Piston Fuel Film Observations in an Optical Access GDI Engine
by
Rolf B. Karlsson
B.S., Mechanical Engineering
GMI Engineering & Management Institute, 1996
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
September 2000
@ 2000 Massachusetts Institute of Technology
All rights reserved
Signature of Author_
]6 epartment of Mechanical Engineering
August 18, 2000
Certified by
Jofin B. Heywood
Sun Jae Professor of Mechanical Engineering
Thesis Supervisor
Accepted by
Ain A. Sonin
Chairman, Department Committee on Graduate Students
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
SEP 2 0 2000
LIBRARIES
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2
Piston Fuel Film Observations in an Optical Access GDI Engine
by
Rolf B. Karlsson
Submitted to the Department of Mechanical Engineering
September 2000 in Partial Fulfillment of the Requirements
for the Degree of Master of Science in Mechanical Engineering
ABSTRACT
Increasing pressure for reduced fuel consumption, particularly in Japan and Europe, has
motivated the development of various engine concepts that improve part-load efficiency. One of
these concepts is Gasoline Direct Injection, or GDI. By injecting fuel under high pressure
directly into the combustion chamber later in the engine cycle and using various piston geometry
and combustion chamber airflow techniques, a locally stratified, globally lean mixture can be
used to obtain part load power with greater efficiency. Emissions concerns, however,
particularly hydrocarbons from unburned mixture and liquid films on the piston and cylinder
walls, are a major concern.
The purpose of this project was to observe the interaction of a GDI fuel spray with the
moving piston crown in an optical access engine, describe the interaction qualitatively, devise a
means to measure the residual fuel film on the piston quantitatively, and compare the results with
a Computational Fluid Dynamics (CFD) simulation. CFD was also used extensively to aid in the
design of the experimental apparatus, particularly in selecting the spray and injection parameters
that would allow stratified charge operation in the simple geometry available in the optical
engine.
The CFD simulations predicted that within the operating window for stratified charge
operation, between 1% and 4% of the injected fuel would remain on the piston as a liquid film.
The most influential parameter for the quantity of residual fuel film was the piston temperature.
The experimental results support the CFD simulations qualitatively, but the amount of fuel film
remaining on the piston is under-predicted. High-speed video footage shows a vigorous spray
impingement on the piston crown resulting in vapor production. The primary GDI combustion
process, a roughly premixed flame, is followed by a turbulent diffusion flame or "pool fire" in
the region of the liquid film once the piston is warm enough to produce vapor at a high enough
rate.
Thesis Advisor: Professor John B. Heywood
Title: Sun Jae Professor of Mechanical Engineering
3
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4
ACKNOWLEDGMENTS
I would like to thank my thesis advisor, Professor John B. Heywood, for his support and
guidance over the past year. David Schmidt, Wai Cheng, and Chris O'Brien deserve credit for
helping me set up an SGI workstation with KIVA-3V. Invaluable technical and administrative
support has been provided by Thane DeWitt, Susan Lutin, and Karla Stryker. I would especially
like to thank Benoist Thirouard for his insightful help with optical diagnostics and hightemperature adhesives. Other Sloan Lab residents who have helped to make my stay at MIT
more enjoyable include Brian Hallgren, Gary Landsberg, Brigitte Castaing, Matt Rublewski, and
my officemate, Ferran Ayala. On campus but not part of the Sloan Lab is Dr. James Bales of the
Edgerton Laboratory, to whom I am grateful for assistance with high-speed video imaging and
equipment. Thanks also go out to Brad VanDerWege, my predecessor, for his many emailed
responses to my questions on the optical engine.
The one person who has probably supported me the most during my time at MIT is my
wife, Amira. She cared enough for me to exchange our family, friends and home in Michigan for
the strange new environment of the Metro Boston area, which I don't think either of us was quite
prepared for.
Without her I would not have come to MIT in the first place, and it is her
continued support that has enabled me to get to where I am now.
Thanks also go to my family, for creating and shaping me as I am, the people who helped
me get into MIT including Ruth Fukuchi, Reggie Bell, Katie Jiang, Karim Nasr, and Fred Shen,
and General Motors for providing my Fellowship.
The research project itself was sponsored by the MIT Sloan Automotive Laboratory Engine
and Fuels Research Consortium, which includes DaimlerChrysler, Ford, General Motors,
ExxonMobil, Shell UK, and Volvo Car.
5
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6
TABLE OF CONTENTS
A BSTRA C T ................................................................................................................................................................. 3
A C KN O W LED GM EN TS .......................................................................................................................................... 5
TA BLE O F C O N TEN TS ........................................................................................................................................... 7
N OM EN CLA TU R E .................................................................................................................................................... 9
LIST OF TA BLES .................................................................................................................................................... 10
LIST O F FIGU R ES .................................................................................................................................................. 11
INTR OD U C TION ........................................................................................................................................... 13
1
1. 1
1.2
1.3
M OTIVATION ................................................................................................................................................. 13
BACKGROUND ............................................................................................................................................... 14
O BJECTIVES ................................................................................................................................................... 16
SIM U LA TION ................................................................................................................................................. 19
2
2.1
2.2
2.3
2.4
2.5
INTRODUCTION ..............................................................................................................................................
K fVA-3V M ODEL ..........................................................................................................................................
SIM ULATION RESULTS ...................................................................................................................................
LIQUID FUEL FILM ON THE PISTON ................................................................................................................
COM PARISON wrrH EXPERIMENTAL RESULTS ...............................................................................................
19
20
22
24
25
EX PERIM ENTA L SETU P............................................................................................................................. 33
3
3.1
3.2
3.3
3.4
INTRODUCTION ..............................................................................................................................................
O PTICAL ACCESS ENGINE ..............................................................................................................................
HIGH-SPEED V IDEO IM AGING ........................................................................................................................
W IDE BEAm LASER-INDUCED FLUORESCENCE .............................................................................................
33
33
35
35
EXPER IM EN TA L R ESU LTS ....................................................................................................................... 43
4
4.1
4.2
HIGH-SPEED V IDEO RESULTS U SING ALum iNum PISTON ............................................................................. .43
H IGH-SPEED VIDEO RESULTS U SING O PTICAL ACCESS PISTON ..................................................................... 46
SUM M A RY A N D C O N CLUSIO N S .............................................................................................................. 71
5
5.1
5.2
SIMULATION WORK ....................................................................................................................................... 71
H IGH-SPEED V IDEO IM AGING ........................................................................................................................ 71
5.3
CONCLUSIONS ............................................................................................................................................... 72
5.4
FUTURE W ORK .............................................................................................................................................. 72
R EFEREN CES.......................................................................................................................................................... 75
APPENDIX A - SAM PLE KIVA INPUT DECK .................................................................................................. 79
APPENDIX B - NEW ENGINE COMPONENT DRAWINGS ............................................................................ 83
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8
NOMENCLATURE
ATDCI
- after top dead center intake
BDC
BTDCF
CAD
CFD
DISI
E0I
GDI
HC
-
NOX
PDPA
PH
PLIF
PW
RPM
SMD
SOI
TDC
bottom dead center
before top dead center firing
crank angle degrees
computational fluid dynamics
direct injection, spark ignition
end of injection
gasoline direct injection
hydrocarbons
oxides of nitrogen
Phase-Doppler particle analysis
port fuel injection
- planar laser-induced fluorescence
-
pulse width
revolutions per minute
sauter mean diameter
start of injection
top dead center
9
LIST OF TABLES
Table 2.1: Kiva-3V model boundary conditions.......................................................................27
Table 3.1: Optical access engine parameters............................................................................
10
40
LIST OF FIGURES
Figure 2.1: Com putational grid resolution. ................................................................................
26
Figure 2.2: C om putational grid details......................................................................................
26
Figure 2.3: Injector orientation relative to intake flow.............................................................
27
Figure 2.4: Fuel vapor distribution at ignition timing, showing injector, ignition, and fuel film
locations in addition to stoichiometric boundary and centerline cross-section of mass
fractio n . .................................................................................................................................
28
Figure 2.5: 300 solid cone projected onto piston at 310' and 330' ATDCI with window area
overlaid. 600 hollow cone also shown in side view for reference, with horizontal lines
indicating the piston crown location marching up from 270' to 360' ATDCI. Top line
above views shows droplet travel over 10' at injected velocity........................................
28
Figure 2.6: Computed spray compared with LIF data under similar conditions.......................29
Figure 2.7: Liquid fuel film resulting from 450 injector orientation, showing film thickness of
order 0.01mm with peak of 0.02mm and film spreading beyond visible window area. ....... 29
Figure 2.8a: Crank angle history of injected fuel mass from EOI to top center for lower piston
temperature (350K). 9% of injected mass remains as film at 360 ..................................
30
Figure 2.8b: Crank angle history of injected fuel mass from EOI to top center for expected
average operating piston temperature (400K).. 4% of injected mass remains as film at 360'.
...............................................................................................................................................
30
Figure 2.8c: Crank angle history of injected fuel mass from EOI to top center for higher piston
temperature (420K). 1% of injected mass remains as film at 360 ..................................
30
Figure 2.9: Comparison of CFD spray development with high-speed video data for 300 SOI... 31
Figure 3.l a: Optical access engine diagram ...............................................................................
38
Figure 3.lb: Optical extension. ................................................................................................
39
Figure 3.2: New Bowditch-type piston allowing optical access from bottom of combustion
ch am b er.................................................................................................................................4
0
Figure 3.3: High-speed video and lighting arrangement..........................................................
41
Figure 3.4: Wide-beam laser induced fluorescence (LIF) concept...........................................
41
Figure 3.5: Optics setup for laser beam expansion. ..................................................................
42
Figure 4.1: Pressure traces indicating cyclic variation and combustion stability.......................51
Figure 4.2a: Reference sketch showing view angle across piston crown..................................
52
Figure 4.2b: Reference sketch showing view angle using mirror and optical access piston (note
the reversal of injector and spark orientation in this view)...............................................
52
Figure 4.3a: Illustration of injected spray structure....................................................................
53
Figure 4.3b: Illustration of spray interacting with liquid film (note: bottom view through mirror is
not same scale as top iso view, and mirror view has been reversed from other data to match
spray direction w ith top iso view ). .....................................................................................
53
Figure 4.3c: Illustration of end of spray interacting with liquid film ........................................
54
Figure 4.3d: Illustration of steady-state liquid fuel film coverage (note: bottom view through
mirror is not same scale as top iso view, and mirror view has been reversed from other data
to m atch spray direction with top iso view). ....................................................................
54
Figure 4.4: Cool and warm spray/piston interaction and combustion sequences (4 pages)..........55
Figure 4.5: Combustion in warm engine (82'C head) showing pool fire (3 pages).................. 59
11
Figure 4.6: Residue on piston imaged by Kodak Ektapro (high-speed video unit) in static engine.
62
...............................................................................................................................................
Figure 4.7: Residue on aluminum piston after stratified-charge operation...............................62
Figure 4.8: First spray impingement and combustion on cool piston (360K)...........................63
Figure 4.9: Film buildup after second injection on cool piston (360K), 360 . ..........................
