A study using Stereoscopic Particle Image
Velocimetry of the Intake Flow in an Engine
Cylinder
J. Rabault, J.A. Vernet, B. Lindgren and P.H. Alfredsson
Abstract The admission flow generated by a parallel valves Diesel engine cylinder
head was investigated by stereoscopic Particle Image Velocimetry in a steady flow
test bench through measurements in the swirl and tumble planes. By combining several sets of measurements a full three-dimensional, three-component reconstruction
of the flow was made. The flow out of the valves was found to form a jet which
collides with the opposite wall before flowing down along the walls into the cylinder, leading to the formation of a recirculation bubble in the tumble plane. In the
swirl plane a complex vortex pair structure is found near the valves from which the
strongest vortex becomes predominant and a single coherent swirling structure can
be observed lower down in the cylinder. Vortex centre tracking was used to analyse
the stability in time and space of the single swirling structure.
1 Introduction and presentation of the cylinder head
Efficient mixing between fuel and air in Diesel engines is critical for the combustion
and the amount of pollutants emitted [1]. The flow structures in the cylinder that are
created during the intake stroke, and evolve during the compression stroke before
combustion occurs, are important for the combustion efficiency [2]. Although engine manufacturers have different design philosophies one of the most significant
structure in Diesel engines is the so called ‘swirl motion’, which corresponds to a
coherent rotating motion of the fluid around the cylinder axis [3]. Swirl is generated
during the admission stroke, and its structure and strength depend on the configuraJean Rabault, Linné Flow Centre, KTH Mechanics, present address, University of Oslo, e-mail:
jean.rblt@gmail.com
Julie A. Vernet, Linné Flow Centre, KTH Mechanics,e-mail: julie@mech.kth.se
Björn Lindgren, Scania CV AB, e-mail: bjorn.lindgren@scania.com
P. Henrik Alfredsson, Linné Flow Centre, KTH Mechanics, e-mail: phal@mech.kth.se
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J. Rabault, J.A. Vernet, B. Lindgren and P.H. Alfredsson
tion of the admission ports (channels) and the choice of the valve seats [4]. Tumble
motion, i.e. the vorticity content perpendicular to the cylinder axis, is also created
during the admission, however in contrast to swirl motion which survives the compression stroke and is therefore present when fuel injection takes place, the tumble
motion is broken down into small scales during compression.
The present experiments were carried out in a collaboration between Scania CV
AB and KTH [5, 6]. The characteristics of the intake flow obtained with a test bench
reproducing the configuration of a new generation of parallel-valves Diesel engine
cylinder (figure 1) was investigated by means of Particle Image Velocimetry (PIV).
The two intake valves are referred to as valve 1 (on the right on figure 1) and 2
(on the left on figure 1). The aim of the present work is to build knowledge about
how the admission port design can be used to set up a specific flow field. Constraints
coming from other considerations, such as cooling and materials resistance, limit the
extent of the design space making it even more challenging to achieve the desired
flow properties in the cylinder.
2 Experimental setup
In the test bench used, a pressure difference over a cylinder head made air flow
through a cylindrical glass section, which is viewed as a simplified version of a
Diesel engine cylinder. The valve lift can be adjusted but was kept fixed during
each measurement. The flow during the admission stroke can be viewed as incompressible and both experimental and numerical studies [5, 7], prove that the value
of the pressure drop does not influence the flow structures in the cylinder, as long
as the Reynolds number is kept high enough. Here a pressure drop of 100 mm H2 O
(981 Pa) was used and leads to cylinder outflow velocities of 5-10 m/s depending on
Fig. 1 Experimental configuration used in the test bench.
The two admission channels
and the cylinder are visible
in blue. Red rods indicate
engineering constraints that
limit the design space of the
channels in the real engine.
The admission channels are in
a so called parallel configuration, which makes it difficult
to get the two valves to work
together in a good way to
create swirl.
Title Suppressed Due to Excessive Length
3
the valve lift. Despite the simplifications of the experimental setup compared with a
real engine cylinder, many years of experience at Scania have proven that the results
are well correlated with tests performed on engines.
PIV measurements have been performed in both the swirl and tumble planes.
A Nd:YAG laser producing a beam of wavelength 532 nm was used together with
a 4 Mpx, 14 bit dynamic range CCD camera (two cameras for S-PIV), at a frame
rate of 7 Hz. Seeding particles of typical size 1-5 µm were obtained from a smoke
generator burning glycol-like oil. Due to the difficulty of getting good optical access
and suppressing laser reflections, S-PIV was used in the tumble planes while only
mono PIV measurements are shown for the swirl planes.
3 Stereoscopic PIV measurements in the tumble plane and 3D3C
reconstruction
Stereoscopic PIV (S-PIV) gives all three velocity components in the measurement
plane but it remains difficult to fully comprehend the three-dimensional (3D) flow
from two-dimensional planes. Instead a full 3D3C (three dimensions, three components) reconstruction in the whole domain was made by using a set of 18 S-PIV
tumble plane measurements [6]. The measurements were performed by rotating the
cylinder head while keeping the same S-PIV cameras configuration, as indicated in
figure 2. The 3D3C reconstruction is then made from the different fields through
a Matlab script. With such a method, the 3D velocity field in a plane is observed
from different relative angles without moving the cameras. This allows all the measurements for a 3D reconstruction to be made without changing the PIV setup and
without the necessity to go through time consuming optics tuning and calibration.
