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Study on ignition mechanism of turbulent jet pre-chamber of H2/CO mixture
... Sadanandan et al. 8 investigated the ignition of H 2 /air mixtures by a hot jet using simultaneous highspeed laser schlieren and OH Planar Laser Induced Fluorescence (PLIF) techniques. Sun et al. 9 performed turbulent jet studies in a 5.5-l optical cylindrical pressure vessel using a 2 ml prechamber. The flame in the prechamber generated by igniting an H 2 /CO mixture was successfully shown to the main chamber without extinguishing. ...
An optically accessible prechamber-equipped constant volume combustion chamber (CVCC) has been designed and installed at the University of Minnesota to investigate turbulent jet ignition and reacting flows. The CVCC features an optically accessible windowed prechamber, allowing the extensive study of the combustion physics inside the prechamber. The prechamber is also modular, allowing multiple internal geometries and nozzle configurations. Two pairs of optical windows in the main combustion chamber, along with a bottom window axisymmetrically viewing the prechamber nozzle, provide exceptional optical accessibility. Optical techniques such as schlieren imaging and chemiluminescence, along with laser diagnostics, can be employed to gain a fundamental understanding of turbulent jet ignition chemistry. The CVCC is designed to withstand a maximum combustion pressure of 100 bars. Multiple standardized and custom ports on the main chamber allow various fueling techniques and sensor integration. The performance and reliability of the CVCC were demonstrated through prechamber and main chamber visualization studies of nanosecond spark-assisted turbulent jet ignition. The optically accessible prechamber-CVCC setup, therefore, provides a platform for highly detailed combustion analysis.
The dynamics and stabilization of fuel lean premixed CO/H2CO/H2/air atmospheric pressure flames in meso-scale channels were investigated numerically, using detailed gas phase chemistry and transport. Experiments in a channel flow reactor by means of chemiluminescence detection of the excited OH radical allowed for model validation at steady conditions and identification of the conditions at which unsteady flame dynamics were present. A detailed parametric study of the influence of wall temperature and CO:H2CO:H2 ratio on the ensuing flame dynamics was performed. The numerical results revealed different flame modes which included oscillatory ignition, random ignition spots, as well as steady weak and V-shaped flames. The wall temperature stability intervals of these modes changed with the CO:H2CO:H2 ratio. The richest variety was found for molar CO:H2CO:H2 ratios between 4 and 10, while at lower ratios the random and the weak modes were absent. At higher ratios all the dynamic modes were suppressed. The Computational Singular Perturbation (CSP) method was used to obtain insights into the physicochemical processes responsible for the weak flames, which were found at relatively high inflow velocities compared to previous studies, and V-shaped flames. A kinetic explanation of the phenomena was supported by the CSP analysis.
The turbocharged downsizing gasoline direct injection (GDI) engines aiming for lower fuel consumption, lower emissions, and higher performance have been regarded as the future hybrid electrical vehicle (HEV) pathway. To improve the thermal efficiency of the HEV engine under lean burn operation, a water direct injection (WDI) system and a high-energy ignition (500 mJ) system were proposed in an Atkinson cycle GDI engine with a high compression ratio of 17. The effect of homogeneous lean burn, water injection ratio (WIR), and spark timing (ST) on engine knock, combustion process, and indicated thermal efficiency (ITE) improvement was studied. The result shows WDI at an earlier stage of the compression stroke (100° CA BTDC) could mitigate knock without significantly increasing combustion instability. When WIR increased to 50%, the engine knock decreased 82.7%, the indicated specific fuel consumption (ISFC) dropped 9.9% as the ST advanced from 1.6° to 9° CA BTDC. The maximum brake torque spark timing (MBT) could be further advanced by extending the lean burn limit (LBL). The wider LBL was achieved with a larger WIR. At λ = 1.1, CO emission reduced by 5 ∼ 7 times and reached 570 ppm, and unburned hydrocarbons were reduced by 9%∼13%. Less than 500 ppm of NOx was noted when λ ≥ 1.5 under each WIR case. The IMEP was extended from test basic 8.3 bar to 13 bar with an increase of 47.73%. The remarkably high ITE of 47.09% can be achieved with a controllable knock level at lean burn condition (λ = 1.9), with an increase of 7.84% compared with the stoichiometric test basic IMEP of 8.3 bar.
