Volume Ignition Research Articles

FLAIM: A reduced volume ignition model for the compression and thermonuclear burn of spherical fuel capsules

We present the “First Light Advanced Ignition Model” (FLAIM), a reduced model for the implosion, adiabatic compression, volume ignition and thermonuclear burn of a spherical DT fuel capsule utilising a high-Z metal pusher. FLAIM is characterised by a highly modular structure, which makes it an appropriate tool for optimisations, sensitivity analyses and parameter scans. One of the key features of the code is the 1D description of the hydrodynamic operator, which has a minor impact on the computational efficiency, but allows us to gain a major advantage in terms of physical accuracy. We demonstrate that a more accurate treatment of the hydrodynamics plays a primary role in closing most of the gap between a simple model and a general 1D rad-hydro code, and that only a residual part of the discrepancy is attributable to the heat losses. We present a detailed quantitative comparison between FLAIM and 1D rad-hydro simulations, showing good agreement over a large parameter space in terms of temporal profiles of key physical quantities, ignition maps and typical burn metrics.

Experimental study on multi-point ignition of NH3/air by high-frequency nanosecond dielectric barrier discharge

Non-equilibrium plasma ignition technology, with its ability to expand ignition limits and increase ignition volume, is considered an important development direction of advanced ignition systems, and has significant advantages in improving ignition stability and combustion rate. This study investigates the ignition and combustion assistance effect of high-frequency nanosecond surface dielectric barrier discharge (nSDBD) on NH3/air mixture in a constant volume combustion chamber (CVCC). The study reveals the influence of high-frequency nSDBD on the growth process of the flame kernel in NH3/air mixture. As the discharge pulse number (PN) increases, the flame kernel develops from the middle position of the discharge filament to the head of the discharge filament, while there is no obvious flame at the root and middle of the discharge filament. Continuous discharge causes the flame kernel to appear concave and the center of the flame kernel to shift towards the wall of the CVCC. In the NH3/air mixture, the initial flame kernels exhibit dissipation phenomenon, resulting in a discrepancy between the number of initial flame kernels and the number of stable combustion flame kernels. As the discharge frequency or pulse number increases, and the number of stable combustion flame kernel increases. The adjustment of discharge frequency and pulse number can effectively control the flame development time (FDT) and flame rise time (FRT) of NH3/air mixture. As PN increases, both FDT and FRT are shortened. When discharge frequency is 20 kHz, the maximum shortening of FDT and FRT are 30 ms and 14 ms, respectively, about 55 % and 30 %. The effects of discharge frequency on FDT and FRT exhibit a non-monotonic variation pattern, and it is necessary to comprehensively consider various discharge parameters to achieve the best ignition effect. The ignition success rate curve of high-frequency nSDBD in NH3/air mixture is steep. The study results of this paper provide new discoveries and experimental data for advanced ignition systems.

Study on the Applicability of Autothermic Pyrolysis In Situ Conversion Process for Low-Grade Oil Shale: A Case Study of Tongchuan, Ordos Basin, China

The feasibility of the autothermic pyrolysis in situ conversion (ATS) process for low-grade oil shale (OS) has not been determined. In this research, the pyrolysis and combustion properties of Tongchuan OS, with a 4.04% oil yield, were systematically analyzed. The findings revealed that temperatures between 350 and 425 °C favored oil production, while temperatures from 450 to 520 °C resulted in a higher rate of gaseous generation. At 300 °C, the volume expansion and ignition coking caused by the large amount of bitumen generated resulted in severe pore plugging, which significantly increased the combustion activation energy of the residue, while the presence of substantial flammable bitumen also significantly decreased the ignition and combustion temperatures. From 300 to 520 °C, the combustion performance of residue decreases continuously. In addition, pyrolysis residues of Tongchuan exhibited a slightly higher calorific value, between 425 and 520 °C, owing to its higher fixed carbon content (10.79%). Based on the ideal temperature screening method outlined for Tongchuan OS, the recommended preheating temperature for Tongchuan OS was 425 °C, while the optimum temperature for the retorting zone should be 510 °C, considering a heat utilization rate of 40%. These findings contribute valuable insights for the application of the ATS process to low-grade OS.

