by
Przemek R. Jamroz
B.S., Mechanical Engineering
University of Maine, 2000
Submitted to the Department of Mechanical Engineering
January 2002 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering at the
Massachusetts Institute of Technology
-January 2002
© 2002 Massachusetts Institute of Technology
All rights reserved
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
Signature of Author
Certified by
Accepted by
Ain A. Sonin
Chairman, Department Committee on Graduate Students

by
Przemek R. Jamroz
Submitted to the Department of Mechanical Engineering
January 2002 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering
Spark ignited gasoline direct injection (GDI) engines are attractive alternatives to the traditional port fuel injection (PFI) engines. The configuration promises increased versatility of engine operation with higher thermal efficiency, reduced pumping losses and improved emission control. Despite many advantages, direct injection has several important drawbacks, such as increased NOx and particulate emissions, as well as elevated rates of deposit accumulation on intake valves and in fuel injectors. Injector deposit is the subject of this study.
Injector deposits are undesirable due to their potentially damaging effects on the injection process. According to published reports, injector fouling has negative impacts on the engine performance, driveability, fuel economy and emissions. The result of injector fouling due to residue buildup is a poor spray quality exhibited by distorted, nonsymmetric flow of fuel into the cylinder. Deposits also impact injector's flow capacity, thus affecting fuel/air mass ratio in the cylinder.
The purpose of this study is to record what happens to the residual fuel in the nozzle passage following an injection. This is accomplished by video imaging the injector passage at different operating conditions for isooctane and commercial gasoline. The setup used a transparent quartz nozzle where evaporation of the residual fuel can be recorded using a high-speed digital camera coupled with a zoom lens.
The images showed that below a critical temperature the evaporation time was long compared to the time between injection events. There was a continuous presence of liquid fuel in the nozzle passage after the injection. The removal of this liquid in the next injection reduced the likelihood of deposit formation. At higher temperatures, fuel flash boiled quickly after the end of injection, leaving very little liquid in the nozzle. The nozzle wall deposit may form from the wall film interacting with the combustion gas.
Thesis Advisor: Professor Wai K. Cheng
Title: Professor of Mechanical Engineering
3

4

The author of this paper would like to express his gratitude to everyone who contributed to the work in this project. This especially includes Prof. Wai K. Cheng, my academic advisor and project supervisor who assisted me with project definition, planning and execution. Specifically, I would like to thank for his help with troubleshooting during test apparatus design and refinement as well as his guidance in analyzing the obtained experimental data.
I would also like to thank Thane DeWitt, whose help was invaluable in solving practical obstacles inherently associated with building and operation of my apparatus. I have acquired much practical knowledge from these experiences.
I would also like to express my appreciation for the valuable input to this project obtained from members of Engine and Fuel Consortium, companies whom they represent in part sponsored this study.
My thanks also go to all students in the Sloan Automotive Laboratory for creating an inspiring atmosphere of camaraderie and cooperation. Special thanks to all, with whom
I had a pleasure of sharing the office space in 31-158, including (in chronological order): Halim Santoso, Christoph Kausch, Jeff Matthews and Jeremy Llaniguez.
Special acknowledgments are also for the efforts of all the engineering faculty I came in contact with during my undergraduate years at University of Maine. Their exceptional commitment to quality teaching made studying pleasurable and allowed me to come to
MIT well prepared.
On a more personal note, I would like to add thanks to my family whose support and encouragement has been invaluable during the period of last three semesters I spent at
MIT. This includes my mother, Barbara Jamroz who had taught me the passion of science and intellectual exploration. Special thanks also go to my sister, Gosia Deja for her kind and enthusiastic spirit as well as my girlfriend, Natania Morrissey for her unconditional love, support and understanding.
5

Abstract
Acknowledgments
Table of Contents
List of Figures
Nomenclature
1.0 Introduction
1.1 Fuel Delivery Systems in Automotive Engines
1.2 Previous Work on Injector Deposits in GDI Engines
1.4 Figures
2.1 Apparatus
2.2 Procedures
2.3 Figures
3.0 Results
3.1 Summary
3.2 Temperature Effect
3.3 Ambient Pressure Effect
3.4 Fuel Properties
3.5 Injector Orientation
3.6 Injection
3.7 Deposit buildup
3.8 Figures
4.1 GDI Injector Deposits
4.2 Summary of the Project
4.3 Future Efforts
21
23
25
10
10
12
14
3
5
7
8
9
54
54
55
31
32
33
35
36
31
31
32
32
7

Fig. 1.1 Typical automotive carburetor
Fig. 1.2 Single point injection system
Fig. 1.3 Multipoint, or port fuel injection (PFI) system
Fig. 1.4 Late injection in a GDI engine
Fig. 1.5 Early injection in a GDI engine
Fig. 1.6 Electron microscope images of GDI injector [8]
Fig. 1.7 Mechanism of injector deposit accumulation
Fig. 1.8 Polymerization theory of deposit formation
Fig. 2.1 Completely assembled test apparatus
Fig. 2.2 Quartz block with a transparent nozzle in the upright orientation
25
26
Fig. 2.3 Quartz block with a transparent nozzle in the inverted orientation
Fig. 2.5 Output window of the Redlake MotionScope@ 8000 S
26
27
Fig. 2.6 High-speed imager Redlake MotionScope@ 8000 S with optical system 28
Fig. 2.7 Plunger mechanism and fuel flow through the nozzle
Fig. 2.8 Fuel supply system and its operation
29
30
Fig. 3.1 Evaporation time of commercial gasoline (upright nozzle) 36
Fig. 3.2 Evaporation time for pure isooctane (upright nozzle) 37
Fig. 3.3 Evaporation of gasoline in subcritical conditions (inverted nozzle)
Fig. 3.4 Evaporation of gasoline in subcritical conditions (upright nozzle)
38
38
Fig. 3.5 Evaporation of gasoline in supercritical conditions (inverted nozzle)
Fig. 3.6 Evaporation of gasoline in supercritical conditions (upright nozzle)
39
40
Fig. 3.7 Evaporation of gasoline in cool end of transition range (inverted nozzle) 41
Fig. 3.8 Evaporation of gasoline in cool end of transition range (upright nozzle) 42
Fig. 3.9 Evaporation of gasoline in mid-range of transition zone (inverted nozzle) 43
Fig. 3.10 Evaporation of gasoline in hot end of transition range (inverted nozzle) 44
Fig. 3.11 Evaporation of gasoline in hot end of transition range (upright nozzle) 45
Fig. 3.12 Evaporation time of gasoline at various ambient pressures (inv. nozzle) 46
Fig. 3.13 Rise of bubbles in subcritical conditions (upright nozzle)
Fig 3.14 Rise of bubbles in subcritical conditions (inverted nozzle)
47
47
Fig. 3.15 Evaporation time of gasoline in two nozzle orientations 48
Fig. 3.16 Turbulent flow patterns during subcritical injection (inverted orientation) 49
Fig. 3.17 Absence of flow patterns during supercritical (inverted orientation) 49
Fig. 3.18 Post-injection distortions of meniscus due to plunger oscillations 50
Fig. 3.19 Video image of an empty nozzle prior to injection (inverted nozzle) 51
Fig. 3.20 Blurred image of injection flow 51
Fig. 3.21 Volumes of liquid fuel during subcritical injection (inverted nozzle) 52
Fig. 3.22 Suggested mechanism of deposit formation process in GDI injectors 53
17
18
15
16
14
15
19
20
8

