# Abbreviations

 ALE Arbitrary Lagrangian-Eulerian aTDC after top dead center bTDC before top dead center BDC bottom dead center CA crank angle CAD crank angle degree CCD charge-coupled device CCV cycle-to-cycle variation CDS central differencing scheme CFD computational fluid dynamics CFL Courant-Friedrichs-Lewy ENO Essentially Non-Oscillatory ERG exhaust-gas-recirculation EVC exhaust valve closing EVO exhaust valve opening FOV field-of-view HS-PIV high speed particle image velocimetry IC internal combustion IVC intake valve closing IVO intake valve opening LES large eddy simulation MRV magnetic resonance velocimetry PIV particle image velocimetry QSOU quasi-second-order upwind QUICK Quadratic Upwind Interpolation for Convective Kinematics RANS Reynolds-averaged Navier-Stokes RMS root mean square RPM rounds per minute SAS scale-adaptive simulation SRS scale-resolving simulation SST shear stress transport TDC top dead center TUBF Technische Universität Bergakademie Freiberg TUD Technische Universität Darmstadt TVD total variation diminishing UDE Universität Duisburg-Essen URANS unsteady Reynolds-averaged Navier-Stokes WG wall-guided

# Description

## Introduction

The TU Darmstadt engine is an optically accessible single cylinder spark-ignition direct injection engine. It is embedded in an especially designed test bench to provide well characterized boundary conditions and reproducible engine operation. A reproducible engine operation is needed to characterize the variety of in-cylinder processes and is a prerequisite for any comparison of experiments and simulations. The in-cylinder processes are characterized using advanced laser-diagnostics to provide measurements at high spatial and temporal resolutions. The aim of this effort is, to build up a comprehensive data set

• to give insights into the underlying physics for a better understanding of the relevant in-cylinder processes and
• for the validation of CFD simulations especially for large eddy simulations (LES).

The validation of CFD simulations for IC engines requires a variety of physical and chemical quantities and a comprehensive dataset consisting of systematic hierarchical sub-datasets. Such a validation sequence typically starts with the comparison of the non-reacting flow field and increases complexity stepwise by the addition of processes such as, combustion of perfect homogeneous air/fuel mixtures, in-cylinder mixture preparation using direct-injection and combustion of these mixtures. The presented test case is part of an ongoing effort at TU Darmstadt aiming for a detailed characterization of engine combustion addressing the non-reacting flow field [4, 5, 47], combustion of homogenous air/fuel mixtures [32] and mixture preparation by direct injection [33]. The here presented test-case includes the sub-dataset on the non-reacting flow field (motored engine operation) providing a comprehensive data set on the first two statistical moments of flow velocities, spatial flow structures, and the dynamics of the turbulent in-cylinder flow field. The following sections summarize the work which has been presented in [4]. Further details on the experimental setup and flow field data can be found in [4, 5, 47]. Additionally, simulation results obtained from three investigations using LES (Large Eddy Simulation) and hybrid URANS (unsteady Reynolds-averaged Navier-Stokes)/LES described in section CFD Simulations are presented and compared with experimental data, see section CFD Simulations.

## Relevance to industrial sector

The engine flow has a governing influence on the key processes of turbulent mixing and combustion, which define engine performance and regarding efficiency and emission. The considered engine features a typical design of a modern spark-ignition direct injection engine. The combination of the four-valve pent-roof cylinder head with the dual-port intake system provides a tumble flow motion, a key flow feature within spark ignition engines. The wall-bounded tumble vortex carries a large amount of the flow's kinetic energy. The tumble vortex interacts with the solid walls, which is recognized as a leading phenomenon that generates turbulence during tumble breakdown late during compression [7]. High turbulence levels are needed to enhance flame propagation for an efficient engine operation. Especially for new combustion concepts for low NOx emissions, increased levels of EGR (exhaust-gas-recirculation) or lean mixtures are of interest. This reduces laminar flame speed, which needs to be compensated for by increased turbulence levels during flame initiation and propagation.

