Individual and Synergistic Effects of Lubricant Additive (Ca, Mg,
Zn) Combinations on Ash Characteristics and DPF Performance
by
Casey Chiou
B.S., Mechanical Engineering
University of California, Los Angeles, 2011
Submitted to the Department of Mechanical Engineering in Partial Fulfillment of
the Requirements for the Degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2013
ARCHIVES
-7Ti)f
@ 2013 Massachusetts Institute of Technology
All rights reserved.
Signature of Author:
/
Department of Mechanical Engineering
June 1, 2013
'If
Certified by:
V
Victor W. Wong
Engineering
in
Mechanical
Principal Research Scientist and Lecturer
Thesis Supervisor
0/1" - I
Accepted by:
David Hardt
Chairman, Department Committee on Graduate Students
(This page intentionally left blank)
2
Individual and Synergistic Effects of Lubricant Additive (Ca, Mg,
Zn) Combinations on Ash Characteristics and DPF Performance
by
Casey Chiou
Submitted to the Department of Mechanical Engineering on June 1, 2013 in Partial Fulfillment
of the Requirements for the Degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
ABSTRACT
Diesel particulate filters (DPF) are devices that trap hazardous particulate matter from diesel
engine exhaust in order to meet increasingly strict particle emissions regulations.
Diesel exhaust particulates mainly include soot and ash. Soot, carbon particles derived from
incomplete fuel combustion, can be oxidized into carbon dioxide after being trapped by the DPF
through a catalytic heating process called regeneration. Ash, however, derived from metallic
additives in the engine lubricant required for robust engine operation, is an incombustible
material and remains within the DPF following regeneration. As ash accumulates over time,
exhaust airflow through the filter becomes restricted and an engine backpressure results. Engine
performance and fuel economy are reduced, requiring the DPF to be cleaned or replaced.
While the detrimental effects of ash on DPF performance and therefore fuel economy can be
illustrated and quantified, there is much to be understood about the specific factors that govern
ash properties like distribution, permeability, and morphology. Several different parameters,
such as engine operating conditions and DPF design, have been found to significantly impact ash
characteristics, and the ultimate goal is to be able to control these parameters to reduce
detrimental ash effects to a minimum and improve DPF service life and performance.
This work addresses the source of ash directly and investigates the effect of lubricant additive
chemistry on ash characteristics and DPF performance. Three lubricant formulations, that differ
only in the type of additives present, are tested and compared using a controlled, accelerated
DPF loading system. Filter pressure drop response and resulting ash property data collected
using an array of experimental and analytical techniques show that very little difference exists
between the tested oils of differing additive content.
Thesis Supervisor:
Title:
Victor W. Wong
Principal Research Scientist and Lecturer in Mechanical Engineering
3
(This page intentionally left blank)
4
ACKNOWLEDGEMENTS
There are many people I would like to thank who have made my MIT career possible. My time
at MIT has been an extremely challenging but rewarding journey that I will forever be proud of.
I will never forget all the things I've learned, the wonderful people I've met, and the work I've
accomplished these past two years, and for that I am enormously grateful for this experience.
First, I would like to thank my research advisor, Dr. Victor Wong, for giving me the opportunity
to work my way through school as a graduate research assistant under his supervision. This
research project has been the heart and soul of my Master's program, and from it I've learned an
immeasurable amount that will have forever strengthened my engineering and leadership
abilities. Dr. Wong has always been supportive, constructive, and understanding throughout all
my efforts, and again I am forever grateful for this opportunity and his supervision.
This project was funded and made possible by the generous contributions of Infineum. Many
thanks to Jai Bansal, Jie Cheng, Jose Gutierrez, and Dan Whyte for their constant support and
input, always providing the necessary resources and direction for the project's success. Also
thanks to other members of the MIT Consortium to Optimize Lubricant and Diesel Engines for
Robust Emission Aftertreatment Systems for their sponsorship and support.
Next I would like to thank Dr. Alexander Sappok for his endless amount of guidance and support
throughout my project. From helping me debug and troubleshoot all my experimental challenges
to providing invaluable feedback on my analyses and presentations, he was always there for me.
Dr. Sappok's leadership constantly provided my work with proper direction, and as a result he
drove me every day to do the best work I could possibly do for the project. I cannot emphasize
enough how much Dr. Sappok's contributions have helped to make my research project possible.
I would also like to thank my lab mates, Dr. Carl Justin Kamp, Michael Bahr, Yujun Wang, Tim
Murray, and Ifran Govani, for their constant support and companionship throughout my research
journey. Together we made it through classes, exams, experiments, meetings, and presentations,
always helping each other out and lending a hand whenever it was needed. It's been a pleasure
working alongside this great group of guys, who helped make this journey fun and enjoyable.
I'm also very grateful for the MIT personnel who made my research efforts possible. First, I
want to thank the rest of the members of the Sloan Automotive Laboratory, including Thane
DeWitt, Raymond Phan, Janet Maslow, and its other graduate students. From handling all the
project logistics, to helping me troubleshoot and repair my experimental test setup, to constantly
teaching me new things about machining and engineering, everyone played a part in aiding me in
my research and providing a friendly face to talk to. Secondly, the members of the Center of
Material Science and Engineering at MIT, whom I closely worked with for much of my data
collection and analysis, deserve my utmost gratitude for their assistance and guidance.
Lastly I would like to thank my family and friends, both at and outside MIT, who have kept me
going throughout this journey. Their constant support and companionship outside of school and
work over the last two years have made this entire experience manageable. They helped me
grow and mature as a person, and I'll never forget the countless memories we shared together.
5
(This page intentionally left blank)
6
TABLE OF CONTENTS
LIST OF FIGURES ........................................................................................................................
9
LIST OF TA BLES ........................................................................................................................
11
N OM EN C LA TURE .....................................................................................................................
13
1INTRODU CTIO N ................................................................................................................
15
1. 1
2
Diesel Engine Fundamentals......................................................................................
S1.1
Diesel Engine Advantages......................................................................................
17
1.1.2
Diesel Engine Applications.................................................................................
18
1.1.3
Diesel Engine Em issions......................................................................................
19
1.2
Diesel Em ission Regulations......................................................................................
20
1.3
Diesel Em ission Reduction M ethods ..........................................................................
22
D IESEL PA RTICULA TE FILTERS .................................................................................
25
2.1
DPF Operation................................................................................................................
25
2.2
A sh Sources....................................................................................................................
28
2.2.1
29
A sh Transport.................................................................................................................
31
2.4
A sh Effects on DPF Perform ance ..............................................................................
32
2.4.1
D PF Pressure Drop ..............................................................................................
32
2.4.2
Lubricant Chem istry Effects...............................................................................
34
Project Objectives ..........................................................................................................
36
FUNDAMENTAL UNDERSTANDING........................................................................
37
3.1
D PF Pressure Drop.........................................................................................................
3.1.1
3.2
4
Lubricant Additives ............................................................................................
2.3
2.5
3
15
D PF Pressure Drop Model.................................................................................
M aterial Properties and Characteristics....................................................................
37
41
. 44
3.2.1
DPF Substrate Properties...................................................................................
44
3.2.2
A sh Properties.....................................................................................................
45
EXPERIMENTAL SET-UP AND TECHNIQUES.........................................................
4.1
DPF Loading..................................................................................................................
49
49
4.1.1
Accelerated A sh Loading....................................................................................
49
4.1.2
Soot Loading...........................................................................................................
51
DPF Post-M ortem Analysis .......................................................................................
52
4.2
4.2.1
Ash Distribution...................................................................................................
52
4.2.2
Ash Porosity............................................................................................................
54
7
4.2.3
4.2.4
4.2.5
5
57
58
58
EXPERIMENTAL TESTING AND RESULTS ..............................................................
61
5.1 Test M atrix and Param eters........................................................................................
Lubricant Formulations.......................................................................................
5.1.1
5.1.2
DPF Properties .....................................................................................................
5.1.3
Ash Loading Param eters .....................................................................................
5.1.4
Soot Loading Param eters ....................................................................................
Test Sum mary .....................................................................................................
5.1.5
61
61
62
63
64
5.2
65
DPF Loading and Pressure Drop Results....................................................................
65
5.2.1
Ash Loading Results............................................................................................
65
5.2.2
Soot Loading Results ..........................................................................................
69
5.3
Post-M ortem Ash Analysis Results................................................................................
73
5.3.1
Ash Distribution Results .....................................................................................
74
5.3.2
Ash Porosity Results............................................................................................
76
5.3.3
Ash Composition Results.....................................................................................
78
5.3.4
Ash Particle Size Results .....................................................................................
81
5.3.5
Ash M orphology Results ....................................................................................
83
5.4
6
Ash Composition ................................................................................................
Ash M orphology .................................................................................................
Ash Particle Size D istribution..............................................................................
DPF M odel Results .....................................................................................................
CONCLUSIO NS .............................................................................................................
86
89
6.1
Lubricant Additive Effects on DPF Performance .......................................................
89
6.2
Lubricant Additive Effects on Ash Characteristics.....................................................
90
6.3
Impact of Ash D epth Filtration ..................................................................................
91
Future W ork........................................................................................................
92
REFERENCES .............................................................................................................................
93
A PPEND IX...................................................................................................................................
97
6.3.1
8
LIST OF FIGURES
Figure 1.1. Relative Concentration of Engine-Out Emissions in Diesel Exhaust ...................
Figure 1.2. Comparison of Regulated Emissions in SI and Diesel Engines ..........................
Figure
Figure
Figure
Figure
Figure
1.3.
2.1.
2.2.
2.3.
2.4.
U.S. EPA Heavy-Duty Emission Standards .......................................................
Cutout of Catalyzed DPF for Heavy-Duty Engine Retrofits ................................
Wall-Flow Substrate Operation ...........................................................................
DPF Filtration Mechanisms ................................................................................
Ash and Soot Distribution in DPF Channels .......................................................
Figure 2.5. Origin of Ash-Related Constituents from DPFs Operated on Various Fuels and
Regeneration Conditions ..............................................................................................................
Figure 2.6. Schematic Representation of Ash Particle Formation and Growth over Repeated
19
20
21
25
25
26
27
28
32
D PF Regeneration ........................................................................................................................
Figure 2.7. Backpressure Increase vs. Simulated Driving Distance for Several DPF Studies .. 33
Figure 2.8. Mass of Accumulated Ash Versus Backpressure Increase for Ten Catalyzed Soot
Filters Exposed to Different 1.8% Sulfated Ash Oils ..............................................................
34
Figure 2.9. DPF Pressure Drop vs. Ash Load for Various Lubricant Formulations ..............
35
Figure 3.1. Soot and Ash Accumulation in a DPF Channel .................................................
Figure 3.2. Soot Depth Filtration in a DPF Channel Wall ....................................................
37
39
Figure 3.3. Soot and Ash Layer Formation ............................................................................
39
Figure 3.4. Ash Layer (Left) and Ash End Plug (Right) .........................................................
Figure 3.5. DPF Pressure Drop Regimes ..............................................................................
40
40
Figure 3.6. Cross Section View of (a) DPF Substrate and Ash Layer (b) Large Ash Agglomerate
45
(c) Porous A sh Particle ...........................................................................................................
Figure 3.7. Number-based Size Distribution and SEM Image of Field-aged DPF Ash ......
46
Figure 4.1. Accelerated Ash Loading System Configuration ....................................................
Figure 4.2. Post-Mortem Sample Extraction .........................................................................
50
53
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
4.3. Measuring Ash Layer Thickness .........................................................................
4.4. Measuring Ash End Plug Formation ..................................................................
4.5. DPF Ash Distribution Profile .............................................................................
4.6. Mounted Filter Samples for SEM Analysis .............................................................
4.7. SEM Micrograph of CJ-4 Lab Ash Layer ...............................................................
4.8. Binary Image of SEM Micrograph ..........................................................................
4.9. XRD Peak Scan for a CJ-4 Lab Ash Sample ......................................................
4.10. TEM Micrograph of a Single CJ-4 Lab Ash Particle ............................................
4.11. Light Obscuration Technique and Example CJ-4 Lab Ash Volume Distribution
53
54
54
55
55
56
57
58
. 59
Figure 5.1. Accelerated Ash Loading and Periodic Regeneration Cycles .............................
64
Figure 5.2. DPF Pressure Drop vs. Ash Load at 0 g/L Soot .................................................
66
Figure 5.3. Pressure Drop Response Normalized and Compared to Previously Tested Oils .... 67
Figure 5.4. DPF Core Testing on Flow Bench System ..........................................................
9
68
Figure 5.5. DPF Core Pressure Drop Response via Flow Bench Testing ..............................
Figure 5.6. Pressure Drop Response to Soot Load at 0 g/L Ash .............................................
69
70
Figure
Figure
Figure
Figure
Figure
5.7. Pressure Drop Response to Soot Load at 10 g/L Ash
5.8. Pressure Drop Response to Soot Load at 18 g/L Ash
5.9. Pressure Drop Response to Soot Load at 25 g/L Ash
5.10. Pressure Drop Response to Ash Load at 3 g/L Soot
5.11. Pressure Drop Response to Ash Load at 6 g/L Soot
..........................................
..........................................
..........................................
70
71
71
..........................................
..........................................
72
73
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
5.12.
5.13.
5.14.
5.15.
5.16.
5.17.
5.18.
5.19.
5.20.
5.21.
5.22.
5.23.
5.24.
5.25.
5.26.
Ash Distribution Profile Results for a Single DPF Channel .............................
Ash Distribution Profiles for Lab, Engine Test, and Field-aged Samples ......
Calcium Oil Engine Test Ash End Plug via X-ray CT .....................................
Ash Layer Porosity Results via SEM Image Analysis ..................
Ash Plug Porosity Results via SEM Image Analysis ...................
Ash Sample Calcium Content via XRD Analysis ............................................
Ash Sample Magnesium Content via XRD Analysis ........................................
Ash Sample Zinc Content via XRD Analysis ...................................................
Volume Basis Particle Size Distribution for Lab CJ-4 Ash, 2012.....................
Number Basis Particle Size Distribution for Lab CJ-4 Ash, 2012.....................
Calcium Oil Lab Ash Morphology .....................................................................
Magnesium Oil Lab Ash Morphology ..............................................................
Lab CJ-4 (Left) and Field CJ-4 (Right) Ash Morphology .................................
Lab Ca (Left) and Base + Ca + ZDDP (Right) Ash Morphology .....................
Lab Mg (Left) and Base + Mg + ZDDP (Right) Ash Morphology ...................
Figure 5.27. Simulated vs. Experimental Porous Media Pressure Drop ................................
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
A.1. Lab CJ-4 Ash Particle Size Number Distribution ...............................................
A.2. Lab CJ-4 Ash Particle Size Volume Distribution ...............................................
A.3. Lab Ca Ash Particle Size Number Distribution ..................................................
A.4. Lab Ca Ash Particle Size Volume Distribution .................................................
A.5. Lab Mg Ash Particle Size Number Distribution ..................................................
A.6. Lab Mg Ash Particle Size Volume Distribution .................................................
A.7. Engine Ca Ash Particle Size Number Distribution ...............................................
A.8. Engine Ca Ash Particle Size Volume Distribution ...............................................
A.9. Engine Mg Ash Particle Size Number Distribution .............................................
A.10. Engine Mg Ash Particle Size Volume Distribution ............................................
A.11. Field CJ-4 Ash Particle Size Number Distribution .............................................
A.12. Field CJ-4 Ash Particle Size Volume Distribution .............................................
10
74
75
75
77
77
79
79
80
81
82
83
84
84
85
85
88
97
97
98
98
99
99
100
100
101
101
102
102
LIST OF TABLES
Table 2.1. Measured and Reported Ash Components, Properties, and Characteristics .............
Table 3.1. Key DPF Pressure Drop Contribution Factors .....................................................
Table 3.2. Cordierite and Silicon Carbide (SiC) DPF Substrate Properties ...........................
Table 3.3. Measured Field and Lab Ash Properties ..............................................................
Table 3.4. Ash Properties for Various Lubricant Additive Formulations .............................
Table 4.1. Accelerated Ash Loading System Specifications ................................................
Table 4.2. Cummins ISB 300 Engine Specifications ............................................................
Table 5.1. Oil Formulation Test Matrix, MIT 2010...............................................................
Table 5.2. Laboratory Testing DPF Properties ......................................................................
Table 5.3. New DPF Property Comparison ............................................................................
Table 5.4. Test Matrix Summary of Previous and New Samples ..........................................
Table 5.5. Ash Loading Summary for Newly Tested Oils ......................................................
Table 5.6. Ash Distribution Summary ..................................................................................
Table 5.7. Ash Layer Porosity Summary ................................................................................
Table 5.8. Ash Particle Size Summary ..................................................................................
