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
For optimum efficiency, the direct injection (DI) gasoline
engine requires two operating modes to cover the full
load/speed map. For lower loads and speeds, stratified
charge operation can be used, while homogeneous
charge is required for high loads and speeds. This paper
has focused its attention on the latter of these modes,
where the performance is highly dependent on the quality
of the fuel spray, evaporation and the air-fuel mixture
preparation.
Previous work by the authors and colleagues has
examined in-cylinder air motion and fuel spray
characteristics [6, 7, 8, 9]. The work described in this
paper continued these studies to examine in-cylinder
mixture formation under early injection conditions using
optical visualisation and fluorescence techniques,
including calibrated LIF measurements of air/fuel ratio.
The results were related to engine performance by
comparing with non-optical fired engine combustion data
obtained under similar operating conditions.
OBJECTIVES
Results of quantitative and qualitative Laser Induced
Fluorescence (LIF) measurements are presented,
together with shadow-graph spray imaging, made within
an optically accessed DI gasoline engine. These are
compared
with
previously
acquired
air
flow
measurements, at various injection timings, and with
engine performance and emissions data obtained in a
fired single cylinder non-optical engine, having an
identical cylinder head and piston crown geometry.
INTRODUCTION
The introduction of direct injection (DI) gasoline engines
into the market place has been a consequence of
continued pressure to improve fuel economy and reduce
CO2 emissions, occurring firstly in Japan [1,2]∗ and more
recently in Europe. The majority of published research
on DI gasoline combustion systems has focused on
understanding
mixture
preparation
under
late
(compression stroke) injection, stratified operating
conditions [3, 4, 5]. Equally important are the processes
involved in producing a homogeneous charge with early
(intake stroke) injection timing. Mixture quality under
these conditions is important for low octane requirement,
low smoke, low cyclic torque variation and high full load
air utilisation.
∗
Numbers in [] denote references
The objectives of the work described within this paper
are to:
•
•
•
•
Perform quantitative laser induced fluorescence
measurements within a DI gasoline engine
Measure and compare the in-cylinder fuel distribution
for a series of injection timings
Examine and assess the mixture formation
processes by comparing the LIF fuel distribution
results with previous air flow measurements
Investigate the correlation between mixture formation
processes and combustion data results
TECHNICAL APPROACH
ENGINE CONFIGURATION - The present analysis has
focused on the homogeneous charge operating mode of
a top-entry Ricardo DI gasoline engine (Table 1 and
Figure 1). Previous investigations have centered on the
experimental examination of the in-cylinder air flow using
both the ‘dynamic flow visualisation rig’ (DFVR) and incylinder laser Doppler anemometry, within the present
optical engine, plus comparisons and analysis of phase
Doppler anemometry, spray imaging and qualitative LIF
data. Subsequent comparison of the experimental data
with the computational fluid dynamic software (VECTIS)
has shown good correlation [8].
Engine speed
1500 rev/min
Bore
74.0 mm
Stroke
75.5 mm
Intake valve opening
16° BTDC (intake)
Intake valve closing
52° ABDC (intake)
Exhaust valve opening
54° BBDC (exhaust)
Exhaust valve closing
18° ATDC (intake)
Max valve lift
8.1 mm
Table 1: Engine parameters
enhancing the intensity of fluorescence and recording
only during its short lifetime, therefore eliminating a lot of
ambient illumination. As the wavelength of the
fluorescence image is Stokes shifted, it can be easily
separated from the excitation energy by an appropriate
band pass filter. Continuous measurement through one
injection or engine cycle is precluded by a maximum
repetition rate of the camera/intensifier and laser
combination of 4 Hz. A picture of the air/fuel mixing
process must therefore be built from several imaged
cycles.
Figure 2a : Photograph of experimental layout
Laser
sheet in
Quartz annulus
Figure 1: Schematic of engine configuration
IN-CYLINDER DIAGNOSTIC TECHNIQUES - In the
present experiments, the techniques of shadow-graph
spray imaging [10] and laser induced fluorescence (LIF)
[11,12,13,14,15] have been employed to investigate the
in-cylinder air/fuel mixing for the DI gasoline operating
conditions under investigation.