Explosive Characteristics Analysis of Gasoline–Air Mixtures within Horizontal Oil Tanks

Horizontal oil tanks, like other oil storage containers, carry the risk of explosion when gasoline–air mixtures are ignited. With the widespread application of horizontal oil tanks in the petrochemical industry, attention to safety risks is increasing. However, currently, a limited amount of experimental research on such tanks exists. To explore the characteristics of gasoline–air mixtures combustion within the confined space of horizontal oil tanks, this study constructed a medium-scale simulated horizontal oil tank (L/D = 3, V = 1.0 m3) platform. By investigating the effects of different initial gasoline–air mixture volume fractions and ignition positions on explosion overpressure characteristic parameters, an analysis of the combustion characteristics was conducted. It was found that the most dangerous gasoline–air mixture volume fraction is 1.9% when ignited at the top position and 2.1% at the middle. It was also observed that the ignition position has a significant impact on the variation in explosion overpressure characteristic parameters, with ignition at the middle position resulting having a greater explosive force compared to ignition at the top position. Furthermore, using ignition at the middle position as an example, a study was conducted on the flame morphology characteristics at initial gasoline–air mixture volume fractions of 1.1%, 1.9%, and 2.7%. The conclusions from this research deepen our understanding of the explosion characteristics of different containers, providing theoretical insights for the safe storage and transportation of oil materials in horizontal oil tanks.

Theoretical study of the effect of fuel injection start pressure on the characteristics of the combustion process, indicators, and braking efficiency of the 2CH10.5/13 type engine

Introduction: Over time, demands on internal combustion engines are increasing, with a focus on fuel efficiency, emissions reduction, improved reliability, and cost-effective production and operation. Studying diesel engines like the 2CH10.5/13 is relevant due to their significant potential in the modern world. The purpose of study is to investigate the theoretical effect of the initial fuel injection pressure on the combustion process, engine braking performance and efficiency. Materials and Methods: The paper describes the methods of choosing injectors for diesel engines with a high-pressure fuel system. Results and Discussion: Using mathematical calculations and modeling is an effective means to prevent engine operation issues. Developing a dedicated calculation method is necessary to understand the impact of diesel fuel ignition volume on engine performance parameters. The results of experimental studies, which included non-motorised, motorised, and operational tests of diesel nozzles, are also presented. Studies have shown that after 4,000 cumulative hours of operation, the residual life of the injector nozzles is insignificant, since the dynamic performance of the diesel engine decreases to the limit value (7%). Conclusions: This may indicate the need for regular maintenance and replacement of nozzles in accordance with the manufacturer’s recommendations.

Advances and Challenges in Combustion Control of Radio Frequency Oscillating Plasma Discharge

Article Advances and Challenges in Combustion Control of Radio Frequency Oscillating Plasma Discharge Linyan Wang , Xiao Yu , Graham Reader , and Ming Zheng * Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, N9B 3P4 Ontario, Canada * Correspondence: mzheng@uwindsor.ca Received: 18 July 2023 Accepted: 13 September 2023 Published: 20 September 2023 Abstract: Oscillating plasma (corona) ignition is a promising technique for producing a larger initial ignition volume for achieving cleaner combustion. The oscillating plasma system provides a quick response to the ignition command compared with the conventional transistor-coil-ignition systems (TCI). Subsequently, the shorter combustion duration contributes to further improvement of engine efficiency under lean/diluted conditions. The challenges of oscillating plasma ignition appear under elevated background pressures, mostly owing to fewer plasma streamers and even void discharge, thus the advantage of oscillating plasma discharge over spark discharge is compromised. To suffice the industrial applications, the challenges of plasma generation and control platforms are investigated and discussed in this work. The challenges include ignition control, background condition impacts, and other external disturbances. A flexible modulation for oscillating plasma generation is established, with the measurements of discharge voltage and current. High-speed imaging, simultaneous with electrical waveform measurements is applied to record the plasma formation and flame propagation. This research provides advances in the ignition control for the oscillating plasma discharge, adding a foundation and reference for the oscillating plasma diagnostics under engine-like conditions with variations of pressure, temperature, gas composition, and flow pattern.

