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To achieve the strict legislative restrictions for emissions from combustion engines, vast improvements in engine emissions and efficiency are required. Two major impacting factors for emissions and efficiency are the reliable generation of an effective mixture before ignition and a fast, stable combustion process. While the mixture of air and injected fuel is generated by highly three-dimensional, time-dependent flow phenomena during the intake and compression stroke, the turbulent flame propagation is directly affected by the turbulence level in the flow close to the advancing flame front. However, the flow field in the combustion chamber is highly turbulent and subject to cycle-to-cycle variations (CCV). To understand the fundamental mechanisms and interactions, 3D flow measurements with combined high spatial and temporal resolution are required.

Stereoscopic particle-image velocimetry (PIV) is used to investigate the non-reacting flow field in the combustion chamber of a motored direct-injection spark ignition (DISI) engine with tumble intake port. The in-cylinder flow is controlled by variable valve timing (VVT), i.e., shifting of the intake cam shaft to earlier or later crank angles (cam phasing). VVT systems are already implemented in production combustion engines, e.g., BMW's Vanos system, to improve the volumetric efficiency and to reduce pumping losses. In the present study, the underlying flow phenomena, i.e., the effect of VVT on the tumble development and turbulent kinetic energy, are analyzed. The flow field is investigated at a set of early, intermediate, and late intake valve opening (IVO) positions during the intake and compression strokes, thus enabling the analysis of the temporal development of the main flow structures.

The development of the flow field in the cylinder of a piston engine possesses a distinct influence on the fuel-air mixing and thus, on the combustion process. In particular, the flow structures that evolve during the intake and compression stroke are of major importance and at constant flow parameters, the intake port geometry influences these structures. To show this impact, the flow field of two engines with different intake port geometries is measured using particle-image velocimetry in the present study. The data are compared regarding the temporal and spatial development of the main flow phenomena and the turbulent kinetic energy. The study focuses on the impact of the two different formation mechanisms of tumble vortices due to the different intake port geometries on the flow structure. Engine A is an optical research engine optimized for high tumble ratios for combustion stability in combustion processes of tailor-made fuels.

The interaction of biofuel sprays from an outward opening hollow cone injector and the flow field inside an internal combustion engine is analyzed by Mie-Scattering Imaging (MSI) and high-speed stereoscopic particle-image velocimetry (stereo-PIV). Two fuels (ethanol and methyl ethyl ketone (MEK)), four injection pressures (50, 100, 150, and 200 bar), three starting points of injection (60°, 277°, and 297° atdc), and two engine speeds (1,500 rpm and 2,000 rpm) define the parameter space of the experiments. The MSI measurements determine the vertical penetration length and the spray cone angle of the ethanol and MEK spray. Stereo-PIV is used to investigate the interaction of the flow field and the ethanol spray after the injection process for a start of injection at 60° atdc. These measurements are compared to stereo-PIV measurements without fuel injection performed in the same engine [19].

The influence of in-cylinder flow on the propagation of 2-Butanone and Ethanol sprays is studied. To solely evaluate the interaction of air flow and fuel, high-speed Mie-Scattering Imaging of hollow cone sprays is conducted both in a single-cylinder optical engine with tumble movement and in a pressure vessel with negligible air flow. The direct comparison reveals an improved spray propagation of 2-Butanone due to the engine’s air flow. The lower viscosity of 2-Butanone causes an enhanced jet breakup compared to Ethanol such that the spray consists of more and smaller droplets. Small droplets possess a lower momentum, which allows the droplets to be more efficiently transported by the air flow. Consequently, the fuel distribution across the cylinder is enhanced. As the liquid fuel is distributed to a larger volume, improved convection accelerates evaporation.

