Flame kernel development period is the time over which the initial flame kernel
burns from the spark gap and begins to interact fully with the turbulent flow
field. This development stage corresponds to the 0-5% mass fraction burned period
(Cho et al., 1992; Aleiferis
et al., 2000, 2004). After the breakdown
of the impedance between the spark plug electrodes by the propagation of ionizing
streamers from one electrode (cathode) to the other (anode), an electrically
conductive path is created between the electrodes. This conductive path is known
as spark or plasma channel. The temperature and pressure of this spark channel
rises very high, around 60000 k and 20 MPa, respectively as described by Willems
and Sierens (2003) and Heywood (1988). This causes
a supersonic expansion of the ignition kernel and a strong shock wave propagates
outward. Hence, the expansion of the plasma kernel at this stage is governed
by the strong shock wave propagation and conduction, rather than combustion.
Gradually, the significance of combustion on plasma kernel growth rate increases
and the thermo-chemical properties of the fluid near the spark plug become the
governing factors of flame kernel development. The combustion reactions at this
stage are then sufficiently powerful to lead the expansion of the kernel without
supplementary energy supply from the spark plug.
From previous studies it was identified that the quality of a cycle in SI engine
can be determined within the short duration of combustion after ignition timing
(Aleiferis et al., 2000, 2004).
Aleiferis et al. (2004) showed the benefits of
having a high convection velocity in the early flame development time. Other
studies of Ting et al. (1995) and Xiong
et al. (2001) portrayed the significance of variable turbulent scale
eddies and vortical flows on the flame kernel development. Ting
et al. (1995) showed the effect of small scale and large scale eddies
on the flame growth rate and displacement speed on single cylinder natural gas
combustion. The outcome of this study, Ting et al.
(1995) verified that the flame growth rate is able to correlate best with
the level of high frequency, small eddy turbulence. On the other hand Xiong
et al. (2001) showed that vortex strength had a greater influence
on kernel flame growth than a vortex size. And also it identifies that vortex
enhances flame growth rate significantly, when its interaction with the flame
kernel is in the early development period.
The current study investigates the significance of variable swirl levels of air intake on flame development characteristics in a direct injection CNG engine. Air-fuel ratio, injection timing and ignition timing are all set to be constant. The objective is to identify flame growth rate and convection speed and parameters that can influence the flame growth rate, such as flame front wrinkles and distortion, of a developing flame at 1800 rpm and partial load by varying the swirl level of the air intake.
MATERIALS AND METHODS
A single-cylinder Hydra research engine with optical access to the combustion chamber is used for the study. This four-valve engine has a cylinder volume of 399.25 cm3, 76 mm bore and 88 mm stroke. Both of the intake valve equipped with butterfly type swirl valves for swirl flow generation at the intake. For this study one of the valves was pinned in an open position (0% closed) and the other one was pinned on 0, 50 and 100% closed positions so that medium tumble, medium swirl and high swirl flows are generated in the cylinder, respectively and yield different characteristic turbulent eddies near spark plug. Optical access into the engine cylinder is available through a 10 mm diameter hole for camera endoscope at the intake side with 30 deg installation angles and another similar size hole for laser access with 16 deg installation angles normal to the camera viewing direction. The laser optical access was not utilized for this specific study. Details of the engine data are given in Table 1. The operating conditions for the study were; engine speed 1800 rpm, injection timing set at 61 BTDC, air-fuel-ratio (A/F) 40.5 (λ = 2.35) and ignition timing was set at 21.5 BTDC.
Fuel stratification is created using a stratified combustion piston, as well
delaying the injection timing. The in-cylinder pressure was measured with a
water cooled piezoelectric pressure transducer connected to KISTLER charge amplifier
and digitized by the National Instruments PCI-6034E device. The pressure data
is used to determine the mass burn fraction in the combustion chamber, IMEP
and COV in IMEP. Flame images are taken by a HiSence 12 bit CCD camera with
image intensifier unit Hamamatsu CA2098. The camera endoscope which is of type
AVL KARL STORZ # M00060 30° is inserted into the engine cylinder and steered
to have a view field parallel to the cylinder axis which is of the tumble plane.
|| Engine specification
Filtered air at 6 bars is supplied to the endoscope cooling channel for the
purpose of cooling.