Figure 2a illustrates the arrangement used within the
optical research engine for the LIF technique. The laser
sheet is introduced along the engine mid-cylinder plane
and is produced and optimised for the fourth harmonic of
the pulsed Nd:YAG laser (266 nm, ultra-violet) by a
series of cylindrical lenses. The result is a laser sheet of
height 20 mm and thickness 1 mm, which is collimated
within the test section, critical to the accuracy of any
quantitative experiments. The laser has been tuned by
the manufacturer to work with a repetition rate of
12.5 Hz, corresponding to an engine speed of
1500 rev/min. The resulting fluorescence signal is
imaged onto a CCD camera (1268x1024) mounted in an
orthogonal plane (Figure 2b) presenting a ‘2D’ slice of incylinder information frozen in time. Since the
fluorescence is weak and short-lived, it is imaged on a
gated intensified camera, having the advantage of
Fluorescence
and spray
images out
Figure 2b: Photograph of experimental optical access
A fuel-tracer mixture was used for the LIF measurements
and comprised 95% iso-octane with 5% acetone by
volume; this mixture was calibrated prior to any
quantitative in-cylinder mixture measurement.
Acetone was used as the fluorescent tracer due to its low
sensitivity to pressure and temperature quenching, hence
minimising errors through cycle-to-cycle variations in
these parameters, plus its high quantum yield and boiling
point of 56 °C. Since the present experiments have been
conducted within a motored engine, spray impingement
will occur upon a relatively cold piston. The low boiling
point of acetone will facilitate evaporation under these
conditions, hence offering a closer simulation of fired
engine mixture preparation conditions.
Quantitative LIF requires extensive calibration of the
correlation between the fluorescence signal and local fuel
concentration, before in-cylinder air/fuel ratios can be
derived. In the present application this calibration was
performed within the optical engine, while motoring the
engine and employing a unique closed-loop approach.
The major benefits of the closed-loop in-cylinder LIF
calibration are:
• identical transmitting and receiving optical paths in
calibration and experiment
• precise matching of the in-cylinder physical
conditions for each crank-angle
• direct air/fuel ratio comparison for each crank-angle
• variations in the laser sheet energy density, optical
distortion and reflections can be filtered.
(A more detailed description and discussion of this
calibration technique will be presented in a future SAE
paper).
Start of Injection
End of Injection
Engine speed
TDC
61° CA ATDC
1500 rev/min
30° CA ATDC
91° CA ATDC
1500 rev/min
60° CA ATDC
121° CA ATDC
1500 rev/min
Table 2: Optical engine operating conditions for the
shadow-graph spray imaging
ENGINE COMBUSTION TESTS - Combustion testing
was conducted in an engine geometry identical to the
optical engine analysis, as part of an investigation into a
lean boost DI gasoline concept [16]. The testing
consisted of globally lean air/fuel ratios, rather than the
rich air/fuel ratios used in the optical engine. However,
the behaviour of the air and the fuel spray in both
engines are comparable. Combustion data collected
consisted of in-cylinder pressure, derived IMEP,
coefficient of variation (CoV) of IMEP, mass fraction
burned and smoke. The fired engine operating conditions
are outlined in table 3.
Engine load
Start of
Engine
injection
Full
4 bar IMEP speed
(rev/min)
(°CA ATDC)
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
access
Injector
mounting
RESULTS AND DISCUSSION
MIXTURE FORMATION
Figure 3: Photograph of optical cylinder head
While a standard cylinder head in conjunction with a
20 mm fused silica annulus was used for the LIF work, a
separate dedicated cylinder-head with pent-roof optical
access was available for the shadow-graph imaging
(Figure 3). This technique allows a quick fuel spray
analysis as opposed to a laser sheet where several 2D
plane data sets are required for a 3D analysis.
A halogen lamp was placed diametrically opposite the
camera, illuminating the combustion chamber. In this
way, the injected spray obscures the transmission of the
light to the camera and forms a shadow. A high speed
intensified CCD camera (IMACON 468) having a spatial
resolution per channel of 576x385 was employed to
obtain the data. After an initial crank angle derived TTL
trigger it was able to acquire up to eight consecutive
images with inter-frame spacing down to 10 ns. The
operating conditions for the shadow-graph spray imaging
and the LIF tests are summarised in Table 2.
In-Cylinder Spray Visualisation - Prior to the analysis of
the fuel/air mixing processes with LIF, the in-cylinder
spray structure was investigated for the full load condition
of SOI at 60° CA ATDC and an engine speed of
1500 rev/min. Figure 4 shows a sequence of images
illustrating the effects of the intake air motion on the
injected spray during the early stages of injection, the
mid-phase, corresponding to the injector’s steady state
condition, and finally the closing stage.
