Laser Spark Ignition Research Articles

Laminar flame speed measurements of ethanol, iso-octane, and their binary blends at temperatures up to 1020 K behind reflected shock waves

A shock tube augmented with a laser-spark ignition system was used to measure the atmospheric-pressure (1 atm ±2%) laminar flame speeds of neat ethanol and iso-octane as well as ethanol/iso-octane blends in a 21% O2-79% Ar (so-called “airgon”) oxidizer. Low-temperature (453 K–524 K) validation experiments of atmospheric-pressure, stoichiometric ethanol/air were first performed for comparison with literature. Two fuel blends with 50% and 85% ethanol (by volume) in iso-octane (E50 and E85, respectively) were then studied in airgon at high unburned-gas temperatures (637 K–1020 K) and at various equivalence ratios (0.75, 1, and 1.2). The low-temperature ethanol/air measurements showed good agreement with literature data, confirming the accuracy of flame speeds measured using a shock tube. High-temperature fuel/airgon flame speeds were simulated with various kinetic models, and significant model disagreement was found, highlighting the value of the new flame speed data in the high-temperature regime. To make the present fuel/airgon measurements compatible with practical engineering applications using air as the oxidizer, the ratio of fuel/airgon and fuel/air flame speeds was calculated using 1D simulations for each experiment, then subsequently used to perform mixture-scaling for the fuel/airgon data to obtain equivalent fuel/air flame speeds. Fuel/air laminar flame speed correlations as a function of the unburned-gas temperature were determined for the studied fuel blends and equivalence ratios. Finally, flame speed sensitivity analyses conducted for high-temperature ethanol/airgon and iso-octane/airgon flames revealed three key governing reactions. Assigning common rate expressions to these reactions across different kinetic models improved the agreement between model-predicted and experimentally measured ethanol flame speeds, but did not have a similar effect for iso-octane, suggesting the need for further exploration of iso-octane-specific reaction rates to facilitate better model agreement.

Effect of spark timing on laser ignition and spark ignition modes in a hydrogen enriched compressed natural gas fuelled engine

In this experimental study, a prototype laser ignited engine was developed and fuelled with hydrogen enriched compressed natural gas (HCNG; mixture of H2 and CNG). Engine performance, emissions and combustion characteristics were compared for laser ignition (LI) and spark ignition (SI) modes. The maximum brake torque (MBT) timing was employed to reduce cycle-to-cycle variations (CCV) and to improve the engine performance, combustion and emission characteristics. Composition of HCNG mixture was changed by dynamically blending H2 with CNG on a volumetric basis. MBT timing was determined for naturally aspirated engine in conventional SI mode, by varying the spark timing (ST) between 22° CA bTDC to 46° CA bTDC and by varying relative air–fuel ratio (λ) from rich-to-lean. Optimum torque for HCNG mixtures was observed at 31° CA bTDC. The 10% mass fraction burn (MFB10) duration reduced continuously with advancing ignition timing up to 31°CA bTDC, which then increased with further advanced ST. For any particular ST, MFB10 and MFB90 decreased with increasing H2 enrichment of CNG. At high λ (lean mixtures), CCV would be higher due to lower engine efficiency and increased emissions. LI mode exhibited lower coefficient of variation in indicated mean effective pressure (COVIMEP) compared to SI mode. The COVIMEP for all HCNG mixtures were within 2% up to λ = 1.2 and it increased further with increasing λ but the variation was within ± 6%. Lower CCV (< 2% of COVIMEP), reduced emissions and improved brake thermal efficiency (BTE) were observed at 31°CA bTDC MBT timing compared to other spark timings. This experimental study indicated that laser ignition is a suitable technology for deployment in HCNG engines.

Effect of transport processes on ignition of stretched diffusion flames using laser spark

Laser spark ignition of methane diffusion flames, in the stagnation region in counter-flow configuration, is studied with a range of transport properties achieved by using different levels of inert dilution with helium and argon as diluents in the fuel stream. The effects of transport properties on ignition strain rate and ignition delay time are investigated and possible effects of effective Lewis number (Lee) are also discussed and compared with those for flame extinction. The following results are obtained:(1) The role of Lee on the critical global strain rate, beyond which ignition is not possible, is qualitatively similar to that on the extinction strain rate. That is, with the same level of dilution, the inert diluent with smaller Lee (therefore relatively smaller heat loss rate) yields larger critical global strain rate.(2) For successful ignition, the ignition delay time decreases with increasing level of helium dilution, while the opposite is true with argon dilution – the experimental results correlated with the calculated results of the diffusion times with these two inert diluents, suggesting that the successful ignition and the ignition delay time is limited by the thermal diffusion time across the flammable layer in the counter-flow region, rather than resulting from the Lewis number effect.