Calibration of the pixel size and camera focus adjustment are done outside the engine cylinder by taking into consideration the distance between the endoscope window tip and the spark electrodes center and keeping the 30° endoscope installation angle. The calibration took into account the image deformations due to the endoscopic imaging and identified that one pixel is equivalent to 0.092 mm within 12.5 mm radius from the center of the endoscope view, whereas, it is equivalent to 0.126 mm out side of this radius of view.
Flame images are collected every 2°CA difference starting from 20 BTDC for 30 CA degrees. To obtain quantitative information from the flame images, a computer algorithm is prepared on Matlab to calculate the projected enflamed area, the perimeter and centroid of the flame area. This flame physical information can be used to identify flame growth rate, flame center displacement and flame distortion as illustrated in Fig. 1.
Flame images taken from the combustion cylinder are very likely accompanied
with back ground noises due to light reflecting surfaces and others. Therefore,
image filtering will be considered as a pre-processing step. The Gaussian filtering
technique is applied for filtering the high frequency noises with the 2D Gaussian
kernel given in Eq. 1. Image enhancement also considered by
stretching the dynamic range of the flame image histogram as discussed by Kabir
et al. (2007):
After filtering out the noises from the raw flame images, the binary image and its boundary are identified by intensity thresholding based on the statistical values of the gray levels of the flame images. For this purpose, let the pixels of the image be represented by L gray levels. The number of pixels in level i is denoted by hi and the total number of pixels is denoted by N. The gray level histogram is normalized and regarded as probability distribution function:
Suppose the pixels are divided into two classes C0 and C1 by a threshold value at k. C0 denotes pixels with levels [0, 1,
, k] and C1 denotes pixels with levels [k+1,
The probabilities of class occurrences ω, class mean levels μ and class variance for both classes are given by:
where, μT, μk and σT2 are given by:
Otsu (1979) suggested minimizing the weighted sum of
within-class variances of the object and background pixels to establish an optimum
threshold, in other word, maximization of between-class variance. The between-class
variance, σB2, can be identified by:
Hence, the optimal threshold is the one that maximizes σB2.
With this threshold value the binary image will be identified and the outer
contour of the binary image is, then, taken as the flame boundary which is expressed
as N by 2 vertices. Freeman chain-code and Elliptic Fourier Analysis (EFA) techniques
are utilized to characterize the flame boundaries. First, the motion along the
boundary is coded with the 8-direction Freeman chain code, as shown on Fig.
2. Using the outputs of the chain code a truncated Fourier series expansion
of the closed contour of the flame projected on the X and Y-axis is identified
using Eq. 8 and 9 as illustrated by Trier
et al. (1996):
where, t is the step required to traverse one pixel along the closed contour, tp-1<t<tp for 1 = p = k and k is the total number of codes describing the boundary contour. n is the number of Fourier harmonics required to generate the approximate flame boundary. T is the basic period of the chain-code, T = tk. A0 and C0 are the bias coefficients. N is the total number of Fourier harmonics needed to generate an accurate approximation of the flame boundary.
Each EF harmonic has four coefficients. The nth set of these coefficients is defined as:
|| The 8-direction Freeman chain code schematic
where, Δxp and Δyp are the spatial changes
in the x and y projections of the chain code, respectively, at link p.
is the step change required to traverse link p of the chain code:
is the number of steps required to traverse the first p components or links of the chain code.
Shape descriptions need to be invariant with rotation, size, translation and starting point on the contour, to use them in any size of flame image. To obtain features that are independent of a specific starting point, it is necessary to identify the angle between the starting point and the first semi-major axis, using the first set of harmonic coefficients:
Then, the coefficients can be rotated to achieve a zero phase shift:
Next, the rotation invariant descriptors can be identified by rotating the semi-major axis of the first harmonic by angle ψ1 until it was parallel to the positive x-axis of the first quadrant:
where, y1*(0) and x1*(0) were found
by applying Eq. 15 into Eq. 8 and 9.
