The document discusses the investigation of mixture distribution and flame propagation in a heavy-duty liquefied petroleum gas (LPG) engine utilizing a liquid phase LPG injection system. It emphasizes the advantages of improved mixture preparation and reduced emissions, particularly in minimizing NOx and particulate matter compared to conventional diesel engines. Experimental methods such as planar laser-induced fluorescence (PLIF) and high-speed imaging were used to analyze fuel distribution and the effects of various injection strategies and piston geometries on combustion characteristics.

1. Mixture distribution and flame
propagation in a heavy-duty liquid
petroleum gas engine with liquid
phase injection
S Oh and C Bae
Department of Mechanical Engineering,
Korea Advanced Institute of Science and
Technology, Daejeon, Korea
Accepted 21 April 2004
Abstract: Enhanced mixture preparation by liquid phase It is especially apparent that most PM (particulate
matter) and NO
x
(nitric oxides) emissions originateinjection on to the port could promote the application
from the diesel engine. Therefore, LPG (liquefiedof liquefied petroleum gas (LPG) in spark ignition (SI)
petroleum gas) is widely used as an alternative fuelengines. Mixture distribution and flame propagation of
in many countries throughout the world to reducethe liquid phase LPG injection (LPLI) engine with a large
the emissions. However, its use in heavy-duty vehiclebore size were investigated in a single-cylinder optical
applications produces many difficulties such asengine which had optical accesses through both sides of the
engine knock, thermal load and partial burning.cylinder liner and bottom of the piston. LPG fuel distri-
Recently, a liquid phase LPG injection (LPLI) systembution was visualized quantitatively by the acetone planar
was introduced [1, 2], which injects LPG fuel into thelaser-induced fluorescence (PLIF) method. In addition,
intake port in the liquid phase rather than in thea series of bottom-view images were taken by direct
gaseous phase.visualization using a high-speed camera. Flow conditions
The LPLI system has many advantages in itswere varied with the shapes of intake port and piston
application for a heavy-duty spark ignition enginegeometries characterized by different swirl and squish
modified from a diesel engine. Compared with aflows. The effects of fuel injection timings were also studied
conventional mixer system, it enhances volumetricto characterize the mixture preparation for open-valve
efficiency by lowering intake air temperature andinjection and closed-valve injection.
reduces toxic emissions such as NO
x
. In addition,Stronger swirl with a swirl ratio of 3.4 and squish flow
power output increases and risks such as backfirewith a cylindrical piston bowl shape showed faster flame
could be reduced significantly as well. The knockpropagations under the open-valve injection condition with
tendency in a spark ignition engine with a large borepreferable mixture formation. On the contrary, closed-valve
size can also be slightly relieved with the LPLI fuelinjection caused the undesirable mixture distribution of
supply system.the mixture at the ignition, which led to a leaner mixture
To ensure fuel economy, performance and emissiondistribution near the spark plug.
characteristics in the LPG engine, optimal combustion
chamber design should be investigated, which couldKey words: LPLI engine, mixture distribution, acetone
be accomplished by adopting a lean burn strategy.PLIF, fuel stratification, flame propagation
Lean burn generally causes unstable engine operation
due to combustion instability such as cyclic variation,
misfire or partial burning. There have been many1. Introduction
attempts to extend the lean operation range in spark
A heavy-duty vehicle with a diesel engine has been ignition engines. Desirable mixture preparation and
rapid combustion can be achieved by flow fieldconsidered as a main source of urban air pollution.
513Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

2. S Oh and C Bae
enhancement through optimal design of the intake access from the side and bottom, one at the upper
part of the cylinder liner and the other at the top ofport and piston shape.
The rich mixture in the vicinity of the spark plug the piston.