Laser-induced breakdown ignition in a gas fed two-stroke engine

Laser-induced ignition for internal combustion engines is investigated intensively after demonstration of a compact ‘laser plug’ possibility. Laser spark benefits as compared to traditional spark plugs are higher compression rate, and possibility of almost any fuel ignition, so lean mixtures burning with lower temperatures could reduce harmful exhausts (NOx, CH, etc). No need in electrode and possibility for multi-point, linear or circular ignition can make combustion even more effective. Laser induced combustion wave appears faster and is more stable in time, than electric one, so can be used for ramjets, chemical thrusters, and gas turbines. To the best of our knowledge, we have performed laser spark ignition of a gas fed two-stroke engine for the first time. Combustion temperature and pressure, exhaust composition, ignition timing were investigated at laser and compared to a regular electric spark ignition in a two-stroke model engine. Presented results show possibility for improvement of two-stroke engines performance, in terms of rotation rate increase and NOx emission reduction. Such compact engines using locally mined fuel could be highly demanded in remote Arctic areas.

Numerical study of threshold intensity dependence on gas pressure in the breakdown of molecular hydrogen induced by excimer laser

In the present work, a numerical analysis is performed to investigate the threshold intensity dependence on gas pressure in laser spark ignition of the H2 plasma. The analysis considered the experimental measurements that were carried out by Yagi and Huo [Appl. Opt. 35, 3183 (1996)]. In their experiment, H2 in a pressure range of 150–3000 Torr is irradiated by a focused excimer laser source using a 96 cm lens at a wavelength of 248 nm and a pulse duration of 20 ns. The study, based on a modified electron cascade model [K. A. Hamam et al., J. Mod. Phys. 4, 311 (2013)], solves numerically a time-dependent energy equation for the distribution of the electron energy as well as a set of rate equations that describe the change in the formed excited molecule population. This model enabled the determination of the threshold intensity as a function of gas pressure. The validity of the model was tested by comparing the calculated thresholds with the experimentally measured ones. Moreover, the calculation of the ele...

Flame-Flow Interaction in Premixed Turbulent Flames During Transient Head-On Quenching

This paper reports on experimental investigations of turbulent flame-wall interaction (FWI) during transient head-on quenching (HOQ) of premixed flames. The entire process, including flame-wall approach and flame quenching, was analyzed using high repetition rate particle image velocimetry (PIV) and simultaneous flame front tracking based on laser-induced fluorescence (LIF) of the OH molecule. The influence of convection upon flame structures and flow fields was analyzed qualitatively and quantitatively for the fuels methane (CH4) and ethylene (C2H4) at ϕ = 1. For this transient FWI, flames were initialized by laser spark ignition 5 mm above the burner nozzle. Subsequently, flames propagated against a steel wall, located 32 mm above the burner nozzle, where they were eventually quenched in the HOQ regime due to enthalpy losses. Twenty ignition events were recorded and analyzed for each fuel. Quenching distances were 179 μm for CH4 and 159 μm for C2H4, which lead by nondimensionalization with flame thickness to Peclet numbers of 3.1 and 5.5, respectively. Flame wrinkling and fresh gas velocity fluctuations proved flame and flow laminarization during wall approach. Velocity fluctuations cause flame wrinkling, which is higher for CH4 than C2H4 despite lower velocity fluctuations. Lewis number effects explained this phenomenon. Results from flame propagation showed that convection dominates propagation far from the wall and differences in flame propagation are related to the different laminar flame speeds of the fuels. Close to the wall flames of both fuels propagate similarly, but experimental results clearly indicate a decrease in intrinsic flame speed. In general, the experimental results are in good agreement with other experimental studies and several numerical studies, which are mainly based on direct numerical simulations.