Table 5.9. Simulated Ash Layer Permeability Summary ........................................................
Table 5.10. Simulated Ash Layer Pressure Drop Summary ...................................................
Table 5.11. Simulated Wall Pressure Drop Summary ...........................................................
Table 5.12. Simulated Porous Media Pressure Drop Summary .............................................
11
31
38
44
47
48
51
52
61
62
63
65
66
76
78
83
86
87
87
87
(This page intentionally left blank)
12
NOMENCLATURE
Al
Ca.
CI
CO
CT
DOC
DPF
Fe
FF
FIB
HC
Mg
Mo
NOx
0
P
PM
Pt
S
SCR
SEM
Si
SI
SiC
TEM
ULSD
XRD
ZDDP
Zn
Aluminum
Calcium
Compression Ignition
Carbon Monoxide
Computed Tomography
Diesel Oxidation Catalyst
Diesel Particulate Filter
Iron
Fully Formulated
Focused Ion Beam
Hydrocarbon
Magnesium
Molybdenum
Oxides of Nitrogen
Oxygen
Phosphorus
Particulate Matter
Platinum
Sulfur
Selective Catalytic Reduction
Scanning Electron Microscope
Silicon
Spark Ignition
Silicon Carbide
Transmission Electron Microscope
Ultra Low Sulfur Diesel
X-ray Diffraction
Zinc Dialkyldithiophosphate
Zinc
Gas Dynamic Viscosity
y,
Gas Wall Velocity
K
Permeability
D,
Pore/Particle Diameter
Porosity
wP
AP
Porous Media Thickness
Pressure Drop
13
(This page intentionally left blank)
14
1
INTRODUCTION
The diesel engine is currently the most efficient type of all internal and external combustion
engines. Developed in 1893 by German inventor Rudolf Diesel, it has today become widely
used in many passenger car and large transport vehicle applications. Diesel engines are desirable
mainly because of their high thermal efficiency, which leads to higher fuel economy and lower
greenhouse gas emissions. One of the main disadvantages of diesel engines, however, is the high
emission of hazardous particulate matter (PM). Diesel particulates have been found to be linked
to many detrimental health effects, and have thus become subject to major worldwide emission
regulations.
Diesel particulate filters (DPF) have become one of the most widespread and effective
technologies used today to control diesel particulate emissions. Placed in diesel engine exhaust
systems, DPFs can trap diesel particulates with high efficiencies, up to 95% and above, helping
to meet increasingly stringent emission regulations. DPFs are vulnerable, however, to their own
set of challenges, providing the motivation behind extensive research to improve and optimize
these crucial pieces of diesel emission control technology.
1.1
Diesel Engine Fundamentals
The main goal of an engine is to convert chemical energy stored in fuel into mechanical power.
In internal combustion engines, a mixture of fuel and air is burned and the resulting force from
the high-pressure gases provide desired power output through the mechanical components of the
engine. For many vehicles, these components consist of the conventional cylinder and piston
arrangement.
The diesel engine is a type of internal combustion engine that relies on compression ignition (CI)
to ignite its fuel-air mixture rather than spark ignition (SI), employed by many other gasoline
engines. In diesel engines, intake air near atmospheric pressure is compressed by the piston to
high pressures of about 4 MPa (600 lb/in 2) and high temperatures around 800 K (1000 *F) [1].
Just before the piston's top dead center, a small amount of fuel is injected where it becomes
15
atomized into small droplets and entrained into the cylinder air. The high temperatures caused
by the compressed air are well above the fuel's ignition point, leading to spontaneous
combustion. The release of burned gas delivers power back to the piston and once it is
exhausted, the cycle can start over again.
In gasoline engines, however, the air and fuel are usually mixed together in the intake system
prior to entry to the engine cylinder, using a carburetor or fuel-injection system [1]. The intake
charge is then compressed to pressures of about 1-2 MPa, and an electrical discharge across a
spark plug just before top dead center is what initiates the combustion process. The in-cylinder
temperatures achieved in SI engines are not enough to ignite the fuel-air mixture, as is the case
for diesel engines, so the use of a spark to initiate burning is necessary.
Although the combustion process seems similar between SI and CI engines, the difference in
ignition strategy alone ends up making the diesel engine substantially different from other
gasoline engines. SI engine compression ratios, the ratio of the volume of the combustion
chamber from its largest capacity to its smallest, are usually around 8-12 [1]. This value is
limited because greater compression, and therefore higher in-cylinder pressures and
temperatures, may lead to auto-ignition of the fuel-air charge before the desired timing and result
in engine knock. Because CI engines don't introduce fuel into the combustion chamber until
after the intake air has already been compressed, significantly higher compression ratios, around
12-24, are able to be reached without fear of auto-igniting the cylinder charge.
As a result of these high compression ratios, diesel engines are further known to have better
torque characteristics than gasoline engines. Since CI engines need to compress air to a much
higher degree than SI engines, diesel engines tend to have longer piston stroke lengths and
therefore larger crankshaft diameters. The applied force on the crankshaft by the connecting
rods is occurring at a greater distance from the crankshaft's axis of rotation, resulting in greater
torque. Consequently, longer stroke lengths along with needing heavier, stronger parts to cope
with the large compression pressures also mean that diesels tend to operate at slower speeds than
gasoline engines.
16
Power output control also differs between diesel and gasoline engines. Gasoline engines control
the load of the engine by restricting intake air through a throttle, which then determines the
amount of fuel that needs to be drawn in order to mix constantly near stoichiometric conditions.
In diesels, on the other hand, the amount of fuel injected into the combustion chamber, rather
than the amount of intake air, achieves load control [1].
1.1.1 Diesel Engine Advantages
The main advantage of the diesel engine is its high efficiency, which can mostly be attributed to
the fact that CI engines can achieve much higher compression ratios. A high compression ratio
is desirable because it allows an engine to reach combustion temperatures using less fuel and
also results in a longer expansion stroke, creating greater power output. Diesel engine efficiency
is also a result of reduced losses, both friction and pumping. Since diesels produce more torque
at lower engine speeds than in gasoline engines, their power is generally produced at lower
engine speeds where friction losses are reduced [2]. And, because diesels avoid intake throttling,
air induction into the engine is not interfered with and pumping losses are reduced [2]. As a
result, diesel engines end up being about 30-35% more fuel efficient than similar-sized gasoline
engines [3].
Another significant advantage of diesel engines is that they emit extremely low concentrations of
unburned hydrocarbons (HG) and carbon monoxide (CO) emissions [4]. The reason behind this
is that diesels operate in very lean regimes where an excess of air, again a result of not needing
intake throttling, ensures that fuel is more able to be fully burned up during combustion. As a
result, diesel engine-out HC emissions are only 19% of gasoline engine-out HC emissions, and
diesel engine-out CO emissions are only 5% of gasoline engine-out CO emissions [2].
Diesel engines are also very durable and reliable, leading to low maintenance costs. Heat is
released earlier in the diesel combustion cycle than in the gasoline cycle and also at a much more
sudden rate, which ultimately requires more robust engine construction [2]. As a result, diesel
engines end up lasting three to four times longer than their gasoline counterparts [2]. In addition,
diesel engines do not use spark plugs, distributors, or ignition systems, necessary for SI engines,
and those parts tend to have more breakdowns than most other mechanical components [2].
17
Lastly, diesel fuel is both cheaper and safer than gasoline. Diesel fuel is more difficult to ignite
and also less volatile than gasoline, making it safer to handle and store [2].
1.1.2 Diesel Engine Applications
Because diesel engines are known to be efficient, reliable, and able to produce high amounts of
torque, they have become extremely popular in many transport vehicle applications. These
include large-scale freight vehicles like ships, trains, and heavy duty trucks, and also passenger
vehicles like buses and light duty trucks. In 1997, diesel engines were found to be used in 80%
of all buses, 85% of heavy duty trucks, 77% of boats and ships, and 89% of all trains in the US
[5]. In total, this accounted for 19% of the total energy used in the US transport sector,
compared to other fuels like gasoline and jet fuels [5].
Diesel engines, however, are not as widely used in passenger cars in the US. They are only used
in 1% of automobiles and 4% of light duty trucks [5]. This is due to some of the disadvantages
that diesel engines possess, that they are generally low speed, loud, and heavy. Diesel engines
are also expensive to manufacture, requiring additional components such as turbochargers for
additional power output, heavier-duty internal components for withstanding higher compression
ratios, and aftertreatment systems for regulating their high emissions. And, in the US where
diesel fuel and gasoline are both very cheap, auto makers simply don't see any economic
advantage in switching to diesel. In Europe, however, fuel prices are much greater than in the
US, making it much easier for consumers to look past those few disadvantages in order to
harness the high fuel efficiencies and long-term cost savings of diesel engines. As a result,
almost half (48%) of new cars in the EU are sold with a diesel engine [6] and in total, diesel
accounts for 30-50% of energy use by the transport sector [5].
Diesel engines are also widely used in construction (83%), agriculture (66%), and mining (22%)
equipment [5], as well as in commercial and industrial generators to provide local power and
meet emergency backup power needs.
18
1.1.3 Diesel Engine Emissions
Diesel fuel is composed of hydrocarbons which, after combustion, should theoretically only yield
carbon dioxide (CO 2) and water vapor (H 20). Indeed, diesel exhaust gas is primarily composed
of CO 2 (2-12%), H2 0 (2-12%), and the unused portion of engine charge air (02 3-17%,
N 2 balance), none of which have any adverse health or environmental effects [8]:
itrogen
-,oxygen
water
Carbn
Pollutant
emissions
vapor
dioxide
Figure 1.1. Relative Concentration of Engine-Out Emissions in Diesel Exhaust, Reproduced
From [8]
Diesel emissions, however, also include a number a hazardous pollutants originating from nonideal processes during combustion, such as incomplete combustion of fuel, reactions between
mixture components under high temperature and pressure, combustion of engine lubricating oil
and oil additives as well as combustion of non-hydrocarbon components of diesel fuel, such as
sulfur compounds and fuel additives [8]. Major pollutants include unburned hydrocarbons (HG),
carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM), comprising a few
tenths of a percent of total diesel exhaust composition [8].
Although the diesel engine has been shown to possess a wide range of advantages, one of its
biggest drawbacks is its high level of NOx and particulate matter emissions. NOx refers to
oxides of nitrogen, generally nitrogen monoxide, also known as nitric oxide (NO), and nitrogen
dioxide (NO 2) [7]. NOx and PM have both been found to be hazardous to human health and as a
result, diesel engines require additional aftertreatment systems in order to curb these harmful
emission levels.
19
Because diesel engines rely on compression ignition where combustion takes place during and
after fuel injection, diesel emissions are quite different from those observed in spark ignition
engines that operate with a premixed mixture [8]. An example comparison of emission levels
between a light-duty diesel vehicle and gasoline vehicle highlights these differences [9]:
300
SI engine with catalyst
-250
S20Diesel
engine (100%)
00
100
50
0
CO
HC
NOx
PM
Figure 1.2. Comparison of Regulated Emissions in SI and Diesel Engines, Reproduced From
[9]
As stated earlier, diesel engines are favorable in that they produce significantly lower levels of
CO and gaseous unburned HC emissions, but again they consequently produce higher levels of
NOx and PM compared to gasoline engines. Diesels produce higher levels of NOx because NOx
is formed by the high-temperature reaction of nitrogen with oxygen, and diesels generally have
higher combustion temperatures as a result of using CI [7]. Furthermore, PM emission is more
prevalent in diesel engines because diesel fuel contains hydrocarbon compounds with higher
boiling points and molecular weights than gasoline [1]. The composition of the unburned
hydrocarbons in diesel exhaust extends over a larger molecular size range, with the heaviest
hydrocarbons condensing into solid-phase soot [1].
1.2
Diesel Emission Regulations
Regulated emissions in the United States, Europe, Japan, and other countries include the four
main aforementioned pollutants: NOx, particulate matter, hydrocarbons, and carbon monoxide.
20
In the United States, emissions standards are managed by the Environmental Protection Agency
(EPA). In addition, the state of California, which possesses one of the largest automotive
markets in the world, has the ability to set even more stringent vehicle emissions standards,
managed by the California Air Resources Board (CARB), that other states can choose to follow
as well.
NOx regulations specifically target nitric oxide (NO) and nitrogen dioxide (NO 2).
Concentrations of NOx in diesel exhaust are typically between 50 and 1000 ppm, with about
15% being NO2 [8]. Nitric oxide (NO) is a colorless and odorless gas, poisonous to humans, and
can cause irritation of the eyes and throat, tightness of the chest, nausea, headache, and gradual
loss of strength [7]. NO 2 is also very toxic. A reddish-brown gas with a suffocating odor, it can
cause delayed chemical pneumonitis and pulmonary edema [7]. As a result, NOx is one of the
most critical pollutants regulated in diesel exhaust. As of 2012, CARB is looking for 75%
reductions in mobile NOx to meet new ozone standards, with the target heavy duty NOx limit
being 0.05 g/bhp-hr by the year 2020 [11]. This is a significant decrease from the 2010 EPA
standard of 0.20 g/bhp-hr, illustrating the constant increase over past years in regulation
strictness [12]:
Emission Rj
Figure 1.3. U.S. EPA Heavy-Duty Emission Standards, Reproduced From [12]
21
As shown above, particulate matter (PM) has also been highly regulated in diesel exhaust in
recent years. Diesel particulates are composed of a mixture of nuclei mode particles and
accumulation mode particles. Nuclei mode particles, about 0.007 to 0.04 um in diameter,
include elemental carbon particles that have adsorbed other species such as hydrocarbon and
hydrated sulfuric acid condensates [8]. Agglomeration mode particles, about 0.04 to 1 um in
diameter, consist of nuclei mode particles that have agglomerated with other solid materials like
metallic ash, cylinder wear metals, and sulfur compounds [8]. Together, these particulates can
penetrate deep into the lungs when inhaled because of their small size and can lead to symptoms
such as headache, dizziness, nausea, coughing, difficulty breathing, and tightness of chest.
Long-term exposure can even lead to more serious health problems such as cardiovascular
disease, cardiopulmonary disease, and lung cancer [10]. Since 2007, heavy-duty diesel PM
emissions have been federally regulated in the US by the EPA to 0.01 g/bhp-hr [13].
Hydrocarbons, gaseous species derived from diesel fuel and from lubricating oil, comprise about
20 to 300 ppm of diesel exhaust [8]. They are toxic and have irritating odors, and some of them,
such as benzene and formaldehyde, are classified as carcinogens. Since 1985, heavy-duty HC
emissions have been regulated by the EPA to 1.3 g/bhp-hr [13].
Lastly, Carbon Monoxide (CO) comprises about 10 to 500 ppm of diesel exhaust concentration
[8]. An odorless, colorless gas, it is very toxic. Symptoms of CO poisoning include headache,
nausea, vomiting, dizziness, and fatigue, and prolonged exposure can even result in seizure,
coma, and fatality. Since 1985, heavy-duty CO emissions have been regulated by the EPA to
15.5 g/bhp-hr [13].
1.3
Diesel Emission Reduction Methods
In order to meet the above strict emission standards, diesel engines have had to implement a
number of emission control technologies. These breakthroughs have transformed the diesel
engine a long way from its reputation in the 1970s as a slow, smoky, dirty, and smelly machine.
As a result of the new wave of stringent regulations from the 1990s and 2000s, diesel engines
have become much, much cleaner and favorable.
22
reduction after the 1970s was more advanced engine design. Significant emission reductions
were achieved through improved mixture formation and higher intake pressures in the diesel
engine. To achieve better fuel and air mixing in the engine cylinder, intake port design and
combustion chamber geometry were optimized to create greater air motion turbulence, and
advanced fuel injector design was adopted to promote increased injection pressure and fuel
motion [8]. In addition, the introduction of exhaust gas recirculation, which recirculates a
portion of an engine's exhaust gas back to the engine cylinders in order to lower combustion
chamber temperatures and reduce the amount of NOx generated, as well as electronic
engine/subsystem control also led to further emissions reductions. As a result, substantial
reductions in PM (90%), NOx (75%), and HC / CO emissions were achieved in the 1980-1990
timeframe [8].
Advances in fuel technology have also significantly aided the reduction of diesel emissions. The
introduction of ultra low sulfur diesel (ULSD) fuels employ greatly reduced sulfur levels, which
help cut down emissions of sulfur dioxide and sulfate particulates [8]. Furthermore, this
reduction also benefits diesel catalytic aftertreatment technologies, whose performances are quite
sensitive and vulnerable to sulfur content. Other fuel characteristics such as higher cetane
number and lower aromatics content have also been proven to reduce emissions [8].