Figure 1 illustrates the schematic of the testedcould be formed through charge stratification by
controlling swirl intensity, squish flow and injection optical engine. A commercial heavy-duty diesel
engine, with a compression ratio of 17, was modifiedtiming [3, 4]. The injection strategy also affects the
stratification so that open valve injection usually to an LPG spark ignition (SI) engine with a lower
compression ratio of 9.3. A spark plug was mountedshows a better mixture preparation [5, 6]. It was
found that the axial stratification is maintained if the at the position of the diesel injector hole around the
centre of the combustion chamber. To minimizeradial component of the swirling motion is stronger
than the axial components [7]. The flow field in the vibration during operation of the single-cylinder
engine, the engine incorporated balance shafts withengine cylinder could be affected by the piston
geometry [8]. Inside the bowl-in-piston combustion counter-rotating weights. The optical engine was
driven by a d.c. dynamometer and controlled by achamber, the interaction between the swirl and
squish flow was intensively investigated [9–11]. programmable engine control unit (ECU). Table 1
shows the summary of its specifications.Planar laser-induced fluorescence (PLIF) using
acetone as a dopant has been used to visualize
2.2 LPLI fuel delivery systemfuel distributions in engines. Quantitative deter-
An LPLI fuel delivery system consists of injectormination of an air/fuel mixture was performed in
module, fuel pump, fuel tank and fuel lines, asengines [12, 13], direct injection engines [14] and
shown in Fig. 2. An LPG injector (DEKA-II injector,natural gas engines [14–16]. Pressure and temper-
Siemens-VDO), which is a bottom-feed type, wasature dependence of a fluorescence tracer was also
used and controlled by the programmable ECU.examined with acetone for an engine application
An injection nozzle specially designed for prevent-[17, 18].
ing ice formation around a nozzle hole due to latentIn the series of heavy-duty LPG engine develop-
heat of LPG fuel is attached at the end of the injector.ment, the optimized piston cavities for the LPLI system
The injector module is mounted 7 cm away from thewere investigated by experimental and numerical
cylinder head in the intake manifold. To keep themethods in a single-cylinder engine [19, 20]. In
fuel in the liquid phase, a fuel tank was alwaysaddition, mixture formation with a spray model for
pressurized with nitrogen gas up to 1.5 MPa. Toliquid LPG injection was simulated under closed-valve
circulate LPG, an external pump, originally readyinjection [21].
for gasoline fuel, was placed between the fuel tankIn the present work, mixture distribution and
and the injector module. The phase of liquid LPGflame propagation inside the LPLI optical engine
was monitored by measuring the temperature andcylinder were observed by direct imaging and the
pressure in the fuel supply line.acetone PLIF method. The fuel fluorescence with
doped acetone was visualized to recognize fuel
2.3 Optical set-updistribution induced by port injection and flow
For the PLIF measurement, a broadband KrF excimerfield interaction prior to ignition. Two-dimensional
laser was used as the light source, which has a maxi-quantification of the equivalence ratio was performed
mum energy of 400 mJ, wavelength of 248 nm andfor the purpose of clarifying the local variation of
pulse width of 20 ns. The laser sheet beam wasfuel distribution. The flame propagation images
shaped both horizontally and vertically, as shownwere compared among various flow field conditions
in Fig. 3. The laser sheet beams were formed by aformulated by different intake port and piston shapes.
combination of four different lenses. The beams wereThe effects of fuel injection timings on the mixture
partially masked by an iris at both edges beforepreparation were simultaneously analysed.
2. Experimental Apparatus
Stroke 140 mm
Bore 130 mm
Compression ratio 9.32.1 Optical engine
Displacement volume 1858 cm3
The experiments were carried out in an optically Quartz piston window size in diameter 77.6 mm
accessible single-cylinder engine. The engine was
equipped with two quartz windows for optical Table 1 Engine specifications.
514 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

3. Mixture distribution and flame propagation in a heavy-duty LPG engine
Fig. 1 Schematic diagram for PLIF and flame propagation imaging.
Fig. 2 LPLI fuel delivery system for the optical single-cylinder engine.
Fig. 3 Laser sheet beam configuration for the bottom and side views.
515Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

4. S Oh and C Bae
entering the cylinder. The horizontal laser sheet fired for every cycle. While this appeared to be a
defect of these kinds of works, acetone has been pre-beam, the width of which is 43 mm, was passed
through the quartz liner approximately 8 mm below ferred as the best choice at least for qualitative fuel
tracing when considering an absorption band access-the bottom of the cylinder head. The vertical beam,
26 mm wide, was directed into the combustion ible with KrF excimer laser, collisional quenching
and toxicity.chamber.
