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Journal Article

Optical Diagnostics for Knock in Compression-Ignition Engines via High-Speed Imaging

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.
Technical Paper

Numerical and Optical Evolution of Gaseous Jets in Direct Injection Hydrogen Engines

This paper performs a parametric analysis of the influence of numerical grid resolution and turbulence model on jet penetration and mixture formation in a DI-H2 ICE. The cylinder geometry is typical of passenger-car sized spark-ignited engines, with a centrally located single-hole injector nozzle. The simulation includes the intake and exhaust port geometry, in order to account for the actual flow field within the cylinder when injection of hydrogen starts. A reduced geometry is then used to focus on the mixture formation process. The numerically predicted hydrogen mole-fraction fields are compared to experimental data from quantitative laser-based imaging in a corresponding optically accessible engine. In general, the results show that with proper mesh and turbulence settings, remarkable agreement between numerical and experimental data in terms of fuel jet evolution and mixture formation can be achieved.
Technical Paper

LES of Flow Processes in an SI Engine Using Two Approaches: OpenFoam and PsiPhi

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.
Journal Article

Influence of the Flow Field on Flame Propagation in a Hydrogen-Fueled Internal Combustion Engine

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.
Journal Article

In-Cylinder LIF Imaging, IR-Absorption Point Measurements, and a CFD Simulation to Evaluate Mixture Formation in a CNG-Fueled Engine

Two optical techniques were developed and combined with a CFD simulation to obtain spatio-temporally resolved information on air/fuel mixing in the cylinder of a methane-fueled, fired, optically accessible engine. Laser-induced fluorescence (LIF) of anisole (methoxybenzene), vaporized in trace amounts into the gaseous fuel upstream of the injector, was captured by a two-camera system, providing one instantaneous image of the air/fuel ratio per cycle. Broadband infrared (IR) absorption by the methane fuel itself was measured in a small probe volume via a spark-plug integrated sensor, yielding time-resolved quasi-point information at kHz-rates. The simulation was based on the Reynolds-averaged Navier-Stokes (RANS) approach with the two-equation k-epsilon turbulence model in a finite volume discretization scheme and included the port-fuel injection event. Commercial CFD software was used to perform engine simulations close to the experimental conditions.
Technical Paper

Imaging of Fuel-Film Evaporation and Combustion in a Direct-Injection Model Experiment

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.
Journal Article

High-Speed Imaging of Early Flame Growth in Spark-Ignited Engines Using Different Imaging Systems via Endoscopic and Full Optical Access

This work investigates the image quality achievable with a large-aperture endoscope system and high-speed cameras in terms of detecting the premixed flame boundary in spark-ignited engines by chemiluminescence imaging. The study is an extension of our previous work on endoscopic flame imaging [SAE 2014-01-1178]. In the present work, two different high-speed camera systems were used together with the endoscope system in two production engines to quantify the time-resolved flame propagation. The systems were cinematography with a CMOS-camera, both with and without an intensifier, the latter variation being used in a four-cylinder automotive engine as well as in a single-cylinder motorcycle engine. An algorithm with automatic dynamic thresholding was developed to detect the line-of-sight projected flame boundary despite artifacts caused by the spark and the large dynamic range in image brightness across each time series.
Technical Paper

Development of a LIF-Imaging System for Simultaneous High-Speed Visualization of Liquid Fuel and Oil Films in an Optically Accessible DISI Engine

Downsizing and direct injection in modern DISI engines can lead to fuel impinging on the cylinder walls. The interaction of liquid fuel and engine oil due to fuel impinging on the cylinder wall causes problems in both lubrication and combustion. To analyze this issue with temporal and spatial resolution, we developed a laser-induced fluorescence (LIF) system for simultaneous kHz-rate imaging of fuel and oil films on the cylinder wall. Engine oil was doped with traces of the laser dye pyrromethene 567, which fluoresces red after excitation by 532 nm laser radiation. Simultaneously, the liquid fuel was visualized by UV fluorescence of an aromatic “tracer” in a non-fluorescent surrogate fuel excited at 266 nm. Two combinations of fuel and tracer were investigated, iso-octane and toluene as well as a multi-component surrogate and anisole. The fluorescence from oil and fuel was spectrally separated and detected by two cameras.
Technical Paper

A Study of ECN ‘Spray B’ in a Light-Duty Optically Accessible Diesel Engine Based on High-Speed Imaging with LED Retro-Reflection

The Diesel-type three-hole Spray B (injector 211201) of the Engine Combustion Network (ECN) was used in a single-cylinder light-duty optically-accessible Diesel engine. A simple optical method was developed to quasi-simultaneously image both liquid and gas phase of the fuel spray as well as combustion at kHz rates by retro-reflection of pulsed LED light from the fire deck. From the images, liquid penetration length, fuel vapor penetration, spray dispersion angle, ignition delay, flame luminosity, and ignition location were determined. Wide-field imaging allowed for studying the nozzle hole-to-hole variation. In addition to a variation of ambient temperature and density to achieve the standard ECN condition, a variation of fuel rail pressure and swirl ratio was also investigated, under both non-reacting and reacting conditions. The results show physically reasonable variations with different operating conditions.