Characterization of gasoline combustion with laser and spark ignition

The combustion of gasoline-air mixtures in a constant-volume combustion chamber with an initial condition of 0.1 MPa pressure and 363 K temperature is experimentally investigated with laser ignition generated by a Q-switched Nd:YAG laser (wavelengths of 532 and 1064 nm). Spark ignition is also tested, and the results are used as the benchmark. Combustion chamber pressure is measured by a piezoelectric pressure transducer, and recorded using a digital oscilloscope. When the equivalence ratio is swept from 1.2 to 1.8, laser and spark ignition show marginal differences in pressure rise rate and peak pressure. The maximum pressure rise rate and the maximum peak pressure are obtained at equivalence ratios of 1.6 and 1.8 for laser ignition and spark ignition, which are 39.4 MPa/µs and 0.68 MPa for the laser wavelength of 532 nm, 38.8 MPa/µs and 0.67 MPa for the laser wavelength of 1064 nm, and 38.1 MPa/µs and 0.67 MPa for spark ignition, respectively. When the equivalence ratio is reduced below 1.2, the pressure rise rate and peak pressure of the laser ignition are significantly higher than those of spark ignition, and the lean limit for laser ignition is also wider than that of spark ignition; therefore, laser ignition is more favorable for lean combustion. For both the laser and spark ignitions, the ignition energy demonstrated a limited impact on both the pressure rise rate and peak pressure. Heat release rate in the combustion chamber is calculated, and the results show that variations of heat release are in accordance with variations of the pressure history.

Fast combustion waves and chemi-ionization processes in a flame initiated by a powerful local plasma source in a closed reactor.

Results are presented from experimental studies of the initiation of combustion in a stoichiometric methane-oxygen mixture by a freely localized laser spark and by a high-current multispark discharge in a closed chamber. It is shown that, preceding the stage of 'explosive' inflammation of a gas mixture, there appear two luminous objects moving away from the initiator along an axis: a relatively fast and uniform wave of 'incomplete combustion' under laser spark ignition and a wave with a brightly glowing plasmoid behind under ignition from high-current slipping surface discharge. The gas mixtures in both the 'preflame' and developed-flame states are characterized by a high degree of ionization as the result of chemical ionization (plasma density n(e)≈10(12) cm(-3)) and a high frequency of electron-neutral collisions (ν(en)≈10(12) s(-1)). The role of chemical ionization in constructing an adequate theory for the ignition of a gas mixture is discussed. The feasibility of the microwave heating of both the preflame and developed-flame plasma, supplementary to a chemical energy source, is also discussed.

内燃機関の熱効率向上に向けた先進着火技術

Natural gas engines have been attracting a lot of attention recently due to the development of unconventional natural gas sources. Achieving lean burn with supercharging is necessary to attain high thermal efficiency. Conventional spark plugs face difficulties with ignition because of the high pressure and lean air/fuel mixture. This paper describes studies on laser spark ignition which has been investigated at AIST as an alternative method for the achievement of stable ignition under such conditions. The extension of lean limit and improvement in thermal efficiency are demonstrated, and the possibilities of advanced laser ignition are also discussed.

Laser spark ignition of premixed methane-air mixtures: parameter measurements and determination of key factors for ultimate ignition results.

In this study, we present an experimental investigation of the parameters of the laser spark ignition of premixed methane-air mixtures and the determination of the key factors for the ultimate ignition result. Ignition is achieved in a mesh honeycomb burner using the 1064 nm output of a neodymium-doped yttrium aluminum garnet (Nd : YAG) laser. All pertinent laser ignition parameters, including the minimum ignition energy, the ignition time and blow out time, and the effects that the variation of experimental conditions, such as the spark energy, ignition position, equivalence ratio (ER), and flow rate, have on these parameters have been addressed systematically. To identify the key factors for the ultimate result of laser ignition, several parameters of the ignition processes are measured simultaneously, with an emphasis given to the temporal behavior of the hydroxyl (OH) radicals in relation to the data regarding the spark energy and the local ER. A clear finding of the study is that successful ignition events are always related to higher OH radical photon emissions, considered to be proportional to the concentration level of the OH radicals present, thus indicating a direct link between the OH level at early times (on a microsecond scale) and the ultimate result of laser ignition. Two-dimensional correlation plots of the spark energy, local ER, and OH radical photon count at early times with the ultimate results of laser ignition indicate that the spark energy and local ER do not play a critical role in determining the success or failure of the ignition and that the OH concentration in the early time range is the key factor in determining the final fate of laser ignition. Finally, on the basis of the results obtained here and in the existing literature, some considerations of the mechanism of laser ignition are presented.