The air-fuel mixture formation in an optical direct-injection internal combustion engine is investigated by numerical simulations for the two biofuels Ethanol and 2-Butanone. The gas phase in the internal combustion (IC) engine is predicted by a large-eddy simulation, in which the fuel phase is determined by a spray model based on Lagrangian particle tracking. A hollow-cone injector is used for which the primary breakup is modeled by a series of small full-cone injections, while the Rosin-Rammler initial droplet size distribution is used. The secondary spray break-up is modeled by the Kelvin-Helmholtz-Rayleigh-Taylor (KHRT) model, and the evaporation of the fuel is determined by the Bellan-Harstad model. The gas phase simulation is based on a finite-volume method formulated for hierarchical Cartesian grids, in which the immersed moving boundaries are resolved using a multiple level-set/cut-cell approach.

Numerical analyses of the liquid fuel injection and the subsequent fuel-air mixing in a high-tumble, long-stroke direct injection engine at operation conditions of 2000 RPM are presented. The Navier-Stokes equations are numerically solved with a finite-volume method for compressible flow based on a hierarchical Cartesian mesh. The solid wall boundaries are represented by a conservative multiple cut- and split-cell method, where a semi-Lagrange level-set solver is used to track the location of the individual moving boundaries. To determine the fuel vapor before ignition, a two-way coupled large-eddy simulation of the turbulent flow field with the spray droplets is conducted. Due to the large number of spray droplets, a Lagrangian Particle Tracking (LPT) algorithm is used to predict the liquid spray penetration and evaporation.

Numerical analyses of the liquid fuel injection and subsequent fuel-air mixing for a high-tumble direct injection engine with an active pre-chamber ignition system at operation conditions of 2000 RPM are presented. The Navier-Stokes equations for compressible in-cylinder flow are solved numerically using a hierarchical Cartesian mesh based finite-volume method. To determine the fuel vapor before ignition large-eddy flow simulations are two-way coupled with the spray droplets in a Lagrangian Particle Tracking (LPT) formulation. The combined hierarchical Cartesian mesh ensures efficient usage of high performance computing systems through solution adaptive refinement and dynamic load balancing. Computational meshes with approximately 170 million cells and 1.0 million spray parcels are used for the simulations.

Direct injection (DI) of compressed natural gas (CNG) is a promising technology to increase the indicated thermal efficiency of internal combustion engines (ICE) while reducing exhaust emissions and using a relatively low-cost fuel. However, design and analysis of DI-CNG engines are challenging because supersonic gas jet emerging from the DI injector results in a very complex in-cylinder flow field containing shocks and discontinuities affecting the fuel-air mixing. In this article, numerical simulations are used supported by validation to investigate the direct gas injection and its influence on the flow field and mixing in an optically accessible ICE. The simulation approach involves computation of the in-nozzle flow with highly accurate Large-Eddy Simulations, which are then used to obtain a mapped boundary condition. The boundary condition is applied in Unsteady Reynolds Averaged Navier-Stokes simulations of the engine to investigate the in-cylinder velocity and mixing fields.

A Holographic Particle-Image Velocimetry (HPIV) system is developed to investigate the in-cylinder flow in a motored four-valve engine operated at 1500 rpm. Image aberrations introduced by the optical liner of the engine are optically eliminated using complex-conjugate hologram reconstruction that allows measurements near the cylinder walls also. High-resolution velocity measurements in different axial planes of the in-cylinder flow are made at crank angles from 60° to 285° ATDC. Prospects and limitations to full three-dimensional extension of the HPIV system are discussed. The results show the development of the in-cylinder flow in two planes with emphasis on large and small-scale flow structures.

New combustion processes require an understanding of the highly three-dimensional flow field to effectively decrease fuel consumption and pollutant emission. Due to the complex spatial character of the flow the knowledge of the development of the flow in an extended volume is necessary. Previous investigations were able to visualize the discrete three-dimensional flow field through multi-plane stereoscopic PIV. In this study, cycle resolved tomographic particle-image velocimetry measurement have been performed to obtain a fully resolved representation of the three-dimensional flow structures at each instant. The analysis is based on the measurements at 80°, 160°, and 240° after top dead center(atdc) such that the velocity distributions at the intake, the end of the intake, and the compression stroke at an engine speed of 1,500 rpm are discussed in detail.