To obtain size invariant descriptors, the coefficients should be divided by the magnitude of the semi-major axis, E*, of the first ellipse:
Setting the bias terms A0 and C0 to zero made the EF coefficients to be invariant to translation
RESULTS AND DISCUSSION
Variable turbulent intensity flows are generated by closing the swirl control
valve at different levels ahead of the intake valves so that the flow would
have a higher velocity than it had in the absence of swirl. The use of swirl
valve control for turbulence generation could increase the pressure drop and
reduces the volumetric efficiency. Consequently, IMEP dropped with the increase
of swirl level as shown in Table 2. The reduction in IMEP
was 1 and 8.5% for medium and high swirl flows, respectively. Heywood
(1988) indicated that there might be a power penalty due to the use of significant
swirl at induction which can reach up to 10% with full-open swirl control valve
(no swirl case), the engine operates with a medium tumble flow. Tumble flows
in an engine cylinder have high rate of turbulence generation than swirl flows.
In a tumble induction case, bulk flow decaying will start early in the compression
stroke. This can create small scale-eddy dominated flow near end of compression.
Ting et al. (1995) showed that small scale eddies
have significant effect on flame-front wrinkling and fuel burning rate enhancement
than the large scale turbulent eddies. Besides, cyclic variation will be less
when burning is in small scale dominated turbulent flow due to homogeneity.
Therefore, fuel burning rate at the early flame development can be faster in
a tumble flow case (no swirl) than the two swirl induction cases. However, since
small scale turbulence has a faster rate of decay than the large scale eddies,
the turbulence intensity will decline faster during fuel burning in the tumble
flow case and hence the overall combustion period becomes longer than the swirl
Whereas, in the swirl intake flow case, due to the conducive nature of the
combustion cylinder design for the swirling flow, decaying of bulk flow into
turbulent flow starts late in the compression stroke. This can create a flow
that retains its bulk flow kinetic energy past TDC and the flow near TDC might
be dominated by large scale turbulence. As this large scale eddies are sources
for small scale eddies, turbulence generation will continue during combustion
and increase burning rate. This is the reason why the overall burning time in
swirl induction case was almost half of the tumble flow case, as shown in Table
2. However, the swirl flow case may have high cycle-to-cycle combustion
variation due to the high proportion of large scale flows in the early flame
development period (Heywood, 1988; Ting
et al., 1995). The high cyclic variability is due to the random motion
of the large scale eddies and their convection of the small flame ball in a
random direction. As the flame ball gets larger in size, all scales of turbulent
eddies enhance the expansion and the burning rate of the flame.
The fuel burn fraction is plotted against CA for the three flow conditions,
as shown in Fig. 3. The RassWeiler model is used to identify
the mass fraction burn of CNG and positive value of fuel fraction burn is observed
after TDC. Effects of turbulent eddies can be observed on the fuel burn fraction
curves of Fig. 3. As discussed before, due to the small-scale
dominated flow of the medium tumble intake case before TDC, the rate of fuel
burn is high in the early flame development period. A similar rate of burning
was observed on the medium swirl level flow which is expected to have mixed
characteristics of tumble and swirl flows as discussed by Zhao
et al. (2002). After 8 ATDC (when 35% of the fuel burned) the fuel
burn rate of the medium tumble slowed down, whereas the medium swirl continued
with the same pace till 80% of the fuel burned. On the other hand, rate of burning
for the high swirl flow case started with a slower rate than the other two cases
and then becomes faster after 7.5 ATDC. That was because the availability of
large scale eddies and the well conserved bulk kinetic energy past TDC in both
swirl intake cases become sources for further turbulence generations.
According to the COVIMEP value shown in Table 2,
the cyclic variation is high with increased swirl level. There might be some
discrepancies on using the average cylinder pressure for identifying fuel burn
rate, especially on the high swirl induction case where combustion instability
is high (13.85% COVIMEP). However, the mass burn curve trends show
that burning rate is much faster with swirl induction than with out. This gives
us short combustion duration in swirl induced flow than with a flow without
swirl. Ninety percent of the fuel is burned in 3.98 and 4.07 msec in the case
of medium and high swirl flow conditions, respectively.
||Mass fuel burn fraction for the different swirl flow condition
|| Engine performance parameter
But in medium tumble case it takes, 7.52 msec, nearly twice the time taken
in swirl flow cases. These show that combustion becomes faster by increasing
swirl level of intake air; even though, there is cyclic variability and high
probability of misfires. Zhao et al. (2002) indicated
that under lean condition an excessive turbulent level might not be desired.
Figure 4 shows the variations of equivalent radius of mean
flame with time. It can be observed that for all flow cases the equivalent flame
radius grows more than 2 mm radius within 0.15 msec After Ignition Timing (AIT).