The engine flow is additionally recognized to be an important cause for cycle-to-cycle variations (CCV). The in-cylinder flow can induce a considerable macroscopic motion of the flame kernel, which significantly affects the combustion process especially for lean combustion [12] and can affect spray formation and mixture preparation, which is important especially for stratified engine combustion [42]. In engine development there is typically a trade-off between efficiency and stable engine operation. An understanding and control of engine CCV is required to move engine operation closer to the limits of stable engine operation in order to further increase engine efficiency.

## Design or assessment parameters

The quantities available for validation are velocities within the central tumble plane providing information on the

• first two statistical moments
• spatial flow structures
• temporal evolution of the flow field over entire engine cycles (crank angle resolved)

The 3D-flow data is available within a 4mm and 8mm thick volume around the central tumble plane, respectively, providing information on the

• first two statistical moments
• spatial flow structures

Furthermore, the in-cylinder, intake-port and exhaust-port pressures are available for comparison.

## Engine test bench

The engine test bench allows conditioning the intake air in terms of pressure, temperature and gas composition, port fuel injection of liquid and gaseous fuels, direct-injection, as well as reliable boundary condition control and measurement. Further, the intake and exhaust systems were designed to provide reproducible thermodynamic and flow boundary conditions and simplified meshing for three-dimensional engine flow simulations. Figure 2 shows a schematic of the engine test bench from the intake to the exhaust plenum. It shows the locations for which thermodynamic conditions (pressure and temperature) were measured during engine operation.

 Figure 2: Engine test bench.

## Engine

The engine is an optically accessible single-cylinder direct injection spark ignition engine with a bore and stroke of 86mm. The engine features a twin-cam, overhead-valve pent-roof cylinder head, a 55mm height quartz-glass cylinder liner with 8mm window extension into the pent-roof, and a Bowditch piston arrangement with flat quartz-glass piston-crown window (75mm diameter). The intake manifold is designed to generate a tumble flow. The engine is operated with a geometric compression ratio of 8.5:1 and has a 499 cm3 displacement volume. At top dead center (TDC), the clearance height is 2.6mm and a clearance volume of 66.61 cm3 remains. The piston ring top-land crevice contributes to the reported clearance volume. The piston ring pack is located 74mm from the piston top to prevent the piston rings from riding over the quartz-glass cylinder liner. The average spacing between the piston and cylinder-liner is 0.5mm. The top-land crevice volume contributes to the overall clearance volume and is included in the geometric compression ratio. For this test-case, a wall-guided (WG) cylinder-head was used equipped with a side-mounted fuel injector, a centrally located spark plug, and dual intake valves (33mm diameter) and exhaust valves (29mm diameter) located on opposite sides of the spark plug, see Figure 3.

 Figure 3: Cross-section of the wall-guided cylinder head in the central tumble plane at z=0mm and a top view of the cylinder head.

## Flow physics and Fluid Dynamics Data

Figure 4 shows the phase-averaged flow field from 2700 cycles during intake at 270° bTDC (for valve timing and definition of TDC see Figure 8). The in-cylinder flow in the central tumble plane is characterized by high velocities generated from the annular flow from the intake valves and the downward motion of the piston. At the left of the image a downward flow comprising of high velocities from the region in-between the intake valves is observed. Beyond the inlet jet to the right, the intake flow is directed downward and is redirected by the piston top. As a result a clockwise tumble motion is formed and a stagnation front is found left of the tumble vortex center where the incoming fluid from the intake valves impinges on the reversing flow from the piston. During the compression stroke at 90° bTDC, the clockwise tumble motion persists with the tumble center present within the exhaust side of the engine. Overall, velocities are significantly lower during compression, with the largest velocities occurring near the piston, which are caused by the upward piston motion.

 Figure 4: Phase-averaged flow fields (based on 2700 consecutive engine cycles) at 270° and 90° bTDC (every 5th vector displayed). Coordinate system is defined with x=0, z=0 at the cylinder axis and y=0 at the beginning of the pentroof. The piston is positioned at y=-51.38mm for these CA. This figure is reproduced with permission from [4].

