MODELING AND COMPARISON OF TRANSIENT EMISSIONS
BEHAVIOR OF HYBRID AND CONVENTIONAL VEHICLES
by
Martin Edward Kosto
Bachelor of Science, Mechanical Engineering
Rensselaer Polytechnic Institute
(1999)
Bachelor of Science, Economics
Rensselaer Polytechnic Institute
(1999)
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
February 2001
BARKER
MASSACHUSTTf INSTITUTE
OF TECHNOLOGY
© 2001 Massachusetts Institute of Technology
All Rights Reserved
JUL 16 2001
LIBRARIES
Signature of Author
Martin E. Kosto
9 January 2001
Certified by
John B. Heywood
of
Mechanical
Engineering
Sun Jae Professor
Thesis Supervisor
Accepted by.
Ain A. Sonin
Chairman, Department Graduate Committee
I
$
N
4
iS'
2
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A
TO MY FATHER,
Timothy J. Kosto:
An Engineer'sEngineer.
3
4
MODELING AND COMPARISON OF TRANSIENT EMISSIONS
BEHAVIOR OF HYBRID AND CONVENTIONAL VEHICLES
by
Martin Edward Kosto
Submitted to the Department of Mechanical Engineering
on January 9, 2001 in partial fulfillment of the
requirements for the Degree of Master of Science in
Mechanical Engineering
ABSTRACT:
Due to increasing emissions and fuel economy legislation, both in the United
States and globally, the search for alternative propulsion systems is escalating. Electric,
hybrid gas-electric, fuel cell, and other possible automotive propulsion technologies are
continually being evaluated for their merit in a more "environmental society".
Investigating these technologies requires both specific evaluation and broad comparative
techniques.
Currently, most codes associated with hybrid vehicle technology focus on fuel
economy, this being the economic driver in the market. Its brother variable, emissions,
has been investigated to a lesser degree. Hence, this study will focus on hybrid gasolineelectric system emissions and will examine what determines the magnitude of hybrid
emissions and compare these to the emissions of a conventional vehicle of similar scope.
Several varieties of emissions estimation methods exist. Many attempt to forecast
such levels using a quasi steady-state approach; looking at each time-step independently
with no memory of previous results. However, engine transient behavior has a large
impact on emissions production. The highest engine-out emissions result from the severe
changes in the engine power the vehicle-driving pattern requires. Additional transients
occur by a gas-electric hybrid's vehicle constant state of change; turning the engine on
and off when necessary. Thus, it is imperative when modeling hybrid vehicle emissions
to use a "transient" investigation method.
In addition to modeling transient engine (emissions) behavior (modeled as a
function of the propellant mixture's air-fuel ratio), a transient catalyst model is included.
This allows different control methodologies and improvements in current automobile
catalytic converters to be assessed. Such technology is only beginning to be studied and
modeled. Yet, it is important to do so, as hybrid technologies will seek to use the best
new products available.
In conclusion, it is shown that hybrid technology is superior to its conventional
counterparts in the emissions arena. Doing so will substantiate criticism of current
transportation technologies in favor of new hybrid technology.
Modeling results,
coupled with real data will substantiate the code and methodology used. Finally, this
study gives the reader an improved understanding of the issues associated with hybrid
vehicle emissions.
Thesis Advisor: Professor John B. Heywood
Title: Sun Jae Professor of Mechanical Engineering
5
6
ACKNOWLEDGEMENTS
Scientifically, this document is a culmination of one-and-a-half years of work.
Philanthropically, however, it is a representation of my life, which has been touched by
so many. These souls are those, who in my time of need, were there for me, whether it
was encouragement, comfort, help or just pure revelry. My deepest and most sincere
thanks go to them, accompanied with best wishes for future success.
Thanks go to the members and faculty at the Sloan Automotive Lab, without
whom this would have been less of an experience. The day-to-day chatter with Kelly
Canales, Brigitte Castaing, loannis Kitsopanidis, Gary Lansdberg, Susan Lutin, Anuscheh
Nawaz, Chad Smutzer, and Shigeyuki Tanaka gave my brain some much-needed rest
from time-to-time. Brian Hallgren deserves my appreciation for never throwing me out
of his office (unless Meredith called).
I'd like to thank Professor John B. Heywood, whose direction and suggestions
have led to the discovery of knowledge and the successful completion of this project.
Thank you Karla (Stryker) for always telling me when my appointments were, no matter
how many times I forgot. I also thank Bruce Tobis, Andy Adamczyk and Rod
Tabaczynski at Ford, who supplied data and made this project possible. Huxian Shen
also deserves a great deal of gratitude for giving me his code when no others were
available.
Additionally, I'd like to thank all my Chemical Engineering (and other) friends
who made weekends (and lunchtime) a lot of fun: Jim Bielenberg, Michael Buchanan,
Kevin Dorfman, Jeb Keiper, Tom Lancaster, John Lock, LaRuth Mcafee, Jason Raff, Ley
(Llewellyn Bentley the third) Richardson, April Ross, Patty Sullivan, Paul Yelvington,
Ian Zacharia, and all those others we often forgot on email lists! I'd especially like to
thank Greg Pollock, whose tastes for fine scotch, symphonic bands, late nights, and bar
management made the past year a great ride.
My gratitude is also deserved by those, who in my life's journey, have played key
roles. Dr. Roberta Love, Principal at Lewiston-Porter High School, who had faith in me
when few others did. Salvatore Paonessa, whose love for the trumpet and sincere
dedication to teaching made me the musician I am today. John P. Cloninger, E.
Benjamin Miller IV, Peter Crisman and Judy Muller, all of whom made my experience at
RPI not just fun, but unforgettable.
Furthermore, I thank my Brother Tim, whose hard work and dedication paved a
road to success for me to follow. Without his experiences in hand, this journey would
have proven much more difficult. To my Sister Sarah, her husband Chris and their
Daughter (my Niece) Elizabeth, who have always been there when I needed help or
someone to talk with.
Most especially, I thank my parents. Thank you for the encouragement and
upbringing that prepared me for life. Your insight and sacrifice have been the underlying
basis for my success and I can only hope to provide for my children as you have for Tim,
Sarah and myself. Dad, your competence as an engineer is unparalleled and Mom, your
unselfishness will always be remembered. To both, your constant support and love have
never failed me.
7
Most importantly, I thank Kim Bryan Kosto, my endearing wife, whose love for
me, even with my imperfections, has no bounds. You have provided the unconditional
love that makes our relationship so special. Only with you am I the happiest man in the
world. Thank you.
Martin E. Kosto
9 January 2001
8
TABLE OF CONTENTS
ABSTRACT
5
ACKNOWLEDGEMENTS
7
TABLE OF CONTENTS
9
LIST OF TABLES
11
LIST OF FIGURES
12
NOMENCLATURE
14
Chapter 1: INTRODUCTION
1.1 Motivation
1.2 Hybrid Technologies Background
1.3 Objectives
17
18
20
Chapter 2: ADVISOR CODE
2.1 Background
2.2 Vehicle Performance Evaluations
2.2.1 Vehicle Calculations
2.2.2 Fuel Converter Calculations
2.2.3 Emissions Calculations
2.2.4 Hybrid Control Strategy
2.3 Limitations
23
24
24
25
25
27
28
Chapter 3: HYBRID/CONVENTIONAL QUASI-STEADY-STATE COMPARISONS
35
3.1 Background
36
3.2 Test Setup
37
3.2.1 Hybrid Configuration
38
3.2.2 Conventional Configuration
38
3.3 Results
39
3.3.1 Engine Operation
39
3.3.1.1 Engine Usage
39
3.3.1.2 More Efficient Operation
40
3.3.1.3 Transient Behavior
41
3.3.2 Engine Tuning
41
3.3.3 Catalyst Cool-Down
42
3.4 Conclusions
Chapter 4: ADVISOR AND ADDITIONAL TRANSIENT MODELS
4.1 ADVISOR Emissions Model Limitations
4.1.1 Transient Operations
59
59
9
4.1.2 Chemical Reactions
4.2 Changes Made to ADVISOR
4.2.1 Air-Fuel Ratio
4.2.2 Emissions Calibration
4.3 NTWC Transient Modeling Program
4.3.1 Modeling Method
4.3.2 Integration with ADVISOR
4.3.3 Limitations
59
60
60
62
63
63
63
64
Chapter 5: ADVISOR-NTWC HYBRID/CONVENTIONAL TRANSIENT MODELING
5.1 Background
71
5.2 Comparison
71
5.2.1 CO Production
71
5.2.2 NO, Production
72
5.2.3 HC Production
72
5.3 Hybrid Configuration Results
73
5.4 Conventional Configuration Results
74
5.5 Hybrid and Conventional Comparison
74
5.5.1 Overall
74
5.5.2 Limitations - Startup and Shutdown
75
5.6 Conclusions
75
5.6.1 Transient Effect
75
5.6.2 Hybrid / Conventional Comparison
75
5.6.3 ADVISOR / NTWC Comparison
76
Chapter 6: CONCLUSIONS AND DISCUSSION
6 1 Conclusions
6.1.1 Steady-State Emissions
6.1.2 Transient Emissions
6.1.3 Overall
6.2 Future Technologies
81
81
83
83
84
REFERENCES
87
10
LIST OF TABLES
Hybrid/ConventionalQuasi-Steady-State Comparisons
Table 3.1:
Vehicle Configuration
44
Table 3.2:
Catalyst Temperature, Engine Off Time, NOx Conversion
Efficiency
45
ADVISOR-NTWC Hybrid/ConventionalTransientModeling
Table 5.1:
Various Compensations - Hybrid & Conventional - Emissions
77
11
LIST OF FIGURES
ADVISOR Code
Figure 2.1:
General ADVISOR Model
29
Figure 2.2:
Conventional Vehicle Configuration
29
Figure 2.3:
Series Hybrid Vehicle Configuration
30
Figure 2.4:
Parallel Hybrid Vehicle Configuration
30
Figure 2.5:
Previous Emissions Temperature Correction Factor
31
Figure 2.6:
New Emissions Temperature Correction Factor
31
Figure 2.7:
ADVISOR Catalyst Model
32
Figure 2.8:
Lumped Capacitance Model
33
Figure 2.9
High SOC
34
Figure 2.10
Low SOC
34
Hybrid/ConventionalQuasi-Steady-StateComparisons
Figure 3.1:
General Engine Map (Torque vs. Speed)
46
Figure 3.2:
Federal Urban Driving Schedule
46
Figure 3.3:
Highway Fuel Economy Test
47
Figure 3.4:
Binned bmep - Hybrid & Conventional - FUDS
48
Figure 3.5:
Binned bmep - Hybrid & Conventional - HWFET
48
Figure 3.6:
Engine Map - Conventional - FUDS
49
Figure 3.7:
Engine Map - Hybrid - FUDS
49
Figure 3.8:
Engine Map - Conventional - HWFET
50
Figure 3.9:
Engine Map - Hybrid - HWFET
50
Figure 3.10:
Brake Specific NO, Map - Conventional - FUDS
51
Figure 3.11:
Brake Specific NO, Map - Hybrid - FUDS
51
Figure 3.12:
Binned NO, BSE - Hybrid & Conventional - FUDS
52
Figure 3.13:
Binned HC BSE - Hybrid & Conventional - FUDS
52
Figure 3.14:
Binned CO BSE - Hybrid & Conventional - FUDS
53
Figure 3.15:
Binned HC FSE - Hybrid & Conventional - FUDS
53
Figure 3.16:
Binned d(bmep)/dt - Hybrid & Conventional - FUDS
54
12
Figure 3.17:
Binned d(bmep)/dt - Hybrid & Conventional - HWFET
Figure 3.18:
d(bmep)/dt vs. Time - Severe - Hybrid & Conventional - FUDS
Figure 3.19:
d(bmep)/dt vs. Time - Minimal - Hybrid & Conventional - FUDS
Figure 3.20:
Difference in Engine-out Brake Specific NO, Emissions - FUDS
Figure 3.21:
bmep vs. Time - Hybrid & Conventional - FUDS
Figure 3.22:
Binned Time Hybrid Engine is Off
Figure 3.23:
Catalyst Efficiency, Temperature and Emissions for
32s Hybrid Engine Shutdown
Figure 3.24:
NOx Catalyst Conversion Efficiency - Hybrid & Conventional
54
ADVISOR and Additional Transient Models
Figure 4.1:
General X vs. bmep
Figure 4.2:
X vs. d(bmep)/dt - Data
Figure 4.3:
Different Compensations for ADVISOR - [X vs. d(bmep)/dt]
Figure 4.4:
General Engine-Out Emissions Characteristics vs. X
Figure 4.5:
Relative NO, emissions versus X
Figure 4.6:
Relative CO emissions versus X
Figure 4.7:
Relative HC emissions versus X
Figure 4.8:
X Based Emissions Adjustment Factor Simulink Model
Figure 4.9:
ADVISOR - NTWC Visual Connection
ADVISOR-NTWC Hybrid/Conventional TransientModeling
Figure 5.1:
X vs. Time - Hybrid - Different Compensations
Figure 5.2:
X vs. Time - Conventional - Different Compensations
Figure 5.3
General post-TWC Emission Levels (w/o 02 Storage) vs. X
13
NOMENCLATURE
bmep
Brake Mean Effective Pressure
bse
Brake Specific Emissions
EAF
Emissions Adjustment Factor
EO
Engine Out
fse
Fuel Specific Emissions
FTP
Federal Test Procedure
FUDS
Federal Urban Driving Cycle
HWFET
Highway Fuel Economy Test
ICE
Internal Combustion Engine
x.