The lower flammability limits of H2/CO/air mixtures with N2 and CO2 dilution were systematic experimentally studied over a wide range of H2 blending ratios (0–100 vol%) with N2 (0–67 vol%) and CO2 (0–67 vol%) dilution in the fuels under various elevated initial temperatures (298 K–473 K) and atmospheric pressure. The experimentation was conducted via an 8 L stainless steel cylindrical explosion vessel and using the metal wire fusing as the ignition source. The corresponding cases were also calculated using Kondo's correlation proposed based on a limiting flame temperature concept. To gain an insightful understanding of the effect of chemical kinetics at different H2 fractions and CO2 dilution ratios, sensitivity analysis and H mole fractions were carried out using Chemkin-Pro. The experimental results showed that the lower flammability limits decreased with the increase of H2 fractions especially when the H2 content was low (xH2 ≤ 0.25). Attributable to the accelerated oxidation of CO by the greater generation of OH from H2/O2 reaction, Le Chatelier's Rule tended to relatively over-estimate the lower flammability limits of H2/CO mixtures with a small amount of H2. Because of the larger heat capacity, and the inhibition effect on the oxidation of CO and the generation of H radicals, CO2 presented a stronger dilution effect on lower flammability limit than N2. Moreover, the lower flammability limits for all measured syngas mixtures displayed great linear temperature dependence. A comparison between the experimental data and calculation results showed that, Kondo's correlation provided the satisfactorily accuracy predictions on the lower flammability limits of diluted syngas mixtures with lower H2 fractions (xH2 ≤ 0.5). However, when the H2 fractions were high and the mixture was highly CO2 diluted, Kondo's correlation over-estimated the lower flammability limits and the prediction error would reach to 30%. The considerably distinctions were not only attributed to the inadaptable assumption against to the growing and lower behaviour of H2 flame temperature at lower flammability limit, but also caused by the preferential diffusion of H2, as well as the variation of the chemical effects under high H2 content and high CO2 dilution conditions.
Effect of turbulent jet ignition induced by pre-chamber sparkplug (PCSP), a simper version of turbulent jet ignition pre-chamber system without fuel injection, on the air-hydrogen combustion characteristics was conducted based on an optical constant volume chamber under varied equivalence ratio conditions. The dynamic pressure sensor and schlieren system were used to evaluate the heat release and flame propagation characteristics. The results confirm the feasibility of PCSP type turbulent jet. The jet increase the flame propagation speed significantly compared to standard ignition, which shorten ignition delay and combustion duration, advance T50 largely, and increase the maximum combustion pressure slightly. As a result, the combustion intensity is increased largely, especially under lean regime, the combustion intensity index can be as high as 1.7 at certain equivalence ratio. In addition, the PCSP turbulent jet reduces the sensitivity of heat release to variation of equivalence ratio, which is helpful to simplify the combustion controlling strategy. Furthermore, with the enhancement of the flame propagation, the tendency of knocking combustion can be suppressed potentially.
A detailed investigation on the ignition characteristics of ultra-lean premixed H2/air mixtures by multiple hot turbulent jets in a dual combustion chamber system was carried out. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. The focus was on the effects of the spark location and fuel/air equivalence ratio within the pre-chamber on the ignition pattern of the main-chamber mixture. The results show that multiple jets resulted in similar lean flammability limit of H2/air as the single jets, given that the total nozzle areas are the same. However, the ignition probability improved significantly near the lean flammability limit using multi-jets compared to single jets. Additionally, depending on the spark location and pre-chamber equivalence ratio, either the side jet, the middle jet, or all the jets can initiate ignition in the main chamber. The ignition characteristics of straight and angled multi-jets were compared, and angled jets produced shorter main chamber burn time. Numerical modeling of the flame propagation process within the pre-chamber was carried out to explain the behavior of individual jets in a multi-jet system. It was found that depending on the pre-chamber spark position, the flame shape inside the pre-chamber changes drastically. The effective L/D ratio governs the flame dynamics in the pre-chamber and subsequently determines which jet (side, middle, or all) will first ignite the main chamber.
