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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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Flying PIV measurements in a driven IC engine flow
Mario Koehler1,*, David Hess2, Christoph Kratzsch1, Christoph Brücker1
1: Institute of Mechanics and Fluid Dynamics, TU Bergakademie Freiberg, Freiberg, Germany
2: Dantec Dynamics, Ulm, Germany* correspondent author: Mario.Koehler@imfd.tu-freiberg.de
Abstract This work focusses on the investigation of the unsteady flow in a transparent IC engine during the intake
cycle. Special focus is laid on evolution of the flow close to the piston crown while the piston is moving. Therefore wedeveloped a Flying TR-PIV method which allows us to follow the flow evolution in a system of reference fixed with themoving piston and where the light-sheet remains at a constant distance to the piston surface. The measurements are
realized in a water-analogue of a 4-valve engine at 1600 rpm engine speed in real situation. The light-sheet is positioned
1mm above the piston crown and moves with the piston thanks to a scanning technique that synchronizes the sheet
motion with the piston motion. A compact high-speed camera is positioned within the piston shaft below the transparent
piston head and records the particle fields within the illuminated planes parallel to the upper plane of the flat piston.
Results are presented for the flow fields above the piston crown that show the strong swirling motion generated in theearly phase of intake due to vortex stretching of horseshoe-type vortices generated at the valves. These vortices givereason for cycle-to-cycle variations due to their intense stretching during the intake phase.

1. Introduction
Many experimental studies have been carried out to investigate the flow during the intake and compression
cycle in IC engines to understand the details during the intake and compression cycle. However, it is only
recently that time-resolved PIV data as well as volumetric PIV measurements could be realized withintransparent engines [1-3]. Details about the flow structure are important for the mixing and combustion
process to reduce fuel consumption in engines. Especially the flow evolution near the walls of the innercombustion chamber is still difficult to achieve with the current techniques [2]. For a detailed investigationof the flow structure near the piston crown, a special time-resolved PIV system is required that allows tofollow the flow structure over the complete motion cycle. Such a system has been developed in our lab and isintroduced herein which is based on a flying TR-PIV camera and a scanning light-sheet method. This allows
us to investigate the evolution of the near-wall flow topology and to determine regions of high wall-shear or
flow separation due to the interaction of the swirl and tumble flow with the piston wall. Those are importantto understand the generation of hot spots, see Hasse et al. 2010 [1].

2. Experimental Set-up
The experiments were carried out in a transparent version of an original four cylinder head with adisplacement of 1.6 liters, see Hess et al. [2]. As working fluid either water or SF6 (sulfur hexafluoride) canbe used. The cylinder head has four valves in total, two at a time for the inlet and exhaust valves. The ICE
has a bore of D = 76.5 mm and an original stroke of S = 85,8 mm. The piston and the valves are driven bysynchronized single linear traverse motors to easily change the valve timings. Cylinder and piston are madefrom transparent Perspex. The linear motors can follow different path-time diagrams as consecutive points,
loaded on each servo drive where the maximum valve stroke is 9 mm. The linear motor (Mannesmann
MDD065) has a maximum theoretical speed of 0.66 m/s. While the downward movement, the piston
aspirates water from a reservoir. After 180°CAD the exhaust valves open and with the beginning piston
upward movement the scavenging of the cylinder starts pushing the liquid back into the reservoir. The water
can be heated up to temperatures of 60°C to reduce the viscosity and match thus higher running speeds.
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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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3. Methods

Fig. 1 Experimental set-up of the IC engine with polygon scanner.

The transparent piston made from Perspex has a plane piston crown and floor so that only small image
distortions are present. A compact high speed camera (Mikrotron Cube4, resolution at 1020x1020 pix² @1000 Hz) is mounted in the piston shaft below the piston crown and is focused through the transparent glass
piston on the light sheet, see Fig. 1. Both, camera and light-sheet move in a reference frame fixed with themoving piston (Flying PIV). A special timing of the laser pulse and the use of a scanning system with arotating polygon mirror ensures that the light-sheet is always at a distance of 1 mm relative to the flat pistonhead and following the piston motion (see Fig. 2). The lens system is adjusted such that sharp particle images
are recorded for the preset position of the measurement plane.