A
surface area, unit: square meter [m2
CO
carbon monoxide d
-- diameter, unit: meter [m] fps
frames pre second
GDI
gasoline direct injection h
-- heat transfer coefficient, unit: Watt per square meter degree Celsius [W/m 2
OC]
HC
-- hydrocarbons hfg
latent heat of vaporization
-- length, unit: meter [m] m
-- mass, unit: kilogram [kg] mm
-- millimeter ms
-- millisecond
NOx
oxides of nitrogen
ON -octane number
PF - port fuel injection
(APn;) - injection pressure difference, unit: [psi] psi - pounds per square inch psig - pounds per square inch of gauge pressure
- heat transfer rate, unit: Watt [W] rpm - revolutions per minute p
-- density, units: kilogram per cubic meter [kg/M 3 ]
T
-- temperature, unit: degree Celsius [SC] t
-- time, unit: second [s]
T -- evaporation time, unit: second [s]
V
--
[M
3
]
9

1.1 Fuel Delivery Systems in Automotive Engines
All spark ignited gasoline engines require a reliable and versatile fuel distribution system to provide an appropriate combustible mixture to the cylinder. A successful fuel delivery system adjusts rapidly to varying conditions in the engine due to transients in speed and torque demand. Since the air-fuel ratio impacts the fuel consumption and engine emissions, the fuel delivery system controls significantly the engine behavior.
The ability to satisfy all of the above requirements is often challenging and subject to intense research in the engine community.
Gasoline direct injection is historically third in a sequence of steps leading to the development of modern fuel delivery systems in internal combustion engines. Originally, automotive engines relied on carburetors (Fig. 1.1), which measured and introduced fuel into the air stream depending on the intake pressure. For long, carburetors were a standard in automotive fuel systems due to their simplicity. They however did not allow for control of fuel delivery on demand, and were phased out when feedback controllers were needed to lower emissions. The development of electronic injectors, which are more precise and controllable than carburetors, leads to improved mixture preparation and lower emissions. Initially, the electronic injection involved a single point injection with one central injector located upstream of the throttle and is shared by all cylinders
(Fig. 1.2). This system was eventually phased out giving way to multipoint injection or port fuel injection (PFI), where each injector delivers fuel directly to the intake port of one individual cylinder (Fig. 1.3). In both cases, the amount of fuel injected as well as injection duration and timing relative to the engine cycle is controlled by an on-board computer, linked to a set of sensors measuring throttle position, intake pressure and oxygen content in the exhaust. These improvements allow for a much greater control over fuel distribution to the engine, allowing to improve fuel economy as well as to minimize emissions.
Despite its advantages, port fuel injection is inherently limited in its ability to fully control fuel delivery because the fuel is not directly delivered to the cylinder. Thus the fuel that appears in the charge may come partly from the fuel metered for the current cycle, and partly from the previous cycles. This makes mixture control in engine transients difficult.
Gasoline direct injection offers a greater versatility in controlling the mixture preparation and combustion process in an automotive engine. The concept has reached production stage in the late 90s, thanks to the development of inexpensive electronic fuel injection
10

systems. With the increasingly restrictive fuel economy and emission regulations in industrialized nations, GDI may soon become a potential replacement for the traditional
PFI technology.
The central focus of GDI technology is direct injection of fuel into the cylinder. A driving factor for the technology is the capability of stratified operation so that the overall lean limit is extended. This extension significantly improves the fuel economy at part load by reducing the throttle loss. For partial load operation the injection is set to occur late (late injection), during compression stroke, when the piston is traveling upward (Fig. 1.4 a and b). The injector delivers only a small amount of fuel into the cylinder, creating a stratified charge sufficient to sustain stable combustion. The fuel is otherwise incombustible in homogenous conditions if it were fully mixed with the cylinder air. The operation of a GDI engine at higher loads resembles that of an ordinary PFI engine.
The injection timing is advanced well into the intake stroke (early injection), allowing for a thorough mixing of the fuel with the incoming air (Fig. 1.5 a and b). Therefore, the resulting mixture is either stoichiometric or slightly rich and it's homogeneous throughout the cylinder to prevent soot formation. The transition point between the early and late injection is an additional design parameter, sometimes distinguished by an intermediate homogeneous lean operation [2, 7].
The use of direct in-cylinder injection has several additional benefits in terms of engine performance and emissions. Vaporization of the fuel cools the cylinder air, effectively increasing its density, leading to an increase in volumetric efficiency [2, 3] by as much as 5% compared to a PFI engine [2]. Subsequently, the peak cylinder temperature is lower, making GDI engines less likely to knock [2]. Higher resistance to knock allows
GDI engines to have higher compression ratio for higher thermal efficiency or more advanced spark timing for increased torque output. Lean operation at part load yields additional gains in thermal efficiency, mostly due to better thermoproperties of lean mixtures (higher y coefficient) and lower heat loss. In addition, the unthrottled operation of GDI engines significantly reduces pumping losses at partial loads. Important drawbacks of GDI technology include elevated NOx [4, 8] and particulate [4, 8] emissions during stratified combustion as well as increased deposit accumulation on intake valves [8] and injector nozzle passage [2, 4, 5, 6, 8]. Additional NOx production is mostly due to lean conditions and higher burned gas temperature at partial load [7], while soot formation is caused by insufficient mixing of fuel and rich burn of the mixture.
Due to the critical nature of fuel system reliability in GDI technology, the problem of injector deposits is especially challenging to resolve.
11