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Los Alamos National Laboratory Double Shell Program Target Development

The Double Shell Program at Los Alamos National Laboratory is studying an alternative platform for achieving robust alpha-particle heating at the National Ignition Facility. Double shells benefit from having a low convergence ratio and lower predicted temperature for achieving volume ignition. The joint required to assemble a double shell has an imperfection in the outer shell that seeds instabilities that can greatly impact the inner capsule’s implosion at bang time. Different variations of the shape and placement of the joint were implemented with improvements in the quality of the machining leading to measurable improvements in yield. High-Z coatings on the outer joint mitigated the impact of the 1- to 2-μm gap sometimes found in double shell assemblies.

High-Fidelity Energy Deposition Ignition Model Coupled With Flame Propagation Models at Engine-Like Flow Conditions

Abstract With the heightened pressure on car manufacturers to increase the efficiency and reduce the carbon emissions of their fleets, more challenging engine operation has become a viable option. Highly dilute, boosted, and stratified charge, among others, promise engine efficiency gains and emissions reductions. At such demanding engine conditions, the spark-ignition process is a key factor for the flame initiation propagation and the combustion event. From a computational standpoint, there exist multiple spark-ignition models that perform well under conventional conditions but are not truly predictive under strenuous engine operation modes, where the underlying physics needs to be expanded. In this paper, a hybrid Lagrangian–Eulerian spark-ignition (LESI) model is coupled with different turbulence models, grid sizes, and combustion models. The ignition model, previously developed, relies on coupling Eulerian energy deposition with a Lagrangian particle evolution of the spark channel, at every time-step. The spark channel is attached to the electrodes and allowed to elongate at a speed derived from the flow velocity. The LESI model is used to simulate spark ignition in a nonquiescent crossflow environment at engine-like conditions, using converge commercial computational fluid dynamics (CFD) solver. The results highlight the consistency, robustness, and versatility of the model in a range of engine-like setups, from typical with Reynolds-averaged Navier–Stokes (RANS) and a larger grid size to high fidelity with large-eddy simulation (LES) and a finer grid size. The flame kernel growth is then evaluated against Schlieren images from an optical constant volume ignition chamber with a focus on the performance of flame propagation models, such as G-equation and thickened flame model, versus the baseline well-stirred reactor model. Finally, future development details are discussed.

Big Data-aided Study of the Physical Process of Volume Ignition

Volume ignition is a method of igniting a fuel as a whole by simultaneously achieving ignition conditions throughout the fuel zone. The basic criterion for ignition is that the thermonuclear energy is greater than the energy leakage at the fuel boundary, resulting in self-sustaining heating and deep combustion. Deuterium-tritium fuels are wrapped in medium to high Z media to reduce radiative leakage and achieve lower-temperature holistic ignition and non-equilibrium combustion, ultimately allowing the fuel to achieve high combustion efficiency. Volume ignition is the use of energy balance relations under the local thermodynamic equilibrium approximation to establish the energy balance equation for thermonuclear systems, and the system ignition threshold is obtained by solving this equation. By understanding the physical process, we believe that the non-equilibrium process is universal to the volume ignition process. Changes in external factors (density, boundaries, albedo, etc.) at the moment of ignition can have a significant impact on the development direction of the system, and an ignition system with a large surface density can nevertheless withstand a large amount of reverse work and continue to burn. The design of the ignition target tries to avoid these factors through margin design, but conversely, the rational use of these laws can further improve the design margin of the capsule. With the aid of big data, the volume ignition method is easier to calculate and has a shorter iteration time. The traditional way is to propose a model, set the material, and then perform the calculation while using big data can set any model and material for calculation. In this paper, a simple comparison will be made to find out that the efficiency of the physical design of a volume ignition target will be effectively improved with the aid of big data. Volume ignition targets can be used not only in Z-pinch-driven systems but also for laser-driven volume ignition.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Photodissociation as a method to increase the ignition volume