Fig. 2 Glass piston with integrated high speed camera system. The optical distance (d=0.3 m) between camera and thelight sheet plane remains constant because of the scanning motion of the light-sheet. The whole set up (high speed
camera, piston, rotating polygon and four valve system) is synchronized such that the flow evolution above the pistoncrown can be followed in detail during the intake and compression stroke.
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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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A pulse laser (Nd:YAG laser, 10 mJ @ 1 kHz, wavelength of 527 nm) with a light-sheet optic is used forillumination of the flow. The light sheet is reflected at the surface of a rotating polygonal mirror with 20
facets while a pulse is generated at each facet but at different timings relative to the center of the facet. The
rotating polygon gives a master signal to the sequencer which synchronizes the high speed camera, the startof the piston and the opening of the valve-system. The time-series of PIV images is recorded at a frame rateof 1000 Hz. The scanning light-sheet has a thickness of w = 1.5 mm. All measurements within the water
analogue refer to typical engine speeds of 1600 U/min. The table shows the different engine speeds in air andwater.
Table 1 Comparison between the engine speed in air and water.

Water

Air

U

0,15 m/s

3,0 m/s

N80 rpm

1600 rpm

Because of the lower kinematic viscosity of the heated water in comparison to air, the engine speed in thewater analogue can be reduced about a factor of 20.
a)

b)
100

pi stonli ghtsheet

piston [mm]

80
60
40
20
0

20

40

60
80
li ghtsheet

100

120

140

Fig. 3 Scanning light sheet unit with Nd:YAG laser for illuminating the transparent glass engine chamber with a lightsheet that is and the constant distance of 1mm between piston and light sheet.

The complete scanning unit is shown in Fig. 3 and results for the position of the scanning light sheet relative
to the piston are given, see also Fig. 4. The profiles in Fig. 3 display the vertical position of the moving lightsheet and the piston head during the intake cycle. Therefore we marked the piston with black paper tohighlight the light-sheet position (Fig. 4). Both the reference edge of the piston head and the light sheet
position are captured during the motion cycle with an external high-speed camera looking from the side. As
seen from the profiles, the light-sheet position remains in nearly constant distance relative to the piston withan average distance of 1mm. The variation of the light-sheet position is less than 0.4mm within the complete
cycle and maximum deviations are observed only at the begin and end of the cycle.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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CAD=40°d) CAD=100°b) CAD=60°e) CAD=120°c)f) CAD=140°

CAD=80°Fig. 4 Visualization recordings of the top of the moving piston (red line) and the moving light sheet (white line) in thephase between CAD = 40° to CAD = 140° during the intake cycle. The results show that the light-sheet remains
approximately in a constant distance relative to the piston head. A black paper sheet is partly wrapped around the piston
to allow the observation of the light-sheet reflections.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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4. Results
A typical PIV results in the plane above the piston crown is given in Fig. 5 to illustrate the flow field in
relation to the position of the inlet and exhaust valves. An exemplary result of the temporal evolution of theflow is shown in Fig. 5 for certain time-steps CAD=6 – 34 °. The iso-surfaces of constant in-plane velocity
magnitude show different areas of low and high-speed motion during the intake phase. These results
represent the phase-averages over 20 recorded cycles.

Fig. 5 Typical flow structure in a plane 1mm above the piston crown inside the engine chamber as seen from the camera
trough the transparent piston. The Spark plug is located in the middle of the engine head. The velocity scale isrepresentative for the following results (Figure 7,8).

Fig. 6 Evolution of the flow above the piston crown, phase-averaged over 20 engine cycles from CAD=6°to CAD=34°.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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Fig. 7 shows the phase-averaged velocity field in the engine chamber over 20 engine cycles from CAD=6° to
CAD=34°. The velocity increases in two areas next to the spark plug. In the separate areas exist two
maximum velocity sections (Fig. 7) on each side of the symmetry plane. The iso-surfaces (Fig. 7) based on
the vorticity magnitude show that the induced flow field is generated by pairs of vortices attached to the wallof the piston crown.

Fig. 7 Evolution of the flow structure at different time steps (CAD=21°,30°) with separate sections of the velocity on
the left and the Vorticity magnitude on the right side.