The DI injector spray can be initially seen to enter the
cylinder at 65° CA ATDC with a narrow pencil structure
having a high penetration velocity (approximately
120 m/s). Between the opening and closing transient
injection flow periods, the mid-injection can be
considered as a steady state flow condition. During this
phase the injected spray can be seen to develop into a
narrow angled hollow cone structure. Since the technique
relies upon the obscuration of light passing through the
liquid droplets, the hollow structure can be easily
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
intake side during this period. The injector is positioned
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
variability was previously noted in [6]. Further tests at the
lower engine speeds of 1000 rev/min and 500 rev/min
indicated reducing degrees of spray deflection and
break-up due to the overall lower intake air velocities and
consequential lower levels of turbulence.
65° CA
ATDC
80° CA
ATDC
105° CA
ATDC
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,
plus entrainment of smaller droplets which are carried
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
125° CA
ATDC
Injection flow rate (approximation)
0
55
60 65
70 75 80 85 90
95 100 105 110 115 120 125 130 135
Crank angle (deg AT DC)
Table 5: Example of typical injection electronic pulse and
fuel flow rate profiles (SOI @ 60° CA ATDC)
Figure 4: Visualisation of injected sprays
(SOI @ 60° CA ATDC)
During the studied injection period of 60 -121° CA ATDC,
the two events of maximum piston speed and maximum
valve lift will occur. The resulting intake air mass flow will
consequently be at a maximum during the injection
event. From the characterisation of the air motion on the
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
compliment the explanations offered for the air / spray
interactions in the previous results. Since the tracer LIF
technique displays information on the fuel concentration,
a very strong signal will be gathered in the presence of
liquid fuel, which could damage the image intensifier. In
the present experiments the camera, intensifier and lens
parameters have been optimised for analysis of fuel
vapour, hence the earliest image acquisition is 15° CA
after the end of the respective injection.
The relationship between the LIF mixture measurements
and the air flow data gathered using the DFVR has been
analysed to lend support to the LIF data and the
corresponding mixture formation mechanism analysis.
Figure 6 shows the mixture distribution and the CoV of
mixture distribution compared to air flow at 90° CA ATDC
for SOI at TDC. In Figure 6a, a rich region with
equivalence ratio values between 1.2 and 1.8 can be
seen on the exhaust side of the chamber. It appears to
have been deflected off the piston bowl and transported
within the prevailing flow out of the piston bowl into the
exhaust side re-circulation region. The high air velocity
entering through the intake valves has resulted in a
dilution of the mixture in the under valve area, down to
equivalence ratio values of less than 0.5. There is a
distinct division between this lean region and the rich
region which correlates with the shear layer between the
piston bowl jet and the intake air flow. From the spray
data, the highly turbulent air flow was seen to cause
spray break-up. The corresponding CoV in the mixture
strength (Figure 6b) indicates a similarly high level of
cycle-to-cycle variation. The lean mixture can be seen to
have a CoV of up to 25% in this under valve region. The
trajectory of the mixture jet from the bowl exit will
additionally be influenced by the cycle-to-cycle variability
of the intake air flow. This has also been captured by the
region with a mixture strength CoV of up to 10% lying
within the piston bowl jet velocity vectors.
An injection timing swing was performed to aid the
understanding of the different mixture formation
processes present during the DI gasoline engine
homogeneous charge operating mode. The initial series
of images acquired 15° CA after the end of injection for
each of the injection strategies are shown in Figure 7.
Figure 7a compliments the result shown in Figure 6 with
a rich region emanating from the lip of the piston bowl.
However, the overall equivalence ratio levels are higher
than in Figure 6 due to less mixture dilution by the intake
air. At 105° CA ATDC, for an SOI of 30° CA ATDC, the
mixture strength can be seen to be globally lean as a
result of the reduced piston impingement for this injection
timing, such that the injected fuel has passed through the
measurement plane by this time. Conversely the SOI
timing of 60° CA ATDC still has a strong mixture
presence on the intake side of the combustion chamber;
an equally rich region is also evident in the exhaust
region. The explanation for the injection tail can be
derived from the spray break-up evident in Figure 4,
entraining liquid fuel droplets in the under valve region
and hence the high liquid portion fluorescence signal.
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 the
exhaust side cylinder wall, and rolled up into the upper
part of the combustion chamber due to the bulk charge
motion.