Relative air-fuel ratio
SOC
State of Charge
TWC
Three-Way-Catalyst
14
15
16
CHAPTER 1
INTRODUCTION
1.1 MOTIVATION
With the passage of the Clean Air Act of 1963 and subsequently the Motor
Vehicles Air Pollution Control Act of 1965, the automobile and transportation industries
embarked on a quest to reduce air pollution. Even prior to 1963 was the creation of the
California Air Resources Board, whose task was the improvement of air quality, mainly
in the Greater Los Angeles Metropolitan Area.
Since then, the automobile industry,
coupled with state, national and global governments have strived to reduce automobile
emissions by various measures.
Beginning with engine controls and simple catalytic converters, emissions
improvement technologies now range to entirely new vehicle system designs. These new
designs attempt to appease both societies' preceding need for a vehicle with proper speed
and range limits in addition to the new necessities for emissions and fuel economy
boundaries. To incorporate such diverse requirements, these new concepts often consist
of both an electric power generation unit and an internal combustion engine together in a
hybrid propulsion system.
This new combination is on the verge of mass production. The Toyota Prius, one
such gasoline-electric car, has been in production for about 2 years.
It achieves a
significant improvement in fuel economy and reduction of total tail-pipe emissions (as
compared to a similar conventional model). Because of the substantial reduction of
emissions (over 8x), the hybrid technology looks promising for the overall cutback of
fleet emissions.
Thus, with increasing legislation, automobile manufacturers are
preparing concept hybrid vehicles as viable alternative to their conventional ICE driven
counterparts.
With more car manufacturers developing such hybrid automobiles, the need to
examine such concepts in the pre-production is clear. To examine novel hybrid concepts,
current methods must be revamped to account for the differences between hybrid vehicles
and their conventional counterparts. For example, gasoline-electric hybrid vehicles run
17
the internal combustion engines in much less extreme ways, therefore incurring less
severe transients because the electric component of the propulsion system can better
provide for the rapid fluctuations in torque.
While revamping current codes to apply to new hybrid vehicle technology, it is
also important to account for improvements in catalytic converter technology, both
associated with conventional vehicles and that design for the specific operation of
hybrids. With options built-in for future advancements, a code will be much more robust
for a far longer period of time than one capable of examining current technology only.
1.2 HYBRID TECHNOLOGIES BACKGROUND
Attempts to design and produce vehicles that use less fuel and produce fewer
emissions have surged and ebbed over the past 40 years.
The fluctuations have
previously correlated with the availability and price of oil.
Only recently has the
realization been made that a more efficient power source will be necessary in the not-sodistant future.
The electric car gained popularity in the early
20
1h
Century. It was much easier to
start, ran quieter and was more reliable than its gasoline internal combustion engine
counterpart. However, it ran into the same problems faced today. As the automobile
infrastructure grew (roads, gasoline stations, etc.), so did the demand for performance
(speed) and range. These two problems are still, by far, the biggest quandaries facing
electric and hybrid electric vehicle designers. In 1990, the specific energy of gasoline
was 3 orders of magnitude greater than that of batteries.
Even with a correction factor
for the electrical system's superior efficiency, gasoline was over 200 times more
powerful per unit mass than batteries.
However, by the year 2000, significant
improvements in battery technology have improved the difference by a factor of 50% to a
level of 100. Still, with current battery technology, it is not possible to build an electric
car that seats five people, travels at speeds up to 100 miles per hour and can trek 300
miles before recharging (which would take only 5 minutes).
To combat such a problem, the automobile industry has crossbred the electric car
with the internal combustion engine. In addition, other technologies such as fuel cells are
seriously being considered, but require further research to be feasible.
18
The gasoline-
electric hybrid appears (currently) to be the best solution. Such a vehicle has an electric
propulsion system, which is utilized at low speeds where high power is not necessary. At
higher speeds and accelerations, the internal combustion engine either assists the motor,
or provides all the torque to the powertrain. In addition, it may also recharge the batteries
through the motor, which acts as a generator.
There exist several variants in the gasoline-electric hybrid family. A series hybrid
configuration uses the electric motor as the lone link to the powertrain.
Any power
supplied by the internal combustion engine will be routed through the motor, then
moving along either to the batteries or driveshaft. A dual (or road-coupled) hybrid ties
each component to one of the two pairs of wheels, where the electric motor would drive
the front wheels and the engine would drive the back. Such an arrangement has rarely
been used and is thought to be inefficient. The third (and most widely used) hybrid design
is the parallel configuration, where the motor and engine are in parallel to the driveshaft.
Unlike the series hybrid, the parallel hybrid's transmission derives power directly from
both the internal combustion engine and the electric motor.
Hybrids are an improvement over their conventional counterparts for several
reasons.
First, the fuel economy of a hybrid is significantly better than most other
vehicles produced.
Some smaller conventional (solely internal combustion engine
driven) cars do come near hybrid numbers, but at the loss of utilization for things other
than minimal human transport.
Fuel economy, being the main driver behind hybrid
development and production (in addition to government legislation), gets a major focus.
This being true, many studies and codes predominately investigate it, leaving out the
other major benefit, reduced emissions.
Reduced emissions levels occur in hybrid vehicles for various reasons. First, the
internal combustion engine, which produces all the emissions, is run significantly less
because the electric motor is used. In addition, conventional vehicle internal combustion
engines must be designed so they can endure severe transients. This transient behavior,
such as hard accelerations and stops, is another significant contributor to high emissions
levels.
Hybrid vehicles do not incur such hard transients, for the electric motor
contributes when such rapidly changing power quantities are called for.
Also, the
19
hybrid's internal combustion engine can be tuned better for emissions at the more steady
power levels.
Current legislation imposes several levels for emissions reduction. The first level
was Tier 1, imposed on all vehicles model year 1994 and newer2 . Tier 2 restrictions
further reduced Tier 1 limitations. The Environmental Protection Agency is imposing
these restrictions on 49 of the 50 states. California (the other state), abides by restrictions
made by its own California Air Resources Board which classifies cars as transitional low
emissions vehicle (TLEV), low emissions vehicle (LEV), ultra low emissions vehicle
(ULEV) and zero emissions vehicle (ZEV).
California also requires each auto
manufacturer to sell a certain number of LEV's each year or face fines.
The Toyota Prius, the first (and currently only) production hybrid vehicle has
been proven in reducing tailpipe emissions and meeting these requirements. Its CO, HC
and NO, emissions all meet the SULEV level (super ultra-low emissions vehicles)
proposed by CARB.
It does so as a hybrid vehicle, but has several novel catalytic
converter improvements, such as a light-off catalyst, a HC-absorption catalyst and
.3
advancements in air-fuel control strategies.
1.3 OBJECTIVES
The foremost target of this study was creating a robust emissions model to allow
for first-cut emissions analysis of various hybrid concepts.
This model accounts for
transient behavior through a catalyst oxygen storage mechanism and changes in internal
combustion engine behavior. It also allows for improvements in the catalytic converter
technologies represented as an improvement in converter efficiency with respect to
temperature.
Coupled with a pre-existing hybrid evaluation code, the total model
provides for a wide variety of configurations and possibilities.
Using this model, one could then pinpoint the "emissions-savings" of hybrid
vehicles.
This comes from the previously posed question: whether the hybrid is
improved because of more stable and better-tuned engines or less ICE running time. The
answer to such question provides insight into which areas need improvement.
In addition, such information was used to prove the viability of hybrid vehicles in
comparison to their conventional counterparts. For example, if the hybrid is better only
20
because it spends less time using an internal combustion engine, then simulations results
will shows the ill importance of fine-tuning the hybrid engine, as doing so produces no
fewer emissions.
21
22
CHAPTER 2
ADVISOR CODE
2.1 BACKGROUND
Over the past several decades, automobile manufacturers, the government and
research institutions have progressively developed computer codes to model vehicle
systems behavior. These models vary from simple one-dimensional lumped-parameter
block diagrams to intricate iterative programs containing many sub-models for various
subsets of a vehicle.
Currently, there exist two different types of vehicle modeling approaches: forward
and backward. The forward approach, for example, determines the vehicle dynamics (i.e.
speed) from the given engine output.
A backwards approach is just the opposite,
determining the engine output and emissions from a desired vehicle speed or other
performance variable.
When analyzing vehicles' emissions levels, a standard "driving cycle" has been
used. This driving cycle concept requires the vehicle to perform at certain speeds and
loads over a set period of time, thus creating an easy standard for which every vehicle can
be compared.
Thus, it is better to use a backward approach when modeling vehiclc
emissions is important.
Additionally, models may be purely theoretical or semi (or fully) empirical.
Theoretical models often allow for a good understanding of the modeled subsection, but
often do not allow for a large variation in dynamics or systems to provide feasible results.
To model many different systems, a semi-empirical approach is superior. Such a setup
allows the end-user to draw from known systems data to further validate a models'
results.
Yet another trait of modeling programs are their time-use and memory
characteristics. Some codes are steady state, calculating one set of results independent
time. These being the easiest of codes, some further developed programs are quasisteady-state, where results from previous time-steps are used, but their paths are assumed
to be inconsequential.
A fully transient model investigates, at all points in time, the
effects of a variable. Pure transient models are often complex and unreliable, because
23
they must be purely theoretical or empirical. Hence, a model including some transient
assumptions and other quasi-steady measures is often the best at predicting outcomes.
ADVISOR (ADvanced VehIcle SimulatOR) is a vehicle modeling code produced
by the Center for Transportation Technologies and Systems of the National Renewable
Energies Laboratory, a federally funded research lab in Colorado. Running in a Matlab
and Simulink environment, it uses a backward, semi-empirical, quasi-steady-state
modeling approach. Comprehensively, the backward setup uses a required and achieved
set of variables, determining what is needed to what could be produced so as to provide a
results check. This project was originally started using version 2.2, but in its course
converted over to version 3.0 (October 2000).
2.2 VEHICLE PERFORMANCE EVALUATIONS
ADVISOR is a set of eight submodules : the desired Drive Cycle, vehicle, wheel
and axle, final drive, gearbox, clutch, fuel converter and exhaust system (Figure 2.1).
Using each model differently, configurations for conventional (Figure 2.2), electric,
parallel and series hybrid gas-electric (Figure 2.3 and 2.4), fuel cell and customs vehicles
can be created. Data files, created by the user using known figures, include generic
information about the vehicle, such as mass, geometry, axle size, frontal area and other
rudimentary values. In addition to the vehicle, information must be supplied about the
powerplant (fuel converter).
This may include engine maps for internal combustion
engines or performance characteristics for a battery-generator-motor powerplant of an
electric vehicle.
2.2.1
VEHICLE CALCULATIONS
The vehicle calculations begin with the calculation of forces using a simple force
balance:
F, = ma
(2.1)
This includes the rolling resistance, aerodynamic drag and the force of gravity that must
be overcome to climb a grade and the force required to achieve certain vehicle
acceleration (determined from the drive cycle). The wheel/axle calculations use lookup
24
tables by vehicle mass to determine how much torque/speed is lost due to friction. In
addition, it calculates the allowable and desired braking force of the vehicle. All the
forces are then combined to determine the vehicle speed, which is then used to comply
with the speed specified at time t of the driving cycle.
2.2.2
FUEL CONVERTER CALCULATIONS
The fuel converter may be the internal combustion engine, motor, fuel cell or any
other power-providing source.
Using the required output as an input, it follows
backwards to determine how much fuel (gasoline, electricity, etc.) is necessary to provide
the requested power. The requested output comes from the transmission, which is linked
to the vehicle force required.
The torque available is calculated using the engine torque from which the
accessories torque is subtracted. The accessories power (and thus torque) is determined
using empirical data from vehicles, and is held constant throughout the cycle. The engine
torque is the minimum of the requested engine torque or the fuel converters' maximum
torque available. This value is computed from look up tables provided by the user.
