Evaluation of CI In-Cylinder Flow using optical and numerical techniques
1 1 2 2
R. Rezaei , S. Pischinger , P. Adomeit , J. Ewald *
1
Institute for Combustion Engines, RWTH Aachen University, Germany
2
FEV Motorentechnik GmbH, Aachen, Germany
In order to evaluate different port concepts for modern Compression-Ignition engines, usually quantities as the swirl
level and the flow coefficient are evaluated, which are measured on a stationary flow test bench. As additional crite-
rion, in this work, the homogeneity of the swirl flow is introduced and defined quantitatively. Different valve lift strate-
gies are evaluated using three-dimensional Particle Imaging Velocimetry in a stationary flow configuration and tran-
sient In-Cylinder CFD simulation using both the Reynolds Averaged Navier Stokes equation and the Large Eddy
simulation approach.
Introduction
New concepts for High-Speed Direct Injection • Basic engine: FEV system engine
205
Compression Ignition (CI) engines are under de- • Bore x stroke: 75 mm x 88.3 mm
velopment due to the increased awareness of the • Injection
200 system: BOSCH 8.0 2000 bar Piezo
ISFC [g/kWh]
mm maximum Valve Lift
CO2 emission impact on global climate change • Compression
195
ratio: 15.3 6.4 mm maximum Valve Lift
4.8 mm maximum Valve Lift
which goes hand in hand with the demand of fur- 3.2 mm maximum Valve Lift
ther reduced fuel consumption as well as the ag- As is190
shown in Figure 1, a reduction of the valve
gravated emission legislation standards. lift provides
185 the best potential for emission beha-
In order to meet theses requirements for future vior by increasing the swirl ratio. The utilization of
180
CI engines, not only the injection system has to be the increased homogeneous swirl by reducing the
2.5
205
suitably defined. Also, for the chosen injection valve lift reduces smoke emission significantly
[g/kWh] [-]
system, the optimal in-bowl swirl has to be gener- without200 any impact on fuel consumption.
2.0 A further
ISFC Number
8.0 mm maximum Valve Lift
6.4 mm maximum Valve Lift
ated. The magnitude of the swirl optimum, howev- reduction 1.5
195 of valve lift leads to a noteworthy in-
4.8 mm maximum Valve Lift
er, is furthermore dependent on the operating point crease of gas exchange losses, which finallyValve
3.2 mm maximum leadsLift
and engine speed. Therefore, in order to provide to increased 1.0
190 fuel consumption without any advan-
Smoke
the corresponding flexibility, a CI engine concept tage concerning 0.5
185 soot emission.
has been developed that features a variable valve
0.0
180
lift and port deactivation concept. By means of this,205
[bar]
2.5
0.0
the optimal trade-off between swirl level and in-
Number [-]
200
ISFC [g/kWh]
cylinder fresh charge filling level can be found. 2.0
-0.1 8.0 mm maximum Valve Lift
Gas Exchange
6.4 mm maximum Valve Lift
It has been shown that different valve lift strate-195 1.5
-0.2 4.8 mm maximum Valve Lift
gies nominally lead to similar filling and swirl le- 3.2 mm maximum Valve Lift
vels. However, differences in combustion behavior190 1.0
-0.3
IMEPSmoke
and engine-out emissions give rise to the assump- 0.5
-0.4
tion that local differences in the in-cylinder flow185
-0.5
0.0
structure caused by different valve lift strategies180
IMEP Gas Exchange [bar]
0.0 0.5 1.0 1.5 2.0
0.0
have noticeable impact. 2.5 NOx-Emission [g/kWh]
In this work, these flow structures were ana- -0.1
Smoke Number [-]
lyzed and quantitatively assessed using both opti- 2.0 Figure 1: 1500 rpm, 6.8 bar emissions [6]
-0.2
cal and numerical techniques.
