Air-Fuel Mixing in a Homogeneous Charge DI Gasoline Engine
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2001-01-0968
Air-Fuel Mixing in a Homogeneous Charge DI Gasoline Engine
Martin Gold, John Stokes, Robert Morgan
Ricardo
Morgan Heikal, Guillaume de Sercey, Steve Begg
University of Brighton
Copyright © 2001 Society of Automotive Engineers, Inc.
ABSTRACT Previous work by the authors and colleagues has
examined in-cylinder air motion and fuel spray
For optimum efficiency, the direct injection (DI) gasoline characteristics [6, 7, 8, 9]. The work described in this
engine requires two operating modes to cover the full paper continued these studies to examine in-cylinder
load/speed map. For lower loads and speeds, stratified mixture formation under early injection conditions using
charge operation can be used, while homogeneous optical visualisation and fluorescence techniques,
charge is required for high loads and speeds. This paper including calibrated LIF measurements of air/fuel ratio.
has focused its attention on the latter of these modes, The results were related to engine performance by
where the performance is highly dependent on the quality comparing with non-optical fired engine combustion data
of the fuel spray, evaporation and the air-fuel mixture obtained under similar operating conditions.
preparation.
OBJECTIVES
Results of quantitative and qualitative Laser Induced
Fluorescence (LIF) measurements are presented, The objectives of the work described within this paper
together with shadow-graph spray imaging, made within are to:
an optically accessed DI gasoline engine. These are
compared with previously acquired air flow • Perform quantitative laser induced fluorescence
measurements, at various injection timings, and with measurements within a DI gasoline engine
engine performance and emissions data obtained in a • Measure and compare the in-cylinder fuel distribution
fired single cylinder non-optical engine, having an for a series of injection timings
identical cylinder head and piston crown geometry. • Examine and assess the mixture formation
processes by comparing the LIF fuel distribution
INTRODUCTION results with previous air flow measurements
• Investigate the correlation between mixture formation
The introduction of direct injection (DI) gasoline engines processes and combustion data results
into the market place has been a consequence of
continued pressure to improve fuel economy and reduce TECHNICAL APPROACH
CO2 emissions, occurring firstly in Japan [1,2]∗ and more
recently in Europe. The majority of published research ENGINE CONFIGURATION - The present analysis has
on DI gasoline combustion systems has focused on focused on the homogeneous charge operating mode of
understanding mixture preparation under late a top-entry Ricardo DI gasoline engine (Table 1 and
(compression stroke) injection, stratified operating Figure 1). Previous investigations have centered on the
conditions [3, 4, 5]. Equally important are the processes experimental examination of the in-cylinder air flow using
involved in producing a homogeneous charge with early both the ‘dynamic flow visualisation rig’ (DFVR) and in-
(intake stroke) injection timing. Mixture quality under cylinder laser Doppler anemometry, within the present
these conditions is important for low octane requirement, optical engine, plus comparisons and analysis of phase
low smoke, low cyclic torque variation and high full load Doppler anemometry, spray imaging and qualitative LIF
air utilisation. data. Subsequent comparison of the experimental data
with the computational fluid dynamic software (VECTIS)
has shown good correlation [8].
∗
Numbers in [] denote referencesenhancing the intensity of fluorescence and recording
Engine speed 1500 rev/min only during its short lifetime, therefore eliminating a lot of
Bore 74.0 mm ambient illumination. As the wavelength of the
Stroke 75.5 mm fluorescence image is Stokes shifted, it can be easily
Intake valve opening 16° BTDC (intake) separated from the excitation energy by an appropriate
Intake valve closing 52° ABDC (intake) band pass filter. Continuous measurement through one
Exhaust valve opening 54° BBDC (exhaust) injection or engine cycle is precluded by a maximum
Exhaust valve closing 18° ATDC (intake) repetition rate of the camera/intensifier and laser
Max valve lift 8.1 mm combination of 4 Hz. A picture of the air/fuel mixing
Table 1: Engine parameters process must therefore be built from several imaged
cycles.
