Ignition Combustion Research Articles

Enhancing ammonia engine efficiency through pre-chamber combustion and dual-fuel compression ignition techniques

In pursuit of unleashing ammonia’s potential in internal combustion engines, an extensive computational study was conducted to explore the effects of two advanced combustion techniques: pre-chamber combustion (PCC) and dual-fuel compression ignition (DF−CI) on a heavy-duty ammonia engine. In the PCC mode, hydrogen was introduced into the pre-chamber (PC) to generate distributed reacting jets, with a small energy fraction of 0.5%. The case featuring a slightly narrower PC throat (diameter of 5.4 mm) yielded the highest indicated thermal efficiency (ITE) of 50.4%, owing to the rapid distribution of reacting jets and advanced combustion phasing. Further increase in hydrogen energy fraction and reduction in throat diameter resulted in a lower ITE because of the enhanced wall heat transfer loss. The DF−CI mode also exhibited significant potential for achieving high efficiency. Through a systemic optimization of parameters such as diesel injection timing, spray included angle, injection pressure, swirl ratio, and piston shape, a significantly enhanced ITE of 50.3% was attained, 7.2% higher than the baseline scenario. The improvement was primarily attributed to the promoted mixing of diesel with air. Among the design parameters, the swirl ratio and spray included angle exhibited the most significant impact on the engine performance. Overall, both the PCC and DF−CI modes were found to yield high ITE and low ammonia emissions. However, because 20% of the diesel energy fraction was utilized to mitigate ammonia slip in the DF−CI mode, notably higher greenhouse gas emissions (carbon dioxide of 96 g/kW-h) were generated than in the PCC mode. In this regard, the ammonia-hydrogen PCC mode should be a more promising solution to achieve zero-carbon emissions for future ammonia engines.

Iron particles ignition in different hot coflow temperatures

Understanding the ignition of iron particle combustion in hot environments is critical for harnessing the use of metal fuels in clean energy production. In this research work, a new single particle burner is implemented to disperse and burn iron particles using hot air coflow. The burner disperses iron particles through Coulomb forces between two air-spaced capacitor electrodes. The oxidizer coflow is heated up as it passes through a newly designed induction gas heater. Electromagnetic induction is used to heat up multiple metallic and porous heat discs by Joule heating. When the coflow gas passes through the pores of the discs, it carries the heat generated within the porous discs and delivers it to the travelling iron particles which provides ultimate heat transfer effectiveness. Coflow temperature homogeneity is examined using Schlieren imaging and confirmed by thermocouple measurements at the outlet of the coflow tube. Particle ignition and combustion occur in open space, providing excellent conditions for optical diagnostics. Two synchronized high-speed cameras were used to simultaneously determine the particle size using green laser shadowgraphy and detect iron particles ignition. Iron particles ignition experiments were conducted for a sieved patch (45–53 μm) at different hot coflow temperatures starting at 700℃ up to 900℃. The burner showed extensive capabilities to maintains a stable and homogeneous hot coflow for a wide range of temperatures up to 900℃. Ignition experiments showed 5.67 % of ejected particles are burnt at 700 ℃, while full ignition (99.33 %) can be achieved at 900 ℃.

The thermal behavior during the co-combustion of bituminous coal and oil palm trunk hydrochars

ABSTRACT Utilizing oil palm trunk (OPT) residue as a co-fuel in coal-fired plants offers an environmentally favorable solution to agricultural waste disposal and fossil fuel combustion impacts. This study investigates the potential of hydrochar from OPT (HTC-OPT) as a coal-like solid fuel for co-combustion. The combustion characteristics of coal, HTC-OPT, and their blends (10–50% HTC-OPT) were analyzed via thermogravimetric analysis with heating rates ranging from 10 to 30°C/min. The Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) models were employed to explore co-combustion interactions and kinetic parameters. Results reveal enhanced combustion efficiency with HTC-OPT addition to coal. Higher HTC-OPT ratios correlate with increased maximum combustion rates, ignition and comprehensive combustibility indices, and decreased ignition and burnout temperatures. Additionally, blended fuels exhibit a proportionally synergistic co-combustion interaction. Both KAS and FWO models indicate similar trends in activation energies, with the lowest values (49.94 kJ/mol for KAS and 60.90 kJ/mol for FWO) at a 10% HTC-OPT ratio.