1.5 -0.3
Increasing the swirl via reduced valve lift pro-
Engine: Variable Charge Motion concept 1.0vides -0.4a slight improvement to the particulate
The intake port of this DI diesel engine consists air/fuel ratio trade-off from 8.0 mm to 3.2 mm valve
-0.5
of tangential and filling ports [1]. Tangential ports 0.5lift. In Figure 0.0 2 in 0.5
particular1.0the NO1.5 x/soot trade-off
2.0
can be used to generate a relatively high swirl ratio is shown in particular for two valve lift strategies,
while the filling ports, as the name already implies, 0.0maximum lift of 4.8NOmm x
-Emission [g/kWh]
vs. 8 mm max. valve lift
IMEP Gas Exchange [bar]
provide a high flow coefficient. Additionally the 0.0with the filling port closed, which have the same
intake charge flow can be directed by machining swirl ratio. It was seen that the soot emissions with
the valve seat rings to yield swirl chamfers. This-0.1a closed filling port are considerably higher than for
concept enables the generation of extremely high-0.2a lift of 4.8 mm. Therefore, the swirl level alone is
swirl numbers with low valve lifts without reducing insufficient to describe the in-cylinder flow. This
the flow for high valve lifts. The impact of different-0.3was also found in [4].
valve strategies on the combustion system using a
single-cylinder engine is assessed [1]. The test-0.4
engine had the following design parameter: -0.5
0.0 0.5 1.0 1.5 2.0
* Corresponding author: ewald_j@fev.de NOx-Emission [g/kWh]
Towards Clean Diesel Engines, TCDE 2009
and also the measured swirl ratio ( CU C A ) at the
PIV test bench.
CU C A RMSVtheta RMSVtheta
CU C A
Filling port deac- 2.28 3.94 1.73
tivated 3.2 mm
Both ports 5.66 3.79 0.67
active 1.6 mm
Table 1: Results of swirl ratio and RMS of the tan-
gential velocity.
As can be seen, the in-cylinder flow field gener-
ated when both ports are active is more homoge-
neous than the case with port deactivation.
Figure 2: 2280 rpm 9.4 bar, emissions [1]
Computational Setup
PIV measurements of stationary intake port In this study, the commercial CFD software
flow STAR-CD is used for the transient calculations of
3D PIV stationary flow analysis of the new port the intake and compression stroke with moving
design was performed for various valve lifts, and valves and piston. On the intake and exhaust port
port deactivation strategies. flange positions, pressure boundary conditions
both ports active - 1.6 mm valve lift from GT-Power gas exchange calculations were
tangential Port
employed. The calculation were performed from
filling Port
360°CA to 720°CA. Two different turbulence mod-
els, the LES Smagorinsky [2] and also the k-ε
model [3] are used for intake flow simulations.
Characterization of In-Cylinder Flow in-
homogeneity
Swirl Velocity Distribution RMS Velocity Fluctuation
In order to quantify the in-homogeneity of the
filling port deactivated - 3.2 mm valve lift
in-cylinder flow field different cut sections perpen-
dicular to the cylinder axis are considered. Each of
the considered cut sections is divided into concen-
tric rings, shown in Figure 4.
1 23 45 6
Swirl Velocity Distribution RMS Velocity Fluctuation
Figure 3: Charge motion analysis by 3D PIV without
and with filling port deactivation at z = 75 mm
Figure 3 shows the flow field in a horizontal Figure 4: Top view of a cut section considering six
section 75 mm below the cylinder head. The aver- rings
age flow distribution is displayed as a vector field
on the left of Figure 3 and the local distribution of For each ring, first a mean value of the tangen-
the flow fluctuation intensity is displayed by the tial velocity component is calculated. Then, for
velocity RMS on the right side of the same figure. each of these rings the RMS of the tangential ve-
In both cases the intake flow rates are similar, but locity is determined.
the resulting swirl flow patterns are strongly differ-
ent. For the filling port deactivation, the swirl flow Simulation results using RANS and LES
structure is less coherent, and fluctuation intensity The in-cylinder angular velocity is defined as
is increased. angular momentum divided by the moment of iner-
Table 1 compares the Root Mean Square tia.