Figure 2a : Photograph of experimental layout
Laser Quartz annulus
sheet in
Figure 1: Schematic of engine configuration
IN-CYLINDER DIAGNOSTIC TECHNIQUES - In the
present experiments, the techniques of shadow-graph Fluorescence
spray imaging [10] and laser induced fluorescence (LIF)
[11,12,13,14,15] have been employed to investigate the and spray
in-cylinder air/fuel mixing for the DI gasoline operating images out
conditions under investigation.
Figure 2a illustrates the arrangement used within the Figure 2b: Photograph of experimental optical access
optical research engine for the LIF technique. The laser
sheet is introduced along the engine mid-cylinder plane A fuel-tracer mixture was used for the LIF measurements
and is produced and optimised for the fourth harmonic of and comprised 95% iso-octane with 5% acetone by
the pulsed Nd:YAG laser (266 nm, ultra-violet) by a volume; this mixture was calibrated prior to any
series of cylindrical lenses. The result is a laser sheet of quantitative in-cylinder mixture measurement.
height 20 mm and thickness 1 mm, which is collimated
within the test section, critical to the accuracy of any Acetone was used as the fluorescent tracer due to its low
quantitative experiments. The laser has been tuned by sensitivity to pressure and temperature quenching, hence
the manufacturer to work with a repetition rate of minimising errors through cycle-to-cycle variations in
12.5 Hz, corresponding to an engine speed of these parameters, plus its high quantum yield and boiling
1500 rev/min. The resulting fluorescence signal is point of 56 °C. Since the present experiments have been
imaged onto a CCD camera (1268x1024) mounted in an conducted within a motored engine, spray impingement
orthogonal plane (Figure 2b) presenting a ‘2D’ slice of in- will occur upon a relatively cold piston. The low boiling
cylinder information frozen in time. Since the point of acetone will facilitate evaporation under these
fluorescence is weak and short-lived, it is imaged on a conditions, hence offering a closer simulation of fired
gated intensified camera, having the advantage of engine mixture preparation conditions.Quantitative LIF requires extensive calibration of the Start of Injection End of Injection Engine speed
correlation between the fluorescence signal and local fuel
concentration, before in-cylinder air/fuel ratios can be TDC 61° CA ATDC 1500 rev/min
derived. In the present application this calibration was 30° CA ATDC 91° CA ATDC 1500 rev/min
performed within the optical engine, while motoring the 60° CA ATDC 121° CA ATDC 1500 rev/min
engine and employing a unique closed-loop approach. Table 2: Optical engine operating conditions for the
The major benefits of the closed-loop in-cylinder LIF shadow-graph spray imaging
calibration are:
• identical transmitting and receiving optical paths in ENGINE COMBUSTION TESTS - Combustion testing
calibration and experiment was conducted in an engine geometry identical to the
• precise matching of the in-cylinder physical optical engine analysis, as part of an investigation into a
conditions for each crank-angle lean boost DI gasoline concept [16]. The testing
• direct air/fuel ratio comparison for each crank-angle consisted of globally lean air/fuel ratios, rather than the
• variations in the laser sheet energy density, optical rich air/fuel ratios used in the optical engine. However,
distortion and reflections can be filtered. the behaviour of the air and the fuel spray in both
(A more detailed description and discussion of this engines are comparable. Combustion data collected
calibration technique will be presented in a future SAE consisted of in-cylinder pressure, derived IMEP,
paper). coefficient of variation (CoV) of IMEP, mass fraction
burned and smoke. The fired engine operating conditions
are outlined in table 3.