Poultry manure combustion parameters to produce bioenergy: A thermogravimetric analysis by isoconventional models and machine learning

Poultry manure has immense potential to generate bioenergy to supply some of producers’ electrical consumption. However, if desired, the combustion of this biomass requires the investigation of important thermal parameters for its efficient and automated industrial-scale application. This study determined combustion parameters such as ignition and burnout temperatures, activation energy, and combustibility indices by thermogravimetric analysis under two approaches: isoconventional models with a set of differential and integral equations and previously trained and validated machine learning models. Poultry manure combustion ignition and burnout temperatures were 212.28 and 443.94, 214.30 and 452.70, and 223.31 and 464.09 °C for 10, 25 and, 20 °C min−1 heating rates, respectively. The combustion index calculated from these temperatures was of 4.78, 6.13, and 4.22 × 10−7. Activation energy was calculated based on isoconventional models, whose values were 99.11 (Friedman), 117.78 (FWO), and 107.50 kJ mol−1 (KAS). Isoconventional models adequately represented mass degradation as a function of temperature. Moreover, after training and testing, our machine learning models (neural networks, decision trees, and random forests) satisfactorily described the combustion of poultry manure, enabling the optimization of parameters such as ignition and burnout temperatures. Poultry manure combustion showed interesting thermal characteristics that can be optimized by machine learning.

A Numerical Investigation on Laminar Flame Speed of Syngas in Air with Ozone Addition

In the context of energy and propulsion systems, in recent years engineering research has increased its efforts to develop more efficient technologies with a lower environmental impact. Specifically, new combustion systems have been investigated, such as ultra-lean combustion and Homogeneous Charge Compression Ignition (HCCI) combustion, and sustainable fuels have been employed. With respect to the latter option, syngas and hydrogen have emerged as attractive choices. However, the low specific energy of syngas and the presence of diluents can cause flame failure and instability. On the other hand, it is well known that ozone-assisted combustion enhances the Laminar Flame Speed (LFS), reduces the ignition delay time, and improves the flame stabilization of fuels such as methane, n-heptane and iso-octane, since it affects the chemistry in the Low Temperature Combustion (LTC) regime. In this context, the aim of the present work is to evaluate the effect of ozone on the LFS of syngas/ozone/air mixtures. Specifically, 1-D simulations have been carried out to compute the LFS of CO/H2 and CO/H2/N2 syngas mixtures. The model has been validated against experimental data available in the scientific literature by considering four different reaction mechanisms. Then, the model has been used to perform a parametric analysis by considering different conditions in terms of syngas composition (three different CO/H2 ratios) and equivalence ratio (from 0.5 to 5). The results show that the addition of 2500 ppm of ozone in air results in an increase in LFS for all equivalence ratios examined. The four different mechanisms give comparable results in terms of LFS and LFS increase for CO/H2 syngas, and are also able to predict ozone effect on LFS enhancement for CO/H2/N2 mixtures. Finally, the results show that the influence of ozone, in terms of LFS percentage increase, is greater for CO/H2 ultra lean mixtures with a lower CO/H2 ratio.

Numerical investigation for enhancing HCCI combustion characteristics in DI-CI engine fueled with butanol/diesel blends

The fast depletion of fossil fuels and the hazardous emissions associated with the use of fossil fuels in IC engines is driving the research in the area of renewable fuels. Homogeneous charge combustion ignition (HCCI) is a new mode of engine combustion technology, used for reducing the emissions without compromising the engine performance. The combination of HCCI and alcohol/diesel blends has the potential to simultaneously address the twin problems of depletion of fossil fuels and engine emissions. Butanol is an attractive biofuel, which is produced from agriculture wastes such as sugar cane bagasses and corn stalk etc. This paper presents numerical and modeling analysis on the emission and performance characteristics of a direct injection- compression ignition (DI-CI) engine operated with butanol/diesel blends. Numerical analysis was carried out using CONVERGE CFD software. Response surface methodology (RSM) was used in developing the response models for three output parameters, viz., nitrogen oxides (NOx), soot emissions and indicated specific fuel consumption (ISFC), in terms of the four input parameters, viz., compression ratio (CR), start of injection (SOI), fuel injection pressure (FIP) and exhaust gas recirculation (EGR). Numerical experiments were conducted by varying these four input parameters, i.e., CR from 14 to 19, FIP from 200 to 280 bar, EGR from 0% to 30% and SOI from 17° to 29° CA bTDC, and with four butanol/diesel blends (0, 20%, 30% and 40% of butanol-by volume, designated as Bu00, Bu20, Bu30 and Bu40). The optimum combination of the input parameters for the four test fuels were found with the objective of minimizing the three output parameters (i.e., ISFC, soot and NOx) using desirability approach. The homogeneity of the air-fuel mixture was estimated using equivalence ratio distribution index, i.e., Target fuel distribution index (TFDI). From the numerical analysis, it was found that the TFDI improved for the optimized case by 20.5%, 21.5%, 24.4% and 27.2% for Bu00, Bu20, Bu30 and Bu40 respectively as compared to their respective baseline configuration.