63
Figure 4.10: Steady state spray impingement sequence and resulting fuel film coverage on cool
p iston (360 K ).........................................................................................................................64
Figure 4.11: Bottom view of first spray impingement on cool piston (360K)...........................65
Figure 4.12: Bottom view at 720' ATDCI for first 5 spray impingement cycles on cool piston.. 66
Figure 4.13: Fuel film at 360' ATDCI for 4 th and 5 th impingement cycles on cool piston..... 67
Figure 4.14: First spray impingement and combustion on warm piston (400K)............68
Figure 4.15: Steady state fuel film coverage on warm piston (400K)........................................69
Figure 4.16: Residue on optical access piston after stratified charge operation........................69
12
1 INTRODUCTION
1.1 MOTIVATION
While many new concepts for automotive propulsion are technically feasible today, such as
electric, fuel cell, and various hybrid systems, the gasoline engine is still by far the most
dominant source of power due to its combination of low cost and high power density. While the
efficiency and emissions of the gasoline internal combustion engine have improved significantly
since its invention, today's environmental concerns and the sheer quantity of engines in use still
warrant constant efforts toward improvement.
One concept that incorporates some benefits of diesel engines into the gasoline spark
ignition engine is gasoline direct injection (GDI), or direct-injection spark ignition (DISI). As
the point of fuel delivery has moved steadily closer to the combustion chamber, the control of
fuel delivery and mixture preparation has improved. The next step from the port fuel injected
(PFI) engine in showrooms today is to inject the fuel directly into the combustion chamber using
high-pressure fuel injectors. This improves control of the amount of fuel delivered, charge
cooling improves full load performance, and stratified charge operation at part load reduces both
pumping and heat transfer losses.
The stratified charge operating mode has produced the most dramatic improvements in
efficiency, with claims of increasing fuel economy roughly 20% under normal operation [18, 19,
23, 24, 25]. As a result, GDI engines have been in production for several years in Japan and,
more recently, Europe, where higher fuel prices contribute to a more significant portion of the
operating costs of vehicle ownership. In the United States, however, tighter requirements on
emissions have kept the GDI engine, which so far has exhibited higher hydrocarbon (HC) and
oxides of nitrogen (NOX) emissions, out of showrooms.
While lower fuel prices minimize
consumer demand for the increase in efficiency, a finite global supply of petroleum and
environmental concerns such as the greenhouse effect of carbon dioxide emissions do make GDI
engines desirable in the longer term. It is therefore desirable to understand and improve the
sources of emissions in GDI engines.
Hydrocarbon emissions in GDI engines in stratified charge mode are believed to stem
primarily from two sources (in homogenous operation, HC emissions can actually be reduced
13
during cold start thanks to more precise fuel metering [23]). The stratified charge preparation
depends on mixing a finely atomized high-pressure spray just enough to obtain an ignitable
pocket of mixture, surrounded by air or extremely lean mixture, at the ignition location. This
mixing process necessarily results in richer regions within the charge pocket, and leaner regions
at the periphery. Both of these regions are subject to incomplete combustion, which is the first
source of hydrocarbon emissions. Incomplete combustion has been studied with conventional
port fuel injection (PFI) engines. Most GDI designs, however, also target the fuel spray directly
onto the piston crown (typically incorporating some type of bowl to direct charge flow), claiming
it enhances mixing, vaporization, and control of charge motion. This intentional impingement of
fuel onto combustion chamber surfaces is thought to result in liquid fuel films that are the second
source of hydrocarbon emissions.
The behavior of in-cylinder liquid fuel films and their
contribution to hydrocarbon emissions has not been as thoroughly studied as incomplete
combustion, although Stanglmaier and Li have shown that for HC emissions purposes, the piston
is the second worst location- for liquid fuel [30, 31]. Of specific interest in this project is the
formation and measurement of liquid fuel films on the piston crown resulting from late injection
directly onto the piston during stratified charge operation.
1.2 BACKGROUND
The concept of injecting fuel directly into the combustion chamber of a spark ignition
engine only trailed the invention of the engine itself by a few decades. It is, however, only with
the recent development of high-pressure fuel injectors and advanced engine control techniques
that the advantages of direct injection have been realized. Pressure-swirl injectors in particular
have proved ideal for use in direct injection applications, since they produce a finely atomized
spray with suitable cone geometry. Techniques have also been developed to customize the cone
geometry of pressure-swirl injectors for specific applications [33, 34].
Virtually every GDI engine in commercial production today uses two operating modes, and
some occasional combinations of these two modes. The first mode is a simple extension of port
fuel injection, and involves direct injection of enough fuel to produce a homogenous,
stoichiometric mixture. Injection in this mode takes place during the intake valve open period in
order to maximize mixing of the fuel with the entering air. Primary benefits over PFI include
14
greater control over the quantity of fuel delivered since there is no fuel storage in the intake port,
and a charge cooling effect whereby the entering air is cooled by the evaporation of the injected
fuel droplets. The charge cooling increases volumetric efficiency and allows the compression
ratio to be raised as high as 12.5:1 [23], which in turn increases engine efficiency.
performance increases on the order of 10% are typically quoted [17, 23, 24].
Peak
The second
operating mode has less in common with PFI engines, and more in common with diesel engines.
Fuel injection takes place much later in the cycle, and the mixture is overall lean. Spark ignition,
however, requires more mixing in order to obtain a roughly stoichiometric mixture by the spark
plug, so injection is earlier than in diesel engines, though still quite late in the cycle, typically on
the order of 60 degrees before top dead center firing (BTDCF). The greatest challenge in this
mode has been obtaining a suitably mixed charge near the spark plug. Early designs injected the
fuel quite close to the spark plug and attempted to ignite the spray as it exited the injector nozzle;
this approach did not provide sufficient mixing, however, and combustion was poor and the
spark plug prone to fouling [3]. The current approach is to inject the fuel spray onto some type
of bowl in the piston crown, and also incorporate strong swirl or tumble airflow. The piston
geometry, airflow, and spray momentum all contribute to the mixing and transport of the fuel, the
end result ideally being a roughly stoichiometric pocket surrounded by air with minimal
quantities of fuel. The primary benefits of this operating mode are reduced pumping and heat
losses, and increased thermodynamic efficiency. Because the fuel is directly injected and the
mixture is lean overall, throttling of the intake is reduced or entirely eliminated for part load
operation. This greatly reduces pumping losses, which can be greater than brake power under
many common operating conditions. Heat losses are reduced because an insulating region of
mostly air, which has not participated in combustion, surrounds the burned charge. Finally, the
thermodynamic properties of a lean mixture (the higher ratio of specific heats, y) result in greater
efficiency of the conversion of fuel energy to useful work compared to a stoichiometric mixture
[1].
Furthermore, various combinations of these two primary operating modes have been
implemented to smooth the transition between them, reduce knock, and improve catalyst light-off
[18, 24, 27].
Despite all these advantages, however, GDI engines are currently only in limited
production, primarily in Japan and Europe where fuel consumption is more important and
15
emissions regulations are not as strict as in the United States. Various new catalytic converter
approaches are being developed to improve the emissions [18, 24], but in stratified charge mode,
where the greatest fuel consumption advantage lies, the overall lean mixture composition poses a
significant challenge to catalyst operation.
While all production GDI engines employ certain tactics to obtain a suitable stratified
charge, such as high-pressure injection, piston crown impingement, and higher vorticity airflow,
the details vary significantly from one application to the next. Mitsubishi, arguably one of the
foremost proponents of GDI, has implemented an intake design that generates strong reverse
tumble in a spherical piston bowl [24, 27]. This design is in relatively large production volume,
but has the drawback of taller packaging requirements compared with a conventional engine,
since the intake manifold extends up from the engine head rather than to the side.
Other
manufacturers have developed swirl systems with involute or other various piston bowl shapes
[18, 23, 32]. Air-assist injectors have also been advocated by Orbital and others [14, 25], as the
added mixing with air allows this type of direct injection to be retro-fitted to existing PFI engine
designs. It has been hypothesized that the difference of these many approaches indicates a lack
of complete understanding of the physics involved. The logic behind this argument is that if the
problem were completely understood, all designs would gravitate toward the single best solution.
The volume of research on GDI engines, particularly stratified charge preparation, also indicates
the presence of unanswered questions. While many studies have been and are being performed
on qualitative mixture preparation using planar laser-induced fluorescence (PLIF), few have
focused on the interaction between the fuel spray and the piston crown.
Even fewer have
attempted to measure the liquid fuel film remaining on the piston.
1.3 OBJECTIVES
The objectives of this project are to observe and describe qualitatively and, if possible,
quantitatively, the interaction between a high-pressure direct injection fuel spray and the moving
surface of the piston crown in a simplified but realistic GDI engine with optical access.
Specifically, high-speed video imaging will be used to observe the fuel spray/piston interaction,
and a laser-induced fluorescence technique will be designed to quantitatively measure the liquid
fuel film thickness on the piston crown. Furthermore, CFD simulations of the experimental
16
setup will be run, and the results from experiment and simulation will be individually evaluated
and compared to each other. The project results will increase the understanding of the fuel spray
interaction with the piston crown, aid designers of GDI engines in the pursuit of optimal systems,
and provide valuable feedback to the computational fluid dynamics community allowing
programmers to tune their spray impingement and wall-film models.
17
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18
2 SIMULATION
2.1 INTRODUCTION
Computational Fluid Dynamics, or CFD, has grown from an esoteric research area into an
everyday engineering tool in the last two decades. One primary enabler for this growth has been
the increase of computing power and decrease of computing cost. Equally important has been
the steady improvement of programming, allowing more geometrically accurate computational
grids, numerous approaches to turbulence modeling, and improved solution convergence time
and solution quality. Original CFD solutions typically involved the steady-state 2D flow around
an airfoil cross-section, and took months of computing time on a mainframe computer. Today,
complex transient 3D flows around detailed realistic geometry can be solved in a matter of days,
sometimes using machines with no more power than the personal desktop computers steadily
appearing in homes across the nation. Full heat transfer, moving grids, and multi-phasic flows
are all routinely implemented. The value of CFD as an engineering tool is incredible. Given the
geometry of a potential design, an engineer with suitable knowledge of the application (boundary
conditions and material properties) can evaluate the design long before the existence of prototype
hardware. Furthermore, greater insight into the peculiarities of the design's performance can be
gleaned from CFD than from physical testing, since the complete flow field and various fluxes
are inherent to the computed solution.
However, despite these many advantages of CFD
simulations, there are caveats. While any CFD software package can provide a solution, the
solution will only be as good as the inputs that generated it.
The program itself must be
physically sound and robust, the material properties must be accurate, and the boundary
conditions must be realistic and stable.