2.2.3
EMISSIONS CALCULATIONS
ADVISOR calculates emissions through a series of steps. Emission levels are
evaluated at the engine, out of the engine, and after a catalytic converter. Included are
Hydrocarbons, Nitric Oxide and Dioxide (NOx), Carbon Monoxide and Particulates. In
the case of a non-diesel fuel converter, Particulates are ignored.
Cold Engine-out emissions are first correlated from data supplied to the code.
This data is indexed by engine torque and speed.
ADVISOR then adjusts the cold
emissions to account for engine warm-up using curve fits. These curve fits (Figure 2.5),
being outdated, were replaced by new data5 (Figure 2.6).
The correction uses a
normalized engine temperature, which is a linear function of the difference between
present and hot (maximum) engine coolant temperatures. The hot engine-out emissions
then proceed through the manifold to a catalytic converter exhaust system sub-model.
The top level of the exhaust system thermal model is shown in Figure 2.7. The
converter thermal model uses two outputs from the fuel converter as its inputs: the
25
exhaust gas flow rate and engine-out temperature. The exhaust gas loses heat to the
exhaust manifold and downpipe prior to reaching the catalytic converter (which in turn
lose heat to the engine and ambient). The catalytic converter is modeled via a three-node
lumped capacitance model including monoliths, inner steel shell, and outer steel shell.
Heat exchange from the gas to the converter nodes, between converter (and pipe) nodes,
and from the converter to the ambient is modeled using advection, conduction,
convection, and radiation thermal resistances, as shown in Figure 2.8. It is composed of
five subsystems.
The first subsystem is a block model analyzing the heat loss from the exhaust gas
as it flows from the engine, through the manifold, downpipe, and catalytic converter. For
each part of the system (manifold, pipes, converter), the convective heat transfer is found
using the surface area, a given heat transfer coefficient, and the temperature difference
between the wall and gas temperatures. A thermal capacitance is determined using the
mass flow and heat capacity of the gas. This is used, in addition to the heat transfer
found, to find the gases temperature as it exits the specific system and enters the next.
The next sub-model accounts for increases in gas temperature within the
converter due to the catalysis of the emissions. First, the catalyzed emissions are broken
down to their molar concentrations as a percentage of the total exhaust gas flow. Then,
using the element's specific heat, a thermal power of the catalyzed is found. This is used
later to determine the converter temperature.
The calculation of heat loss in the entire catalyst system is the third sub-model.
This includes heat exchanges between the manifold and ambient (convection and
radiation), pipes and ambient (convection and radiation), pipes and converter interior and
exterior shells (conduction), pipes and converter monolith (radiation), monolith and
interior (conduction), interior and exterior (conduction and radiation), and exterior and
ambient (convection and radiation). These values are all calculated using user supplied
heat capacitances, thermal conductivities, convective heat transfer coefficients and
emissivities. In addition, heat flows are based on temperatures from the previous time
step.
Next, a converter temperature is calculated to provide for the catalyst efficiency.
This is assessed first by adding together the heat transfer from the exhaust gas to
26
converter, the total heat transfer from the monolith, and the heat released from the
catalyzed emissions. The temperature change is found by dividing this heat transfer by
the monolith's thermal capacitance.
In addition, applying the same methodology, the
other system's temperatures are calculated to be used in the next iteration.
Finally, the monolith's temperature is used to find the converter's efficiency.
Again, data supplied by the user, indexed by monolith temperature and specific emission
(HC, NOX, CO, or PM) provide the emissions reduction percentage. This is then applied
to the engine-out emissions to determine what the tail-pipe emissions will be.
Additionally, a "break-through" limit is imposed. If the tail-pipe emissions exceed a
certain level (ADVISOR suggests 5 times the federal Tier I regulations), then the pre-set
number will be used in place of the calculated emissions level. This provides a "realitycheck" to the process.
2.2.4
HYBRID CONTROL STRATEGY
ADVISOR has a control methodology to assess, in a gasoline-electric hybrid
configuration, when the internal combustion engine and electric motor should operate
(both independently and together for added output).
ADVISOR provides for several
different hybrid control strategies, ranging from series, to electric assist to fuel cell
technology.
Examining the parallel hybrid arrangement, the code operates as shown in
Figures 2.9 and 2.10.
When the batteries state-of-charge (SOC) is greater than the minimum allowed
(Figure 2.9), the engine will only run above a certain engine speed (called the electric
launch speed) and required torque (a constant fraction of the maximum torque). Below
these levels, only the motor will provide power to the wheels. The user determines both
the launch speed and the off torque envelope.
If the SOC falls below the allowed minimum (Figure 2.10), the engine or motor
must provide power to the generator to increase the SOC level. If the required torque
(torque to wheels plus torque to generator) is below the off torque envelope, then the
motor can supply enough power to increase the SOC. However, if the required torque
falls above it, the engine must supply power in addition to the motor.
27
2.3 LIMITATIONS
Advisor has several limitations to its users. First, the program was designed as a
tool to aid in understanding hybrid fundamentals, rather than a reliable predictor of
hybrid vehicle behavior.
Additionally, the market focus for hybrid vehicles is their
improved fuel economy rather than their reduced emissions levels. Hence, ADVISOR
provides much more accurate results for fuel economy, where more time has been spent
improving the code to reflect reality.
Secondly, being quasi-steady state, it doesn't allow for a true transient analysis of
vehicle performance; the foremost problem this study sought to tackle. Emissions are
transient by nature and occur mostly (after catalyst light-off) during severe transient
engine behavior.
Thus, if engine behavior is smoother than reality suggests, the
emissions data will be inaccurate. It is also true that transient emission modeling is still
in an adolescence stage.
Finally, ADVISOR is limited by the amount of data held by the user. Because it
is an empirical code, specific vehicle configurations require large amounts of
information, which may differ from those provided in the code. However, those provided
allow for a "back-of-the-envelope" calculation.
In all, these limitations may prove
ADVISOR inefficient in the vehicle design area, but of exemplary value in an initial
feasibility study.
28
total fuel used (gal)
gal
--
exscalc
fc
drive cycle
<cyc>
emis
vehicle controls
<vc>
-
AND
->Clutch
ri
fuelHCC,
vehicle <veh>
wheel and
axle <wh>
final drive <d>
gearbox <gb>
clth
exhaust sys NOx, PM (g/s)
<ex>
f>cere
Figure 2.1: The general ADVISOR model. This shows the different vehicle components
of the model used.
Wheel
Gearb ox
'ftI
:Final
Dnive
Cl itch
J...
Jfc:
Fuel Converter
Figure 2.2: Conventional vehicle configuration in ADVISOR.
29
Wheel
Gearbox
Jfa :Final
D rive
61172...Energy
Ja
Converer
Geerato
-Mto
Fuel
Power
...
KiStorage
Bus
N...
System
Figure 2.3: Series gasoline-electric hybrid vehicle configuration in ADVISOR.
Wheel
......
Gearbox.........
Torque Coupler
J&
J
Final
:Drive
.........
.....
Motor
..................................
Fuel Converter
Energy
Storage
System
Figure 2.4: Parallel gasoline-electric hybrid vehicle configuration in ADVISOR.
30
Hot Engine Out Emissions Correction Factor
ADVISOR
20
18 16 14 12 H0 10wU 8
642
0
260
a
Y
'4
--
---
280
%
--
30 0
320
340
360
380
Engine Coolant Temp [K]
--
HC -
-
- CO
NOx
Figure 2.5: Previous ADVISOR hot engine-out emissions correction factor. (EO emis =
Cold EO emis * CF)
Hot Engine Out Emissions Correction Factor
SAE Paper 971603
1.6
1.4
1.2
-
0
1 0.8
0.6
0.4
260
280
300
320
340
360
380
Engine Coolant Temp [K]
--
HC -
-
- CO
NOx
Figure 2.6: Study updated hot engine-out correction factor. (From SAE 971603) Note the
significant change from Figure 2.5 in both magnitude and deviation from unity.
31
OLD cat
temp.
OLD Temp
Correction
OLD tail
pipe emis
Trn-
, EU
Terminator
ex-cat_ eff I
~ThFJ*E~sat~
Heat Transfer as ga s flows from engine to catalytic converter
emisca'd
i
Tex,gas
ex-gas-tmp
Efficiency assigned
using gas temperature
ex gas flow
creakthru
Ocat
EO emis
info
Tex,eo
$
ex cat th_pwr
-
HC
ncti
temp eff
Xmax
ex-cat-tmp
exoeem
temp elf
-K
ex gas heat flow
...............
Ocm
m
1/Ccm
Qcat-i
+
--
Ocat
ex temp
u)M
c
temp eff
-.....
-/
/c
Enable
1Cp
x
Omni
ex sys heat flow
tailpipe
emis (g/s)
Max of breakthrough
or catalyzed gas
/
-K
Qcp
catalyst using
cond, cony & rad
...
temp eff
Qci
Ocal-p
Calculates HT in
Ox
T mN
veh spdpc
Tfc,x
T
'-U
.......
Ocat
.
/Ccx
m
/C
Figure 2.7: ADVISOR catalyst model. Emissions coming in are temperature corrected engine-out emissions with data such as
temperature and mass flow rate. Temperatures are found using heat exchanges between the gas, the catalytic converter and the
environment and thermal capacitances (shown in Figure 2.8). Conversion efficiency is found using the catalyst temperature and
applied to the engine-out emissions entering.
Cl
.
Inlet pipe
.CLdulyIl
wIwtILtn
cor eter
S
Exhaust gas
.
...
.U
........
....
........-_ ---
hm-
"
___W
i.
Exhaust gas
ct
Rc,2
Rd1 22
Tip
dt
Rcn1
R--2
'TdE
R4
RAv
R:v2
Rda2
Ren3
TdC
Rd3
R>6s
..
Rd4
-
Amb
ient
Figure 2.8: Lumped Capacitance Thermal Network Model of Catalyst System, where:
A - Monolith
B - Interior
C - Exterior
33
For SOC > cslo_soc
j
Engine
Torque
Maximum Torque Envelope
Engine
ON
Off Torque Envelope
Lcsoff trq fiac*(max torque )
.......
....
......
,4
Electric
Launch
Speed
w
Engine Speed
Figure 2.9: When the SOC is high enough such that the batteries need not be recharged,
the engine will commence operation only when the motor cannot fully provide the
total torque required.
For SOC < cslosoc
Engine
Torque
Maxim iu Torque Envelope
Aiditional Charge Torque
cs_chargeftrq *(avg(cso _soc, cs_hi_soc)-SOC)
*Case 1 (blue): Operation
point isrequire d tor que plus
charge torque
*
Engine
ON
"
...
- -
Minimum Torque Envelope
cs_mintrcLfrac*(max torque)
.
ea
Required
Case 2 (red): Operation
point lies along mniimum
torque envelope because
re quire d tor que plus char ge
torque is too low
Torque
Ii
Engine Speed
Speed
Figure 2.10: When the SOC is such that the batteries need recharged, the engine will
commence operation when the motor cannot fully provide the total torque required
(which is the vehicle torque + torque to charge the batteries to increase the SOC).
34
CHAPTER 3
HYBRID/CONVENTIONAL
QUASI-STEADY-STATE COMPARISONS
3.1 BACKGROUND
Fundamentally, a gasoline-electric hybrid vehicle (referred herein as hybrid)
incurs fewer emissions because it operates the internal combustion engine less than a
conventional vehicle under the same speed and range requirements. In addition to this
simple proviso, it is important to note when the hybrid does not use its ICE. Most hybrid
control strategies call for engine output only when the motor is not powerful enough to
drive the vehicle at the requested speed (usually above 45 miles per hour).
70% of
emissions come during the first few minutes of driving, before the catalyst system can
warm-up. Thus, if the hybrid is not using its ICE during this time period, a substantial
reduction in emissions occurs. Also, emissions systems improvements such as light-off
and electrically heated catalysts and HC absorption chambers improve the time to lightoff and overall efficiency of the catalyst.
With the hybrid control strategy, however, it is necessary to investigate catalyst
cool-down. A hybrid vehicle may not use its ICE for large periods of time. In doing so,
the catalyst temperature may fall to where it wields the system inefficient versus a
continuous conventional system.
In addition to the original proviso that hybrids emit less because they use the ICE
less, the hybrid's engine performs in the more efficient portions of the engine map (at
high engine speed and torque) as shown in Figure 3.1.
Because the hybrid uses the
electric motor below (approximately) 45 miles per hour, it operates the ICE not just less,
but less in the more inefficient areas of the map. Furthermore, engines placed in hybrid
vehicles can be better tuned for improved emissions output. Because hybrid vehicles
only use their engine to provide power at high levels (save when the batteries need
charged), the engine can be designed such that, the performance map for engine out
emissions is centered about the average operating points.