An intensified charge-coupled device (ICCD) As mentioned above, pure LPG fuel does not
fluoresce in the excited electronic state by lasercamera (4Quik05A, Stanford Computer Optics) was
synchronized with the laser output to take images of excitation. The acetone, 20 per cent by volume of the
injected amount of fuel, was doped. To quantify fuelthe fluorescence at any given crank angle by the
delay generator and signal counter. The master signal concentration from the fluorescence signal, several
sources of error should be corrected, i.e. (a) variationfor the synchronization of all equipment came from
the optical encoder linked to the engine. A WG305 of the laser power, (b) spatial non-uniformity of the
laser sheet beam, (c) distortion due to the curvaturecut-off filter was attached in front of the camera to
suppress the undesirable signals such as Mie scatter- of the quartz window, (d) the variation of CCD
element sensitivities in the camera and (e) back-ing and stray light inside the cylinder. A high-speed
charge-coupled device (CCD) video camera (Kodak ground signal noise. A specially written program
was applied to process raw images with the detailedSR-c) was also used for direct flame imaging,
coupled with a high-speed gated image intensifier to considerations for quantification as follows.
compensate for the weak flame illumination. The
3.1 Normalizationintensifier gain and the exposure time were adjusted
While the average energy output of the laser is stablefor lean operation.
over a long period of time, the shot-by-shot energy
can vary significantly. The variation of the laser
3. Acetone PLIF for LPG Fuel Imaging power output was usually measured directly with a
power meter. In this work, a portion of the laser
A pure LPG, a mixture of butane and propane, emits output is diverted with a beam-splitter and comes
no fluorescence. It needs the seeding of a fluorescing into the corner of view in the camera with attenuated
additive. In the current study, acetone (dimethyl intensity. Because the fluorescence image and laser
ketone) was chosen as the dopant because it is much intensity were recorded simultaneously, every raw
less dependent on oxygen quenching and is less toxic fluorescence image can be normalized using the
than other dopants [22]. The physical properties of reference intensity of the laser in the same image.
LPG and acetone are summarized in Table 2.
Even though acetone may not be suitable as a flow 3.2 Background image subtraction
There are various sources of interference inside thefield tracer because its boiling point is higher than
that of LPG fuel, it should be noted that all the LPG cylinder that increase the uncertainty of quantification,
such as fluorescence from the quartz window itself,and acetone must be vaporized by the end of the
intake process. This was verified by fluorescence deposits on the wall and strayed light. These noises
were observed under an engine motoring conditionimaging without the optical filter. No elastic scatter-
ing from droplets was found in the tested images. without any fuel injection. To remove the background
noise, 50 images under the motoring condition wereTo minimize this kind of error due to unequal boiling
points, the temperature of the engine coolant was averaged at the corresponding crank angle and
subtracted from the raw image.kept at 80 °C and also the engine was simultaneously
Properties LPG Acetone
Molecular formula C
3
H
8
(60 wt %), C
4
H
10
(40 wt %) CH
3
KCOKCH
3
Specific gravity at 25 °C 0.5247 0.7880
Boiling point at 1 atm (°C) −34.30 56.13
Vapour pressure at 20 °C (MPa) 0.61 0.02
Density at 20 °C (kg/m3) 531.6 789.8
Viscosity at 25 °C (cps) 0.1175 0.3075
Table 2 Physical properties of LPG and acetone.
516 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

5. Mixture distribution and flame propagation in a heavy-duty LPG engine
3.3 Flat field correction investigate the effects of swirl intensity. The swirl
ratio was measured in a steady flow rig. The RicardoAfter normalization and background subtraction, it
is necessary to account for the variation of pixel swirl number (Rs) was defined by the following
equation [23]:sensitivity and non-uniformity of the laser sheet
beam. This correction was performed for every pixel
by dividing the fluorescence images of homogeneous
mixtures. During this quantification procedure a Rs=
L
D P
a2
a1
C
f
N
R
da
AP
a2
a1
C
f
da
B
2
(1)
mixing tank was placed between the intake manifold
and the fuel injector to improve air/fuel mixing [4].