Furthermore, to obtain an exact solution, direct
numerical simulation (DNS) is required, which involves solving the Navier-Stokes equations
with a grid fine enough to capture the smallest scale of turbulent eddies. For the typical flow
around an automobile, for example, this would require a grid with many billions of cells, which
cannot easily be solved even with today's formidable available computing power. For this
reason, turbulence models have been developed which typically solve the Reynolds-Averaged
Navier-Stokes equations, using various two- or three-equation turbulence models to account for
the generation and dissipation of turbulence within the flow. While efficient and surprisingly
19
accurate, these turbulence models are not perfect and do not apply universally to every flow
situation. For this reason, a sample solution is typically validated with physical data for a class
of flows. Once the software and approach have been proven for one case, they can be applied
with reasonable confidence to similar cases.
2.2 KIVA-3V MODEL
Because CFD is a widely used tool in engine development, and no less so for GDI engine
development, it was desirable to have CFD results to compare with the experimental data. Since
few studies have attempted to validate the physics of spray-wall interaction under in-cylinder
conditions, this comparison would provide invaluable data either validating the approach used or
indicating required adjustments to the approach.
Another reason for using CFD in this project was as a design tool for the experimental
setup. The optical engine in question has been used at the MIT Sloan Automotive Laboratory for
several decades, but it has only recently been converted to a GDI engine and then only used
under homogenous operation with early injection. The engine's optical access design results in a
square cross-section and flat piston and head surfaces, preventing exact implementation of any
one of the current approaches to stratified mixture preparation. Furthermore, the concern existed
that if one of the existing approaches was specifically adopted, the results would only be relevant
for similar approaches. The variance of approaches has been discussed in section 1.2. For these
reasons, a simplified approach to stratified mixture preparation was implemented, in order to
make the results more generally applicable and less specific to any particular existing design.
The unprecedented approach limited the usefulness of lessons learned from existing designs,
however.
CFD was therefore used to model the stratified mixture preparation before major
engine modifications were made in order to obtain rough design specifications.
At the same
time, as mentioned, this type of application has not been well validated, so the engine
modifications were purposely designed to allow flexibility in several parameters to fine-tune
operation in stratified charge mode.
Kiva-3V was selected as the CFD code, being a widely available research code with a long
history in both direct injection (mostly diesel but recently gasoline as well) and port injection
engine applications, and a functional built-in spray and wall-film model. The bulk fluid (gas)
20
solution is Eulerian, meaning that the grid is stationary for a given geometry and the fluid
variables are solved for at each grid point. This is the approach used in virtually every CFD
package currently available. The spray (liquid) solution, however, is Lagrangian, meaning that
every material particle is tracked in space. This is necessary to accurately solve for the motion
and interaction of the extremely small droplets produced by modem fuel injectors. In order to
conserve computing resources, Kiva-3V lumps fuel droplets into parcels of droplets with
identical velocity, position, and temperature, the total number of which can be specified to
increase spray model accuracy or reduce solution time. In this manner, potentially millions of
droplets can be modeled with only a few thousand parcels being tracked, and still provide
reasonable accuracy. Typically the total number of parcels is in the range of 3000 to 5000; for
this project, 8000 parcels were used to provide increased accuracy. Sub-models are included to
allow the parcels to disperse, coalesce, and evaporate based on their position, velocity, size, and
environment. The fuel model is single-component, which may not be entirely representative of
gasoline in wall-impinging situations, but should serve the project well enough since
experimental setups often use iso-octane doped with single fluorescing hydrocarbons. The wallfilm treatment includes a splash model, which takes into account the impinging droplet
characteristics including Weber and Reynolds numbers. Based on this information, the droplet is
either reflected (with a random adjustment to velocity to account for the momentum exchanged
during the bounce) or contributes to the wall-film.
Kiva-3V does not account for primary
atomization, so the spray's Sauter Mean Radius (and other nozzle parameters such as cone angle)
must be specified based on existing measurements. Finally, Saffman lift forces are not modeled,
so the droplets' aerodynamic behavior does not vary as they approach the piston crown.
The grid builtfor the Kiva-3V modeling contains a maximum of approximately 107,000
cells at bottom dead center. The collapsing and expansion of the combustion chamber (and
lifting of valves) in Kiva-3V is treated by "snapping" layers of cells. The bottom layer of cells is
distorted as the piston or valve moves for each time step, and once the distortion exceeds a
specified fraction of the original cell height, the layer is removed or a new layer is added
depending on whether compression or expansion is taking place. The grid resolution and detail
are shown in Figure 2.1 and Figure 2.2. Only the intake port and valve are included in the model
since combustion and exhaust are not simulated.
21
In addition to the geometry of the optical access engine, various boundary conditions must
be prescribed for model simulations. The major parameters are listed in Table 2.1, and a typical
full input deck is provided for reference in Appendix A. The details of the engine can be found
in Chapter 3.
2.3 SIMULATION RESULTS
KIVA-3V was run on a Silicon Graphics 02 workstation with an R10000 processor and
128MB of memory. Typical running time for a case resuming from 250' ATDCI, simulating
injection and all fuel modeling, and completing at 360' ATDCI, was on the order of 3-4 days.
Post-processing was executed on the same machine using GMV.
Several scenarios were simulated to determine the operating window, if one indeed existed,
for late injection, stratified charge operation of the optical access engine.
As discussed in
Chapter 1, typical production GDI engines employ various piston bowl geometry and charge
vorticity effects to mix and transport the fuel to the point of ignition. The hypothesis for the
optical engine setup was that with the injector and spark plug located on opposite sides of the
combustion chamber, as opposed to a centrally located plug as in most production engines, the
injected fuel would travel farther before the point of ignition and mix sufficiently by diffusion
and the momentum of the spray itself, which would entrain substantial quantities of air. The
initial mixture preparation strategy was based on typical production injection timing, modified to
take the extra distance and spray's initial velocity into account. A few iterations were necessary
to fine-tune the strategy, but the CFD model predicts that an operating window does indeed exist
for stratified charge mode with the optical engine.
The intake stroke generates a mild forward tumble flow, which persists despite the corners
in the combustion chamber, which tend to be parasitic to both tumble and swirl. The tumble
becomes more pronounced during the compression stroke, when injection takes place. The most
suitable injection timing was found to be 290' after top dead center intake (ATDCI), or 700
BTDCF, which is roughly 10' earlier than typical production designs.
At this point in the
compression stroke, the tumble results in a mild flow across the piston crown surface,
perpendicular to the injector orientation. This presumably enhances the mixing of the fuel, and
also results in the center of the fuel mass distribution being slightly off-center once the fuel vapor
22
reaches the opposite side where the spark plug is located. Figure 2.3 illustrates the orientation of
the intake flow relative to the injector and ignition locations, and Figure 2.4 shows (from the
same isometric viewing direction) the resulting fuel vapor distribution at 350' ATDCI, roughly
when ignition would take place. Significant deviation of injection timing and geometry results in
under- or over-mixing of the fuel vapor, or unsuitable locations of the ignitable mixture.
The fuel injector mounting angle was simulated at 450 and 30' inclined from vertical (the
previous spray behavior studies utilized a vertical injector mounting, since operation was
primarily homogenous). The rough orientation of the injector was based on the geometry of the
engine, positions of the injector and spark plug, and estimates of the fuel vapor transport as well
as the necessity to produce a liquid fuel film within the region of the piston crown observable
through the optical access design. Figure 2.5 shows the projection of a 300 solid cone onto the
piston crown at 310' and 330' ATDCI.
Within the limitations of fuel vapor transport, this
simple geometric study indicated that a narrow spray, later injection, or a lower orientation (such
as 300 from vertical instead of 450) would be necessary to result in impingement within the
observable area of the piston crown.
Initial simulations of the late injection indicated a strong collapse of the hollow cone,
primarily dependent on the ambient density (or pressure). This trend is supported by several
experimental studies [3, 21, 32, 35] including VanDerWege's previous work using the optical
engine, and is an improvement in spray modeling over previous versions of KIVA [21]. The
cone angle collapse is a result of the spray's momentum entraining the bulk fluid; with increasing
ambient density, the entraining flow has a more pronounced effect on the spray droplets. Also,
the smaller droplets of hollow cone injectors are more strongly effected by ambient flows.
Figure 2.6 shows a calculated spray compared to LIF data from VanDerWege under similar
conditions, confirming the spray results generated with the CFD model. It is worthwhile to
notice in the CFD prediction that the majority of droplet parcels closer to the axis are smaller,
having partially evaporated and thus becoming more susceptible to the entrained airflow.
The collapse of the spray under the higher pressures present during late injection provides
the small effective cone angle required to produce impingement in the area visible through the
piston window. The only potential concern was that at the 450 injector orientation, a fraction of
the resulting liquid fuel film would travel as a result of momentum from the impingement
23
location to an area beyond the visible area, as shown in figure 2.7. For this reason, a 300 injector
orientation was also simulated. The resulting liquid film was indeed more centrally located, but
the reduction of the spray's horizontal momentum was enough to prevent effective vapor
transport to the spark plug. For this reason, the 450 injector mounting was chosen.
2.4 LIQUID FUEL FILM ON THE PISTON
As mentioned earlier, one of the major advantages of CFD simulations is the ability to
examine the resulting solution in many different ways. For example, while the liquid droplets
collapse into a narrower angle that more closely resembles a solid cone, the majority of the
injected fuel remains at the outer regions of the spray in vapor form. The rapid vaporization of
the fuel is one of the advantages of pressure-swirl type injectors, and is a by-product of the
extremely small droplets produced. Kiva-3V provides information in the solution files listing the
mass of fuel present as liquid droplets, vapor, and wall films. Plotting this data versus crank
angle illustrates quite clearly that under typical operating conditions, by the end of injection the
majority of the fuel has already vaporized. As the spray travels through the hot, compressed air
in the combustion chamber, it continues to vaporize rapidly. Some droplets do survive long
enough to impinge on the piston, however. The piston temperature is expected to be on the order
of 400K during typical operation, while the Leidenfrost temperature (the point at which vapor
production prevents wall wetting; above this temperature droplets hover on a cushion of their
own vapor until they completely evaporate) of gasoline even at room temperature is closer to
600K [36]. Therefore, the droplets will form a liquid film on the piston. The piston temperature
is, however, a major variable in determining what fraction of the liquid fuel remains as a liquid
film. It should be reiterated at this point that this behavior is a result of the Kiva-3V sub-models
governing droplet impingement on walls and wall-film mechanics, and not necessarily
representative of the actual physics. It is instructive, nonetheless, to observe the variation in fuel
mass remaining as liquid film on the piston after spray/wall interaction. Figure 2.8 shows the
crank angle history of injected fuel mass from the end of injection to top center for three different
piston temperatures, breaking the mass down into liquid droplets, vapor, and liquid film on the
piston. The fuel mass history clearly illustrates that cooler piston temperatures result in more
fuel remaining as film; as the temperature is raised from 350K to 400K and then 420K, the
24
fraction of injected fuel remaining as film is reduced from 9% to 4% and then 1%. Closer
examination of the history also reveals that not only does the fuel evaporate more quickly from
the hotter surfaces, but also a smaller fraction is likely to remain on the surface following the
initial "splash."
It should also be mentioned that combustion and exhaust were not simulated, and that the
fuel film formed is representative only of a single injection onto a dry piston.