35
Severe engine transients is yet another extensive contributor to engine-out
emissions. When the catalyst is fully warmed up (usually by 1-2 minutes into a driving
cycle), it reduces engine-out emissions between 90% and 98% except when there is a
significant change in the engine-out emissions composition. The hybrid vehicle does not
incur as many severe engine transients for several reasons. First, the foremost significant
transient occurs from a stopped position. When the vehicle accelerates from 0-20 or 0-45
miles per hour (even 0-60), these transients are handled by the motor. Similarly, the
hybrid's ICE operates between vehicle speeds of 40-85 miles per hour, a difference of 45
mph instead of an 85 mph spread (0-85 mph). Finally, the hybrid control strategy allows
the motor to provide dampening to any unusual power requests to the engine, such that
the engine has ample time to react and adjust.
Thus, there are several investigations to be made using the simple quasi-steadystate approach built in to ADVISOR. First, determine why the hybrid produces fewer
emissions than its conventional counterparts. This will show the relative advantages of
fewer emissions (due to engine shut-off) versus the operation in a more efficient (less
engine-out emissions) area of the engine map. In addition, it is necessary to test the long
engine shut-off periods to ensure the catalytic converter temperature does not drop below
an efficient level. Finally, a by mile-engine-on comparison will be made to determine if
the hybrids engine is better or worse than its conventional counterpart.
3.2 TEST SETUP
As prefaced, ADVISOR simulates vehicle performance according to specified
conditions (driving cycles).
To compare a conventional vehicle with a hybrid
counterpart, two such configurations were created (see 3.2.1 and 3.2.2). Furthermore, a
set driving cycle was needed to compare these vehicles; one which represented typical
driving in the United States.
Such a cycle exists, and is known as the Federal Test
Procedure Cycle. This procedure is comprised of two driving cycles: the Federal Urban
Driving Schedule (FUDS) (Figure 3.2) and the Highway Fuel Economy Test (HWFET)
(Figure 3.3).
The FUDS driving cycle is used to simulate city driving. It is comprised of
several vehicle accelerations and decelerations of different amplitude, a rest period and
36
the same acceleration/deceleration levels repeated.
These represent the start-and-stop
nature of urban driving. The cycle is 1372 seconds long with a top speed of 56.7 miles
per hour and an average speed of 19.5 mph. The total distance traveled is 7.5 miles.
The HWFET driving cycle is used to simulate highway driving. This test consists
of a warm-up phase followed by a test phase. The driver follows the same driving trace in
both the warm-up and the test phase. However, since ADVISOR is coded to include hot
starts, the first phase is redundant. The cycle (as used in ADVISOR) is 770 seconds long
with a top speed of 59.9 miles per hour and an average speed of 47.6 mph. The distance
traveled is 10.3 miles.
Several simulations were run, changing the vehicle type, initial conditions (hot or
cold start-up), physical and geometrical aspects of each vehicle configuration, and the
driving cycles. A time-step of 0.1 seconds was used. To keep post-processing time to a
minimum, only 35% (480 seconds) of the FUDS driving cycle and 65% (500 seconds) of
the HWFET driving cycle were analyzed when comparing vehicles. The catalyst cooldown simulations used the entire cycles. A physical comparison of vehicles can be found
in Table 3.1.
3.2.1
HYBRID CONFIGURATION
The hybrid vehicle used for the simulations was created as something similar to
the PNGV (Partnership for a New Generation of Vehicles) goals 6, being a mid-sized
vehicle with enough power to perform as a typical conventional vehicle would. Using
this as a baseline, a medium car frame (a 1998 Ford Taurus) was used.
The internal combustion engine is an inline 4 cylinder engine, similar to one
found in a Ford Contour. The engine's power output is 77 horsepower and emissions
data was scaled from the 105 horsepower engine data received from the Ford Motor
Company. The motor was selected to provide the hybrid vehicle with approximately the
same maximum horsepower from an average car, such as the Taurus.
Thus, a 90
horsepower motor was used, bringing the total vehicle power to 167 horsepower.
Additionally, a manual shift transmission was selected from ADVISOR's set of data.
This 5-speed model was edited to include gear ratios also supplied by Ford.
37
3.2.2
CONVENTIONAL CONFIGURATION
A 2000 Ford Taurus was modeled as the "average" family vehicle built at the time
of the study. Using the same frame as the hybrid vehicle, only the powerplant and
transmission were changed.
The internal combustion engine was a 153 horsepower
Vulcan V-6 cylinder engine with a displaced volume of 3 liters. All data for this engine
was, again, supplied by the Ford Motor Company. The transmission was modeled as a 4
speed automatic rather than a 5 speed due to judgments that it better suited a mid-size
family car, whereas a 5 speed manual transmission would be used in a hybrid to increase
efficiency.
3.3 RESULTS
Upon running each scenario, a spreadsheet was created to view and analyze the
data as prescribed above. Output variables from ADVISOR included (on a lo second
basis): hot and cold start engine-out and tail-pipe emissions, internal combustion engine
speed and torque, motor speed and torque (hybrid only) and vehicle speed. From this
data, the brake mean effective pressure (bmep) was calculated using Formula 3.17:
Bmep[kPa] =
6.28 x n, x Torque[Nm]
(3.1)
Vd [liters]
In addition, the power produced by the internal combustion engine, was found using
Formula 3.2:
bmep[kPa] x Vd[liters] x N rev
Power[kW] =
x
1
(3.2)
Total Torque = Engine Torque + Motor Torque
(3.3)
The total torque was calculated using Formula 3.3:
The change in bmep with respect to time was calculated for each time step in addition to
all three torques (engine, motor, and total).
38
Bmep is subsequently used as a comparison factor because of its good qualities.
First, it compares the engines only and has no component for the hybrid vehicle's motor,
which is unimportant because it produces no emissions.
More importantly, it scales
torque with respect to engine displacement (size), allowing for reasonable assessments
between the 1.25-liter hybrid and 3-liter conventional engines.
Furthermore, brake specific emissions (bse) were used, as opposed to normal
emissions calculations that present emissions as a mass fraction of the total output. Bse
data is formulated as mass per power output of the engine, which also normalize the
results, so as not to mis-compare emission predictions from two different engines.
3.3.1
ENGINE OPERATION
3.3.1.1 ENGINE USAGE
First examined were emissions savings due to less than whole engine usage for
the hybrid vehicle. The engine runs only 21% of the total time in the hybrid vehicle as
compared to 95% of the time in the conventional (an initial starting delay of -20 seconds
occurs) over the first 480 seconds (35%) of the FUDS driving cycle (Figure 3.4).
Summing the emissions produced by the conventional vehicle at times when the hybrid
engine was off, a 72% reduction in engine out emissions occurs. Additionally, the hybrid
runs its engine during only 40% of the first 475 seconds (65%) of the HWFET driving
cycle (Figure 3.5) compared to the conventional engine running 98% of the time. A 50%
reduction in engine-out emissions is seen by the hybrid from its conventional counterpart
comparing engine-off emissions.
3.3.1.2 MORE EFFICIENT OPERATION
The hybrid vehicle uses the engine at less polluting operating conditions.
Comparing Figures 3.6 and 3.7, the conventional vehicle operates at all levels of the
engine map, whereas the hybrid engine only performs at the more efficient points, nearer
to the "center " of the map. This phenomenon is true for the HWFET driving cycle as
well
(Figures 3.8 and 3.9).
For the conventional vehicle, only 30% of the engine
39
operating points are above 30% efficiency.
On the other hand, the hybrid operates
exclusively above the 30% mark.
Furthermore, the effects of engine efficiency and emissions production are
considerably similar. Looking at the bse maps for the FUDS cycle (Cold Start), the
conventional vehicle (Figure 3.10) shows that only 36% of the NOx emissions are below
10g/kWh, whereas 80% of the hybrid vehicle's (Figure 3.11) NOx emissions are below
10g/kWh. Moreover, 20% of the conventional vehicle's NOx emissions lie outside 34
g/kWh as compared to only 18% of the hybrid. This trend can also be seen in Figure
3.12, where the hybrid has significantly less outliers. Changing the data to reflect only
times when emissions are being produced, 51% of the hybrids occur below 10 g/kWh as
compared to 40% of the conventional vehicle. Beyond 20 g/kWh, the vehicles produce
nearly the same percentage of emissions when the engine is producing emissions.
For HC (Figure 3.13) and CO emissions (Figure 3.14), the same, but more
pronounced, phenomenon occurs.
The no-emissions levels are much higher for the
hybrid vehicle. Its emissions profile is similar to a "normal" distribution about the lower
i. The profile for the conventional vehicle is also similar to a normal distribution, but has
a higher offset and a significant tail that extends out past the hybrid's profile.
A fuel-specific approach to tail-pipe emissions production shows the hybrid to produce
similar (but reduced) amounts of HC, CO, and NO,. Figures 3.15 shows similar emission
profiles when the HC emissions levels are divided by the fuel used. The center of the
hybrid distribution is slightly smaller, which is to be expected for a slightly smaller
engine. The profiles are not tremendously different because the engine technologies are
similar.
The offset is again attributed to the disproportionate engine use by the
conventional vehicle.
3.3.1.3 TRANSIENT BEHAVIOR
Reducing the transients of a vehicle greatly reduces the level of emissions
produced.
Figure 3.16 shows the change in bmep with respect to time for both the
conventional and hybrid vehicles for the FUDS driving cycle. The hybrid vehicle has
significantly fewer large (10-1000 kPa/s) transients because the motor aids to control
such events. The HWFET driving cycle shows similar results, however, the hybrid incurs
40
more sizable positive torque (bmep) changes (Figure 3.17).
There are fewer large
negative torque changes, because the extra power is used to charge the hybrid's electrical
system.
Thus, there are fewer emissions and more available power.
Additionally,
Figures 3.18 and 3.19 show the difference in transient engine operation during the FUDS
driving cycle. Figure 3.18 shows the difference in severe engine transients while Figure
3.19 shows the difference in smaller transients.
The reduced transients transform into reduced emissions looking at the difference
in engine-out emissions (Figure 3.20). Most of the differences are negative (difference =
hybrid - conventional), thus there is a significant decrease of engine out emissions
between the conventional and hybrid vehicles. The graph only accounts for those times
when both engines are running and creating emissions. Over 70% of the time the hybrid
engine generates less (normalized) emissions than the conventional vehicle's engine.
Only 13% of the time does the hybrid produce significantly (>5g/kWh) more NO,
emissions.
3.3.2
ENGINE TUNING
The hybrid vehicle also incurs fewer emissions because it is not required to
perform at such vastly different operating points.
Figure 3.21 shows the difference
between the bmep along the FUDS driving cycle for each vehicle configuration. The
conventional vehicle must be able to vary between 0 and 900kPa. The hybrid must vary
only between 400 and 800kPa with a few outliers at points where the program "hiccups".
This alleviates the need design for power output at inefficient points (the phenomenon
described in Section 3.3.1), and therefor the hybrid's internal combustion engine need not
be as rigorous as its conventional counterpart. ADVISOR, however, only takes this into
account only upon the user defined engine emissions output data.
3.3.3
CATALYST COOL-DOWN
The temperature of the catalytic converter determines how efficiently the catalyst
will perform its function. As the engine is run, hot gas warms the converter making it
more efficient. Thus, it is important to see how the catalyst temperature and efficiency
41
vary both over the entire driving cycle and, more specifically, during periods when the
hybrid does not run its internal combustion engine.
Over the entire 1372 seconds of the FUDS driving cycle, ADVISOR shuts the
hybrid's engine off 181 times. The length of shut off (shown in Figure 3.22) varies
between 0.1 and 60 seconds. The shut-offs of length less than 2 seconds probably would
not occur in real life and will be ignored. This assumption leaves 55 occurrences of
average length 14.3 seconds.
Although the catalyst temperature may not drop in an
average stop, a 60 second (or longer) stop may prove detrimental to the catalyst
temperature, thus efficiency, thus reduced emissions.
For example, Table 3.2 shows a sampling of engine shutdown periods.
The
largest decrease in NO, catalytic efficiency occurs during a 57-second shutdown, where it
drops by 5.3%. It is the largest because the catalyst has not been heated to its maximum
efficiency.
However, the shutdown is for a considerable amount of time, and those
emissions not catalyzed are far more desirable than those produced by the conventional
vehicle. Once the catalyst is warmed above -300'C, even a 60-second shutoff results in
a loss in efficiency of only 2%. Figure 3.23 shows a typical 32-second shutoff profile.
The catalyst temperature drops 16'C while the efficiency drops less than 3%.
Figure 3.24 is a close-up look at the effect of catalyst cool-down. ADVISOR sets
a maximum limit for NOx catalyst efficiency, which is 91% in this case.