In addition to the above technologies, diesel engines require the use of a number of exhaust gas
aftertreatment devices in order to further comply with strict NOx and PM regulations. The most
important and effective technologies used today include urea-SCR devices to control NOx
emissions, and diesel particulate filters (DPF) to control PM emissions. SCR refers to selective
catalytic reduction of NOx, which is a way of converting nitrogen oxides into nitrogen (N 2) and
water (H 20) with the aid of a catalyst such as ammonia or urea:
NO + N0
2
+ 2NH 3 -* 2N 2 + 3H 2 0
(1.1)
SCR catalysts are capable of high (90%) reductions of NOx emissions, and are currently used in
many heavy- and light-duty diesel engine applications [8]. Other newer, less widespread
technologies include NOx adsorbers, which trap nitrogen oxides in the catalyst washcoat and use
23
spikes of rich air/fuel mixtures to regenerate the stored NOx, and lean NOx catalysts, which
reduce nitrogen oxides through selective reactions with hydrocarbons.
Diesel particulate filters have been used as the most effective and popular solution for curbing
PM emissions. Diesel oxidation catalysts (DOC) are often used to oxidize the hydrocarbon
portion of diesel particulates, but they are unable to remove much of the carbonaceous fraction.
DPFs, therefore, constitute another crucial component of the diesel aftertreatment system. These
ceramic traps, present in most diesel applications since 2007, capture PM from diesel exhaust
with high efficiencies above 90%, and are able to remove much of the particulates through
catalytic and heating processes called regeneration [8]. Although very effective, DPFs are
vulnerable to durability and reliability problems that eventually result in high exhaust gas
pressure drop, leading to a greater energy demand for regeneration and a decrease in overall fuel
economy. As a result, extensive research has gone into improving and optimizing DPF
performance as well as developing new pieces of diesel emission control technology at a time
when regulation standards grow tighter and tighter.
24
2
DIESEL PARTICULATE FILTERS
Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent
their release to the atmosphere. First introduced in 1985 on Mercedes cars equipped with a 3.0 L
turbocharged IDI engine sold in California, DPFs have now become the most effective
technology for the control of diesel particulate emissions [8]. They possess very high filtration
efficiencies, in excess of 90%, as well as good mechanical and thermal durability [8].
Figure 2.1. Cutout of Catalyzed DPF for Heavy-Duty Engine Retrofits, Reproduced From [8]
2.1
DPF Operation
The most common DPF design is the wall-flow monolith, which features a cylindrical ceramic
structure with many small, parallel channels running in the axial direction [8]. The channels are
plugged on each end in an alternating checkerboard pattern, forcing the inlet gas to flow through
the porous walls of the DPF:
ocl_*
_OF
oQ
otm
O
U
ba
Figure 2.2. Wall-Flow Substrate Operation, Reproduced From [8]
25
The porous channel walls of the DPF, therefore, act as the main filter medium. They are made
from porous ceramic materials, most commonly cordierite, used in heavy-duty applications, and
silicon carbide (SiC), used in diesel passenger cars [8]. These ceramics possess material
porosities typically between 45 and 50%, with medium pore sizes around 10 to 20 um [8].
Particulates initially deposit inside the porous channel walls, known as depth filtration. As the
soot load increases and the walls become saturated, a particulate "cake" layer begins to develop
along the surfaces of the walls, constituting cake filtration:
[
porous barrier
.
particle
4?UI6,
Cake Filtration (Sieving)
Depth Filtration
Figure 2.3. DPF Filtration Mechanisms, Reproduced From [8]
Eventually, the DPF will collect a large amount of soot, restricting exhaust gas flow and
resulting in engine back pressure. This impairs engine operation and reduces fuel economy, so
the soot must somehow be removed. This takes place via filter regeneration, the thermal process
by which solid soot hydrocarbons are oxidized into gaseous products like carbon dioxide.
Carbon oxidation can make use of either oxygen or nitrogen dioxide. Oxygen is present in diesel
exhaust at sufficient concentrations at practically all operating conditions, but relatively high
temperatures are necessary to achieve appreciable regeneration rates with
02
[8]:
C + 0 2 -+ CO2
(2.1)
For this type of regeneration, called active regeneration, additional fuel needs to be spent in order
to periodically increase filter temperatures to the required "active" regime, typically above 550
*C, where soot is able to be oxidized effectively using only oxygen in the exhaust. Nitrogen
26
dioxide based regeneration can be conducted at lower temperatures, as low as 250 *C, but this
requires increased levels of NO 2 likely achieved by the use of an oxidation catalyst [8]:
NO+
0 2 =NO2
N02 + C -
2
N2 + C0 2
(2.2)
(2.3)
This second strategy is called passive regeneration. Here, exhaust gas temperatures are often
near or above the temperatures required for regeneration, so little to no additional heating is
required and the filter is able to regenerate continuously.
In addition to the aforementioned carbon-based soot, diesel PM also includes a small fraction,
less than 4%, of inorganic ash particles and other trace metals and elements [8]. Like soot, these
particles are trapped inside the DPF, but they are not, however, able to be removed from the DPF
through filter regeneration. Over time, ash will gradually accumulate along the DPF channel
walls and toward the back of the filter, reducing its overall performance and contributing even
more to the filter pressure drop [15]:
Figure 2.4. Ash and Soot Distribution in DPF Channels, Reproduced From [15]
27
As a result, DPFs require periodic maintenance, typically not more than once per year, in order
to clean out residual ash and restore filtration efficiency. DPFs have proven themselves to be
effective, valuable pieces of technology for diesel emission control, but they are still susceptible
to certain drawbacks. One major obstacle is the aforementioned ash accumulation problem,
which has been studied extensively in recent years. Much research and resources have been
spent towards understanding the fundamental mechanisms governing the accumulation of ash, its
properties, and effect on DPF performance, with the ultimate goal being to control its detrimental
effects to a minimum and extend DPF service life.
2.2
Ash Sources
The chemical composition of DPF ash suggests that it derives mainly from additives in the
engine lubricant. Ash can also include various engine wear and corrosion particles as well as
trace metals found in diesel fuels, but lubricant-derived ash comprises the majority of the ash
found in the DPF [21]:
100Fuel
80
Wear
60
CoM[K[I
DPF
40
Lube
20
1P-ULSD
2A.ULSD
3A.B20
4A-B100
Figure 2.5. Origin of Ash-Related Constituents from DPFs Operated on Various Fuels and
Regeneration Conditions, Reproduced From [21]
During typical engine operation, small amounts of lubricant will inevitably enter the combustion
chamber on the cylinder lining by passing over the piston rings. After combustion takes place,
the inorganic, incombustible materials from the lubricant will be emitted to the DPF as ash
particles where they will remain trapped even after filter regeneration. Studies also show that oil
consumption is directly related to the amount of ash accumulated in a DPF [19]. Engine
28
lubricants consist of a base oil (75-83%), viscosity modifier (5-8%), and an additive package
(12-18%) [16]. While the base oil is primarily responsible for lubrication, the additive package
is used to provide a number of crucial and beneficial properties that preserve and improve the
performance of the base oil.
2.2.1 Lubricant Additives
The additive package consists of many different components. First, detergents are used to
prevent corrosive wear through their acid-neutralizing properties. They are metal salts of organic
acids that frequently contain associated excess base, usually in the form of carbonate [17]. They
serve to neutralize corrosive combustion acids as well as suspend oxidation products in order to
control deposit formation [17]. As a result, detergents control rust, corrosion, and resinous
buildup in the engine. Detergents exist in different types based on the common polar groups
present in the detergent molecules. These include mainly sulfonate, phenate, and carboxylate
groups, but also sometimes salicylate and thiophosphonate groups [17]. Detergents are also
paired with metals, most commonly calcium and magnesium, in order to become oil-soluble and
thus able to be carried by the base oil. It is because of these metals, inorganic and incombustible,
that ash is produced and emitted to the DPF. Calcium-based detergents are more common due to
their high anti-corrosion performance and low cost, but magnesium-based detergents are also
used in combination because their smaller molecular weight produces less ash [17]. As of 2003,
most detergents consist of basic calcium sulfonate (65%) and basic calcium phenate (31%) [17].
Dispersants are another major class of lubricant additives. Dispersants and detergents together
make up about 45-50% of the total volume of the lubricant additives manufactured [17].
Dispersants are metal-free, however, so upon combustion they do not lead to ash formation. The
goal of the dispersant is to suspend oil insoluble contaminants and degradation products, which
include sludge, resin, varnish, and hard deposits. By suspending these products in the bulk
lubricant and preventing them from attacking the base oil and other additives, dispersants serve
to minimize particulate-related abrasive wear and viscosity increase [17].
The other main group of additives and source of ash formation is antiwear additives. Antiwear
additives are necessary to prevent damage and wear to parts experiencing boundary lubrication
29
[8]. These additives have a polar structure that forms a protective layer on metal surfaces,
preventing direct contact between sliding surfaces [8]. The most commonly used and cost
effective antiwear additives are zinc dialkyldithiophosphates (ZDDP). ZDDP can also exist in
different types, depending on the structure of its alkyl groups from synthesis, which ultimately
affect its thermal stability and performance [8].
As stated earlier, it's understood that ash is primarily derived from additives in the engine
lubricant because of its chemical makeup. Compositional analysis of DPF ash show that it is
composed mostly of Ca, Mg, Zn, P, and S compounds in the form of various phosphates,
sulfates, and oxides [14]. These correlate with the widely used Ca- and Mg-based detergents and
Zn-based antiwear additives, as well as the S- and P-based polar and alkyl groups with which
those additives are synthesized with. Even though these additives provide numerous benefits to
the base oil, their consequential ash production has become a very concerning issue that has been
addressed more and more in recent years. In 2006, the CJ-4 oil specification was developed to
minimize the production of lubricant-derived ash. Lubricants were required to have the
following chemical limits [18]:
"
Maximum sulfated ash content:
1.0%
"
Maximum phosphorous content:
0.12%
"
Maximum sulfur content:
0.4%
e
Maximum volatility:
13%
The sulfated ash restriction refers to the limiting of additive content in the lubricant, as defined
by the sulfated ash test (ASTM D874) [14]. In this laboratory test, the lubricant's calcium and
magnesium additives are converted to their sulfates and its zinc additives are converted to their
oxides in the absence of phosphorus [14]. This provides a quantitative measure of additivederived ash production, so that the resultant ash delivered to the DPF may be limited.
It should be pointed out, however, that real ash obtained from DPFs after on-road use is much
different and more complex than lab ash generated by the ASTM test. Below is a small list of
species typically found in DPF ash [14]:
30
Ash Constituent
Calcium Sulfate
Magnesium Oxide
Magnesium Sulfate
Zinc Oxide
Zinc Magnesium Phosphate
Zinc Phosphate
Zinc Pyrophosphate
Chemical Formula
CaSO 4
MgO
MgSO 4
ZnO
Zn2 Mg(PO 4 ) 2
Zn3 (PO4 )2
Zn2 (P2 0 7 )
D
3
gIcm
2.96
3.58
2.66
Description
C
5.61
1,460
2,832
1,124
1,975
3.60
4.00
900
Anhydrous, decomposes -1,250'C
Anhydrous, decomposes: 900-1,1 00C
3.75
Table 2.1. Measured and Reported Ash Components, Properties, and Characteristics,
Reproduced From [14]
Because real ash is formed under much different conditions than those from the sulfated ash test,
subject to various temperatures, substrate interactions, flow conditions, and regeneration modes,
its composition can contain a much wider and complex array of species and compounds
compared to the mere three that the sulfated ash test aims to limit. The presence of phosphorus,
true for real ash, is also known to affect sulfated ash test results [14]. As a result, while sulfated
ash limits defined by oil specifications are one step in limiting the amount of lubricant-derived
ash production, they do not comprehensively define all aspects of ash accumulation inside the
DPF and therefore cannot fully predict the resulting effects on DPF performance.
2.3
Ash Transport
Studies show that lubricant-derived ash starts off in the form of small ash precursors, well below
100 nm, bound to carbonaceous soot particles [14]. Sampling analysis is able to identify
lubricant additive-related elements, such as zinc and phosphorus, in individual soot agglomerates
collected upstream of the DPF [14]. As soot accumulates in the DPF, ash precursors will in turn
be suspended along the channel walls held by the carbon particles. During filter regeneration,
the soot particles separating the ash precursors are removed and the ash becomes more
concentrated. Over repeated regeneration cycles, the ash precursors agglomerate together with
new incoming ash to form larger micron-sized ash particles, typically found in the DPF [14]:
31
Figure 2.6. Schematic Representation of Ash Particle Formation and Growth over Repeated
DPF Regeneration, Reproduced From [14]
These fundamental mechanisms of ash transport and agglomeration are the basis of what
determines ash properties such as particle size, structure, packing, and distribution. A number of
parameters, though, can affect these events, such as engine operating conditions, regeneration
strategy, and DPF design, in turn affecting the characteristics of ash found in its final form inside
the DPF.
2.4
Ash Effects on DPF Performance
Due to the detrimental effects of ash on DPF performance, much research and work has gone
into trying to fully understand all the fundamental mechanisms governing ash properties and
behavior, such as composition, morphology, and distribution. Some characteristics of DPF ash
are understood, for instance that it is primarily lubricant-derived and that ash production
increases with higher sulfated ash levels and oil consumption rates, whereas others like
distribution and packing in the DPF are less defined. There are a number of different factors too,
ranging from engine operating conditions to DPF design to lubricant formulation, that could
affect some or all of these ash properties, turning ash accumulation into a very complex yet
important field worth studying.
2.4.1 DPF Pressure Drop
Ash accumulation has been shown to lead to an increase in DPF pressure drop. Pressure drop
refers to the difference in pressure across the DPF, between its inlet and outlet faces. As ash
32
builds up in the filter over time, exhaust flow becomes more and more restricted. This restriction
leads to a buildup of backpressure on the inlet, engine side of the DPF. As a result, the
differential pressure, or pressure drop, across the filter increases. Studies have shown that for
lubricant formulations at various sulfated ash levels, engine backpressure directly increases with
simulated driving distance and therefore ash accumulation [19]:
2.4
-
2004-01-1955 (1.6%
-m-2003-08-0408 (2.0%
--- 2003-08-0408 (1.6%
2003-08-0408 (1.0%
--- 910131 (1.6% Ash)
-o-910131 (Ashless)
2.2
7;
U)
2.0
1.8
E
.
I
2004-01-3013 (1.48% Ash)
Ash)
Ash)
Ash)
Ash)
1.6
0
CD
1.4
U)
Z
1.2
&age=
1.0
0.8
-
50,000
100,000
150,000
200,000
250,000
Simulated Distance (km)
Figure 2.7. Backpressure Increase Versus Simulated Driving Distance for Several DPF Studies,
Reproduced From [19]
Depending on the lubricant sulfated ash level, engine backpressure was expected to double at
distances anywhere from 75,000 km to over 250,000 km. At increased backpressure levels, the
engine has to compress the exhaust gases to a higher pressure which involves additional
mechanical work and less energy extracted, leading to an increase in fuel consumption [20].
Literature shows that a 10 kPa increase in particulate filter pressure drop can result in fuel
consumption increases ranging from 1%for newer engines to 4.5% for older engines [20].
In the above study, however, it can be seen that sulfated ash level does not correlate directly with
backpressure increase. This suggests that more total ash does not necessarily mean more
detrimental backpressure. In another study, a similar conclusion is reached. For ten different
33
oils at the same sulfated ash level of 1.8%, there was no correlation found between the amount of
ash present in the DPF and the resulting filter pressure drop [19]:
16
-
y = -0.033x + 10.404
R2 =0.0326
14.
12-
+
100
8-
0
6-
E
+
420 i
40
50
60
70
80
90
100
110
120
AP (mbar)
Figure 2.8. Mass of Accumulated Ash Versus Backpressure Increase for Ten Catalyzed Soot
Filters Exposed to Different 1.8% Sulfated Ash Oils, Reproduced From [19]
Again, this suggests that more ash does not necessarily lead to higher backpressure increase,
implying that there may be differences between types of ash, that one type can be more harmful
than another. Although sulfated ash level can affect the total amount of ash generated and
emitted to the DPF, it is not the sole factor controlling ash-derived backpressure increase. Ash
morphology, packing, and distribution, potentially affected by parameters such as lubricant
additive chemistry, engine operation, and DPF design, are some of the many additional variables
that could explain the above findings, why not all ash behaves the same.
2.4.2 Lubricant Chemistry Effects
A study conducted in 2010 at MIT sought to investigate the effects that ash derived from
different types of lubricant additives had on resulting DPF performance. Six different lubricant
formulations, synthesized with base oil and different combinations of Ca, Mg, and Zn additives
at the same sulfated ash level of about 1%, were used to generate ash loaded DPFs. In the
absence of soot, the following DPF pressure drop results were obtained [22]:
34
3.5
3.0 -ase
-r
ua
CJ-4
Q 2.5
02.
