Peter O. Witze

Conditionally-Sampled Velocity and Turbulence Measurements in a Spark Ignition Engine

Abstract Laser Doppler velocimeter measurements have been made in a homogeneously-charged spark ignition engine. With ignition at the side wall of the disc-shaped combustion chamber, the fluid motion in the direction of flame propagation was measured at the center of the chamber. A simultaneous ionization probe measurement was used to identify the time of flame arrival at the velocimeter probe volume. Phase-averaged measurements recorded from many engine cycles were conditionally sampled according to flame arrival time. The results presented show an increase in the unburned gas turbulence from compression, and strongly suggest that cyclic variation in burn duration is caused by cyclic variation in the bulk turbulence intensity ahead of the flame.

Time-resolved laser-induced incandescence of soot: the influence of experimental factors and microphysical mechanisms

We present a data set for testing models of time-resolved laser-induced incandescence of soot. Measurements were made in a laminar ethene diffusion flame over a wide range of laser fluences at 532 nm. The laser was seeded to provide a smooth temporal profile, and the beam was spatially filtered and imaged into the flame to provide a homogeneous spatial profile. The particle incandescence was imaged onto a fast photodiode. The measurements are compared with the standard Melton model [Appl. Opt. 23,2201 (1984)] and with a new model that incorporates physical mechanisms not included in the Melton model.

Time-Resolved Laser-Induced Incandescence and Laser Elastic Scattering Measurements in a Propane Diffusion Flame

Laser-induced incandescence (LII) and laser elastic-scattering measurements have been obtained with subnanosecond time resolution from a propane diffusion flame. Results show that the peak and time-integrated values of the LII signal increase with increasing laser fluence to maxima at the time of the onset of significant vaporization, beyond which they both decrease rapidly with further increases in fluence. This latter behavior for the time-integrated value is known to be characteristic for a laser beam with a rectangular spatial profile and is attributed to soot mass loss from vaporization. However, there is no apparent explanation for the corresponding large decrease in the peak value. Analysis shows that the peak value occurs at the time in the laser pulse when the time-integrated fluence reaches approximately 0.2 J/cm(2) and that the magnitude of the peak value is strongly dependent on the rate of energy deposition. One possible explanation for this behavior is that, at high laser fluences, a cascade ionization phenomenon leads to the formation of an absorptive plasma that strongly perturbs the LII process.

Measurements and Predictions of the Precombustion Fluid Motion and Combustion Rates in a Spark Ignition Engine

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured in a motored research engine. The compression of complex bulk motions created during induction produces turbulence as the piston approaches top dead center. The turbulence field is shown to be isotropic but nonhomogeneous. A zero-dimensional computer simulation based on an averaged k-epsilon model is shown to adequately predict the decay of turbulence at a point in the flow after the production phase is completed. Cylinder pressure measurements were recorded for homogeneous stoichiometric combustion for a range of engine speeds and ignition locations. A two-zone (burned and unburned gases) thermodynamic model accurately predicts the measured pressure histories when the turbulence results determined from the motored tests are used to establish initial conditions for the combustion model.

Fiber-Optic Instrumented Spark Plug for Measuring Early Flame Development in Spark Ignition Engines

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.

Direct Measurement of the Turbulent Burning Velocity in a Homogeneous-Charge Engine

An experiment is described for the direct measurement of the turbulent burning velocity during premixed combustion in a spark ignition engine. The gas velocity is measured using a high data rate laser Doppler velocimeter system that resolves the unburned gas motion on an individual cycle basis. The ensemble-averaged flame speed is determined from ionization probe measurements of the time of flame arrival at discrete positions along the path of flame propagation. The difference between the cycle-resolved unburned gas velocity and the ensemble-averaged flame speed gives a direct measurement of the turbulent burning velocity that is unbiased by cycle variations in the combustion rate.

Fluid Motion during Flame Propagation in a Spark Ignition Engine

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured during combustion in a research engine. Simultaneously with each LDV measurement, the cylinder pressure and gas state (unburned or burned) were measures so that conditional sampling techniques could be used in the data-averaging procedure. Measurements of the mean velocity component in the direction of flame propagation agree well with a computer simulation of the induced velocities generated by the volume expansion of the burned gases. Mean velocities measured parallel to the flame surface are shown to be complex because a small amount of swirl was present. Conditional sampling on the time of flame arrival at the LDV probe volume revealed a thirty percent cyclic-variation bias error in the turbulence component normal to the flame. The turbulence field ahead of the flame appears to be enhanced by compression, with the component normal to the flame increased twice as much as the parallel component. 28 references, 16 references.

Velocity Measurements in the Wall Boundary Layer of a Spark-Ignited Research Engine

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 ..mu..m 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 ..mu..m, and the turbulence intensity increased as the wall was approached. With low swirl the 700-1000 ..mu..m 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.

Two-Component Laser Velocimeter Measurements in a Spark Ignition Engine

Abstract Simultaneous two-component laser Doppler velocimeter measurements for homogeneous charge combustion in a spark ignition engine are used to calculate ensemble-averaged turbulence stresses. Cyclic variation hias in the data is reduced by conditional sampling. 011 the flame arrival time at the probe volume. Results are presented for engine speeds of 300. 600. and 1200 rpm. The lower speed is believed to be transitional because the general character of the fluid motion is found to he unique. For the two higher engine speeds. the following conclusions can he made: (1) The precombustion turbulence is highly anisotropic. with the component parallel to the piston surface nearly double the component in the direction of piston motion; (2) both the norrnul and shear turbulence stresses scale with mean piston speed; (3) combustion does not have an appreciable effect on either the preflame or postflame normal turbulence stresses. hut compression in front of the flame docs appear to reduce the degree of correl...

Concurrent Quantitative Laser-Induced Incandescence and SMPS Measurements of EGR Effects on Particulate Emissions from a TDI Diesel Engine

A comparison of scanning mobility particle sizer (SMPS) and laser-induced incandescence (LII) measurements of diesel particulate matter (PM) was performed. The results reveal the significance of the aggregate nature of diesel PM on interpretation of size and volume fraction measurements obtained with an SMPS, and the accuracy of primary particle size measurements by LII. Volume fraction calculations based on the mobility diameter measured by the SMPS substantially over-predict the space-filling volume fraction of the PM. Correction algorithms for the SMPS measurements, to account for the fractal nature of the aggregate morphology, result in a substantial reduction in the reported volume. The behavior of the particulate volume fraction, mean and standard deviation of the mobility diameter, and primary particle size are studied as a function of the EGR for a range of steady-state engine speeds and loads for a turbocharged direct-injection diesel engine. Both the SMPS and LII techniques demonstrate good repeatability and consistency with each other. Increasing the EGR results in a sharp rise in the volume fraction of particulates for all engine speeds and loads. At all speed and load conditions the primary particle size decreases with increasing EGR. (Less)