In reality,
combustion and exhaust will diminish the liquid film mass (liquid fuel on the piston is thought to
contribute directly to HC emissions [30, 31]), and each successive cycle will add a small fraction
of the injected fuel mass to the film. Potentially a steady-state equilibrium could be reached,
although cooling issues and varying fuel component volatility (the heavier components would be
more prone to remaining as film) place this beyond the scope of the current project.
2.5 COMPARISON WITH EXPERIMENTAL RESULTS
As mentioned earlier, while one motive for using CFD was as a design tool for the
experimental setup, another reason, which is perhaps more important in the long term, was to
compare the CFD simulations with actual experimental data.
High-speed video imaging
provides an excellent qualitative fuel droplet distribution picture which can be compared almost
directly with KIVA-3V results.
Figure 2.9 follows the development of the spray and the resulting fuel film as predicted by
CFD and observed with high-speed video on a dry 400K piston. The CFD simulation is for a
start of injection at 300' ATDCI and a pulse-width of 3.3ms, or 20', while the experimental data
uses a pulse-width of 4ms, or 240, in order to fire with more stability. The only other difference
is a slightly lower intake pressure for the experiment; the CFD simulation used an intake pressure
of 0.99bar while the experimental setup, though unthrottled, was 0.9bar. Exact matching of
spray development time is difficult since the CFD data was written every 10' while the highspeed video timing results in frames every
30,
so the high-speed images closest in terms of
injection timing are shown. Despite this, the qualitative features of the developing spray and
impingement region are in excellent agreement. Penetration rates, which can be quantified by the
time for the tip of the spray to reach the piston crown (just over 10'), are also in good agreement.
Only the fuel film area after spray interaction is underpredicted by CFD.
25
Figure 2.1: Computational grid resolution.
Figure 2.2: Computational grid details.
26
Boundary Condition
Ambient Temperature
Intake Manifold Pressure
Initial In-Cylinder Temperature
Cylinder Wall Temperature
Piston Temperature
Injected Droplet Sauter Mean Radius
Injected Droplet Initial Velocity
Ambient Injector Cone Angle (Outer)
Ambient Injector Cone Angle (Inner)
Start of Injection
Injection Duration
Table 2.1: Kiva-3V model boundary conditions.
End of injection, 310ATDC
Tracers
radius
(walls colored by
fuel mass fraction)
W 0-00325
-0.00293
-0.00261
-0.00228
-0.00196
-0.00164
-0,00131
-0.000988
(8 microns)
-0.000665
2
i-0.0003 Q
F1.82e-05
Figure 2.3: Injector orientation relative to intake flow.
27
Default Value
293 K
99 kPa
293 K
350 K
400 K
8 pm
80 m/s
500
250
2900 ATDCI
200 CAD (3.3ms pw)
0.03g fuel injected from 290-310ATDC onto 4001 piston
Fuel distribution at 350ATDC favorable for ignition
Global AFR = 38
Stoichiometric boundary
Fuel Mass Fraction
0.159
Rich
0.143
-0.127
Spark
Ocatlon
-0.111
Lean
-0.0952
1
-0.0793
or location
Lean
-0.0476
-0.0159
Figure 2.4: Fuel vapor distribution at ignition timing, showing injector, ignition, and fuel film
locations in addition to stoichiometric boundary and centerline cross-section of mass fraction.
Figure 2.5: 300 solid cone projected onto piston at 3100 and 3300 ATDCI with window area
overlaid. 600 hollow cone also shown in side view for reference, with horizontal lines indicating
the piston crown location marching up from 270* to 360' ATDCI. Top line above views shows
droplet travel over 100 at injected velocity.
28
60 (25 hollow) cone, Vinj=80 mis, halfway through 290-310
injection duration or 1.67ms ASOI (0.016 of0.03 grams)
(
V
14a
Tracers
LIF data,
VanDeawege
widar cone,
eariler 1nJ.)
(cm)
radius
.0027
.00244
-0.00217
-0.0019
-
-0.00163
o
-0.00137
-0.0011
a*"e
a
-0.000831
<-81microns
I'-0.000563
.000295
2.78e-0S
Figure 2.6: Computed spray compared with LIF data under similar conditions.
0.03g fuel Injected from 290-310ATDC
400K piston
350ATDC
(no ignition)
Film Height
(cm) 00 2
0.
Window outline superimpcsed
(0.02mm)
.0018
-0.0014
.0012
.001 (0.01mm)
.0008
-0.0006
-0.0004
-0.0002
M-
0
Figure 2.7: Liquid fuel film resulting from 450 injector orientation, showing film thickness of
order 0.01mm with peak of 0.02mm and film spreading beyond visible window area.
29
Fuel Distribution v. Crank Angle, Tpiston=350K
3.OQE-02
2.50E-02
2.OOE-02 J
c
1.50E-02
5.OOE-03
O.OOE+OO4"
0.00E+00
31ATDC
320ATDC
340ATDC
330ATDC
350ATDC
360ATDC
Figure 2.8a: Crank angle history of injected fuel mass from EOI to top center for lower piston
temperature (350K). 9% of injected mass remains as film at 360'.
Fuel Distribution v. Crank Angle, Tpiston=400K
3.OOE-02
2.50E-02
2.OQE-02
E 1.50E-02
1.OOE-02
5.OOE-03
O.OOE+00
31 OATDC
320ATDC
340ATDC
330ATDC
350ATDC
360ATDC
Figure 2.8b: Crank angle history of injected fuel mass from EOI to top center for expected
average operating piston temperature (400K). 4% of injected mass remains as film at 360'.
Fuel Distribution v. Crank Angle, Tpiston=420K
3.OQE-02
2.50E-02
2.OOE-02
1.50E-02
Vapo
1.OOE-02
5.OOE-03
O.OOE+00
31 OATDC
320ATDC
330ATDC
340ATDC
350ATDC
360ATDC
Figure 2.8c: Crank angle history of injected fuel mass from EOI to top center for higher piston
temperature (420K). 1% of injected mass remains as film at 3600.
30
Figure 2.9: Comparison of CFD spray development with high-speed video data for 300' SOI.
310
3200
30*
360
340
31
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32
3 EXPERIMENTAL SETUP
3.1 INTRODUCTION
The heart of the experimental portion of this project is the optical access engine, which has
been modified over the years to shed light on any number of internal combustion engine
phenomena. Most recently it has been operated as a homogenous charge direct injection engine
with optical access on three walls. To implement stratified charge operation and a view of the
combustion chamber from below, several modifications were made to the engine. High-speed
video imaging was used to qualitatively study the entire spray/piston interaction. A wide-beam
form of laser-induced fluorescence is also designed to measure the fuel film thickness
quantitatively, although equipment limitations placed validation of this technique beyond the
timeframe of this project. In the future, planar laser-induced fluorescence and phase-Doppler
particle analysis (as in [3]) could also be used to further explore stratified GDI sprays interacting
with the piston in this experimental setup.
3.2 OPTICAL ACCESS ENGINE
The optical access engine consists of an optical extension added to a Cooperative Fuels
Research (CFR) Engine, which is a single-cylinder engine with the capability of adjustable
compression ratio. An illustration of the optical access engine is shown in Figure 3.1, and the
engine specifications are listed in Table 3.1. The square cross-section optical extension allows
complete optical access through two opposed quartz walls and limited optical access on a third
wall through a quartz window. The quartz walls and windows are sealed with sheet silicone
rubber gaskets attached with high-temperature RTV. A fourth mode of optical access was added
for this project, by designing and constructing a Bowditch-type piston which allows optical
access through the piston via a 450 mirror and a quartz window held in the piston crown with
two-component high-temperature silicone RTV adhesive (Dow Coming Sylgard* 275).
The
two-component RTV cured properties (elastomeric) allow it to absorb differential thermal
expansion of the quartz and aluminum, unlike the fairly rigid properties of high-temperature
filled epoxy products.
Figure 3.2 illustrates the new optical access piston assembly with a
photograph of the major components, and complete drawings for the piston components are
33
included in Appendix B. A UV-coated mirror is fixed with epoxy to a supporting plate, which is
held in the piston with 6 machine screws. A thermal stress analysis was conducted for the mirror
and plate assembly since the mirror has poor tensile properties; the results indicated that the
mirror was at risk of fracture should the operating temperatures rise from the equilibrium
temperature by more than approximately 20'C, depending on the cured epoxy material
properties. Cooling below equilibrium was not expected to pose a problem, since the higher
coefficient of thermal expansion of the steel plate would place the mirror under compression
rather than tension. Therefore the mirror and plate assembly was cured at roughly 75'C, so that
typical operation would result in temperatures cooler than the assembly equilibrium point. As an
additional precaution the epoxy used (Emerson & Cuming Eccobond® 45/Catalyst 15) was
selected for high-grade adhesion and strength yet semi-rigid cured properties in order to both
provide positional stability for the mirror and absorb some of the thermal stresses.
A
thermocouple was also embedded in the exposed face of the quartz window using the twocomponent high temperature RTV (which has a thermal conductivity similar to quartz) in order
to obtain a piston temperature measurement.
The thermocouple location is schematically
indicated in Figure 3.4. Wiring for the thermocouple was embedded in the RTV holding the
window, and then run through small holes drilled in the piston behind the mirror. A quickdisconnect was provided in the cavity behind the mirror in order to easily replace the segment of
wire subject to reciprocating motion.
Sealing of the square cross-section is accomplished with three sets of overlapping graphite
bars, which provide a sliding seal without oil lubrication.
Teflon bars have been tested
previously but were found to leave a residue, which not only impedes visual access but fluoresces
under UV laser excitation. The limitations of sealing with graphite bars and silicone gaskets
prevent compression ratios significantly higher than 8:1.
The engine is uncooled, and can therefore only be run for a period of several minutes before
the silicone gaskets sealing the quartz walls are compromised. Engine speed is controlled with
an AC electric motor with the capability to provide brake load, although the optical access engine
experiences such high friction that the motor rarely needs to absorb energy.
Based on the CFD simulation results, a 50' hollow-cone fuel injector with nominal flow
rate of 9.2 g/s was selected (Siemens Automotive SN 33623, measured cone angle 56' and
34
measured flow rate 9.14 g/s) and mounted at 45'. Parameters were matched with one of the
injectors provided to Sandia National Laboratories to facilitate future comparisons. An insert
was necessary to mount the injector at an angle; a drawing is provided in Appendix B. Fuel
injection pressure was 5Mpa, provided by a hydraulic accumulator (Tobul 3AT30-2-S) and highpressure nitrogen. A more thorough description of the fuel system and filling procedure can be
found in [3].
3.3 HIGH-SPEED VIDEO IMAGING
High-speed video was used in order to obtain qualitative imaging of the spray-piston
interaction event without the limitations of PLLF, which would restrict the view to a single crosssection as well as bringing into play the various fluorescence variables discussed in the following
section.
The extensive modifications to the engine allow excellent optical access for both
lighting and camera viewing angles, making it a suitable candidate for high-speed video in
addition to LIF.
A Kodak Ektapro system was used, which contains enough memory to store 2,300 images,
each containing 192x239 8-bit pixels (256 shades of gray), from its NMOS array in 2.3 seconds.