Fig. 8 Scheme of the horseshoe-type vortices formed in the wake of the valves during the early intake phase. Each pairof the legs is attached to the piston crown and stretched in vertical direction due to the motion of the piston.

These vortices belong to the horseshoe-type vortices generated at the inlet valves during intake, see thesketch illustrated in Fig. 8. While the piston is moving down, the legs of the horseshoe-type vortices are
stretched in axial direction which increases rotational speed and generates strong streamwise vortices with
their legs attached to the piston wall. Fig. 8 depicts the structure of the horse-shoe type vortices attached to
the valves and with counter and clockwise rotation of each leg that is attached to the piston crown. The
vortices finally move laterally towards the cylinder wall with increasing CAD. At CAD=170° the vortices
disappeared as seen in Error! Reference source not found.. The comparison validates the results of tumble
visualization in the middle plane. The flow shown in Fig. 9 left represents a footprint of the tumble on the
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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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piston wall.

Fig. 9 Comparison of near-wall flow in CAD=170° with the tumble visualization in [2].

As seen in Fig. 9 the tumble touches the piston ground. This area shows the highest velocity. The cycles(CAD=148°-178°, Fig. 10) show the flow structure from the outlet side to the inlet. Recirculation areas (redlines) in the region near the cylinder wall and the high velocity section in the middle of the piston ground(black line) are highlighted. The recirculation-sections are caused by the reflection of the flow against thecylinder-wall.
CAD=148°

CAD=178°

Fig. 10 Phase-averaged flow field, averaged over 20 engine cycles from CAD=148° to CAD=177°.

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17 International Symposium on Applications of Laser Techniques to Fluid Mechanics
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5. Conclusion
With the developed flying PIV method using a scanning light-sheet technique it is possible to investigate thetemporal evolution of the near-wall flow topology and to determine regions of high wall-shear or flow
separation at the piston crown in IC engine flows. Of special interest is the interaction of the swirl andtumble flow with the piston wall and the generation of hot spots in the combustion chamber. The system has
been tested in a water analogue of a 4-valve BMW engine for the intake phase. For the investigations of thecompression phase with the same system, we will use in future measurements SF6 as a working fluid. SF6 isa gas with a 6x higher density than air at comparable dynamic viscosity. The use of the gas allows
investigating the intake as well as the compression phase in the same engine chamber.
It is to the first time that TR-PIV results of the flow evolution near the piston crown are measured in detailthanks to the developed technique. The results of the flow evolution near the piston crown show that intense
vortex-structures are present which play an important role on the near wall flow topology and which
influence the flow structure in the engine chamber. Those vortices are understood as the footprints of the legsof horseshoe-type vortices generated in the wake of the valves. Because of the downwards motion of thepiston, the legs attached to the piston wall are stretched and therefore increase in strength during the intake
phase similar to a tornado vortex. The comparison of different intake cycles shows that the motion paths ofthese vortices along the piston crown vary in time and give reason for cycle-to-cycle variations even if theflow is still at weak turbulence levels. We expect that the development of lighter and more compact highspeed cameras in future will allow the adaptation of the developed technique to higher running speeds andhigher spatial resolutions.

6. Acknowledgements
The study is funded within a joint FVV and DFG project (Deutsche Forschungsgemeinschaft, DFG) within
the special research group “Numerische Simulation und Validierung der Selbstzündung und Verbrennung“.
The present project is funded by the DFG under the contract code BR1494/27-1 and funding is gratefully
acknowledged.

7. References[1] Hasse C, Sohm V, Durst B (2010): Numerical investigation of cyclic variations in gasoline engines usinga hybrid URANS/LES modelling approach. Computers & Fluids 39 (2010) 25–48.
[2] Hess D., Tag S., Brücker C (2012) Volumetric Flow Studies in a 4-stroke Water-Analogue IC-Engine
Using High-Speed Scanning-PIV. Proc. 16th Int Symp on Applications of Laser Techniques to Fluid
Mechanics, Lisbon, Portugal, 09-12 July, 2012, ISBN[3] Dannemann J., Klaas M., Schröder W. (2010) Three-dimensional flow field within a four-valve
combustion engine measured by PIV. Proc. International Symposium on Flow Visualization June 21-24,
2010, EXCO Daegu, Korea, ISFV14 – Daegu / Korea – 2010.

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