which is a glyceride with three fatty acids, represented as R1, R2 andR3)into a fatty acid methyl ester. ....................................................................... 8

Figure II-2: NOx, PM, CO and HC % changes as biodiesel blending percentage increaseas determined by the EPA through statistical regression of publicallyavailable data on highway heavy-duty truck engines [18, 51]. Taken from[44] ............................................................................................................. 14

Figure II-3: Diesel soot agglomerate composed of spherical primary particles, taken fromRef. [168] .................................................................................................... 37

Figure II-4: A sketch of limit-pressure cycle .................................................................... 46Figure II-5: Indicated efficiency map with and .......................................................... 47

Figure III-1: FAME composition of B100 soybean oil methyl ester ................................ 52

Figure III-2: Molecular structures[217], names, abbreviations, chemical formulas, andChemical Abstracts Service (CAS) numbers of compounds comprising thebiodiesel used in this work. ........................................................................ 53

Figure III-3: Thermophoretic sampling apparatus ............................................................ 57

Figure III-4: FTIR spectrums for oils with different biodiesel dilution............................ 58

Figure III-5: Correlation between absorbance of Peak 1746 cm-1from FTIR and dilutionlevel ............................................................................................................ 59

Figure III-6: Demonstration of curve fitting for Raman spectrum ................................... 63

Figure IV-1: NOx emissions at 25% load. Four injection timings (9, 5, 1 before topdead center and 3 after top dead center) were tested for both ultra lowsulfur diesel (ULSD) and B40 blend. Three fuel injection pressures (52, 60and 80 MPa) were tested at each injection timing ...................................... 65

Figure IV-2: NOx emissions at 50% load. Four injection timings (9, 7, 5, 3 1 beforetop dead center and 1, 3 after top dead center) were tested for both ultralow sulfur diesel (ULSD) and B40 blend. Three fuel injection pressures (72,90 and 108 MPa) were tested at each injection timing for ULSD, and sixfuel injection pressures (72, 80, 90, 98, 108 and 118 MPa) were tested ateach injection timing for B40. .................................................................... 66


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Figure IV-3: NOx emissions at 75% load. Four injection timings (9, 5, 1 before topdead center and 3 after top dead center) were tested for both ultra lowsulfur diesel (ULSD) and B40 blend. Three fuel injection pressures (87,109 and 131 MPa) were tested at each injection timing. ............................ 67

Figure IV-4: PM emissions at 25% load. Four injection timings (9, 5, 1 before top deadcenter and 3 after top dead center) were tested for both ultra low sulfurdiesel (ULSD) and B40 blend. Three fuel injection pressures (52, 60 and 80MPa) were tested at each injection timing. ................................................ 69

Figure IV-5: PM emissions at 50% load. Four injection timings (9, 5, 1 before top deadcenter and 3 after top dead center) were tested for both ultra low sulfurdiesel (ULSD) and B40 blend. Three fuel injection pressures (72, 90 and108 MPa) were tested at each injection timing. ......................................... 71

Figure IV-6: PM emissions at 75% load. Four injection timings (9, 5, 1 before top deadcenter and 3 after top dead center) were tested for both ultra low sulfur

diesel (ULSD) and B40 blend. Three fuel injection pressures (87, 109 and131 MPa) were tested at each injection timing. ......................................... 72

Figure IV-7: NOx-PM emission trade-off at 25% load. The legend shows the fuel andinjection timing for each test. Three fuel injection pressures (52, 66 and 80MPa) were tested at each injection timing for each fuel. Fuel injectionpressure is not distinguished in the plot. .................................................... 73

Figure IV-8: NOx-PM emission trade-off at 50% load. The legend shows the fuel andinjection timing for each test. Three fuel injection pressures (72, 90 and108 MPa) were tested at each injection timing for each fuel. Fuel injection

pressure is not distinguished in the plot ..................................................... 74

Figure IV-9: NOx-PM emission trade-off at 75% load. The legend shows the fuel andinjection timing for each test. Three fuel injection pressures (87, 109 and131 MPa) were tested at each injection timing for each fuel. Fuel injectionpressure is not distinguished in the plot ..................................................... 75

Figure IV-10: PM-NOx emission trade-off at the start of injection timing at 9 before topdead center for ultra low sulfur diesel and B40. (a) 25% load; (b): 50% load;(c): 75% load; (d): 75% load after Soxhlet of PM filters ........................... 77

Figure IV-11: Brake thermal efficiency at 25% load for ultra low sulfur diesel (left) andB40 (right). The multiple points at each start of injection timing are due tothe multiple fuel injection pressures. .......................................................... 79



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Figure IV-14: Apparent heat release profile of different fuels and different fuel injectionpressures at 25% load. The SOI: a) 9BTDC, b) 5BTDC, c) 1BTDC, d)3ATDC. ..................................................................................................... 85

Figure IV-15: Apparent heat release profile of different fuels and different fuel injectionpressures at 50% load. The SOI: a) 9BTDC, b) 5BTDC, c) 1BTDC, d)3ATDC. ..................................................................................................... 88

Figure V-2: Oxygen equivalence ratio () field for ULSD and B40 at different injectionpressures at the SOI of 7BTDC, mid-load ................................................ 99

Figure V-3: Oxygen equivalence ratio () field for ULSD and B40 at different injectionpressures at the SOI of 9BTDC, mid-load .............................................. 100

Figure V-4: The correlation between brake specific NOx emissions and average oxygenequivalence ratio of the fuel-air mixture at lift-off length at 25% load,regardless of fuel type. The legend: xB is x degree before TDC; xA is xdegree after TDC. ..................................................................................... 102



Figure V-7: semi-ln plot of absolute value of slope vs. SOI .......................................... 105

Figure V-8: semi-ln plot of Y-axis intercept (at X=0) vs. SOI ....................................... 106

Figure VI-1: Emissions of B20 engine test with various post injection strategies. Samplesare differentiated with post injection conditions while rest parameters werekept the same. The fuel for the non-ULSD test was B20. ........................ 109

Figure VI-2: Exhaust temperature and BSFC profile of the 100-hour test. Each point isthe average of one test period. The dash lines are used to divide zones ofeach post injection strategy. ..................................................................... 111

Figure VI-3: Pressure traces of test with different post injection strategies at both TDC(top) and late crank angle (bottom) .......................................................... 113


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Figure VI-5: Bulk cylinder temperature of tests with different post injection strategies.The arrow points to the lowest acceptable combustion temperature. ....... 117

Figure VI-7: GC-MS results of diesel, B100, lighter cut from 432C distillation of freshand used lubricant (top); Region of interest in the GC-MS results (bottom).The numbers at the diesel zone means the number of carbon in relevantparaffin. .................................................................................................... 124

Figure VI-8: GC-MS results of hydrocarbons collected from exhaust gases for variouspost injection strategies ............................................................................ 126

Figure VI-9: FAME composition of B100 and of the biodiesel in used lubricant. ........ 127

Figure VII-1: A transmission electronic microscopy (TEM) image of diesel soot at 30%load and 50 MPa fuel injection pressure .................................................. 130

Figure VII-2: A transmission electronic microscopy (TEM) image of B20 soot at 30%

load and 50 MPa fuel injection pressure .................................................. 131

Figure VII-3: Primary particle sizes for both diesel and B20 soot ................................. 133

Figure VII-4: Fractal dimensions for both diesel and B20 soot...................................... 134

Figure VII-5: Number of primary particles for both diesel and B20 soot ...................... 135

Figure VII-6: Particle size distribution of diesel and B20 combustion exhaust, 30% load,before thermodenuder ............................................................................... 136

Figure VII-7: Particle size distribution of diesel and B20 combustion exhaust, 30% load,

after thermodenuder ................................................................................. 137

Figure VII-8: Particle size distribution of diesel and B20 combustion exhaust, 60% load,before thermodenuder ............................................................................... 138