Experimental and numerical studies of the ignition and combustion of an H2-O2 mixture (ϕ=1.5, T0=713 K, P0=1 atm) initiated by the radiation of the pulsed excimer ArF-laser with the wavelength of λ=193.3 nm, leading to the dissociation of O2 molecules, have been carried out. The possibility of increasing the ignition region by changing the focusing of laser beam was demonstrated. When a lens with focus length of f = 20 cm is used, the ignition region is close to spherical, and the flame propagation mode is realized. When a lens with f = 50 cm is employed, the ignition region substantially extends along the laser beam. Due to the propagation of the ignition wave in the longitudinal direction in this case, the combustion front velocity turned out to be much higher in the longitudinal direction than in the radial one. When the mixture ignites at different focusing at the identical time (at the energy in the pulse Ep=20 mJ with f = 20 cm and Ep=75 mJ with f = 50 cm), the volume of the ignition region turned out to be ∼16 times larger when using a long-focus lens. At the same time, the pulse energy is only 3.75 times higher. The minimal pulse energy required for the ignition of the H2O2 mixture depends on focusing and at considered mixture conditions is equal to Ep∼5 mJ when using lens with focal length of f = 20 cm. At f = 50 cm, the minimal pulse energy increases to Ep∼20 mJ. However, the minimal mass fraction of laser-produced O atoms, at which the H2O2 mixture ignites at T = 700 K, does not depend on focusing and is equal to YO∼0.001.

Effect of Hydrogen Blending on the Combustion Performance of a Gasoline Direct Injection Engine.

To explore the effect of hydrogen blending on the combustion of a gasoline direct injection engine, a three-dimensional model of the engine is built. The effects of some hydrogen volume fractions (HVFs) and ignition timings (ITs) on the engine performance parameters are studied. Furthermore, the microstructure and mechanism of combustion are analyzed. The simulation results reveal that when the gasoline engine is blended with hydrogen, the active hydroxyl radical concentration increases, and the combustion process is accelerated. When the IT is fixed, with the HVF rising, the peak heat release rate and cylinder pressure will increase. The ignition delay, combustion duration, and crank angle when cumulative heat release reaches 50% decrease. Additionally, the autoignition is shifted to an earlier time as the IT advances. Under the studied conditions with the increase in the HVF, the knock resistance is enhanced because hydrogen has a high knock resistance and octane number.

Critical temperature for volume ignition of deuterium–tritium fuel in inertial confinement fusion: Effects of hydrodynamic instabilities

Volume ignition is an alternative approach to inertial confinement fusion. Due to igniting the whole fuel region rather than the central hot spot compared with the central hot-spot ignition, more laser energy is needed for volume ignition. Therefore, it is much desirable to examine the ignition margin for volume ignition. Hydrodynamic instabilities are major factors responsible for degrading inertial confinement fusion implosion performance. Hydrodynamic instabilities usually bring dramatic deformations of the fuel target, and accordingly, more radiation energy loss leaks from the fuel region. Therefore, the focus of this paper is on how they influence the radiation energy loss and increase critical temperatures for volume ignition. The present results show that critical ignition temperature increases both with the perturbation mode number and the perturbation amplitudes. What is more, we find that perturbations with longitudinal mode have a greater impact than those with latitudinal mode, and targets with lower deuterium–tritium mass are more vulnerable to perturbations. The present results are important and offer support for subsequent ignition-target design.