After all corrections were made as described above,
where L
D
is the engine shape parameter, C
f
the flow
the linear correlations between acetone fluorescence
coefficient, N
R
the non-dimensional rig swirl, a
1
the
intensity and the equivalence ratio were obtained at
opening time of the intake valve and a
2
the closing
each crank angle, as shown in Fig. 4.
time of the intake valve.
In addition, flame propagation measurements
were performed only in the lean condition, with4. Experimental Condition
overall equivalence ratios (w) of 0.8 but 1.0 for laser-
induced fluorescence (LIF) measurements becauseAll through this work, the optical engine was
its signal was too weak to acquire mixture distri-operated at an engine speed of 500 r/min, with an
bution in the lean region. Sometimes it is not easy tointake manifold pressure of 86 kPa and a coolant
distinguish the fluorescence signal from the noise.temperature of 80 °C. The LPG, formed as the mixture
Because the LIF signal is directly proportional toof 60 per cent propane and 40 per cent butane by
mixture concentration, the mixture distribution undermass, was used as a fuel. PLIF images were taken
lean conditions could be qualitatively representedduring the compression process, at BTDC (before
by observations under richer conditions. For bothtop dead centre) 120, 90, 60, 40 and 30° CA, described
measurements of mixture concentration and flameas the relative timings from compression TDC
propagation, ignition timings were always fixed at(top dead centre).
BTDC 20° CA.As for the injection timing, open-valve injection
Two geometrically different pistons were used(OVI) and closed-valve injection (CVI) were com-
to investigate the effects of piston geometry. Eachpared, which were BTDC 240 and 100° CA respect-
piston has a different squish area. Figure 5 illustratesively. The engine cylinder heads with different swirl
the shape of the pistons. One piston has a sphericalratios (Ricardo swirl number) of 2.3 and 3.4 were
bowl shape and the other has a cylindrical bowlimplemented by two different helical ports to
shape.
5. Results and Discussion
5.1 Mixture distribution
Figure 6 shows nine individual images selected
from a set of 50 images at BTDC 30° CA for w=1.0,
Rs=2.3 and open-valve injection. They show the
cyclic variations of the mixture distribution. In the
averaged images, a dominant trend in the mixture
distribution can be finally observed when the cyclic
fluctuating images are ensemble averaged.
The base engine used in the experiments is a
two-valve diesel engine with a helical intake port.
The flow field inside the cylinder mainly consists
of swirl flow that moves around the vertical axis of
the cylinder. Therefore, it is supposed that the fuel
Fig. 4 Relationship between the equivalence ratio (w) (or excess moves along the main stream, the swirling flow,
while the air/fuel mixing process proceeds.air ratio, l) and relative fluorescence intensity.
517Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

6. S Oh and C Bae
Fig. 5 Combustion chamber geometries of the tested pistons.
Fig. 6 Cyclic variation of fuel distribution at BTDC 30° CA
Fig. 7 The averaged bottom-view images of fuel distribution
for w=1.0, Rs=2.3 and open-valve injection.
during the compression process at an engine speed of
500 r/min, overall equivalence ratio (w)=1.0, open-
The images of LPG fuel distribution during com- valve injection and a spherical piston bowl.
pression from the bottom view are shown in Fig. 7
in the case of open-valve injection with w=1.0. Each
image at every crank angle was averaged from 50 fuel is found in the lower left side of the image at
BTDC 120° CA for Rs=2.3. The fuel is transportedsingle-cycle images to show the global effect of swirl
strength. Since the swirl flow moves in a clockwise to the upper side of the viewing area in the clockwise
direction. The cloud of fuel seemed to disappeardirection in this engine, the dense bulk motion of
518 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

7. Mixture distribution and flame propagation in a heavy-duty LPG engine
in the window at BTDC 60° CA, but thick fuel injection period is accumulated in the intake port and
vaporized easily, its mixture preparation is relativelyconcentration is observed again at a later stage
(as in BTDC 40 and 30° CA). In the case of a high homogeneous. At the early stage of the intake pro-
cess the fuel in the volume of the intake port is firstswirl ratio, Rs=3.4, the dense fuel concentration is
observed at the upper side of the viewing area and induced into the cylinder and then the bottom of
the cylinder becomes richer than the other partsalso rotates clockwise. The overall fuel concentration
in the viewing area is much higher than that of of the cylinder [7]. As the compression process
advances, the homogeneity of the equivalence ratioRs=2.3. This indicates that a strong swirl flow
hinders air/fuel mixing. This can be observed in side is improved. However, it seems that the average
equivalence ratio near the spark plug is still lowerview measurements more clearly.
