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Mixture formation in an optically accessible hydrogen-fueled engine was investigated using Planar Laser-Induced Fluorescence (PLIF) of acetone as a fuel tracer. The engine was motored and fueled by direct high-pressure injection. This paper presents the evolution of the spatial distribution of the ensemble-mean equivalence ratio for six different combinations of nozzle design and injector geometry, each for three different injection timings after intake-valve closure. Asymmetric single-hole and 5-hole nozzles as well as symmetric 6-hole and 13-hole nozzles were used. For early injection, the low in-cylinder pressure and density allow the jet to preserve its momentum long enough to undergo extensive jet-wall and (for multi-hole nozzles) jet-jet interaction, but the final mixture is fairly homogeneous. Intermediately timed injection yields inhomogeneous mixtures with surprisingly similar features observed for all multi-hole injectors.

In an optically accessible single-cylinder engine fueled with hydrogen, acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) are used to evaluate in-cylinder mixture formation. The experiments include measurements for engine operation with hydrogen injection in-cylinder either prior to or after intake valve closure (IVC). Pre-IVC injection is used to produce a near-homogeneous mixture for PLIF calibration experiments and to establish a baseline comparison for post-IVC injection. Calibration experiments and a temperature correction allow conversion of the acetone fluorescence signal to equivalence ratio. For post-IVC injection with start of injection (SOI) coincident with IVC, PLIF results are similar to pre-IVC injection. With retard of SOI from IVC, mixture inhomogeneities increase monotonically, with high hydrogen concentration spatially located near the injector and within a smaller volume.

Flame propagation in an optically accessible hydrogen-fueled internal combustion engine was visualized by high-speed schlieren imaging. Two intake configurations were evaluated: low tumble with a tumble ratio of 0.22, corresponding to unmodified intake ports, and high tumble with a tumble ratio of 0.70, resulting from intake modification. For each intake configuration, fueling was either far upstream of the engine, with presumably no influence on the intake flow, or the fuel was injected directly early during the compression stroke from an angled single-hole injector, adding significant angular momentum to the in-cylinder flow. Crank-angle resolved schlieren imaging during combustion allowed deducing apparent flame location and propagation speed, which were then correlated with in-cylinder pressure measurements on a single-cycle basis. In a typical cycle, flame shape and convective displacement are strongly affected by the in-cylinder flow.

Knocking combustion is associated with extremely high in-cylinder pressure rise rates, strong pressure oscillations, destructive engine vibration, as well as audible noise. It not only exists in spark-ignition (SI) engines but also in compression-ignition (CI) engines, for both conventional Diesel and more premixed modes of combustion. Recent work showed that during Diesel knock the flame’s motion synchronizes with the in-cylinder pressure ringing. To improve the optical method and investigate knock in CI engines further, we imaged the flame luminosity with n-dodecane as a Diesel surrogate in an optically accessible engine during knock at very high frame rates (60 kHz). First, the knocking time interval was determined based on the temporal variation of the mean image intensity. Within this time interval, the instantaneous flow fields were calculated by “optical flow” based on cross-correlation.

The in-cylinder charge motion during the compression stroke of an optically accessible engine equipped with direct injection of hydrogen fuel is measured via particle image velocimetry (PIV). The evolution of the mean flow field and the tumble ratio are examined with and without injection, each with the unmodified 4-valve pent-roof engine head and with the intake ports modified to yield higher tumble. The measurements in the vertical symmetry plane of the cylinder show that intake modification produces the desired drastic increase in tumble flow, changing the tumble ratio at BDC from 0.22 to 0.70. Either intake-induced flow is completely disrupted by the high-pressure hydrogen injection from an angled, centrally located single-hole nozzle. The injection event leads to sudden reversal of the tumble. Hence the tumble ratio is negative after injection. However, the two intake configurations still differ in tumble ratio by about the same magnitude as before injection.

