Conventional Ignition Research Articles

Fast Ignition: Overview and Background

Fast ignition is an approach to inertial fusion in which precompressed fuel is ignited with an external heat source. This arrangement can, in principle, lead to higher gains than conventional ignition produced by stagnation of convergent flows. In addition, because ignition is separate from the implosion in fast ignition, hydrodynamic mix has less opportunity to quench ignition than in the conventional process. This paper introduces some of the basic ideas of fast ignition: ignition requirements, gain curves based on simple energetic models, and integrated gain models including hohlraum and implosion physics. Because possible gains in this approach are so large, it is possible to examine the use of fuels with small tritium fractions, the so-called “advanced fuels.” In addition, the historical background of this field is discussed.

Laser Ignition of Methane-Air Mixtures at High Pressures and Diagnostics

Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean-side ignition limit of methane-air mixtures was found to be λ=2.2. However, only λ<2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ=1.8. Only with excessive efforts λ=2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd:YAG laser at 1064 nm. The engine worked successfully at λ=1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NOx emission were achieved at λ=2.05 NOx=0.22 g/KWh. Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.

B214 水素火炎ジェット点火法による燃焼制御 : ノズル部の熱流束(OS-4 内燃機関の燃焼I)

A jet ignition method by hydrogen flame jet is proposed to control the combustion of lean mixtures. Hydrogen flame jet is supplied from the cavity whose volume is less than one percent of the main chamber volume through the nozzle. It is worried that the heat loss of the jet ignition method becomes bigger than that of conventional ignition, because this method creates strong and high temperature flame jet. Especially at the nozzle, the heat flux might become bigger than other place. So the heat flux at the nozzle were measured and the relation between the jet velocity and heat flux were shown. The faster the flame jet is, the bigger the heat flux at the nozzle is. And the heat flux increases with the temperature of the jet. The maximum heat flux is proportional to the velocity and the temperature of the flame jet.

Surface characterization of energetic materials using point-to-point contact FTIR-ATR

Surface characterization of energetic materials and their components has been a useful tool for resolving many research and quality control issues. An important application for gun propulsion is the understanding of plasma-propellant interactions (PPIs) in order to exploit plasma ignition, which offers the potential for rapid, controlled ignition of high-energy density charges. Toward this end, surface characterization using Fourier-transform infrared (FTIR) microscopy has been performed on propellants exposed to plasma in extinguished closed-bomb and open-air plasma experiments. These experiments are being performed to elucidate the properties of plasmas that enhance energy transfer and ignition, ultimately with the goal of designing an electrothermal-chemical (ETC) igniter-propellant charge for the next-generation weapon for the Future Combat System. However, analytical techniques capable of low detection limits for propellant degradation products are needed for characterizing the nature (e.g., radiation, convective heat transfer, hot particle flow, etc.) and distribution (in a packed bed or charge to ensure uniform and repeatable ignition) of plasma interaction with the propellant. Knowledge of such phenomena is valuable for validating simulations of plasma dynamics and optimizing igniter design. Reflectance spectroscopy has been used previously in our laboratory in closed-bomb studies to characterize the morphology and chemical composition of the grains from extinguished closed-bomb and open-air plasma experiments, but degradation products had proved difficult to detect. Recently, the same samples were reanalyzed using point-to-point contact attenuated total reflectance (PPC-ATR) in order to improve diagnostics for plasma-propellant interaction. Commercially available attachments have been designed to exert greater pressure on the sample for greater contact with the ATR crystal, improving sensitivity. With PPC-ATR, it was possible not only to detect but to identify the components of JA2 most likely associated with plasma degradation. Decomposition products were limited to less than a millimeter into the sample, involved secondary carbon denitration, and apparently did not migrate, strongly suggesting that plasma-propellant interaction involved denitration of nitrocelluose (NC). A chemical species with infrared absorption at /spl sim/1570 cm/sup -1/ is apparently specific to plasma ignition (i.e., not observed with conventional ignition) and could serve as a marker compound for the extent of plasma interaction in charges with ETC igniters.