Use of lean or ultra-lean air-fuel ratios is an efficient and proven strategy to reduce fuel consumption and pollutant emissions. Previous works indicate that lean burn mixtures improves engine thermal efficiency by improving combustion quality, reducing heat transfer losses and increasing the possibility of apply higher compression ratios. However, lower fuel concentration in cylinder hinders mixture ignition, requiring greater energy to start combustion. To favor the ignition process, several high energy providers methods have been studied. Between them, prechamber ignition system presents potential reductions in emission levels and fuel consumption, operating with lean burn mixtures and expressive combustion stability. In this paper, a literature review has been made about prechamber ignition systems as lean combustion technology, focusing in the several investigations regarding combustion and emissions characteristics and presenting the key advantages and challenges in prechamber ignition technology application. From this review can be observed that the pre-chamber ignition system is an important way to increase thermal efficiency, reduce fuel consumption and emissions in spark ignition engines.
Direct injection of fuel into the cylinder of an engine leads to the problem of particulate matter (PM) emissions. Lean-burn gasoline direct-injection (GDI) engines are known to emit higher levels of ultrafine particles than do conventional engines. The level of PM emissions by lean-burn GDI engines is unlikely to meet the EURO-VI emissions standards. In this study, the effects of combustion strategy and excess air ratio on the PM concentrations and particle size distribution were evaluated for a naturally aspirated lean-burn GDI engine. The engine operating conditions—including the fuel-air mixture and load—were varied in order to analyze the PM formation and the particle size distribution. The PM concentration was found to increase dramatically at an excess air ratio of 1.5, at which ratio lean combustion with a stratified mixture occurred. This was regarded as being the transition region between the premixed flames and the stratified mixture flames. Further, an increase in the excess air ratio to beyond 2.0 caused the PM concentration and particle number to increase again, possibly as a result of the relatively high ambient pressure and lower combustion temperature.
An experiment was developed to investigate the ignition mechanisms of premixed CH4/air and H2/air mixtures using a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition process inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produced a jet comprising of only hot combustion products from pre-chamber combustion. The latter produced a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and spark position on the ignition mechanisms and probability. A global Damköhler number was defined to remove parametric dependency. The limiting Damköhler number, below which ignition probability is nearly zero, was found to be 140 for CH4/air and 40 for H2/air. Lastly, the ignition outcomes were plotted on the turbulent premixed combustion regime diagram. All non-ignition cases fell within the broken reaction zone regime, whereas flame and jet ignition mostly fell within the thin reaction zone regime. These results can provide useful guidelines for future pre-chamber design and optimization.
The performance of a combustion initiation system for the Otto cycle engine is described which uses a pre-chamber of 1.5% main chamber clearance volume, fuelled with minute quantities of hydrogen. The order of magnitude faster flame kernel growth, obtained with hydrogen mixtures compared with hydrocarbon fuels, provides enhanced jet momentum from a small proportion of the total charge energy, and active radicals and intermediate species which assist in initiating combustion in mixtures significantly leaner than the lean flammability limit with normal ignition. The performance results demonstrate the ASTM CFR engine working at up to 70% maximum torque with NOx emissions close to ambient levels. The hydrogen assisted jet ignition, HAJI, use to achieve this also confers extremely stable combustion with coefficients of variation of peak cylinder pressure and specific work per engine cycle reduced by 50 to 80% from those normally achieved with this engine. This allows increased thermal efficiency at ultra lean operation, at relative air/fuel ratios of around 2, and about two numbers increase in the highest useful compression ratio. There is no lean limit of combustion within the range of usable engine torque. Operation with relative air/fuel ratios of 5 have been achieved. With methanol as the main chamber fuel, the results presented here demonstrate an improvement in maximum indicated thermal efficiency of 15 per cent simultaneously with the low NOx emissions. The hydrocarbon emissions remain high which is a characteristics of this research engine.