1.2 Previous Work on Injector Deposits in GDI Engines
The problem of injector deposit formation has been previously studied by a number of automotive and gasoline manufacturers [1, 2, 4, 5, 6]. Two types of injector deposits have been identified: carbonaceous sediment produced from the soot and lubricating oil during the normal work cycle, and those believed to be formed from fuel matter leftover in the hot environment of the injector passage following injection [2, 6]. Deposits of the latter type typically accumulate as thin layers of waxy residue on the injector's internal surfaces, near the injector tip and at the nozzle outlet [6, 8]. The residue typically forms a characteristic circular pattern around the plunger tip as well as the outlet (Fig. 1.6 a and b). Figure 1.7 a, b and c offers a plausible scenario for the internal deposit formation. When the injector nozzle temperature is sufficiently high, the bulk of the residual liquid fuel in the nozzle after an injection flash boils, leaving a thin liquid film of the heavier fuel components on the wall. This film interacts with the charge gas and forms a deposit layer. In the next cycle, however, part of this deposit is washed away by the fluid flow. the deposit pattern is thus result of the formation and wash away process.
Injector deposits are highly undesirable, especially if they reside on the internal surfaces of the injector. Such accumulation is likely to cause damage to the carefully designed and calibrated injection process in GDI engines. While some deposits, concentrated near the tip area are believed to affect the spray geometry and aiming [6], the accumulation around the minimum metering area cause reduction in gasoline flow
[6, 8]. All researchers reported loss of injector flow as one of the principle effects of the deposit accumulation [2, 4, 5, 6, 8] leading to change of effective air-fuel ratio in the cylinder. All immediate effects of deposit formation listed above have been linked to subsequent increase in vehicle NOx and particulate emissions [4, 8], some increase in
CO and HC emissions [4, 8], and general decrease in vehicle's performance in terms of its driveability [4], acceleration [8] and fuel economy [4, 8].
There have been several attempts to explain the deposit formation process on injector surfaces. According to one theory, the accumulation in GDI injectors results from fuel polymerization or coking in the hot environment of the injector passage [6]. This condition, believed to be caused by high-boiling point residual fuel leftover from the injection process leads to the formation of a waxy residue on the injector's internal surfaces (Fig. 1.8 a, b and c).
Operating conditions in the cylinder are believed to affect the deposit accumulation process. Injector tip temperature has been found to have by far the greatest effect on deposit formation [1, 5, 6]. In general, loss of fuel flow rate due to the accumulation
12

increases sharply for tip temperatures exceeding 1500C, peaking around 1750C. On the contrary, maintaining the tip temperature always below 150
0
C has been found to significantly reduce the residue formation [6]. Most of the deposit build-up occurs early
[4, 5], suggesting that injector surface might play a significant role in the initial sedimentation. Indeed, surface coatings have been found to inhibit the initial accumulation but had little effect on residue build-up once the process has started [5,
6]. Low fuel injection pressure is indirectly related to the deposit formation in that it limits the washing action of a fresh charge [1, 6]. Fuel properties, such as low T
90 point
(90% distillation temperature), high olefin and aromatic content in gasoline have been found to increase the deposit build-up rate, resulting in a faster and more severe loss of fuel flow rate [4, 5, 6, 8]. Gasoline additives, such as polyehteramines and polyisobutylene amines detergents significantly reduce accumulation of deposits and even leads to partial recovery of fuel flow rate in heavily deposited injectors [4, 5, 8].
1.3 Objectives
In response to the need of better understanding deposit buildup in injector nozzle, this study observed and recorded processes involving the residual fuel in the time following the injection event. Despite the renewed interest in GDI technology in the past decade, majority of researchers have concentrated their efforts on evaluating only the cumulative effects of injector deposits on engine performance and emissions, leaving relatively little in the literature describing processes leading to such accumulation based on real-time observations. This project is therefore intended to fill this gap by providing detailed real-time recordings of injection process as well as fuel evaporation immediately following the injection.
The focus of the observation is in the two regimes in the post-injection evaporation process of the fuel: the slow, diffusive evaporation at lower temperatures and the more rapid, flash boiling process observed at higher temperatures. The results of previously conducted studies in the area of GDI injector fouling indicate the existence of a temperature threshold [5, 6], above which deposit formation rates are dramatically increased. The detailed observations of the processes associated with fuel injection are aimed to verify that transition temperature for these two regimes is likely to be the threshold of deposit formation process.
13

1.4 Figures
All figures presented below refer to the textual information in this chapter.
N
Fig. 1.1 Typical automotive carburetor.
-~4I
14

(Ir
Fig. 1.2 Single point injection system.
Fig. 1.3 Multipoint, or port fuel injection (PFI) system.
15

(a)
(b)
Fig. 1.4 Late injection in a GDI engine: (a) Atomization of the liquid spray and impingement on piston crown bowl surface, (b) Vaporization and transport towards the spark plug.
16

(a)
(b)
Fig. 1.5 Early injection in a GDI engine: (a) Atomization surrounding intake air.
of the liquid spray, (b) Mixing of the fuel with the
17

(a)
(b)
Fig. 1.6 Electron microscope images of a deposited GDI injector after 20,100 km of lean operation. (a)
Deposits accumulated in a circular pattern around the injector tip, (b) Cross section of the nozzle passage of the same injector revealing increased residue buildup at the outlet of the nozzle [8].
18

F
(a)
(b)
mw~~
(c)
Fig. 1.7 Mechanism of injector deposit accumulation: (a) Fuel flow through a clean nozzle during an injection event, (b) A layer of deposit forms during the interval between injections, (c) Some of the deposit is washed off during the next injection event leaving characteristic circular pattern around the tip and on the nozzle walls.
19