Consideration of the energy transfer mechanisms involved in SPETC ignition systems

Solid propellant electrothermal-chemical (SPETC) gun concepts have evolved over the past decade. A common difficulty experienced by many of the "large-scale" applications of SPETC technology has been the excessive electrical energy and power requirements needed to produce significant performance benefits. However, one aspect of SPETC gun technology that has attracted a lot of interest is in the area of plasma ignition due to its lower power requirements. It has been shown by various groups that plasma ignition offers benefits over a conventional gunpowder ignition system at large scale. High-energy electrical discharges previously used for ignition can potentially lead to adverse internal ballistics. Effort has recently been directed at reducing the ignition energy stimulus with the result that electrical discharge energies of a few tens of kilojoules are now routinely used to ignite large caliber gun system propellant charges. This not only gives improved internal ballistics but also reduces the cost, size, and complexity of SPETC gun components. An understanding of the energy transfer mechanisms involved during the ignition phase, which allows the electrical energy to be transferred to the surface of propellants, is central to reducing the electrical energy requirements, and these mechanisms are the subject of this paper. There is some evidence for radiative energy transfer being the dominant transfer mechanism for some higher power plasma igniters. However, with low power density and low energy ignition, the radiant flux is negligible in the timescale under consideration, suggesting that radiation is not the dominant process. Supporting experimental evidence from measurements of the radiant flux incident upon the propellant grain surface is offered. These measurements have been performed with the use of embedded optical fibers in translucent propellant formulations. The light collected by these fibers is input to an absolutely calibrated time-resolved spectrograph. A new hypothesis is put forward, which proposes that the energy transfer mechanism for low-power SPETC ignition is dominated by a process of metallic vapor deposition. Modeling and experimental results are presented, which support this hypothesis.

Effects of selected parameters on performance and emission of biomass-fired cookstoves

The effects of different parameters on performance and emissions of three biomass-fired stoves have been investigated. The parameters considered were moisture content of fuel, size of fuel, size of pot and method of ignition, while the selected stoves were an improved Indian stove, a Vietnamese traditional stove and an improved stove developed by the Royal Thai Forestry Department. It was found that increase in fuel moisture content resulted in decrease in stove efficiency, increase in the emission factor of CO and decrease in the emission factor of NO x ; a slight decrease in CO 2 emission factor was also observed, while emission of CH 4 was not significantly effected. The fuel size did not show any significant influence on the efficiency of the stove, however, it showed significant influence on the emission of CO for the size range investigated. The size of pan did not affect the efficiency of the stoves tested. Two methods of stove ignition—conventional bottom ignition and top ignition—were investigated. In general, emission of CO and NO x was significantly less in case of top ignition in comparison with conventional bottom ignition.

An experimental study on engine performance with high ignition energy

The objective of this research is to investigate the improvement of engine performance with high ignition energy. In order to study this objective, two different spark ignition engines are employed and four different ignition systems are adopted. The ignition energy, combustion pressure, exhaust gases and fuel consumption are measured to evaluate engine performances. In addition, the heat release rate, mass fraction burned and coefficient of variation (COV) of indicated mean effective pressure (IMEP) are calculated with the measured pressures. The results reveal that high ignition energy shortens the combustion duration, improves the engine stability and reduces fuel consumption. When a higher ignition energy is applied to a lean mixture, the emission of hydrocarbons (HC) is reduced while an engine with the conventional ignition energy produces higher HO emission. A faster heat release rate and a higher mass fraction burned also result. When the same amount of ignition energy is applied to a lean mixture, the burning velocity is slow as the discharge duration becomes longer, but the COV of IMEP is improved.