This provides 1.Orms temporal resolution, which can be increased by storing images with half
resolution or less. At 1000 RPM this translates to 6' of crank resolution for full size images and
30 resolution for half-height images, which were the two frame sizes used. A 90mm f/2.5 camera
lens was used with the digital array. In order to obtain adequate lighting with minimal direct
reflection for the extremely short exposure times, a 600W halogen light was arranged as shown
in Figure 3.3. Some footage was also taken through the new piston window, using the mirror.
Similar component spacing was used for this setup (the camera distance was reduced by
approximately 40cm), and the camera was simply placed on the other side (exhaust) of the engine
and aimed at the mirror.
3.4 WIDE BEAM LASER-INDUCED FLUORESCENCE
Planar laser-induced fluorescence (PLIIF) has been used extensively in the past few years to
obtain qualitative images of fuel vapor and liquid distribution in various stages of mixture
preparation. With optical access, a sheet of laser light with suitable wavelength is introduced to a
35
cross-section of interest, and a gate-intensified CCD camera observes the fluorescence normal to
the sheet of light. To excite electrons of the molecules typically representative of fuel, UV laser
light is required, although the fluorescence is typically in the visible region. Gasoline contains
heavier aromatics which fluoresce readily, but for more consistent results a non-fluorescing base
such as iso-octane is typically doped with known fractions of fluorescing compounds
representative of the distillation fraction of interest, such as acetone (Tb = 56'C, roughly Tio), 3pentanone (Tb
=
102'C, roughly T5 0 ), or cyclohexanone (Tb
=
155'C, roughly T 90). Drawbacks
of this method include the need for optical access along two directions (one for the laser sheet,
and one for the camera view) and high energy UV laser manipulation, but most significant is the
fact that, although fluorescent yield is in general proportional to the mass of a compound present,
fluorescence images are semi-quantitative at best. Much work is in progress (see, for example,
[22] and [37]) generating calibration data in order to obtain more quantitatively accurate results
from PLIF, but the varying intensity of excitation energy, dependence of fluorescent yields on
temperature and pressure, and collision quenching by oxygen and other compounds still makes
accurate calibration extremely difficult [3]. A new method that provides in-situ calibration for
certain types of measurements is a wide-beam laser-induced fluorescence, which is described in
this project.
The new method of LIF measurement requires that the compound to be studied exist as an
optically thin film on a surface with optical access. In this case, the objective is to measure the
liquid fuel film on the piston, so the piston requires optical access.
Excitation energy is
introduced through the optical access, and the film fluoresces proportionally to its thickness [12].
The film must be optically thin, since increasing thickness of the film eventually extinguishes the
excitation light and produces a fluorescence signal proportional to the area rather than the
thickness of the film. Liquid phase LIF is much stronger than vapor phase, and is therefore
expected to dominate [20]. The key to providing an accurate quantitative interpretation of the
image, which is viewed through the same optical access as the excitation light is provided
through, is in-situ calibration using grooves etched or milled into the surface of the window. The
depth of the grooves must be of the same order of magnitude as the film thickness, and the width
must be sufficient to provide a distinguished region in the resulting CCD image. The assumption
must be made that the film will maintain a uniform surface, resulting in a band of increased
36
thickness over the groove. This will result in a stripe of brighter fluorescence, and the step
increase in brightness will be proportional to the depth of the groove. Using this information, the
thickness of the film can be calculated everywhere it is visible in the CCD image.
Figure 3.4 shows a schematic of the wide-beam LIF concept (with a calibration groove
expanded out of scale for illustration). Since the CFD results predict a film thickness on the
order of 0.01mm, the grooves must be approximately this deep. While machining an exact depth
is not critical, is it necessary to know what the final depth is in order to calibrate the images.
Accurate resolution of the groove width with typical CCD cameras requires the grooves to be on
the order of 1-2mm wide.
Finally, the laser light exciting the fluorescent compounds must be expanded to the same
cross-section as the window area to be studied with LIF, which is a 2" square in this case. The
laser beam exits the laser as a rectangular beam 23mm high and 7mm wide. Two sets of lenses
are necessary to expand this to ja minimum of 50.8mm square, as illustrated in Figure 3.5.
Apertures are also included in the optic setup to contain divergent laser beams. Finally, a longpass filter, which reflects UV light between 260nm and 320nm (the laser light is 308nm), is
positioned to reflect the beam toward the optical access engine. Since the resulting fluorescence
is visible (above 320nm), a camera placed on the opposite side of the long-pass filter from the
engine will detect the fluorescence signal through the filter.
At the time of this writing, the optical access piston assembly and laser beam expansion
optics setup had been completed, but a suitable gate-intensified CCD camera with controlling
and timing hardware was not available to conduct a proof-of-concept run of the wide-beam LIF
concept. It is anticipated that subsequent work using the same facilities will continue work on
this diagnostic strategy.
37
;
S
1~
sta ndard
head
ass embly
Co mbustion chamber
$0
jare piston
:0
0'
0
-
E
42o
wi ndow
ndard piston
sta
ndard single
inder engine
embly
Figure 3.la: Optical access engine diagram.
38
--
-
Head
Clamping Wall
Weld
Piston Rings
Aluminum Square
Piston
--
Quartz Glass Walls
1
-- - - - Split Connecting Rod
Weld
Teflon 0-ring
---
-
Adaptor Base Plate
n
CFR Piston
CFR Cylinder
Figure 3.1b: Optical extensioi
39
I
Bore
Stroke
Value
82.6mm (square)
114.3mm
Displacement
Valve Train
Head
Piston
Compression Ratio
Connecting rod length
0.77 liter
2 valves, push-rod driven
Flat
Flat
8:1
254mm
Intake Valve Open
Intake Valve Close
50 BTC
550 ABC
Parameter
480 BBC
Exhaust Valve Open
120 ATC
Exhaust Valve Close
parameters.
Table 3.1: Optical access engine
4q
zmr
Aluminum piston block
MIVN BIDNG
REV
D~liR11DN
DIR.
AfrDED
Graphite bar seal lands
Epoxy
Mirror
Mirror support
4-
Aluminum piston block
Connecting rod
OPTICAL PISTON
SLOAN
LABORATORY
R. Karlsson
"
...
11/20/1999
PISTON ASSY
"""a'OP-000
sR.&1
OF 1
Figure 3.2: New Bowditch-type piston allowing optical access from bottom of combustion
chamber.
40
4 --- - A~m - Camera
- -
Ka
Intake
side
Exhaust
side
-20cm
comb.
chamber
600W
lamp
-
Tripod
Figure 3.3: High-speed video and lighting arrangement.
Calibration the
Combustion Chamber
(cl )
Fuel Film
Quart-Window
Fue- Film
Quartz
Graphite Seal Lands
From Laser
and
To Camera
Piston
(Modified)
Connecting Rod
Figure 3.4: Wide-beam laser induced fluorescence (LIF) concept.
41
UV
UV
irror
7x7mm beam
mirror
1~
C..
33mm
sph. lens
Aperture
:mm
Cyl. lens
250mm
sph. lens
Aperture -
-
55x53mm
beam
Plan view of engine
7 5mm
C:yl.
*
intake Exhausi
n0
-
Laser light reflected into engine
Digital
lens
23x7mm
beam
mera
Visible fluorescence passed to ca mera
Djecto2R7
Long-pass
A"-
filter
Figure 3.5: Optics setup for laser beam expansion.
42
Laser
Lambda Physik
Compex 102
XeCI 308nm
4 EXPERIMENTAL RESULTS
4.1 HIGH-SPEED VIDEO RESULTS USING ALUMINUM PISTON
During construction of the optical access piston, high-speed video imaging was conducted
using the older solid aluminum square piston.
While the CFD simulations indicated a
combustible mixture resulting from injection starting at 2900 ATDCI using a pulse-width of
3.3ms, stratified combustion was found to be more stable using approximately 300' SOI and a
4.Oms pulse-width.
This resulted in a global relative air-fuel equivalence ratio of X=2, as
compared to the CFD simulations which resulted in X closer to 2.5. Based on high-speed video
imaging taken during setup of the injection and spark timing, the primary reason for this was
ambient density in the combustion chamber. The KIVA-3V model was initially run with an
intake pressure of 0.99bar and does not model any type of leakage from the unusual sealing
techniques used in the optical access engine. The intake manifold pressure, while unthrottled,
was in fact monitored at 0.9bar, and some leakage past the graphite bar seals is inevitable. As a
result of these two facts, the slightly earlier injection resulted in a wider cone than predicted,
since the ambient density was not high enough to collapse the spray cone angle. The fuel vapor
resulting from this geometry was too highly dispersed to form an ignitable mixture. By injecting
10' later, which is actually closer to the average SOI for production stratified GDI operation, the
ambient density had increased enough to collapse the cone angle and provide a more localized
fuel vapor distribution. Increasing the pulse-width provided a small increase in the amount of
fuel, further improving the local fuel mass fraction. Despite the 10' delay in injection, spark
timing was only retarded to 5' BTDCF. This may be due to a higher spray velocity than
estimated for the CFD simulations, which in turn could be a result of the lower ambient density
(and newer fuel injector than used for the estimate), but may also have been in part a result of the
higher quantity of fuel injected. Amoco Indolene High Octane Motor Fuel (brand code 15211), a
multi-component petroleum fuel formulated like gasoline but tested to meet certain property
restrictions, was used for the initial high-speed video imaging to capture as accurately as possible
the behavior of a real-world fuel spray.
Figure 4.1 shows 50 pressure traces overlaid to illustrate cyclic variation and stability of
combustion.
Some non-physical noise is present in the signal from the pressure transducer,
43
which is somewhat dated. Peak pressure is observed to occur at approximately 385' ATDCI,
which is 10' later than expected for optimized ignition timing, confirming that the ignition
timing is significantly retarded for stratified combustion due to the geometric constraints of the
optical access engine. Combustion variability is present, as expected for the unusual geometry
and retarded ignition, but is not overly excessive. These pressure traces were taken sequentially
in a "medium-warm" engine, with a piston temperature estimated at 380K. The engine will not
fire with stability when cold (room temperature) since impingement on the cold walls and piston
prevent sufficient vapor production, despite the pressure-swirl injector's highly atomized spray.
Combustion stability generally improved as the engine warmed up. While stratified combustion
did prove both feasible and reasonably stable, the combustion chamber geometry did not contain
the fuel-air mixture in any way. The flame was also highly visible through the quartz walls and
yellowish, indicating rich regions and soot production. This under-mixing may be ameliorated in
production GDI designs by the higher charge vorticity. The result in the optical access engine,
however, was a gradual build-up of soot on the windows (and other surfaces) in addition to the
slight residue left by the graphite seal bars. Frequent disassembly and cleaning was found to be
necessary to maintain suitable optical access.
The collapsed cone angle is clearly visible in the footage, with the fuel droplet pattern in
excellent agreement with existing LIF studies and the CFD predictions as shown in Figure 2.9.