Figure VII-9: Particle size distribution of diesel and B20 combustion exhaust, 60% load,after thermodenuder ................................................................................. 139

Figure VII-10: Thermogravimetric analysis result of 30% load diesel and B20 soot, m/m0indicates the residual mass weight percentage. ........................................ 140

Figure VII-11: Thermogravimetric analysis result of 60% load diesel and B20 soot, m/m0indicates the residual mass weight percentage. ........................................ 141

Figure VII-12: Apparent oxidative reactivity of 30% load diesel and B20 soot ............ 143

Figure VII-13: Apparent oxidative reactivity of 60% load diesel and B20 soot ............ 144

Figure VII-14: AD1/Agratio with fuel injection pressure for both diesel and B20 soot . 146


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Figure VII-15: D1 FWHM values with fuel injection pressure for both diesel and B20soot ........................................................................................................... 147

Figure VIII-1: comparison of simulated motor pressure trace with engine data, running at1500 rpm, 30% load, single injection timing at 1 before top dead center,100 MPa fuel injection pressure ............................................................... 158

Figure VIII-2: Representation of apparent heat release (AHR) at 1500 rpm, 30% load, 1before top dead center injection, 100 MPa fuel injection pressure with aconvolution of two peaks: one Gaussian peak and one ExpConvExp peak.Top: difference between the engine data and fit curve; middle: comparisonof engine data and fit curve; bottom: deconvolution of artificial peaks. .. 159

Figure VIII-3: Representation of apparent heat release (AHR) at 1500 rpm, 60% load, 1before top dead center injection, 100 MPa fuel injection pressure with aconvolution of two peaks: one Gaussian peak and one ExpConvExp peak.Top: difference between the engine data and fit curve; middle: comparison

of engine data and fit curve; bottom: deconvolution of artificial peaks. .. 160

Figure VIII-4: comparison of simulated pressure trace with engine data, running at 1500rpm, 30% load, single injection timing at 1 before top dead center, 100MPa fuel injection pressure ...................................................................... 163

Figure VIII-5: Examples of apparent heat release with: h1/h1b = 0.2, h2/h2b=1.29;h1/h1b = 0.6, h2/h2b=1.17; h1/h1b = 1, h2/h2b=1 .................................. 166

Figure VIII-6: Effect of start of combustion on indicated thermal efficiency, h1/h1b = 0.2,h2/h2b=1.29; h1/h1b = 0.6, h2/h2b=1.17; h1/h1b = 1, h2/h2b=1 ............ 167

Figure VIII-7: Effect of start of combustion on brake thermal efficiency, h1/h1b = 0.2,h2/h2b=1.29; h1/h1b = 0.6, h2/h2b=1.17; h1/h1b = 1, h2/h2b=1 ............ 168

Figure VIII-8: Examples of apparent heat release with: h1/h1b = 1, h2/h2b=1; h1/h1b = 2,h2/h2b=0.66; h1/h1b = 3, h2/h2b=0.34; h1/h1b = 4, h2/h2b=0.02 .......... 169

Figure VIII-9: Effect of start of combustion on indicated thermal efficiency, h1/h1b = 1,h2/h2b=1; h1/h1b = 2, h2/h2b=0.66; h1/h1b = 3, h2/h2b=0.34; h1/h1b = 4,h2/h2b=0.02 .............................................................................................. 170

Figure VIII-10: Effect of start of combustion on brake thermal efficiency, h1/h1b = 1,

h2/h2b=1; h1/h1b = 2, h2/h2b=0.66; h1/h1b = 3, h2/h2b=0.34; h1/h1b = 4,h2/h2b=0.02 .............................................................................................. 171

Figure VIII-11: Examples of apparent heat release with: w1/w1b = 0.6, h2/h2b=1.17;w1/w1b = 0.8, h2/h2b=1.11; w1/w1b = 1, h2/h2b=1 ............................... 172

Figure VIII-12: Effect of start of combustion on indicated thermal efficiency, w1/w1b =0.6, h2/h2b=1.17; w1/w1b = 0.8, h2/h2b=1.11; w1/w1b = 1, h2/h2b=1 . 173


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Figure VIII-13: Effect of start of combustion on brake thermal efficiency, w1/w1b = 0.6,h2/h2b=1.17; w1/w1b = 0.8, h2/h2b=1.11; w1/w1b = 1, h2/h2b=1 ........ 174

Figure VIII-14: Examples of apparent heat release with: w1/w1b = 1, h2/h2b=1; w1/w1b= 2, h2/h2b=0.66; w1/w1b = 3, h2/h2b=0.34; w1/w1b = 4, h2/h2b=0.02 175

Figure VIII-15: Effect of start of combustion on indicated thermal efficiency, w1/w1b = 1,h2/h2b=1; w1/w1b = 2, h2/h2b=0.66; w1/w1b = 3, h2/h2b=0.34; w1/w1b= 4, h2/h2b=0.02 ...................................................................................... 176

Figure VIII-16: Effect of start of combustion on brake thermal efficiency, w1/w1b = 1,h2/h2b=1; w1/w1b = 2, h2/h2b=0.66; w1/w1b = 3, h2/h2b=0.34; w1/w1b= 4, h2/h2b=0.02 ...................................................................................... 177

Figure VIII-17: Examples of apparent heat release with different peak distances ......... 178

Figure VIII-18: Effect of start of combustion on indicated thermal efficiency with

different peak distance, L = l2-l1............................................................ 179

Figure VIII-19: Effect of start of combustion on brake thermal efficiency with differentpeak distance ............................................................................................ 180

Figure VIII-20: Examples of apparent heat release profiles for sharp peak (h1/h1b = 8)versus low and wide peak (w1/w1b = 8) .................................................. 181

Figure VIII-21: Indicated thermal efficiency for sharp peak (h1/h1b = 8) versus low andwide peak (w1/w1b = 8) ........................................................................... 182

Figure VIII-22: Brake thermal efficiency for sharp peak (h1/h1b = 8) versus load andwide peak (w1/w1b = 8) ........................................................................... 183

Figure VIII-23: Heat transfer loss for sharp peak (h1/h1b = 8) versus load and wide peak(w1/w1b = 8) ............................................................................................ 184

Figure VIII-24: Comparison of cylinder pressure for sharp peak (h1/h1b = 8) versus loadand wide peak (w1/w1b = 8) apparent heat release .................................. 185

Figure VIII-25: Indicated work for sharp peak (h1/h1b = 8) versus low and wide peak(w1/w1b = 8) ............................................................................................ 186

Figure VIII-26: P-V diagram for sharp peak (h1/h1b = 8) versus low and wide peak(w1/w1b = 8) ............................................................................................ 187

Figure VIII-27: Comparison of two apparent heat release (AHR) profiles with the samestart of combustion timing at 362.2 (2.2 after top dead center), indicatedefficiency of AHR1 = 49.74%, indicated efficiency of AHR2 = 47.23% 188


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Figure VIII-28: Comparison of cylinder pressure traces of two apparent heat release(AHR) profiles with the same start of combustion timing at 362.2 (2.2after top dead center), indicated efficiency of AHR1 = 49.74%, indicatedefficiency of AHR2 = 47.23% .................................................................. 189

Figure VIII-29: Comparison of two apparent heat release (AHR) profiles with the samestart of combustion timing at 350 (10 before top dead center), indicatedefficiency of AHR1 = 50.02%, indicated efficiency of AHR2 = 50.84% 190

Figure VIII-30: Comparison of cylinder pressure traces of two apparent heat release(AHR) profiles with the same start of combustion timing at 350 (10before top dead center), indicated efficiency of AHR1 = 50.02%, indicatedefficiency of AHR2 = 50.84% .................................................................. 191