Temperature fields during the development of autoignition in a rapid compression machine

Temperature and concentration fields have been investigated in the cylindrical combustion chamber of a rapid compression machine (RCM) by schlieren photography, chemiluminescent imaging and planar laser induced fluorescence of acetone and of formaldehyde in a 2-dimensional sheet across the diameter. The timescale of particular interest was up to 10 ms after the piston has stopped. Experiments were performed in non-reactive and reactive conditions. Acetone was seeded in non-reactive mixtures. Combustion was studied first in a system containing di-tert-butyl peroxide vapour in the presence of oxygen. The decomposition of di-tert-butyl peroxide generates methyl radicals, which are then oxidised if oxygen is present. The overall reaction is exothermic and is characteristic of a conventional thermal ignition. In addition, chemiluminescence, resulting from CH2O*, accompanies the oxidation process. The combustion of n-pentane was then investigated at compressed gas temperatures that spanned the range in which there is a negative temperature dependence of the overall reaction rate, typically 750-850 K. The response to thermal feedback in this more complex thermokinetic system can be the opposite of the "thermal runaway" that accompanies di-tert-butyl peroxide combustion. The purpose of making comparisons between these two types of systems was to show how the temperature field generated in the RCM is modified in different ways by the interaction with the chemistry and to discuss the implications of this for the spatial development of spontaneous ignition. As the piston of the RCM moves it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects gas from the walls and moves it across the cylinder head pushing it forward into a plug at the centre. Thus, soon after the end of compression there is an adiabatically heated gas which extends virtually to the wall, but this incorporates a plug of colder gas at its core. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows until at least 10 ms post-compression. The consequence of "thermal runaway" on a timescale that is compatible with the development of this temperature field is that the reaction rate in the adiabatically compressed toroidal region accelerates faster than in the core, and goes to completion first. A somewhat similar pattern emerges during n-pentane combustion when the initial condition is set at the lower end of the negative temperature dependent range. By contrast, at adiabatically compressed gas temperatures close to the upper end of the negative temperature dependent region, the reaction rate in the cooler core develops faster than that in the surrounding zone, and the temperature difference is rapidly smoothed out. This does not lead to spatial homogeneity in all respects, however, because different rates and extents of reaction generate different concentrations of intermediates. This stratification has implications for the eventual spatial evolution of spontaneous ignition.

Chemical characterization of plasma-treated propellant samples

The morphological and chemical characterization of M30 propellant grains recovered after conventional or plasma ignition in interrupted closed bomb experiments has been performed. The capillary material used during plasma ignition was either polyethylene (PE) or polyethylene terephthalate (Mylar). Propellants were extinguished at pressures between 35 and 100 MPa. The samples are of interest due to previous reports of apparent burning rate augmentation with plasma ignition of M30, compared to conventional ignition, in non-interrupted closed bomb experiments [1]. Birk [2] has reported that burning rate augmentation appears to occur during the plasma event, but there was no evidence for post-plasma augmentation. Birk has also analyzed vivacity curves from plasmaignited M30 and the data indicated that the grains appear to burn regressively, suggesting an increase in surface area was generated beyond that expected from normal grain burning [2]. In the current work, differences between extinguished grains from conventional and plasma ignition were primarily physical or morphological in nature. For the extinguished grains from PE plasma ignition there appeared to be a stripping of NQ crystals in the perforations. Along the outer surfaces of the grain ignited with plasma, hot embedded particles and increased surface area were observed, and the melt layer was immeasurably thin. Although extensive chemical characterization has been performed, there appears to be very little chemical difference between the burned surfaces of the plasma and conventionally ignited samples. The SEM analysis of the perforations showed that NQ depletion in the perforations was most evident at the lowest blowout pressure used (35 MPa) and seemed to diminish at higher pressure, apparently becoming obscured by “normal” burning processes. Thus, it appears that plasma-propellant interactions and associated effects occur only very early during ignition. This is consistent with the fact that few differences in the chemical composition are observed for plasma and conventionally ignited samples. A depletion of NQ in the perforations may contribute to a true burning rate augmentation through chemical reactions. However, a true chemical burning rate augmentation as the only mechanism for the M30 non-interrupted behavior would not be consistent with regressive burning.