Evaporation of the spray was difficult to quantify, but it appears that evaporation did not take
place as rapidly as the CFD simulations predicted. Droplets are highly visible due to light
scattering, while fuel vapor appears in the footage as a slight blur. No droplets were observed to
leave the piston; rather all impinging droplets contribute to and interact vigorously with a thin
liquid film on the piston, which in turn produces sufficient vapor to allow stratified combustion.
The liquid fuel film is observed to persist after combustion.
Once the engine is warm,
combustion appears more vigorous in the footage, presumably due to enhanced vapor production
on the hotter piston. Pool-fires are also clearly visible once the engine is warm enough for the
piston to continue vapor production, after the spray interaction, at rates comparable to its
combustion.
Figure 4.2a shows a reference sketch of the viewing angle used for the high-speed video
imaging results discussed in this section. Tumble flow across the piston is away from the viewer,
44
or from intake toward exhaust. Figure 4.3 shows four illustrations, also for reference, pointing
out key characteristics of the following images.
For example, the collapsed cone structure
exhibits a darker core region where the entrained air flow has created sufficient droplet density to
reduce lighting, while the periphery of the cone is not as dense and therefore much lighter. The
vortex structure generated by the entrained flow at the leading edge of the spray is visible from
approximately 309' to 318' ATDCI, depending on the lighting (for example, in the warmed up
case with the aluminum piston the increase in scattered light drowns out the vortex). Spray
interaction with the liquid fuel film is typically visible as a shadowy pattern within the film in the
top iso views, while the bottom view is better for viewing secondary breakup and droplet
production, which is visible as increased scattered light in these views. Figure 4.4 shows two
sequences of the spray/piston interaction followed by combustion during steady state operation,
one for a relatively cool engine (55'C head, estimated 350K piston) and one for a warmed up
engine (82'C head, estimated 400K piston). Half resolution was used for these sequences to
allow a frame rate of 0.5ms, or 3CAD apart. To save space, only select frames have been
included from the combustion sequence, since little of interest is visible for the cool engine
combustion. Note, however, the small lighter streaks propagating away from the spark location
in frames 387O-408' for the cool piston; these are believed to be soot particles generated from
rich pockets or possibly residual fuel droplets.
They are burned in the premixed flame as it
propagates through the combustion chamber. The pool fire drowns out any such features for the
warm case. Figure 4.5 shows a sequence of combustion only for the warmed up case using full
resolution (1.Oms frame rate, or 6CAD apart) to more completely capture the vigorous
combustion and pool fire, which appears to persist for at least half of the expansion stroke. The
brighter overall image quality of the warm case may be due to increased vapor production, but it
is also at least in part a result of a cleaner piston. This imaging session was the first time
stratified operation was attempted in the optical access engine, and the original square piston was
nearly black from deposits of prior operation. During the course of the session, the repeated
impingement of Indolene near the center of the piston actually cleaned the surface, increasing the
reflectivity of the piston crown and adding to the light transmitted to the camera. Figure 4.6
shows a static image taken after several runs, with the intake side quartz wall removed, using the
high-speed video camera (the piston and camera angle have been lowered to provide a better
45
view of the piston crown). The impingement zone is clearly visible as a bright region on the
piston, which has literally been washed by the fuel. Figure 4.7 shows a photograph of the piston
removed from the engine after the entire session. An edge-filtered copy of the photograph has
also been included with the deposit pattern overlaid for reference. The lighter region near the
center of the piston crown is a relatively clean surface, where previously existing deposits have
been rinsed away by repeated fuel spray impingement. Surrounding this region are several rings
of new deposits left by the fuel film, possibly comprised of partially oxidized heavier
components in the fuel, since they are less prone to evaporation and combustion. Lighter spots
toward the periphery of the new deposit layers are not "clean", as the center is, but merely lighter
colored deposits.
It is interesting to note the remarkable similarity between the fuel film area, marked by the
rings of residue, and the simple projection of a cone onto two surfaces in Figure 2.5, which was
performed with a pencil and paper in a few minutes before any CFD or experimental work was
done. Notice also that as the fuel film propagated away from the impingement site toward the
ignition location, lateral momentum appears to have been added by mild charge tumble exactly
as predicted by the CFD simulations. This is evidenced by the spreading of the deposit rings
(which mark the edge of the fuel film) away from the viewer in Figures 4.6 and 4.7.
4.2 HIGH-SPEED VIDEO RESULTS USING OPTICAL ACCESS PISTON
Once the optical access piston was completely assembled, high-speed video imaging was
conducted using it in place of the existing aluminum piston, since the quartz window allowed
more flexibility of lighting and viewing angles, and the thermocouple embedded by the window
provided a temperature measurement of the piston during operation. Operation was found to be
similar to that using the solid aluminum piston. Some minor differences included a much more
rapid rise in operating temperatures, particularly the measured piston temperature, due to the
poor thermal conductivity of the quartz window.
Once a stable operating temperature was
reached, continuous firing brought the piston temperature from roughly 50'C to 1500 in less than
90 seconds. For this reason run times were quite short, on the order of one minute, though with
some practice capturing a particular thermal state on video did not prove too difficult. Injection
timing also proved to be quite critical to stable operation, with piston temperature being an input
46
variable. Lower piston temperatures resulted in slower vapor production and therefore needed
SOI advances of between one and three CAD, while with a fully warmed up piston (400K or
above) SOI timing of 3000 ATDCI could be used. The most universally stable SOI timing was
found to be 298 -299' ATDCI.
Finally, the quartz window reflects light at low angles of
incidence, so the halogen lighting used to illuminate for high-speed video had to be adjusted
slightly to one side to prevent direct reflection, which would overpower any other imaging.
Footage was taken at two thermal states, similar to those estimated for the aluminum
piston, one for a cool "early warm-up" piston temperature of 360K and one for a warmer piston
temperature of 400K, which is more typical of the expected steady-state operation piston
temperature expected for production GDI engines. Again, since the quartz window is such a
poor conductor, the piston temperature rose rapidly. A typical run consisted of ensuring stable
operation, shutting off injection, then capturing injection again after several "dry" cycles to allow
existing fuel film to evaporate.
Even in rapid succession, these steps allowed the piston
temperature to rise to 360K. The warmed up 400K piston was reached in less than one minute.
Figure 4.8 shows spray impingement on the cool piston followed by combustion for the
"first" cycle, meaning that the engine was motored for at least 10 cycles prior to this injection to
allow any liquid fuel on the piston to evaporate. At 360' ATDCI all of the spray has interacted
with the piston, and the mixture above the piston has been sparked just under ims earlier (3550
ATDCI) so the flame is not yet visible from this angle. Since the velocity of the piston is briefly
zero at this point, the image is quite clear and shows the extent of the liquid fuel film after the
first impingement. The visible fuel film area is significantly larger than predicted by the CFD
simulations, as shown in Figure 2.7, although the general shape and location are in qualitative
agreement. Figure 4.9 shows the same top center image after the second spray has impinged on
the piston crown, showing some growth of the liquid film. From the high-speed video data, it is
apparent that qualitative equilibrium is reached after approximately five cycles. Figure 4.9 shows
spray impingement and the resulting fuel film on a cool piston after more than five cycles. Note
the increase in lighting in this sequence compared to the "first" impingement, despite the fact that
the lighting arrangement, including camera aperture setting, was not changed between these
images.
The reason for the increase in light is an increase in droplet breakup and vapor
production. The droplets of the spray interact with the thin fuel film already existing on the
47
piston in this case, producing secondary droplets as they splash down. These secondary droplet
splashes and the added vapor production resulting from them add to the scattering of light, the
result being a more well lit combustion chamber. The added fuel vapor is also visible as a
general blurriness of the images leading up to top center. At the top center position the extent of
the fuel film on the piston is once again clearly visible, and increased significantly from the first
two cycles. One revolution later, during the valve open period, the fuel film is still visible and
appears the same size. This is also illustrated in Figure 4.10, with the label 7200 indicating one
full revolution after top center firing. Note in this image that the lighting has decreased to the
level of the first spray impingement, since there is no spray or production of secondary droplets
during the valve open period. This image proves that the fuel film survives after combustion,
even pool fire. After five cycles, the area covered by the liquid film does not appear to change
significantly. It is impossible to determine at this point, however, whether the film thickness or
composition is changing after five cycles.
It is likely, for example, that for each spray
impingement the lighter components of the fuel are more quickly vaporized and combusted,
while the heavier components are gradually transported toward the periphery of the fuel film
area, where they may be partially oxidized into the darker residue rings found on the piston after
stratified operation.
Figure 4.11 shows the first spray impingement on a cool piston again, but now from a
bottom view using the mirror and piston window as illustrated in Figure 4.2b. The mirror is the
primary camera target, and the tumble flow is toward the view across the piston crown, which
translates to down in the mirror view. Note that the spray direction is reversed from the top iso
views since the camera has been moved from the intake side to the exhaust side. The region of
spray impact on the piston crown is clearly visible, particularly at 3360. The final three images in
this sequence show a slightly darker region, which corresponds to the fuel film area. Also visible
in the same images are lighter streaks, which may be breakup of the liquid film caused by
turbulent shear at the surface. The location and appearance of these "white-caps" is in agreement
with the direction of the tumble predicted by the CFD simulations, that is, the airflow is shearing
the fuel film in the direction of the exhaust valve. It is also possible, however, that these streaks
are residue build-up at the fuel film periphery lit from above. Note that Tabata et al found
airflow, i.e. swirl and tumble, to be important mainly for fuel vapor transport, while only spray
48
momentum and in-cylinder temperatures had primary impacts on liquid droplet and film behavior
[32].
Figure 4.12 shows the bottom views of the liquid film at 7200 ATDCI, or top center during
valve open, for the first 5 cycles. The faint dark region is observed to grow during each cycle,
consistent with the increase in fuel film coverage observed obliquely from above the piston.
More of the lighter regions are visible during the 3 rd and
4 th
valve open periods, again possibly
indicating breakup on the film surface as a result of the gas exchange process.
Increased
secondary droplet and vapor production from the fuel film on the piston during the exhaust valve
open period could have serious implications for hydrocarbon emissions mechanisms, although
most current designs have the piston-bowl or other confining piston geometry located away from
the exhaust valve.
Finally, Figure 4.13 shows the 360' position (immediately after spray
impingement) for the
4 th
and
5 th
cycles. The lighter "white-cap" regions are quite prominent in
these images, indicating that once a liquid fuel film has built up on the piston, primary spray
droplets impinging on the film cause more secondary droplets or film break-up than does the
tumble flow across the film. It is possible, of course, that conventional GDI designs with higher
charge vorticity generate more fuel film break-up and therefore store less fuel on the piston.