This high rate of growth is achieved due to expansion of the extremely high
temperature and pressure ignition kernel by a strong shock wave as discussed
in Heywood (1988). After that, the ignition kernel cools
down and shrinks to a smaller size of about 1.5 mm at 3 msec AIT. This flame
size is the minimum size recorded in all flow cases and this period can be taken
as ignition kernel development period. In this period flame growth is not a
function of turbulent flow characteristics. It is the function of the ignition
system. At the end of the ignition kernel formation process, Zhao
et al. (1994) found flame size to be 1 mm. Starting from 3 msec after
ignition onset, the effect of fuel combustion on flame growth becomes apparent
and the flame growth rate curve for the different swirl level intake follows
different trend depending on the turbulent flow field characteristics surrounding
the flame kernel.
||Equivalent flame radiuses at different crank angle after ignition
for variable swirl level intakes
It is observed that the medium and high swirl flow cases gain more increase in flame size than the tumble flow case. However, the increase in equivalent radius of the flame is not uniform and the non uniformity of the curves is increasing with swirl level rise. The high-frequency small scale eddies in the tumble induction seem to yield a uniform flame growth than the large scale dominated flow of the other two swirl induction cases, as shown in Fig. 4.
Flame growth rate is plotted against time as shown on Fig. 5.
It is taken to be the rate of change of mean flame equivalent radius. It can
be observed from the figure that the growth rate for the three flow cases starts
at a very high value and drops sharply till it attains the minimum rate. Then
it shows a short rise and after a certain time it attains a constant rate of
growth. A similar trend is recorded by Willems and Sierens
(2003). The initial gain of a very high rate of growth is due to the expansion
by strong shock wave of the plasma kernel as discussed earlier. The flame growth
rate in the development period is high for swirl induction cases and lower for
medium tumble intake case, despite the high burning rate of the later in the
early flame development period (Fig. 3, 5).
This is probably due to quenching of the flame kernel in the tumble induction
case in contact with cool surfaces such as spark plug electrodes which is to
be discussed later.
Flame convection is another important parameter that can influence combustion
performance. Flame convection magnitude and direction can be affected by the
bulk flow velocity and the motion of large scale eddies that have random motion
(Josefsson et al., 2001; Ting
et al., 1995). When the flame kernel is smaller than the average
eddy size, the small flame ball will be randomly convected by the flow due to
the random motion of the eddies. Besides, as the size of the flame increases
and the piston reaches TDC the squish flow and the bowl on piston crown will
have significant influence on flame convection direction (Ghasemi
and Djavareshkian, 2010).
||Flame growth rate after ignition timing for variable swirl
Figure 6-9 are plotted to convey flame
convection and flame center displacement phenomenon. The random displacement
of flame centroid at various times is plotted against crank angle on Fig.
6. It depicts that in the early period of development, the flame starts
to grow and expand on the left side of the ignition point for all the three
flow cases. Then the flame is displaced to the right of the ignition point in
later time for all flow cases. However, time of displacement to the right of
spark center for each cases are different. In the case of both swirl flow intakes,
the flame moves to the right early before TDC, starting from 8 BTDC (1.25 msec
AIT), as shown in Fig. 6b and c. Whereas
for the medium tumble case the flame moves to the right after 2 ATDC (2.18 msec
AIT) (Fig. 6a). The eccentric bowl on the piston crown, shown
in Fig. 7 and the squish flow generated near TDC which is
able to intensify the swirl flow as discussed by Zhao et
al. (2002), favor the displacement of the flame towards the right of
the ignition point in the early development period of the two swirl induction
As it is observed from Fig. 8 and 9, the
farness of the flame kernel from the spark center literally shows significant
influence on flame growth rate. Based on the flame image analysis within 30CA
(3 msec) after ignition onset, the average flame position in medium tumble,
medium swirl and high swirl flow cases is 3.14 mm at 94.67°, 9.78 mm at
84.27° and 5.11 mm at 74.91° away from the spark center, respectively.