S1.5
-Base
1.5
1.o
+ Mg + ZDDP
Base + ZDDP
-----
Base + Mg
0.5
0
5
10
15
I
I
I
I
I
0 .0
20
25
25 *C, Space Velocity = 20,000 hri
I
I
I
30
35
40
45
Ash Load [g/L]
Figure 2.9. DPF Pressure Drop as a Function of Ash Load for Various Lubricant Formulations,
Reproduced From [22]
The study shows that for the same DPF ash level, oils containing Ca additives resulted in DPF
pressure drops approximately two times greater than oils containing only Mg or Zn additives,
and that for the same soot load oils containing only Mg and Zn additives actually displayed a
pressure drop benefit relative to the commercial CJ-4 formulation [22]. Analysis of the different
ash samples showed that while ash distribution in the DPF was roughly similar, there were
distinct differences in ash morphology as well as in ash packing density and porosity, with the
calcium-based ash being the least porous [22]. Lastly, there were differences noticed in ash
composition, with the calcium-based oils yielding mostly calcium sulfate ash, zinc-based oils
producing mostly zinc phosphate ash, and magnesium-based oils producing mostly magnesium
oxide ash [22].
The above study clearly highlights a significant impact of lubricant additive chemistry on ash
characteristics and DPF performance. For the same sulfated ash content and consumption rate,
oils that differed only in the types of additives present produced very different results that were
extremely interesting and worthy of further investigation.
35
2.5
Project Objectives
Although many studies, like the one above, have been able to clearly illustrate the impact of
lubricant additive chemistry on ash characteristics and DPF performance, many of the underlying
mechanisms governing why there are such differences remain less understood. There are many
unanswered questions pertaining to what exact ash properties are affected by variations in
additive formulations, and why those changes occur.
The objective of this project is twofold. First, three additional lubricant formulations will be
tested using the same setup and procedures from the previous 2010 study at MIT in order to see
if those observed trends can be reproduced and reaffirmed. Secondly, and more importantly, this
work will aim to best explain the previously seen findings, why there was such a large difference
in performance between calcium- and magnesium-based ash. To do so, a better understanding of
the mechanisms that determine ash properties like distribution, permeability, particle size, and
composition must be achieved so that any differences in resulting DPF pressure drop can
quantitatively be pinpointed to certain fundamental factors.
In doing so, this work ultimately hopes to contribute further knowledge to understanding the
effects of lubricant additive chemistry on ash accumulation in DPFs so that lubricant
formulations, and therefore diesel aftertreatment systems, may be optimized to reduce the
negative fuel economy and health hazard effects from lubricant-derived ash to the greatest
degree.
36
3
FUNDAMENTAL UNDERSTANDING
Earlier, it was shown that ash accumulation in DPFs directly results in engine backpressure
increase, or a rise in filter pressure drop. Pressure drop is a much more complex subject,
however, as it is a component consisting of several variables associated with different types of
flow and friction losses. To better understand the effects of ash and soot accumulation on DPF
pressure drop, the fundamental mechanisms governing these parameters must first be examined
in detail.
3.1
DPF Pressure Drop
The main filter medium of the DPF is again its porous channel walls. As soot and ash
accumulate in the DPF, however, along the channel walls and even inside the wall pores as well,
the filtration efficiency of the DPF naturally worsens. Below is a simple schematic representing
typical soot and ash accumulation within a DPF [23]:
F3]r
Figure 3.1. Soot and Ash Accumulation in a DPF Channel and Pores, Reproduced From [23]
The number labels in the above figure correspond to different types of flow restrictions and
losses that arise due to soot and ash accumulation inside the DPF. Each contributes to the DPF's
total overall pressure drop, and are all outlined in greater detail in the following table [23]:
37
PressReynDldp
Key Parameters
Contribution(R)
Pressre
rop
Contribution
Reynolds
Number
Controlling Properties
to Total DPF
toesure
Pesr
Drop
Filter geometry, ash and
soot layer thickness
Filter geometry, ash and
Transition
< 3%
soot layer thickness
Filter geometry, ash end
plug formation
< 2,100
5%/-30%
1
Inlet losses
(Contraction)
Open frontal area
2
Frictional losses
along inlet
channel walls
Channel hydraulic
diameter
Available channel
length
Wall permeability
ash ad sot epth filtraion
3
Frictional losses
from flow through
wall
Wall thickness
Filtration area
4
Frictional losses
from flow through
ash layer
Filter geometry
Filter geometry
Ash porosity, pore size
Ash packing density
Ash layer thickness, end
plug formation
Soot porosity, pore size
Soot packing density
Soot layer thickness
Ash permeability
Ash thickness
Filtration area
5
Frictional losses
from flow through
soot layer
Soot permeability
Soot thickness
Filtration area
6
Frictional losses
along outlet
6 al oult
diameter
7
Outlet losses
(Expansion)
<< 1
Channel hydraulic Filter geometry
cha
l<
Filter geometry
Available channel
500/
2,100
-
90%
~5%
Outletllosse
I
Open outlet area
I
Filter geometry
I
Transition
I
< 3%
_I
Table 3.1. Key DPF Pressure Drop Contribution Factors, Reproduced From [23]
Exhaust gas flow entering the DPF must contract in order to fit within the many, much smaller
filter channels. This loss leads to a small pressure drop increase and is governed by the available
frontal channel area, as determined by the ash and soot layer thicknesses. That reduction in
channel frontal area, or hydraulic diameter, also results in frictional losses along the channel
walls. There are also similar frictional losses along the outlet channel walls and due to flow
expansion. But studies show that the majority of DPF pressure drop contribution can be
attributed to frictional losses from flow through the filter's porous media [23]. As soot and ash
enter the DPF, they deposit inside the channel pores as well as form layer membranes along the
walls. These layers are porous themselves, so at any given time, the filter's porous medium
through which flow passes consists of the DPF substrate wall and/or any soot or ash layers
present.
38
The first soot that enters a clean DPF initially deposits within a portion of the substrate wall
pores. This is known as "depth filtration," and typically doesn't exceed more than about a third
of the total substrate wall thickness [14]:
Wait surface
0Depth
Wall
wurfac
Particulates trapped
inside the surface pore
-10 pm
Figure 3.2. Soot Depth Filtration in a DPF Channel Wall, Reproduced From [14]
After this early soot is oxidized over the first filter regeneration periods, the remaining ash will
end up filling the pores to the surface. At this point, with the substrate pores full of ash,
additional soot entering the DPF will begin to accumulate on the surface of the channel walls.
This is called "cake filtration," as soot starts to accumulate in a cake-like layer throughout the
DPF. Ash left behind from subsequent regenerations will then remain in a layer along the
channel walls [14]:
Figure 3.3. Soot and Ash Layer Formation, Reproduced From [14]
39
As this ash layer grows over time, it will eventually reach a "critical thickness" at around 12 g/L
DPF ash load, which designates grams of ash per liter of DPF volume, at which its maximum
allowable shear stress is reached [26]. Layer ash will begin to break off and deposit toward the
back of the filter, forming the ash "end plug," which covers the entire cross-sectional area of the
channel [26]:
Figure 3.4. Ash Layer (Left) and Ash End Plug (Right), Reproduced From [26]
At higher ash loads above 20 g/L, this end plug starts to become a significant factor as it has
effectively reduced the available length of the DPF channels. The filtration area of the substrate
is diminished, restricting exhaust gas flow and contributing to increased filter pressure drop.
These two filtration regimes, depth and cake, however, have very different effects on DPF
pressure drop as confirmed by most experimental research results [24]:
Loaded
filter
AP after initial
loading phase
APInitial loading
phase
Time
Figure 3.5. DPF Pressure Drop Regimes, Reproduced From [24]
40
The initial depth filtration phase begins with a sharp increase in filter pressure drop over just a
small amount of time. This mechanism is the most detrimental to the substrate wall, as soot and
ash particles directly clog the pores of the substrate and reduce its permeability. Once depth
filtration has taken place and cake filtration has started, however, DPF pressure drop increases
roughly linearly as a function of soot and ash layer thickness. This regime's contribution to total
DPF pressure drop is much less dramatic.
Again, frictional losses from flow through the substrate wall and soot and ash layers are the
largest contributors to overall DPF pressure drop. The magnitude of these pressure drop
components are largely affected by the properties of these layers, mainly permeability, a function
of porosity, pore size, and packing, as well as layer thickness.
3.1.1 DPF Pressure Drop Model
Quantifying DPF pressure drop in terms of fundamental mathematical equations is essential for
the comprehensive understanding of soot and ash effects. Breaking down total pressure drop
into a function of fewer underlying parameters such as porous media permeability and thickness
can help provide insight into the driving factors governing DPF pressure drop, highlighting
which properties contribute the greatest significance. DPF pressure drop can initially be
represented by the separate pressure drop terms discussed in Table 3.1, attributed to the different
modes of flow and friction losses throughout the DPF [23]:
APTotat = APin + APout + APcfannei + APwal + APAsh + APsoot
(3.1)
APin/out represents the inlet flow losses due to contraction and expansion as the diesel exhaust
gases enter and leave the DPF. APChanne, represents the frictional losses along the channel
walls, and
APWalt/Ash/Soot
represents the frictional losses due to flow through the filter's porous
media, consisting of the substrate, soot, and ash layers.
The pressure drop associated with the filter inlet and outlet losses, due to gas expansion and
contraction, can be described as
41
(3.2)
APintout = Z1nout (eV2
where p is the exhaust gas density and v is the exhaust gas velocity. Z1n/out defines the
frictional coefficients for inlet channel gas contraction and outlet channel gas expansion as
Z1n = -0.415 (if) + 1.08
(3.3)
Zout = (1 -
(3.4)
)2
where Af refers to the filter frontal area, and A represents the filter's total surface area. The
above equations assume laminar flow at the filter inlet and exit, but if this is not the case, then
the appropriate friction coefficients for turbulent or transition flows must be used [25].
Pressure drop due to frictional losses for gas flow along the channel walls can be described as
APchannet = 4f
(a:
where L is the available length of the channel and DH is the channel hydraulic diameter.
(3.5)
f
denotes the Fanning friction factor, which can be estimated by
f
= Re
(3.6)
where Re is the exhaust gas Reynolds number and C is a constant equal to 14.23 for square cross
section DPF channels and 16.00 for round cross section channels [25]. Exhaust gas flow within
the DPF channels is typically laminar, and as the ash load of the filter increases over time and the
ash layer thickness increases, the cross sectional opening of the channel generally transitions
from square to circular [23]. Although pressure drop due to channel friction is typically small in
comparison to total DPF pressure drop, this term can become quite significant at high soot and
ash loads when the channel hydraulic diameter is greatly reduced.
42
Studies show that pressure drop due to flow through the filter's porous media, which include the
substrate, soot, and ash layers, contributes the most to total DPF pressure drop [23]. Flow
through porous media can be described starting with the Forchheimer-Extended Darcy Equation:
()ww
APWall/Ash/Soot
+
pv 2
(3.7)
t denotes the exhaust gas dynamic viscosity, K, the permeability of the porous medium, v, the
# the inertial resistance. In most
gas wall velocity, w the porous medium thickness, and
practical applications for diesel particulate filters, the Reynolds number for flow through the
porous media is much less than one, meaning that the inertial terms can be neglected as a firstorder approximation [23]. As a result, porous media pressure drop can be described as:
APWaill/Ash/Soot
=
Y
(3.8)
VWw
The permeability of a porous medium can generally be characterized by the Rumpf and Gupte
equation, valid for 0.35
E
0.7 [23]:
K = S.6
Le55Dp2
(3.9)
E denotes the porosity of the porous medium and Dp the mean pore diameter. Permeability is
particularly straight forward to calculate in this manner for a clean DPF, as porosity and pore
diameter information can be supplied by the filter manufacturer. Once soot and ash have begun
to deposit within the substrate pores, however, this quantity becomes much more difficult to
quantify as two very different porous media are now present and intertwined.
Because soot and ash layers have been found to generally have higher porosities, the KozenyCarman equation (E > 0.8) is instead used for calculating ash and soot layer permeability [23]:
K=
E
18O(1-E)2
43
Dj
P
(3.10)
In this case, it may sometimes be more appropriate to use the mean particle size for Dp, as the
soot and ash particles themselves are often the dominating flow characteristic within the layer.
3.2
Material Properties and Characteristics
The characterization of DPF pressure drop in terms of fundamental mathematical equations
shows that total pressure drop is most significantly a function of a few key properties related to
the filter porous media, which include the substrate, soot, and ash layers. It's important to
understand, then, the nature of these material properties, what mechanisms determine and affect
them, and to what degree their variation influences overall DPF pressure drop.
3.2.1 DPF Substrate Properties
The two most common materials used to make DPF substrates are cordierite and silicon carbide
(SiC). Cordierite is characterized by good thermal shock resistance and relative low cost, but has
a lower melting temperature that is vulnerable to runaway regeneration events [23]. SiC, on the
other hand, has a much higher operating temperature but displays low thermal shock resistance
and high manufacturing cost. Table 3.2 compares various material properties between cordierite
and SiC filters [23]:
Property
Channel Width
Wall Thickness
Permeability
Porosity
Mean Pore Size
Melting Temperature
Thermal Expansion
Elastic Modulus, Axial
Strength, Axial
Thermal Shock Parameter
Thermal Conductivity
Relative Cost
[mm]
[mm]
[x 10~2 mz]
[%]
[pm]
[*C]
[x 10-" 1/*C]
[GPa]
[MPa]
[W/m-K]
Cordlerite
1.3-2.1
0.3 - 0.5
0.5
45-50
13-34
1450
0.7
4.7
2.6
790
<2
Low
Silicon Carbide (SIC)
1.0- 1.6
0.3 - 0.8
1.24
42 - 58
8- 17
2400
4.5
33.3
18.6
124
20
High
Table 3.2. Cordierite and Silicon Carbide (SiC) DPF Substrate Properties, Reproduced From
[23]
44
In regards to DPF pressure drop, open channel width and wall permeability change over time due
to increasing soot and ash loading. As soot and ash deposit within the substrate wall pores,
porosity and mean pore size are reduced and exhaust gas flow becomes more restricted.
Although it's known that DPF substrates are roughly 50% porous initially, their porosity and pore
size characteristics after ash and soot depth filtration are difficult to quantify and measure
experimentally. Still, it's clear from the previous model that the substrate wall permeability, to
whatever degree of reduction, directly influences total DPF pressure drop.
3.2.2 Ash Properties
Ash layer accumulation and depth filtration can be observed in Figure 3.6 (a) below. While soot
depth filtration can penetrate up to about a third of the filter wall thickness, ash typically is only
found within the first few surface pores and therefore doesn't seem to penetrate to any significant
depth into the filter wall [14]. A closer examination of DPF ash shows that it consists mostly of
agglomerates. These agglomerates, formed from nano-sized ash precursors that have undergone
a number of high temperature filter regeneration events, can vary greatly in size. For example,
Figure 3.6 (b) shows a large ash agglomerate above 80 pm in diameter while Figure 3.6 (c)
shows smaller agglomerates of about a few microns in diameter. In either case, though, it's
interesting to note that the ash agglomerate structures are fairly porous in nature. Applying
focused ion beam (FIB) technology to cut open the ash agglomerates reveals that the seemingly
round and solid ash agglomerates actually contain significant interior void regions [14].
I-
Figure 3.6. Cross Section View of (a) DPF Substrate and Ash Layer (b) Large Ash Agglomerate
(c) Porous Ash Particle, Reproduced From [14]
45
Although ash agglomerate size can vary pretty significantly on the micron scale, experimental
findings in literature show that a large majority of ash agglomerates are actually sub-micron [18]:
is.
100.0
0
0.
0.(20 0.100
1.00W
10.00
100.0 500.0
Diameter (pin)
Figure 3.7. Number-based Size Distribution and SEM Image of Field-aged DPF Ash,
Reproduced From [18]
In general, though, ash agglomerates present in the DPF are still larger than any incoming soot
particles. Soot primary particles only range from about 10 to 40 nanometers in diameter, and any
soot agglomerates that have formed are typically on the order of 100 nm [27].
A number of studies have also measured other DPF ash properties such as packing density,
porosity, and particle size. A summary of these findings for various field aged and lab generated
samples are provided below [26]:
46
Source (SAE Paper)
2000-01-1016
2001-01-0190
2004-01-0948
2005-01-3716
2006-01-0874
2006-01-3257
2008-01-0331
2009-01-1086
2010-01-1213
Materl~
Density
[g/cmI
3.13
2.5
2.85
3.4
Packing
Density
[g/cm!