Figure B-1: Repeatability of thermogravimetric analysis for diesel soot, 30% load and 75MPa .......................................................................................................... 238

Figure E-1: Repeatability of particle size distribution of exhaust gases running with B20at 30% load and 50 MPa fuel injection pressure ...................................... 242


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List of Tables

Table I-1: Emission regulation for light and heavy duty diesel engines [3] ....................... 2

Table II-1: Chemical structure of common fatty acids, taken from [38] ............................ 9

Table II-2: Chemical composition of biodiesel derived from different feedstock, takenfrom [39] ..................................................................................................... 10

Table II-3: Physical and chemical properties of common fatty acid methyl esters(FAMEs), taken from [39] .......................................................................... 10

Table II-4: The effects of property differences between soy methyl ester biodiesel or itsblend and petroleum diesel on engine parameters and fuel combustion. Apositive/negative sign in the difference column means increase/decrease;a positive/negative sign in the rest column means increase (oradvance)/decrease (or retarding) of the property. Taken from [44] ........... 12

Table III-1: Engine specifications ..................................................................................... 49

Table III-2: Engine operating parameters in Task 1 and 2 ............................................... 54

Table III-3: Engine operating parameters in Task 3 ......................................................... 55

Table III-4: Engine operating parameters in Task 4 ......................................................... 56

Table V-1: Input parameters during calculation ............................................................... 94

Table VI-1: Equivalence ratio , mean pressure P mean temperature Tand ignition delay

ID calculated for each post injection case. .............................................. 118

Table VI-2: Calculated UHC emission rate and consumed HC by combustion at latecrank angle. B20 is a fuel mixture blended with 20% v/v SME based B100and 80% v/v ultra low sulfur diesel. ......................................................... 120

Table VIII-1: Engine basic parameters used by friction model. ..................................... 155

Table C-1: Repeatability of Raman analysis for B20 soot, 30% load and 50 MPa fuelinjection pressure ...................................................................................... 239

Table D-1: Repeatability of number of primary particles and fractal dimension for B20soot, 60% load at 75 MPa fuel injection pressure .................................... 240

Table D-2: repeatability of measurement of diameter of primary particles of diesel soot,60% load at 125 MPa fuel injection pressure ........................................... 241


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Nomenclature

AZ = autoignition zone

AHR = apparent heat release

CO = carbon monoxide

PM = Particulate Matter

NOx = Nitrogen Oxides

IVC = inlet valve close

EVO = outlet valve open

LCA = late crank angle

LOL = Lift-Off Length

l1,l2= locations of artificial heat release peaks

h1, h2= height of artificial heat release peaks

w1= width of artificial Gaussian heat release peaks

l1b, l2b, h1b, h2b, w1b= location, height and width of apparent heat release at baseline

TDC = top dead center

TAN = total acid number

FAME = Fatty Acid Methyl Esters

SME = Soybean Methyl Esters

SMPS = scanning mobility particle sizer

SOI = Start of Injection

SOC = Star of Combustion

ULSD = Ultra Low Sulfur Diesel

UHC = Unburned Hydrocarbon


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mfMEP = mechanic fraction mean effective pressure

IMEPg = gross indicated mean effective pressure

ID = ignition delay

= overall equivalence raito

x = axial coordinate

r = radical coordinate

(x) = average equivalence ratio of a fuel spray at x

(A/F)st = stoichiometric air fuel ratio

= oxygen equivalence ratiox+= penetrate length scale of fuel jet

f= fuel density

a= air density

d = diameter of orifice

Ca = area-contraction coefficient

nC = nuber of carbon atoms in fuel

nH = number of hydrogen atoms in fuel

nO = number of oxygen atoms in reactants

H = lift-off length

Uf= velocity of injected fuel

Zst = stoichiometric mixture fraction of fuel

p = mean pressure

P = in-cylinder pressure

V = chamber volume


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T = in-cylinder temperature

T = mean temperature

postm = rate of the mass of post injected fuel

= engine thermal efficiency

= crank angle

= Cp/Cv, ratio of constant pressure heat capacity and constant volume heat capacity

qt= total heat added

qw= engine wall heat transfer


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Acknowledgements

I want to express my sincere gratitude to my academic advisor, Dr. Andr L.

Boehman, for his support, guidance, enthusiasm, patience and encouragement during this

study. Thanks to his excellent direction, I could explore new field of knowledge and

complete my Ph.D. study.

I would also like to express my gratitude to Dr. Daniel C. Haworth, Dr. Randy L.

Vander Wal and Dr. Yaw D. Yeboah for their valuable comments and suggestions to

improve the quality of my research and dissertation as my committee.

I also want to thank Dr. Magin Lapuerta Amigo from University of Castilla-La

Mancha, Spain and Dr. John R. Agudelo from Universidad de Antioquia, Colombia for

their generous help and instructions in the work of soot oxidative reactivity and engine

efficiency analysis.

I also want to thank Dr. Joseph M. Perez and his tribology group in the

Department of Chemical Engineering for their generous help in the lubricant analysis.

I also want to thank my group members at the Diesel Combustion and Emissions

lab for their help. In particular, I am grateful to Vince Zello, Greg Lilik and Bhaskar

Prabhakar for their help in the engine test stand, to Chenxi Sun for her help in soot TEM


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imaging, to Eduardo Barrientos for his help in TGA operating, to Dongil Kang and

Vickey Kalaskar for their general help and discussions during my research.

I also want to thank Dr. Dania A. Fonseca for her generous help in GC analysis

and Joe Stitt for the Raman spectroscopy analysis in the Material Characterization Lab.

I want to give my special thanks to my family for their support during my work.

The financial support and valuable discussions from Infineum, USA, GE

Transportation, Volvo Powertrain and Oak Ridge National Lab are heartilyacknowledged.


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Chapter I. Introduction

Since it was first developed in the late 19th century, the diesel engine has been

widely used in heavy duty transportation for its higher output torque and brake thermal

efficiency than gasoline engine [1]. Through the past century, although the fundamental

design of the diesel engine hasnt been significantly changed, the regulation for diesel

engine exhaust emissions, including carbon monoxide (CO), unburned hydrocarbon

(UHC), nitrogen oxides (NOx) and particulate matter (PM), are becoming increasingly

strict. Among these exhaust gases, the amount of CO and UHC from conventional engine

operating mode are much lower than NOx and PM, therefore the latter are more of

interest to regulators and researchers. Table I-1presents the current and future planned

emission regulations for both light duty and heavy duty vehicles from authorities

including environmental protection agency (EPA) and California government. Significant

difficulties can be expected for current emission control technologies. For example, the

EPA Tier 2 Bin 5 NOx regulation is likely beyond the reach of todays de-NOx

aftertreatment considering cold start, and in 2010 only 30% of current new cars in

California are able to meet the SULEV program [2]. For heavy duty transportation, very

few engines before 2010 are able to meet the US 2010 regulation [3], i.e., most on-road

engines need to be improved in emission control.


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Table I-1: Emission regulation for light and heavy duty diesel engines [3]

Light duty

(EPA Tier2)

NOx(g/mile)

PM(g/mile)

CO(g/mile)

HCHO(g/mile)

Bin 5 (2009

effective)

0.07 0.01 4.2 0.018

Bin 4 0.04 0.01 2.1 0.011Bin 3 0.03 0.01 2.1 0.011Bin 2 0.02 0.01 2.1 0.004Bin 1 0 0 0 0

Light duty(California)

SULEVprogram

0.02 0.01 1 0.004

Heavy dutyNOx(mg/kWh)

PM(mg/kWh)

CO(mg/kWh)

NMHC(mg/kWh)

US 2010 260 13 - 182

In addition to the emission regulation, current engine technologies are also facing

the challenge of sustainable development. Given that it is widely believed that the fossil

energies coal and petroleum, which have supported the past 200 years industrial

development may not be sufficient to satisfy the increased need for energy in the future,

studies on energy sustainability are more and more popular. Typically, sustainable

energy development involves three aspects: energy saving on demand side, replacement

of fossil fuel with renewable fuels and efficiency improvement in energy applications [4].

