Todd D. Fansler

Piston Fuel Films as a Source of Smoke and Hydrocarbon Emissions from a Wall-Controlled Spark-Ignited Direct-Injection Engine

Thin films of liquid fuel can form on the piston surface in spark-ignited direct-injection (SIDI) engines. These fuel films can result in pool fires that lead to deposit formation and increased hydrocarbon (HC) and smoke emissions. Previous investigations of the effects of piston fuel films on engine-out HC and smoke emissions have been hampered by their inability to measure the fuel-film mass in operating direct-injection engines. In this paper, a recently developed high-speed refractive-index-matching imaging technique is used for quantitative time- and space-resolved measurements of fuel-film mass on a quartz piston window of an optically-accessible direct-injection engine operating over a range of fully-warmed-up stratified-charge conditions with both a high-pressure hollow-cone swirl-type injector and with a high-pressure multihole injector. Measured fuel-film mass is a small percentage of the total fuel injected with the high-pressure swirl injector (maximum of ∼1% with gasoline fuel and ∼0.1% with isooctane fuel). Most of the piston fuel-film mass evaporates during the cycle and burns as a pool fire. These pool fires are observed by endoscopic and through-the-piston imaging, and the occurrence and location of the pool fires are consistent with the measured piston fuel films. The fuel-film data are also correlated with engine-out HC and smoke emissions measurements from a conventional all-metal single-cylinder engine of the same design. Smoke emissions from the engine with a high-pressure swirl injector increase linearly with the measured fuel-film mass. Fuel films are found to be the dominant source of smoke emissions with the swirl injector in this engine, with ∼10% of the wall-film mass converted to emitted smoke mass. Smoke emissions from the engine with a high-pressure multihole injector are very small or zero, consistent with the much smaller measured fuel-film mass (∼0.05% of the injected gasoline fuel volume). In contrast, engine-out HC emissions do not correlate with fuel-film mass. For optimum injection timings, the measured fuel-film mass is so small that even in the unlikely event that all of the fuel film mass was converted to engine-out HC emissions, fuel films could account for less than 15% of the total HC emissions for the swirl injector and less than 2% for the multihole injector. For off-optimum injection timings, the HC emissions are significantly larger, but wall films can account for at most 35% of the unburned HC emissions. This is contrary to some previous studies that claimed fuel films were the largest contributor to HC emissions (∼80%) in stratified SIDI engines. The data from this engine support overmixing as the dominant source of HC emissions for optimum engine operating conditions. However, fuel films may be a significant source of HC emissions for cold start or low-speed engine-operating conditions.

Spray measurement technology: a review

Sprays are among the most intellectually challenging and practically important topics in fluid mechanics. This paper reviews needs, milestones, challenges, and a broad array of techniques for spray measurement. In addition, tabular summaries provide cross-referenced entry points to the vast literature by organizing over 300 citations according to key spray phenomena, physical parameters and measurement techniques for each of the principal spray regions (nozzle internal flow, near-field spray-formation region, far-field developed spray, and spray-wall interaction). The article closes with perspectives on some current issues in spray research, including the cost and complexity of apparatus for spray physics and spray engineering, the need for simultaneous diagnostic measurements under application-relevant conditions, and the effective comparison of spray measurements and numerical simulations.

Cycle-Resolved Laser-Velocimetry Measurements in a Reentrant-Bowl-in-Piston Engine

Cycle-resolved measurements of in-cylinder air velocity have been made in a motored engine (600-r/min engine speed, 10.6:1 compression) having a high-squish reentrant-bowl piston. The velocity fluctuations have been analyzed both by low-pass/highpass frequency filtering and by evaluation of non-stationary velocity autocorrelation functions. These two complementary analyses and general issues pertaining to their interpretation are carefully examined

Invited Review: Combustion instability in spray-guided stratified-charge engines: A review