Once the
conventional vehicle reaches 91%, it remains constant throughout the remainder of the
driving cycle.
The hybrid's catalyst, when the engine disengages, does lose some
efficiency, albeit often less than 1%. This proves there is little or no effect (especially
after catalyst light-off) of temperature loss affecting the catalyst efficiency.
3.4 CONCLUSIONS
There is a considerable emission savings when comparing a hybrid vehicle to its
conventional counterpart. From the data above, one can point to several reasons: reduced
transients, less asked of the internal combustion engine, both overall and particularly in
bad emissions areas, and the overall design and tuning of the powerplant itself. Clearly,
emissions reductions will occur if the engine is better tuned to operate in a specific region
of torque output, albeit a minimally. Furthermore, the reduction of engine transients
42
trims the engine-out emissions by 10-20%, thus creating a noticeable impact on the total
emissions output. However, the engine operating procedure is the primary reason for
decreased tail-pipe emission levels.
Although the lack of emissions (due to engine shut-off) does provide for sizable
decreases, the fact that the hybrid does not operate the engine in high emissions areas
(low torque and low speed) is probably the most significant improvement in the vehicle
design and control. If the engine operated at the low end of the engine map, a significant
increase in emissions would occur (comparing just the hybrid to itself). Hence, it is of
great importance to design the engine for minimum emissions output at high engine
operating conditions.
43
Table 3.1: Vehicle configuration in ADVISOR for hybrid and conventional vehicles.
venicle ueometry
1000
2.04
0.335
Mass [kg] (w/o engine)
Frontal Area [mz]
Cd
Wheelbase [m]
2.6
Height [m] (Center of Gravity)
Cargo Mass [kg]
Transmission
Speed
Shift
0.5
140
4
Automatic
5
Manual
Internal Combustion Engine
Displacement [L]
Valves [per cylinder]
Maximum Torque [Nm]
Maximum Power [kW]
Mass [kg]
Electric Motor
Maximum Power [kW]
Maximum Torque [Nm]
Mass [kg]
44
1.25
3
4
4
300 @ 4500 rpm 110 @ 4300 rpm
58
130
186
416
75
273 @ 6000 rpm
91
Table 3.2: Sample Results for various engine off intervals.
-
I
Citdty~t
Ten1p~rature
ILL. I~
701.8
2
381.2
2.2
464.1
2.2
598.6
2.2
706.7
2.2
311.3
4.2
560.0
4.2
674.0
4.2
688.0
4.2
309.8
6.2
386.0
6.2
675.8
6.2
566.8
6.3
60.1
10.2
337.4
12.2
36.2
15.5
645.6
15.6
301.5
16.2
507.0
16.2
459.9
22.4
684.9
22.7
687.4
29.7
Engine
Off Time
I
31.6
479.6
33.1
36.9
529.9
691.7
442.5
394.3
659.7
452.7
249.0
323.7
615.3
39.1
39.3
40.2
49.1
57.3
59.2
60
I
U -U-
A'ncNO~
ATc
-1.7
-1.9
-0.6
-1.8
-1.5
-1.2
-3.8
-2.4
-3.2
-2.8
-4.8
-3.2
-4.3
-2.5
-8.0
-0.4
-9.3
-12.4
-9.8
-7.7
-12.8
-15.7
I-
-0.5%
-0.2%
0.0%
0.0%
-0.3%
0.0%
0.0%
-0.1%
-4.3%
0.0%
-0.4%
-0.1%
-4.0%
0.0%
0.0%
-0.5%
-0.8%
-3.4%
0.0%
0.0%
-1.3%
-1.4%
-0.1%
-2.8%
-3.6%
0.0%
-2.0%
-5.3%
-2.7%
-14.6
-16.7
I
-18.4
-9.8
-15.2
-19.3
-16.1
-8.3
-11.2
-9.5
0.0%
-1.7%
I-
45
Torque (bmep)
Ak
More
Hybrid
Speed
Figure 3.1: Speed vs. T orque engine map - hybrid and conventional vehicles. The
conventional vehicle operates in more regions while the hybrid's operation is more
concentrated and in a more efficient region.
Federal Urban Driving Schedule
30
25
20
0.
E
15
10
5
0
0
200
400
800
600
1000
1200
1400
Time [s]
This test protocol is for city
Figure 3.2: Federal Urban Driving Schedule (FUDS).
driving and includes 2 (identical) acceleration/deceleration periods in addition to a
shut-off period. The red line shows the first 35% (480 seconds).
46
Highway Fuel Economy Test
30
25
20
,
n
0.
'
r F
15
10
5
0
0
200
400
600
800
Time [s]
This test protocol is for
Figure 3.3: Highway Fuel Economy Test (HWFET).
highway driving. The red line shows the first 65% (476 seconds).
47
bmep
vs. Binned Timesteps - 35% FUDS
10004
940
3922
900
741
800
700
574
600
595
458
500
412
4
400-
8
351
255
316
300
18
193
200
200-
125
57
100-
76
5
71
34
31
0 0
-4
0
0
8
8
CD
S
D
0
0
0
0
bmep [kPa]
13 3.OLEN 1.25L]
Figure 3.4 (top) and 3.5 (bottom): Engine bmep output for the FUDS (3.4) and HWFET
(3.5) Driving Cycles. In both, there is a significant amount of time the hybrid's
engine is off (and not producing emissions) and the hybrid does not have negative
bmep because the engine shuts off during such time periods.
bmep
vs. Binned Timesteps - 65% HWFET
2925
2000
1752
1600
0L
1200
979
800
656
655
E
762
448
420
400
289
263
180
151
0
21
47
0
0
31
0
0
0
0
cc
CD
[ 3.OL M 1.25L
48
C,)
0
0
0
0
0;
0
8
bmep [kPa]
0
40
A
IC Engine Torque vs. Engine Speed
Ford 3. OL 35% FUDS
351
301
251
z
201
0* 151
101
-
-
-2
----
-_
-
--
51 +1
1
6000
5000
4000
3000
2000
1000
0
Engine Speed [rpm]
*2.02% u 30.99% A 31.10% X 17.24% K 2.79% * 5.72% + 10.12%
Figure 3.6 (top - Conventional) and 3.7 (bottom - Hybrid): Engine operating (efficiency
as a decimal) points indexed with engine torque and speed for FUDS driving cycle.
The hybrid operates solely within the 30% efficiency contour while the conventional
vehicle operates only i of the time above 30% engine efficiency.
IC Engine Torque vs. Engine Speed
Ford 1.25L 35% FUDS
121
101
81
E
z
--3
61
0
41
+
.02
21
0.15
-~~-0-I
0
2000
1000
4000
3000
6000
5000
7000
Engine Speed [rpm]
+
6.32%
m 47.30%
A
33.44%
X
12.95% -
-0.00%
0
0.00%
49
IC Engine Torque vs. Engine Speed
Ford 3.OL 60% HWFET
351
301
251
E
z 201
U)P
151
101
0.26
X
51
1
5000
4000
3000
2000
1000
0
6000
Engine Speed [rpm]
0.41%
U 24.94%
A 52.77% x 16.85% x 0.21% * 3.51% + 1.31%
Figure 3.8 (top - Conventional) and 3.9 (bottom - Hybrid): Engine operating (efficiency
as a decimal) points indexed with engine torque and speed for HWFET driving cycle.
The hybrid operates solely within the 30% efficiency contour while the conventional
vehicle operates only i of the time above 30% engine efficiency.
IC Engine Torque vs. Engine Speed
Ford 1.25L 60% HWFET
121 -
101
81
E
j~3
0)
61
41
-
21
0.2
-----
1
101
0
1000
2000
3000
4000
6000
5000
Engine Speed [rpm]
+
50
11.29%
m
38.32%
a
43.08%
< 7.31% -0.00%
*
0.00%
7000
ICE Torque vs. Engine Speed - NOx Map
Ford 3.OL 35% FUDS
351
301--
are Brake
-Contours
251
Specific Emissions
[g/kWh]
E 20X
E 201-
51
Figure 3.10 (top - Conventional) and 3.11 (bottom - Hybrid): NOx emissions [g/kWh]
points indexed with engine torque and speed for FUDS driving cycle. In addition to
fewer points (less emissions), 80% of the hybrid's emissions are better than 15 g/kWh
whereas only 55% of the conventional points are within that limit.
ICE Torque vs. Engine Speed - NO. Map
Ford 1.25L 35% FUDS
121
101
1P
81
-A
E
61
~'
~N
0
4IPf~
4
.9'
I
4
~
41
21
~~4.9I
N
7.
i|I
I
1
0
1000
-~
2000
3000
4000
5000
6000
7000
Engine Speed [rpm]
* 43.12% 0 12.44%
13.86%
9.17% X 2.75%
6.01% + 12.64%
51
brake specific NO, Emissions
1738
vs. Binned Timesteps - 35% FUDS - Cold Start
3822
1200
1040
1000
846
800
w
600
1384
343
400
26
200
200
j
192
23
200
1
136 116
165
110
0"
0
0m
n
?
r
7
0n
0
-0
i.
Zo
0n
0
-
.:
bs emissions [g/kWh]
3 3.OL 0 1.25L
Figure 3.12: Binned brake specific NO, emissions for hybrid and conventional vehicles.
Note undefined brake specific emissions occur when the power is zero or negative.
These emissions would be "idling" emissions and range between 0 and 5 g/kWh.
Similar to the emissions maps above, there are fewer total occurences of emissions.
brake specific HC Emissions
vs. Binned Timesteps - 35% FUDS - Cold Start
3822
1738
800-
737
737
700
600
500
454
408
400
259
300 200
170
157
200
140
134
165
164
131
56
49
68
100
15
2
0
o3.OL l 1.25L
-
bs emissions [g/kWh]
Figure 3.13: Binned brake specific HC emissions for hybrid and conventional vehicles.
The hybrid's distribution is smaller in both offset (time) and width (severity) of
emissions production.
52
brake specific CO Emissions
vs. Binned Timesteps - 35% FUDS - Cold Start
1938 3822
10 00
00 - 900 808
8
00 - 7
0
DO 00
II
0
0.
0
500-
0
4 00 -
30
419
395
E
p
331 346
47H
300
196
2 0
0
150
108
114
87
33
231
0
01870
.R
0 .
98
76
1
4
-4
-El
-4
0
AL
bs emissions [g/kWh]
1
0.LE 1.25L
Figure 3.14: Binned brake specific CO emissions for hybrid and conventional vehicles.
The hybrid's distribution is smaller in both offset (time) and width (severity) of
emissions production.
fuel specific HC Emissions
vs. Binned Timesteps - 35% FUDS - Cold
3822
-
10 00
1429
9 00
8 00
750
663
7 00
6 00
521
482
5 00
469
437
CL
I-
-
4 00
298
A:
30
0-200
2(
174
4159
0-
101
10
17 1
19 2
12
18
5
21
1
10
111
0
-.
0 3.OL M 1.25L
fs emissions [g/
]
0
.
A
-
Figure 3.15: Binned fuel specific HC emissions for hybrid and conventional vehicles. The
hybrid's distribution is again smaller in both offset (time) and width (severity) of
emissions production. The lack of large levels (100+ g/kg) is because the hybrid
rarely uses small amounts of fuel.
53
Transient Behavior
dbmep/dt vs. Binned Timesteps - 35% FUDS
1258 3792
600
584
5897
533
545 525
500-
400
296
(
-
300 246
197
200 -
134
114
110
95
100 -
75
67
43
31
52
7
3
2
5
9
13
4
6
2
2
0
n
0r\, -
~
Lnt
o Cn -
-LN
~dbmep/dt
13
0 3.0Luo1.25L
N
0
51
A
0,
0
1
[k as]J
-~-
Figure 3.16 (top - FUDS) and Figure 3.17 (bottom - HWFET): Severity of Transients.
Binned changes in bmep with respect to time for hybrid and conventional vehicles.
This shows the hybrid has less severe changes in bmep over time, thus less severe and
overall transients which leads to better emissions.
Transient Behavior
dbmep/dt vs. Binned Timesteps - 65% HWFET
3137 1392
1000 -r
901
900 -
836
800 -
732
700614
*.
600-
(
500 -
0
400-
566
413
E
300164
200 68
100-
7
8
54
8
102
219n28 12
7
8
7~
7
8
0
7
0 '. 0 -
'
II
(7
151
98 3
[]
83
0
0
000
0
A
S03.OL E 1.25L
3
2
50L
9
-
-
-N
?
dbmep/dt [kPa/s]
-.
I
0
oo
on
o
§
t
525
dbmep/dt vs. Time
Ford 3.OL & 1.25L 35% FUDS
--
500
400+
300
-
200
Z!
100w50
-0.
C0
-100No
0
-200
*
-300*
.
