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The effects of imaging system resolution and laser sheet thickness on the measurement of the Batchelor scale were investigated in a single-cylinder optical engine. The Batchelor scale was determined by fitting a model spectrum to the dissipation spectrum that was obtained from fuel tracer planar laser-induced fluorescence (PLIF) images of the in-cylinder scalar field. The imaging system resolution was quantified by measuring the step-response function; the scanning knife edge technique was used to measure the 10-90% clip width of the laser sheet. In these experiments, the spatial resolution varied from a native resolution of 32.0 μm to 137.4 μm, and the laser sheet thickness ranged from 108 μm to 707 μm. Thus, the overall resolution of the imaging system was made to vary by approximately a factor of four in the in-plane dimension and a factor of six in the out-of-plane dimension.

High resolution planar laser-induced fluorescence (PLIF) measurements were performed in an optically accessible internal combustion (IC) engine to investigate the behavior of scalar dissipation and the fine-scale structures of the turbulent scalar field. The fluorescent tracer fluorobenzene was doped into one of the two intake streams and nitrogen was used as the carrier gas to permit high signal-to-noise ratio fluorescence measurements without oxygen quenching effects. The resulting two-dimensional images allowed for an analysis of the structural detail of the scalar and scalar dissipation fields defined by the mixing of the two adjacent intake streams. High levels of scalar dissipation were found to be located within convoluted, sheet-like structures in accordance with previous studies. The fluorescence data, which were acquired during the intake stroke, were also used to examine the scalar energy and dissipation spectra.

Planar laser-induced fluorescence (PLIF) has become a useful diagnostic for the quantification of in-cylinder flowfield conditions, and in many applications determining the homogeneity of the in-cylinder flowfield is of primary importance. In some cases, noise associated with this imaging technique (i.e., camera noise, shot-to-shot laser energy variation, and laser sheet profile variations) can dominate the flowfield inhomogeneities, leading to biased mixture stratification statistics. Presented herein are three data normalization schemes (global-, image-, and ray-mean) that can be used to correct for these noise sources when assessing mixture stratification from PLIF data. The normalization schemes are applied to in-cylinder PLIF data obtained over a wide range of inhomogeneity levels, and the conditions over which the use of each normalization scheme is appropriate are discussed.

The mixing between the flows introduced through different intake valves of a four-valve engine was investigated optically. Each valve was fed from a different intake system, and the relative sensitivity to different flow parameters (manipulated with the goal of enhancing the bulk in-cylinder stratification) was investigated. Flow manipulation was achieved in three primary ways: modifying the intake runner geometry upstream of the head, introducing flow-directing baffles into the intake port, and attaching flow break-down screens to the intake valves. The relative merits of each flow manipulation method was evaluated using planar laser-induced fluorescence (PLIF) of 3-pentanone, which was introduced to the engine through only one intake valve. Images were acquired from 315° bTDC through 45° bTDC, and the level of in-cylinder stratification was evaluated on an ensemble and cycle-to-cycle basis using a novel column-based probability distribution function (PDF) contour plot.

The noise characteristics of four camera systems representative of those typically used for laser-imaging experiments (a back-illuminated slow-scan camera, a frame-straddling slow-scan camera, an intensified slow-scan camera and an intensified video-rate camera) were investigated, and the results are presented as a function of the signal level and illumination level. These results provide the maximum possible signal-to-noise ratio for laser-imaging experiments, and represent the limit of quantitative signal interpretation. A calibration strategy for engine data that limits the uncertainties associated with thermodynamic and optical correction was presented and applied to engine data acquired with two of the camera systems. When a rigorous analysis of the signal is performed it is seen that shot noise limits the quantitative interpretation of the data for most typical laser-imaging experiments, and obviates the use of single-pixel data.

