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Mixture distribution and flamepropagation in a heavy-duty liquidpetroleum gas engine with liquidphase injection
SOh and C BaeDepartment of Mechanical Engineering,Korea Advanced Institute of Science andTechnology, Daejeon, Korea
Accepted 21 April 2004
Abstract: Enhanced mixture preparation by liquid phase It is especially apparent that most PM (particulate
matter) and NOx
(nitric oxides) emissions originateinjection on to the port could promote the applicationfrom 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 ofin many countries throughout the world to reducethe liquid phase LPG injection (LPLI) engine with a largethe emissions. However, its use in heavy-duty vehiclebore size were investigated in a single-cylinder opticalapplications produces many difficulties such asengine which had optical accesses through both sides of theengine 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 planarwas 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 directgaseous 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 pistonapplication for a heavy-duty spark ignition enginegeometries characterized by different swirl and squishmodified from a diesel engine. Compared with aflows. The effects of fuel injection timings were also studiedconventional mixer system, it enhances volumetricto characterize the mixture preparation for open-valveefficiency 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 flamecould be reduced significantly as well. The knockpropagations under the open-valve injection condition withtendency in a spark ignition engine with a large borepreferable mixture formation. On the contrary, closed-valvesize can also be slightly relieved with the LPLI fuelinjection caused the undesirable mixture distribution ofsupply 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, acetonebe accomplished by adopting a lean burn strategy.PLIF, fuel stratification, flame propagationLean burn generally causes unstable engine operation
due to combustion instability such as cyclic variation,
misfire or partial burning. There have been many1. Introductionattempts 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.
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 fuel2.3 Optical set-updistribution induced by port injection and flowFor the PLIF measurement, a broadband KrF excimerfield interaction prior to ignition. Two-dimensionallaser was used as the light source, which has a maxi-quantification of the equivalence ratio was performedmum energy of 400 mJ, wavelength of 248 nm andfor the purpose of clarifying the local variation ofpulse width of 20 ns. The laser sheet beam wasfuel distribution. The flame propagation imagesshaped both horizontally and vertically, as shownwere compared among various flow field conditionsin 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 mixturepartially masked by an iris at both edges beforepreparation were simultaneously analysed.
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. The3.1 Normalizationintensifier gain and the exposure time were adjustedWhile 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 laserA pure LPG, a mixture of butane and propane, emits output is diverted with a beam-splitter and comesno fluorescence. It needs the seeding of a fluorescing into the corner of view in the camera with attenuatedadditive. In the current study, acetone (dimethyl intensity. Because the fluorescence image and laserketone) was chosen as the dopant because it is much intensity were recorded simultaneously, every rawless dependent on oxygen quenching and is less toxic fluorescence image can be normalized using thethan 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 C3H8(60 wt%), C
4H10(40 wt%) CH
3KCOKCH
3Specific gravity at 25 °C 0.5247 0.7880Boiling point at 1 atm (°C) −34.30 56.13Vapour pressure at 20 °C (MPa) 0.61 0.02Density at 20 °C (kg/m3) 531.6 789.8Viscosity at 25 °C (cps) 0.1175 0.3075
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, thelaid 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 coincidesflame 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 inRs=2.3. This implies that introducing a high levelopen-valve injection is due to the suppression ofof 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
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
a1
opening time of the intake valve
a2
closing time of the intake valve
BTDC before top dead centre
Cf
flow coefficient
CVI closed-valve injection
ICCD intensified charge-coupled deviceFig. 13 Flame areas from ignition to 30° CA after ignitionLD
engine shape parameterfor two different piston geometries with overallLPG liquefied petroleum gasequivalence ratio (w)=0.8, open-valve injection andLPLI liquid phase LPG injectionRs=2.3.NR
non-dimensional rig swirl
NOx
nitric oxides
OVI open-valve injection6. ConclusionsPLIF 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
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