1!
-400
-500
Time [s]
+ 3.OL a 1.25L
Figure 3.18 (top - Severe) and Figure 3.19 (bottom - Minimal): Transient magnitude.
Changes in bmep with respect to time for hybrid and conventional vehicles using
different scales. The hybrid incurs fewer severe transients (top) and the magnitude of
smaller changes in bmep are 50-75% less then those of the conventional vehicle
(bottom).
dbmep/dt vs. Time
Ford 3.OL & 1.25L 35% FUDS
20
-
*
+
**f++ -
15
+
10
40
1L *%
+.iBL
5
E*
-10-
-15
-20
-*
* 3OL
1 .25L
U
Time [s]
55
Difference of Engine Out brake specific NO, Emissions
(When both engines are on - 35% FUDS)
160
Hybrid Emissions Better
140
123
139
H
100
H
80
H7
120
120
1]
120
143
Hybrid
Emissions
Worse
62
39
40
20
10
n
I
F
39
41
28
25
4
8
I
7
0
0
0
1
Occurences
difference of bs emissions [g/kWh]
Figure 3.20: Difference in engine-out brake specific NO, emissions for hybrid and
conventional vehicles on the FUDS driving cycle. When both engines are running,
the hybrid produces, most often, less brake specific emissions than the conventional
vehicle. Only 13% does the hybrid produce significantly more emissions, thus, the
hybrid calls on its engine in less polluting areas of operation.
bmep vs. Time
Ford 3.OL & 1.25L 35% FUDS
- -
-
1000
-
900
800
m-
700
MM
,
600
C
500
.0
400
-
300
00
0
+3.OL m 1.25q
100
300
200
400
500
Time [s]
Figure 3.21: Bmep vs. time for hybrid and conventional vehicles on the FUDS driving
cycle. Again, the hybrid vehicle's engine concentrates on the larger power
production requirements leaving smaller, more inefficient for the electric motor.
56
Time Hybrid Engine is Off
vs. Binned Timesteps - 100% FUDS - Cold Start
88
25
19
20n
2S15
0
00
8
7
E
6
6
5
z5
4
4
3
0
-A.~.
0)
-
A
U1
0
U.1
0
C)
0)
-C
-
-4
-
-
-
-
o
Time Engine Off [s]
U1.25L
Figure 3.22: Binned length of timesteps in which the hybrid vehicle is using only electric
power. Those engine-off periods below 2 seconds exist only in simulations and
would not occur in reality. There are however, significant lengths of time the engine
is not running.
Emissions & Catalyst Info vs. Time
-32 second Engine Off Period
- 380
1 92.7%
0.9 -.
375
89.9%
0.8 --
370
369.9
0.7
365
0.6
0.
E
360 1-
0.5
0.4-
355 352.5
0.3
350
0.2
345
0.1
0
806.6
340
II
810.6
---
814.6
818.6
Cold HC Emis
Cold NOx Emis
822.6
826.6
830.6
834.6
-
Time [s]
Catalyst Efficiency
-
Catalyst Temperature
-
838.6
842.6
846.6
850.6
Cold CO Emis
Figure 3.23: Catalysts efficiency, temperature, and tail-pipe emissions for hybrid vehicle
during 32 second engine shut off period on FUDS driving cycle. This "average"
shut-off shows the unimportance of catalyst cooldown and its affect on conversion
efficiency.
57
NOx Catalyst Efficiency
Hybrid and Conventional Vehicles - 35% FUDS (Small Scale)
92%
91%
'-PIJ
C
I
1riv
i,
0
uLJ
90%-
89%
0
-
Conventional
200
400
Hybrid
600
800
1000
1200
1400
ime
Figure 3.24: Catalyst efficiencies for hybrid and conventional vehicles on FUDS
driving cycle. The graph shows there is little difference between the hybrid and
conventional vehicles conversion efficiency even though the hybrids engine is
turned off from time to time.
58
CHAPTER 4
ADVISOR AND ADDITIONAL
TRANSIENT MODELS
4.1 ADVISOR
EMISSIONS MODEL LIMITATIONS
ADVISOR has several limitations that prevent it from producing results without
qualifications. It is important to bolster ADVISOR where these faults exist to sustain and
strengthen those conclusions.
The two largest inadequacies are the (lack of) transient
modeling of engine behavior and the rudimentary chemical reaction modeling of the
catalytic converter.
4.1.1
TRANSIENT OPERATIONS
As stated in Chapter 3, transient engine behavior is an important factor in postcatalyst-light-off emissions creation. ADVISOR does not provide for such behavior. For
any given level of engine output (and speed), the emissions level is constant and the mix
of air to fuel is always stoichiometric. Therefore, the engine-out emissions are only a
function of torque (bmep) and engine speed whereas they should be a function of torque
(bmep), engine speed and the change in torque (bmep) with respect to time (Formula
4.1).
Engine out emissions = f(bmep, N, d(bmep)/dt)
(4.1)
Including the change in bmep will account for high level transients that account for a
considerable share of engine-out emissions, and are the cause of a majority of the tail
pipe emissions after the catalysts has fully warmed up.
4.1.2
CHEMICAL REACTIONS
The three-way catalyst model ADVISOR uses does not account for chemical
reactions between the exhaust gas (and contained emission compounds) and the catalytic
converter.
This interaction is non-essential for the quasi-steady-state engine out
59
emissions modeling currently used. However, to properly show the effects of transients,
the model must account for air-to-fuel ratios other than stoichiometric (X =1).
Furthermore, the conversion efficiency calculated in ADVISOR is only a function of
catalyst temperature.
Modem day catalytic conversion efficiencies are functions of
several variables, including temperature, space velocity, age, and oxygen storage
capability.
Hence, it will be important to incorporate such parameters into the new
model.
4.2 CHANGES MADE TO ADVISOR
Several code modifications were made to the ADVISOR program. The following
section (with supporting figures) is supplied for those who wish to modify ADVISOR in
the same fashion. These revisions were done in the simple block diagram models, which
are the underlying backbone of the ADVISOR code.
4.2.1
AIR-FUEL RATIO
To account for transient engine behavior, it was necessary to use a parameter,
which as a function of the change in bmep with respect to time, could be used to alter
engine-out emissions.
Creating new 3-dimensional matrices indexed by all three
parameters in Formula 4.1 would be a sizable, if not impossible task for a simple initial
investigation.
ADVISOR calculates emissions based on a specified stoichiometric relative airfuel ratio (herein referred to as lambda or X). Ideally, when X=1, the fuel is completely
converted to the oxidized byproducts of combustion. When the engine is called upon for
more or less power, a small excursion away from stoichiometry occurs and the engine
bums fuel-lean (X>1) or fuel-rich (k<1).
Figure 4.1 shows the change in bmep with respect to time and its relationships to
X. A perfect (ideal) engine would always operate at stoichiometry where emissions are
minimized. However, due to engine transients, it is impossible to do so. For example, if
a vehicle reduces its velocity (i.e. slows down), there is less demand from the engine.
There is, however, a short (<1 second) lag time when there exists a "puddle" of fuel in
the intake port even though the quantity of air is reduced, thus making the mixture fuel-
60
rich (X<1), when there is no compensation.
The same reasoning applies to a positive
engine transient, when the mixture, with no compensation, becomes is fuel-lean (X>1).
Modem control systems, however, compensate these fluctuations of X thus requiring
large transients to produce changes from stoichiometry.
Data in Figure 4.2 relates d(bmep)/dt to X. from an uncompensated fuel metering
Five separate curve fits were
This represents the worst-case scenario.
system.
implemented into the fuel converter model of ADVISOR (Figure 4.3). They include
uncompensated,
1 compensated, I compensated, full compensation and perfect
compensation (stoichiometric). The worst case scenario (Figure 4.2) is considered the
uncompensated curve fit. The
the uncompensated.
and - compensated conditions are simple scaled fits of
For example, the
2
compensated scenario implies at any given
bmep, the difference from stoichiometry (AX) will be half that of the uncompensated
scenario. Additionally, the full compensation fit implies that for small changes in bmep,
the fuel metering system will react in time to prevent any excursions from stoichiometry.
However, as the magnitude of transients increase, changes in X will occur. Each of these
compensations will be compared with the stoichiometric to see the effect of transients on
emissions.
Determining X from a 1 second time step change in bmep led to anomalies in the
results due to simulation-related jumps in the d(bmep)/dt. To make corrections for this
phenomenon, a time-averaged relative air-fuel ratio was found using the past three timestep values for changes in X, as shown in Formula 4.2:
A
1A
1
2
4
(4.2)
3
14
where the change in X is not the change in time-step, but the change from stoichiometry.
61
4.2.2
EMISSIONS CALIBRATION
Engine-out emissions vary with respect to changes in X. Figure 4.4 shows the
general trend of the three emissions tested and simulated: NOx, HC, and CO. The first
step was to create curve fits from general data for each emissions type with respect to X.
Nitric oxide and nitrogen dioxide combined vary as a parabola with respect to X.
Data taken8 provides Figure 4.5. From the two curve-fits (Formulas 4.3 and 4.4), a
lambda based emissions adjustment factor (EAF) is found and the true transient
emissions are found using Formula 4.5.
If X < 1: EAF = 73.517X3 - 174.15X2 + 137.54X - 36.021
(4.3)
EAF = -36.827X 2 + 81.928X - 44.089
(4.4)
If X
1:
E-O EmissionsTransient = EAF * E-O Emissionsoriginai
(4.5)
Similarly, the curve-fits (Formulas 4.6 and 4.7) for carbon monoxide are found using the
data in Figure 4.69. There is a simple linear regression when the mixture is fuel-rich
which tails off near stoichiometry.
EAF = -31X + 31.8
(4.6)
1: EAF = 6.551E8e20 592
(4.7)
If X < 1:
If X
Hydrocarbons behave in a linear fashion with respect to X as shown in Figure 4.71". The
EAF for HC is shown below in Formula 4.8.
For all X: EAF = -2.5X + 3.5
(4.8)
Figure 4.8 shows the ADVISOR Simulink block diagram added to adjust hot engine-out
emissions for transient behavior using the curve fits from above to find the EAF and thus
the new emission levels.
62
4.3 NTWC TRANSIENT MODELING PROGRAM
Although ADVISOR was configured to compute transient engine-out emissions,
no provisions could easily be added for transient behavior in the catalytic converter. It
was then essential to find a simulation code, which properly analyzed the transient
emissions with consideration for the transient behavior of a catalyst. NTWC11 ' 12 was
selected for its robust calculation of catalytic transient behavior.
4.3.1
MODELING METHOD
The model, similar to ADVISOR, uses a transient heat transfer simulation
technique with a transient catalyst chemical kinetic mechanism, which includes
provisions for oxygen storage chemistry.
Specifically, the governing equations provide
for convective heat and mass transfer from the exhaust gas to the catalytic converter with
heat conduction through the converter itself and losses to the surrounding environment,
the reactions occurring on the converter's surface and the capacity to store oxygen under
fuel-lean conditions and release it under fuel-rich conditions. From the temperature and
species time derivatives come the reaction scheme, which, using those reactions, produce
rates of catalysis for the system, thus providing the level of emission reduction.
The oxygen storage model (See Chapter 5) allows for proper representation of the
modern three-way catalyst. Excess oxygen increases the conversion efficiency for CO,
which produces other elements, which in turn, increase the conversion efficiency for
NOx.
4.3.2
INTEGRATION WITH ADVISOR
NTWC uses engine-out emissions levels, exhaust gas flow rate and temperature
and relative air-fuel ratio as inputs.
This data is obtained through ADVISOR for a
specific vehicle configuration per time-step.
It is then fed into NTWC, which
independently calculates transient tail-pipe emissions.
Because NTWC does not
calculate the heat and temperature losses between the catalytic converter and the engine
exhaust manifold, the input temperature is that of the exhaust gas in the pipes, which is
calculated by ADVISOR. Figure 4.9 shows the interaction between the two simulation
codes as they apply to the actual vehicle setup.
63
4.3.3
LIMITATIONS
Several limitations exist with the current modeling setup (of ADVISOR
NTWC).
-
First, with all "corrected" programs, there are some integration issues, the
largest of which is the actual temperature of the exhaust gas as it enters the catalytic
converter. ADVISOR does not calculate this value, but rather estimates it using basic
heat transfer principles.
Furthermore, there is no feedback between ADVISOR and NTWC. Because of
programming issues, the complete integration was not possible and the temperatures and
mass flow rates coming from ADVISOR are not bounded by NTWC.
Additionally, ADVISOR's calculations of transient emissions is merely a
regression fit as opposed to an actual calculation.
Unfortunately, transient engine-out
emissions are significantly hard to model in a general sense, and require a great deal of
insight into a particular engine-vehicle configuration - something this study aimed to
prevent.