Temperature fields during the development of combustion in a rapid compression machine

Temperature and concentration fields have been imaged by Rayleigh scattering in one-dimension on a line and by laser induced fluorescence (LIF) of acetone in a 2-D sheet across the diameter of the cylindrical combustion chamber in a rapid compression machine. Experiments were performed in non-reactive and reactive conditions. To investigate the development of combustion, the exothermic decomposition of di-t-butyl peroxide vapor diluted by inert gas was studied. This reaction is characteristic of a conventional thermal ignition. Acetone is a major product. Inert gas mixtures, to study the temperature field in the absence of reaction, were seeded with acetone. The evidence from the experimental results supports the following interpretation. As the piston of the machine moves, it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects cool gas from the walls and moves this gas across the cylinder head pushing it forward into a plug at the center. Once the piston stops, there is a stratified component at the center, which is slightly colder than the bulk of the gas, and for a short time afterwards there is very limited mixing by bulk transport of gas from one part to another, because the gas velocity is not very high. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows before 20 to 25 ms after the end of compression. The effect (on the combustion of di-t-butyl peroxide) of this “temperature stratification” at the core of the cylinder is that the reaction develops more slowly in the center than elsewhere. The onset of reaction in a toroidal region is shown unambiguously, and thermal runaway is initiated there. This is demonstrated by LIF measurements through the central plane of the reaction cylinder. From the study of inert mixtures seeded with acetone, it is shown also that the colder core lies just ahead of the piston crown, but it does not reach the central plane until 1 ms after the piston has stopped. Rayleigh scattering on a 1-D line in the central plane proved to be insufficiently sensitive to show the presence of the cooler zone resulting solely from the physical compression. However, the evidence for temperature stratification becomes unequivocal from Rayleigh scattering measurements made in the later stages of the peroxide decomposition. Limits of sensitivity of the scattering technique may be inferred from this. The physical characteristics of the compression are likely to be replicated in other rapid compression machines and are relevant to understanding the spatial development of autoignition in such systems, which has implications also for numerical modeling. There are rather more complicated consequences, than is the case for thermal ignition, for chain-thermal interactions which involve development through the negative temperature-dependent regimes, as occurs in combustion of the alkanes and that of other organic compounds.

Analysis of burning rate phenomena and extinguished solid propellants from an interrupted closed bomb with plasma igniter [ETC launchers

The morphological and chemical characterization of M30 propellants recovered after conventional and plasma ignition with polyethylene (PE) and polyethylene terephthalate (Mylar) in an interrupted closed bomb (extinguished at pressures between 35 and 100 MPa) has been performed. This work was undertaken to investigate mechanisms associated with the apparent burning rate augmentation of M30 under conditions of plasma ignition compared to conventional ignition. Burning rate augmentation of solid propellants with plasma ignition was found to occur during, but not after, the plasma event. The following factors are evidence of a more forceful process for plasma ignition compared to conventional ignition: a) the initial pressure rise was greater with plasma ignition; b) metals from the closed bomb hardware, and stripping of NQ from M30 propellants was observed in the perforations; c) irregular surface burning (possibly due to ablation). And hot embedded particles have been observed in grains after plasma, but not conventional, ignition. Extensive chemical analyses of extinguished grains have been performed, but no significant differences were revealed between plasma and conventional ignition, quite possibly because any early chemical interactions are obscured by normal burning processes occurring later.

Control of Combustion in a Closed Chamber by a Hydrogen Flame Jet Ignition. Heat Flux in a Cavity.

