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Spark plugs instrumented with a ring of optical fibers in the threaded-body region have seen considerable use in the past ten years, and it is expected that their application to unmodified production engines will increase in the years to come. Interpretation of the optical signals obtained with the probe is often difficult, particularly under lean operating conditions where the low luminosity of the flame leads to imprecise flame arrival detection. A systematic look at the optical signals, along with direct imaging of the flame, has been undertaken to calibrate and optimize the determination of flame arrival times. In addition, an evaluation of the different models available for the analysis of the flame arrival data is made. Data fits are compared with real flame images, to determine which model best estimates the convective velocity of the flow and the expansion speed of the flame kernel.

A new diagnostic technique is described that has the capability of making real-time, in situ measurements of the volatile fraction of diesel particulate matter (PM). LIDELS uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse produces elastic light scattering (ELS) that gives the volume of the total PM, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction. Calibration is required for the individual total PM and solid fraction to be quantitative. Applicability of the technique is demonstrated for load and EGR sweeps for a turbocharged, direct-injection diesel engine.

Laser-induced incandescence (LII) is a sensitive diagnostic technique capable of making exhaust particulate-matter measurements during transient operating conditions. This paper presents measurements of LII signals obtained from the exhaust gas of a 1.9-L TDI diesel engine. A scanning mobility particle sizer (SMPS) is used in fixed-size mode to obtain simultaneous number concentration measurements in real-time. The transient studies presented include a cranking-start/idle/shutdown sequence, on/off cycling of EGR, and rapid load changes. The results show superior temporal response of LII compared to the SMPS. Additional advantages of LII are that exhaust dilution and cooling are not required, and that the signal amplitude is directly proportional to the carbon volume fraction and its temporal decay is related to the primary particle size.

Ionization probes installed in the head gasket of an engine have been used to infer the shape of the burned volume from measurements of when the flame contacts the gasket. It is demonstrated that the technique can be extended to infer fluid motion by using one of the ionization probes as the ignition site, with the ensuing flame serving as a flow marker. It is shown that swirl motion, and its direction, can be detected, and that flame propagation velocities can be measured. A comparison of estimated swirl velocities with laser Doppler velocimeter measurements show remarkably good agreement. The most valuable feature of the technique is that it can be applied to any production engine without modification.

A recently developed spark plug equipped with fiber-optic flame-arrival detectors has been used to measure the motion and rate of growth of the early flame kernel. The cylinder pressure and gas velocity in the spark gap were measured simultaneously with the flame kernel measurements, permitting the data to be analyzed on a cycle-by-cycle basis to identify cause-and-effect correlations between the measured parameters. The data were obtained in a homogeneous-charge research engine that could be modified to produce three very different flow fields: (1) high swirl with high turbulence intensity, (2) tumble vortex with moderate turbulence intensity, and (3) negligible bulk motion with low turbulence intensity. The results presented show a moderate correlation between the combustion duration and the rate of growth of the flame kernel, but virtually no correlation with either the magnitude or direction of movement of the flame kernel away from the spark gap.

Double-pulse laser-induced desorption with elastic laser scattering (LIDELS) is a diagnostic technique capable of making time-resolved, in situ measurements of the volatile fraction of diesel particulate matter (PM). The technique uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse of a pulse-pair produces elastic laser scattering (ELS) that gives the total PM volume, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction. In an earlier study, we used a single laser to make real-time LIDELS measurements during steady-state operation of a diesel engine.

A photographic study is presented illustrating the influence of mixture motion on flame propagation in an internal combustion engine. Variation in swirl and turbulence levels was achieved by rotating the orientation of a shroud on the intake valve. Laser Doppler velocimetry was used to characterize the precombustion fluid motion. A flexible shadowgraph system was developed for visualizing in-cylinder events. The results show that cyclic variation is not necessarily decreased by increasing the burn rate. The fastest burn achieved in this study occurred with high swirl, when the flame remained attached to the spark plug. If random detachment of the flame occurred, however, cyclic variation was greatly enhanced.

Hot-wire anemometer and laser Doppler velocimeter measurements have been taken in a motored reciprocating engine and compared to assess the validity of hot-wire measurements. The procedure used to account for the sensitivity of the hot wire to changes in the gas temperature is extensively investigated. The results presented show that for the optimum conditions of known flow direction, low turbulence level, and low compression ratio, the hot-wire anemometer can provide useful mean velocity results. Accurate hot-wire turbulence intensity measurements appear to be possible only for the intake and exhaust strokes.

Measurements are presented showing the effect of swirl level and spark location on burn duration in a homogeneous-charge engine. Laser shadowgraph photographs of the flame structure were used to help interpret the observed results. As expected, without swirl the burn duration was a direct function of flame travel distance, such that central ignition was optimal. When swirl was introduced, off-axis ignition was aided by flame-holder effects that enhanced the flame speed in the circumferential direction. However, only for the highest swirl level studied (swirl number = 8.3) was the burn rate increased by moving the ignition point toward the cylinder wall. For lower swirl levels, central ignition was still preferable.

A two-dimensional axisymmetric numerical model is used to illustrate how the presence of walls can significantly influence the shape of a spark-ignited premixed gas flame, even when wall boundary layers are neglected. This in-viscid model is based on tracking the flame interface as it interacts with the combustion-generated flow field. Comparisons made with a quasi-dimensional phenomenological model show that the assumption of a spherical flame surface held centered at the ignition location can lead to a large underprediction of the flame area. This occurs because the spherical flame is prematurely truncated by the chamber walls, and because the spherical assumption constrains the flame surface-to-volume ratio to be the absolute minimum. In contrast, results from the two-dimensional model show that the flame slows down as it approaches a wall since the velocity induced by volume expansion must be zero normal to the boundary.

An optical probe for measuring the motion and rate of growth of the early flame kernel in spark ignition engines is described. The probe consists of a standard spark plug with eight optical fibers installed in a ring at the base of the threaded region of the plug. The fibers collect the light emitted from the flame as it crosses the field of view of the fibers, and transmit the light to photomultiplier tubes. The time from ignition until detection of the flame is used to compute the average flame velocity in the direction of each fiber relative to the spark location. The real-time data acquisition system permits statistical analysis of cycle-by-cycle variations in the combustion rate. Because the probe was built using a standard 14 mm spark plug, it can be used in unmodified production automotive engines.

Laser Doppler velocimetry has been used to measure velocity and turbulence intensity profiles in the wall boundary layer of a spark-ignited homogeneous-charge research engine. By using a toroidal contoured engine head it was possible to bring the laser probe volume to within 60 μm of the wall. Two different levels of engine swirl were used to vary the flow Reynolds number. For the high swirl case under motored operation the boundary layer thickness was less than 200 μm, and the turbulence intensity increased as the wall was approached. With low swirl the 700-1000 μm thick boundary layer had a velocity profile that was nearly laminar in shape, and there was no increase in turbulence intensity near the wall. When the engine was fired the boundary layer thickness increased for both levels of swirl.