Constant volume combustion properties of Al/Fe2O3/RDX nanocomposite: the effects of its particle size and chemical constituents

A potential novel lead-free primary explosive:Al/Fe2O3/RDX nanocomposites, were prepared by the ultrasonic mixing method, and their ignition and constant volume combustion properties in the electric detonator were analyzed. Results show the constant volume combustion properties are a function of the specific surface area and proportions of their ingredients. A small amount of Al/Fe2O3 nanothermite could accelerate the constant volume combustion of nanocomposites, and make the ignition delay time and pressure rise time decrease. When nano-Fe2O3 with high specific surface area was used as oxidizer, or both cyclohexane and acetone as the solvent, the ignition and combustion properties show a dramatic improvement. Al/Fe2O3/RDX nanocomposites with faster combustion velocity than RDX and enough detonation output power to explode other explosives could be obtained by adjusting their particle sizes and the amount of their constituents.

Experimental study on the propagation characteristics of hydrogen/methane/air premixed flames in a narrow channel

This work is focused on the explosion characteristics of premixed gas containing different volume fractions of hydrogen in a narrow channel (1000 mm × 50 mm × 10 mm) under the circumstance of stoichiometric ratio. The ignition positions were set in the closed end and the middle of the pipeline respectively. The results showed that when the gas was ignited at the pipeline closed end, the propagating flame was tulip structure for different premixed gas. When the hydrogen volume fraction was less than 40%, the flame propagation speed increased significantly with the rise of hydrogen volume fraction, and the overpressure peak also appeared obviously in advance. However, when the volume fraction of hydrogen was more than 40%, the increase of flame propagation speed and the overpressure peak occurrence time varied slightly. Furthermore, when the ignition position was placed in the middle of the pipeline, the flame propagation speed propagating to the opening end was much faster than that propagating to the closing end, and there was no tulip shape when the flame propagates to the opening end. The flame propagating to the closed end appeared tulip shape under the influence of airflow, and high-frequency flame oscillation occurred during the propagation. This work shows that the hydrogen volume fraction and ignition position significantly affected the flame structure, flame front speed, and explosion overpressure.

Effects of distribution form and location of different branch tunnels on overpressure characteristics of ventedgasoline-air mixture explosion in closed vessels

The branch structure of the tunnel significantly affects the overpressure characteristics of combustible gas explosions in confined space. However, most of previous studies involved explosions in branch vessels were limited to single branch structure, effects of the distribution form and location of the branch structure were rarely considered. In order to explore the influence of various branch distribution forms and locations on overpressure characteristics of vented gasoline-air mixture explosion in closed vessels, the experiments were carried out using three kinds of branch tunnel distribution forms (linear/staggered/symmetrical) and three kinds of branch tunnel locations (near to the spark plug/far from the spark plug/evenly distributed along the main tunnel), under the condition of the same tunnel volume (0.24 m3), initial fuel volume concentration (1.2%) and ignition energy (5 J). The maximum explosion overpressure pmax, the time to reach maximum explosion overpressure, the maximum rates of pressure rise (dp/dt)max, and the deflagration index KG were examined. Moreover, the effects of distribution form and location of branch tunnels on overpressure characteristics were discussed. Results show that explosion overpressure characteristics are strongly influenced by branch tunnels' distribution form and location. In terms of the symmetrical distribution, the maximum explosion overpressure, the maximum rates of pressure rise, and the deflagration index KG are the lowest among the three types of distribution forms of branch tunnel. Linear and staggered distributions have similar overpressure characteristics, whose explosion overpressure, maximum rates of pressure rise, deflagration index KG are 1.14, 1.52 and 1.52 times of those in the symmetrical situation respectively. Time to reach the maximum explosion overpressure and time to reach the maximum rates of pressure rise in the symmetrical situation are delayed, which are 1.31 and 1.30 times of those in the linear and staggered situations respectively. The maximum rates of pressure rise and the deflagration index KG descend in the following distribution locations: far from the spark plug, near to the spark plug, evenly distributed along the main tunnel. The results indicate that the farther the branch tunnel from the ignition end, the larger the explosion intensity index, and the closer the branch tunnel from the ignition end, the earlier the time to reach the maximum explosion overpressure rising rate.