For engineering practice, the evaluations of applications of renewable fuels and

efficiency improvements are especially important, which is therefore the focus of the

present work. With respected to diesel engines, evaluation of biodiesel (primarily on the

emission impact as discussed previously) as a renewable fuel and engine thermal

efficiency improvements are investigated.

Biodiesel is a fuel which is comprised of monoalkyl esters of long chain fatty

acids derived from vegetable oils or animal fats [5], and it has been studied as an

alternative fuel to petroleum diesel for compression-ignition engines, both in neat form


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(i.e., 100% esters) or in blended form with diesel fuel [6]. Compared with petroleum

diesel fuel, biodiesel has many advantages: it is renewable and biodegradable; it has a

high flash point which improves transportation safety [7]; and it can reduce exhaust

levels of some regulated emissions including CO, UHC and PM [8-11]. However, some

disadvantages of biodiesel are limiting the growth of its usage, such as decreased

oxidative stability [12], degraded cold-flow performance compared with petroleum diesel

[7], and increased emissions of NOx [8-11, 13]. Among these, the increase of NOx

emissions has especially drawn researchers attention and attempts to explain the origin

of the biodiesel NOx effect have been made [14-18], which may contribute to potentialapproaches to biodiesel NOx reduction.

Traditionally, there are two ways for control of tailpipe NOx emissions: control of

in-cylinder combustion, and after treatment devices like NOx absorber catalysts and

selective catalytic reduction [19, 20]. The former can be affected by a variety of factors

including the physical and chemical properties of fuel, the engine control strategy (e.g.

fuel injection control), exhaust gas recirculation (EGR) ratio, and boost pressure. The

latter can be affected by the type of catalyst, exhaust gas composition and temperature of

the treatment device. The interest of this work focuses mainly on the engine in-cylinder

combustion control via fuel injection strategy, i.e., the impact of start of injection (SOI),

fuel injection pressure, fuel injection duration and multiply injections on the combustion

and, consequently, engine emissions.

Experiments conducted by previous researchers have shown that fuel injection

strategy can significantly affect engine emissions [21, 22]. For instance, it has been

shown that advancing the SOI can increase NOx emissions and decrease PM emissions


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[21, 23]. Note this observation was also claimed to contribute to the biodiesel NOx effect

in traditional mechanical (pump line nozzle type) fuel injection systems [23] given that

biodiesel has a higher bulk modulus of compressibility. Nevertheless, the modern

common-rail injection system will not be affected by the fuel compressibility [13]. Fuel

injection pressure can also significantly affect the engine emissions. Increased fuel

injection pressure can increase NOx emissions and decrease PM emissions [19, 20]. This

observation is of special interest with biodiesel fuel. Since biodiesel has a lower heating

value than petroleum diesel [24], a higher brake specific fuel consumption (increased

injection quantity per unit time) for biodiesel is observed [6], and the default enginecalibration will achieve the increased injection quantity by increasing both the injection

pressure and the injection duration. Therefore, this work also intends to investigate which

is the more significant factor for the biodiesel NOx effect: biodiesels chemical properties,

increased injection pressure or increased injection duration. Furthermore, it has been

reported by many researchers that multi-injection strategy, especially multi-injection with

post injection [a fuel injection considerable after top dead center (ATDC)], can improve

engine emissions [13, 25-30]. Therefore, a thorough investigation of the post injection

strategy is also conducted in this work.

The other aspect of sustainable energy development efficiency improvement in

energy applications (i.e., brake thermal efficiency of a diesel engine) is also an interesting

topic. Although biodiesel fueling introduces higher brake specific fuel consumption, the

brake thermal efficiency of a diesel engine operating with biodiesel has been observed to

be the same level as with diesel fuel [31]. Meanwhile, variation of fuel injection strategy,

which consequently affects combustion phasing, can be a significant factor to affect


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engine brake thermal efficiency [22]. Theoretical investigation by Hoppie et al. [32]

suggests that neither the Otto nor the Diesel cycle is optimum for realistic engines, and an

optimum rate of heat release may be attainable via hypergolic combustion. However, the

artificial heat release profiles employed by them are not suitable for practical engineering.

Hence in this work, the author applies an analysis of optimized heat release pattern,

which can be controlled by fuel properties and engine injection strategy, intending to

provide suggestions for high efficiency engine design. While it is acknowledged that

other factors such as turbo, EGR system and aftertreatment devices can also affect the

total brake thermal efficiency of a diesel engine, they are not included in the discussion ofthis work.

In summary, this work investigates the impact of biodiesel fueling and fuel

injection strategy, including start of injection, fuel injection pressure/duration and split

injection on diesel engine emissions and performance. The origins of the biodiesel NOx

effect and the change of NOx emissions by fuel injection strategy are investigated in this

work. In addition, a theoretical attempt to achieve high engine efficiency by optimizing

the apparent heat release pattern is also conducted in this work.

Objective:in this work the intention is to understand the relation between engine

emissions, biodiesel fueling and fuel injection strategy. The significance of factors

contributing to the biodiesel NOx effect is evaluated. In this work it is also intended to

understand the impact of post injection on lubricating oil dilution with biodiesel fueling,

and the impact of fuel injection pressure on diesel engine soot characteristics. The

optimized heat release pattern toward high engine thermal efficiency is also investigated.


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Hypothesis:

Main hypotheses of this work include:

Biodiesel NOx penalty can be eliminated by engine fuel injection control

The impact on NOx emission from engine fuel injection strategy control and

biodiesel fueling can be explained by the equivalence ratio at autoignition

zone near the flame lift-off length

Biodiesel fueling can introduce lubricating oil dilution with post injection

strategy

Soot from high injection pressure has higher oxidative reactivity since the

decreased soot precursor concentration can prolong the soot inception time.

Optimized heat release design is a sharp peak at top dead center

To achieve the objectives of this research plan, the study is divided into 5 tasks:

Task 1: Investigation of the impact on engine emissions of both biodiesel fueling

and injection strategy at different engine speeds and loads.

Task 2: Investigation of the origin of engine NOx emission variations from

biodiesel or injection strategy by evaluating the adapted spray model.

Task 3: Investigation of the impact of post injection on engine emissions and

lubricating oil dilution.

Task 4: Investigation of the impact of fuel injection pressure on diesel engine soot

oxidative reactivity with both diesel and biodiesel blends.

Task 5: Investigation of optimized heat release pattern to achieve high engine

thermal efficiency.


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Chapter II. Literature Review

2.1 Biodiesel background

Throughout this thesis biodiesel implies a fuel composed of glyceride-free mono-

alkyl esters of long-chain fatty acids converted from triglycerides such as biologically-

based fats and oils [33-35]. In fact, attempts to use biologically derived hydrocarbons

(not only biodiesel) in internal combustion engines have been conducted for more than a

century. For instance, in 1900 when Dr. Diesel exhibited his great invention the Diesel

engine at the Paris World Exposition for the first time, the engine was running on 100%

peanut oil [33, 34]. From the 1920s to the 1940s numerous applications of vegetable oils

as diesel engine fuels were reported simply based on the theme of providing tropical

colonies of European countries with an independent source of fuel [36]. The more

appropriate form of biologically-derived fuel (ethyl esters of palm oil), which meets the

present definition of biodiesel, was initially included in a Belgian patent issued to

Chavanne as early as 1937 [37]. However, the petroleum derived diesel fuel immediately

became the primary fuel for diesel engines, and the concept of biologically-derived fuel

hadnt drawn peoples attention until the 1970s due to the energy crises. Since then

vegetable oil-based fuels have been reconsidered among the alternative energy sources.