A comparative study of laser ignition and spark ignition with gasoline–air mixtures

Abstract The ignition probability and minimum ignition energy (MIE) of premixed gasoline–air mixture for different equivalence ratio was experimentally studied using a nanosecond pulse at 532 nm and 1064 nm from a Q-switched Nd:YAG laser in a constant-volume combustion chamber (CVCC) The result was compared with the spark ignition. The initial pressure and temperature of the mixture was 0.1 MP and 363 K, respectively. The research indicates that within the flammable range, the probability increases when the ignition energy increases and the distribution of MIE with the equivalence ratios is U-shape for both laser and spark ignition. For laser ignition with 532 nm, when the incident energy is higher than 110 mJ or the absorbed energy is high than 31 mJ, 100% of ignition could be obtained within equivalence ratios of 0.8–1.6. For 1064 nm it is 235 mJ and 30 mJ. To get the same ignition probability of mixture with identical equivalence ratio, the incident energy of 1064 nm is twice more than the incident energy of 532 nm, while the absorbed energy values are virtually the same. It indicates that significant wavelength dependence is expected for the initial free electrons but irrelevant for the process of absorbing energy. The initial free electrons are produced from impurities in gasoline–air mixture because the intensity in the focus (10 12 W/cm 2 ) is too low to ionize gas molecules via the multi-photon ionization process, which requires higher irradiance (≥10 14 W/cm 2 ). The MIE obtained with a laser-spark ignition is greater than that measured by electrical sparks. The MIE for laser ignition was obtained at equivalence ratio of 1.0 both of 532 nm and 1064 nm, and it was 13.5 mJ and 9.5 mJ, respectively. But for spark ignition, the MIE is 3.76 mJ with equivalence ratio of 1.6. What׳s more, laser ignition extends the lean flammability limit from 0.8 to 0.6.

Hollow-core photonic crystal fibre for high power laser beam delivery

AbstractWe review the use of hollow-core photonic crystal fibre (HC-PCF) for high power laser beam delivery. A comparison of bandgap HC-PCF with Kagome-lattice HC-PCF on the geometry, guidance mechanism, and optical properties shows that the Kagome-type HC-PCF is an ideal host for high power laser beam transportation because of its large core size, low attenuation, broadband transmission, single-mode guidance, low dispersion and the ultra-low optical overlap between the core-guided modes and the silica core-surround. The power handling capability of Kagome-type HC-PCF is further experimentally demonstrated by millijoule nanosecond laser spark ignition and ${\sim }100~\mathrm{\mu} \mathrm{J} $ sub-picosecond laser pulse transportation and compression.

Advancing lean combustion of hydrogen–air mixtures by laser-induced spark ignition

We investigate how ignition through laser-induced plasma can improve the application of lean combustion, in particular in environmental conditions relevant to hydrogen internal combustion engines (ICE). Major design goals when developing combustion engines are increasing thermal efficiency and decreasing combustion emissions. High compression ratios, lean combustion and precise ignition timings are contributing factors in ICE optimization. In our studies, several gains from laser spark ignition are investigated. The high energy content of laser-induced ignition kernels are shown to speed up the development of the early flame kernels. These extended ignition kernels transfer into self propagating flames even in lean mixtures. Leaner mixtures are ignited in our experiments using laser spark ignition in comparison to conventional electrical spark plugs. Precise ignition timing is realized. Multi-point ignitions are synchronized on the timescale of microseconds to enhance the progress of combustion. We modified the locus of ignition in a mixture flow to decrease the temporal extent of flame contact with the wall. Therefore, burning duration and heat loss can be reduced.

Laser-induced optical breakdown applied for laser spark ignition

AbstractIn the present article, the experimental investigation of optical breakdown induced by ns/mJ pulses at two wavelengths, 1064 nm and 532 nm, in air of atmospheric pressure is reported and discussed. The obtained breakdown thresholds were compared with theory and are in good agreement. The generated plasmas have been characterized by their amount of scattered laser light, energy transmission, and change of the transmitted temporal shape. Laser-induced plasma formation in a gas, in air, also generates an acoustic pressure wave. The acoustic energy is compared to the laser pulse energy and is found to be linearly dependent. Moreover, the frequency distribution of the characteristic acoustic pressure wave was analyzed. The experiments described were accomplished in order to optimize a laser ignition system with regard to efficiency and costs. The laser system employed for these investigations is a compact high peak power, passively Q-switched, longitudinally diode-pumped solid-state laser. Such a “laser spark plug” should replace conventional spark plugs in internal combustion engines because conventional ignition has reached its limits in terms of efficiency and durability. Thereby, a reduction of pollutant emission should also be feasible.