Effect of ammonia addition on combustion characteristics of hydrogen/air using passive turbulent jet ignition

Hydrogen (H2) and ammonia (NH3) are ideal carbon-free fuels, and the application of NH3/H2 in internal combustion engines offers the potential to reduce carbon emissions. And turbulent jet ignition (TJI) is an advanced ignition strategy that can effectively enhance the combustion. This work aims to experimentally investigate the effect of NH3 addition on the combustion characteristics of H2/air mixtures under passive TJI conditions. Combustion images and instantaneous pressure were collected for processing and analysis. The effect of NH3 substitution and equivalence ratio were investigated. The results indicate that the peak combustion pressure decreases with the addition of NH3, accompanied by longer combustion duration and ignition delay, as well as lower jet velocity and flame area growth rate. The hot jet will be quenched by the strong heat transfer, causing the change in the ignition mechanism as the NH3 substitution ratio increases. In addition, compared to lean conditions, the combustion appears to be less sensitive to the equivalence ratio on the rich side and always shows high jet velocity and flame propagation speed. However, a relatively small amount of NH3 addition changes the ignition mechanism on the rich combustion side compared with lean conditions, resulting in a longer ignition delay.

Experimental Study on the Impact of Hydrogen Injection Strategy on Combustion Performance in Internal Combustion Engines.

Hydrogen is an ideal alternative fuel for internal combustion engines due to its fast combustion rate and near-zero carbon emissions. To further investigate the impact of the injection strategy on combustion performance under various excess air coefficients and loads, experimental methods were employed to systematically study injection timing, injection pressure, and dual injection. The results demonstrate that appropriately delaying hydrogen injection timing can improve the thermal efficiency by up to 2.6% and reduce NOx emissions of equivalent combustion by nearly 88%. While increasing the hydrogen pressure to 8 MPa does not directly enhance the thermal efficiency and emissions, it can reduce the injection pulse width by approximately 75%, allowing for more flexibility in delaying the injection timing. Delaying the secondary end of injection (SEOI) leads to a reduction of over 47% in NOx emissions for λ (excess air coefficient) values of 1.5 and 1.0. The combustion duration and ignition delay initially increase and then decrease with the delay in SEOI, depending on the movement state of the fuel jet. With the increase in secondary injection proportion (SIP), brake thermal efficiency increases by no more than 1%, but NOx is reduced by more than 43% for λ values of 1.5 and 1.0. In the case of ultralean conditions (λ = 2.3), increasing the SIP from 0 to 30% results in a nearly 14% increase in the peak heat release rate (HRR) and a 19% reduction in combustion duration.

Construction of hollow heterogeneous microspheres containing energy storage fibers by electric spray to promote combustion of nano aluminum

To solve the problem of nano-aluminum (nAl) powder easy agglomeration and improve its thermal reaction, nAl was combined with adhesive (Viton A, F2602) and oxidizer (Ammonium perchlorate, AP) forming microspheres by electrostatic spray method. The morphology and structure were analyzed by SEM, EDS and XPS. The results showed that the microspheres (diameter≈Φ10–15 μm) were hollow structures formed by winding of fluoropolymer fibers, while AP and nano aluminum particles were embedded on the fibers. By controlling the electric spray process, the well-dispersed AP and nAl nanocrystals were assembled on the fiber. Thermal analysis indicated that the gas phase products decomposed by AP were absorbed by porous microspheres, and part of the heat was stored in polymer fibers, thus promoting the breakdown of the alumina shell and accelerating the pre-ignition reaction. The combustion and laser ignition experiments proved hollow heterogeneous microspheres have excellent thermodynamic properties. The assembled microspheres not only solved the problem of poor rheology caused by the agglomeration of nAl, but also improved the insufficient combustion of nAl and the condensation of products.