Images of the side view, as presented in Fig. 8, than that of the other parts of the cylinder. This
implies that the axial fuel distribution is not desirableshow vertical distribution of LPG fuel in the case of
open- and closed-valve injections with Rs=3.4 and and might cause a misfire or partial burn for a lean
operation. On the contrary, the open-valve injectionw=1.0. The closed-valve injection case shows a
higher equivalence ratio in the lower part of the demonstrates a higher concentration at the top of
the cylinder where the swirling motion governs theimages during compression, even though there is a
small spatial variation. Because LPG fuel during the formation of a rich mixture, as shown in Fig. 7. The
Fig. 8 The averaged side-view images of fuel distribution during the compression process at an engine speed of 500 r/min, Rs=3.4,
overall equivalence ratio (w)=1.0 and a spherical piston bowl.
519Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

8. S Oh and C Bae
cloud of fuel observed at BTDC 120° CA is steadily from the spark plug while it increases with closed-
valve injection (CVI). However, for (b) Rs=2.3, thesustained and the rich zone near the spark plug is
finally formed near TDC, as shown in the images of vertical fuel concentrations present are relatively
constant for both injection timings. As the swirlBTDC 60–30° CA in Fig. 8.
In the case of Rs=2.3, as shown in Fig. 9, even intensity becomes strong, the axial fuel concentration
varies to the extent that the equivalence ratio (w) nearthough there is small variation, the overall equivalence
ratio in the images is nearly homogeneous compared the spark plug is over 1.3 with Rs=3.4.
The comparison between open- and closed-valvewith the stronger swirl condition. The fuel cloud
found in the left side of the image at BTDC 120° CA injections shows that the fuel stratification could be
achieved by the injection timing control with a strongis also spread out during the compression process.
Figure 10 shows the averaged equivalence ratio for swirl flow of Rs=3.4. It is noted that the fuel distri-
bution is significantly sensitive with injection timingsBTDC 30° CA along the z direction from the spark
plug to describe the axial mixture stratification, where in the case of strong swirl. A low fuel concentration
is observed near the spark plug in the case ofeach value represents the averaged equivalence ratio
overall through the x direction. The axially stratified the closed-valve injection while a rich mixture is
distributed in the lower part of the cylinder. Thefuel distribution could be quantitatively obtained.
For (a) Rs=3.4 with open-valve injection (OVI), the overall fuel concentration in the vertical imaging
area, when Rs=3.4, also seems to be higher than forequivalence ratio shows a gradual decrease away
Fig. 9 The averaged side-view images of fuel distribution during the compression process at an engine speed of 500 r/min, Rs=2.3,
overall equivalence ratio (w)=1.0 and a spherical piston bowl.
520 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

9. Mixture distribution and flame propagation in a heavy-duty LPG engine
geometry as for Rs=3.4. This again indicates that a
stronger swirl flow is more useful for rich mixture
formation near the spark plug. The cylindrical piston
bowl causes a stronger tumble motion than the
spherical piston bowl inside the cavity [21]. This
tumble motion pushes the mixture upwards with a
high equivalence ratio and finally results in a higher
fuel concentration near the ignition location. It is
supposed to enhance the combustion process for lean
operation.
5.2 Flame propagation
Raw flame images of different swirl ratios are pre-
sented in Fig. 11 for the cylindrical piston bowl under
the lean mixture condition (w=0.8). Flame images are
a series of frames in a cycle. The flame propagation
patterns are shown differently for both swirl ratios.