Start of Engine load Engine
injection Full 4 bar IMEP speed
(°CA ATDC) (rev/min)
0 X 1500
15 X 1500
30 X X 1500
45 X 1500
60 X X 1500
90 X 1500
Table 3: Fired engine operating conditions
optical Injector
mounting RESULTS AND DISCUSSION
access
MIXTURE FORMATION
Figure 3: Photograph of optical cylinder head In-Cylinder Spray Visualisation - Prior to the analysis of
the fuel/air mixing processes with LIF, the in-cylinder
While a standard cylinder head in conjunction with a spray structure was investigated for the full load condition
20 mm fused silica annulus was used for the LIF work, a of SOI at 60° CA ATDC and an engine speed of
separate dedicated cylinder-head with pent-roof optical 1500 rev/min. Figure 4 shows a sequence of images
access was available for the shadow-graph imaging illustrating the effects of the intake air motion on the
(Figure 3). This technique allows a quick fuel spray injected spray during the early stages of injection, the
analysis as opposed to a laser sheet where several 2D mid-phase, corresponding to the injector’s steady state
plane data sets are required for a 3D analysis. condition, and finally the closing stage.
A halogen lamp was placed diametrically opposite the The DI injector spray can be initially seen to enter the
camera, illuminating the combustion chamber. In this cylinder at 65° CA ATDC with a narrow pencil structure
way, the injected spray obscures the transmission of the having a high penetration velocity (approximately
light to the camera and forms a shadow. A high speed 120 m/s). Between the opening and closing transient
intensified CCD camera (IMACON 468) having a spatial injection flow periods, the mid-injection can be
resolution per channel of 576x385 was employed to considered as a steady state flow condition. During this
obtain the data. After an initial crank angle derived TTL phase the injected spray can be seen to develop into a
trigger it was able to acquire up to eight consecutive narrow angled hollow cone structure. Since the technique
images with inter-frame spacing down to 10 ns. The relies upon the obscuration of light passing through the
operating conditions for the shadow-graph spray imaging liquid droplets, the hollow structure can be easily
and the LIF tests are summarised in Table 2. identified by the two darker regions representing the top
and bottom surfaces of the cone.cylinder centre-line using the DFVR, it was shown that air
velocities of greater than 20 m/s are present on the
65° CA intake side during this period. The injector is positioned
ATDC between and directly below the intake valves, where the
two air-steams from each valve will meet, creating an
area of high turbulence and flow fluctuation. This high
velocity perturbating air flow will have a direct impact
upon the injected fuel. Once the spray has become fully
established, shown in the timings of 80° CA ATDC and
105° CA ATDC, the spray can be seen to be deflected,
indicating reverse tumble influence on the small fuel
droplets. A greater degree of break-up can be seen on
the upper edge of the injected spray, since this is the
intake air/spray interface. Evidence of this upper edge
80° CA variability was previously noted in [6]. Further tests at the
lower engine speeds of 1000 rev/min and 500 rev/min
ATDC indicated reducing degrees of spray deflection and
break-up due to the overall lower intake air velocities and
consequential lower levels of turbulence.
At 125° CA ATDC the closing stages of the injection
process are represented (Figure 5), with evidence of a
more significant degree of spray structure break-up. The
lower droplet velocities present during these latter stages
will result in the air motion being the dominant driving
force. High speed video taken under the same injection
conditions showed similar highly variable injection spray,
105° CA plus entrainment of smaller droplets which are carried
ATDC into the cylinder centre. For homogeneous operation,
interactions between the air and droplets can be
favourable in the mixing process, although the high
cycle-to-cycle variability could ultimately prove
detrimental to the combustion stability, even at these
early crank angles.
Electronic pulse
Injection flow rate (approximation)
125° CA 0
55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135
ATDC
Crank angle (deg AT DC)
Table 5: Example of typical injection electronic pulse and
fuel flow rate profiles (SOI @ 60° CA ATDC)
Local Air/Fuel Ratio Measurements - In order to offer
explanations for the mixture formation processes for
various injection timings within the DI engine, the LIF
data has been calibrated to provide the local air/fuel
ratios across the cylinder centre-line. These results will
Figure 4: Visualisation of injected sprays
compliment the explanations offered for the air / spray
(SOI @ 60° CA ATDC)
interactions in the previous results. Since the tracer LIF
technique displays information on the fuel concentration,
During the studied injection period of 60 -121° CA ATDC,
a very strong signal will be gathered in the presence of
the two events of maximum piston speed and maximum
liquid fuel, which could damage the image intensifier. In
valve lift will occur. The resulting intake air mass flow will
the present experiments the camera, intensifier and lens
consequently be at a maximum during the injection
parameters have been optimised for analysis of fuel
event. From the characterisation of the air motion on thevapour, hence the earliest image acquisition is 15° CA exhaust side cylinder wall, and rolled up into the upper
after the end of the respective injection. part of the combustion chamber due to the bulk charge
motion.
