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SPRAY IMPINGEMENT CHARACTERIZATION OF A SWIRL
TYPE INJECTOR FOR GASOLINE DIRECT INJECTION
ENGINES
LI Bo, LI Yunqing, WANG Defu.
School of Transportation Science and Engineering BUAA, Beijing, 100083, Peoples R China
Keywords: Gasoline Direct Injection engine, swirl type
injector, spray impingement characterization.
Abstract
Engine efficiency and emissions depend on the initial fuel
spray, evaporation, mixture preparation and distribution
inside the chamber. This phenomenon is based on the
interaction of the in-cylinder air motion, tumble or swirl. One
drawback of the gasoline direct injection (GDI) engine is that
the spray impingement on the piston and cylinder wall often
leads to high emissions of unburned hydrocarbons and soot
[1]. To further understand the spray impingement
characteristics that are related to the GDI engine performance
of economy, emission and the combustion, the sprays into
constant volume chamber within a flat wall installed under
different back pressure value from a swirl type GDI injector
were visualized. The spray impinging development, spray
penetration at normal and radius direction of wall, spray tip
velocity and droplet characteristics were measured at various
injection pressure, ambient back pressure, wall distance from
the injector tip to the impinging wall, and wall inclination
angle. Experimental results showed that the spray structure
exhibited a large hollow cone angle and wide spreading at
low back pressure, therefore the fuel film adhered on wall
after being impinged is thin and it is favourable for fuel
evaporation and diffusion, however, it is unfavourable for fuel
evaporation and diffusion under high back pressure. The
spray growth velocity is sharply decreased, also the spray
height and spray radius growth rate decreased after
impingement of the spray. Impinging sprays exhibit longer
spray penetration behaviour at normal and radius direction of
the wall under higher injection pressure, shorter wall
distances and larger angles of wall inclination. The spray
height and spray radius decreased with the increase of
ambient pressure. Higher injection and lower ambient
pressure promote an increase of the spray area, and causes
more rapid movement of the spray. Higher ambient pressure
slows down the spray movement. The formed vortexes under
high back pressure become one of factors that increase the
spray height and decrease the spray radius and spray tip
velocity growth rate.
1 Introduction
Gasoline Direct Injection (GDI) has been an emerging and
successful technology in cutting down emissions, saving fuel
consumption and improving the thermal efficiency of engines.
GDI engines have potential to achieve greater fuel economy
compared with a diesel engine at partial loads and burst forth
favourable performance than port fuel injection engines at
high loads. In particular, under partial load operation when
the fuel is injected late during the compression stroke, the
goal is to quickly transport the fuel-air mixture towards the
spark plug with no fuel impingement on surfaces, and to
achieve complete droplets evaporation in the short time
available between the end of injection and the start of ignition
[2]. At present, GDI engines with the function of fuel
stratified injection are in mass production, and it offers lower
fuel consumption up to 15% in the case of overall-lean partial
load operation. The spray characteristics from the high
pressure swirl injector which are generally employed for these
engines. Hence, the relative location of the injector and spark
plug, piston geometry, cylinder geometry and injection timing
are important criteria in the design of GDI engines, air motion
and fuel spray delivery characteristics become important
factors for investigation. It is necessary to obtain a detailed
understanding of the spray wall impingement process and its
effects on the engine emissions. Therefore, experimental
techniques and numerical methods should be consolidated to
investigate the fuel spray atomization, the vaporization and
the wall impingement processes of the hollow-cone fuel spray.
To maintain stable, stratified combustion and acquire the
thermodynamic potential for reducing specific fuel
consumption, coupling with the advantages of quicker starting,
improving transient response and formation of the fuel-air
mixture in the cylinder must be more precisely controlled [3].
In this paper, the effects of the fuel injection pressure,
ambient back pressure, impinging distance from injector
nozzle to impinging wall and impinging wall inclination on
the impinging spray development were investigated. The
impinging spray growth velocity and spray penetration from
normal and radius directions along the wall for various wall
conditions were analyzed through the experiment.
2 Experiment arrangement and instrument
2.1 Spray impingement visualization system
The high pressure fuel supply system used in this study is
shown schematically in Figure 1 with the swirl type injector
installed inside a constant volume chamber. The high
pressurized nitrogen gas is responsible for delivering highpressure fuel up to 15Mpa to the injector. The chamber is
equipped with two quartz windows and connected to a
pressurized bottle of mixture gas composed of nitrogen and
carbon dioxide (N2:93.75%, CO2:6.25%) that the molecular
weight of mixture is equal to the molecular weight of air. The
maximum pressure value inside the chamber is 1.2Mpa. A flat
wall plate was installed into the chamber. The distance
between the injector nozzle tip and the flat wall and the
inclined angle of the flat wall can be adjusted conveniently.