With Rs=2.3 the initial flame shape starts like a
circle and the flame front propagates towards the
exhaust valve side. The flame kernel is developed
around the spark plug and grows to larger flames in
an oval shape where the flame propagation direction
coincided with the swirl direction.
In the case of a high swirl ratio (Rs=3.4), the flame
continued to propagate in a circular shape. The initial
flame propagation is observed in the vicinity of the
spark plug. The flame is developed symmetrically
Fig. 10 The line averages of equivalence ratio along the vertical from the centre of the flame kernel. After the flame
direction from the spark plug at BTDC 30° CA with front exceeds the visualization limit, the piston
different injection timings and piston bowl shapes for window border, bright flames are observed around
w=1.0. the centre of the combustion chamber, rotating along
the swirl direction.
The flame stretches out along the major flow
direction in a weak swirl condition. However, whenRs=2.3. This means that, in the case of Rs=3.4, the
laid in a strong swirl flow field, trapped in the centre,rich region is centred and its fuel is not radially trans-
the flame developed into the circular flame with aported as much as in case of Rs=2.3. This coincides
flame front in a saw-tooth shape. The flame withwith the results of the bottom-view images in Fig. 7.
high swirl, Rs=3.4, shows a larger flame area thanIt is recognized that the advantage of strong swirl in
Rs=2.3. This implies that introducing a high levelopen-valve injection is due to the suppression of
of swirl increases the level of turbulence in theradial convection. A rich mixture remains in the
engine [24, 25].vicinity of the spark plug with the aid of a strong
Even though there are still uncertainties in theswirl flow. Mixing along the cylinder axis should be
flame images because they are two–dimensionaldelayed to retain axial stratification if the radial com-
projected ones of a three-dimensional propagatingponent of the swirling flow is stronger than the axial
flame, the flame propagation reflects the fuel distri-component. This confirms the report that the swirl
bution as presented in Figs 7 to 9. The late flameplays a key role in preserving axial fuel stratification
propagation in the centre with a high swirl ratio[7]. Figure 10 also shows the averaged equivalence
(Rs=3.4) is observed (e.g. 36° CA after ignition inratio with different piston bowl shapes. For both swirl
Fig. 11). It is supposed that the fuel burning isratios, the cylindrical piston bowl presents a higher
retained in the centre because the fuel concentrationequivalence ratio. For Rs=2.3, however, the fuel con-
centration is not affected as much by the piston in the centre of the cylinder is higher with a strong
521Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

10. S Oh and C Bae
Fig. 11 Flame images from ignition to 48° CA after ignition with overall equivalence ratio (w)=0.8, open-valve injection and a
cylindrical piston bowl.
swirl (Rs=3.4) and open-valve injection as shown in (Rs=3.4) is developed more rapidly than the weaker
swirl flow (Rs=2.3). However, the opposite trend isFig. 8b. In the case of a low swirl ratio (Rs=2.3), any
found in the case of closed-valve injection (CVI),noticeable burning flame in the centre is not found
even if a higher turbulence intensity for Rs=3.4 thannear the end of flame propagation.
for Rs=2.3 is supposed. As mentioned above, theFigure 12 shows the comparison of two-dimensional
axial fuel stratification under the strong swirl con-flame areas for two different swirl ratios, 2.3 and 3.4,
dition is strongly affected by fuel injection timing.at each crank angle. The flame area along the y axis
The flame propagation with Rs=2.3 does not dependrepresents the sum of the number of image pixels in
very much on its injection timing. The developmentthe inner region of the flame. The flame area was
of the flame area shows a very similar history forestimated by counting the pixels, the grey level of
each case, though the open-valve injection gives awhich is over the predetermined threshold value
slightly higher value. It should be noted that thererepresenting the clear border of the flame front with
is a large cyclic variation of the flame area formanifest contrast. In the case of open-valve injection
Rs=3.4 and closed-valve injection. It is most likely(OVI), the flame area for the stronger swirl flow
due to reverse stratification, as stated above. A leaner
mixture strength at the ignition timing may lead
to unstable initial flame development, resulting in
cyclic combustion variations. Unstable combustion,
such as misfire and partial burning, could be
expected in a real engine with this condition.