Figure 4.14 shows the full spray impingement and combustion sequence for the warmed up
piston at 400K. Notice in this sequence that the lighting is increased despite the lack of an
existing fuel film on the piston, indicating a more vigorous droplet break-up and vapor
production on the piston surface as a result of the higher piston temperature. The combustion
sequence clearly shows first a roughly spherical flame propagation, consistent with premixed
operation, again with luminous streaks indicating rich regions or fuel droplets producing soot
which is subsequently burned in the premixed flame front. These features are well illustrated in
frames 372' and 378'. Once the flame front catches up with the descending piston crown, this
primary GDI combustion mechanism is followed by a transition, as seen in frame 3840, to a poolfire on the piston crown, which can be seen in frames 390' and 3960. Figure 4.15 shows the
equilibrium fuel film area for the warmer piston. The fuel film is more difficult to discern in this
case, mainly because the film lies almost completely on the quartz window, which reflects light
in a similar manner. The closer edge of the film can be seen, however, due to refraction at the lip
49
of the pool, and the left tip of the film (closer to the spark plug) is also visible as a darker region
extending from the window RTV.
As with the aluminum piston, a photograph was taken of the optical access piston after
stratified operation to illustrate the deposits left by the fuel film.
Figure 4.16 shows this
photograph with several items labeled for clarity, and also includes an edge-filtered copy of the
picture with the deposit structure overlaid for reference. Note that these significant deposits are
the result of relatively little operating time, less than one hour in total, since the piston window
was cleaned frequently during the high-speed imaging sessions. The existence of similar deposit
structures on other GDI pistons would tend to indicate similar spray impingement, fuel film
buildup, and pool-fires.
As an addendum to the discussion over the images taken from the high-speed video, it is
instructive to describe certain qualities of the spray impingement and combustion process that are
not apparent in static images. Following assembly of the optical access piston, a small lip of
cured two-component RTV surrounded the quartz window. This was an expected by-product of
ensuring complete filling of the gap between the window and the aluminum piston frame. The
lip was very carefully removed using a razor blade, resulting in an almost perfectly smooth piston
crown. No protrusions were visible on the surface, and running a finger over the RTV joint
indicated a barely perceptible shift in the surface. Combustion in the high-speed video, however,
repeatedly shows a small flame structure surrounding the section of the RTV joint where the
spray momentum carries the fuel film across the RTV. Figure 4.14, particularly 3360 and 3420,
also shows a brighter region immediately to the left of the RTV joint, resulting from a higher
secondary break-up and vapor production in this area. The conclusion to be drawn from this
evidence is that surface roughness, as well as existing liquid film, is very important in
determining the behavior of spray impingement, as proposed by Han et al [36].
simulations did not account for surface roughness effects.
50
The CFD
cIQ
-t
CD
-t
Pressure traces overlaid for cyclic variation
CD
-t
CD
-t
C)
CD
CO
C)
en
C)
C)
C)
3\
C
I
C)
C
or
C
C,,
Tlj, -Uijilj,
7TIT7
C
C,,
180
225
270
315
360
CAD ATDCI
385
405
450
495
540
Far quartz wall
Figure 4.2a: Reference sketch showing view angle across piston crown.
..
.........
[luminum piston (partially As Wei
Figure 4.2b: Reference sketch showing view angle using mirror and optical access piston (note
the reversal of injector and spark orientation in this view).
52
Spark plug
n ector
Far quartz wall
-Spray core
(dense from entrainment)
Piston
315 ATDCI (2/3 through injection)
Pre-existing fuel film
spray periphery and
vortex structure
Figure 4.3a: Illustration of injected spray structure.
Piston
330 ATDCI
(6 degrees after EQI)
Spray interaction
Film
with fuel film
Spray
Mirror looking up through piston window
tlumhinum piston structure (below rrtror
Figure 4.3b: Illustration of spray interacting with liquid film (note: bottom view through mirror is
not same scale as top iso view, and mirror view has been reversed from other data to match spray
direction with top iso view).
53
Last of injected fuel
Piston
Film
333 ATDCI
(9 degrees after EQI)
Spray interaction with
fuel film (ripples)
Figure 4.3c: Illustration of end of spray interacting with liquid film.
Piston
720 ATDCI
(top center gas exchange)
Steady-state fuel film coverage area
Mirror looking up through piston window
Aluminum piston structure (below mirror)
Connecting rod (glare from radius)
Figure 4.3d: Illustration of steady-state liquid fuel film coverage (note: bottom view through
mirror is not same scale as top iso view, and mirror view has been reversed from other data to
match spray direction with top iso view).
54
Figure 4.4: Cool and warm spray/piston interaction and combustion sequences (4 pages).
82*C Head
55*C Head
3030
(SOI
3000)
3060
3090
3120
3150
3180
55
3210
324*
(EOI)
I .
3270
3300
3330
3360
3390
56
34201
3450
348*0
3510
3540
3570
3600
57
387*
3930
3990
4080
58
e 4.5: Combustion in warm engine (82'C head) showin
-I1111w
3600 (50 after spark)
3660
3720
3780
3840
390U
59
396*0
4020
4200
4260
60
4380
4500
4620
4560
61
Figure 4.6: Residue on piston imaged by Kodak Ektapro (high-speed video unit) in static engine.
Sootfresidue rings
"Cleaner' reg ion
Figure 4.7: Residue on aluminum piston after stratified-charge operation.
62
3930 (pool fire below diffusion flame)
3600 (film visible)
Figure 4.8: First spray impingement and combustion on cool piston (360K)
Figure 4.9: Film buildup after second injection on cool piston (360K), 3600.
63
324
330*
336"
3420
3480
3540
3600
7200
Figure 4.10: Steady state spray impingement sequence and resulting fuel film coverage on cool
piston (360K).
64
3300
3360
3470
3480
3540
3600
Figure 4.11: Bottom view of first spray impingement on cool piston (360K).
65
1st
qrd
5" (approx. equilibrium)
I
Figure 4.12: Bottom view at 7200 ATDCI for first 5 spray impingement cycles on cool piston.
66
5"' (approx. equilibrium)
Figure 4.13: Fuel film at 3600 ATDCI for 4h and 5' impingement cycles on cool piston.
4th
67
-.,.,.,IIIm
-.
Figure 4.14: First spray impin,
4/1
if
.nt and combustion on warm
-
iston (400K).
_I
iJ9U"
/I
4k
I
L-
I_______________________
68
Figure 4.15: Steady state fuel film coverage on warm piston (400K).
ReM ue nrgs
Dar er
a
-~
r
Mtt
IN.
PII
Figure 4.16: Residue on optical access piston after stratified charge operation.
69
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70
5 SUMMARY AND CONCLUSIONS
5.1 SIMULATION WORK
Computational fluid dynamics simulation, using KIVA-3V, of the rather unorthodox GDI
engine with optical access proved quite useful in the design of the engine for stratified operation.
The operating window needed some fine-tuning for precise injection and spark timing, but given
the imperfect boundary conditions initially provided to the program, the predictions were quite
good. Qualitatively the behavior of the fuel spray as it impinges on the piston crown is captured
well, but the amount of fuel remaining as a liquid film is under-predicted.
Adjustment of the
splash model may be in order, although further experimental data, possibly provided by the widebeam LIF concept discussed in section 3.4, would greatly aid the precise modifications. Further
improvements to the code in the future might include modeling surface roughness effects and
their interaction with potential existing liquid films.
5.2 HIGH-SPEED VIDEO IMAGING
High-speed video imaging proved very useful for qualitative description of the spray/wall
interaction, combustion, and liquid fuel film buildup. The spray structure is visible, despite the
extremely small droplets, due to light scattering, which also reveals the location and production
of fuel vapor. The optical access piston allows for two views of the impingement event, and the
liquid fuel film is clearly visible. The fuel film area is seen to grown over the first few cycles,
reaching a steady-state coverage area after approximately 5 cycles.
After the first spray
impingement, the existence of a fuel film on the piston crown promotes secondary break-up and
vapor production from both the spray and the film due to interaction of the primary droplets with
the liquid film (splashing).
Minor surface roughness can also have a significant effect on
secondary break-up and vapor production.
For operation during warm up with piston
temperatures on the order of 360K, the area covered by liquid fuel film is quite large,
approaching half of the piston crown.
For operation closer to the estimated steady-state
operating point of production GDI engines, with a piston temperature of 400K, the liquid fuel
film coverage diminishes due to more rapid evaporation, but the fuel film still survives
combustion and is exposed to the open valves between injections. There is also evidence that the
71
gas exchange process may cause secondary break-up of the liquid fuel film, resulting in
additional droplets and/or vapor being transported into the exhaust manifold and contributing
directly to engine-out hydrocarbon emissions.
Finally, vapor production off the piston varies significantly with temperature, as is
evidenced by the combustion behavior observed.
With a room temperature piston, stable
operation is not possible since most of the impinging fuel forms a liquid film and insufficient
vapor is produced to reliably form an ignitable mixture cloud. As the piston warms up, stable
operation becomes possible with the increased vapor production, but pool fires are not
immediately observed.
Closer to the estimated steady-state stratified operating point for
production GDI engines, however, the warmer piston produces fuel vapor from the liquid film at
a rate that will feed a pool fire for approximately half of the expansion stroke. The oxygen
available in cylinder may limit the duration of the pool fire. It is expected that with high enough
piston temperatures, the liquid fuel film will diminish to the point of preventing pool fires, as the
impinging spray is more completely vaporized.
5.3 CONCLUSIONS
While GDI engines do provide substantial advantages in fuel metering control, efficiency,
and performance, there is still much room for improvement.
Mixture preparation during
stratified charge operation in particular is not entirely understood, with fuel spray impingement
on the piston crown resulting in a liquid fuel film and, under certain conditions, a turbulent
diffusion flame or pool fire on the piston crown fed by the liquid fuel film. This liquid fuel film
has yet to be precisely quantified, as do the mechanisms by which the fuel film evaporates during
combustion and gas exchange, all of which determine engine-out hydrocarbon emissions levels.
5.4 FUTURE WORK
One definite area for future work is the wide-beam laser-induced fluorescence diagnostic
concept designed during this project and described in section 3.4. Several key pieces have
already been constructed and assembled, including the laser beam manipulation setup and optical
access piston with in-situ calibration grooves and thermocouple. Once a suitable gate-intensified
camera and controlling and timing hardware are obtained, the concept can be tested.
72
If
successful, quantitative fuel film thickness measurements can be conducted for a wide range of
operating conditions such as piston temperature, intake temperature, fuel temperature, fuel
composition, spray geometry, and piston surface roughness effects. This would provide a more
concrete basis for updating various CFD sub-models.
Other experimental work in the future could include planar laser-induced fluorescence, or
PLIF, possibly to study the fuel vapor distribution after combustion is complete in an attempt to
quantify evaporation and contribution to hydrocarbon emissions.
Phase-Doppler particle
analysis, or PDPA, could also be used to more accurately measure the size and velocity of the
fuel droplets, both primary (exiting from the injector nozzle) and secondary (resulting from
break-up of the fuel film surface due to primary droplet impingement, turbulent shear stress, or
surface roughness).
Finally, new computer models can be implemented in CFD codes to capture the phenomena
revealed by this project, and compared with both the qualitative and semi-quantitative data now
available and any further data generated by the approaches listed above.
73
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74
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76
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35. Gold, M., Li, G., Sapsford, S., and Stokes, J., "Application of Optical Techniques to the
Study of Mixture Preparation in Direct Injection Gasoline Engines and Validation of a CFD
Model," SAE Paper 2000-01-0538, 2000.