Cycle-to-cycle variability in mixture strength is illustrated
in Figure 8 by the CoV in the LIF measurements, where
all three injection timings show regions of variability
above 25%. Figure 8a illustrates the variability in
transportation of fuel mixture out of the piston bowl, with
evidence of the influence of the intake air flow
perturbation in the under valve region. A similar under
intake valve variability is seen in Figure 8b for the SOI
timing of 30° CA ATDC. As the injection timing is
retarded to 60° CA ATDC the level of spray break-up and
entrainment has increased due to the increased intake
air mass flow and variability. Cycle-to-cycle variation of
over 25% on the exhaust side indicates the extent of
injection roll-up into the upper regions of the chamber.
Moving through the stroke, Figure 9 shows the mixture
distribution for the three injection timings, at intake BDC,
superimposed with the DFVR air-flow measurements.
For the two earlier timings, a higher equivalence ratio can
be seen in the central region of the upper cylinder due to
the fuel/air mixture carried in the reverse tumble vortex
out of the piston bowl. For the timing of SOI at TDC, the
rich region has been drawn into the lower cylinder with
the prevailing air motion. Conversely the upwardly
moving injection roll up for SOI at 60° CA ATDC is still
evident at BDC due to both its upward motion and the
reduced time between end of injection and BDC when
compared to SOI at TDC.
Since the piston geometry interferes with the laser sheet
for crank angles after 280° CA ATDC this timing is the
latest in-cylinder mixture distribution presented, and is
shown in Figure 10. The rich region carried into the lower
part of the chamber for the SOI at TDC can be seen to
re-appear on the exhaust side during compression
(Figure 10a). A more even global mixture distribution is
evident in Figure 10b for SOI at 30° CA ATDC. However
the reduced evaporation and mixing time has resulted in
exhaust side enrichment for SOI at 60° CA ATDC due to
the initial injection roll-up region.
Figure 11 illustrates the effect of reducing evaporation
and mixing time available from the end of injection. The
later start of injection (and hence end of injection due to
fixed pulse width) shows an increasing level of cycle-tocycle variability in the in-cylinder mixture strength. These
variability effects and the global mixture distribution
processes will influence the combustion performance of
the fired engine. The next section will address some of
these effects.
AFR(φ) CoV
Rich
High
1.8
25%
20%
1.2
15%
10%
0.6
5%
0
Lean
0%
Low
Figure 6 (a): Mixture distribution @ 90° CA ATDC for a
SOI @ TDC; superimposed DFVR air-flow
Figure 6 (b): CoV in the mixture distribution @ 90° CA
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
Figure 7 (a): Mixture distribution
@ 75° CA ATDC for a SOI @
TDC (φ range = 0 – 2.55)
Figure 7 (b): Mixture distribution
@ 105° CA ATDC for a SOI @
30° CA ATDC (φ range = 0 – 2.55)
0.5
Figure 7 (c): Mixture distribution @
135° CA ATDC for a SOI @ 60°
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
distribution @ 75° CA ATDC for a
SOI @ TDC
Figure 8 (b): CoV in the mixture
distribution @ 105° CA ATDC for
a SOI @ 30° CA ATDC
Figure 8(c): CoV in the mixture
distribution @ 135° CA ATDC for
a SOI @ 60° CA ATDC
0%
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 cylinder
AFR (φ)
Rich
2.5
2.0
1.5
1.0
0.5
0
Lean
Figure 9 (a): Mixture distribution @ 180°
CA ATDC for a SOI @ TDC with
superimposed DFVR derived air-flow
Figure 9 (b): Mixture distribution @ 180°
Figure 9 (c): Mixture distribution @ 180°
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
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
@ 280° CA ATDC for a SOI @
TDC (φ range = 0 - 1.5)
Figure 10 (b): Mixture distribution
@ 280° CA ATDC for a SOI @
30° CA ATDC (φ range = 0 - 1.5)
Figure 10 (c): Mixture distribution
@ 280° CA ATDC for a SOI @
60° CA ATDC (φ range = 0 - 1.5)
0
Lean
Figure 10: Mixture distribution @ 280° CA ATDC
CoV in
AFR (φ)
High
15%
10%
Figure 11 (a): CoV in the
mixture distribution @ 280° CA
ATDC for a SOI @ TDC
Figure 11 (b): CoV in the
mixture distribution @ 280° CA
ATDC for a SOI @ 30° CA
ATDC
Figure 11c): CoV in the mixture
distribution @ 280° CA ATDC
for a SOI @ 60° CA 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 cylinder
FULL LOAD OCTANE REQUIREMENT - Figure 12
shows the knock-limited ignition advance versus start of
injection timing at 1500 rev/min wide open throttle with a
constant 22:1 air/fuel ratio. The changes in ignition
advance reflect changes in octane requirement.