(a)
(b)
(c)
Fig. 1.8 Polymerization theory of deposit formation: (a) Dispersion of deposit precursors in liquid fuel immediately after the injection event, (b) Accumulation of deposit, (c) Cohesion of deposit particles into larger groups, forming waxy residue of polymerized fuel.
20

2.1 Apparatus
The experimental apparatus used to conduct this study was designed to match the geometry and operating conditions of an actual injector. The test rig consisted of an injector body with a plunger mechanism and transparent quartz nozzle, where evaporation of the residual fuel was recorded using a high-speed video camera equipped with a zoom lens. The injector body was obtained from a common-rail PFI system used in a 2.OL 4-cylinder Saturn production engine and was fitted into the assembly and sealed against any leaks to the outside using a set of o-rings (Fig. 2.1).
The plunger seat was replaced with a precision-machined channel for small clearance fit to prevent any lateral movement of the plunger, which could result in a subsequent fuel leakage through the nozzle during the interval between injections. Such leakage would be highly undesirable as it would lead to significant data distortion. The quartz piece was fixed in position by an aluminum cover plate and was placed between two silicon rubber gaskets (Fig. 2.2 and 2.3), which provided a good contact seal with the plunger head and prevented any damage to the glass due to impact loading.
The most important part of the apparatus was the nozzle passage (Fig. 2.4). A precision-machined and polished hole inside a quartz block was located downstream of the injector assembly. The nozzle simulated conditions that existed inside the internal injector passage during the running cycles. The hole was 0.5 mm in length and approximately 0.25 mm in diameter, matching the geometry of a typical injector nozzle.
The passage was illuminated by a background light source positioned in such way as to allow a clear distinction of a surface during evaporation when liquid fuel filled the nozzle. In such condition, the gaseous phase (fuel vapor) appeared dark and liquid phase (liquid fuel) appeared transparent. During injection, there were further optical distortions due to density gradients (shadow graph effect). All images of the evaporation processes were captured by a Redlake MotionScope® 8000 S high-speed video imager
(Fig. 2.6) and subsequently stored on a videotape. The camera was additionally equipped with a 50-mm zoom lens with a 40-mm spacer for close-up imaging. Due to lighting constraints, the optimum image was obtained at a recording rate of 4000 frames per second, allowing viewing of events as short as one quarter of a millisecond with a magnification of over 100:1. The effective imaging area on the video screen was about 32.67 cm2.
21

Imager
Max recording speed
Available playback speeds:
Image size of 0.25-mm diameter nozzle
Redlake MotionScope@ 8000 S
8000 fps (frames per second)
1, 2, 3, 4, 5, 10, 30, 60, 125, 250, 500,
1000, 2000, 4000 and 8000 fps
Resolution at 4000 fps
Sensor array
Exposure time at 4000 fps
Imaging area
100 x 98 pixels
656 x 496 pixels
Maximum recording time 8.2 sec.
Maximum number of frames per recording 32,768
0.230 ms
0.25 mm
2
Optical system
Effective magnification
50-mm zoom lens with 40-mm spacer
Over 100:1
Approximately 1.0 in. (2.54 cm)
Table 2.1
The output from the high-speed video imager also included a frame counter, which allowed an exact determination of time and number of frames elapsed in the measurements. Other information provided by the output window included the current recording and playback speeds as well as identification number (event number) assigned to each test run. Fig. 2.5 illustrates a typical output window for the Redlake
MotionScope@ 8000 S.
During the experiment, the injector coil was activated by an electrical pulse produced by a signal generator. The injection event (Fig. 2.7 a and b), simulated that in a running engine. Both the pulse width and pulse spacing (for intermittent injection) can be adjusted. From the optical point of view, given the lighting constraints at the chosen recording speed, best evaporation results were expected for long injection pulse, allowing more fuel to be delivered to the nozzle. This was likely to be especially advantageous at higher temperatures, when evaporation time eventually plunges below equipment recording capacity. Therefore, for the purpose of this experiment, the pulse width was set to about 20-25 ms and pulse spacing to 90 ms. In terms of an actual engine operation, these settings correspond to a 2000 injection pulse in an engine running at high load at a speed of 1500 rpm.
Fuel was supplied to the injector assembly by means of a simple pressurized system, shown on figure 2.8 a, b and c. The supply system was composed of a floating-piston hydraulic cylinder, vacuum pump, fuel tank and compressed nitrogen supply, all linked
by a system of valves, which allowed for emptying, charging and cleaning of the system. The injection pressure was controlled by gas pressure of compressed nitrogen
22

in the upper part of the floating-piston cylinder and was maintained at a constant value during each test run. The outlet of the quartz nozzle was also maintained at a constant pressure by being connected, through an outlet channel to a very large vessel, which, owing to its size can be considered essentially isobaric. Varying the outlet atmosphere pressure allows for a quantitative analysis of the effects of nozzle pressure on evaporation time.
The high temperature environment in the engine cylinder was simulated by electrically heating the walls of the nozzle. The temperature of the apparatus was continuously monitored by a thermocouple embedded in the aluminum cover of the quartz block to ensure an intimate contact with the glass surface. The nozzle temperature was controlled by adjusting voltage supplied to two 500-Watt cartridge heaters inserted in the assembly (Fig. 2.1). Similarly to a functioning engine, the injected fuel was essentially at the room temperature. Preheating takes place in the reservoir located in the assembly upstream of the plunger head where fuel resided prior to injection.
2.2 Procedures
The experimental part of the project comprised a series of test runs as a function of two independent parameters: nozzle wall temperature and pressure at the nozzle passage.
These experiments were performed for two different fuels and two injector orientations.
The fuels were isooctane and commercial gasoline (ON 87). While isooctane is a pure hydrocarbon, the commercial gasoline is a mixture of several hydrocarbons whose chemical composition is typically represented by an averaged formula: CnH
1
.
87 n.
The neat fuel was only used to simplify the analysis and interpretation of data.
Two injector orientations were selected for the experiments: upright with fuel injected upwards and inverted, with fuel injected downward at an angle of 200 off the vertical axis. The data produced using the latter orientation more closely resembled injector orientation in a typical GDI engine with fuel sprayed down towards the piston crown bowl. Using the two orientations is nonetheless important as it is likely to provide clues to the role of gravity in the evaporation process, when liquid fuel is present in the nozzle because of the bubble formation. Injector orientation is not likely to be as important for experiments performed at higher temperatures, where flash boiling occurred.
Since the nozzle wall temperature is a major factor affecting deposit formation in GDI engines, the dependence of evaporation time on temperature was tested for each selected pressure level at the nozzle exit in sweeps ranging from 90 0C to 130 C, at roughly 5 0C intervals. The experiments for each data point are repeated three times to
23