In this study two different simulation approaches to large eddy simulation of spark-ignition engines are compared. Additionally, some of the simulation results are compared to experimentally obtained in-cylinder velocity measurements. The first approach applies unstructured grids with an automated meshing procedure, using OpenFoam and Lib-ICE with a mapping approach. The second approach applies the efficient in-house code PsiPhi on equidistant, Cartesian grids, representing walls by immersed boundaries, where the moving piston and valves are described as topologically connected groups of Lagrangian particles. In the experiments, two-dimensional two-component particle image velocimetry is applied in the central tumble plane of the cylinder of an optically accessible engine. Good agreement between numerical results and experiment are obtained by both approaches.

Late-evaporating liquid fuel films within the combustion chamber are considered a major source of soot in gasoline direct-injection engines. In this study a direct-injection model experiment was developed to visualize and investigate the evaporation of fuel films and their contribution to soot formation with different diagnostic techniques. A mixture of isooctane (surrogate fuel) and toluene (fluorescent tracer) is injected by a multi-hole injector into a wind tunnel with an optically accessible test section. Air flows continuously at low speed and ambient pressure through the test section. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. Combustion is initiated by a pair of electrodes within the fuel/air-mixture. The turbulent flame front propagates through the chamber and ignites pool fires near the fuel films, leading to locally sooting combustion.

Large Eddy Simulations (LES) and tracer-based Laser-Induced Fluorescence (LIF) measurements were performed to study the dynamics of fuel wall-films on the piston top of an optically accessible, four-valve pent-roof GDI research engine for a total of eight operating conditions. Starting from a reference point, the systematic variations include changes in engine speed (600; 1,200 and 2,000 RPM) and load (1000 and 500 mbar intake pressure); concerning the fuel path the Start Of Injection (SOI=360°, 390° and 420° CA after gas exchange TDC) as well as the injection pressure (10, 20 and 35 MPa) were varied. For each condition, 40 experimental images were acquired phase-locked at 10° CA intervals after SOI, showing the wall-film dynamics in terms of spatial extent, thickness and temperature.

Flame penetration into the top-land crevice of a combustion engine's cylinder is investigated by large-eddy simulation (LES) and high-speed visualization experiments. This penetration is of practical relevance as it leads to the formation of unburned hydrocarbons (UHC) wherever the flame is quenched inside the crevice. In optically accessible engines, the crevice is particularly large, so that it must be considered in simulations, which also creates an opportunity to study flame penetration in detail. The high-speed imaging shows a luminous front, subject to cyclical variation, penetrating into the top-land crevice, but cannot distinguish between a flame burning into the crevice or just being pushed into it by increasing pressure in the cylinder. The LES of the process permits to turn off the reaction source term, so that the effect of convection and free flame propagation can be separated.

Flame penetration into the top-land crevice of a combustion engine's cylinder is investigated by large-eddy simulation (LES) and high-speed visualization experiments. This penetration is of practical relevance as it leads to the formation of unburned hydrocarbons (UHC) wherever the flame is quenched inside the crevice. In optically accessible engines, the crevice is particularly large, so that it must be considered in simulations, which also creates an opportunity to study flame penetration in detail. The high-speed imaging shows a luminous front, subject to cyclical variation, penetrating into the top-land crevice, but cannot distinguish between a flame burning into the crevice or just being pushed into it by increasing pressure in the cylinder. The LES of the process permits to turn off the reaction source term, so that the effect of convection and free flame propagation can be separated.

Mixture formation in a hydrogen-fueled heavy-duty engine with direct injection and a nearly-quiescent top-hat combustion chamber was investigated using laser-induced fluorescence imaging, with 1,4-difluorobenzene serving as a fluorescent tracer seeded into hydrogen. The engine was motored at 1200 rpm, 1.0 bar intake pressure, and 335 K intake temperature. An outward opening medium-pressure hollow-cone injector was operated at two different injection pressures and five different injection timings from early injection during the intake stroke to late injection towards the end of compression stroke. Fuel fumigation upstream of the intake provided a well-mixed reference case for image calibration. This paper presents the evolution of in-cylinder equivalence ratio distribution evaluated during the injection event itself for the cylinder-axis plane and during the compression stroke at different positions of the light sheet within the swirl plane.