Probing the combustion kinetics of dimethylformamide: A theoretical study of H-atom abstraction reactions

The DMF vapor in the exhaust gas is needed to be recovered and incinerated to eliminate NOx emissions. This process will also provide the required heat for the production process. To verify the combustion chemistry of DMF in control of its ignition in fuel-lean combustion, 16H-atom abstraction reactions of DMF were systematically studied by high-level quantum chemical calculation in this work. The reactions include abstracting two types of H-atom from DMF molecule: H-atom of aldehyde and methyl groups respectively. Moreover, eight abstractors: Ḣ, ȮH, O2, ĊH3, CH3Ȯ, CH3OȮ, NO2, and HȮ2 were involved. In ab initio calculation, the M06-2X/6–311++G(d,p) method was used for geometry optimization, vibration frequency, and dihedral scan calculation. CCSD/cc-pVXZ (X = T, Q) method was employed for the single point energy (SPE) calculation. The rate constants of all abstraction reactions and the thermochemical quantities of the DMF and corresponding two radical products were carried out simultaneously. The calculation results show that abstracting the H-atom of the aldehyde group is dominant over the methyl group H-atom abstraction. Moreover, for both the aldehyde and methyl groups, H-atom abstraction, ȮH, Ḣ, and CH3Ȯ abstractors produce the fastest rates. From the application point of view, one detailed kinetic model of DMF was developed by adopting our calculation results. The model accurately predicts the ignition delay times (IDTs) of DMF at a wide range of temperatures and equivalence ratios. The sensitivity analysis indicates that the branching ratio of abstracting aldehyde group H-atom and methyl group H-atom is crucial for the reactivity prediction of DMF.

Numerical investigations on hydrothermal flame characteristics of water-cooled hydrothermal burner

Abstract Supercritical hydrothermal combustion, as a quick homogeneous oxidizing process, offers a promising treatment option for industrial wastewater. This paper established a computational fluid dynamics model of a water-cooled hydrothermal combustion burner to investigate the thermal flame characteristics. The effects of the fuel mass flow rate, fuel concentration, initial reactor temperature, reaction pressure, and oxidant temperature on the thermal combustion ignition were revealed. The results indicate that the fuel concentration (from 10 wt% to 60 wt%) and initial reactor temperature (from 623 to 773 K) had less effect on the ignition temperature. In contrast, the ignition temperature increases by 398 K with increasing fuel mass flow rate (from 24 kg h−1 to 1080 kg h−1). As the oxygen temperature increases (from 273 to 673 K), the ignition temperature gradually decreases to 573 K and then increases. An increase in reaction pressure can facilitate a decrease in ignition temperature to a certain extent, and the optimal reaction pressure is 25 MPa. This study provides a vital reference for a hydrothermal burner’s scale-up design and ignition operation.

Review and Perspectives of Novel Flue-Gas Internal Recirculation Combustion Technology for Low Nitrogen Emission: Fundamentals, Performance, Method, and Applications for Conventional Fossil Fuels and Sustainable e-Fuels

Novel flue-gas internal recirculation (FIR) is gaining increasing attention owing to its overwhelming stable combustion and nitrogen oxide (NOx) reduction advantages. This paper aims to reveal the nitrogen reduction mechanisms in the exciting new FIR methods and discuss the main FIR techniques in terms of combustion performance. In particular, the basic concept and the nitrogen reduction mechanism of FIR are first introduced. Besides, three main categories of FIRs are reviewed in accordance with the characteristics and operation status of different devices. The analysis of 16 FIR systems suggests that the recirculation space, effective recirculation rate, and combustion ignition limit must be considered during the design process, in order to balance NOx reduction and combustion efficiency. The integrated FIR and external FIR can achieve NOx emission reductions of at least 20% and 15.5%–20%, respectively. The embedded FIR exhibits more outstanding NOx emission reduction performance, with NOx reduction ratios of up to 26.09%–65%. In addition, the influence elements of FIR on combustion efficiency and nitrogen reduction are also discussed. In conclusion, strategies aimed at enhancing the combustion performance of novel FIR burners are thoroughly delineated, and the challenges that need to be addressed are succinctly highlighted in a forward-looking perspective. This review is expected to clarify the NOx reduction and combustion characteristics of current FIR techniques and suggest a development direction for FIR design.