The relationship between the LIF mixture measurements
and the air flow data gathered using the DFVR has been Cycle-to-cycle variability in mixture strength is illustrated
analysed to lend support to the LIF data and the in Figure 8 by the CoV in the LIF measurements, where
corresponding mixture formation mechanism analysis. all three injection timings show regions of variability
Figure 6 shows the mixture distribution and the CoV of above 25%. Figure 8a illustrates the variability in
mixture distribution compared to air flow at 90° CA ATDC transportation of fuel mixture out of the piston bowl, with
for SOI at TDC. In Figure 6a, a rich region with evidence of the influence of the intake air flow
equivalence ratio values between 1.2 and 1.8 can be perturbation in the under valve region. A similar under
seen on the exhaust side of the chamber. It appears to intake valve variability is seen in Figure 8b for the SOI
have been deflected off the piston bowl and transported timing of 30° CA ATDC. As the injection timing is
within the prevailing flow out of the piston bowl into the retarded to 60° CA ATDC the level of spray break-up and
exhaust side re-circulation region. The high air velocity entrainment has increased due to the increased intake
entering through the intake valves has resulted in a air mass flow and variability. Cycle-to-cycle variation of
dilution of the mixture in the under valve area, down to over 25% on the exhaust side indicates the extent of
equivalence ratio values of less than 0.5. There is a injection roll-up into the upper regions of the chamber.
distinct division between this lean region and the rich
region which correlates with the shear layer between the Moving through the stroke, Figure 9 shows the mixture
piston bowl jet and the intake air flow. From the spray distribution for the three injection timings, at intake BDC,
data, the highly turbulent air flow was seen to cause superimposed with the DFVR air-flow measurements.
spray break-up. The corresponding CoV in the mixture For the two earlier timings, a higher equivalence ratio can
strength (Figure 6b) indicates a similarly high level of be seen in the central region of the upper cylinder due to
cycle-to-cycle variation. The lean mixture can be seen to the fuel/air mixture carried in the reverse tumble vortex
have a CoV of up to 25% in this under valve region. The out of the piston bowl. For the timing of SOI at TDC, the
trajectory of the mixture jet from the bowl exit will rich region has been drawn into the lower cylinder with
additionally be influenced by the cycle-to-cycle variability the prevailing air motion. Conversely the upwardly
of the intake air flow. This has also been captured by the moving injection roll up for SOI at 60° CA ATDC is still
region with a mixture strength CoV of up to 10% lying evident at BDC due to both its upward motion and the
within the piston bowl jet velocity vectors. reduced time between end of injection and BDC when
compared to SOI at TDC.
An injection timing swing was performed to aid the
understanding of the different mixture formation Since the piston geometry interferes with the laser sheet
processes present during the DI gasoline engine for crank angles after 280° CA ATDC this timing is the
homogeneous charge operating mode. The initial series latest in-cylinder mixture distribution presented, and is
of images acquired 15° CA after the end of injection for shown in Figure 10. The rich region carried into the lower
each of the injection strategies are shown in Figure 7. part of the chamber for the SOI at TDC can be seen to
Figure 7a compliments the result shown in Figure 6 with re-appear on the exhaust side during compression
a rich region emanating from the lip of the piston bowl. (Figure 10a). A more even global mixture distribution is
However, the overall equivalence ratio levels are higher evident in Figure 10b for SOI at 30° CA ATDC. However
than in Figure 6 due to less mixture dilution by the intake the reduced evaporation and mixing time has resulted in
air. At 105° CA ATDC, for an SOI of 30° CA ATDC, the exhaust side enrichment for SOI at 60° CA ATDC due to
mixture strength can be seen to be globally lean as a the initial injection roll-up region.