0.4- 1.0
0.54
0.4
83
85
0.31 - 0.52
0.45
0.17-0.34
0.3
90-95
91.1
Porosity
Permeability
[%
mP[m]
2.8-7.4 x 1014
5 x 10
Particle Size
1 - 10
0.1 - 0.5
2.4 - 37.6
0.4-8
1
Table 3.3. Measured Field and Lab Ash Properties, Reproduced From [26]
The results show that ash, as it exists in its layer form inside the DPF, is considerably more
porous (80 - 95%) than the filter substrate itself (-50%). It can also be seen that these
measurements have a fairly high amount of variation, with ash porosity ranging from 83 to 91
percent and particle size ranging from 0.1 to over 10 microns in diameter. A quick sensitivity
calculation using the previous DPF mathematical model reveals that just a 5% reduction in ash
layer porosity can result in a dramatic 77% increase in pressure drop through the ash layer. But a
5% increase in ash particle size only results in about a 9% increase in ash layer pressure drop,
suggesting that ash permeability is more highly sensitive of ash porosity.
Several factors have been found to affect ash properties and therefore DPF pressure drop.
Regeneration strategy, for example, is able to affect ash distribution. Filters employing periodic
regeneration tend to accumulate ash plugs in the back of the DPF over time while filters being
continuously regenerated tend to accumulate thicker ash layers and less of a plug [23]. High
exhaust and DPF temperatures can also subject the ash to increased agglomeration and sintering,
affecting overall packing and porosity in the layers. But in 2010 at MIT, it was also found that
lubricant additive chemistry can affect a variety of ash properties. Six lubricant formulations
involving various Ca, Mg, and Zn additives at the same 1% sulfated ash level were used to load
different DPFs with ash under similar loading conditions. The consequent ash properties were
measured and quantified, with the results shown in the table below [26]:
47
Lubricant
Base
Base
Base
Base
Base
CJ-4
Ash
Lubrcant
Ash
Load
Lod
+ Ca
+ Mg
+ ZDDP
+ Ca + ZDDP
+ Mg + ZDDP
[g/L]
29
24
28
25
23
42
Ash
sh Layer
t~rer
Thickness
Tikes
Layer
Packing
Plug
Packing
Material
Density
Density
Density
Ash
Prst
Porositesyt
Prst
[cm]
0.015
0.009
0.013
0.011
0.009
0.013
[g/cm3]
0.27
0.19
0.19
0.30
0.27
0.30
[g/cm3j
0.20
0.17
0.17
0.13
0.14
0.17
[g/cm3n3j
3.0
3.6
3.9
3.1
3.5
3.4
[
90.9
94.6
95.1
90.5
92.1
91.1
Table 3.4. Ash Properties for Various Lubricant Additive Formulations, Reproduced From [26]
Although ash distribution was roughly the same for all test cases, there were noticeable
differences found in ash porosity. Considering how sensitive ash layer pressure drop is to
porosity, these differences could potentially explain the differences in total DPF pressure drop
that were seen during the ash loading tests, with the formulations with the highest pressure drops
having the lowest ash porosities and vice versa. It can be seen that lubricant additive chemistry
can directly affect important ash properties such as packing and porosity, which in turn can have
significant effects on DPF pressure drop. It's important, then, to investigate exactly why ash
from different additives behave differently and fully understand the underlying mechanisms
governing these observed differences.
48
4
EXPERIMENTAL SET-UP AND TECHNIQUES
The objective of this study is to build off of previous work related to the effects of lubricant
additive chemistry on ash characteristics and DPF pressure drop performance. This work aims to
test additional lubricant additive formulations to see if similar results and trends can be
reproduced, to relate these discrepancies to differences in key ash properties such as porosity and
composition, and then finally to explain the underlying mechanisms governing the observed
differences.
4.1
DPF Loading
In order to compare ash characteristics and pressure drop performance between DPFs, filter
samples must be loaded to a sufficient ash load. DPF tests typically load filters to about 25 - 40
g/L ash in order to build up a sufficient amount of ash layer formation. Only then can the filter
pressure drop profiles fully take shape and any differences between ash samples be measured.
These high ash loads, however, can only be reached after a vehicle has driven some hundreds of
thousands of miles on the road. DPF sample generation, therefore, is extremely time consuming
and expensive. One solution is to accelerate the ash loading process, reducing the time and cost
necessary to generate ash loaded DPF samples.
4.1.1 Accelerated Ash Loading
There are a few ways DPF ash loading can be accelerated. The first is fuel doping, in which
lubricant is doped into the engine fuel to directly increase lubricant consumption in the power
cylinder [26]. Another technique involves injecting oil mist into the engine's intake manifold to
be burned inside the combustion chamber [26]. Although these strategies greatly speed up the
ash loading process, they do not resemble natural oil consumption and ash loading as accurately
as on-road vehicle use. There can be differences in exhaust soot-ash proportions and lubricant
burning mechanisms, for example, that may result in inaccurate pressure drop response, so the
challenge is to develop a technique that simulates real field-aged ash loading as closely as
possible.
49
The experiments conducted in this study all employ the MIT Accelerated Ash Loading System,
which has been in use for the past several years to run tests related to lubricant additive effects
on DPF ash accumulation. In addition to being able to accelerate the DPF ash loading process,
one of the main benefits of this system is its flexibility. A number of the simulated variables
such as exhaust temperature and flow rate, oil consumption, lubricant chemistry, and DPF
geometry can be independently changed in order to address the many different mechanisms
affecting DPF ash accumulation. Although this system is again merely a simulation resembling
real on-road ash loading, in-depth studies have shown that the resulting ash is very comparable in
morphology, distribution, and elemental composition to that found in field-aged samples [28].
The Accelerated Ash Loading System fundamentally consists of a customized diesel burner and
Cummins ISB engine coupled together to allow both engine-out soot and accelerated lubricantderived ash to be loaded in a DPF [23]:
t
Backpressure
Control
Centrifugal
Blower
Exhaust
Diesel Supply toBumer
To Emissions Analyzers
Cummins ISB Exhaust Flow
-
Exhaust
t
Figure 4.1. Accelerated Ash Loading System Configuration, Reproduced From [23]
The oil supply line is filled with the engine lubricant to be tested, where it is then delivered by a
computer-controlled constant volume pump to the oil injector. The air-assisted oil injector
50
sprays and atomizes the lubricant inside the combustion chamber where it is then burned by the
hot burner exhaust. A centrifugal blower at the rear of the system then pulls the exhaust to a
downstream DPF while an intermediate heat exchanger is able to control the exhaust temperature
before it does so. Lastly, digital thermocouples and pressure transducers on either side of the
DPF measure inlet and outlet temperatures as well as filter pressure drop. The following table
summarizes the system's operating specifications [26]:
Speciftcation
System Parameter
[L/h]
1.5 - 7.6
Fuel Consumption
[mL/min]
0.94 - 9.4
Oil Consumption
[kPa]
700 - 1400
Injection Pressure
[SLPM]
266 - 1130
Air Flow
DPF Inlet Temperature
200 - 800
[*C]
Table 4.1. Accelerated Ash Loading System Specifications, Reproduced From [26]
Because this system employs secondary oil injection as its main accelerated ash loading
mechanism, even lubricant formulations that are not fully formulated, and therefore unsafe to be
used inside an engine, are able to be tested. Furthermore, the diesel burner does generate a small
amount of soot, so throughout the accelerated ash loading process some type of filter
regeneration strategy must be employed via the exhaust heat exchanger. Ultimately, this system
is able to simulate a 40 g/L DPF ash load, which equates to roughly 240,000 miles of driving
distance, in only about 100 hours of laboratory testing [23].
4.1.2 Soot Loading
The Accelerated Ash Loading System is also coupled to a Cummins ISB engine so that the DPF
may be loaded with real engine-out soot. This is important for studying how ash accumulation
affects a filter's soot filtration capabilities. The Cummins ISB is a 6-cylinder, 4-stroke,
turbocharged, direct-injection diesel engine that employs a number of emission regulation
technologies such as common rail fuel injection and cooled exhaust gas recirculation. A
summary of the engine's specifications are provided below [26]:
51
Specification
890 N-rn @ 1600 rpm
224 kW @ 2500 rpm
6, in-line
4-stroke, direct injection
Common rail
Variable geometry
turbocharger and intercooler
5.9 L
17.2:1
4 valves / cylinder
O.D. = 158 pm
L = 1.00 mm
8 sac-less nozzles / injector
800 - 1600 bar
System Parameter
Maximum Torque
Maximum Power
Number of Cylinders
Combustion System
Injection System
Aspiration
Displacement Volume
Compression Ratio
Cylinder Head Layout
Injection Nozzle
Injection Pressure
Table 4.2. Cummins ISB 300 Engine Specifications, Reproduced From [26]
4.2
DPF Post-Mortem Analysis
Once filter samples have been loaded with sufficient ash and the corresponding pressure drop
response has been collected, a number of advanced diagnostic techniques can then be applied in
order to measure and quantify important ash properties such as porosity, distribution, particle
size, composition, and morphology. The accurate characterization of these ash properties is
crucial for being able to systematically compare the different lubricant formulation tests.
4.2.1 Ash Distribution
Post-mortem analysis unfortunately is destructive of the filter and ash samples, as core samples
must be cut and sectioned away for closer examination. This process typically begins by cutting
a length-wise core from the center region of the DPF and then sectioning it into several, usually
four, equal length segments. The center of the DPF is chosen because it possesses the filter's
most representative ash distribution, and the square cross section core is typically sized to about
3/4 inch or 7-9 channels in both width and height:
52
Figure 4.2. Post-Mortem Sample Extraction
Ash distribution characterization consists of measuring the ash layer thickness throughout the
DPF length as well as the point at which the ash end plug starts to form. At each of the
segmented sample faces, a high resolution picture is taken of all the channels and an image
processing software (ImageJ) is used to measure layer thickness in two directions:
Figure 4.3. Measuring Ash Layer Thickness
Measurements are averaged across all channels in both X and Y directions as well as for both
faces at each partition between samples. Next, ash end plug formation is measured using a thin
rod again averaged for all available channels:
53
I
Figure 4.4. Measuring Ash End Plug Formation
Once layer thickness and end plug formation data has been collected at various points throughout
the DPF length, an ash distribution profile can be generated to simulate approximate ash
accumulation for one typical channel:
Flow
I
1.6
-
E
1.4
E 1.2
C
ai1
0.8
0.6
0.4
0.2
0
0
50
100
150
Distance (mm)
Figure 4.5. DPF Ash Distribution Profile
4.2.2 Ash Porosity
Once ash distribution has been measured, the core segments can then be mounted for
examination under a scanning electron microscope (SEM). First, the samples are further
sectioned so that a well defined ash layer and the ash end plug are present immediately after the
point of segmentation. These samples are next mounted in a low-viscosity resin and then ground
and polished so that the ash layer and plug are flush with the top surface:
54
Figure 4.6. Mounted Filter Samples for SEM Analysis
Finally, the sample surfaces are coated with a thin layer of carbon (50 nm) and then viewed
under an SEM to provide high-magnification cross section images of the ash layer and end plug:
Figure 4.7. SEM Micrograph of CJ-4 Lab Ash Layer
The equipment used throughout this study to obtain SEM micrographs is the Jeol 5910 General
Purpose SEM. These micrographs provide a two-dimensional snapshot of a given point within
the ash layer or plug. One can then directly measure ash porosity by using an image processing
software to calculate a particle area fraction for a given selection region. The grayscale
micrograph, though, must first be thresholded into a binary image so that all pixels can be
considered as either particle area or void space:
55
Figure 4.8. Binary Image of SEM Micrograph
In addition to applying binary thresholding to the original grayscale SEM micrograph, it also
helps to apply ImageJ's Watershed segmentation filter, which separates seemingly distinct
particles based on roundness factor. Once the binary image is obtained, particle area fraction can
be calculated for a given selection region, averaged over several different channels and locations,
and then expanded to characterize ash porosity in all three dimensions according to the following
relation:
Particles)3/2
Aselection
(4.1)
Alternatively, ash porosity can also be measured according to
1-Pi~ackinq
PMaterial
Where packing density Ppacking equals M for a given volume of ash layer or plug and
VAs
Pmaterial refers
hsh
to the true material density of the ash.
56
(4.2)
4.2.3 Ash Composition
Remaining ash from the other core segments can then be extracted for additional analysis. First,
composition can be measured for a loose ash powder sample using X-ray diffraction (XRD).
The following figure shows a typical peak scan for an ash sample using the PANalytical X'Pert
Pro Multipurpose Diffractometer:
cX~mbf
1000
2500
30
40
PPa
n [*zft a (Ln r a
5b
s(hS)p
Figure 4.9. XRD Peak Scan for a CJ-4 Lab Ash Sample
The above example scan reveals the extremely complex nature of ash samples, that their
composition consists of an incredible amount of different elements and compounds coming from
a multitude of sources such as the engine lubricant, fuel, and wear and corrosion particles. For
simplicity, subsequent analysis typically narrows down the candidate list to compounds
containing Ca, Mg, Zn, S, P, 0, Fe, Si, Al, and Mo elements. Furthermore, once an ash sample's
composition has been determined, its true material density can be estimated by taking a weighted
average of all the material densities of its constituents.
57
4.2.4 Ash Morphology
Next, loose ash samples can be viewed under a transmission electron microscope (TEM) at very
high magnifications so that individual ash particles can be seen. The intent is to be able to
characterize ash morphology and note any differences between lubricant formulations in particle
size, shape, or agglomeration. Below is an example TEM micrograph taken using the Jeol
200CX General Purpose TEM:
Print Nag:
G
to
00 IOUB6.*1
AWC-.sy.t..
Figure 4.10. TEM Micrograph of a Single CJ-4 Lab Ash Particle
4.2.5 Ash Particle Size Distribution
Particle size is another quantifiable characteristic of ash that can be used to systematically
compare different samples. Although a TEM is able to provide very detailed and accurate
information regarding a single ash particle's size and shape, particles can only be viewed and
analyzed one at a time. Ideally, a very large sample set of ash needs to be characterized in order
to comprehensively represent the whole test sample, but doing so would be extremely time
consuming and tedious with the TEM. Alternatively, particle size for an entire sample of ash
powder can be measured using light obscuration. First, a small amount of ash is suspended and
dispersed in solution with a solvent such as ethanol or isopropanol. This solution is passed
58
through a light source essentially as a very dilute stream of single particles, and for each particle
a detector measures the degree of light obscuration to determine its size:
DIWENTIAL OITRUION
VOLUME-WT
Ret %
4A0
Sr0
Detecor
3M
2.50
2.00
1.50
1.00
0.50
0.00
laser
MSuce
0.5
1
2
5
10
20
80
100
200
00
Dim.-(um)-
Figure 4.11. Light Obscuration Technique and Example CJ-4 Lab Ash Volume Distribution
Thus, this technique is able to provide true particle counts for the whole sample as all individual
particles are analyzed. The distribution results can be presented either number-based, where
particle sizes are classified purely on their frequency of occurrence, or volume-based, where
particle sizes are classified based on their volume percentage of the entire sample set.
Furthermore, mean, median, and mode particle size information can be obtained. The equipment
used for these analyses is the Particle Sizing Systems AccuSizer Model 770.
This technique, however, is not without limitations. First, it is based on the Equivalent Spherical
Diameter assumption, which assumes the detected particles are spherical in nature. TEM data
shows that ash typically isn't, though, so the data could potentially be skewed. Also, the lower
detection limit for this technique is only about half a micron, which sometimes cuts off part of
the number-based distributions that contain much smaller particle sizes. Alternative methods
such as laser diffraction can be used to reach smaller particle sizes down to 0.02 microns, but the
number-based distributions cannot be directly measured. They have to be estimated using
calculations and assumptions based on the volume-based distribution results, which don't yield
direct and true measurements like in the light obscuration method.
59
(This page intentionally left blank)
60
5
EXPERIMENTAL TESTING AND RESULTS
By applying the above techniques, this work aims to test and characterize new lubricant
formulation samples and compare the results to previous data, ultimately hoping to better
understand the effects of lubricant additive chemistry on ash characteristics and DPF
performance.
5.1
Test Matrix and Parameters
The previous MIT lubricant additive studies tested a total of six different oil formulations with
varying additive content. This work will contribute seven more filter samples for further
comparison and analysis. Three additional oil formulations will be tested using the MIT
Accelerated Ash Loading System. Two of those same oils will also be used in non-accelerated,
engine-derived ash loading to compare laboratory and engine testing. Lastly, Two field-aged
sample from on-road use with commercial oil formulations will be analyzed as a baseline
comparison.