In addition, the feedstocks for biodiesel have also been expanded to a variety of vegetable

oils including soybean, rapeseed, sunflower, etc., which also creates a variation in both

physical and chemical properties of biodiesel [34].

The production of biodiesel mainly follows the transesterification process, which

can crack the large molecules (triglyceride) of vegetable oils/fats and transform them to


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fatty acid methyl esters (FAME) with similar molecular weight as diesel fuel. Figure II-1

presents a traditional transesterification process for biodiesel production. This reaction

requires methanol as a reactant and alkali or acid as catalyst, and the by-product of the

reaction is glycerol. Demirbas [35] proposes that the same reaction can also be promoted

with supercritical methanol, with which no catalysts are needed, and consequently

purification of the final product can also be simplified.

Figure II-1: A diagram representing the transesterification process, replicated from[6]. The general scheme of the process is to modify a triglyceride (a fat or oil whichis a glyceride with three fatty acids, represented as R1, R2 andR3) into a fatty acidmethyl ester.


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With different structures (length, saturation) in the fatty acid, the physical and

chemical properties of FAME may vary. Table II-1presents the chemical structure of

common fatty acids.

Table II-1: Chemical structure of common fatty acids, taken from [38]

Fatty acid Systematic name Structure* FormulaLauric Dodecanoic 12:0 C12H24O2Myristic Tetradecanoic 14:0 C14H28O2Palmitic Hexadecanoic 16:0 C16H32O2Stearic Octadecanoic 18:0 C18H36O2Arachidic Eicosanoic 20:0 C20H40O2Behenic Docosanoic 22:0 C22H44O2

Palmitoleic

cis-9-Hexadecenoic

Acid

16:1 C16H30O2

Oleic cis-9-Octadecenoic 18:1 C18H34O2

Linoleiccis-9,cis-12-Octadecadienoic

18:2 C18H32O2

Linoleniccis-9,cis-12,cis-15-Octadecatrienoic

18:3 C18H30O2

- cis-11-Eicosenoic 20:1 C20H38O2*xx:yindicatesxxcarbons in the fatty acid chain withydouble bounds

Although these FAMEs are found in biodiesel, the chemical composition of

biodiesel is different if the biodiesel is derived from different feedstock, as shown in

Table II-2. It can be seen that generally C16:0, C18:0, C18:1 and C18:2 are the major

FAMEs species in biodiesel. In the United States, the primary feedstock for biodiesel is

soybean oil [12], which predominantly contains unsaturated FAME C18:1, C18:2 and

C18:3, as well as, a small amount of saturated FAME C16:0 and C18:0. The difference

in physical and chemical properties of FAME can also be significant, as shown in Table

II-3. Saturated FAME with a long chain always leads to higher melting point and

kinematic viscosity, which is expected. However, the degree of saturation can

significantly affect certain properties including melting point, oil stability index and

cetane number.


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Table II-2: Chemical composition of biodiesel derived from different feedstock, tak

feedstockFatty acid composition (%)

14:0 16:0 18:0 20:0 22:0 16:1 18:1 18:2

Canola oil 0 4 2 0 0 0 61 22

Palm oil 1 45 4 0 0 0 39 11

Soybean oil 0 11 4 0 0 0 23 54

Sunflower oil 0 6 5 0 1 0 29 58

Corn oil 0 11 2 0 0 0 28 58

Cottonseed oil 1 23 2 0 0 1 17 56

Chicken fat 1 25 6 0 0 8 41 18

Beef tallow 4 26 20 0 0 4 28 3

Table II-3: Physical and chemical properties of common fatty acid methyl esters (FAME

FAMEMelting pointa

(C)

Heat value

(MJ/kg)

Kinematic

viscositybat 40 C

(mm2/s)

Oil stability indexc

(h), at 110 CCetane N

C14:0 19 39.45 3.3 >40 -

C16:0 31 39.45 4.38 >40 86

C16:1 34 39.3 3.67 2.1 51

C18:0 39 40.07 5.85 >40 101

C18:1 20 40.09 4.51 2.5 59

C18:2 35 39.7 3.65 1 38

C18:3 52 39.34 3.14 0.2 23

a: Ref. [7, 40]; b: Ref. [41]; c: Ref. [7]; d: Ref. [7]; e: Ref. [42]


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Although cetane numbers of the C18:2 and C18:3 FAMEs are relatively low, as a

mixture, biodiesel is often observed to have a higher cetane number than petroleum diesel

[39], which improves its performance in diesel engines. High unsaturation can improve

the cold weather performance, but also significantly decrease the oxidation stability.

Chemical properties of a fuel (e.g., hydrocarbon structure, aromatic content,

cetane number) dominate the fuels effect on combustion and pollutant formation [43].

Physical properties (e.g., viscosity, density) generally affect injection timing, fuel

atomization and fuel evaporation, which eventually have indirect effects on combustion

and emission formation. In other words, diesel engine may have a different responsewhen biodiesel is used. Table II-4presents a summary by Sun et al. [44] on the impact of

soy methyl ester (SME) based biodiesel and its blend with petroleum diesel on physical

and chemical properties and engine response. This summary also reveals the difficulty in

biodiesel research that multi-parameters can change simultaneously with biodiesel

fueling, so that the understanding of each parameters impact is not straightforward. For

instance, the lower heating value of biodiesel can lead to an increase injection pressure

and duration by the default engine calibration, which may also have impact on engine

emissions [21, 22]. This issue will also be addressed in this work.


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Table II-4: The effects of property differences between soy methyl ester biodiesel or its blend andparameters and fuel combustion. A positive/negative sign in the difference column mean

positive/negative sign in the rest column means increase (or advance)/decrease (or retarding) of thDifference Injection

timing

Injection

pressure

Fuel spray

penetration

Fuel spray

angle

Fuel spray

atomization

Igniti

delayPhysical Properties

Liquid densitya + + + - - +

Bulk modulus of

compressibilityb

+ + + +

Speed of sound + + + +

Liquid viscositya + + - - +

Surface tensiona + + + -

Vapor pressurea - - - +

Volatilityc - -

Liquid specific

heata

- + -

Vapor specific

heata

- + -

Heat of

vaporizationa

+ -

Chemical Properties

Chain length + -

Oxygen contentc + + -

Aromatics contente - +

Sulfur content -

Saturation (iodinevalue)d - +

Cetane numberg + -

Heating valuec -

a: Ref [45]; b:Ref. [23, 46]; c: Ref. [47]; d: Ref.[48]; e: Ref. [49]; f: Ref.[15]; g: Ref. [50]


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at the same time, the sustainability concept of biodiesel has also been criticized

since the methanol needed for biodiesel production majorly comes from petroleum [35].

Nevertheless, approximately 95% of the carbon in biodiesel are derived from renewable

sources such as oil and fat [33]. Hence, it is still of interest to investigate the impact of

biodiesel on engine emissions and performance to approach the future of sustainable

energy.

2.2

Impact of biodiesel fueling on engine emissions

The review by Graboski and McCormick in 1998 [6] and a more recent review by

Lapuerta et al. in 2008 [15] show consistently that the majority of diesel engine tests with

biodiesel fueling show reductions of exhaust levels of some regulated emissions

including unburned hydrocarbons (UHC), carbon monoxide (CO), and particulate matter

(PM), but increases of emissions of nitric oxides (NOx). For example, Figure II-2shows

the statistical analysis of trends of the impact of biodiesel blending on emissions from

heavy-duty engines, conducted by the Environmental Protection Agency (EPA) [51].

However, despite the statistical analysis by the EPA, the impact of biodiesel on engine

emissions as observed in Figure II-2may be limited to certain engine conditions: in this

case it is a heavy-duty truck engine on highway. The understanding of biodiesels impact

on diesel engine emissions requires a thorough investigation of previous engine studies.