Power to Gas has attracted much attention as a solution to handle the electricity overproduced from renewable energy. This overproduced electricity can be converted into H2 and injected into the existing natural gas grid. Therefore, efficient and safe usage of H2 enriched natural (HNG) gas is a key issue to spread Power to Gas further. In this paper, the influences of H2 addition on the engine combustion process is discussed, focusing on large-bore lean burn gas engines operated at high specific loads. The experimental setup consists of a single cylinder research gas engine with a displacement of 4.77L. The results show a big influence of the addition of H2 on the operating range and emissions even with small amounts of H2. The misfire limit extends to the leaner side, while knocking must be prevented by a later ignition timing (IGT). Pre-ignition phenomena are limiting the operating range at richer conditions with rising H2 amounts. While NOx emissions increase, unburned hydrocarbon (THC) and formaldehyde (HCHO) emissions decrease due to an enhanced combustion. This and the leaner operating conditions, which can overcompensate the actual NOx increase, facilitate a higher efficiency, which is discussed based on a detailed loss analysis.
A methodology is presented for studying the influence of using alternative fuels on the cycle-to-cycle variations of a spark ignition engine which has been fuelled with mixtures of natural gas and hydrogen in different proportions (0–100%). The experimental facility consists of a single-cylindrical spark ignition engine coupled to an asynchronous machine with a constant engine rotation speed of 1500 rpm. A thermodynamic combustion diagnostic model based on genetic algorithms is used to evaluate the combustion chamber pressure data experimentally obtained in the mentioned engine. The model is used to make the pressure diagnosis of series of 830 consecutive engine cycles automatically, with a high grade of objectivity of the combustion analysis, since the relevant adjustment parameters (i.e. pressure offset, effective compression ratio, top dead center angular position, heat transfer coefficients) are calculated by the genetic algorithm. Results indicate that the combustion process is dominated by the turbulence inside the combustion chamber (generated during intake and compression), showing little dependency of combustion variation on the mixture composition. This becomes more evident when relevant combustion variables are plotted versus the Mass Fraction Burned of each mixture. The only exception is the case of 100% hydrogen, due to the inherent higher laminar speed of hydrogen that causes combustion acceleration and thus turbulence generation.
The multiple mapping conditioning (MMC) method is used to extend the stochastic PDF approach to the flamelet regimes of premixed combustion. MMC permits for a desired degree of flamelet-like conditions while reflecting the fluctuating nature of turbulent flames and preserving the integrity and universality of the chosen mixing model. The model is implemented in the context of the partially stirred reactor (PaSR), which is therefore generalised to have a wider range of applicability. A stochastic formulation of original MMC is deployed, where mixing of particle scalar values is conditioned on a Markovian reference variable which emulates an implied particle position relative to a flame. The model interactions with the reference variable are controlled through the flamelet localness parameter, Λ, which is also related to the ratio of diffusive to convective time scales. The model is implemented in a Monte Carlo numerical scheme using detailed GRI3.0 chemical kinetics without adjustments of kinetic coefficients. Predictions of NOx emissions are validated against experimental data for a lean premixed high-pressure combustor in which reactions fall between the flamelet and distributed regimes. There is good agreement between model predictions and experimental data.
The paper mainly focuses on applying the two-stage combustion system with a pre-chamber into the stationary internal combustion spark ignited engine. It especially concentrates on applying throttle less operation at partial load and reduction of the NOx emission. Considerations conducted in the paper are based on the in-cylinder combustion progress analysis. Additionally, analysis of tailpipe toxic emission, with particular focus on the NOx formation in the engine equipped with the pre-chamber, is also performed. The paper presents both results of 3-D combustion modeling in the SI engine and results conducted on a test SI engine. The 3-D modeling was performed in the KIVA-3V code. Next, results from modeling were compared with results obtained from tests. Finally, satisfactory good consistency between modeled and experimental courses of both pressure, temperature and NOx were obtained. Thus, the engine model with the proposed two-stage combustion system properly simulates engine working conditions on the test bed. Results from both analyses confirmed that the two-stage combustion system significantly shortens combustion duration of an ultra lean gasoline–air mixture and contributes to reduction in NOx.