5.1.1 Lubricant Formulations
The additive concentrations of the six oil formulations tested in the previous MIT lubricant
additive studies are presented below. All oils are formulated to the same 1% sulfated ash level
[26]:
Lubricant
CJ-4
Base oil + Ca
Base oil + Mg
Baseoil+ZDDP
Base oil + Ca + ZDDP
Baseoil+ Mg+ZDDP
Zn
P
Ca
Mg
S
[ppm
1226
<1
<1
2612
1280
1280
[ppm]
985
2
<1
2530
1180
1180
[ppm]
1388
2928
<1
<1
2480
<1
[ppm]
355
5
2070
<1
<1
1730
[ppm]
3200
609
460
6901
2750
2840
Table 5.1. Oil Formulation Test Matrix, MIT 2010, Reproduced From [26]
Note that only the commercial CJ-4 lubricant is a fully formulated oil, which includes all the
other components such as viscosity modifiers and anti-foam agents in addition to the additive
61
package that make it safe to run in a real engine. Again, from these tests there were significant
differences in filter pressure drop observed mainly between oils containing calcium and those
that did not.
To investigate these results further, an emphasis is placed on the possible detrimental effects of
calcium additives, and this is to be addressed by the testing of three additional oil formulations
using the MIT Accelerated Ash Loading System. The first of the three fully formulated oil
formulations is another commercial CJ-4 lubricant. The second is similar in formulation but
contains a much higher level of Ca additives and essentially no Mg present. Similarly, the third
oil would instead contain a much higher level of Mg additives and essentially no Ca present. All
three oils would be used to load DPFs to an ash load of 25 g/L. Again, the objective here is to
isolate the detergent variable and see if there are any differences between using calcium or
magnesium.
The same fully formulated high-Ca and high-Mg oils would additionally be used in engine
testing at an independent facility employing natural oil consumption. These samples are
provided with the intent of comparing ash generated by different means, either through real
engine testing or in an accelerated laboratory setting.
Lastly, the new fully formulated CJ-4 oil would be the same lubricant used in the two field-aged
samples that are provided as a baseline comparison.
5.1.2 DPF Properties
The previous MIT lubricant additive tests all used conventional cordierite DPFs manufactured by
Coming [26]:
Substrate
Catalyst
Cordierite
Pt
Dimensions
D5.66" x 6"
(D14.38 x 15.24 cm)
Cell Density
200 cpsi
(31 cells/cm 2)
Wall Thickness
0.012"
(0.03 mm)
Filter Volume
Table 5.2. Laboratory Testing DPF Properties, Reproduced From [26]
62
2.47 L
Similar Pt-catalyzed cordierite filters with the same geometries above are used for this work's
additional oil tests. They are provided by the same DPF supplier, but involve a new and
different manufacturer, NGK. Properties of the DPFs used in both sets of testing are compared
below:
Property
Bulk Density
Cell Density
Material Density
Open Frontal Area
Geometric Surface Area
Mean Pore Size
[g/cmI
[CPSI]
[g/cmI
[%]
[cm2/cml
[pm]
Corning DuraTrap CO 200/12
0.42
200
N/A
34
9.25
13
NGK DHC-558
0.38
200
1.2
34
9
15
Table 5.3. New DPF Property Comparison
The new DPFs used in this study are said to have a 5-10% reduction in filter pressure drop
compared to the previously manufactured filters. Although the above properties are somewhat
similar between both types of filters, it's important to note that the DPFs used in previous and
current tests are not identical.
Furthermore, the provided engine test and field-aged samples use cordierite filters that are
similar in the above DPF properties but have a much larger overall geometry. Dimensions are
instead D12" x 12", making the filter volume 22.2 L.
5.1.3 Ash Loading Parameters
Samples generated via the MIT Accelerated Ash Loading System all employ periodic, active
filter regeneration. Ash is loaded into the DPF in one hour intervals at inlet temperatures around
250-275 *C. In between these loading cycles would be a 15-minute regeneration period, in
which the DPF inlet temperature rises to about 600-650 *C in order to oxidize any soot produced
by the diesel burner fuel. As a result, the filter would see the following typical temperature and
pressure drop cycles. As expected, one can notice a gradual rise in filter pressure drop with
increasing ash load [26]:
63
Temperature Cycles
750 600 450-
-
]JhfIli
jiII IJI
1RJ]
CL300 -0)0)
0
AP Cycles
-
2 .5
--- -
:-
-- -- --- -
2.0-
1 .5 ----
0.0
---
- :--
1
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Time [hr]
Figure 5.1. Accelerated Ash Loading and Periodic Regeneration Cycles, Reproduced From [26]
The provided engine test and field-aged samples also employ similar periodic, active filter
regeneration. Ash loading would occur at approximately 350 *C and regeneration would take
place at about 650 'C over 20 minutes.
5.1.4 Soot Loading Parameters
At various points throughout a filter's ash loading timeline, the DPF is loaded with soot in order
to measure the effects of certain ash levels on soot filtration performance. For this work's
testing, filters are loaded to about 6 g/L soot at 0, 10, 18, and 25 g/L ash load and the resulting
filter pressure drop is measured at each benchmark. Once the soot loading is finished, the filter
would be fully regenerated for the ash loading process to commence again. Soot loading is
conducted using the coupled Cummins ISB engine in one-hour intervals at 224 N-m engine load,
with DPF inlet temperatures at approximately 275 'C. Both the burner and Cummins ISB use
ultra-low sulfur diesel fuel.
64
5.1.5 Test Summary
A test matrix is provided below summarizing the parameters for all new samples to be tested as
well as all previous samples to be compared to. Note that the commercial CJ-4 oil used in the
old and new tests differ in manufacturer, and that FF denotes a fully formulated oil for those that
aren't commercial CJ-4 formulations:
Source
Lubricant
CJ-4 (1)
Base oil + Ca
Base oil + Mg
Base oil + ZDDP
Base oil + Ca + ZDDP
Base oil + Mg + ZDDP
Filter
I
I
Lab
Lab
Lab
Lab
Lab
Lab
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite
Lab
Lab
Lab
Engine
Engine
Field
Field
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite
Cordierite
Regeneration
I
MIT (2010)
Ash
Load
Equivalent
[g/LJ
[muJ
4
4
4
4
4
4
42.0
29.3
23.8
27.8
25.0
22.7
-315,000
3
3
3
N/A
N/A
N/A
N/A
25.4
25.1
24.9
-25
-25
-75
-85
-175,000
-175,000
-175,000
-175,000
-175,000
-425,000
-350,000
Oiltlnjectlon
Rate
I[mL~mln]
Periodic
Periodic
Periodic
Periodic
Periodic
Periodic
Mileage
-220,000
-170,000
-210,000
-175,000
-165,000
MIT (2012)
CJ-4 (2)
High-Ca (FF)
High-Mg (FF)
High-Ca (FF)
High-Mg (FF)
CJ-4 (2)
CJ-4 (2)
Periodic
Periodic
Periodic
Periodic
Periodic
Periodic
Periodic
Table 5.4. Test Matrix Summary of Previous and New Samples
5.2
DPF Loading and Pressure Drop Results
First, pressure drop results will be presented for the three additional oil tests generated using the
MIT Accelerated Ash Loading System. The objective is to see if the same trends from past
studies, noting significant pressure drop differences between oil formulations containing calcium
additives and those that did not, can be reproduced using three new fully formulated oils.
5.2.1 Ash Loading Results
Even before analyzing the pressure drop data, it's important to note that all three oil tests differ in
the amount of time and oil consumption needed in order to reach the target ash load of 25 g/L:
65
Lubricant
CJ-4 (2)
High-Ca (FF)
High-Mg (FF)
Time
[hr]
82.5
76.25
86
Oil Consumption
Ash Load
[kgl
[g/L]
11.5
10.7
11.9
25.4
25.1
24.9
Table 5.5. Ash Loading Summary for Newly Tested Oils
The high-Ca oil reaches the target ash load in less time compared to the baseline CJ-4 test and
also consumes less oil in the process, while the high-Mg oil requires both more time and oil.
These differences can be attributed to the fact that the additives form different ash compounds.
It will be shown later that Ca-based ash consists mostly of sulfates [CaSO 4] while Mg-based ash
contains mostly phosphates [Mg 3(PO4)2]. For the same unit amount of additives, then, the
calcium additives gain more additional molecular weight from the sulfates per mole than the
magnesium additives do from the phosphates, meaning the target ash load is reached sooner.
Ash loading results for the three new oils are presented below. At each ash level, DPF pressure
drop is measured in the absence of soot at room temperature and at 20,000 1/hr space velocity:
MIT Accelerated Ash Loading Tests (2012)
0.6
-
Ca (FF)
0.5
0.12 kPa
0.4
CL
%40
.
0.3
0.2
0.1
25 *C, Space Velocity = 20,000 hri
0
0
5
10
15
Ash Load (g/L)
20
25
Figure 5.2. DPF Pressure Drop vs. Ash Load at 0 g/L Soot
66
30
The results show two major findings. First, there appears to be very little to no difference in
filter performance across the three oils. At 25 g/L ash load and no soot present, pressure drop for
an oil formulation containing a high level of calcium additive content is only slightly (0.12 kPa)
worse than that for an oil formulation containing no calcium and instead all magnesium-based
detergents. This contradicts previous findings, which saw Ca-based oils yield filter pressure
drops approximately two times greater than oils without calcium below 25 g/L ash. Note that the
current and previous tests, however, were not conducted 'back-to-back." Although done on what
should have been the same test setup, the previous studies were completed two years prior by
another student using a different set of DPFs, test oils, and pressure transducers. Discrepancies
could be inherent in these variables or possibly in some other unknown changes.
Secondly, the overall magnitudes of the filter pressure drops appear to be much lower than those
measured in the previous studies. Plotting the above results together with the 2010 data and
normalizing all curves to the same starting pressure drop
(APNormalized = APMeasured/APO g/L Ash), it seems that the new oils' performance roughly
equals that of the lower pressure drop, non-Ca-containing oils from the previous set of tests:
MIT Accelerated Ash Loading (2010 & 2012)
12
Base + Ca + ZDDP
10
Base + Ca
CJ-4 (2010)
x4
8
M
4U06
Lab Ca
Lab CJ-4.
CL
4
ab Mg
Base + Mg
25 *C, Space Velocity = 20,000 hri
011
0
10
30
20
40
50
Ash Load (g/L)
Figure 5.3. Pressure Drop Response Normalized and Compared to Previously Tested Oils
67
Even though the newly tested oils include a full CJ-4 formulation and a high calcium content oil,
their pressure drop responses do not exhibit the same behaviors as seen in the past. The new
calcium-based oils do not result in significantly higher filter pressure drops for a given ash load
below 25 g/L when compared to formulations that don't contain calcium.
Further pressure drop testing is conducted in order to verify the above results. Small cores are
extracted from the centers of two of the fully loaded DPFs (the third had cracked in half and
therefore is unusable) and these cores are tested on an alternate flow bench system:
Figure 5.4. DPF Core Testing on Flow Bench System
Pressure drops across the high-Ca and high-Mg cores are measured using an entirely different set
of analog and digital pressure transducers, and then compared to data from previous studies
collected on the same flow bench apparatus. Since the previous flow tests were done at different
ash loads, however, we normalize the pressure drop measurements by the ash load, i.e.
APNormalizedt
= APMeasured/
DPFAsh Load * 10. Plotting the new results together with the
previous 2010 test samples, we see that similar trends are reproduced. Again note that the
previous samples used filter cores similar in size and material (cordierite) to those used in this
current study, although not completely identical:
68
Pressure Drop Normalized to Ash Load
1.0
0.9
-
0.8
-
A
0.7
8
Ca(FF)
---------Mg (FF)
-- - - 0
0.6
0.5
-- -----------------
0.4
------- -----
0.3
0.2
0.1
-I
20,000
80,000
60,000
40,000
Space Velocity [1/hr]
100,000
120,000
Figure 5.5. DPF Core Pressure Drop Response via Flow Bench Testing
Once again it's shown that the newly tested fully formulated oils show very little to no difference
in filter pressure drop at the same 25 gIL ash load, and that those pressure drops appear to be
noticeably lower in magnitude compared to those measured in the previous 2010 oil tests.
5.2.2 Soot Loading Results
At four points throughout the ash loading process, filters are loaded with 6 g/L of soot in order to
study the effect of a given ash level on DPF soot filtration performance. Filters are then fully
regenerated before continuing on with further ash loading. The first soot loading is conducted at
the very start of ash loading, at 0 g/L ash load. As expected, with no ash introduced yet, the
three filters all behaved similarly like new, clean DPFs would perform. They displayed very
little to no difference (<0.5 kPa) in pressure drop response due to increasing soot load. Note,
then, that the series labeling by oil formulation is arbitrary in the following plot and only serve to
relate each data set to the subsequent oil-dependent results:
69
0 g/L Ash
3.5
CJ-4
3
0.38 kPa
2.5
MCa
2
1.5
1
0.5
0
25 *C, Space Velocity = 20,000 hr
I
0
1
2
3
4
Soot Load (g/L)
5
6
7
Figure 5.6. Pressure Drop Response to Soot Load at 0 g/L Ash
At 10 g/L ash for the three different oils, however, the filters continue to show very little to no
difference (< 0.5 kPa) in pressure drop response to increasing soot load:
10 g/L Ash
2.5
CJ-4
Ca
0.36 kPa
2
Mg
1.5
0.
43
1
0.5
25 *C, Space Velocity = 20,000 hr'
0
0
1
2
4
3
Soot Load (g/L)
5
6
Figure 5.7. Pressure Drop Response to Soot Load at 10 g/L Ash
70
7
Similarly, little difference in pressure drop response continues to be observed at 18 g/L ash load
(< 0.5 kPa) as well as at the final ash load of 25 g/L (0.65 kPa):
18 g/L Ash
3.5
Mg
-}- .47kPa
3
'
2.5
CJ-4
2
S1.5
0.5
25 *C, Space Velocity = 20,000 hr
0
0
1
2
4
3
5
7
6
Soot Load (g/L)
Figure 5.8. Pressure Drop Response to Soot Load at 18 g/L Ash
2S g/L Ash
4.5
Ca CJ-4
4
0.65 kPa
3.5
Mg
3
.
at-
2.5
2
1.5
1
0.5
25 *C, Space Velocity = 20,000 hr'
0
0
1
2
4
3
SootLoad (g/L)
5
6
7
Figure 5.9. Pressure Drop Response to Soot Load at 25 g/L Ash
71
The acquired ash and soot loading data can be rearranged to instead plot pressure drop as a
function of ash load for a given soot level. The same trends are seen, however. Very little
difference exists in performance between the three oil tests at 3 g/L soot (<0.5 kPa):
3 g/L Soot
14
Ca
LCJ-4
0.14 kPa
Mg
1.4
<i 1.2
0
0.1
~0.8
0.4
0.6
0.4
0.2
0 -T
0
25 *C, Space Velocity
- -
5
15
10
20
25
= 20,000
hri
-
30
Ash Load (g/L)
Figure 5.10. Pressure Drop Response to Ash Load at 3 g/L Soot
The dip in pressure drop response seen in all three curves refers to the pressure drop "benefit"
associated with early ash accumulation. Initially, incoming soot is deposited within the DPF
wall pores, contributing greatly to filter pressure drop. But with the presence of an established
ash layer, this soot depth filtration is avoided and only cake filtration proceeds. Thus, filter
pressure drop response to soot loading for a filter with a thin ash layer is actually lower than that
for a completely clean filter. After a certain point, however, the ash layer becomes thick enough
where its contribution to porous media pressure drop outweighs the benefits of reducing soot
depth filtration.
At 6 g/L soot, filter pressure drop response to ash load also shows very little difference
(< 0.5 kPa) between the three test oils:
72
6 g/L Soot
4.5
Ca
4
0.37 kPa
_4
3.5
3
~2.5
'~2
1.5
1
0.5
25 "C, Space Velocity = 20,000 hr'
0
i
0
I
5
i
10
I
I
20
15
Ash Load (g/L)
25
30
Figure 5.11. Pressure Drop Response to Ash Load at 6 g/L Soot
DPF pressure drop response to ash and soot loading shows very little difference in performance
between the three newly tested oils below 25 g/L ash and 6 g/L soot. Previous trends showing
oils containing higher levels of calcium additives leading to significantly increased filter pressure
drops at a given ash or soot load are not observed. To further support and explain the above
findings, in-depth analysis of the filter and ash samples would be conducted and compared to
previous data.
5.3
Post-Mortem Ash Analysis Results
Previous studies saw noticeable differences in filter performance between oils of different
additive formulations, and these findings were supported by the fact that the resulting ash
characteristics also differed to some degree. Although ash distribution was roughly the same for
all oils, the calcium-based ash associated with higher pressure drop responses was measured to
be generally less porous and was found to have different ash compound forms. For the three
newly tested oils, however, filter performance was observed to be very similar. One would
expect, then, their resulting ash properties to be very similar as well.