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Figure II-2: NOx, PM, CO and HC % changes as biodiesel blending percentageincrease as determined by the EPA through statistical regression of publicallyavailable data on highway heavy-duty truck engines [18, 51]. Taken from [44]

In addition, as discussed in the introduction, since NOx and PM emissions from

current technology diesel engines are close to the limits permitted by regulation and both

limits will be even more stringent in the near future, these two emissions will be critical

factors in the development of new diesel engines and aftertreatment systems. For the

other regulated emissions, CO and UHC, no further development in engine technology

seems to be necessary to meet future limits [15].


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2.2.1 Nitric oxides (NOx)

In their comprehensive review, Lapuerta et al. [15] states that a majority of the

literature report an increase of NOx emissions with biodiesel use and a minority of other

observations. In this sense, they divide the impact of biodiesel fueling on NOx emissions

into 4 groups:

Group I: Increase.

Group II: Increase only in certain operating conditions

Group III: No significant difference

Group IV: Decrease with biodiesel fueling

Similar categorization is also adapted in another review by Sun et al. [44], except

that they claim that the majority of the literature they reviewed reported inconsistent

trends in the effect on NOx emissions from the use of biodiesel, in other words,

considering the distribution of engine configurations and operating conditions, it is

difficult to arrive at a deterministic conclusion on the impact of biodiesel fueling on NOx

emissions. Nevertheless, observations associated with engine configurations and

operating conditions are still meaningful for biodiesel research.

Group I: In Graboski and McCormicks review [6] of ten older two-stroke engines

running with biodiesel derived from soy and rapeseed, all engines running with rapeseed-

oil biodiesel show a consistent increase in NOx emissions, and two third of the engines

running with soybean-oil biodiesel show a consistent increase in NOx emissions. The

remaining one third engines running with soybean-oil biodiesel showed both increases

and decreases in NOx emissions depending on the operating conditions, which should be

categorized into Group II. FEV Engine Technology [52] carried out an experimental


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study in a 7.31L Navistar engine running the 13-mode US Heavy-Duty test cycle using

different soybean blends and observed a maximum of 8% increase in NOx emissions

with B100. Marshall et al. [53] tested a Cummins L10E engine under steady conditions

with different biodiesel blends, and a maximum of 16% increase in NOx emissions was

observed with B100. However, in their transient test condition B20 blend gave a higher

increase in NOx emissions than B30. Szybist et al. [14] tested a Yanmar L70 EE air-

cooled, four-stroke, single cylinder direction injection diesel engine operating at steady

3600 rpm and 75% load and reported around 10% increase in NOx emissions with pure

soybean-oil biodiesel. Other experiments have reported increased NOx emissions withbiodiesel fueling [18, 54, 55].

Group II: Some authors claimed that the effect of biodiesel on NOx emissions

depends on the type of engine and its operating conditions. Serdari et al. [56] tested on-

road emissions from three different vehicles using high sulfur diesel fuel (1800ppm) and

10% sunflower-oil biodiesel blends. They found both increases and decreases in NOx

emissions, and attributed such differences to the different engine technology and

maintenance conditions. Hamasaki et al. [57] tested a single-cylinder engine at 2000 rpm

and different loads with three waste-oil biodiesel fuels. They measured slight decreases in

NOx emissions at low loads but increases at high loads. Similar observations have also

been reported by Zhang and Boehman [13]. They conducted an experiment in a

DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty

diesel engine and found that NOx increases with biodiesel fuel were only observable at

high engine load. At low engine load, no significant difference in NOx emissions can be

observed between the baseline diesel, B20 and B40. Yehliu et al. [25] conducted


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experiments in the same engine with a thorough investigation of different load and speed

conditions, and found that at high speed and low load, biodiesel fueling decreased NOx

emissions. Staat and Gateau [58] tested a 6-cylinder engine following the ECE R49 test

cycle and an urban transient cycle named AQA F21 established by the French Agency of

Air Quality. They observed a 9.5% increase in NOx emissions in the ECE R49 cycle, yet

a 6.5% reduction in the transient urban cycle. The above observations appear to be

consistent with the review by Tat [59], that NOx emissions with biodiesel fuels are

usually higher than those from diesel fuel when they are measured in an engine test bench

but not when they are measured from vehicles. Tat [59] attributed this observation to thereason that engine loads are usually lower in vehicles than those imposed in experimental

test rigs. Similar observations have also been reported by McCormick [60, 61] who

measured NOx emission reductions of around 5% when using 20% soybean-oil biodiesel

blends.

Attempts have been made to explain these conditional biodiesel NOx effects. Li

and Gulder [62] claimed that the higher cetane number of biodiesel has a more sensible

effect on NOx emission at low load than at high load. However, this understanding may

no longer work when high-cetane ultra low sulfur diesel (ULSD) fuel is used instead of

the conventional low sulfur diesel fuel, i.e., blending of biodiesel with ULSD may not

significantly change the cetane number of fuel. Tat [59] proposed another explanation:

the injection pump tended to advance the injection timing at low load, but he observed

that this advance was higher with diesel than with biodiesel fuel in a certain load range,

leading to increased NOx emissions with diesel fuel at these load condition. This

explanation, however, is not applicable for diesel engines with a common-rail injection


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system, whose start of injection is well controlled and consistent for both petroleum

diesel and biodiesel, as in [13, 25, 63]. Although the same authors attributed the NOx

reduction with biodiesel to the specific engine configuration, it might arise from the

decreased apparent heat release from premixed combustion with biodiesel [63], due to its

lower energy content and a shortened ignition delay.

Group III: Durbin et al. [64] tested four different engines with diesel, pure

biodiesel and a 20% biodiesel blend. The engines were chosen to represent a wide variety

of heavy-duty engines: turbocharged and naturally aspirated, direction and indirect

injection, and no significant differences in NOx emissions were found. Mandpe et al. [65]tested a Euro III emission certified common-rail diesel engine on road with Jatropha

Curcas seed oil biodiesel and found no significant difference in NOx emissions between

biodiesel and petroleum diesel. Nabi et al. [66] conducted an experiment with a single-

cylinder 9.8 kW engine at steady-state with neem-oil biodiesel at different EGR ratios.

Although they observed decreased NOx emissions with increased EGR ratio, they found

no significant differences in NOx emissions between diesel and biodiesel. As suggested

by Lapuerta et al.[15], the observation by Nabi et al. [66] may be actually due to the low

unsaturation level of neem oil, since Graboski et al. [67] have shown that NOx emissions

increased with unsaturation.

Group IV: A minor number of papers have reported decreases in NOx emissions

when using biodiesel fuels. Armas et al. [63] conducted an experiment in a DDC/VM

Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel

engine at high speed and low load, with different start of injection. For each start of

injection they also fine-tuned the ECU to match the combustion phasing between


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biodiesel blends and baseline diesel. They found slight NOx reduction with biodiesel

consistently for all start of injection timings. Lapuerta et al. [68] tested pure and blended

biodiesel from sunflower and cardoon oils in an indirection injection 1.9L engine

operating at five selected steady modes, and they observed a slight decrease in NOx

emissions with biodiesel.

Overall, although no unanimity has been found, the majority of observations in

the literature show a slight increase in NOx emissions with biodiesel fueling. Under

certain conditions, no significant difference or decreased NOx emissions were observed

with biodiesel, which was likely due to specific engine configuration or fuel composition,which suggests that such results are not generally representative.

2.2.2 Particulate Matter (PM)

It has been consistently reported in the literature that a noticeable decrease in

diesel PM emissions can be observed with biodiesel fueling [6, 8, 9, 11, 58, 69-72]. The

Environmental Protection Agency collected a number of engine emissions data with

biodiesel [51] and proposed a statistical correlation between PM emissions level and

biodiesel blending:

0.006384 %B

D

PMe

PM

= (1)

whereD

PM

PMindicates the PM emission ratio between biodiesel and diesel, and %B is the

biodiesel blending level in percentage. The shape of Equation (1) can be also seen in

Figure II-2.


