Air-Fuel Mixing in a Homogeneous Charge DI Gasoline Engine

 
CONTINUE READING
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 references
enhancing 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 the
vapour, 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 the
AFR(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 cylinder
AFR (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 cylinder
COMBUSTION 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 injection
CONCLUSION                                                     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
REFERENCES                                                         engines”, Progress in energy & combustion science,
                                                                   1998, pp297-336.
1. Kume T.; Iwamoto Y.; Lida K.; Murakami M.;                  14. Ipp W., Wagner V., Krämer H, Wensing M., Leipertz
   Akishino K.; Ando H. (1996); “Combustion control                A., Arndt S., Jain A.K., (1999); “Spray formation of
   technologies for direct injection SI engines.” SAE              high pressure swirl gasoline injectors investigated by
   paper 960600.                                                   two-dimensional Mie and LIEF techniques”; SAE
2. Ando H. (1996) “Combustion control technologies for             paper 1999-01-0498
   gasoline engines,“ Proceeds IMechE conference -             15. Gold M R, Arcoumanis C., Whitelaw J H, Gaade J.,
   ’Lean burn combustion engines’ S433/001/96.                     and Wallace S. (2000); “mixture preparation
3. Ekenberg M. and Bengt J., (2000); “Fuel Distribution            strategies in an optical four-valve port-injected
   in an air assist direct injected spark ignition engine          gasoline engine.” International Journal of Engine
   with central injection and spark plug measured with             Research, 2000, Vol. 1, No. 1, pp41-56.
   laser-induced fluorescence”, SAE paper 2000-01-             16. Stokes J., Lake T.H., Osborne R.J., "A Gasoline
   1898                                                            Engine Concept for Improved Fuel Economy - The
                                                                   Lean Boost System", SAE paper 2000-01-2902
You can also read