Optical diagnosis study of fuel volatility on combustion characteristics of spray flame and wall-impinging flame

Finding suitable alternative fuels and reducing carbon emissions are important development directions for future engines. Many carbon-neutral fuels, such as alcohol fuels are widely used, and they are more volatile than traditional diesel fuel. Therefore, exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics and the development of efficient and clean combustion technology. In this paper, the flame structure, flame development characteristics and soot evolution of three fuels, including n-hexane, 74%v n-heptane mixed 26%v isooctane (PRF26) and 53%v n-dodecane mixed 47%v isooctane (D53) were tested, that the cetane number of three fuels is essentially same. High speed self-luminous imaging and RGB two-color method were used to investigated the effect of fuel volatility on combustion. Results can be seen as following. Firstly, during the early stages of n-hexane combustion, a blue premixed flame can be found, and there is a pale-yellow flame in the later stage. As the boiling point increases, the area of the blue flame in the first stage gradually decreases, the area of the yellow flame in the later stage gradually increases, and the spatially integrated natural luminosity (SINL) of flame gradually increases. In addition, as the boiling point increases, the initial spontaneous combustion ignition point is away from the nozzle, the ignition delay time reduces and flame area becomes larger. Furthermore, the flame of high boiling fuels result in higher soot emissions. Secondly, in the event of flame wall-impingement, the near-wall flame of the three fuels has higher flame SINL and more soot formation in comparison to the free spray flame. The flame development of the three fuels is similar. However, there are some differences in the flame brightness distribution. The bright flame areas in n-hexane and PRF26 are concentrated near the wall, while the bright flame area in the D53 flame is concentrated on the wall and the flame head area rolled up with a larger distribution area. The flame near the wall is impacted by the wall temperature that the lower wall temperature leads to prolonged fuel evaporation and mixing time, which increases the ignition delay time. In conclusion, fuel volatility has less influence on flame structure and development, but a significant impact on ignition delay time and emissions. Regardless of whether wall impingement occurs, higher volatile fuels can extend the ignition delay time, increase the flame area and decrease soot emissions from combustion.

Effect of red mud additive on the combustion performance of lignite and sulfur fixation behaviors

ABSTRACT To reveal the role of red mud (RM) as the catalyst during the combustion behavior of lignite, two kinds of RM from Bayer red mud (SR) and sintering red mud (GR) were chosen to investigate the catalytic combustion behaviors of lignite under different oxygen contents, the action mechanism and the sulfur fixation performance by the thermogravimetric analysis (TGA) and fixed-bed reactor. Flynn–Wall–Ozawa (FWO) method was used to study the combustion kinetics. The results showed that RM can promote the release of pyrolysis volatiles and significantly improve the ignition performance of coal. Moreover, the improvement effect of RM becomes more noticeable with the increase of O2 content. RM decreased the ignition and burnout temperatures of the char combustion. The types of RM exert different catalytic properties owing to different compositions. Under oxygen-rich conditions, the apparent activation energy in the early stage of coal combustion can be significantly reduced by RM. In addition, RM enhances the peak temperature during constant combustion of coal, improves combustion rate, shortens the burnout time, and remarkably reduces SO2 emission in exhausted gas. About 83.7% SO2 in combustion gas produced at 700°C was reduced at the SR addition of 25 wt%, indicating RM is a potential and cheap catalyst for high-effective coal combustion.

Temperature and flame structure imaging in kerosene swirl-stabilized spray flames at low air flow using TLAF and OH-PLIF

Air flowrate is an important parameter for airbrast nozzles and low air inlet flow rate in airbrast type combustors will lead to worse atomization, combustion instability, and increased pollutant emissions. Using non-linear excitation regime two-line atomic fluorescence (NTLAF) thermography imaging and planar laser-induced fluorescence of hydroxyl radical (OH-PLIF), temperature and flame structure in the near-nozzle area are investigated at different low air flow rate to study the combustion deterioration of kerosene swirl-stabilized spray flames. In this concerned area, combustion occurs mainly at the near-wall region but a few OH signals are present at the centerline region except for the kerosene LIF particles. With the air supply getting low, the near-wall OH signals get low, corresponding to the change of indium Stokes and anti-Stokes fluorescence. The temperature in the centerline region is presented to be reduced and the region for the temperature threshold filter expands, indicating the reduction of the downstream hot combustion products brought back by weak swirl, which is closely related to the extremely low air flowrate. Under extremely low air flow rate, the highly fluctuating temperature and weak gas-liquid mixed combustion at the centerline area may tend to cause combustion instability or ignition failure.