This article reviews systematic research on combustion instabilities (principally rare, random misfires and partial burns) in spray-guided stratified-charge (SGSC) engines operated at part load with highly stratified fuel -air -residual mixtures. Results from high-speed optical imaging diagnostics and numerical simulation provide a conceptual framework and quantify the sensitivity of ignition and flame propagation to strong, cyclically varying temporal and spatial gradients in the flow field and in the fuel -air -residual distribution. For SGSC engines using multi-hole injectors, spark stretching and locally rich ignition are beneficial. Combustion instability is dominated by convective flow fluctuations that impede motion of the spark or flame kernel toward the bulk of the fuel, coupled with low flame speeds due to locally lean mixtures surrounding the kernel. In SGSC engines using outwardly opening piezo-electric injectors, ignition and early flame growth are strongly influenced by the sprays characteristic recirculation vortex. For both injection systems, the spray and the intake/compression-generated flow field influence each other. Factors underlying the benefits of multi-pulse injection are identified. Unresolved questions include (1) the extent to which piezo-SGSC misfires are caused by failure to form a flame kernel rather than by flame-kernel extinction (as in multi-hole SGSC engines); (2) the relative contributions of partially premixed flame propagation and mixing-controlled combustion under the exceptionally late-injection conditions that permit SGSC operation on E85-like fuels with very low NOx and soot emissions; and (3) the effects of flow-field variability on later combustion, where fuel-air-residual mixing within the piston bowl becomes important.

Quantitative liquid and vapor distribution measurements in evaporating fuel sprays using laser-induced exciplex fluorescence

Fully quantitative two-dimensional measurements of liquid- and vapor-phase fuel distributions (mass per unit volume) from high-pressure direct-injection gasoline injectors are reported for conditions of both slow and rapid vaporization in a heated, high-pressure spray chamber. The measurements employ the coevaporative gasoline-like fluorobenzene (FB)/diethylmethylamine (DEMA)/hexane exciplex tracer/fuel system. In contrast to most previous laser-induced exciplex-fluorescence (LIEF) experiments, the quantitative results here include regions in which liquid and vapor fuel coexist (e.g. near the injector exit). A unique aspect is evaluation of both vapor- and liquid-phase distributions at varying temperature and pressure using only in situ vapor-phase fluorescence calibration measurements at room temperature and atmospheric pressure. This approach draws on recent extensive measurements of the temperature-dependent spectroscopic properties of the FB–DEMA exciplex system, in particular on knowledge of the quantum efficiencies of the vapor-phase and liquid-phase (exciplex) fluorescence. In addition to procedures necessary for quantitative measurements, we discuss corrections for liquid–vapor crosstalk (liquid fluorescence that overlaps the vapor-fluorescence bandpass), the unknown local temperature due to vaporization-induced cooling, and laser-sheet attenuation by scattering and absorption.

Local fuel concentration, ignition and combustion in a stratified charge spark combustion in a stratified charge spark ignited direct injection engine: Spectroscopic, imaging and pressure-based measurements

Abstract A recently developed spark emission spec-troscopy technique has been used to measure the effects of fuel injection timing, spark timing and intake swirl level on the individual-cycle fuel concentration at the spark gap in a wall-guided spark ignited direct injection (SIDI) engine. The fuel-concentration measurements were made simultaneously with measurements of individual-cycle spark discharge energy and cylinder pressure. Endoscopic imaging of the fuel spray and high-speed imaging of combustion (both broadband and spectrally resolved) augment these quantitative data. For optimum engine operation, the fuel-air equivalence ratio at the spark gap just after spark breakdown is rich on average (〈φ〉 ≈1.4–1.5) and varies widely from cycle to cycle (∼25 per cent). The evolution with crank angle of the mean equivalence ratio and its cycle-to-cycle fluctuations are correlated with the cylinder pressure, heat release and imaging data to provide insights into fuel transport and mixture preparation that are important to understanding and optimizing ignition and combustion in SIDI engines. For example, causes of misfires and partial burns have been determined.