73
5.3.1 Ash Distribution Results
Similar to the previous MIT lab-generated DPF samples, overall ash distribution is very similar
between the three different oil formulations:
1.6
. 1.4
E
.E.. 1. 2
IA
#A
U
'2 0.8
E0.6
.c
0.4
0.2
0
0
50
100
150
Distance (mm)
Figure 5.12. Ash Distribution Profile Results for a Single DPF Channel
Layer thickness ranges from 0.07 to 0.10 mm and plug length only varies between 31 and
32 mm. These figures are on par with those obtained from the previous 2010 samples, which
saw layer thicknesses between 0.08 and 0.13 mm and plug lengths between 22 and 29 mm. After
comparing the above results to the other engine and field-aged samples, some differences can be
seen. Because the samples differ in DPF size, the following profile data has been normalized to
DPF length (% DPFLength = Distanceeasured/TotalDPFLength * 100):
74
1.6
E 1.4E
; 1.2
U
0.8
Engine
Lab
Field
0.60.40.2
0
0
20
40
60
80
100
Distance (% of Total DPF Length)
Figure 5.13. Ash Distribution Profiles for Lab, Engine Test, and Field-aged Samples
It appears that the engine test samples display a much thinner ash layer as well as a much longer
end plug than both the lab and field-aged samples. A closer examination of the engine test
sample end plugs via X-ray CT scanning, however, reveals that the ash plugs are actually very
discontinuous:
Figure 5.14. Calcium Oil Engine Test Ash End Plug via X-ray CT
75
This extremely insightful view inside the DPF channels shows that the engine test ash plugs,
which would have appeared to be extremely long via the standard procedures of measuring plug
length, are in reality broken up much lesser in volume. These plug discontinuities could be a
result of ash sintering at unintentionally high DPF temperatures, or possibly due to incomplete
filter regenerations with soot forming intermediate sections between the ash.
The field-aged samples, however, are more comparable in distribution to the lab ash samples.
Distribution data for the lab, engine, and field-aged ash are summarized below and compared to
previous 2010 figures:
Thickness
SampleLayer
[mm]
[mM]
Plug Length
[% DPF Length]
(2012)
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4 1
Field CJ-4 2
Engine Ca
Engine Mg
____
Base
Base
Base
Base
Base
0.076
0.104
0.071
0.092
0.125
0.022
0.028
31
32
31
24
35
20
21
20
8
11
--
--
--
--
0.136
0.085
0.136
0.087
0.085
15.6
28.7
18.9
21.9
27.3
10.2
18.8
12.4
14.4
17.9
___ ___ ___(2010)
____
+ Ca
+ Mg
+ ZDDP
+ Ca + ZDDP
+ Mg + ZDDP
Table 5.6. Ash Distribution Summary
Again, consistent with past data, ash distribution is roughly the same between different oil
formulations as well as when compared to field-aged samples. This suggests that porous media
thickness as well as available filter channel length are comparable between samples.
5.3.2 Ash Porosity Results
Ash layer porosity measured via SEM image analysis is mostly similar for the newly tested oils.
Similar to past data, porosities are high in magnitude, ranging from about 93 to 95%:
76
Ash Layer Porosity (2012)
100
98
Mg
96
94
0
88
86
84
Figure 5.15. Ash Layer Porosity Results via SEM Image Analysis
Note that the engine test samples, as previously shown, display very little to no ash layer present,
making the layer porosity measurements unfeasible.
Similarly, ash porosity in the end plug is also comparable between the new oils, ranging from
about 94 to 97%:
Ash Plug Porosity (2012)
100
98
Ca
Mg
Field
96 -
94
0
L-
92 -
0 90
88
86
84
Figure 5.16. Ash Plug Porosity Results via SEM Image Analysis
77
Note that in general, consistent with past results, ash plugs yield higher porosities than in the ash
layer. This is attributed to the fact that flow velocity inside the DPF is greatest near the walls,
which are the main flow through mechanisms, so material packing is much greater in the wall
layer than in the center of the channel where the end plug forms.
Ash layer porosity results are summarized below. Again, figures are similar among the newly
tested oil formulations, and are also more or less on par with those from previous studies:
Ash Layer Porosity
Sample
(2012)
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4
CJ-4
Base
Base
Base
Base
Base
93.0
93.0
95.0
193.0
(2010)
91.1
90.9
94.6
95.1
90.5
92.2
+ Ca
+ Mg
+ ZDDP
+ Ca + ZDDP
+ Mg + ZDDP
Table 5.7. Ash Layer Porosity Summary
The spread of porosity figures in the previous studies, however, correlated with much more
significant differences in filter pressure drop response. Those differences, though, are not seen
with the newly tested oil formulations.
5.3.3 Ash Composition Results
XRD analysis of the newly tested ash samples show that, similar to previous findings, Ca-based
ash exists primarily in the sulfate and phosphate form. Note that when sulfate or phosphate
compound forms are referred to, they include any intermediary elements such as "calcium zinc
phosphate" or "calcium iron phosphate," for example:
78
Calcium Sulfate
C 100
80I
60
.
=
m
-
0 020-
U
n
S0Engine Ca Field 0-4
Lab Ca
Lab 0-4 Base + Ca
+ ZDDP
C-4
Field (2010)
(2010)
Calcium (Zinc, Iron) Phosphate
Q 100
80
60
40
m
0820
r' 0
_m.
Lab Ca
Engine Ca Field 0-4
Lab CJ-4 Base + Ca
+ ZDDP
Field (2010)
0-4
(2010)
Figure 5.17. Ash Sample Calcium Content via XRD Analysis
Magnesium content found in the ash, however, exists mainly as a phosphate form only:
Magnesium Sulfate
100 -
.0 80 S60
0-
0
40
20
0-
-
-
Engine
Mg
Field
0-4
Lab Mg Lab CJ-4
Base+
Mg [1]
Base +
Mg [2]
0-4
(2010)
Field
(2010)
Magnesium (Zinc, Iron) Phosphate
100
.80,60
0
C- 40 0 20 0
--
M
M
Engine
Mg
Field
0-4
-
Lab Mg Lab C-4
Base+
Mg [1]
M M
Base+
Mg [2]
0-4
(2010)
Figure 5.18. Ash Sample Magnesium Content via XRD Analysis
79
Field
(2010)
Previous studies found a majority of Mg-based ash existing as magnesium oxide, but this applied
only to the Base Oil + Mg lubricant formulation, which contained no amount of sulfur or
phosphorus. It makes sense, then, that primarily an oxide form would dominate with exhaust
being abundant with oxygen. In the newly tested, fully formulated oils, though, which contain
adequate levels of sulfur and phosphorus, magnesium phosphate dominates, suggesting that an
affinity for phosphorus is stronger than that for oxygen. Still, there is the question why calcium
ash also forms sulfates while magnesium ash does not. It appears that magnesium additives,
unlike calcium, have a stronger affinity for phosphorus and do not prefer the sulfate form.
Lastly, similar to previous studies, zinc content found in ash exists primarily as a phosphate:
(Calcium, Magnesium) Zinc Phosphate
- 100
.2 80
c 60
Lc 40
E 20
0
U
0
Engine Engine
Ca
Field
Lab Ca Lab Mg Lab 0-4 Base +
Ca +
0-4
Mg
Base +
0-4
Field
Mg
(2010) (2010)
Base+
Mg
(2010) (2010)
ZDDP
Zinc (Iron) Oxide
100
.2 80
U)
60
0 40
E 20
0
0
,
,M
-
Engine Engine
Ca
Mg
Field
0-4
,
Lab Ca Lab Mg Lab CJ-4 Base+
Ca+
ZDDP
0-4
Field
Figure 5.19. Ash Sample Zinc Content via XRD Analysis
Previous studies considered differences in ash composition, between calcium sulfate and
magnesium oxide, as a potential factor governing filter pressure drop. While there are noticeable
differences in composition between the newly generated Ca-based and Mg-based ash types, it's
80
unclear what consequences follow. For the newly tested oils, Ca-based ash contains both sulfate
and phosphate forms while Mg-based ash only contains only phosphates, yet their pressure drop
responses to overall ash load remain similar.
5.3.4 Ash Particle Size Results
Ash particle size is measured for the new lab, engine, and field samples. Below is an example
volume-based size distribution result for the 2012 lab CJ-4 ash:
VOWME-WT DIFFERENTiAL DSTRIBUTION
ROL %
4.00
3.00
250
2.00
1.50
1.00
0.50
0.00
0.5
1
2
5
10
20
50
100
200
500
Dam.(um)->
Figure 5.20. Volume Basis Particle Size Distribution for Lab CJ-4 Ash, 2012
The volume basis distribution shown above implies that most of the sample set's total volume
consists of particles that are about ten microns in diameter. That does not necessarily mean,
however, that there are more ten-micron particles than any other size. A number basis
distribution for the same ash sample is provided below:
81
NUMBER-Wr DIFFERENTLAL DISTRIBUTION
Re. %
7.0
rr
I
-rrTT
~~~*1
I
_____________
I
-
-t
-~
1.0
4.0
2.0
1,0
0.0
0.5
1
2
5
10
20
50
100
200
500
Diam.(um)->-
Figure 5.21. Number Basis Particle Size Distribution for Lab CJ-4 Ash, 2012
The number basis distribution instead shows that most of the particles of the sample set, simply
when counted by frequency, are much smaller in diameter.
Also note that the number distribution gets cut off at the lower detection limit of 0.5 pm. This is
very commonly seen for number basis distributions, since a majority of powder samples do
consist of much smaller particles. No equipment, however, can currently measure down to a
diameter of "zero." Alternate techniques with higher resolutions can be used to measure those
smaller, nano-sized particles, but they in turn have upper detection limits of only about one
micron and are also unable to directly measure number-based distributions.
A summary of ash particle size for the newly tested samples is provided below. The full
distribution graphs for the other ash samples can be viewed in the appendix. Although the
number basis distribution data are only partial, they still provide very useful information that can
be used to make systematic comparisons between sample sets.
* Note, then, that the mean particle size results for the partial number distributions are skewed
and inaccurate, as the full distribution curve was not able to be captured:
82
Sample
Number Mean Diameter
Volume Mean Diameter
13.65
10.29
10.99
10.62
11.26
15.66
1.39
1.39
1.35
1.14
1.03
0.97
Lab CJ-4
Lab Ca
Lab Mg
Engine Ca
Engine Mg
Field CJ-4
*
Table 5.8. Ash Particle Size Summary (* See Above)
Both number and volume basis distribution results show that overall ash particle size is mostly
consistent between the newly tested lab, engine, and field samples. The field sample shows a
slightly higher volume-based mean diameter, but that is likely attributed to a higher level of
larger engine wear and corrosion particles accumulated from extended on-road use. The above
consistencies, though, support the similarities previously observed in the filter pressure drop
response data.
5.3.5 Ash Morphology Results
TEM images of individual ash particles are collected for the newly tested samples. First, the
high-Ca oil produces ash that is a combination of round and branch-like structures:
UffComera
System
Figure 5.22. Calcium Oil Lab Ash Morphology
83
syt.
Nf Cmers
Similarly, the high-Mg oil yields ash that also contains both spherical and branched elements:
07.0in
Hag 5100
44:is p 0/3o2/1 3
rint
500 Iw
NV=120.0.v
t
1rint Mogm79500x 1.0 in
4:36sl p 04/12/13
500 ran
HV=120.0kV
6800X
sypt.
Direct MN"g
AT Ca.ue
DIrc Keg: 500I
ANTCamerdsystem
Figure 5.23. Magnesium Oil Lab Ash Morphology
Similar morphology can also be observed for CJ-4 lab and field sample ash:
Tlt
3:
I
07.0 in
3N0Ag71300w0
000
e
)s .:3
MW-130.0ky
1/0/13
i
6nt
mg! 10403003
10:31:0
Dirct3 Ng: S000
AJhT
Camer.System
s
0
261
7.0to
100
no
V 120.ekv
Direct Hg: 10000x
.syste
ANTCer
Figure 5.24. Lab CJ-4 (Left) and Field CJ-4 (Right) Ash Morphology
84
Results show that ash morphology looks roughly the same across the different oils, involving
combinations of both round and branch-like structures. Not only do the fully formulated Cabased and Mg-based ash resemble each other, but they also resemble ash from previous studies:
HIT
t -mm2.9 t
O5
m! ;
prit
4C10A4 V 64,11/1
7.0033.
in
500 M
HV 11,.*W
Print Has: 100007.0,I00
0.17,1)
74f.A44 a
So,
HV. 204Akv
irect 140. Syzoex
*s
ViecCOOOOZ
Figure 5.25. Lab Ca (Left) and Base + Ca + ZDDP (Right) Ash Morphology
~I0K
07.
,Imt2Uag
4-44,20 V04"1/~13
.
S*
000
00120.6k0
Dir0- e
Print N":~ 1640*0z 97.0 in
I0'00:1 P 01/17,13
$700.
100
"VI.
bl...D
rt
M
AW
Mas
0 104000.
AwTCamet sytem
system
ARTCamera
Figure 5.26. Lab Mg (Left) and Base + Mg + ZDDP (Right) Ash Morphology
85
The observed similarities in ash morphology for the newly tested oils once again support the
similarities previously recorded for filter pressure drop response.
5.4
DPF Model Results
Earlier, it was shown that DPF pressure drop could be modeled according to fundamental
mathematical equations, which were functions of several key parameters such as ash layer
thickness and porosity. By inputting values into the model according to measurements taken
from post-mortem analysis as seen above, one can attempt to work backwards from given ash
characteristics to simulate filter pressure drop, which can then be compared to actual
experimental ash loading data for further confirmation. So far, experimental ash loading and
post-mortem analysis data both have shown very similar results for the newly tested oils, and it
will help to see if these results make sense mathematically according to the fundamental model.
Equation 3.10 shows ash permeability as a function of ash porosity and mean particle diameter.
Ash porosity can be inputted from the recorded measurements via SEM image analysis. Mean
particle diameter, however, as explained earlier, was not able to be fully measured. But, volume
and number distribution data shows similarities in particle size for the newly tested oils and
literature states that average particle diameter is about 0.3-0.5 pm. So, particle diameter will be
estimated and held constant for each sample, resulting in the following ash permeability figures:
Sample
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4
Ash Layer Porosity
Mean Particle Diameter.
93.0
93.0
95.0
93.0
0.3
0.3
0.3
0.3
Ash Layer Permeability
8.22
8.09
1.72
8.32
E-14
E-14
E-13
E-14
Table 5.9. Simulated Ash Layer Permeability Summary
Ash permeability can then be used in Equation 3.8 to determine pressure drop through the ash
layer. Ash layer thickness is inputted from recorded measurements, and gas dynamic viscosity
(18.5 x 106 N-s/m2) as well as gas wall velocity (0.00727 m/s) are both constants estimated by
typical values from experiments and literature:
86
Sample
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4
Ash Layer Thickness
Ash Layer Permeability
8.22
8.09
1.72
8.32
0.076
0.104
0.071
0.125
Ash Layer Pressure Drop
0.124
0.172
0.056
0.202
E-14
E-14
E-13
E-14
Table 5.10. Simulated Ash Layer Pressure Drop Summary
Pressure drop through the filter substrate walls can be calculated in the same way. Wall
thickness is constant in all cases. Wall permeability, however, is difficult to measure
experimentally once ash has filtered into the pores. It is instead estimated using a theoretical
model based off of the experimental ash loading pressure drop response data, as outlined by
Wang and Sappok in SAE 2013-01-1584 [29]. Wall pressure drop is calculated below:
Sample
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4
Wall Thickness
Wall Permeability
2.02
2.12
2.54
1.50
0.3
0.3
0.3
0.3
Wall Pressure Drop
E-13
E-13
E-13
E-13
0.199
0.190
0.159
0.268
Table 5.11. Simulated Wall Pressure Drop Summary
Summing up the pressure drop terms for the ash layer and substrate wall yields total porous
media pressure drop for the DPF:
Sample
Porous MedlaPressure Drop
0.323
0.362
0.215
0.470
Lab CJ-4
Lab Ca
Lab Mg
Field CJ-4
Table 5.12. Simulated Porous Media Pressure Drop Summary
These figures should correspond with the measured pressure drop increase due to experimental
ash loading:
87
Porous Media Pressure Drop (Calculations)
400.00
Cj_4
Ca
300.00
200.00
0.21 - 0.36 kPa
100.00
0.00
Porous Media Pressure Drop (Ash Loading)
0.6
0.5
Ca
0.4
--
C. 0.3
A
Mg
0.2
.26 - 0.38 kPa
0
0.1
0
0
5
10
20
15
Ash Load (g/L)
25
30
Figure 5.27. Simulated vs. Experimental Porous Media Pressure Drop
Filter pressure drop increase due to ash loading is of the same order between mathematical
simulations and actual experimental data. Clearly the theoretical model is not quite able to
perfectly capture the exact figures measured in the experiments, but all three calculations are on
the same order as their experimental counterparts. Overall pressure drop increase is about 0.2 to
0.4 kPa for both simulated and measured results. Furthermore, the magnitudes of ash layer
pressure drop versus substrate wall pressure drop as calculated by the simulation are roughly
equal, which can also be seen by the relative magnitudes of the depth and layer filtration regions
of the experimental pressure drop curves. This data validation by fundamental mathematical
model adds further support to the experimental findings of the three oil tests conducted in this
study, confirming that the observed trends are true and real occurrences.