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This trend proposed by EPA suggests that the maximum PM emission reduction is

around 50% with pure biodiesel. This observation is consistent with several other

investigations [54, 69, 73-75]. Other authors found even higher PM emission reductions

with biodiesel. Van Gerpen and Canakci [76] observed as high as 65% reductions of PM

emissions with soybean oil and waste oil based biodiesel fuels. Schumacher et al. [77]

tested PM emissions from a DDC series 60 diesel engine and reported more than 60%

PM emission reduction with biodiesel.

The reductions in PM emissions have been shown in general, as being more

effective with lower biodiesel concentrations in the blends [15]. In other words, thereduction of PM emissions for each percentage of biodiesel blending, i.e., relative

reduction, has been observed to be the highest for partial blends [78-81].

The effects of biodiesel on PM emissions together with other parameters, such as

engine configurations and load conditions have also been investigated. In the review by

Krahl [82] on diesel engine tests with rapseed-oil biodiesel, the authors found that the PM

emission reductions were lower ( or there were no significant reductions) in heavy-duty

engines than in light-duty engines. Lapuerta et al. [79] investigated the PM emissions of a

typical European passenger car diesel engine operating at low-middle load conditions

and found that the PM emission reduction with biodiesel is larger at low load. Contrary

observations have also been reported [83]. Leung et al. [84] tested a single-cylinder

engine with a diesel fuel and a pure rapeseed oil based biodiesel at different load

conditions. They found larger decreases for biodiesel at high load. In addition, the

property of diesel fuel can also affect the NOx reduction by biodiesel. Boehman et al. [75]

reported a PM emission of 20% blending soybean oil based biodiesel with both low


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sulfur diesel (325 ppm sulfur) and ultra low sulfur diesel (15 ppm sulfur) and found that

the reduction of PM emissions by biodiesel is more significant for low sulfur diesel.

Similar observations have also been reported [51].

A small number of studies did not find significant reductions (or even found

slightly increases) in PM emissions. Armas et al. [63] found increased PM emissions in

their experiment with a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail,

direct injection light-duty diesel engine at high speed and low load. Boehman et al. [75]

found a slight increase of PM emission for soybean oil based biodiesel blending with

ultra low sulfur diesel. Similar observations that PM emissions were increased withbiodiesel have also been reported in other places [74, 85]. This phenomenon has been

generally attributed to the higher soluble organic fraction (SOF) of the particulate [63,

75], which is widely accepted to be increased when using biodiesel [6, 67, 69, 74, 77] due

to its lower volatility of the unburned hydrocarbons (Table II-4). Meanwhile, the

insoluble fraction (ISF) was decreased with biodiesel fueling [75].

In summary, it is widely accepted that biodiesel fueling can decrease the PM

emissions of diesel engine. Therefore, it has been suggested that this impact of biodiesel

on PM emissions may be combined with engine injection strategy to optimize the NOx-

PM emission trade-off [15].


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2.3 Origin of biodiesel emission effect

2.3.1 Biodiesel NOx effect

NOx formation chemistry:

To identify the factors that can affect NOx emissions, it is necessary to understand

the mechanism of NOx formation. Although a large number of reactants and reaction

pathways have been found to participate during NOx formation [86, 87], the key

reactions at the pressure, temperature and time scales in a diesel engine consist of three

major reactions [86], referred as to the extended Zeldovich mechanism:

1

2

3

2

2

k

k

k

N O NO N

O N NO O

OH N NO H

+ +

+ +

+ +

(2)

Typically the time scales in a diesel engine do not allow these reactions to reach

equilibrium. Therefore, longer residence time of in-cylinder gases at high temperature

(above 1800K since the thermal NO formation below 1800K is generally negligible [87])

can lead to higher NOx emissions. A simple second order kinetic expression can be used

to describe the NO formation when its concentration is below equilibrium:

1 2 2 2 3

[ ][ ][ ] [ ][ ] [ ][ ]

d NOk O N k O N k OH N

dt= + + (3)

where [NO], [O], [N2] and [OH] are the molar concentrations of NO, O, N2 and OH,

respectively. The rate constant k can be generally expressed as [88]:

exp( ), 1,2,3i aii iu

Ek BT i

R T

= = (4)


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where Bi, i, Ruand Eaiare constants for each ki. Equations 2 and 3 suggest that the key

parameters that control the NOx formation include: 1) concentration of O-atom, N2, N-

atom and OH, 2) in-cylinder temperature, or 3) the residence time of the in-cylinder

mixture at high temperatures. Hence, any variation in NOx emissions must be related to

one or more of the above factors.

Biodiesel NOx effect:

To date, a significant number of studies have been conducted in the attempt to

explain the NOx variation and to identify the key parameters that affect it when

alternative fuels (e.g., biodiesel) are used, both from experiments [14, 15] and numerical

simulation [16, 17, 89]. Literature reviews by Mueller et al. [18], Lapuerta et al. [15] and

Sun et al. [44] have discussed the various proposed hypotheses that may account for

engine NOx variation with biodiesel fueling, which can be summarized into seven

categories:

(1) Injection timing. Alternative fuels have different compressibility than

petroleum diesel. For example, biodiesel has higher bulk modulus of

compressibility while FT diesel has lower bulk modulus of compressibility [23].

A higher bulk modulus can lead to a faster traveling of the pressure wave in the

fuel line of pump-line-nozzle fuel injection systems, causing an advance in the

start of injection (SOI) that can consequently lead to a longer residence time

and/or higher in-cylinder temperature. The advanced SOI of biodiesel and

retarded SOI of FT diesel have been observed to affect the NOx emissions [23, 90,

91]. Although this factor has been considered as the most solid argument for the


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biodiesel NOx effect [15], the injection timing of a modern common-rail injection

system is not affected by the bulk modulus of compressibility of the fuel [13].

(2) Injection pressure and fuel spray. The lower heating value of biodiesel

results in an increase in the brake specific fuel consumption, which will be treated

as an increase of the fuel injection pressure/duration according to default engine

calibration [55]. This change in fuel injection parameters can yield an increase in

NOx emissions [21, 22]. Furthermore, the change of fuel properties, e.g. viscosity,

density and surface tension when biodiesel is used can increase the average

droplet diameter [92], leading to a lengthened ignition delay as well as a possiblylonger diffusion combustion period [6].

(3) Combustion phasing. That biodiesel can increase NOx emissions also can

be due to its impact on combustion phasing. The higher cetane number of

biodiesel can advance the start of combustion (SOC), which may lead to longer

residence time at high temperature for the combustion products [18]. However, as

discussed previously for FT diesel, some authors also claim that the NOx

reduction is due to its higher CN [25, 63, 91, 93] since very high CN indicates

short ignition delay, leading to less premixed burning [94, 95]. This inconsistency,

that both biodiesel and FT diesel have increased CN, leads to different direction in

change of NOx emissions, suggests that the impact of high cetane number on

NOx emissions is a combination of two effects.

(4) Premixed-burn fraction. The fact that biodiesel contains oxygen increases

the premixing quality (since fewer oxidizer molecules are required to reach

stoichiometric proportion) during the ignition delay, resulting in better


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combustion and higher temperature. The oxygen content in biodiesel also

increases the local oxygen concentration, which can promote NOx formation as

discussed previously. Song et al. [96, 97] tested oxygenated diesel fuel (via

oxygenated fuel additives) and found increases in NOx emissions. However, this

explanation is questioned by other authors [68, 69, 89]. In addition, this

explanation is not suitable for FT diesel since it does not contain oxygen in the

fuel.

