Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 401-411).

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xiii, 160 p.; also includes graphics (some col.). Includes bibliographical references (p. 156-160). Available online via OhioLINK's ETD Center

In this dissertation, some practical ignition techniques are presented that show how some problems of lean-burn combustion can be overcome. Then, to shed light on the effects of the ignition techniques described, the focus shifts to the more specific problem of the early phase of spark ignition. Thermal models of ignition are reviewed. These models treat the energy provided by the electrical discharge as a point source, delivered infinitely fast and creating a spherically symmetric ignition kernel. The thesis challenges the basis of these thermal models by reviewing the work of many investigators who have clearly shown that the temporal characteristics of the discharge have a profound effect upon ignition. Photographic evidence of the early phase of ignition, as well as other evidence from the literature, is also presented. The evidence clearly demonstrates that the morphology of spark kernels in the early phase of development is toroidal, not spherical as suggested by thermal models. A new perspective for ignition, a fluid dynamic point of view, is described. The common ignition devices are then classified according to fluid dynamics. A model describing the behaviour of spark kernels is presented, which extends a previously established mixing model for plasma jets, to the realm of conventional axial discharges. Comparison of the model behaviour to some limited data is made. The model is modified by including the effect of heat addition from combustion, and ignition criteria are discussed. / Graduate

This study was concerned with improvement and validation of a thermodynamic spark ignition engine simulation code developed in Leeds. Experimental validation data were generated using a central ignition, disc-shaped combustion chamber variant of a ported single-cylinder research engine with full-bore overhead optical access. These data included simultaneous measurement of cylinder pressure and flame position at different operating conditions. The engine was skip fired (fired once every five cycles), to remove residuals and ensure well defined in-cylinder fuel-air mixture for simulation. Flames were imaged using a digital camera capturing the light emitted from the flame ("natural light"). New methods were developed to process the pressure and film data. Flame pictures were processed to determine enflamed area, mean flame radius and flame centroid. Parameters were also developed to describe flame "circularity" ("shape factor") and to describe asymmetry of flame approach to the cylinder walls ("active perimeter fraction", APF). Time-base crank angle records allowed evaluation of engine speed variation within a cycle and mean engine speed for a cycle. Although generated principally for model validation, the experimental results proved interesting in their own right. Middle, slow and fast cycles were defined for each condition. Analysis of these cycles suggested that there was no correlation between the initial flame centroid displacement, its locus over the flame propagation period or the flame "shape factor" and the speed of combustion and pressure development. As the flame approached the wall, the active perimeter fraction fell in a similar manner for all the middle cycles. Substantial modifications were made to a pre-existing thermodynamic engine cycle code. Deficiencies in the blowby, heat transfer and thermodynamic aspects were corrected. An additional ("Zimont") turbulent burning velocity sub-model and a new routine for the influence of engine speed variation within a cycle were incorporated into the code. The active perimeter fraction parameter function determined in the experiments was encoded to allow for the effects of flame-wall contact on entrainment rate during the late flame propagation. A radial stratified charge model was also developed. Burned gas expansion over the flame propagation period was shown to significantly change the unburned gas charge stratification from the initial variation. Two types of initial stratification (linear and parabolic distributions, rich of the centre and lean close to the wall) were imposed. Faster combustion development was observed in both cases, c. f that for equivalent homogeneous charge. Good agreement was observed between experimental results and "Zimont model" predictions at different equivalence ratios and engine speeds. Other computations using the pre-existing Leeds K and KLe correlations gave reasonable predictions at the various engine speeds and at rich conditions; however, they yielded slower results than experimentally observed for lean conditions.

Studies were conducted into several aspects of the recovery of a spark gap consisting of 6 mm. diameter, flat tungsten electrodes, after an initial discharge of 32 kamp. maximum, and duration 11 sec. The controlling effect of electrode heat transport on the intermediate recovery of the spark gap was demonstrated using air at 760 mm. Hg as a discharge medium, and gap lengths of 3, 4, and 5 mm. In the intermediate stages of recovery, the increase in breakdown strength of the gap was found to be proportional to t½, and to be nearly independent of gap length. These results are explained on the basis of uniaxular heat flow, through the electrodes. A previously observed long-time or delayed recovery in a hydrogen spark gap between clean tungsten electrodes was investigated to establish the possible role of hydrogen-tungsten adsorption in this effect. The temperatures of the electrodes were varied by means of small heating coils, and the recovery characteristics for heated electrodes measured. By comparing these results to the normal recovery characteristic, the phenomana of delayed recovery was shown to be at least partially attributable to the adsorption of hydrogen onto the electrode surfaces. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An attempt has been undertaken to enhance turbulence in an S.I. engine at the final stage of the compression stroke, without affecting the intake process. The method employed to control the turbulence level made use of an original design called the squish-jet combustion chamber. The design had potential to generate jets in the chamber before CTDC and thus create dramatically different turbulent flow patterns. Natural gas, a slow burning fuel, was used for performance tests, and different levels of turbulence were expected to markedly affect the combustion process. A flow visualization experiment was performed under conditions similar to a motored engine. As a result, the jet development in the squish-jet type combustion chamber was documented. A new type of a flat cylinder head, and a set of squish-jet pistons were designed and manufactured. Experiments conducted on the redesigned Ricardo Hydra, single cylinder research engine, evaluated the influence of the squish-jet chamber on the mixture motion and the engine performance over a wide range of operating conditions. The jet velocities were measured with a hot wire probe located in the piston bowl, and turbulence parameters with a probe inserted through a cylinder head. The squish-jet design was evaluated for 6 different configurations. As a result it has been established that the squish-jet design does not create jets strong enough to dramatically enhance the turbulent flow field. The design, however, diminished the squish effect which is shown to be very important for the middle part of flame development. The simple squish design produces faster burning rate in the first half of the combustion process and develops the highest peak pressures. Variabilities of both cyclic IMEP and peak pressure are found to be unaffected by the presence or absence of strong squish motion. This suggests that the most important phase of combustion for the cyclic variation is the initial stage of the flame development. A comparison of ensembled pressure signals between combustion chamber designs, conducted at RAFR=1.00 and at RAFR=1.25 shows less dispersion in the latter case. It appears that at lean operation mixture motion influences combustion process to a lesser degree than at stochiometric conditions. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