A phenomenological model for plasma-assisted combustion with NRP discharges in methane-air mixtures: PACMIND

The recent advances in plasma-assisted combustion must now be strengthened through high-fidelity numerical simulations. However, detailed plasma-combustion coupled simulations in complex geometry and turbulent flow involving hundreds of discharges are still unaffordable due to prohibitive computational costs. In this work, a Plasma-Assisted Combustion phenoMenological modelIng for Non-equilibrium Discharges (PACMIND) is proposed. The model aims to avoid computing in detail the plasma phase and consists of incorporating the leading order plasma effects on the reactive compressible Navier–Stokes equations. It consists of two ultra-fast effects with radical productions and gas heating, as well as a slower gas heating due to vibrational energy relaxation. Based on detailed kinetic simulations, the model has been applied to air and methane-air mixtures and contains respectively 5 species, 3 reactions and 12 species, 7 reactions. The model has been validated against experiments in air and detailed plasma-assisted combustion ignition simulations in methane-air at various equivalence ratios and reduced electric fields. Significant improvements regarding ignition delay times have been observed by comparison with an existing model. In addition, the model accounts for NOx production by plasma discharge and covers the full range of conditions encountered in a premixed flame. Hence, it opens the way to the simulation of realistic 3D turbulent configurations with minimal over-costs.

Numerical study of the effect of pressure on the combustion characteristics of ammonia/coal-derived syngas mixture under gas turbine operating conditions

This study investigates the influence of pressure variation on combustion properties, including laminar burning velocity and ignition delay time, as well as emissions in gas turbine conditions. Based on numerical simulations and using Matlab, the impact of pressure on the relationship between laminar burning velocity and free radicals, as well as the relationship between ignition delay time and laminar burning velocity, were analyzed. The results of Matlab curve fitting suggest that the decrease in free radical concentration due to increased pressure is the main cause of the reduction in laminar burning velocity. The presence of reactions such as H + O2 = O + OH and all ammonia-related reactions results in a linear relationship between the laminar combustion velocity and max(H) or max(NH2 + H), which varies with pressure in mixed ammonia combustion. Moreover, the linear correlation coefficient between the laminar combustion velocity and free radicals is a function of pressure. Using a one-dimensional laminar premixed model and a zero-dimensional closed reactor model, this study has identified a power function relationship between the laminar combustion velocity and ignition delay time in synthetic gas mixed with ammonia combustion, where the coefficients and exponents of the power function are functions of pressure. Moreover, these two fitting relationships are independent of equivalence ratio and fuel ratio. Finally, the analysis of NO emissions indicates that the NO produced by chemical pathways affects the chemical equilibrium of NO produced by thermal pathways. Additionally, the reaction CO + OH = CO2 + H initially produces some NO in the thermal pathway·NH3 participates in reactions after H2 and CO have reacted. Lastly, it was found that the increase in pressure promotes the reaction NH + NO = N2O + H, which leads to a decrease in the molar fraction of NO with increasing pressure.