account for statistical variability of injection processes. The goal was to cover the transition from the normal evaporation to the flash boiling regime.
In all of the experiments, evaporation was determined as follows. The time t = 0 was defined at the end of injection. Evaporation time is then assumed equal to t = 'r when no liquid fuel was visible inside the nozzle. At lower temperatures with surface clearly visible and moving slowly, the end of evaporation was visually well-defined. At higher temperatures, measurements of evaporation time proved more challenging and required careful review of the data acquired with the high-speed video equipment.
In the course of repeated test runs, the silicon rubber gasket materials were subjected to repeated stress due to the pounding action of the plunger head. The compressive force distorted the gasket material around the opening area, eventually disrupting the injection process. To ensure reliable data, gaskets were replaced after the recordings for each data point are completed.
With each fuel change, the supply system was carefully purged with nitrogen. To prevent possible contamination of the nozzle, the entire injector as well as the quartz piece were completely disassembled and then likewise cleaned with a jet of nitrogen.
All silicon rubber gaskets supporting the quartz piece were removed and replaced. The nozzle passage was carefully inspected for any residue and subsequently cleaned with a stream of nitrogen. The test rig was then reassembled and refilled with new fuel.
Once the injection pressure was set, the cartridge heaters were inserted into the rig and slowly brought to the desired temperature, at which the measurements of evaporation time were to be carried out.
24

2.3 Figures
All figures presented below refer to the textual information in this chapter.
0-Ring Seals
Quartz Block
Aluminum Cover
Plunger Mechanism
I
Injection Signal
Input Socket
Fig. 2.1 Completely assembled test apparatus.
*
FUEL FLOW
25

x
Fig. 2.2 Quartz block with a transparent nozzle, where fuel injection and evaporation processes are observed (here for the upright orientation).
O-Ring
Fig. 2.3 Quartz block with a transparent nozzle, observed (here for the inverted orientation).
where fuel injection and evaporation processes are
26

d =
0.25 mmNz
L
=
0.50 mm
z e
Out
I
0
Fig. 2.4 Geometry of the test nozzle (here in inverted orientation).
Fig. 2.5 Output window of the high-speed Redlake MotionScope@ 8000 S imager used in this experiment.
27

Fig. 2.6 High-speed imager Redlake MotionScope@ 8000 S with optical system used to capture images of evaporating fuel for this experiment.
28

(a)
(b)
Fig. 2.7 Plunger mechanism and fuel flow through the nozzle: (a) Plunger is blocking the nozzle opening, forcing the fuel to remain in the reservoir, (b) Plunger is activated allowing the fuel to flow through the nozzle.
29

N2C
2W_-
Floating piston
W
Floating
(a) piston
Y
Vacuum pump
(b)
F loating piston
Fuel tank
Vacuum pump
Injector
N2
Fuel tank
,S;2
(c)
Fig. 2.8 Fuel supply system during 3 stages of operation: (a) Purging, (b) Charging, (c) Injecting.
30

3.1 Summary
For both the isooctane and gasoline, the evaporation time of the residual fuel in the nozzle after injection decreased sharply when the nozzle temperature was above a critical value. The evaporation process changed from surface mass transfer to bulk flash boiling as described in section 2.2. The shift decreased the evaporation time from minutes and seconds to less than a millisecond within a temperature interval of less than 15 0C. This behavior was observed in all test cases, regardless of the ambient pressure, fuel type or nozzle orientation.
3.2 Temperature Effect
The nozzle temperature was found to have by far the most dramatic effect on the evaporation of fuel left in the nozzle after injection. In all cases, the data followed a trend of sharp decline in evaporation time with temperature increase within a well defined transition zone. Figures 3.1 and 3.2 clearly illustrate this trend, for the two fuels used in this study: commercial gasoline and isooctane. Below the critical temperature, the evaporation time is long compared to the period between injections. This results in a continuous presence of liquid in the nozzle passage. This region generally below
110 OC for isooctane and below 95 OC for commercial gasoline. The images obtained at this temperature range (Fig. 3.3 and 3.4) show very slow evaporation process with complete vaporization of the residual fuel in over a minute. The liquid forms a clearly defined surface with a meniscus, which slowly moves toward the plunger as the fuel evaporates. Any vapor bubbles are small and move slowly.
At higher temperatures, the evaporation rate is dramatically increased to the point where it is difficult for the high-speed imaging system to detect any liquid during and after the injections (Fig. 3.5 and 3.6). For isooctane this occurs at temperatures above
120 0C and for gasoline above 105 0C - 110 0C, depending on the nozzle orientation.
Such sharp transition implicates occurrence flash boiling of fuel.
Images acquired in the transition range show increased vapor presence with increasingly vigorous boiling as temperatures increase. At the low end of the transition region, the evaporation time is still large compared with injection spacing. The liquid fuel fills up the cavity with a clearly formed meniscus, which quickly recedes towards the plunger (Fig. 3.7 and 3.8). Vapor bubbles created during this process are generally much larger and more buoyant than these generated during the diffusive evaporation.
As the temperature is further increased, the meniscus structure disappears giving way
31