Jet ignition method by hydrogen oxygen flame is proposed to control the combustion of lean mixtures. Hydrogen flame jet is supplied from the cavity whose volume is less than one percent of main chamber volume through a nozzle. Generally, the heat loss of the jet ignition method at the chamber wall is thought to be bigger than conventional ignition. In this report, the heat flux at the cavity wall are measured. The heat flux in the cavity is larger than that in the main chamber. The leaner the mixture in the main chamber is, the smaller the heat flux is. When hydrogen oxygen mixture is used in the cavity, the heat flux is much bigger than that using hydrogen air mixture. The smaller the nozzle diameter is, the bigger the heat flux becomes. The heat flux depends on the velocity of the flame in the cavity.

Microwave Initiated Self-Propagating High-Temperature Synthesis of SiC

The use of microwave energy to initiate self-propagating, high-temperature synthesis (SHS) of Si + graphite mixtures has been investigated. The results indicate that, unlike with conventional ignition techniques, green densities in excess of 80% of theoretical can be ignited and the combustion wavefront can be crudely controlled. It was found that the induction time for ignition increased with increasing green density and that a higher microwave power level was required with the denser green pellets to achieve the same ignition time. Combustion front velocity increased with green density. The degree of densification was found to decrease with increasing green density. For a given green density, the degree of densification increased with increasing microwave power. The product contained a significant proportion of ultrafine (36–72 nm diameter) SiC whiskers; despite this, final densities as high as 83.6% of theoretical could be obtained without the use of applied pressure. This compares with the ∼50% densities obtained via conventional ignition techniques.

Control of Combustion by Hydrogen Flame Jet Ignition in a Closed Chamber. Heat Flux and NOx.

A hydrogen jet ignition method is proposed to control the combustion of lean mixtures. The hydrogen jet is supplied from the cavity whose volume is less than one percent of main chamber volume through a nozzle. The heat flux at the wall in the main chamber and the NOx emission are measured. It is shown that the heat flux becomes smaller as the combustion duration is longer. On the contrary, the heat flux of hydrogen jet ignition is smaller than that of conventional ignition when the combustion durations are equal. The NOx emission of hydrogen jet ignition is bigger than conventional ignition when the equivalence ratios are the same. The NOx emission of very lean mixture whose combustion duration is reduced less than that of conventional ignition of stoichiometric one is smaller than that of conventional ignition of it. It is shown that the NOx emission is reduced keeping the fast combustion duration by using hydrogen jet ignition method.

Combustion Characteristics of Spark Ignition and Pilot Flame Ignition Systems in a Stratified Charge Wankel Type Rotary Engine.

A pilot flame ignition system, which has superior characteristics in the high ignition energy and the large flame contact area to a conventional spark ignition system, is experimentally examined by the indicated pressure analysis and the high speed direct photography. A model combustion chamber, which simulates a Wankel-type direct injection stratified charge rotary engine, was employed to test the ignition performance of both the pilot flame ignition and spark ignition systems. As a result, it was found that the pilot flame system successfully ignites the very lean charge stratified mixture which the spark system fails to ignite and that the combustion characteristic difference using different ignition systems becomes small as the overall equivalence ratio is increased.

An efficient, power-enhanced ignition system

Spark ignition engines typically have ignition systems that develop 20-30 kV for breakdown of the spark plug gap and deliver current pulses in the range of 30-100 mA. These systems are characterized by very low energy transfer efficiency, 1% or less. Most of the energy is dissipated by the resistances in the transformer, spark plug wires, and spark plugs. This operating mode is adequate (but not necessarily optimum) for most engines, but will not meet the needs of future lean burn and some alternate fuel engines that require higher energy discharges to effectively ignite air/fuel mixtures. Requirements for increased ignition power and energy will have to be met with higher efficiency ignition systems. Improved efficiency can be achieved by utilizing peaking capacitors in the secondary circuit, preferably with a low-loss spark coil. With these modifications, the transfer efficiency can be increased to the 50-75% range. This paper summarizes the results of high-power ignition experiments and related electrical system analyzes for so-called "breakdown ignition" conditions. It includes descriptions of conventional ignition systems, a method for upgrading, and a new higher energy system that features multiple drivers and an energy recovery circuit. The effects of power-enhanced ignition on mileage and emissions for a variety of vehicles tested in laboratory and on the road are summarized sufficiently to show a range of typical results. The paper does not include comprehensive test data for a wide range of vehicles and operating conditions.