88
6
CONCLUSIONS
The objective of this study is to supplement previous investigations of lubricant additive effects
on ash characteristics and DPF performance. Three lubricant formulations of the same sulfated
ash level, that differ only in the type of additives present, are tested and compared to an array of
additional samples with different oil formulations and generation means in order to quantify and
characterize any significant differences that result from varying additive chemistry.
6.1
Lubricant Additive Effects on DPF Performance
For the three fully formulated lubricants tested in this study, a commercial CJ-4 oil, a high
calcium detergent oil, and a high magnesium detergent oil, very little differences are seen in
filter pressure drop response to ash and soot loading and the slight differences that do appear
don't always yield the same relative disparity between the oils. Namely, one oil doesn't always
produce higher filter pressure drops than the other two.
At soot levels of 0, 3, and 6 g/L, DPF pressure drop increase as a function of ash load below
25 g/L is very similar between the oils, yielding less than 0.4 kPa (33%) difference at any given
ash load.
At ash levels of 0, 10, 18, and 25 g/L, DPF pressure drop increase as a function of soot load
below 6 g/L is also very similar between the oils, yielding less than 0.7 kPa (18%) difference at
any given load.
The high calcium detergent oil, however, reached its final ash load on the same accelerated lab
system in approximately 11% less time than the high magnesium detergent oil. Ash generation
rate, therefore, is quicker for oils with higher calcium content due to calcium having a greater
atomic weight than magnesium for roughly the same ash compound forms, leading to earlier
pressure drop increases in the filter for the same amount of on-road use.
89
6.2
Lubricant Additive Effects on Ash Characteristics
Similarities in DPF performance for the three tested oil formulations are supported by additional
similarities observed in resulting ash properties. Fundamental mathematical models, which
quantify DPF pressure drop as a function of key parameters related to its porous media, confirn
that similarities in ash layer and wall properties also result in similarities in DPF performance.
Overall ash distribution is similar between the three oil formulations, with ash layer thickness
ranging from 0.07 to 0.13 mm and ash plug length ranging from 31 to 32 mm.
Ash layers are highly porous and also comparable, ranging from 93 to 95%.
Overall ash particle size is very similar according to partial number distribution data. Volume
distribution data also show similarities, with mean particle size ranging from 10.3 to 13.7 pm.
Ash morphology is also similar for the three oil formulations from visual comparisons of TEM
images. Ash particles and agglomerates generally consist of combinations of spherical and
branched structures. Quantitative analysis of particle shape factors has yet to be completed.
There are, however, slight differences in ash composition. The high calcium detergent ash exists
mostly as calcium sulfate and calcium (zinc, iron) phosphate, while the high magnesium
detergent ash exists mostly as magnesium (zinc, iron) phosphate only.
Inputting these measured ash properties into the DPF mathematical model yields simulated filter
pressure drops on the same order as those recorded from experimental ash loading.
The measured ash properties for the three oil formulations above are also comparable to figures
from previous lubricant additive studies that used similar testing methods and conditions. Past
samples recorded layer thicknesses of 0.08 to 0.13 mm, ash layer porosities of 90 to 95%, as well
as similarly shaped ash particle morphologies. Ash composition, though, was found to consist
mostly of calcium sulfate and magnesium oxide compounds.
90
6.3
Impact of Ash Depth Filtration
So far, it's been shown that the three oil formulations tested in this study, differing only in
calcium and magnesium detergent content, display very little difference in both DPF
performance and overall ash properties. The ash properties, though, are also not too different
from those measured in previous lubricant additive studies, but those previous studies saw much
more pronounced differences in DPF performance between different oil formulations. For some
reason, even with similarly characterized ash, the three oils from this study do not exhibit the
same differences in filter pressure drop response to ash and soot loading.
There are a couple of possible reasons to explain the discrepancies seen between current and
previous test results. First, the three oils tested in this study are a completely new set of oils.
While their additive levels are mostly similar to those from previous tests, there could be some
unknown differences somewhere in the oil formulation that cause these oils to behave much
more similarly than previously seen. Secondly, the oil tests in this study use new filters from an
entirely different DPF manufacturer. Again, while they share mostly similar geometries and
properties with the filters from previous tests, they do note a 5-10% reduction in pressure drop
performance and any other differences in the filters could somehow be significantly affecting ash
loading.
Finally, differences between current and previous observations could be explained by possible
differences in ash depth filtration. Figure 5.3 shows all previous and current pressure drop data
normalized to clean DPF pressure drop and plotted on the same graph. It's clear that the new oil
tests display similarly low pressure drop responses relative to some of the other previous
calcium-based oil tests. The difference between these profiles, though, appears to primarily take
place within the initial depth filtration region of the ash loading curves. After the beginning,
steep rise in pressure drop due to the substrate wall pores directly filling with ash and once ash
layer formation has commenced, the more gradual increase in pressure drop that follows from
subsequent ash loading appears to be more or less similar between all test cases. The previous
calcium-based oil tests do show slightly steeper pressure drop increases in the layer filtration
91
region, but the point at which layer filtration starts in the first place is significantly higher,
sometimes twice as high, in pressure drop compared to the other oil tests.
It seems, then, that the mechanisms governing ash depth filtration could somehow be differing
significantly, resulting in dramatic differences in initial filter pressure drop increase. In that
case, one would expect substrate wall permeability to be quite different between samples that
have different degrees of depth filtration. Filter wall permeability, however, once it has already
undergone ash depth filtration, is very difficult to measure experimentally as it involves one
highly porous medium suspended partially within another, less porous medium. In order to fully
understand these apparent differences in ash depth filtration, therefore, and to know if they
account for the differences observed between past and current pressure drop response data, this
issue must be investigated further.
6.3.1 Future Work
Currently no experimental technique is used to quantify ash-loaded wall permeability. It is
instead estimated from a theoretical model that is based on experimental ash loading pressure
drop data. For this reason, though, any differences in experimental pressure drop response
between oil tests will inherently be reflected in the model's determination of substrate wall
permeability. According to the model, wall permeability for the new oils tested in this study is
an entire order of magnitude greater than that of the previous samples.
The exact opposite correlation, however, is what really should be determined. Does real wall
permeability actually differ between these samples as apparent in the pressure drop response
data, and are these differences the main factors governing overall pressure drop increase? A next
step, therefore, is determining a systematic method of experimentally measuring substrate wall
permeability after ash depth filtration has occurred. Much progress has been made recently
using techniques such as FIB that can cut into substrate wall pores and provide detailed views of
ash deposits, offering a very viable method for achieving wall permeability information.
Furthermore, additional oil tests involving the same oil formulations used in the previous
lubricant additive studies can be repeated in order to see if previous trends can be reproduced.
92
REFERENCES
[1]
Heywood, J.B., Internal Combustion Engine Fundamentals, McGraw-Hill, Inc., New
York, 1988.
[2]
Khair, Magdi K. "The Case for the Diesel Engine." DieselNet Technology Guide.
Ecopoint Inc., 2000. Web. 4 Apr. 2013.
<http://www.dieselnet.com/tech/diesel-case.php#adv>.
[3]
"Diesel Vehicles." www.fueleconomy.gov. U.S. Department of Energy, n.d. Web. 4 Apr.
2013. <http://www.fueleconomy.gov/feg/di-diesels.shtml>.
[4]
Schindler, K-P., 1997. "Why Do We Need The Diesel?", SAE Technical Paper 972684,
doi: 10.4271/972684
[5]
Charles River Associates, "Diesel Technology and the American Economy," Diesel
Technology Forum Report no. D02378-00, 2000.
[6]
"Diesel Market Highly Developed in Europe." ACEA. European Automobile
Manufacturer's Association, 9 June 2010. Web. 5 Apr. 2013.
<http://www.acea.be/news/news-detail/dieselmarket-highly-developed-ineurope/>.
[7]
Baukal, Charles. "Everything You Need to Know About NOx." John Zinc Co. LLC, n.d.
Web. 5 Apr. 2013. <http://www.johnzink.com/wp-content/uploads/everything-about-
nox.pdf>.
[8]
Majewski, W. Addy "What Are Diesel Emissions." DieselNet Technology Guide.
Ecopoint Inc., 2012. Web. 5 Apr. 2013.
<http://www.dieselnet.com/tech/emi-intro.php>.
[9]
Bosh, 1994. "Diesel Fuel Injection", Robert Bosch GmbH
[10]
Gallagher, James. "Diesel exhausts do cause cancer, says WHO." BBC News, 12 June
2012. Web. 5 Apr. 2013. <http://www.bbc.co.uk/news/health-18415532>.
[11]
Johnson, Tim. "Vehicle Emissions Review - 2012." Corning Environmental
Technologies. Detroit. 16 Oct. 2012. Lecture.
[12]
"Meeting EPA 2010." FactsAboutSCR.com. North American SCR Stakeholders Group,
2008. Web. 8 Apr. 2013. <http://www.factsaboutscr.com/environment/epa2OlO.aspx>.
[13]
"Emission Standards Reference Guide." United States EnvironmentalProtectionAgency.
US EPA, n.d. Web. 8 Apr. 2013. <http://www.epa.gov/otaq/standards/heavy-duty/hdciexhaust.htm>.
93
[14]
Sappok, Alexander. "Ash Accumulation in Diesel Particulate Filters." DieselNet
Technology Guide. Ecopoint Inc., 2012. Web. 17 Apr. 2013.
<http://www.dieselnet.com/tech/dpfash.php>.
[15]
Aravelli, K., Jamison, J., Robbinson, K., Gunasekaran, N., and Heibel, A., "Improved
Lifetime Pressure Drop Management for DuraTrap RC Filters with Asymmetric Cell
Technology (ACT)," Diesel Engine Efficiency and Emissions Reduction Conference,
Detroit, MI, 2006.
[16]
Boschert, T., 2002. "The Lubricant Contribution to Future Low Emission Engine
Design", Diesel Particulate and NOx Emissions Course (University of Leeds), Ann
Arbor, MI, October 2002.
[17]
Rudnick, Leslie R. LubricantAdditives: Chemistry and Applications.Wilmington: CRC
Press, 2003. Print.
[18]
McGeehan, J., "API CJ-4: Diesel Oil Category for Both Legacy Engines and Low
Emission Engines Using Diesel Particulate Filters", SAE 2006-01-3439, 2006.
[19]
Bodek, B., and Wong, V., "The Effects of Sulfated Ash, Phosphorus and Sulfur on Diesel
Aftertreatment Systems - A Review", SAE 2007-01-1922, 2007.
[20]
Jaaskelainen, Hannu. "Engine Exhaust Back Pressure." DieselNet Technology Guide.
Ecopoint Inc., 2012. Web. 18 Apr. 2013.
<http://www.dieselnet.com/tech/diesel-exhpres.php>.
[21]
Morcos, M., Ayyappan, P., and Harris, T., "Characterization of DPF Ash for
Development of DPF Regeneration Control and Ash Cleaning Requirements", SAE
2011-01-1248, 2011.
[22]
Sappok, A., Wong, V., and Munnis, S., "Individual and Synergistic Effects of Lubricant
Additive Components on Diesel Particulate Filter Ash Accumulation and Performance",
ICES2012-81237, 2012.
[23]
Sappok, A., "The Nature of Lubricant-Derived Ash-Related Emissions and Their Impact
on Diesel Aftertreatment System Performance," PhD Dissertation, Massachusetts
Institute of Technology, 2009.
[24]
DieselNet Technology Guide, 'Wall-Flow Monoliths", Revision 2005-09b,
DieselNet. <http://www.dieselnet.com/tech/dpf wall-flow.php>, 2005.
[25]
DieselNet Technology Guide, "Cellular Monolith Substrates", Revision 1998-08c,
DieselNet. <http://www.dieselnet.com/tech/cat-substrate.php>, 2005.
94
[26]
Munnis, S., "Synergistic Effects of Lubricant Additive Chemistry on Ash Properties
Impacting Diesel Particulate Filter Flow Resistance and Catalyst Performance," S.M.
Dissertation, Massachusetts Institute of Technology, 2011.
[27]
Lee, K., Zhu, J., Ciatti, S., Yozgatligil, Al, and Choi, M., "Sizes, Graphitic Structures,
and Fractal Geometry of Light-Duty Diesel Engine Particulates," SAE 2003-01-3169,
2003.
[28]
Sappok, A., Beauboeuf, D., and Wong, V., "A Novel Accelerated Aging System to Study
Lubricant Additive Effects on Diesel Aftertreatment System Degradation," SAE 2008-
01-1549, 2008.
[29]
Wang, Y., Sappok, A., and Wong, V., "The Sensitivity of DPF Performance to the Spatial
Distribution of Ash inside DPF Inlet Channels," SAE 2013-01-1584, 2013.
95
(This page intentionally left blank)
96
APPENDIX
NUMBER-WT DIFFERENTIAL DISTRIBUTION
Ret. %
7.0
.0
5.0
4.0
3.0
2.0
1.0
0.0
0.5
1
2
5
10
20
s0
100
200
500
Dim.(umn)->*
Figure A.1. Lab CJ-4 Ash Particle Size Number Distribution
VOLUME.WT DIFFERENTIAL DSRIBUTION
R&L %
4.00
__
3.50
3.00
2.50
2.00
1.50
1.0
0,50
0.00
0.5
1
2
5
10
20
50
100
200
500
Dam.(um)-
Figure A.2. Lab CJ-4 Ash Particle Size Volume Distribution
97
Rej. %
NUMBER.WT DIFFERENTIAL DiSTRIBUTION
4.0
-_-
5.0
-
-
-
-
-
-
4.0
Zo
1.0
0.5
1
2
5
10
20
50
100
200
50
igL(um)-u
Figure A.3. Lab Ca Ash Particle Size Number Distribution
VO1.UMW4NT OFFERENTIAL DISTRIBUTION
Rel. %
4.00
3.50
&00
Z50
200
1.50
1.00
0.50
0.00
0.5
1
2
5
10
20
50
100
200
500
DIam.(un)->
Figure A.4. Lab Ca Ash Particle Size Volume Distribution
98
NUMBERWT DIFFERENTIAL DISTRBUTION
Rat %
7.0
-
-rr
-
-
-
---
6.0
5.0
-
--
--
-
- -- -
- -
-
-
-
-
4.0
3.0
010
L
0.5
1
2
5
10
20
50
LL
100
200
500
Diarm(um)->
Figure A.5. Lab Mg Ash Particle Size Number Distribution
VOLUME-W DIFFERENTIAL DIfTRIBUTION
Ret %
4.00
3.50
2.50
2.00
1.50
1.00
0.50
0.00
0.5
1
2
5
10
20
50
100
200
500
Diam.(ump>
Figure A.6. Lab Mg Ash Particle Size Volume Distribution
99
DIFFERENTIAL DISTRIDUTION
NUMBER.WT
NUMBER-WT DIFFERENTIAL DISRIBTION
%
ReL
Ret %
9.0
13.0
7.0
5.0
-
_
_
_
_
_
_
_
5.0
4.0
3.0
2.0
0.0,
.
0's
1
2
5
10
20
, ,
50
,,
100
,
200
500
Diem.(um>
Figure A.7. Engine Ca Ash Particle Size Number Distribution
Figure A.8. Engine Ca Ash Particle Size Volume Distribution
100
Figure A.9. Engine Mg Ash Particle Size Number Distribution
VOWME-WF DIFFERENTIAL DISTRIBUTION
Re. %
5.0
4.5
-
4.0
3-5
3.0
2.5
2.0
1.0
0.0
0.5
1
2
5
10
20
50
100
200
500
Dim,(um)-11
Figure A.10. Engine Mg Ash Particle Size Volume Distribution
101
NUMBER-WT DIFFERENTIAL DISTRBUTION
Rel. %
9.0
8.0
2.0
3.0
_
_
_
__
_
-__
2.0
0.0
1
2
5
10
20
60
100
20
50
Diam.(um)-.
Figure A.11. Field CJ-4 Ash Particle Size Number Distribution
VOLUME.WI DIFFERENTIAL DISTRIBUTION
Reo %
7.0
--
-
5.0
-
--
4.0--
2.0
- -
-
___
1.0
0.0
0.5
1
2
a
10
20
50
100
200
600
Dim.(um)-'
Figure A.12. Field CJ-4 Ash Particle Size Volume Distribution
102