The injection signal is controlled by ST Microelectronics
L9707 module which also generates 5V TTL pulse with
provided time delay, and the pulse will be transmitted to high
speed camera synchronously. The spray images were
recorded by high speed camera from Phantom Company with
512×512 resolution, frame rate of 10,000 frames per second.
The black thick line indicates the fuel or air, whereas the solid
line represents the signal. The macroscopic structure of the
fuel spray impingement, such as spray penetration and spray
development can be acquired directly from the images
captured by the high speed camera. The schematic of
inwardly opening, single fluid and swirl type direct injection
injector applied is shown in Figure 2.
Figure 2 Schematic of Swirl type injector
2.2 Spray impingement experimental conditions
In this paper the injection fuel mass of each time is set to
26.66mg for lean burn destination and the pulse width of
injection was set according to different injection pressure.
The spray development of impingement was conducted in
terms of the following conditions as Table 1.
Injection system
Injector
Injection pressure(Mpa)
Ambient pressure(Mpa)
Fuel mass injected each time(mg)
Wall distance(mm)
Wall inclination angle(degree)
Description
Swirl-type
2, 3, 4, 5, 6
0.05-1.2
26.66
30, 40, 50, 60
0, 30, 45, 60
Table 1: Experimental conditions for impinging spray
2.3 Spray impingement penetration definition
Figure 1 Constant volume chamber and fuel injection system
for spray impingement investigation
In order to describe the spray development behaviour after
impingement, here the spray height and spray radius
parameter are introduced from Figure 3 for vertical spray
impingement and figure 4 for inclined impingement. Spray
radius R represents the spray penetration development along
wall surface after impingement, while spray height H
represents the spray penetration development along vertical
wall surface direction after impingement. L represents
impinging distance between injector nozzle and wall and ș
means wall inclination angle.
Line 1: Pinj=6.0Mpa, Pback=0.1Mpa;
Line 2: Pinj=5.0Mpa, Pback=0.1Mpa;
Line 3: Pinj=4.0Mpa, Pback=0.1Mpa;
Line 4: Pinj=6.0Mpa, Pback=1.0Mpa;
Line 5: Pinj=5.0Mpa, Pback=1.0Mpa;
Line 6: Pinj=4.0Mpa, Pback=1.0Mpa;
Figure 3 Vertical spray impingement definitions
Figure 5 Vertical spray impingement development
Figure 4 Inclination spray impingement definitions
3 Results and discussion
3.1 Spray impingement at 0 degree wall inclination angle
Figure 5 shows the effects of injection pressure, ambient back
pressure, and at wall distance L=50mm on the macroscopic
behaviour of the fuel spray impingement.
0ms
1ms
2ms
3sm
4ms
Once the spray tip touch the wall, some of droplets wet the
surface of the wall and others especially from outside of the
hollow cone spray sharply splash and rapidly move toward
two sides. At the same time, there are vortexes formed at the
front of spray development. With the increase of injection
pressure, the spray impingement height, spray radius, sprays
tip speed and vortexes intensity increased. The higher
velocity at the point of impingement due to the high injection
pressure causes a higher radial velocity near the wall and
results in a sharper tip of spray. The smooth wall surface with
less friction is also a reason for this phenomenon. Under low
ambient back pressure, the spray structure showed a hollow
cone type formation which impinged on the wall with thin
droplets fuel film adhering and covering comparatively large
area. It is favourable for droplets evaporation and diffusion.
With the increase of ambient back pressure, the spray is
compact and with a solid cone and decreased spray angle, and
fuel just distributes in a small range which resulted the
formation of thick droplets fuel film adhering on the
impingement wall, and it is hard for droplets evaporation
during quite short time. After it impinges the wall, the spray
momentum is reduced and the spray tip velocity of moving
two sides is slowed down obviously due to the resistance
induced by high ambient pressure. The spray height and spray
radius are smaller than at lower ambient pressure. There are
vortexes formed during spray development and the vortexes
are stronger under high ambient back pressure than under low
ambient back pressure. These also become the factors which
slow down the spray tip velocity.
Figure 6 Influence of wall distance on spray height
Figure 6 shows that the spray height decreased with the
increase of wall distance. The spray height increased in the
beginning of impingement until to the maximum and then
starts to decrease due to droplets evaporation, spread and the
decreased spray tip momentum.
Figure 9 Influence of wall distance on spray radius
Figure 7 Influence of wall distance on spray radius
Figure 7 indicates that the spray radius increased with the
increase of wall distance but the spray tip velocity of moving
toward two sides is decreased which is the result of spray
momentum is slow down with the increase of wall distance
and most of the droplets evaporating and spreading during
spray development.