A study on the effects of increased ignition energy on cold start hydrocarbon emissions was conducted. The tests were conducted on a single cylinder ASTM-CFR engine. The engine was outfitted with EFI, exhaust analyzers, and temperature probes. The engine was also modified to produce cold start conditions rapidly after each run. For the experiment the engine was started from 18° C using three increasing ignition energy levels. The first level of ignition energy was the ignition energy produced by the stock CFR engine's ignition system. The second and third increased ignition energy levels were obtained by adding 0.387 joules and 1.187 joules to the stock output through a supplementary ignition system. Startup emissions and the number of cycles until the first successful fire were measured. The results of the tests show a 14% decrease in the average peak hydrocarbon (HC) concentration levels at the highest ignition energy. Overall reduction in HC was less. The variance in the peak HC levels was reduced at the highest ignition energy setting. CO production was increased in response to the increase in HC consumption. The spread in measured number of cycles until first fire was decreased at the highest ignition energy level. Although positive results were obtained, the test apparatus had some problem areas that may have reduced the effectiveness of the high energy ignition system. Based on what was learned recommendations on apparatus refinements and further tests were included. / Master of Science

The ignition process is a critical consideration for combustion devices. External energy transfer to the combustor is required for ignition in common combustion systems. There are many ways to deposit energy into the flow but a standard method is a spark discharge because it is simple, compact, and reliable. Sparks can be categorized as either inductive or capacitive sparks that use a coil or an electrical resonance circuit with capacitor, respectively, to amplify the voltage. The creation of a successful ignition event depends on the spark energy deposited into the flow, the initial composition, pressure, temperature, turbulence level of flow etc. The deposited energy by the spark into the flow is critical for estimation of initial energy available for ignition of the mixture. Therefore, the electrical characteristics of the sparks were investigated under various flow conditions. Then measurements of deposited energy into the flow were conducted using a very accurate experimental procedure that was developed in this research. The results showed considerable electric energy losses to the electrodes for the relatively long, inductive sparks. However, the short, capacitive spark deposits electric energy into the flow with minimal loss (above 90% deposition efficiency). In addition, the characteristics of inductive spark are affected by flow velocity and by the existence of a flame. However, variations in the flow conditions do not affect the characteristics of the capacitive spark such as voltage-current time trace and energy deposition efficiency. Two ignition systems using above mentioned two spark types were developed. First, the capacitive spark energy was directly deposited into the premixed flow. Most researchers have not concentrated on the early initiation process but on the flame growth. Therefore, the generated kernel formed by the energy deposition was observed and characterized using optical methods, immediately following the spark. In addition, the mixing effect for this ignition kernel with surrounding gas was simulated using a numerical method. Based on the time trace of the OH* chemiluminescence, the reaction starts with the discharge and it is continuous until combustion begins. This means that in the presence of a high density spark in premixed flow, there exists no traditional delay as defined by other researchers for auto ignition. A simple Radical Jet Generator (RJG) was developed that is able to ignite and stabilize a flame in a high-speed flow. The inductive spark initiates the combustion in the RJG chamber. The RJG then injects the partially-burned products carrying large amounts of heat and radicals into a rapidly moving flammable main stream. Then it ignites and stabilizes a flame. The RJG requires low levels of electrical power as long as the flow velocity is relatively low since most of the radicals are produced by the incomplete combustion in its chamber. The importance of radicals was analyzed by RJG experiments and numerical methods. The reaction zone for RJG using a rich mixture was located both inside and outside of the RJG chamber. Therefore, the RJG using a rich mixture performed better in the ignition and stabilization of combustion in the main flow. According to an analysis using the CHEMKIM simulation software combined with the San Diego chemical mechanism, the RJG jet resulting from a rich mixture contains more radicals and intermediates than that produced by a lean mixture for the same sensible enthalpy. In addition, the burned gas contains less radicals and intermediates than the partially burned gas. If the RJG is operating with a high speed main flow, the flow rate through the RJG chamber must be increased to allow the radical jet to penetrate well into the rapid flow due to their higher injection velocity. Unfortunately, this leads to unsteady combustion in the RJG, which results in the pulsation of the radical jet. This reduces the number of radicals injected into the main flow. To investigate this operating condition, special attention was focused on four possible factors: unburned reactant pockets caused by motion of the spark channel, spark frequency, flame propagation speed and ignition delay. It was shown that the unsteadiness is affected by the flame speed and ignition delay because the frequency of pulsation in the chamber is highly dependent on the equivalence ratio. In addition, the interaction between the RJG operation and the combustion dynamics in the main combustor was documented. The acoustic pressure oscillations in the main combustor were suppressed when the RJG jet was turned on because the reaction region is relocated by the operation of the RJG.

Thesis (M.S.)--West Virginia University, 1998. / Title from document title page. "December 3, 1998." Document formatted into pages; contains xii, 110 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 63-65).

Thesis (M.S.)--Michigan State University. Dept. of Mechanical Engineering, 2008. / Title from PDF t.p. (viewed on July 27, 2009) Includes bibliographical references (p. 119-122). Also issued in print.