Comparisons of Computed and Measured Three-Dimensional Velocity Fields in a Motored Two-Stroke Engine

Computer simulations are compared with measurements of the three-dimensional, unsteady scavenging flows of a motored two-stroke engine. Laser Doppler velocimetry measurements were made on a modified Suzuki DT-85 ported engine. Calculations were performed using KIVA-3, a computer program that efficiently solves the intake and exhaust port flows along with those in the cylinder. Measured and computed cylinder pressures and velocities are compared. Pressures agree well over the cycle as do the velocities at the intake ports. In-cylinder velocities differ in detail, but the tumbling motion in the cylinder is well replicated in vertical plane passing through the cylinder axis. 20 refs., 7 figs., 3 tabs.

Advanced diagnostics for minimizing hydrocarbon emissions from a direct-injection gasoline engine

Minimizing unburned-hydrocarbon (HC) emissions at light load is essential for realizing the potential fuel-economy, cold-start, and transient-HC advantages of direct-injection (DI) stratified-charge engines. This paper summarizes the application of several advanced diagnostics to understand and quantify HC sources in an experimental DI two-stroke engine. Single-cycle (two-dimensional) and multicycle-averaged (two-dimensional and reconstructed three-dimensional) laser-induced-fluorescence (LIF) imaging of gasoline (1) characterizes the highly stratified fuel distribution at the time of ignition, (2) identifies cyclic variations in the fuel concentration near the spark gap as a principal cause of misfires and partial burns, (3) reveals regions of fuel-air mixture around the periphery of the fuel cloud that are too lean to burn, and (4) detects the outgassing of unburned fuel from the fuel injector nozzle-exit crevice late in the engine cycle. Cyclic variations are investigated further by collecting continuous, time-resolved data on liquid fuel distributions, combustion, and exhaust hydrocarbon emissions over many consecutive engine cycles. Specifically, high-speed (4000 frames/s) video imaging of the fuel spray and of spectrally resolved combustion luminosity is combined with simultaneous exhaust-HC sampling using a close-coupled fast-response (∼2 ms) flame-ionization detector. Cylinder pressure is also digitized simultaneously, so that the imaging results can be correlated with the heat released and the exhaust HC mass for each engine cycle. The results (1) show that combustion begins as partially premixed flame propagation and ends as slower mixing-limited or diffusion burning, (2) reveal quantitatively the fate of unburned fuel in misfire and partial-burn cycles, and (3) provide strong evidence that the dominant HC sources are incomplete combustion of the injected fuel cloud and late release of fuel trapped in the injector nozzle-exit crevice (rather than fuel trapped in the piston top-ring-land crevice, which is the dominant HC source in conventional homogeneous-charge four-stroke engines).

Investigation of pre-mixed flame-kernel/vortex interactions via high-speed imaging

To reduce cycle-to-cycle variations in SI-IC engines, knowledge of early flame kernel growth in a turbulent flow field is required. Understanding the interaction between a flame kernel and a vortex is an important fundamental step toward this goal. This paper presents high-speed movies of combustion luminosity during the interaction of a laminar vortex with a spark-generated pre-mixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts.

‘‘Designer diagnostics’’ for developing direct-injection gasoline engines

The primary motivation for stratified-charge spark-ignited direct injection (SC-SIDI) engines is to maximize fuel economy by operating the engine with minimal (preferably no) throttling at part load. This requires control of the fuel-air mixing process to create a fuel cloud around the spark plug that is favorable for ignition and complete combustion in every engine cycle. This paper illustrates experimental techniques that have been developed to measure key aspects of the incylinder fuel-injection, ignition, combustion and emissions-formation processes. Most of these techniques rely on high-speed digital imaging and in-situ calibration in order to characterize dynamic in-cylinder phenomena that vary substantially from cycle to cycle.