result of the reduced piston impingement for this injection
timing, such that the injected fuel has passed through the Figure 11 illustrates the effect of reducing evaporation
measurement plane by this time. Conversely the SOI and mixing time available from the end of injection. The
timing of 60° CA ATDC still has a strong mixture later start of injection (and hence end of injection due to
presence on the intake side of the combustion chamber; fixed pulse width) shows an increasing level of cycle-to-
an equally rich region is also evident in the exhaust cycle variability in the in-cylinder mixture strength. These
region. The explanation for the injection tail can be variability effects and the global mixture distribution
derived from the spray break-up evident in Figure 4, processes will influence the combustion performance of
entraining liquid fuel droplets in the under valve region the fired engine. The next section will address some of
and hence the high liquid portion fluorescence signal. these effects.
While the fuel presence on the exhaust side of the
chamber was shown to come out of the bowl for the
injection timing starting at TDC, with the strategy of SOI
at 60° CA ATDC the piston will be too far down the bore
to have a similar influence. Under these conditions it is
proposed that the fuel spray has impinged upon theAFR(φ) CoV
Rich High
1.8 25%
20%
1.2
15%
10%
0.6
5%
0 0%
Lean Low
Figure 6 (a): Mixture distribution @ 90° CA ATDC for a Figure 6 (b): CoV in the mixture distribution @ 90° CA
SOI @ TDC; superimposed DFVR air-flow ATDC for a SOI @ TDC; DFVR air-flow
Figure 6: Comparison of in-cylinder mixture distribution and DFVR derived air-flow
AFR (φ)
Rich
2.5
2.0
1.5
1.0
0.5
Figure 7 (a): Mixture distribution Figure 7 (b): Mixture distribution Figure 7 (c): Mixture distribution @
@ 75° CA ATDC for a SOI @ @ 105° CA ATDC for a SOI @ 135° CA ATDC for a SOI @ 60°
TDC (φ range = 0 – 2.55) 30° CA ATDC (φ range = 0 – 2.55) CA ATDC (φ range = 0 – 2.55) 0
Lean
Figure 7: In-cylinder mixture distribution 15° CA after the end of injection
CoV in
AFR (φ)
25%
20%
15%
10%
5%
Figure 8 (a): CoV in the mixture Figure 8 (b): CoV in the mixture Figure 8(c): CoV in the mixture
0%
distribution @ 75° CA ATDC for a distribution @ 105° CA ATDC for distribution @ 135° CA ATDC for
SOI @ TDC a SOI @ 30° CA ATDC a SOI @ 60° CA ATDC
Figure 8: CoV of in-cylinder mixture distribution 15° CA after the end of injection
NB: Different scales have been used to maintain a visible contrast between the differing mixture strength regimes
within the cylinderAFR (φ)
Rich
2.5
2.0
1.5
1.0
0.5
0
Lean
Figure 9 (a): Mixture distribution @ 180° Figure 9 (b): Mixture distribution @ 180° Figure 9 (c): Mixture distribution @ 180°
CA ATDC for a SOI @ TDC with CA ATDC for a SOI @ 30° CA ATDC; with CA ATDC for a SOI @ 60° CA ATDC; with
superimposed DFVR derived air-flow superimposed DFVR derived air-flow superimposed DFVR derived air-flow
Figure 9: Mixture distribution @ 180° CA ATDC with superimposed DFVR derived air-flow
AFR (φ)
Rich
1.5
1.0
0.5
Figure 10 (a): Mixture distribution Figure 10 (b): Mixture distribution Figure 10 (c): Mixture distribution
@ 280° CA ATDC for a SOI @ @ 280° CA ATDC for a SOI @ @ 280° CA ATDC for a SOI @ 0
TDC (φ range = 0 - 1.5) 30° CA ATDC (φ range = 0 - 1.5) 60° CA ATDC (φ range = 0 - 1.5)
Lean
Figure 10: Mixture distribution @ 280° CA ATDC
CoV in
AFR (φ)
High
15%
10%
Figure 11 (a): CoV in the Figure 11 (b): CoV in the Figure 11c): CoV in the mixture
mixture distribution @ 280° CA mixture distribution @ 280° CA distribution @ 280° CA ATDC
ATDC for a SOI @ TDC ATDC for a SOI @ 30° CA for a SOI @ 60° CA ATDC
ATDC 5%
Low
Figure 11: CoV in the mixture distribution @ 280° CA ATDC
NB: Different scales have been used to maintain a visible contrast between the differing mixture strength regimes
within the cylinderCOMBUSTION AND LIF COMPARISON timing is retarded, less piston spray impingement occurs
and less fuel is carried over to the exhaust side
FULL LOAD OCTANE REQUIREMENT - Figure 12 combustion chamber wall.