A Comprehensive Review of the Free Piston Engine Generator and Its Control

Review A Comprehensive Review of the Free Piston Engine Generator and Its Control Patrick Brosnan 1, Guohong Tian 1, * , Umberto Montanaro 1, and Sam Cockerill 2 1 School of Mechanical Engineering Sciences, University of Surrey, Surrey GU2 7XH, UK 2 Libertine FPE Ltd., Sheffield S9 1DA, UK * Correspondence: g.tian@surrey.ac.uk; Tel.: +44-1483-300-800 Received: 2 February 2023 Accepted: 10 March 2023 Published: 23 March 2023 Abstract: The Free Piston Engine (FPE) is a unique machine with a higher thermal efficiency than its counterpart, the Conventional Reciprocating Piston Engine (CPE). The unique piston motion of the FPE is not constrained kinematically like the CPE with its connecting crankshaft and rotational inertial masses. Moreover, when directly coupled to the Linear Electric Machine (LEM) to harness electric energy production, the Free Piston Engine Generator’s (FPEG) characteristic motion, now being dynamically constrained, permits an extensive range of piston trajectory profiles to be exploited during operation. In addition, exploring varied piston trajectories during the development stages may be vital in reducing in-cylinder combustion emissions through strategies such as Low-Temperature Combustion (LTC) and Homogeneous Charge Compression Ignition (HCCI). This review paper will focus on the key motivations and drivers for continued FPEG development. It will also highlight and review its distinct advantages and challenges in being a viable solution as a future zero-carbon engine technology. Finally, FPE fundamentals, alongside its rich history, will be introduced, clearly presenting how academia and industry have described and controlled its intrinsic non-linear dynamics.

Combustion characteristics and concentration measurement of ADN-based liquid propellant with electrical ignition method in a combustion chamber

An experimental study was performed to investigate combustion characteristics of ammonium dinitramide (ADN) based liquid propellant with electrical ignition method in a combustion chamber. The experiment was conducted in different gas atmospheres such as argon, air and nitrous oxide under an on-load voltage range of 60–100 V. The influence of the gas atmosphere, on-load voltage on the ignition delay time, peak combustion pressure, ignition energy, total energy and key reaction products mole fractions were studied. The ignition delay time of propellant decreases gradually with the increase of ignition voltage, while the ambient gas has little effect on the ignition delay time because the flame appears inside the propellant earliest. The peak combustion pressure of propellant is highest when the ambient gas is nitrous oxide, whereas lowest in argon atmosphere. The ignition energy and total energy are insensitive to gas atmosphere and decrease gradually with the increasing on-load voltage. The concentration of N2O increases with the increase of on-load voltage. The mole fraction of CO decreases with the increase of on-load voltage in general, especially obviously at relatively high on-load voltage of 100 V.

Formation of products upon ignition, combustion and melting of mixtures of high-entropy alloy FeNiCoCrCu with titanium and carbon

The dependence of the ignition temperature, combustion rate and composition of the resulting products on the concentration of Ti + C in mixtures with powder of a high-entropy alloy (HEA) FeNiCoCrCu and the initial mixture of metals forming it (MIX) has been studied. HEA was obtained by mechanical activation (MA) of a mixture of metal powders in argon. At the melting temperature, the high-entropy FeNiCoCrCu alloy decomposes into several phases, but the basis of the HEA alloy, as well as the alloy obtained by melting and crystallizing MIX, is a 5-component phase with an average formula Cu1.2Fe1.4Ni1.4Co1.4Cr. In addition, 5, 4, and 3-component phases with averaged formulas Cu2Ni2Co2Fe2Cr, Cu3Ni3Co2.9Fe2.5Cr, Cu4.8Ni4.5Co4.6Fe4.2Cr, Cu40Fe2Ni4Co2C, Cr12.5Fe3.2Co2.6Ni and Co3.2 Fe3,5 Cr are present in small amounts in the binder. Experiments on the ignition and combustion of mixtures of MIX and HEA with Ti + C were carried out in argon at atmospheric pressure. The combustion rate, ignition temperature, and maximum temperature reached in the thermal explosion of MIX and HEA mixtures with Ti + C increase with increasing Ti + C concentration. Due to the low exothermicity of the mixtures, the experiments were carried out at an initial temperature of 500 °С. At this initial temperature, the combustion limit of the samples occurs when the Ti + C concentration in the HEA and MIX mixtures is less than 30 %. Based on the results of scanning electron microscopy, the volume concentration of the number of titanium carbide (TiC) particles in molten samples was calculated. In an alloy with a HEA binder, the number of TiC particles per unit volume is 1.5-3.0 times greater than in an alloy with a MIX binder, and their size is correspondingly smaller. With an increase in the concentration of Ti + C from 30 to 40 % in a mixture with HEA, the number of TiC particles per unit volume decreases. In a mixture with MIX, the number of TiC particles per unit volume passes through a minimum. This is due to two opposite processes: on the one hand, the probability of the generation of TiC particles increases, on the other hand, their coagulation occurs.