Bmep, engine speed, and d(bmep)/dt are all factors in engine-out emissions
levels, but other variables such as ambient temperature, fuel injection control logic and
air quality also have an impact on emissions production.
Although these (and other) limitations provide some uncertainty and error, they
do not impede the general characteristics of the results nor reduce significantly the
validity of the conclusions based on the results.
64
X=1
* "us,,
/
/
/
/
/
dbmep
dt
/
/
/,
/6...--
/
Ideal (Perfect) Compensation (X=1)
-
------... Best Real Compensation
- .
-
-
No Compensation
-
Figure 4.1: Ideal X vs. change in bmep with respect to time. Ideally, X would never stray
from stoichiometric. However, realistically, with no control compensation, the airfuel mixture will go lean when there is a request for power due to the slight time lag
and "under-injection" of fuel. Optimal compensation reduces the excursions from
X=1 for most transients except those too severe to offset.
Transient Behavior
as a function of uncompensated
A
200150
100
50
++
V~
0
6
0.7
0.8
0.9
e
1.1
1.2
1.3
14
E
10
*-100
y = 592.02x - 590.53
R2 = 0.8672
-150
X (uncompensated)
Figure 4.2: Real (data) X vs. change in bmep with respect to time. This shows the
relationship between an uncompensated X and transient engine behavior. When the
vehicle slows down, there is a "puddle" of excess fuel leading to a rich mixture.
65
Compensations
300
200
-
100
02
E
-100
-200
-300
0.6
-
0.8
Stoichiometric
-
Uncorrpensated -
1.4
1.2
1
1/4 -
1/2 -
Best
Figure 4.3: Various compensations (lambda models) made in ADVISOR.
The
uncompensated model assumes no feedback loop alters the air-fuel ratio. The 25%
and 50% models are simple slope changes to the uncompensated model whereas the
best compensation assumes a feedback loop which can control X between certain
transients (changes in tourque)
Emissions Behavior
0.7
0.8
0.9
1
1.1
1.2
1.3
x
NOx emis
H emrs -
-CO
ems
Figure 4.4: General characteristics of engine-out emissions with respect to X (note y-axis
not to relative scale).
66
Relative NO. Emissions vs. )
1.6
x
1.4
z0
1.2
1
0
0a
0
y = 73.517x 3
0.8
.0
a.
'
0
-
y = -36.827x 2 + 81.928x - 44.089
R 2 = 0.9804
-
0.6
:
N
174.15x 2 + 137.54x - 3 6.021
R 2 = 0.9797
04
0
0.2
0.9
0.8
0.7
0.6
1.3
x
Lower End -u-- Upper End
-+--
1.2
1.1
1
Figure 4.5: Relative NO, emissions versus X with regression analysis. When runnning
rich, the engine-out emissions will be reduced. When slightly lean, more NOx will be
produced.
Relative CO Emissions vs. X
8
7
0
6
0 5
0
0
0.
E
w
0
y = -31x + 31.8
R 2 =1
4
3
0
0
I.-
1x
y = 6.55 1E+08e -2.059EO
2
R = 9.921E-01
2
1
0
I-
0.7
-- +- Lower End -=-
0.8
Upper End
0.9
1
1.1
1.2
x
Figure 4.6: Relative CO emissions versus X with regression analysis. When runnning
rich, the engine-out emissions will be significanlty more.. When lean, less CO will be
produced.
67
Relative HC Emissions vs. X
1.6
1.4
1.2 1
0
y = -2.5x + 3.5
0
1
R2
=1
0.8
0
w
0
a0.
0.6
0.4
0.2
0 I0.7
0.8
0.9
1.1
1
1.2
1.3
Figure 4.7: Relative HC emissions versus X with regression analysis. The linear data
shows there will be more HC engine-out emissions when the air-fuel mixture is rich
(too much fuel to burn) and less when it is lean.
h., \
lam hcremiscorr
HC Lambda Corr
lam-co-emis-corr
CO Lambda Corr
m-emiscorr
am-noxemiscorr
Lambda emis adj
lambda
N%
NOx Lambda Corr
F
Mux
PM Corr
Figure 4.8: Lambda based emissions adjustment factor (EAF) Simulink model. Using
lambda found based on chosen compensation, the engine-out emissions correction
factor is calculated.
68
NTWC input:
utput:
Seissions
ncxhaust
Transient
emissions levels
Texhaust
T
Texhaust
-T-
-- - - - - - - - - - -
Engine
(Fuel
Catoy,
Pipes
Converter)
Engine-Out Data:
Additional Data:
(from ADVISOR)
emissions
Texhaust
Muffler/ Tail-pipe
(from ADVISOR)
'------------------
nfexhaust
Texhaust
-l
- - - - - - - -
Figure 4.9: ADVIOSR - NTWC visual connection. ADVISOR's results at the catalytic
converter's entrance are fed into NTWC and analyzed.
69
70
CHAPTER 5
ADVISOR-NTWC
HYBRID/CONVENTIONAL
TRANSIENT MODELING
5.1
BACKGROUND
Using the "combination" ADVISOR-NTWC simulation code, the effect of
transients can be seen using perfect compensation (stoichiometric) as the baseline (see
Figure 4.3). Four test cases were made against the baseline as degrees of compensation.
They include a worst-case (no compensation),
Acompensation,
2
compensation, and full
compensation. These were made for both the conventional vehicle and hybrid vehicle
using the full 1400 seconds of the Federal Urban Driving Schedule (FUDS).
Additionally, these results are compared to those provided by the ADVISOR model.
5.2 COMPARISON
As stated in Chapter 4, two main factors affect the conversion efficiency and thus
tail-pipe emissions during transients.
These are the engine-out emissions, which are
affected by X, and the reductions in emissions achieved through oxygen storage. Table
5.1 shows the emissions levels for each test in grams per mile, the difference between
methods and vehicles and changes from stoichiometry modeling results.
5.2.1
CO PRODUCTION
CO is a by-product of combustion and is thus closely related to the amount of
excess fuel supplied to the engine. When the air-fuel mixture is lean, engine-out CO
emissions are significantly less than at or above stoichiometry.
Hence, again only
positive transients harmfully affect the magnitude of engine-out CO emissions.
While an engine is running lean, a ceria coated catalyst stores oxygen as per
Formula 5.113:
02+ 2Ce 2 O 3 -+
4CeO 2
(5.1)
71
While running lean, CO is completely oxidized by the oxygen present in the air. When
the engine runs rich, excess CO is left unoxidized, thus the ceria (Formula 5.2) is reduced
and releases oxygen which is used to oxidize the remaining CO.
(5.2)
CO + 2CeO 2 -+ CO 2 + Ce 2O 3
Additionally, a CeO 2 catalyst is a good steam reforming catalyst, and thus catalyzes the
reactions of CO and HC with H2 0 during rich conditions (Formula 5.3).14
CO + H 2 0 -+ H 2 + CO 2
5.2.2
(5.3)
NOx PRODUCTION
NO, is an additional by-product of combustion. It has a unique relationship to
changes in air-fuel mixtures. The stoichiometry point is just rich of the peak in the EAF
(see Figure 4.5). Thus, under more rich conditions, fewer engine-out emissions will be
produced, while slightly lean conditions will lead to an increase in NOx. Significantly
lean conditions will also lead to a reduction in such emissions.
When the engine runs lean and oxygen is being replenished, there is an increased
availability of reactant sites, which allows for further reduction of excess NOx. Also, the
H 2 produced (Formula 5.3) allows for additional reduction of NOx by producing N2 and
H 20 from H 2 (Formula 5.4).
NO, + xH 2 -- N 2 + xH 2O
5.2.3
(5.4)
HC PRODUCTION
HC production occurs when the fuel in the combustion chamber is not fully burnt.
As air-fuel conditions go rich, HC emissions increase.
Inversely, if less than
stoichiometric fuel is available, it will all burn, leaving little or no remnants. Therefore,
with increased positive transients come excursions into lean air-fuel mixtures and thus
less HC emissions.
72
Excess oxygen does not affect the tail-pipe emissions.
However, unburned
hydrocarbons may react with water further reducing them the H2 gas (Formula 5.5).
CxHy + 2H 20
-+
(2+y/2)H 2 + xCO 2
(5.5)
5.3 HYBRID CONFIGURATION RESULTS
The hybrid vehicle's engine rarely slows down (negative d(bmep)/dt). Instead of
reducing its speed (and thus output) during a decrease in vehicle speed, it shuts down.
Therefore, there are more increases in bmep than decreases, thus it is most often lean
(Figure 5.1). Consequently, according to Figures 4.5 - 4.7, the engine-out emissions will
be greater for NOx and less for CO and HC for any of the compensation routines.
Additionally, the oxygen storage capability increases the conversion efficiency for the
unburned HC and CO produced (Table 5.1).
As compensation level decreases, lean excursions by X become greater in
magnitude. Thus, the amount of engine-out HC emissions diminishes. Oxygen storage,
however, has little effect on such emissions as it is seen in Table 5.1. At stoichiometry,
the hybrid vehicle produces .0796 grams per mile, whereas it produces 12% less during
the no-compensation transient run. As the compensation increases, the vehicle performs
closer to stoichiometry and has worse HC emissions (because there are fewer lean
excursions). This would imply the hybrid vehicle, in terms of HC emissions, performs
better during transient behavior because it rarely "overfuels" the engine which causes
high HC emissions.
Additionally, NTWC results compare closely to ADVISOR's
outcomes implying little or no effect of oxygen storage on HC emissions.
The hybrid's CO production also gets worse with better compensation, which is
attributed to the same phenomenon as HC emissions.
As X goes more lean (less
compensation), there is a significant reduction in engine-out emissions. Additionally, the
oxygen storage capacity decreases CO emissions as the compensation gets better.
Because these two trends are opposite, it is necessary to determine which has the bigger
effect. Although the difference between no compensation and best compensation is 7%
or .12 grams per mile, the worst compensation is still 5% less than stoichiometry,
73
implying the reduction in engine out emissions wins over the oxygen storage capacity of
the catalyst.
NO, emissions become better as compensation improves.
Because of lean
conditions, engine-out emissions levels grow with decreasing compensation.
compensation, there is -30% more emissions than at stoichiometry.
At no
As compensation
improves, NTWC predicts better improvements in emission levels over ADVISOR due to
the further reduction of NO, via the steam reformation and increased reaction sites. For
example, NTWC predicts the no compensation levels to be 24% higher than
stoichiometric as opposed to ADVISOR's 29% increase.
5.4 CONVENTIONAL CONFIGURATION RESULTS
The conventional vehicle's performance over the driving cycle indicates an equal
number of excursions from X, both rich and lean (Figure 5.2).
Thus, the results from
NTWC come closer to those of ADVISOR.
Hydrocarbon
emissions
are
worse
stoichiometry, which should be expected.
for
all
compensations
compared
to
Best compensation emission levels are 6%
higher than stoichiometric levels whereas no compensation is 13% higher. Again, since
oxygen storage plays a relatively small role in HC emissions reduction, there is little
difference between ADVISOR and NTWC.
CO and NOx production for the conventional vehicle also improves with
compensation. Oxygen storage capabilities of the catalyst are key in decreased tail-pipe
emissions. The excursions away from X come close to canceling the effects of changes in
engine-out emissions making the NTWC marginally (-5-10%) better across the board.
5.5 HYBRID AND CONVENTIONAL COMPARISON
5.5.1
OVERALL
The hybrid vehicle produces fewer emissions. HC emissions levels are roughly
40% of conventional vehicles. CO emissions are between 20 and 30% better. There is a
5x reduction in NOx emissions.
Although these are absolute levels (g/mile) and not
normalized for engine size (brake specific emissions), the hybrid still offers a significant
reduction in total vehicle emissions.
74
5.5.2
LIMITATIONS - STARTUP AND SHTDOWN
The modeling methodology used does not necessarily account for the increase in
HC emissions created during the significant number of engine starts the hybrid performs.
The hybrid incurs 55 (Figure 3.22) hot engine restarts over the entire FUDS driving
cycle. If the initial fuel injected into the combustion chamber is larger than necessary (by
10 mg), a total of 0.55 grams extra would be produced. Normalized by distance, an extra
0.73 grams/mile in engine-out emissions would occur. Although some may be catalyzed,
more tailpipe emissions would occur.
5.6 CONCLUSIONS
5.6.1
TRANSIENT EFFECT
Transient emissions
do increase the
levels for most of the scenarios
(compensations) with the exception of HC and CO hybrid emissions. In these two cases,
the reduced emissions levels are explainable. HC emissions are reduced in the hybrid
because less fuel is being burned and thus, less fuel is left in crevices and the combustion
chamber (which plays a role in the magnitude of HC emissions). Additionally, there is an
excess amount of air (oxygen) which implies the HC's are fully burned and there is
excess oxygen to further oxidize any additional CO emissions.