The current work investigates the in-cylinder mixing of a fluorescent tracer species inducted into the engine through a small-diameter tube mounted along the inner port wall and the remaining inlet stream in a small-bore utility engine. Planar laser-induced fluorescence (PLIF) measurements were acquired on a single plane, parallel to and approximately 4 mm below the cylinder head deck, throughout the intake and compression strokes. The data were analyzed to qualitatively and quantitatively describe the evolution of the mixture stratification. The highest degree of stratification in the mean field was observed at a timing of 90 crank angle (CA) degrees after top dead center (aTDC) of the intake stroke, which corresponds closely to the point of maximum intake valve lift (105 CA degrees aTDC).

Schlieren visualization was performed to investigate hydrogen injection into a quiescent chamber. The injection pressures investigated were 52 and 104 bar, and the chamber density ranged from 1.15 to 12.8 kg/m3, giving rise to underexpanded jets for all conditions. The expansion waves outside the nozzle were clearly visible with hydrogen, and the effect was confirmed with studies of nitrogen injected into a nitrogen environment. The distance between the expansion wave fronts was found to scale directly with the ratio of the exit pressure to the chamber pressure. The jet tip penetration rate was measured and was found to increase with injection pressure, and decrease with chamber density as expected. A mass- and momentum-preserving scheme was developed to relate the underexpanded jet to a subsonic jet of larger diameter.

Combustion under stratified operating conditions in a direct-injection spark-ignition engine was investigated using simultaneous planar laser-induced fluorescence imaging of the fuel distribution (via 3-pentanone doped into the fuel) and the combustion products (via OH, which occurs naturally). The simultaneous images allow direct determination of the flame front location under highly stratified conditions where the flame, or product, location is not uniquely identified by the absence of fuel. The 3-pentanone images were quantified, and an edge detection algorithm was developed and applied to the OH data to identify the flame front position. The result was the compilation of local flame-front equivalence ratio probability density functions (PDFs) for engine operating conditions at 600 and 1200 rpm and engine loads varying from equivalence ratios of 0.89 to 0.32 with an unthrottled intake. Homogeneous conditions were used to verify the integrity of the method.

High-resolution planar laser-induced fluorescence (PLIF) measurements were performed on the scalar field in an optical engine. The measurements were of sufficient resolution to fully resolve all of the length scales of the flow field through the full cycle. The scalar dissipation spectrum was calculated, and by fitting the results to a model turbulent spectrum the Batchelor scale of the turbulent flow was estimated. The scalar inhomogeneity was introduced by a low-momentum gas jet injection. A consistent trend was observed in all data; the Batchelor scale showed a minimum value at top dead center (TDC) and was nearly symmetric about TDC. Increasing the engine speed resulted in a decrease of the Batchelor scale, and the presence of a shroud on the intake valve, which increased the turbulence intensity, also reduced the Batchelor scale. The effect of the shrouded valve was less significant compared to the effect of engine speed.

High frame rate particle image velocimetry (PIV) measurements were performed in a motored engine at speeds of 600 and 1200 rpm under both throttled and unthrottled conditions. Data were acquired at 1 kHz throughout the entire engine cycle, allowing the temporal and spatial evolution of the flow to be observed. The data were both temporally and spatially filtered to study the turbulent flowfield. The mean (over the spatial domain) kinetic energy of the high-pass filtered data, and its evolution with cutoff frequency or length, was used to quantitatively compare differences between operating conditions and different cycles at the same condition. The difference in fluctuation kinetic energy, when normalized, between different operating conditions was found to be comparable to the difference between cycles. A comparison between spatially and temporally filtered data at the same level of fluctuation kinetic energy was performed.

Valve deactivation followed by multiple compression-expansion strokes was employed to remove intake-generated turbulence from the bulk gas in an internal combustion engine. The result was a nearly quiescent flowfield that retains the same time-varying geometry and, to a first approximation, thermodynamic conditions as a standard engine. Mass loss, and more significantly heat loss were found to contribute to a reduction in the peak cylinder pressure in the cycle following two compression-expansion strokes. The reduction of the turbulence was verified both computationally and by performing premixed combustion studies. Mixing studies of both liquid spray jets and gaseous jets were performed. Laser-induced fluorescence images of high spatial resolution and signal-to-noise ratio were acquired, allowing the calculation of the two in-plane components of the scalar dissipation rate.