to a combination of large vapor pockets and a thin film covering the nozzle walls
(Fig. 3.9). At the high end of the transition region, these changes further progress in the direction of increased vapor content. The liquid no longer fills up the nozzle passage but splatters on the walls forming a quickly disappearing thin film (Fig. 3.10 and 3.11). With increasing temperatures, the size of the wetted wall area decreases until no liquid can be observed.
3.3 Ambient Pressure Effect
In general, ambient pressure affects the value of the temperature which marks the onset of the transition in evaporation process (Fig. 3.12). Experimental results showed an increase in average evaporation time at a given temperature as a result of increased ambient pressure. Qualitatively, the vaporization process inside the nozzle was essentially unchanged; the transition temperature increased with elevated ambient pressures. This phenomenon is tied directly to that to have flash boiling, the vapor pressure has to exceed the ambient pressure to upset the mechanical equilibrium.
Thus, at higher ambient pressure, the required temperature for flash boiling is higher.
This dependence may explain the discrepancy between the transition point in flash boiling of gasoline observed in this study (at around 110 (C) and the increased deposit accumulation threshold observed in engine studies to occur around 150 0C. In-cylinder pressures of a typical engine cycle (2.5 to 3.0 MPa), being greater than the nozzle outlet pressures used in this study (50 to 200 kPa) would therefore require higher temperature for the leftover fuel to flash boil.
3.4 Fuel Properties
Experimental results also show a limited effect of fuel composition on the evaporation process (Fig. 3.1 and 3.2). Both fuels exhibit similar behavior with a clearly defined decline in evaporation time around their respective critical temperatures. Fuel physical properties has clearly affected the value of these critical points, suggesting that evaporation properties of individual fuel components, at least to some degree, may play a significant role in shifting the transition zone on the temperature scale. The neat fuel has a rather abrupt transition (Fig. 3.1), while in the multi-component gasoline, the evaporation time changes were not as abrupt.
3.5 Injector Orientation
The significance of nozzle orientation in the conducted experiments reflects the effect of gravity in fuel evaporation process. In the absence of forced convection motion the vapor bubbles, by buoyancy may potentially enhance the heat transfer between liquid fuel and nozzle walls, leading to an appreciable change in evaporation time. Naturally,
32

this process relies on the presence of vapor pockets suspended in liquid with a clearly defined free surface. This requirement limits buoyancy effects to a temperature range below the critical level or, at the very least to the low-end of the transition region where liquid phase still dominates. As the wall temperature increases, vapor pockets gradually grow, leading to the disappearance of the free surface, thus making nozzle orientation essentially irrelevant.
To observe gravitational effects, one must therefore focus on the low temperature range, where liquid presence is significant. In such conditions, vapor bubbles generated on the glass surface and on the plunger face universally drift toward the top. In case of the upright nozzle, this direction corresponds to the nozzle outlet, aiding vapor removal and inducing a buoyant flow, which enhances heat transfer with the surrounding walls.
The bubbles float to the top, where they escape the free surface meniscus which gradually advances toward the plunger as the liquid evaporates (Fig. 3.13). For the inverted configuration, buoyancy forces bubbles to rise, creating a growing vapor pocket near the plunger. A second meniscus also forms at the bottom, near the nozzle outlet.
The liquid fuel, trapped in the middle, gradually evaporates releasing vapor bubbles, which eventually escape through by either of the two menisci (Fig. 3.14). The heat exchange rate for the inverted nozzle is slightly lower when compared with the upright case, due to decreased convective flow and reduction in heat transfer coefficient at the surface of the metal plunger, which is now exposed to vapor.
Experimental results indeed show a slight difference in evaporation time between the two nozzle orientations for tests conducted at the same temperature and injection pressure (Fig. 3.15). In case of the upright nozzle, evaporation time is shortened by as much as 15 to 20% when compared to the inverted configuration. This difference diminishes with increasing temperature. The rapid boiling process, which takes place at higher temperatures makes gravitational effects all but negligible. Then, the length of evaporation process is essentially identical for both nozzle orientations.
Overall, gravitational effects are shown to be either small or negligible and can be comfortably ignored from the standpoint of fuel evaporation time. The data collected for the upright nozzle can therefore well approximate evaporation process inside an inverted nozzle, which more closely resembles injector geometry of an actual engine.
3.6 Injection
Several comments have been made earlier suggesting a strong connection between the rate of injector fouling in GDI engines and the presence of liquid fuel in injector nozzle. Consequently, the evaporation process has occupied a central place in this
33

research project. Nevertheless, the characteristics of the injection process is also essential. Conditions during injection set the stage for fuel vaporization, making the two processes inherently connected. Visual data of injections collected during this study serve not only as a convenient time reference for the start of evaporation but also provide valuable clues about the fluid behavior during and after injection. This information can potentially contribute to a better understanding of fuel vaporization by establishing a set of similarities between the two processes. Video images gathered during individual test runs indeed reveal that injection process, like evaporation changes most dramatically with increasing temperature of the nozzle walls, resembling the shift observed in the evaporation data. While the timing of injection can not vary because it is fixed by the pulse duration, there is a sharp contrast in relative content of liquid phase as the injector temperature increases above the critical value. At low temperatures, the presence of liquid fuel is very evident and demonstrated by presence of turbulent patterns and small vapor bubbles in the fuel flow (Fig 3.16). Above the critical temperature, these patterns are far less visible, possibly due to a significant vapor content (Fig 3.17), whose flow does not produce observable effects. At temperature levels high enough, no change can be observed, leaving virtually no liquid fuel present in the nozzle through most of the injection process. Since the test rig design eliminates any vapor presence in the line prior to injection (vapor bubbles would drift upstream inside the line), these results further support previous conclusions about fuel flash boiling on contact with the hot nozzle walls.
Injection pressure had no visible effect on the process. This is to be expected since the only pressure-parameter involving injection flow is APin;, which stays constant throughout the study. Other parameters, such as flow orientation and fuel type have likewise produced no significant differences.
An interesting phenomenon takes place at the end of injection pulse as the plunger closes the opening through which the fuel flows into the nozzle. The plunger face hits the surface of the rubber gasket, which rests between the quartz block and main body of the apparatus, subsequently inducing an oscillation of the liquid fuel inside the nozzle. As a result, interesting images of flattened or extruded meniscus surface appear immediately after the end of injection (Fig. 3.18). It is important to note that such oscillations would not take place in engine-mounted injectors, where plunger seat is metal and absorbs most of the shock associated with deceleration of the plunger.
A few words should be devoted in this section to the quality of the recorded injection images. Unlike during evaporation, high velocities of the observed flow make it very hard to track translation of individual objects such as dust particles or vapor bubbles as
34