The effects of CO2 dilution on NOx emissions and combustion instability are studied in an effort to use biogas as a fuel in dual lean premixed gas turbines. More specifically, the influence of the radiation heat transfer characteristics of CO2, which is a major component of biogas, on the combustion characteristics of a dual lean premixed flame of biogas is investigated. OH* chemiluminescence images are used to observe the flame structure for various CO2 dilution rates. The results show little difference in the flame structure for CO2 dilution rates of up to 40%, while higher dilution rates lower the flame intensity. Also, dual lean premixed flames show very different flame structures and temperatures depending on the pilot fuel mass fraction. The results show a decrease in the flame temperature when the dilution rate is increased, resulting in a reduction in the thermal NOx emissions. The present paper also shows that the radiation heat release, which is due to the high heat release rate of CO2, promotes a further drop in the flame temperature, in addition to the thermal effects of CO2. These findings are numerically modeled, empirically verified, and added to the NOx prediction model. CO2 dilution also changes the combustion oscillation frequency. To estimate this, the flame temperature is calculated as a function of the CO2 dilution rate and the pilot fuel mass ratio. The accuracy of the flame temperature calculation can be improved with a more accurate estimation of the radiation heat loss. In calculating the radiation heat loss for an unstable flame, considering the heat release fluctuation (i.e. the temperature fluctuation) improves accuracy, whereas taking the average temperature results in an underestimation. With an improved estimation of the temperature, the combustion oscillation frequency is more accurately predicted.
The oxidation of syngas mixtures was investigated experimentally and simulated with an updated chemical kinetic model. Ignition delay times for H2/CO/O2/N2/Ar mixtures have been measured using two rapid compression machines and shock tubes at pressures from 1 to 70 bar, over a temperature range of 914–2220 K and at equivalence ratios from 0.1 to 4.0. Results show a strong dependence of ignition times on temperature and pressure at the end of the compression; ignition delays decrease with increasing temperature, pressure, and equivalence ratio. The reactivity of the syngas mixtures was found to be governed by hydrogen chemistry for CO concentrations lower than 50% in the fuel mixture. For higher CO concentrations, an inhibiting effect of CO was observed. Flame speeds were measured in helium for syngas mixtures with a high CO content and at elevated pressures of 5 and 10 atm using the spherically expanding flame method. A detailed chemical kinetic mechanism for hydrogen and H2/CO (syngas) mixtures has been updated, rate constants have been adjusted to reflect new experimental information obtained at high pressures and new rate constant values recently published in the literature. Experimental results for ignition delay times and flame speeds have been compared with predictions using our newly revised chemical kinetic mechanism, and good agreement was observed. In the mechanism validation, particular emphasis is placed on predicting experimental data at high pressures (up to 70 bar) and intermediate- to high-temperature conditions, particularly important for applications in internal combustion engines and gas turbines. The reaction sequence H2+HO˙2↔H˙+H2O2 followed by H2O2(+M)↔O˙H+O˙H(+M) was found to play a key role in hydrogen ignition under high-pressure and intermediate-temperature conditions. The rate constant for H2+HO˙2 showed strong sensitivity to high-pressure ignition times and has considerable uncertainty, based on literature values. A rate constant for this reaction is recommended based on available literature values and on our mechanism validation.
1.The rate of yield of hydrogen atoms under the conditions of the high-temperature isothermal combustion of methane and hydrogen with air was calculated.2.A comparison of the calculated and experimentally measured [H] values as a function of the composition of the starting mixtures shows their satisfactory agreement.
Ignition and burning mechanisms of the main chamber mixture by a torch jet were experimentally investigated using a divided chamber bomb. The effects of the nozzle diameter and volume ratio on the structure of the torch jet, ignition process, and subsequent burning process in the main chamber were minutely examined, in both uniform and stratified charges, by measurements of ion current, light emission, OH-emission (306.4 nm), initial torch jet velocity, and main chamber pressure histories and by schlieren photography. The structure of the torch jet was greatly influenced by the nozzle diameter and volume ratio independently of the main chamber mixture ratio. According to the physical and chemical characteristics obtained for it, the structure of the torch jet could be classified into four types, and the ignition and burning processes in the main chamber could also be classified into four patterns depending on the torch jet structure: pattern I, chemical chain ignition and well-dispersed burning; pattern II, composite ignition and well-dispersed burning followed by wrinkled laminar burning; pattern III, flame kernel torch ignition and wrinkled laminar burning; and pattern IV, flame front torch ignition and wrinkled laminar burning. Combustion characteristics such as main chamber pressure and net burning time in the main chamber also showed their own peculiar features. Examination of the lean flammability limit gave a possibility of lean burning outside of the normal flammability limit by using divided chamber systems. From these results combustion pattern II was found to be most favorable for lean burning.