High‐Performance Positive Paste for Lead‐Acid Batteries

Positive lead‐acid plates with high porosity and surface area, aiming to deliver a very high current density, about 1 A/cm2, were developed. The high porosity and surface area were achieved by using a combination of fine particles of lead oxide and/or basic lead sulfates with an adequate amount of persulfate and water. The relationship between the positive paste phase composition and the high rate performance of the plate was studied. The highly porous plate is able to deliver a very high current owing to more acid being available in the plate structure. In the low rate applications when acid diffusion from the bulk becomes the limiting factor, the high‐performance plate is not more advantageous than the conventional starting, lighting, and ignition (SLI) plates. The cycle life of the high‐performance plate is sensitive to depth of discharge. The deep discharge high rate capacity of the high‐performance plates falls faster than that of the SLI plate. Nevertheless, the high‐performance paste delivers at least 30% more energy, either to the same depth of discharge per cycle or for the entire service life with constant capacity removal in each cycle. One failure mode of the high‐performance plates is the change of material morphology during deep discharge cycling, which results in material shedding.

Experimental studies of the plasma-propellant interface for electrothermal-chemical launchers

A plasma-propellant interaction experiment PIPE has been constructed to investigate the plasma-propellant interface physics, and plasma augmentation to the propellant burn rate of electrothermal-chemical launchers. The experimental facility consists of an electrothermal plasma source that injects plasma to the surface of solid or liquid propellants. The source is powered by a 340 /spl mu/F Maxwell capacitor (17 kJ at 10 kV, up to 100 kA, 400 /spl mu/s pulse duration) via a pulse forming network. Experiments with JA2 granular solid propellant under vacuum operation (20 Torr base pressure) showed incomplete burn of the propellant. However, the overall chamber pressure is a factor of two higher than that without propellant. Preliminary measurements showed that the burn rate with plasma injection has been increased by a factor of three at a pressure of 0.7 kbar compared to conventional ignition. >

Torch Ignition: Ideal for Lean Burn Premixed-Charge Engines

Sluggish flame initiation and propagation, and even potential misfiring, become major problems with lean-fueled, premixed-charge, spark-ignited engines. This work studies torch ignition as a means for improving combustion, fuel economy, and emissions of a retrofitted, large combustion chamber with nonideal spark plug location. A number of alternative configurations, employing different torch chamber designs, spark-plug locations, and materials, were tested under full-load and part-load conditions. Results indicate a considerable extension of the lean operating limit of the engine, especially under part-load conditions. In addition, torch ignition can lead to substantial thermal efficiency gains for either leaner or richer air-fuel ratios than the optimum for the conventional ignition system. On the richer side, in particular, the torch-ignited engine is capable of operating at maximum brake torque spark timings, rather than compromised, knock-limited spark timings used with conventional ignition. This translates into thermal efficiency improvements as high as 8 percent at an air-fuel ratio of 20:1 and full load.

Plasma Jet Ignition of Methanol at Sub-Zero Temperatures

Use of a pulsed plasma jet igniter to improve low-temperature starting of 100 per cent methanol (M100) fuelled engines is explored. A simple theoretical model of heat transfer from plasma to liquid methanol shows that sufficient energy is transferred for fuel vaporization in cold starting. Two laboratory experiments test liquid plane surface and droplet modes of ignition. An ordinary four cylinder auto engine fitted with plasma jet igniters is tested for cold starting in a Canadian winter, and cold starting is improved from +10°C (conventional ignition) to −16°C.