they move through the nozzle. This leaves many frames blurred as the initially empty nozzle, which appears dark (Fig. 3.19), fills up with a chaotic image of turbulent flow
(Fig. 3.20). Gray or dark areas (once again Fig. 3.20) possibly correspond to vapor or mixed-phase regions. Similar images with dark areas are also prevalent at very low temperatures. This suggests a flow-related behavior, where fuel may vaporize due to cavitation. Density gradient may could also contribute to the dark image due to shadowgraph effects. Heat transfer, increasingly important at higher temperatures naturally enhances this process, leading to even greater vapor content. The evidence of liquid presence is concluded due to occasional increases in clarity of the recorded data. It is then possible to observe transparent volumes of liquid fuel occupying the nozzle cavity
(Fig. 3.21), similarly to evaporation data. The size and frequency of these transparent regions decreases sharply with temperature, agreeing with the previously reported results. Nevertheless, the interpretation of the collected injection data is highly speculative at the very least, making it impossible to state much with absolute certainty.
3.7 Deposit buildup
The absence of a liquid fueled nozzle passage in between injections as a contributing factor in injector fouling has been previously suggested in this report. It has also been suspected by other researchers [5, 6] who linked the absence of liquid phase to increased residue buildup. The polymerization theory, introduced in section 1.2 further supports this hypothesis, claiming that reduced liquid presence leads to more difficult removal of already accumulated deposits. While this is certainly true, the role of fuel vapor in the nozzle may contribute directly to the deposit formation process. The fuel vapor reacts chemically with combustion products, which are forced into the nozzle due to increasing cylinder pressure early during expansion stroke. The resulting product could accumulate on injector surfaces. This mechanism is schematically illustrated on figure 3.22 a, b and c.
35

3.8 Figures
All figures presented below refer to the textual information in this chapter.
Evaporation Time For Gasoline
120
0
100
80 cc
Cu
60
40
20
180
-
160
140
GasolineRUN1
GasolineRUN2
-e- GasolineRUN3
--GasolineAVERAGE
85 95 105
C]
115
Fig. 3.1 Evaporation time as a function of nozzle temperature for commercial gasoline (upright nozzle)
36

Evaporation Time For Isooctane
120
U)
E
CI
0
100
80
0.
CL w
60
180
160
140
-+
-+-IsooctaneRUN2
,6- IsooctaneRUN3
=Isooctane_AVERAGE
40
20
0
100 105 110
C]
115 120
Fig. 3.2 Evaporation time as a function of nozzle temperature for pure isooctane (upright nozzle)
37

Fig. 3.3 Evaporation process below critical temperature as observed in the transparent nozzle for gasoline at 25 0C in inverted nozzle configuration. The dark region in the bottom of the nozzle represents the vapor.
The entire passage is almost filled with liquid, which slowly evaporates. The vapor bubbles seen in the picture are small, slow and very few. The bright area at the center of the dark region represents a glare due to background light shining on the cylindrical surface of the nozzle.
Fig. 3.4 Evaporation process in subcritical temperature as observed in the transparent nozzle for gasoline at 30 C in upright nozzle configuration.
38

-- i -". h I- ---
Fig. 3.5 Evaporation process for gasoline above critical temperature (112 0C) as observed in the test nozzle for inverted orientation. The two images are taken less than 20 ms apart. The upper photograph shows a small splatter of fuel film, which has clearly evaporated by the time the bottom image was taken.
No liquid could be observed beyond the bottom frame. The bright area at the center of the dark region represents a glare due to background light shining on the cylindrical surface of the nozzle.
39

Fig. 3.6 Evaporation process for gasoline in supercritical conditions for upright nozzle orientation
110 OC. Similar vaporization of small fuel volume (visible in the glare area in the upper photograph) can at be observed for these two images, taken approximately 17 ms apart.
40

Fig. 3.7 Evaporation of gasoline in the inverted nozzle in the low-temperature end of transition region (at
89 0C). As increasing temperatures approach the critical point, the vapor content increases dramatically.
The vapor bubbles increase in size, speed and number compared to the conditions well below the critical temperature. The two images are taken 3ms apart. Relatively low heat transfer rates associated with diffusive evaporation ensure continuing presence of liquid fuel, leading to longer evaporation times.
41

Fig. 3.8 Analogous images of fuel evaporation in the low-temperature end of transition region (at 89 0C) upright nozzle orientation. The blurred bubble images are due to their increased velocity.
in
42

Fig. 3.9 Development of large vapor pockets at mid-range of the transition region, here captured for inverted nozzle at 95 0C. The disappearance of clearly defined liquid phase is accompanied by abundance of thin and quickly disappearing film on the walls (hence the glare brightness is diminished).
43

Fig. 3.10 Further progress towards increased vapor content in the nozzle at high-temperature transition zone (here for inverted nozzle at 102 0C). end of the
The size of areas covered with fuel film as well as its evaporation time is sharply decreased.
44

Fig. 3.11 Analogous changes observed for upright nozzle orientation at 103 OC.
45

Evaporation Time of Gasoline
320-
300-
280
~-
0 180-
260-
240-
220-
200-
9
160 0
0.
cc
140
120r
100
0-
75
80
60
40 -
20
-
K
85
-cm
- -* ~aRN
-
P = 50 kPaRUN1
P = 50 kPa-RUN2
P = 50 kPaRUN3
-0 P = 100 kPaRUN1
P = 100 kPaRUN2
P = 100 kPaRUN3
0-P
=
200 kPaRUN1
-P = 200 kPa RUN2
-U-P = 200 kPaRUN3
-=5
95 105
Temperature [deg. C]
115 125
Fig. 3.12 Evaporation time as a function of nozzle temperature for commercial gasoline at of ambient pressure (inverted nozzle).
different levels
46

Fig. 3.13 Rise of bubbles in upright nozzle in subcritical evaporation conditions; one meniscus develops.
Fig 3.14 Two menisci present in inverted nozzle in subcritical evaporation of gasoline.
47