However it has limitations as compared to tomographic PIV, since snapshots are
only available in each measurement plane and for the whole domain reconstruction
has to be done before the mean flow is available.
An uncertainty analysis based on results from one plane of S-PIV data was first
made to determine the number of independent snapshots required to reach an ac-
Fig. 2 The method used
to efficiently perform 3D3C
reconstruction consists in rotating the cylinder head rather
that the PIV instruments. 18
different S-PIV measurements
in tumble planes are used to
reconstruct the full velocity
field.
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J. Rabault, J.A. Vernet, B. Lindgren and P.H. Alfredsson
Fig. 3 3D3C reconstruction of the mean flow field in the cylinder for a 15 mm valve lift, close to
the valves (left) and lower down in the cylinder (right). The arrows indicate velocity in the swirl
plane and colour indicates the velocity in the Y direction (ms−1 ) where red is flow towards the
cylinder head and blue is flow down the cylinder.
curacy in the mean flow better than 5 percent. Following this analysis, a 3D3C reconstruction based on 300 S-PIV pictures was performed for a 15 mm valve lift at
two different swirl planes in the cylinder, one close and one further away from the
valves as can be seen in Figure 3. The arrows indicate the velocity in the swirl plane
and the colour scheme shows the vertical velocity. The jet structure is visible near
the valves and the downward motion of the fluid is localised near the wall opposite
to the valves, in the region where the jets collide with the walls. A small counterrotating jet, coming from valve 2, is also observed in the figure. To obtain strong
swirl this counter-rotating jet is unwanted, however it originates from the difficulty
to introduce the flow tangentially to the cylinder wall in a parallel valves configuration. Further down in the cylinder a clear coherent swirling motion is observed. The
vertical velocity shows that a strong recirculation bubble is present in both planes
and this is consistent with previous results [7, 8]. However it is the first time as far
as the authors are aware that the full 3D picture can be obtained from an experiment
in a complex experimental setting representing a real engine cylinder head.
4 Measurements in swirl plane and flow analysis
3D3C reconstruction is a valuable method for getting general understanding about
the flow, but the time required to perform the measurements necessary to obtain one
3D picture makes it difficult to perform an analysis of the influence of parameters
such as valve lift on flow features. The reconstruction process also implies that the
accuracy of the information available in each swirl plane is limited. As a complement detailed influence of both the valve lift and the distance to the valves on the
structures present in the swirl plane will be shown from Mono PIV measurements.
Title Suppressed Due to Excessive Length
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As presented in figure 4 (top), the flow pattern in the swirl plane evolves from
a complex situation where the jets are active near the valves, to a counter-rotating
double vortex structure further away from the valves, from which the strongest vortex becomes predominant and a single coherent swirling structure can be observed
lower down in the cylinder. This observation is consistent with the 3D3C flows of
figure 3.
The valve lift has an influence on how the flow in the cylinder develops, as shown
in figure 4 (bottom). The higher the valve lift, the closer to the valves the double
vortex structure and the single vortex structure can be observed. While a double
vortex structure can still be observed at moderate valve lifts, a clean swirling motion
is observed for the maximum valve lift.
Time stability of the swirling centre is also important for combustion stability.
Based on the snapshots it was possible to track the swirling centre position using a
two-step algorithm written in Matlab. The algorithm first identifies the region of the
swirl plane where the modulus of velocity is lowest, before determining the exact
swirl centre location by finding the point around which flow circulation has a maximum. The distribution of the swirl centre position at 15 mm valve lift, 40 cm from
the valves is presented in figure 5. The distribution of the swirl centre is concentrated in a peak of regular shape near the centre of the cylinder, which indicates that
the swirling structure position is stable in time.
Fig. 4 Top: evolution of flow patterns in the swirl plane depending on the distance from the valves,
for a valve lift of 6 mm. From left to right, the distance to the valves is 2, 10 and 40 cm. Bottom:
Influence of the valve lift on the development of the swirl plane structures. Comparison of the
patterns obtained with valve lifts of 3, 9 and 15 mm (from left to right), at a distance of 12 cm from
the valves. Axis in mm.
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J. Rabault, J.A. Vernet, B. Lindgren and P.H. Alfredsson
Fig. 5 Distribution of the
position of the instantaneous
swirl centre, based on the
analysis of 1500 snapshots,
40 cm from the valves at a
valve lift of 15mm.
5 Conclusion
Detailed PIV analysis of the admission flow in a parallel valves Diesel engine is reported. 3D3C reconstruction of the velocity field allows to identify the key features
present in the flow. Detailed studies of the patterns present in the swirl planes are
conducted, and the swirling structure proves to be well defined and stable, which is
a desired property for good engine performance.
The present results are in good agreement with the conclusions of other studies
[7], including those of different engine configurations such as spark engines with
only one admission canal [8] and implies that the features described are quite general.
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