shows the knock-limited ignition advance versus start of
injection timing at 1500 rev/min wide open throttle with a 2
constant 22:1 air/fuel ratio. The changes in ignition
advance reflect changes in octane requirement. 1.5
Optimum start of injection for octane requirement was at
FSN
30° CA ATDC, with octane requirement increasing for 1
more advanced or retarded injection timings. When
operating at a mean air/fuel ratio of 22:1, fuel rich areas 0.5
in the combustion chamber, particularly in the end-gas
regions, would be detrimental to octane requirement. 0
0 15 30 45 60 75 90
Octane requirement Improvement
SOI
40
Figure 13: Engine out smoke (FSN) correlated to SOI
Ign (deg BTDC)
30 (Full load 1500 rev/min)
20
10 PART LOAD CYCLIC COMBUSTION STABILITY -
Figure 14 shows cycle to cycle combustion stability,
0 measured as coefficient of variation of IMEP, versus start
0 15 30 45 60 75 90 of injection timing at 1500 rev/min 4 bar IMEP. In this
SOI (degATDC) case the air/fuel ratio was 14.5 and 10% external EGR
was applied. Later injection timings produced an
increase in cycle to cycle combustion variation, with a
more rapid deterioration beyond 45° CA ATDC. This
Figure 12: Knock limited ignition timing (°CA ATDC) correlates with the increase in coefficient of variation of
correlated to SOI (Full load 1500 rev/min) AFR observed in the LIF results, shown in Figure 11. It
would appear that, although impingement on the bowl at
early start of injection results in some mixture in-
Turning to the LIF results shown in Figure 10, a start of homogeneity, the reliability of this mode of fuel transport
injection timing of 30° CA ATDC appears to be optimum and evaporation results in low cycle to cycle AFR
for mixture homogeneity. With start of injection at TDC variation. As injection timing is retarded, less fuel
or 60° CA ATDC there is a rich area on the exhaust side impingement occurs and more reliance is placed on air
of the combustion chamber, more so for TDC start of motion for fuel transport and evaporation. This results in
injection. This would explain the increased octane improved fuel evaporation rate and mixing. However, the
requirement at these injection timings. The increase in cycle to cycle variation in air motion, combined with
coefficient of variation of AFR with start of injection reduced SOI-to-ignition interval, leads to increased cycle
60° CA ATDC would also have a detrimental effect on to cycle AFR variation.
octane requirement. In a fired engine operating at 22:1
air/fuel ratio, cycles containing locally fuel rich areas 3
would be more likely to knock.