Optimum start of injection for octane requirement was at
30° CA ATDC, with octane requirement increasing for
more advanced or retarded injection timings. When
operating at a mean air/fuel ratio of 22:1, fuel rich areas
in the combustion chamber, particularly in the end-gas
regions, would be detrimental to octane requirement.
timing is retarded, less piston spray impingement occurs
and less fuel is carried over to the exhaust side
combustion chamber wall.
2
1.5
FSN
COMBUSTION AND LIF COMPARISON
1
0.5
0
30
20
10
0
0
15
30
45
60
SOI (degATDC)
75
90
Figure 12: Knock limited ignition timing (°CA ATDC)
correlated to SOI (Full load 1500 rev/min)
Turning to the LIF results shown in Figure 10, a start of
injection timing of 30° CA ATDC appears to be optimum
for mixture homogeneity. With start of injection at TDC
or 60° CA ATDC there is a rich area on the exhaust side
of the combustion chamber, more so for TDC start of
injection. This would explain the increased octane
requirement at these injection timings. The increase in
coefficient of variation of AFR with start of injection
60° CA ATDC would also have a detrimental effect on
octane requirement. In a fired engine operating at 22:1
air/fuel ratio, cycles containing locally fuel rich areas
would be more likely to knock.
FULL LOAD SMOKE - Figure 13 shows smoke versus
start of injection timing at 1500 rev/min wide open throttle
with a constant 22:1 air/fuel ratio. For more information
on the lean boost DI gasoline concept, please refer to
[16]. Start of injection at TDC produces the highest
smoke emissions, with smoke reducing as injection
timing is retarded.
The LIF results provide an
explanation for these observations. With start of injection
at TDC, fuel impinges on the wall of the bowl and is
carried by its own momentum and the strong air motion
over to the exhaust side of the combustion chamber,
where some probably impinges on the cylinder wall. Any
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 injection
15
30
45
60
75
90
SOI
Figure 13: Engine out smoke (FSN) correlated to SOI
(Full load 1500 rev/min)
PART LOAD CYCLIC COMBUSTION STABILITY Figure 14 shows cycle to cycle combustion stability,
measured as coefficient of variation of IMEP, versus start
of injection timing at 1500 rev/min 4 bar IMEP. In this
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
correlates with the increase in coefficient of variation of
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 inhomogeneity, the reliability of this mode of fuel transport
and evaporation results in low cycle to cycle AFR
variation. As injection timing is retarded, less fuel
impingement occurs and more reliance is placed on air
motion for fuel transport and evaporation. This results in
improved fuel evaporation rate and mixing. However, the
cycle to cycle variation in air motion, combined with
reduced SOI-to-ignition interval, leads to increased cycle
to cycle AFR variation.
3
CoV IMEP (%)
Ign (deg BTDC)
40
Octane requirement Improvement
0
2.5
2
1.5
1
0.5
0
0
15
30
45
60
SOI (degATDC)
Figure 14: CoV in IMEP (%) correlated to SOI (Part load
1500 rev/min)
CONCLUSION
The use of in-cylinder diagnostic techniques in a singlecylinder DI gasoline engine has revealed strong
correlation between data from different optical techniques
and combustion performance. The following conclusions
can be drawn from the observations:
•
•
•
•
•
•
•
The high velocity spray from a DI injector will be
deflected by intake air during the homogeneous
operating mode.
Different injection timings result in different mixture
formation processes.
Fuel vapour is carried out of the piston bowl by the
reverse tumble air motion for early injection timings
There is evidence of fuel spray impingement and
’rolling-up’ on the exhaust side cylinder wall for later
timings.
Vaporised fuel is carried in the prevailing reverse
tumble air motion out of the piston bowl towards the
spark-plug. This process is evident at 180° CA ATDC
for all injection timings.
A start of injection of 30° CA ATDC offers optimum
mixture conditions for engine octane requirement.
This can be explained by the areas of enrichment
evident at 280° CA ATDC for the SOI timings of TDC
and 60° CA ATDC.
As the time between injection and ignition increases,
the variability in combustion stability improves, a
direct consequence of increased time for fuel
evaporation and air/fuel vapour mixing.
ACKNOWLEDGMENTS
The authors would like to thank the University of Brighton
and Mr. R. Osborne (Ricardo) for providing data for this
paper and the EPSRC for the use of the IMACON 468
CCD camera. We would also like to thank the directors of
Ricardo Consulting Engineers for allowing the paper to
be published.
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