(5) Kinetics. Different chemical kinetic pathways for NOx formation exist for

different fuels. For example, it is argued that an increased level of CH exists at theautoignition zone (AZ) during biodiesel combustion [98], which leads to the

production of N-atoms in the jet core as well as the promotion of NOx formation.

(6) Adiabatic flame temperature. Some authors claimed that biodiesel has a

slightly higher adiabatic flame temperature than conventional diesel, so thermal

NOx production is higher [16, 69, 99]. Although the decreased NOx emissions for

FT diesel appears to be consistent to this theory: FT fuel has a lower adiabatic

flame temperature [100], due to its lower C/H ratio [101], than the petroleum

diesel fuel. No unanimity can be found for the biodiesel since others state that the

adiabatic flame temperature is higher for diesel fuel [44, 66, 102].

(7) Radiation heat transfer. Soot radiation is the primary means of heat loss

from an in-cylinder flame. It has been suggested that since biodiesel can decrease

the PM emissions, the actual flame temperature is higher, leading to an increase

of NOx formation [103]. However, it has been suggested that soot emissions may


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not correlate with flame luminosity and the soot radiation appears to be smaller

than other effects [18].

Mueller et al. [18] also pointed out that the biodiesel NOx increase is related to a

number of coupled mechanisms, which also have influences on each other, rather than a

single phenomenon. Note that many of those mechanisms are affected by both fuel

properties and engine operating conditions. For example, the premixed-burn quality can

be affected by the droplet size, which is controlled not only by the injection strategy and

fluid surface tension [1], but also by the difference in the rate of mass transfer during the

injection of the multi-component fuel [104]. Among those mechanisms, Mueller et al. [18]suggest that the most important ones responsible for the biodiesel NOx increase are

longer residence time and higher in-cylinder temperature from either advances in

combustion phasing or lower radiative heat loss. They proposed, based on the models

established by Siebers and Naber [105, 106], that the origin of the NOx increase with

biodiesel is based on reacting mixtures that are closer to stoichiometric (less rich) for

biodiesel-containing fuels: a) during ignition (i.e., during the premixed volumetric

autoignition event from the start of combustion to the end of premixed burning), and b) in

the standing premixed autoignition zone (AZ) near the flame lift-off length (LOL) at

higher loads. According to Mueller et al. [18], this is so far the most generalized

explanation which is consistent with the observations from biodiesel, high cetane number

diesel and engine load conditions. In additional to their heavy-duty single-cylinder engine

test, in this work this theory will be tested with a light-duty diesel engine for the impact

of both biodiesel and engine fuel injection strategy.


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2.3.2 Biodiesel particulate matter effect

As discussed previously, it is widely accepted that biodiesel can decrease the PM

emissions. It is also believed that the factors contributing to the PM emission reductions

include:

Oxygen content in biodiesel: the oxygen content of the biodiesel molecule,

which can promote complete combustion even in regions of fuel-rich diffusion

flames, has been believed to contribute to the PM emission reduction with

biodiesel [107]. Song et al. [96, 97] tested oxygenated diesel fuel (via oxygenated

fuel additives) in a VW turbocharged direct injection diesel engine, and found

decreased PM emissions with increased oxygen content of fuel for both engines.

Similar observation has also been reported by Frijters and Baert [108]. Sison et al.

[109] tested different oxygenates, including biodiesel, with diesel fuel in a single-

cylinder optically accessible diesel engine and observed decreased soot formation.

The kinetic model developed by Flynn et al. [110] based on the conceptual model

of Dec [111] suggested that a sharp decrease in the formation of soot precursors

would be observed when the oxygen content in the fuel increased: as the oxygen

content of the fuel increased, larger fractions of the fuel carbon were converted to

CO in the rich premixed region, rather than to soot precursors.

Absence of aromatics in biodiesel: The aromatic content in petroleum

diesel fuel has been considered to serve as soot precursors [69, 71, 112, 113]. This

can be investigated in two ways: 1) decrease the aromatic content in petroleum

diesel, (Schmidt and Van Gerpen [113] artificially blended diesel fuel with

octadecane (C18H38) and observed a significant reduction in PM emissions.); 2)


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increase the aromatic content in biodiesel. Mueller et al. [18] doped the pure

biodiesel with phenanthrene, a 3-ring aromatic species, and compared the

emissions at both decreased oxygen environment and normal air environment

with a heavy-duty single-cylinder optically accessible diesel engine. They found

increased PM emissions for all load conditions for the doped biodiesel in a

decreased oxygen environment. With a normal air environment, the doped

biodiesel has an increased PM emission only at low-moderate load condition.

Other authors have treated both the oxygen content and absence of aromatics in

the biodiesel to the PM emission reductions [114, 115].

Higher oxidative reactivity of biodiesel soot: Boehman et al. [75] collected

the particulate matter from the exhaust of a six-cylinder Cummins ISB 5.9L direct

injection turbocharged diesel engine, with both diesel fuel and a 20% blend

soybean oil based biodiesel, and then they employed thermogravimetry to analyze

the oxidative reactivity of the soot particles (de-volatized particulate matter).

They found that the soot particles for B20 are more reactive than for diesel fuel,

which suggests that the particulate matter will be more rapidly consumed by the

oxidation in the exhaust, leading to decreased PM emissions. Song et al. [116]

expanded this observation to pure biodiesel and used high resolution transmission

electronic microscopy (TEM) technique to confirm the faster oxidation of the soot

for pure biodiesel. Yehliu et al. [117] applied a counting algorithm to identify the

initial nanostructure of soot particles from high resolution TEM images, and they

found more short and curvy graphene layers in soot from biodiesel combustion,


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i.e., it has a higher oxidative reactivity due to its higher degree of disorder [118-

120].

There have been other explanations proposed for the biodiesel PM effect, as well.

For instance, biodiesel can introduce an advance of combustion either due to advanced

injection timing [90] or higher cetane number [102] can prolong the residence time of

soot particles in a high-temperature atmosphere, which in the presence of oxygen

promotes further oxidation [72, 112]. Others also claimed that the usually lower final

boiling point of biodiesel provided lower probability of soot or tar being formed from

heavy hydrocarbon fractions unable to vaporize [69]. High sulfur content of diesel fuelhas also been suggested as a reason for its high PM emissions [121]. Nevertheless, these

explanations have been believed to be insignificant [108, 115, 122], as well as not

applicable since new fuel standards came into effect.

2.4

Fuel Injection strategy with biodiesel fueling

Fuel injection strategy is an important aspect in the optimization of diesel engine

missions. For instance, in earlier discussion the significant impact of injection timing and

injection pressure on engine NOx emissions has been shown. Some studies have

attempted to optimize diesel engine combustion with biodiesel fuel by adjusting the

injection strategy to improve exhaust emissions [123, 124]. Generally speaking, for

conventional diesel combustion, the control of fuel injection strategy includes three

aspects 1) injection timing (or start of injection); 2) injection pressure or injection

duration; 3) number of fuel injection. Among those, although the advance of injection


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timing with biodiesel fueling was a concern for earlier engine research, it is now believed

as an insignificant factor for modern engines with common-rail fuel injection systems

[13]. The application of common-rail fuel injection systems also provides availability of

multiple fuel injection design, which has become a fuel injection strategy. A large

number of researchers have found that multiple fuel injections can reduce engine NOx

emissions [125-129]. However, some authors observed increased hydrocarbon emissions

and slight decreased engine thermal efficiency with multiple fuel injections [25]. In

addition, the complex combustion behavior of multiple fuel injections is not suitable for

fundamental engine combustion study, and therefore it is not a main focus in the fuelinjection strategy investigated here. Nevertheless, multiple fuel injections with post

injection, which is a fuel injection event considerably after top dead center (TDC) will be

investigated in this work and discussed in a later section.

Meanwhile,

Documents

PengYe Dissertation 2011