2.5
CoV IMEP (%)
FULL LOAD SMOKE - Figure 13 shows smoke versus 2
start of injection timing at 1500 rev/min wide open throttle 1.5
with a constant 22:1 air/fuel ratio. For more information 1
on the lean boost DI gasoline concept, please refer to
[16]. Start of injection at TDC produces the highest 0.5
smoke emissions, with smoke reducing as injection 0
timing is retarded. The LIF results provide an 0 15 30 45 60
explanation for these observations. With start of injection
at TDC, fuel impinges on the wall of the bowl and is SOI (degATDC)
carried by its own momentum and the strong air motion
over to the exhaust side of the combustion chamber, Figure 14: CoV in IMEP (%) correlated to SOI (Part load
where some probably impinges on the cylinder wall. Any 1500 rev/min)
fuel which does not evaporate from the piston surface
and cylinder walls will be ignited by the pre-mixed flame
and burn by diffusion, producing smoke. As injectionCONCLUSION 4. Sacadura J.C, Robin L., Dionnet F., Gervais D.,
Gastaldi P. and Ahmed A. (2000); “Experimental
The use of in-cylinder diagnostic techniques in a single- investigation of an optical direct injection SI engine
cylinder DI gasoline engine has revealed strong using fuel/air ratio laser induced fluorescence”, SAE
correlation between data from different optical techniques paper 2000-01-1794.
and combustion performance. The following conclusions 5. Zhao H, Williams J, Damiano L., Bryce D.,
can be drawn from the observations: Ladommatos N and Ma T. (2000); “Optical engine
and laser diagnostics for stratified charge and
• The high velocity spray from a DI injector will be controlled auto-ignition combustion studies”, IMechE
deflected by intake air during the homogeneous - International conference on computational and
operating mode. experimental methods in reciprocating engines, 1-2
• Different injection timings result in different mixture Nov 2000
formation processes. 6. Comer M.A., Bowen P.J., Sapsford S.M. Johns
• Fuel vapour is carried out of the piston bowl by the R.J.R., (1998), “The transient effects of line pressure
reverse tumble air motion for early injection timings for pressure swirl gasoline injectors”, ILASS 98,
• There is evidence of fuel spray impingement and Manchester, July 1998.
’rolling-up’ on the exhaust side cylinder wall for later 7. Comer M.A., Bowen P.J., Bates C.J. Sapsford S.M.,
timings. “CFD modelling of direct injection gasoline sprays”,
• Vaporised fuel is carried in the prevailing reverse ILASS 99, Toulouse, July 1999.
tumble air motion out of the piston bowl towards the 8. Faure M.A., Sadler M., Oversby K.K., Stokes J.,
spark-plug. This process is evident at 180° CA ATDC Begg S.M., Pommier L.S., Heikal M.R., (1998) “ A of
for all injection timings. LDA and PIV techniques to the validation of a CFD
• A start of injection of 30° CA ATDC offers optimum model of a direct injection gasoline engine,” SAE
mixture conditions for engine octane requirement. paper 982705.
This can be explained by the areas of enrichment 9. Gold M., Li G., Sapsford S., Stokes J., (2000)
evident at 280° CA ATDC for the SOI timings of TDC “Application of optical techniques to the study of
and 60° CA ATDC. mixture preparation in direct injection gasoline
• As the time between injection and ignition increases, engines and validation of a CFD model,” SAE paper
the variability in combustion stability improves, a 2000-01-0538
direct consequence of increased time for fuel 10. Arcoumanis C., Whitelaw J.H., Hentschel W. and
evaporation and air/fuel vapour mixing. Schindler K.P. (1994); “Flow and combustion in a
transparent 1.9 litre direct injection diesel engine
(I.Mech.E. Proceedings, Part D, Journal of
ACKNOWLEDGMENTS Automotive Engineering, 1994, Vol. 208, No. D3,
pp191-205.)
The authors would like to thank the University of Brighton 11. Seitzman and Hanson (1993), ,"Planar fluorescence
and Mr. R. Osborne (Ricardo) for providing data for this imaging in gases." Instrumentation for flows with
paper and the EPSRC for the use of the IMACON 468 combustion, (1993), Academic Press Ltd, London, pp
CCD camera. We would also like to thank the directors of 405-466
Ricardo Consulting Engineers for allowing the paper to 12. Baritaud T.A. and Heinze T.A. (1992) “Gasoline
be published. distribution measurements with PLIF in a SI engine,”
SAE paper 922355.
13. Zhao and Ladommatos, (1998) “Optical diagnostics
for in-cylinder mixture formation measurements in IC
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