(RMS) value of the measured tangential velocity –
- (1)
4.8 mm valve lift 4.8 mm valve lift
0 Filling port closed Filling port closed
0
Distance from cylinder head [mm]
Distance from cylinder head [mm]
2
The dimensionless swirl ratio for each operating 4
2
4
6
point is then obtained according to 8
6
8
10 10
12 12
in cylinder 14 14
Swirl ratio . (2) 16 16
Engine
18 18
20 20
22 22
0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10
Results of CFD simulations using the different RMSVtheta [m/s] RMSVtheta [m/s]
valve lift and port strategies are shown in Figure 5. Figure 7: RMS value of tangential velocity using
RANS(left graph) and LES(right graph)
It can be seen that the in-cylinder swirl ratio can be
increased to the same level either by reducing the As can be seen, differences in in-cylinder flow
maximum valve lift to 4.8mm or by port deactiva- field between these valve strategies can be distin-
tion. guished using the LES turbulence model rather
9 8.0 mm maximum valve lift
than the k-ε model. The investigations with the
8
6.4 mm maximum valve lift
4.8 mm maximum valve lift
LES model show that the 4.8 mm valve lift produc-
7 3.2 mm maximum valve lift
Filling port closed es more homogeneous swirl than port deactivation.
6
Swirl ratio [-]
This is also in agreement with experimental inves-
5
4 tigations at the 3D PIV flow test.
3
2
Summary and conclusions
1 Differences in emission behavior for different
0 valve lifts and with and without deactivated filling
360 420 480 540 600 660 720
Crank angle [°CA ATDC]
port were observed for a single-cylinder engine.
Measurements on a stationary flow bench and
Figure 5: Swirl ratio over crank angle as calculated CFD calculation both assessed the swirl level for
by RANS CFD simulations.
the different concepts. While in particular a maxi-
mum valve lift of 4.8 mm for both intake ports pro-
Inhomogeneity of in-cylinder flow
duces the same swirl level as the maximum valve
Figure 6 compares the cut sections of the tan-
lift of 8 mm with the filling port deactivated, the
gential velocity field of each strategy at the middle
soot emissions are significantly different and high-
of the piston bowl which is simulated using the k-ε
er for the latter configuration
model in STAR-CD at 2280 rpm.
Therefore it was argued that next to the swirl ra-
Using RANS model tio, another important parameter, describing these
Using LES model
Tangential velocity Tangential velocity discrepancies, is required. An approach to eva-
[m/s] [m/s]
luate the in-homogeneity of in-cylinder flow by
Filling port
Filling port
closed
closed
means of PIV and CFD simulation was developed
and presented. While in CFD, the RANS approach
could not show visible differences in the in-
homogeneity of in-cylinder flow, the LES approach
valve lift
valve lift
4.8 mm
4.8 mm
in CFD and the 3D-PIV method showed differenc-
es between two cases in a way that the in-
homogeneity in the case filling port closed is higher
Figure 6: Cut section of the tangential velocity field in than a maximum valve lift of 4.8 mm for both
the middle of the bowl using the RANS model (left) and
valves.
the LES model (right) at -30°CA ATDC
This work presented here has been submitted
As from the RANS simulation, there is almost to the SAE ICE conference in September 2009.
no difference between the two cases in terms of References
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A cut section of the tangential velocity field in Körfer, A. Kolbeck, M. Lamping, D. Tatur, D. Tomaz-
the middle of the bowl is shown also in Figure 6 for ic, Gas exchange optimization and the effect on
the same valve lift strategies using the LES model. Emission reduction for HSDI diesel engines, SAE
The LES turbulence model captures turbulent flow Paper 2009-01-0653 (2009).
[2] J. Smagorinsky, General Circulation experiments
structures while in RANS only the mean flow is
with the primitive equations I. The basic experiment,
resolved. Monthly Weather Review, 91(3),99-164 (1963).
The RMS values of the tangential velocity from [3] B.E. Launder, D.B. Spalding. The numerical compu-
RANS and LES are compared for both valve strat- tation of turbulent flow. Computer Methods in Applied
egies in Figure 7. Mechanics and Engineering 3,269-289 (1972)
[4] P. Adomeit, S. Pischinger, M. Becker, H. Rohs, A.
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