Figure 5.315 shows the general characteristics of a TWC with respect to X (without
oxygen storage). These curves agree with the findings in Table 5.1. The conventional
configuration incurs minor increase in emissions levels when a best-fit compensation is
applied. HC emissions are 6% higher than ideal compensation while NO, is a mere 4%
greater. Additionally, the hybrid's NOx best-compensation levels are also only slightly
greater than stoichiometry. This implies the transients incurred do provide additional
emissions production, but not at an extreme level when compared to the simple
stoichiometric case runs.
5.6.2
HYBRID/CONVENTIONAL COMPARISON
The Hybrid configuration's emissions levels are within an ULEV rating for CO
and NO, emissions and between ULEV and LEV for HC emissions. They range between
20 and 65% of their conventional counterparts. The hybrid has significant improvements
75
in NOx and HC emissions with smaller savings in CO levels. This provides the needed
data to prove the less severe transient behavior of the hybrid provides the much-needed
emissions savings to qualify as a LEV.
5.6.3
ADVISOR VERSUS NTWC
ADVISOR and NTWC emissions predictions vary slightly.
difference is the estimation of CO emissions levels.
The only sizble
Although ADVISOR lacks the
oxygen storage model of NTWC, both use a similar approach to quantifying temperatures
and heat transfers to and from the exhaust system. HC and NOx emission levels are
forecasted within 15% of each other. Thus, using NTWC's oxygen storage model just
adds more exoneration to an already robust emissions calculation code.
76
Table 5.1: Emissions results for various compensations and changes from stoichiometry. The difference in results between ADVISOR
and NTWC is minimal, thus bolstering ADVISOR's methodology. Additionally, NTWC predicts decreasing emissions as
compensation improves. Furthermore, the hybrid configuration has significantly fewer emissions than its conventional
counterpart. HC emissions are less because the hybrid does not over-fuel when it slows down, rather it shuts the engine off. CO
emissions experience the same phenomenon while NOx emissions methodically decrease.
Compensation
Stoichiometry
None
25%
50%
Best
A% From Stoich
None
25%
50%
Best
ADVISOR
HC
CO
NOx
[g/mile]
0.0728 1.080 0.032
0.0653 0.746 0.041
0.0690 0.794 0.040
0.0705 0.823 0.039
0.0720 0.893 0.037
Hybrid
NTWC
% Difference
HC
CO
NOx
HC CO NOx
[g/mile]
[%]
0.0796 1.689 0.031
9% 56% -5%
0.0701 1.599 0.038 7% 81% -1%
0.0728 1.672 0.036 6% 86% -8%
0.0747 1.714 0.034 6% 83% -12%
0.0751 1.721 0.032 4% 77% -12%
-10%
-5%
-3%
-31%
-26%
-24%
29%
26%
20%
-12%
-9%
-6%
-5%
-1%
2%
24%
19%
11%
-1%
-17%
14%
-6%
2%
5%
ADVISOR
HC
CO
NOx
[g/mile]
0.1580 1.567 0.187
0.1812 2.034 0.239
0.1801 1.967 0.226
0.1793 1.917 0.211
0.1737 1.873 0.206
1
Conventional
NTWC
HC
CO
[g/mile]
0.1706 2.160
0.1934 2.609
0.1902 2.549
0.1855 2.451
0.1811 2.325
15%
14%
14%
30%
25%
22%
28%
21%
13%
13%
11%
9%
21%
18%
13%
10%
19%
10%
6%
8%
% Difference
NOx
HC
CO
NOx
[g/mile]
0.176
0.204
0.195
0.189
0.183
16%
11%
7%
4%1
8%
7%
6%
3%
4%
38%
28%
30%
28%
24%
-6%
-15%
-14%
-10%
-11%
I
X vs. Time
X vs. Time
No Compensation
1/4 Compensation
1.2
1.2
1.1
ee
0.9
1
0.9
0.8-
0.8
0
200
400
600
800
1000
1200
1400
0
200
400
Time [s]
1.2-
600
Time
800
X vs. Time
X vs. Time
1/2 Compensation
Best Compensation
1.2
1.1
1000
1200
1400
1000
1200
1400
[s]
1.1
S1
0.9-
0.9
0.8-
0.8
0
200
400
600
800
Time [s]
1000
1200
1400
0
200
400
600
800
Time [s]
Figure 5.1: Various X compensations for hybrid configuration for FUDS driving cycle. As compensation improves there are less
excursions from X. Additionally, note excursions are centered near stoichiometry, but there are more significant lean excursions.
1.2
X vs. Time
X vs. Time
No Compensation
1/4 Compensation
1.2
1.1
er-
1.1
1
U
S1
I
ilir 1w
rr Ill. 71
i di ,11IL Fi6L
111177
0.9
r
1111
7
0.9
0.8
0.8
0
200
400
600
800
1000
1200
1400
0
200
400
Time [s]
1.2
600
800
1000
1200
1400
1000
1200
1400
Time [s]
) vs. Time
X vs. Time
1/2 Compensation
Best Compensation
1.2-
lia
LALL i,
ir *
1-1717Trif
HO
0.9
0.9-
0.8
0.8
0
200
400
600
800
Time [s]
1000
1200
1400
0
200
400
600
800
Time [s]
Figure 5.2: Various X compensations for hybrid configuration for FUDS driving cycle. As compensation improves there are less
excursions from X. Additionally, excursions are centered about stoichiometry.
Conversion Efficiency vs. X
Typical TWC w/o 0 2 Storage
100
0
80
0
C
60
C:
0
40
0
20
0
ooo
0
0.98
0.97
-
NOx -
HC -
0.99
CO
1
1.01
1.02
1.03
Figure 5.3: Typical Conversion Efficiency for Three-Way Catalyst without oxygen storage
capabilities. NTWC, showing the same general trends, predicts CO to be worse for the
Conventional vehicle (which hovers about stoichiometry) and better for the hybrid (which stays
to the lean side). HC and NOx also compare with NTWC results.
80
CHAPTER 6
CONCLUSIONS
6.1 CONCLUSIONS
ADVISOR and NTWC, working separately and together, provide reasonable
explanations to the phenomena tested. ADVISOR, which is under constant improvement
by NREL, still needs additional work to provide unquestionable emission results, but
performs well in general trend evaluation and initial back-of-the-envelope calculations.
Although not a commercial code, NTWC does an excellent job predicting the effects of
oxygen storage and three-way catalyst performance under various operating conditions.
As shown in its supporting published documents, it forecasts transient emissions within a
slim margin of error. Unfortunately, actual hybrid emissions data was not available at the
time of this study to prove the validity of the ADVISOR/NTWC findings, but the author
feels this should not have an impact on the trends found.
6.1.1
STEADY-STATE EMISSION LEVELS
As stated in Chapter 3, the hybrid all-around outperforms its conventional
counterpart in steady-state emissions analysis. The four main reasons:
*
The hybrid uses its engine for less time, thus producing less total emissions.
" When the hybrid engine is producing emissions, it produces less because of its
reduced engine size.
*
The hybrid's engine operates in more efficient areas of the engine map, both
in terms of emissions and fuel economy.
*
Fewer and less severe transients are absorbed by the hybrid's engine.
Because the hybrid has an electric motor, it uses the engine 4 times less often than
its conventional counterpart. Thus, the hybrid produces emissions for only 15 seconds
versus a conventional engine's minute.
This alone provides for the much-needed
reduction of emissions. It is not, however, the only reason for emissions reduction.
81
Additionally, most hybrid vehicles have comparatively small engines.
For
example, the standard family sedan's engine in 2000 was about 2.8L 6 . Most hybrid
engines are ~to 3 the size. (The Toyota Prius, weighing in around 2800 pounds, has an
undersized 1.5L engine).
Hence, when the hybrid produces emissions, they are
considerably less, for only half the conventional vehicle's amount of fuel is being burned.
However, when hybrid emissions are investigated on a brake specific basis, they are
found to be marginally less than their conventional counterparts.
Again, a smaller
powerplant accounts for emissions reductions, but is not the crucial piece.
The crux to the hybrid vehicle's emissions savings is engine performance. The
hybrid's engine only operates when the motor cannot provide the power necessary to
achieve the requested speed. This occurs at higher vehicle speeds, where the engine is
more efficient. Because it does not operate at low speeds and torque, it does not produce
the high magnitude emissions that are created by conventional vehicles. For example, the
conventional vehicle creates significant emissions when idling and starting from a
stopped position. These two scenarios, which are not incurred by the hybrid, attribute a
sizable chunk to vehicle emissions. Without them, there is a significant reduction in tailpipe emissions.
A side-study was performed to investigate the possibility of catalyst cool-down as
a complication of hybrid engine shut-off periods.
The conversion efficiencies are
significantly related to catalyst temperature, which is related to gas flow through the
exhaust system. If the engine is off, then the catalyst temperature might fall and reduce
its efficiency. This, however, does not play a key role, since even the longest shut-down
periods (- 1 minute) result in minor (5*C) temperature changes in the catalytic converter.
Finally, the two main causes for increase in engine-out emissions (other than
normal driving) are the pre-light-off period where the catalyst has not fully warmed up
and severe transients caused sizable changes in required torque (both positive and
negative). Such transients are controlled by the hybrid, as it can allocate power from
both its motor and internal combustion engines. Thus, the hybrid incurs less of these
behavioral issues because the motor often damps such changes in requested torque.
82
6.1.2
TRANSIENT EMISSION LEVELS
ADVISOR is a quasi-steady-state empirical emissions model. It does not take
into account changes in air-fuel ratio nor is its catalyst model "chemically robust". It
lacks any sort of oxygen storage capacity of the catalyst. These issues required an indepth look at other programs that predict transient emissions and have oxygen storage
models. NTWC did an excellent job using data from ASDVISOR as input to further
calculate and corroborate hybrid and conventional emission levels.
Transient analysis of the hybrid vehicle showed improvements in HC and CO
emissions. Because the hybrid rarely decelerates (it turns the engine off), its transient are
often lean. Lean excursions provide a reduction in HC emissions because there exists
more air than needed to completely bum the fuel. Additionally, under lean conditions,
there is an excess of oxygen needed to oxidize CO further reducing the emissions. NO,
emissions do increase, but only by 5% under a good air-fuel compensation regime. The
conventional vehicle incurred an equal distribution of lean and rich excursions, and thus,
increased emissions levels across the board.
Although emissions improved with
improved compensation, they still exceeded the stoichiometric levels.
The analysis, however, did not include a specific investigation of the extensive
startup and shutdown of the engine. Such an investigation may show the true extent of
HC emissions. However, back of the envelope calculations show these emission levels
may vary at most, 30-40%. Thus, to find general trends, it was not necessary to perform
such an inquiry.
6.1.3
OVERALL
Overall, there were not significant changes between a reasonable "worst-case"
compensation level (50% compensation) and stoichiometry. At most, emissions were
15% higher using this transient analysis.
This implies there is minimal benefit in
considering transient behavior when performing basic initial feasibility calculations.
Additionally, when performing back-of-the envelope calculations, it is important to
properly size all vehicle components in ADVISOR, but it does not require actual data.
Although it is beneficial to have raw engine and motor numbers, resizing current modules
by power does not appreciably change results.
83
6.2 FUTURE TECHNOLOGIES
Hybrid vehicles have a bright future. Current versions are providing the basis by
which automotive makers will build upon. Although hybrids measure up to emissions
and fuel savings standards, they still require a great deal of work. The "ideal" hybrid that
can transport 6 people and their luggage is far off, but not unforeseeable.
Additional technologies, such as light-off catalysts, HC absorption chambers,
better controls will further the hybrid field. Improvements in catalyst technology are
being made daily and allow for better conventional vehicle emissions, which will be used
up until that day when the ideal hybrid is introduced.
84
85
86
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and Fuel Consumption", SAE Paper 971603, 1997.
6. "PNGV Program Plan", Partnership for a New Generation of Vehicles, 1995.
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8. Komiyama, K., Heywood, J. B., "Predicting NO, Emissions and Effects of Exhaust
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Fuel Type, Fuel Stoichiometry, and Hydrogen-to-Carbon Ratio and CO, NO, and HC
Exhaust Emissions", SAE Paper 730476, 1973.
10. IBID
11. Shen, H., Shamim, T., Sengupta, S.: "An Investigation of Catalytic Converter
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Simulations of Catalytic Converters during the Federal Test Procedure", Proceedings
of 33 rd National Heat Transfer Conference, 1999.
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Automotive Catalysis and OBD-II Catalyst Monitoring", SAE Paper 931034, 1993.
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