Figure 13 shows the flame areas for two different
piston geometries. Each piston has its own flow
characteristics, and it is squish intensity that causes
different flame propagations. The squish area of the
cylindrical piston bowl is 50 per cent larger than that
of the spherical piston bowl, as shown in Fig. 5. The
faster flame propagation of the cylindrical piston
bowl is clearly observed beyond variance. In addition
to a strong interaction between swirl and squish flow
Fig. 12 Flame areas from ignition to 30° CA after ignition for to accelerate in-cylinder combustion [3–6], it is con-
different swirl intensities and injection times with sidered that a higher concentration also exists near
overall equivalence ratio (w)=0.8 and a cylindrical the ignition location of the cylindrical piston bowl,
as shown in Fig. 10.piston bowl.
522 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

11. Mixture distribution and flame propagation in a heavy-duty LPG engine
5. The cylindrical shape piston bowl with the larger
squish area showed faster flame propagation than
the spherical shape piston bowl with a smaller
squish area.
Notation
a
1
opening time of the intake valve
a
2
closing time of the intake valve
BTDC before top dead centre
C
f
flow coefficient
CVI closed-valve injection
ICCD intensified charge-coupled deviceFig. 13 Flame areas from ignition to 30° CA after ignition
L
D
engine shape parameterfor two different piston geometries with overall
LPG liquefied petroleum gasequivalence ratio (w)=0.8, open-valve injection and
LPLI liquid phase LPG injectionRs=2.3.
N
R
non-dimensional rig swirl
NO
x
nitric oxides
OVI open-valve injection6. Conclusions
PLIF planar laser-induced fluorescence
PM particulate matterFuel distribution and flame propagation character-
istics in a two-valve heavy-duty engine with an LPLI Rs Ricardo swirl number
fuel supply system were investigated in a single- SI spark ignition
cylinder optical engine. LPG fuel distribution was TDC top dead centre
measured by the acetone PLIF method quantitatively
l excess air ratioand the flame development was acquired by direct
w equivalence ratioflame imaging according to piston shape, swirl
intensity and injection timing. Fuel concentration was
quantified from the PLIF images and the flame areas
were also calculated from the direct flame images.
This study leads to the following conclusions: References
1. Quantitative images of LPG fuel distribution during
1 Sirens, R. An experimental and theoretical study ofthe compression process were obtained using the
liquid LPG injection. SAE Paper 922363, 1992.
acetone planar laser-induced fluorescence method. 2 Jaasma, S. The development of heavy duty LPi systems.
2. With a higher swirl strength (Rs=3.4) and open- Vialle Technical Paper, 1998.
3 Moriyoshi, Y., Morikawa, H., Kamimoto, T. andvalve injection, the cloud of fuel followed the
Hayashi, T. Combustion enhancement of very lean pre-
flow direction and the radial air/fuel mixing was
mixture part in stratified charge conditions. SAE Paper
limited by a strong swirl flow. 962087, 1996.
3. The axial fuel stratification and concentration 4 Johansson, B., Neij, H., Alden, M. and Juhlin, G.
Investigation of the influence of mixture preparationwere sensitive to fuel injection timing in the case
on cyclic variations in a SI-engine, using laser induced
of Rs=3.4 but were relatively independent of the
fluorescence. SAE Paper 950108, 1995.
injection timing in the case of Rs=2.3. Thus, a 5 Arcoumanis, C. and Kim, J. W. Flow and combustion
strong swirl flow could promote desirable axial characteristics in a four-valve spark-ignition engine
fuelled by CNG. In Proceedings of the 4th Internationalfuel stratification and, as a result, may extend the
Conference on Internal Combustion Engine: Experiments
lean operation limit in a real engine.
and Modeling (ICE 99), Capri, Italy, September 1999,
4. In the case of Rs=3.4, faster flame propagation pp. 267–275.
was obtained with open-valve injection. However, 6 Gold, M. R., Arcoumanis, C., Whitelaw, J. H., Gaade, J.
and Wallace, S. Mixture preparation strategies in anin the case of closed-valve injection the trend was
optical four-valve port-injected gasoline engine. Int. J.
reversed and Rs=3.4, furthermore, showed slower
Engine Res., 2000, 1(1), 41–56.
flame propagation and greater cyclic variation of 7 Quader, A. A. The axially-stratified-charge engine. SAE
Paper 820131, 1982.the flame area.
523Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE

12. S Oh and C Bae
8 Johansson, B. and Olsson, K. Combustion chambers 17 Fujikawa, T., Hattori, Y. and Akihama, K. Quantitative
2-D fuel distribution measurements in an SI enginefor natural gas SI engines. Part 1: fluid flow and com-
bustion. SAE Paper 950469, 1995. using laser-induced fluorescence with suitable com-
bination of fluorescence tracer and excitation wave-9 Arcoumanis, C., Bicen, A. F. and Whitelaw, J. H.
Squish and swirl-squish interaction in motored model length. SAE Paper 972944, 1977.
18 Ghandhi, J. B. and Felton, P. G. On the fluorescentengines. J. Fluids Engng, 1983, 105, 105–112.
10 Fansler, T. Turbulence production and relaxation in behavior of ketones at high temperatures. Exp. Fluids,
1996, 21, 143–144.bowl-in-piston engines. SAE Paper 930479, 1993.
11 Nagayama, I., Araki, Y. and Iioka, Y. Effects of swirl 19 Kim, C., Lee, D., Oh, S., Kang, K., Choi, H. and Min, K.
Enhancing performance and combustion of an LPGand squish on S.I. engine combustion and emission.
SAE Paper 770217, 1997. MPI engine for heavy duty vehicles. SAE Paper
2002-01-0449, 2002.12 Wolff, D., Beushausen, V., Schluter, H., Andresen, P.,
Hentschel, W., Manz, P. and Arndt, S. Quantitative 20 Oh, S., Kim, S., Bae, C., Kim, C. and Kang, K. Flame
propagation characteristics in a heavy duty LPG engine2D-mixture fraction imaging inside an internal com-
bustion engine using acetone-fluorescence. In Proceedings with liquid phase injection. SAE Trans., 2002, 111(3),
2284–2296.of the International Symposium on Diagnostics and
Modeling of Combustion in Internal Combustion Engines 21 Lee, E., Park, J., Huh, K., Choi, J. and Bae, C.
Simulation of fuel/air mixture formation for heavy(COMODIA 94), Yokohama, Japan, 1994, pp. 445–451.
13 Wolff, D., Schluter, H., Beushausen, V. and Andresen, P. duty liquid phase LPG injection (LPLI) engine. SAE
Paper 2003-01-0636, 2003.Quantitative determination of fuel air mixture distri-
butions in an internal combustion engine using PLIF 22 Lozano, A., Yip, B. and Hanson, R. K. Acetone: a tracer
for concentration measurements in gaseous flows byof acetone. Ber. Bungenges. Phys. Chem., 1993, 97(12),
1738–1741. planar laser-induced fluorescence. Exp. Fluids, 1992,
13, 369–376.14 Rubas, P. J., Paul, M. A., Martin, G. C., Coverdill, R. E.,
Lucht, R. P. and Peters, J. E. Methane jet penetration 23 Ricardo steady state flow bench port performance
measurement and analysis technique. DP 93/0704,in a direct-injection natural gas engine. SAE Paper
980143, 1998. Ricardo, 1993.
24 Witze, P. and Vilchis, F. Stroboscopic laser shadow-15 Hiltner, J. and Samimy, M. A study of in-cylinder
mixing in a natural gas powered engine by planar laser- graph study of the effect of swirl on homogeneous com-
bustion in a spark-ignition engine. SAE Paper 810226,induced fluorescence. SAE Paper 961102, 1996.
16 Hiltner, J. and Samimy, M. The impact of injection 1981.
25 Witze, P. The effect of spark location on combustion intiming on in-cylinder fuel distribution in a natural gas
powered engine. SAE Paper 971708, 1997. a variable-swirl engine. SAE Paper 820044, 1982.
524 Int. J. Engine Res. Ω Vol. 5 Ω No. 6
JER 01304 Ω © 2004 Ω IMechE