Analytical physical models for cryogenic double-shell capsule design driven by Z-pinch dynamic Hohlraum

The Z-pinch dynamic Hohlraum (ZPDH) is a promising indirect-drive approach for inertial confinement fusion. The volume ignition capsule is more robust than the hot-spot ignition capsule for ZPDH due to the fact that the ZPDH radiation drive source has a high energy but low symmetry. Focusing on the ignition design of cryogenic double-shell volume ignition capsules using ZPDH radiation sources, three analytical physical models, including the ablation and implosion model, the shell collision model, and the burn fraction model, are established to quantitatively characterize the relation of capsule parameters. Robust capsule designs are then determined based on these analytical models together with 1D radiation hydrodynamics simulations. The results show that under the 10 ns, 308 eV radiation drive source produced by ZPDH with 50 MA load current, capsules with a large range of parameters can ignite. The fusion yield of the recommended capsule is 16.0 MJ, and the absorbed energy is 1.28 MJ.

Cycle-skipping strategy with intake air cut off for natural gas fueled Si engine.

In this study, cycle-skipping was investigated for a natural gas engine which has single cylinder, unsupercharged with 1.16 L volume and spark ignition. Additionally, inlet manifold air was switched off during cycle-skipping to minimize pumping losses. Thus, cycle-skipping strategy was carried out, and its effects on emission and engine performance were investigated. Indicated mean effective pressure, indicated efficiency, specific emissions (CO, HC, and NOX) and combustion characteristics (in-cylinder pressure and rate of heat release) were investigated in the study. As a result of performed study, it is predicted that a significant improvement can be achieved in indicated thermal efficiency as 22.8% and 13.4% by different cycle-skipping strategies. However, there is not a continuous change in emissions for different cycle-skipping strategies. While CO and NOX emissions increased in 3N1S (three normal, one cycle-skip) condition, HC emissions decreased in accordance with normal condition. For both cycle-skipping strategies, all the emissions have an increase in accordance with normal condition. In 3N1S and 2N1S (two normal, one cycle-skip) cycle skip engine operating conditions, compared to engine operating under normal condition, CO emissions increased by 14.7 and 51.7 times, respectively. In terms of HC emissions, while emission values decreased by 27.8% under 3N1S operating conditions, they increased by 67.2% under 2N1S operating conditions. Finally, in 3N1S and 2N1S cycle skip engine operating conditions, NOx emissions increased by 3.7 and 6.9 times, respectively, compared to normal operating condition. Another significant result of this study is that peak in-cylinder pressure increased as the cycle-skipping rate increased.

Numerical studies on minimum ignition energies in methane/air and iso-octane/air mixtures

In this study, the dependence of minimum ignition energies (MIE) on ignition geometry, ignition source radius and mixture composition is investigated numerically for methane/air and iso-octane/air mixtures. Methane and iso-octane are both important hydrocarbon fuels, but differ strongly with respect to their Lewis numbers. Lean iso-octane air mixtures have particularly large Lewis numbers. The results show that within the flammability limits, the MIE for both mixtures stays almost constant, and increases rapidly at the limits. The MIEs for both fuels are also similar within the flammability limits. Furthermore, the MIEs of iso-octane/air mixtures with a small spherical ignition source increase rapidly for lean mixtures. Here the Lewis number is above unity, and thus, the flame may quench because of flame curvature effects. The observations show a distinct difference between ignition and flame propagation for iso-octane. The minimum energy required for initiating a successful flame propagation can be considerably higher than that required for initiating an ignition in the ignition volume. For iso-octane with a small spherical ignition source, this effect was observed at all equivalence ratios. For iso-octane with cylindrical ignition sources, the phenomenon appeared at lower equivalence ratios only, where the mixture's Lewis number is large. For methane fuel, the effect was negligible. The results highlight the significance of molecular transport properties on the decision whether or not an ignitable mixture can evolve into a propagating flame.