Evaporation Time for Gasoline
0
200
180
-
160
140
120
0
0
C
100
80 a
V
\A
UPRIGHT_RUN1
-U-EUIGHTRUN2
----
UPRIGHTRUN3
-
-UPRIGHTAV ERAGE e- INV ERTED_RUN
INV ERTEDRUN2 o
INVERTEDRUN3
-U -NVERTEDAVERAGE xv
60
40
20
0
85
-a
N NR~
95 105
115
Fig. 3.15 Evaporation time as a function of nozzle temperature orientations: inverted and upright.
for commercial gasoline in two nozzle
48

LZ
Fig. 3.16 Turbulent flow patterns associated with subcritical (liquid-dominated) injection flow inside the nozzle (inverted orientation).
Fig. 3.17 Absence of flow patterns in vapor-dominated injection at supercritical conditions (inverted orientation).
49

Fig. 3.18 Distortions to meniscus shape due to post-injection plunger oscillations. In the upper photograph, the free surface surges upward and then is driven down towards the outlet (bottom image).
The two images were taken 3 ms apart.
50

Fig. 3.19 Video image of an initially empty nozzle prior to injection (inverted nozzle). The bright area at the center of the dark region represents a glare due to background light shining on the cylindrical surface of the nozzle.
Fig. 3.20 Blurred image of injection flow. Rapid changes in image quality make any observations of injection flow highly speculative.
51

Fig. 3.21 Clearly visible volumes of liquid fuel observed during injections in subcritical conditions (inverted nozzle).
52

(a)
(b)
,-7.~
(c)
Fig. 3.22 Mechanism of deposit formation process in GDI injectors: (a) Below a critical temperature, the long evaporation time results in the continuous presence of liquid fuel after the injection thus preventing deposit formation, (b) At higher temperatures, fuel flash boils quickly after the end of injection, leaving fuel vapor in the nozzle, which reacts with combustion products forming waxy residue, (c) which accumulates on the nozzle surfaces.
53

4.1 GDI Injector Deposits
Injector deposits have been identified as especially undesirable in GDI combustion systems because of their detrimental effect on fuel delivery to the cylinders. Most researchers have thus recognized injector deposits to be a major obstacle in achieving the full potential of lean-burning engine in terms of improved fuel economy, performance and lowered emissions [2, 4, 5, 6, 8]. Injector deposits cause general deterioration of spray quality [6, 8] and reduction in fuel flow [4, 5, 6, 8]. Several attempts to identify the mechanisms responsible for deposit formation concluded the residue to originate from partial degradation of fuel in the presence of extreme conditions in the cylinder early during expansion stroke [4, 6]. Much effort has been spent on establishing dependencies between the degree of injector fouling and operating conditions as well as fuel chemistry. The evidence presented points to the evaporation of the leftover fuel as the key to formation of injector deposits [4, 5, 6].
4.2 Summary of the Project
The principle objective of this study was to observe in real time, the fuel evaporation in an injector nozzle under various operating conditions. These observations were used to explain the injector deposit formation and accumulation in GDI engines. A specially designed apparatus was constructed to perform a series of experiments in order to qualitatively link the speed of evaporation to the temperature and ambient pressure. An essential part of the assembly was a transparent quartz nozzle, which allowed for observation of the evaporation processes with use of a high-speed video recorder equipped with a magnifying optical system. The system was successful in obtaining good-quality images of the injection process and of the subsequent evaporation of leftover fuel. Based on the experimental data, it is concluded that:
1. Nozzle temperature was by far the most important factor influencing the evaporation process. At temperatures below a critical level, a slow vaporization process led to continuous presence of liquid fuel in the nozzle for a duration much longer than the time between injections. The presence of the liquid fuel and the subsequent fuel flushing out in the next injection minimized deposits from forming. At higher temperatures, the fuel flash boiled on contact with the nozzle walls, leaving no bulk liquid in the passage. The transition from a slow diffusion-dominated evaporation process at the lower temperatures to a very fast flash boiling occurred over a narrow range of temperature.
54

2. Ambient pressure as well as other factors such as gravity and fuel type only affected the critical temperature value that marked the transition region.
4.3 Future Efforts
In the course of conducting this research, several valuable suggestions were made by the members of the sponsoring consortium regarding possible improvements to the experiment, as well as potential leads to related studies in the future. Among others, it was recommended that the experiment be repeated for a larger variety of fuels to address the effects of fuel properties. Other suggestions included introducing independent temperature controls for the nozzle walls and fuel, and modifying the test rig geometry to resemble engine environment even more closely.
Due to the necessary presence of combustion products, it would be difficult to continue this study in its present form. For most meaningful results, a combustion chamber would be needed with some way to optically monitor the evaporation process and observe the deposit buildup. Other, more involved setups might prove even more successful, avoiding such deficiencies of transparent nozzles as inability to test surface coatings. In any usable form, these future efforts are likely to bring more answers to the problem of injector deposits.
55

[1] Harada, J., Tomita, T., Mizuno, H., Mashiki, Z., Ito, Y., "Development of Direct
Injection Gasoline Engine," SAE 970540
[2] Iwamoto, Y., Noma, K., Nakayama, 0., Yamauchi, T., Ando, H., "Development of
Gasoline Direct Injection Engine," SAE 970541
[3] Anderson, R. W., Yang, J., Brehob, D. D, Vallance, J. K., Whiteaker,
R. M.,
"Understanding the Thermodynamics of Direct Injection Spark Ignition (DISI)
Combustion Systems: An Analytical and Experimental Investigation," SAE 962018
[4] Arters, D.C., Macduff, M. J., "The Effect On Vehicle Performance Of Injector
Deposits In A Direct Injection Gasoline Engine," SAE 2000-01-2021
[5] Aradi, A. A., Colucci, W. J., Scull Jr., H. M., Openshaw, M. J., "A Study of Fuel
Additives for Direct Injection Gasoline (DIG) Injector Deposit Control," SAE 2000-01-
2020
[6] Zhao, F. et al. "Progress in Energy and Combustion Science," SAE 1999
[7] Heywood, J. "Internal Combustion Engines," McGraw-Hill 1988
[8] Arters, D.C., Bardasz, E.A, Schiferl, E.A., Fisher, D.W., "A Comparison of Gasoline
Direct Injection Part I - Fuel System Deposits and Vehicle Performance," SAE 1999-01-
1498
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