The spray radius under high ambient back pressure from
figure 9 showed the same development tendency as under low
back pressure but the spray radius values are smaller than
under low back pressure which also can be found the reason
that higher back pressure with higher resistance on spray
development.
3.2 Spray impingement at 30 degree wall inclination angle
In this following Figure 10, only the downward spray
penetration after impingement was measured because the
upward penetration is very small in comparison to the
downward penetration. Spray penetration increases with an
increase of the wall inclination angle. This tendency indicates
that the effect of wall impingement on spray is reduced owing
to the reduction in radial droplets momentum when increasing
the inclination angle of the wall.
0ms
Figure 8 Influence of wall distance on spray height
Figure 8 describes the same development as low back
pressure that the spray height decreased with the increase of
wall distance, but the spray height is obviously lower than
under low ambient back pressure conditions which can be
explained by that higher ambient back pressure with higher
resistance on spray itself. The thicker droplets film adhering
on the wall is unfavourable for fuel evaporation.
1ms
2ms
3sm
4ms
Line 1: Pinj=6.0Mpa, Pback=0.1Mpa;
Line 2: Pinj=5.0Mpa, Pback=0.1Mpa;
Line 3: Pinj=4.0Mpa, Pback=0.1Mpa;
Line 4: Pinj=6.0Mpa, Pback=1.0Mpa;
Line 5: Pinj=5.0Mpa, Pback=1.0Mpa;
Line 6: Pinj=4.0Mpa, Pback=1.0Mpa;
Figure 10 Vertical spray impingement development
Figure 11 Influence of wall distance on spray height
Figure 14 Influence of wall distance on spray radius
In the case of the inclined wall from Figure 11-14, the
downward penetration of the spray is very large. It may
indicate that the increased angle of wall inclination reduces
the effect of impingement on the macroscopic spray
characteristics. As the injection pressure increases, so does
spray velocity. With the increase of wall inclined angle, the
spray path behaviour is enhanced. The reason for this is that
it was affected by the droplets gravitation, drag and friction
forces. The spray pattern under high ambient back pressure is
compact and the spray height, spray radius and spray tip
velocity are smaller than low back pressure. When the
impingement angle is 0 deg, it has effects on the drag and
friction force as well as the ambient gas density.
4 Conclusions
Figure 12 Influence of wall distance on spray radius
Figure 13 Influence of wall distance on spray height
This experimental study was performed investigating the
effects of injection pressure, ambient back pressure, wall
distance from the injector tip to impinging wall, and wall
inclination angle on the macroscopic behaviour of fuel sprays
impingement. The conclusions of this study can be
summarized as follows:
(1) Providing physical understanding and experimental data
for validating CFD simulations.
(2) Providing support for designing the relative position of
injector and spark plug, piston and cylinder geometry.
(3) Impinging sprays exhibit longer spray penetration
behaviour at normal and radius direction of the wall
under higher injection pressure, shorter wall distances
and larger angles of wall inclination. However, the spray
height and spray radius decreased with the increase of
ambient pressure.
(4) The spray growth velocity is sharply decreased after
impingement of the spray. The spray height and spray
radius growth rate decreased due to ambient mixture gas
resistance and decreased droplets momentum.
(5) Higher injection and lower ambient pressure promote an
increase of the spray area, and causes more rapid
movement of the spray. Higher ambient pressure slows
down the spray movement.
(6) The spray structure showed a large hollow cone angle
and wide spreading at low back pressure, therefore the
fuel film adhered on wall is thin and it is favourable for
fuel evaporation and diffusion, but it is converse under
high back pressure and it is unfavourable for fuel
evaporation and diffusion.
(7) The formed vortexes under high back pressure are
stronger than under low back pressure. This became one
of factors that increase the spray height and decrease the
spray radius and spray tip velocity growth rate.
Acknowledgements
Financial support from Engine Department of R&D Centre,
China FAW Group Corporation is gratefully acknowledged.
The authors would like to thank the students from BUAA for
their valuable technical support during the course of this work.
References
[1] Sang Yong Lee, Sung Uk Ryu. Recent Progress of SprayWall Interaction Research, Journal of Mechanical Science
and Technology (KSME Int.J.), Vol.20, No.8, PP. 11011117, 2006.
[2] N ABANI, SBAKSHI and RVRAVIKRISHNA.
Multidimensional modelling of spray, in-cylinder air
motion and fuel-air mixing in a direct-injection engine,
Sadhana Vol.32,Part 5, October 2007, pp.597-617.
[3] S.KIM, J.M.NOURI, Y.YAN and C. ARCOUMANIS.
EFFECTS OF INTAKE FLOW ON THE SPRAY
STRUCTURE OF A MULTI-HOLE INJECTOR IN A
DISI ENGINE, International Journal of Automotive
Technology, Vol. 10, No.3, pp. 277-284(2009).
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