36. Han, Z., Xu, Z., and Trigui, N., "Spray/Wall Interaction Models for Multidimensional Engine
Simulation," International Journal of Engine Research, vol. 1, no. 1, pp. 127-46, 1999.
37. Yang, J. and Melton, L., "Fluorescence-Based
Method Designed for Quantitative
Measurement of Fuel Film Thickness during Cold-Start of Engines," Applied Spectroscopy,
vol. 54, no. 4, pp. 565-74, 2000.
77
(This page intentionally left blank)
78
APPENDIX A - SAMPLE KIVA INPUT DECK
ITAPE5
MIT GDI Optical Engine
irest 2
nohydro 0
1
lwall
lpr
0
irez
2
ncfilm 9999
nctap8 9999
nclast 9999
ncmon 10
ncaspec 0
gmv
1.0
cafilm 10.0
cafin 360.0
angmom 0.0
pgssw 0.0
dti 1.00000e-5
dtmxca 1.0
dtmax 1.000OOe-4
tlimd 0.0
twfilm 9.99e+9
twfin 9.99e+9
fchsp 0.25
bore 11.6743
stroke 11.43
squish 1.633
1.0e+3
rpm
0.0
atdc
datdct 0.0
revrep 2.0
conrod 26.67
swirl 0.0
swipro 3.11
thsect 300.0
sector 0.0
deact 0.0
1.0e-3
epsy
1.0e-3
epsv
1.0e-4
epsp
1.0e-3
epst
1.0e-3
epsk
1.0e-3
epse
Critical parameter descriptions
Restart segment
Crank angle frequency of post-processing files
Engine speed in rev/min
79
gx
0.0
0.0
gy
0.0
gz
tcylwl 350.15
thead 350.15
tpistn 400.15
pardon 0.0
0.0
aO
1.0
bO
artvis 0.0
ecnsrv 0.0
0.0
adia
anu0
0.0
visrat-.66666667
tcut 800.0
tcute 1200.0
epschm 0.02
omgchm 1.0
turbsw 1.0
0.0
sgsl
trbchem 0.0
capa 18.0
pmplict 2.0
lospeed 0.0
airmul 1.457e-5
airmu2 110.0
airlal 252.0
airla2 200.0
0.74
prl
rpr
1.11
rsc
1.11
xignit 0.00
tlign -1.0
tdign -1.0
calign 999.0
cadign 9.6
xignl1 0.25
xignrl 0.75
yignfl 0.0
yigndl 0.238
zignbl 11.75
zigntl 12.50
xign12 0.0
xignr2 0.0
yignf2 0.0
Cylinder temperature
Head temperature
Piston temperature
80
yignd2 0.0
zignb2 0.0
zignt2 0.0
kwikeq 0
numnoz 1
numinj 1
numvel 1
tlinj -1.0
tdinj -1.0
calinj 300.0
cadinj 20.0
tspmas 0.03
tnparc 8000.0
pulse 2.0
injdist 1
kolide 1
tpi 350.0
turb
1.0
breakup 1.0
evapp 1.0
drnoz 3.1
dznoz 13.021934
dthnoz 0.0
tiltxy 180.0
tiltxz 45.0
cone 37.5
dcone 12.5
1.0
anoz
8.00e-4
smr
0.0
ampO
8000.0
nsp
3
gasoline
o2 mw2 32.000 hf2 0.0
n2 mw3 28.016 htf3 0.0
stoifuel 4.0
stoio2 49.0
2
nreg
"presi" 9.9000e+5 1.0353e+6
"tempi" 293.15
293.15
0.10
"tkei" 0.10
"scli" 0.0
0.0
"er"
0.0
0.0
"mfracfu" 0.0
0.0
"mfraco2" 0.2300000000 0.2300000000
Start of injection in crank angle degrees
Duration of injection in crank angle degrees
Total mass of injected fuel in grams
Number of parcels used to model injection
Square wave injection pulse
Collision model on
Turbulence model on
Dispersion model on
Evaporation model on
Distance of injector nozzle from cylinder axis in cm
Vertical position of injector nozzle in cm
Position of injector nozzle relative to model X-axis
Plan orientation of injector nozzle relative to X-axis
Elevation of injector nozzle from vertically down
Mean injector cone angle
Differential cone angle
Injected Sauter mean radius in cm
Injected parcel velocity in cm/s
Initial pressures in regions 1 and 2 in g/cm-s2
Initial temperatures in regions 1 and 2, in K
81
"mfracn2" 0.7700000000 0.7700000000
0
nrk
0
nre
nvalves 1
vliftmin 0.05
skirtth 0.13125
tmove 293.15
vtiltxz 0.0
nlift 069
isoot
0
distamb 6.858
pamb 9.9000e+5
Ambient pressure (intake) in g/cm-s 2
tkeamb 423.0
sclamb
4.8
velin 0.0
reedin 0.0
reedout 0.0
nregino 2
nregamb 1
numpcc
2
0.0 1.0353e+6
720.0 1.0353e+6
numpex
0
82
ImIrI
0.300t
7
oli I wim I
7
0.300047
0.475
.
goam
//
2.250
z
0.502 0.652
0
1
S1.054
00
-
C-
1.01.506
--
L
3.756
77-T
z
z
2.000
I
- 0.600 1-
2.000
-
0,848
1.344
-1.841-
-3.200
DrtL
and
-_Ii
1
~
I
-
Drill and courrterslnk for
6-32 Mochl fl screw
6 ploces
0.250
--
top for 1/4x28
4 places
M
LOAN LABORATORY
O
PTICAL PISTON
PISTON BLOCK
R. Karisson
I'Iril.|n"DP-001
11/20/1999
1All dimensions in inches |.* 1 OF 1
1"
0
RESIONS
ZONE
REY
Oot"
ENP1FN
APF0VE
DriLl and tap
6-32 x 0, 50" deep
6 pl aces
A
i
00
0,744
IzII
0. 248
A
45 0000
4-
1.241
ti
72484
0- 2,
0, 744 -
)00
0,.250
1,241
M T
OPTICAL
PISTON
SLOAN LABORATORY
MIRROR SUPPORT
R, Karl.sson
OT STEEL
11/2O/1999
,1 /20/.99
WTH
A~~
E
MNO.OP-002
All dimensions fn inches ISh
1
RLV
np
1
DNE
REY
Dal
CEMCIPrD4
AMROM
i
RO, 2
Thread 0, 5"xONF
+
00
3.00
h0
-
4-
*
---- 0, 25
- 1.75 - -
1,00
-
MT
SLOAN
OPTICAL PISTON
ROD
LABORATORY CONNECTING
R, Karlsson
11/20/1999
1 STEEL
I
All dimensions in inches
_OP-003
Isbht 1 OF 1
4
PE
ZNE
0 N$
OESCRE
flN
IEY
Date
APPRED
NOTE: Material thickness to be
determl ned by sprlng rate:
Compressi on -Prom 0. 15' - o 0, 075'
shoutcI require oapproximotel y
2 lbP,
0, 25
00
40.15
~-
2.50
-m
M -
OPTICAL PSTON
SEAL SPRING
SLOAN LABORATORY
R.
MY
Karlsson
11/20/1999
-1
I.
M
GI
ND.
O
O P
12 SPRING ST=L
P -004
All dimensions in inches Iht 1
I
-DR4
~EY
OF I
'-,
I
4
ZME
0. 1250
-J
REY
DOb
DEDCIPTMON
WRK.D
0. 2500
4-
00
-3-
0, 28000
0,2800
--
-
3, 2500
SLDAN
R.
T
LABORATORY
Karlsson
11/20/1999
OPTICAL
SEAL
PISTON
BAR
15 GRAPHITE
I OO P-005
All dimensions in inches 1so@" 1 OF 1
2DNE
IWY
DEMCRI~flN
D-E)5
WPWMD
R0, 0625
r-
2. 200
4-
00
00
1,000
--e
2. 200
M T
OPTICAL PISTON
WINDOW
SLOAN LABORATORY
R. Karlsson
11/20/1999
QTY
1
WMAL
10
R17
SOP-006
All dimensions in inches
Ia-'.
1 OF 1
I
EV
WK
-
-
250
PCNPTON
AfV
MI
-
>7
0,050
0.250
--
--
---
10,845
~Njfj0.750
00
0.
0,1875
-9 0.1875
A-
T
1.000
i.7
0,375
-
0.29 w
-
--
0.290
3/4 x 14 Stralght thread
Vith 3/16' deep clearance Flat
CAUgned with Injector hole)
Refer
to injector mounting
drawing for hole dimensions
ALL dimensions +/-0.00 5"
M
I
SLOAN LABORATOR
R. Karlsson
6/5/2000
INJECTOR INSERT
PLUGS(30 & 45)
All dism
ItNJ-001
All dimensions in tnches Ist-
I OF I
4-
4
3
3
4
" t- CLMP LOAD --------
n2
50s. 9 --------
1
2
;172
4.8 N/BE CYLINR
FEVIO I "N
PRESSFE
A IELAE 155.E IW D-S IPV D4( 03/15"0
J
GV PPV 914 07/17/9
B IDO '19443
DEA..HVNRAVING 021-EN
lES OTHQWISE BEFFED PLL D
ON
CeNTEFLIlE TO BE WMTIN
j
~j~
3- ALL FU~EFfNK VPLLE5 AF
AT n-E M]NI"k
AXED~ TCLERNC 10. 6TO 1a. 3 DI M
ONDIT[N
D
D
FOR,
/-AAM
c
c
TYPICAL NOUNTING
C
31. 5MAX
18. 3to. 5-
(943
I I
/7"
ULS D11EC
/
B
n3 AM
55
B
/x
I
TI
--
0 7.82 t.D
ED m
I-i
I
--
I
1
DtARI D'S
(022.5
4f J2. 7
w-EDD
0.8
/
DWK
MK
Ot/15s/3
01/15/
(017.5
.25
A
"O-R]NG CAV]TY
mom
.
t0. L3
.IMA
Ot/1"Sn
-
mum
WIN
(45~
91fIFIED P.Ar WRIST FIT
MET rSMInE ramBwc WRi PLATJM3
3n
aLE
*
ICw ALL
T" L-" imz
IEMIV
MeS
~ W ~DT EwEs 4W6
1D M1S FIN Wr
iinPmF
t54 IR
TO
ceo
Bsears AMAntve
Mmrlt NB, Va. (LeA)
PISA (ETALY)
MOUNTING
A
A
PEMMMflI JON
CAVIFY DETAIL
SO&E
4
2/1
c4614
1 03 Wr MN
1
Is-Efi
I m
CATIA F11LE
lB
ZGE
DuPTmH
REY
Dm
mFdE
U.351)Mia
90 745
0 25Dia
45'
0 12SDia
U416D1i
0 175
RO 5
AA
21'
05
's15
1
22B
13125
45"
13125
3/1 x 24 thread
4-
W=77
Section A-A
MI
INJECTOIR MOUNT
SLOAN LABORATORY BRACKET
R. K<drlsson
MiNJ-D02
T
All dimensions in inches ahw 1 OF 1
6/5/2000