The 3D structure of the in-cylinder flow field was determined by Tomographic PIV. Figure 5 shows phase-averaged flow fields during intake (270° bTDC, 180° bTDC) and compression (90° bTDC). The vectors (every 4th vector displayed) represent the local 3D-velocity (${\displaystyle {u}}$, ${\displaystyle {v}}$, ${\displaystyle {w}}$) at the central tumble plane of the measurement volume. Discrete levels of velocity magnitude computed by all three velocity components are displayed by 3D iso-surfaces. At 270° bTDC, iso-surfaces in the high-intake velocity region are aligned parallel to the flow direction. Beyond this region (for y<−20mm), the iso-surfaces are aligned perpendicular to the flow. At 180° bTDC the flow field is no longer characterized by large velocities of an inlet flow, but is rather characterized by smaller velocities due to the lack of piston movement. As a result, the phase-averaged flow field does not exhibit any preferential direction regarding the 3D velocity iso-surfaces. During compression (Figure 5, c), the flow is reorganized. A portion of the tumble motion is visible during compression with the tumble center located near the top right corner. The flow field shows a perpendicular direction to most 3D velocity iso-surfaces and velocity magnitudes remain on a similar level as for 180° bTDC. The phase averaged velocity image does not reveal drastic 3D flow structures compared to phase averaged flow fields during intake.

 Figure 5: Phase-averaged flow fields; Iso-surfaces of the 3D-velocity magnitude with vectors (every 4th vector displayed) at the central tumble plane (z=0mm). This figure is taken from [5]

The vectors in Figure 6 show the local instantaneous 3D-velocity at the central tumble plane at a particular cycle during intake at 270° bTDC. The flow features, such as the inlet flow and the tumble vortex locations, revealed within the mean flow (see Figure 4) are easily identifiable in the instantaneous cycle. Additionally vortical structures are visualized using the Q-criteria, revealing the complexity of the turbulent 3D flow. A variety of small scale vortical structures (≈2–6mm in diameter) of different size and orientation are found in the region of the inlet flow. Although many structures are cut due to the limited thickness of the measurement volume (Δz=±2mm) the formation of vortex tubes is visible.

 Figure 6: Snapshot at 270° bTDC showing the local 3D-velocity in the central tumble plane. Vortical structures are visualized using the Q-critera.

Finally, time-resolved PIV (2D2C) was used to capture the transient behavior of the in-cylinder flow. Figure 7 shows the phase-averaged flow (72 cycles) at 4 selected CAD (bottom) with an averaged trace of the velocity magnitude (top) at a monitor point (y = x = 0mm). Although the spark plug was removed for these experiments, this monitor point is within the vicinity of the spark plug region. The flow within this region is important to characterize as it directly impacts the flow defining the spark-ignition event. The velocity amplitude peaks when the intake and the exhaust valves open. Another peak is found ∼ 40° bTDC. This is close to ignition timing or for stratified combustion typical injection timing.

 Figure 7: Averaged (black) and instantaneous (grey) trace of the velocity magnitude (top) at a monitor point (y=x=0mm) with a few corresponding 2D flow fields (bottom) at selected CAD.

Contributed by: Carl Philip Ding, Rene Honza, Elias Baum, Benjamin Böhm, Andreas Dreizler — Fachgebiet Reaktive Strömungen und Messtechnik (RSM),Technische Universität Darmstadt, Germany

Contributed by: Brian Peterson — School of Engineering, University of Edinburgh, Scotland UK

Contributed by: Chao He, Wibke Leudesdorff, Guido Kuenne, Amsini Sadiki, Johannes Janicka — Fachgebiet Energie und Kraftwerkstechnik (EKT), Technische Universität Darmstadt, Germany

Contributed by: Peter Janas, Andreas Kempf — Institut für Verbrennung und Gasdynamik (IVG), Lehrstuhl für Fluiddynamik, Universität Duisburg-Essen, Germany

Contributed by: Stefan Buhl, Christian Hasse — Fachgebiet Simulation reaktiver Thermo-Fluid Systeme (STFS), Technische Universität Darmstadt, Germany; former: Professur Numerische Thermofluiddynamik (NTFD), Technische Universität Bergakademie Freiberg, Germany