1813 レーザー着火におけるレーザービームの時間・空間プロファイルの影響(J14-1 環境モニタリングと負荷低減のための熱流体・環境工学の展開(1),J14 環境モニタリングと負荷低減のための熱流体・環境工学の展開)

1813 レーザー着火におけるレーザービームの時間・空間プロファイルの影響(J14-1 環境モニタリングと負荷低減のための熱流体・環境工学の展開(1),J14 環境モニタリングと負荷低減のための熱流体・環境工学の展開)

Laser spark ignition of two-phase monodisperse mixtures

Experimental measurements of the minimum ignition energy for two-phase reactive mixtures involving a liquid fuel (n-heptane, JP4), using a Q-switched Nd-YAG laser operating at 1064nm, are reported in this study. Ignition energies were measured for many different mixtures. We find that the minimum incident energy level leading to ignition lies for both fuels between 16 and 30mJ. We also point out the high uncertainties obtained for such studies, due to the limited number of realizations possible in practical measurements. While incident energies are obtained with a good precision, transmitted energies present considerable dispersions. For comparison purposes, it might be important to take into account these uncertainties.

Minimum ignition energy for laser spark ignition

Combustion ignition by electrical sparks and nonresonant laser sparks is preceded by breakdown, electron heating, relaxation of internal plasma energy, shock wave generation and propagation, and subsonic flows. Measurements and model calculations of these processes (which occur over times of 10 −12–10 −1 s) in laser sparks show that the minimum ignition energy (MIE) for laser sparks is higher than for electrical sparks, fundamentally because of the higher energy cost of laser spark formation (both breakdown and heating), but also because of more efficient removal of the energy absorbed in laser sparks to regions outside the nominal ignition kernel by the shock. The MIE is bounded below by the breakdown energy, which depends strongly on the focal length of the lens in laser systems; this partly explains the wide range of reported laser spark MIEs. Another possible factor leading to a lower MIE for electrical sparks is supplemental heating of the kernel region long after emission of a shock wave, an effect entirely absent in laser sparks. Energy removal by shocks is more efficient for both types of spark than convective energy transport or diffusion out of an ignition kernel, so the spark ignition MIEs inevitably exceed the thermal MIE based on chemical kinetics for local heating of the kernel region.

LASER IGNITION IN A LEAN PREMIXED PREVAPORIZED INJECTOR

Laser-induced spark ignition has proved to be an alternative way of achieving ignition. In this paper, we demonstrate the feasibility of laser spark ignition at the outlet of a lean premixed prevaporized injector, similar to those used in low-NOx air jet engines. This injector uses liquid n-heptane as fuel in preheated air with a high level of turbulence. After a description of the combustion facility and of the experimental setup, experimental results are presented and discussed. Special attention is paid to ignition energy thresholds for various positions of the focusing point in the combustion chamber. The influence of energy level on ignition probability is also presented. Finally, shadowgraphy visualization with a high-speed camera of one ignition event is presented and discussed.

An optical and spectroscopic study of laser-induced sparks to determine available ignition energy

This work conducts an optical and spectroscopic study of laser-induced sparks created in air. It is aimedat determining the minimum ignition energy associated with laser spark ignition. Gas breakdown was produced using a single-mode, Q-switched Nd:YAG laser. It produced a 0.6 cm diameter beam at a wavelength of 1064 nm with a 5.5 ns pulse duration. The beam deflection and the absolute two-line intensity ratio techniques were used for measuring the spark expansion and temperature. The results were used to determine the shock propagation energy, the radiation energy losses, and the energy of the hot gas that is left over after the shock has been propagated away. With spark energies ranging from 15 to 50 mJ, we found that the shock energy was from approximately 70% to 51%, the radiation energy losses were from 22% to 34%, and the energy of the remaining hot gas was about 7% to 8% of the spark total energy. As far as ignition is concerned, the shock propagation and the radiation energies are wasted. The energy of the hot gas is the energy source that causes ignition. Thus, within the limits of error of the present measurements and calculations, the ignition energy obtained by the laser spark ignition does not differ greatly from that obtained by the electric spark ignition as has been reported.

レーザーアブレーション着火方式を用いた水素・酸素混合ガスにおける着火特性

Irradiating a nano-second laser pulse on a solid surface, a GH2/GO2 combustion chamber was operated to measure the minimum ignition energy for three parameters: the spot diameter, the thruster pressure, and the target material. It was confirmed that this method is useful since the minimum ignition energy can be much less than that of the laser spark ignition. It turned out that the minimum ignition energy increases as the spot diameter increases, and decreases monotonously as the thruster pressure increases. In addition, it was suggested that the ignition is possible even after the irradiation of a million laser pulses by applying the metallic targets (tungsten and stainless). On the other hand, the carbon target provided poor ignition probability.