Their respective mean flame growth rate within this period of time is 3.38,
4.57 and 3.92 msec. The flame kernel which is far from the spark plug might
avoid flame surface quenching, hence get better growth rate than the one which
is very near to spark electrodes. The convection of the flame kernel away from
the spark center, on the other side, is a function of flow characteristics around
the spark plug. As its observed from the discussed results, the turbulent
flow in the medium swirl flow case convects flame kernel far away from the spark
center relative to the other two flow cases.
||Flame center displacement from the center of the spark plug
electrode gap at different time AIT in (a) medium tumble (no swirl) flow,
(b) medium swirl flow and (c) high swirl flow cases
||Schematic diagram of piston, spark plug and injector arrangement
with off-center bowl
As a result, a higher flame growth rate is recorded in the medium swirl flow
case than the other two cases (Fig. 5, 8).
The measure of flame distortion for the three flow cases is plotted against
time as shown on Fig. 10. It is calculated by the ratio of
flame perimeter to perimeter of a circle whose radius is equal to the equivalent
radius of the flame. It can be noticed that the flame distortion generally follows
a similar trend for all flow cases and there is no flame increases in size and
starts to interact with the flame distortion up to 0.5 msec AIT.
||Cycle average flame centroid distance from ignition center
||Relation between flame center displacement and flame growth
Thereafter, the surrounding turbulence.
||Flame distortions at different crank angle after ignition
||Schematics of distances between flame contours vertices
With the size increase flame distortion can be exhibited due to some reasons,
such as combustion irregularity caused by local air-fuel variability, flame
surface quenching in contact with cool surfaces such as spark plug electrodes
and flame stretch by large scale eddies. However, general observation on Fig.
10 shows that the least distorted flame has the better growth rate (in this
case medium swirl flow type). The flame distortion is moderately correlated
with the flame growth rate.
Flame front wrinkles are approximated from the irregularity of the boundary
of the flame. This boundary irregularity can be found from the standard deviation
of the distances measured between the vertices of the mean flame contour and
the turbulent flame contour. The second harmonics of the elliptic Fourier approximation
for the flame boundary yields the mean flame contour, as shown in Fig.
11. The standard deviation of these distances, thus, used to measure degree
of flame front wrinkledness. Standard deviation of boundary point distances
for the three different cases of swirl levels are plotted against flame equivalent
radius as shown in Fig. 12.
||Standard deviation of distances measured between vertices
of turbulent and mean flame contours at variable swirl induction
It can be observed that in general speaking the flame wrinkles have direct
relation with size increase throughout the flame development period. However,
the correlation of the wrinkles measure with flame size rise is weaker for medium
swirl level case than the other two cases.
In this study flame development characteristics are investigated by varying the Swirl Control Valve (SCV) position just before the two intake valves. The following conclusions can be derived from the study.
From the nature of swirl and tumble flow in engine cylinders and from the effect
of the generated turbulent eddies, it is possible to conclude that medium swirl
flow case showed the best characteristics of the two types of flow to have a
high fuel burning and flame expansion rate. Flame displacement away from the
spark center, especially in the early period of combustion, is higher in medium
swirl flow case than the two other flow cases. This helps to reduce flame quenching
by spark plug electrodes when the flame ball is very small in early period.
However, the reason why the medium swirl flow case has high flame convection
than the high swirl flow case is not clear. The large scale eddies, mean bulk
flow and the generated squish near TDC might have their own influence on flame
convection, particularly in the early flame development stage. This situation
is expected to be clear in the next part of the study when the flow field near
spark plug and surrounding the flame is dealt using PIV technique.
Turbulence decaying in tumble flow starts early in compression stroke due to its bulk flow structure and as a result charge flows near TDC will be dominated by small scale eddies that can ultimately increase fuel burning rate. This is the reason to get a fast fuel burning rate in the early combustion time of the medium tumble case than the two swirl flow cases. However, as a result of its nearness to the spark plug, flame growth rate is lower than the swirl flow cases probably due to quenching by the cool surfaces of the electrodes.
Though turbulence generation is late in swirl flow cases, kinetic energy is conserved well past TDC to generate turbulent eddies which increased burning rate and as a result yields lower overall combustion time than the no swirl case (medium tumble).
Even though the two swirl flow cases enhanced the fuel burning rate and reduce the overall combustion time, the cyclic variability is increased and keeps on increasing with swirl level rise.
Flame front wrinkledness and flame distortion show direct effect on flame growth rate.
The variable level of swirl intake can have different air-fuel mixing rate and as a result, the local air-fuel ratio might be variable at the time of ignition. This can result a variable flame growth rate among the different swirl level intakes in the early flame development period.