Reaction of lean to ultra-lean mixtures supported by high-temperature burned gas can resolve the dilemma between complete combustion versus ultra-low NOx emissions in lean premixed gas turbine combustors. The combustion characteristics and NOx emissions in “lean–lean” two-stage combustion were investigated for premixed–prevaporized kerosene–air mixtures using a co-axial flow configuration. Secondary prevaporized kerosene–air mixtures of lean to ultra-lean compositions were injected into the stream of hot burned gas prepared by the combustion of lean prevaporized kerosene–air mixtures stabilized on an annular perforated flame holder in the primary stage. The progress of mixing and reactions of the secondary mixture jets of prevaporized kerosene–air mixture injected into the co-axial primary hot burned gas flow were investigated by spatial gas sampling and direct photography. The effects of the ratio of secondary to primary air flow rates and equivalence ratios of the primary and secondary mixtures on the emissions from the two-stage combustor were studied. Imparting swirl to the secondary mixture jets resulted in an enhancement of the mixing of the jets with the primary burned gas. It was also shown that the use of reaction of lean to ultra-lean secondary mixtures supported by the hot burned gas from the primary stage is much advantageous in extending the operating range of ultra-low NOx emissions of the 10 ppm level and complete combustion as compared with other approaches such as fuel staging and variable geometry. The proposed ultra-low NOx combustion concept has a potential of suppressing combustion instabilities that is often experienced with lean premixed combustion.
The problem of measuring mixture fraction (defined here as the fractional part of mass originating from the exhaust gas jet)
during the mixing of an unsteady hot exhaust gas jet impinging into a quiescent premixed hydrogen/air mixture is addressed.
The diagnostic method is based on planar Laser-Induced Fluorescence of nitric oxide (NO) that is seeded to the hot jet and
used as a tag for mixture fraction. The research work presented in this paper focusses on the analytical and experimental
data employed for the estimation of mixture fraction using NO as a tracer. Since the NO LIF-signal does not solely depend
on the amount of NO, but also on temperature and overall chemical composition, additional information is normally required
to transform LIF-signals into mixture fraction values. To provide this additional information, the concept of reduced state
spaces is applied: Correlations between state variables (temperature and species) are exploited to establish relationships
between the measured signal and the mixture fraction. Simple numerical model simulations are used in combination with spectroscopic
calculations to check for correlations between NO-LIF signal and mixture fraction. The result is that sharp correlations between
NO-LIF signals and mixture fraction exist, provided that chemical reactions have not yet significantly influenced the flow.
These correlations display only little sensitivity with respect to changes of the jet temperature, amount of NO seeded to
the jet, and jet velocity. Measurements of NO-LIF were then performed for a non-reacting case before ignition, and mixture
fraction maps were obtained from the measured LIF-signals by exploiting the correlations. The resulting data reveal the different
driving forces behind mixing at regions near to the jet tip and at the radial shear layers.
KeywordsLaser Induced Fluorescence-Hot jet ignition-Hydrogen explosion-Reduced state spaces
Experimental and numerical investigations of the ignition of hydrogen/air mixtures by jets of hot exhaust gases are reported. An experimental realisation of such an ignition process, where a jet of hot exhaust gas impinges through a narrow nozzle into a quiescent hydrogen/air mixture, possibly initiating ignition and combustion, is studied. High-speed laser-induced fluorescence (LIF) image sequences of the hydroxyl radical (OH) and laser Schlieren methods are used to gain information about the spatial and temporal evolution of the ignition process. Recording temporally resolved pressure traces yields information about ambient conditions for the process. Numerical experiments are performed that allow linking these observables to certain characteristic states of the gas mixture. The outcome of numerical modelling and experiments indicates the important influence of the hot jet temperature and speed of mixing between the hot and cold gases on the ignition process. The results show the quenching of the flame inside the nozzle and the subsequent ignition of the mixture by the hot exhaust jet. These detailed examinations of the ignition process improve the knowledge concerning flame transmission out of electrical equipment of the type of protection flameproof enclosure.