Fuel-air Equivalence Ratio Research Articles

Experimental and simulation study on lean combustion characteristics of passive pre-combustion chamber ignition boosted GDI engine

In order to verify the potential application of passive pre-combustion chamber technology in commercial GDI engines, the efficiency characteristics, combustion characteristics (CA50, Knock Index and COV) and emission characteristics of a pre-combustion chamber ignition engine in a boosted environment were studied by three-stage injection strategy to form a lean burn environment (λ = 1.3). A 3D simulation model of pre-combustion chamber ignition engine is established. Combined with the engine bench test results, the process of intake, mixture formation, combustion and emission generation under different working conditions is simulated by simulation method. The development process of TKE (turbulent kinetic energy), air-fuel ratio, temperature and combustion emissions in the main/pre-combustor is studied. It is found that under high load conditions in boosted pre-combustion chamber engine, the average knock index can be controlled between 2 and 3.4 bar, and the COV can be controlled between 3% and 3.5%. The indicated thermal efficiency reaches over 40% within the load range of IMEP from 10 to 14 bar. When the load increases, NOx and soot emissions show a downward trend, the HC and CO emissions slight increase. The pre-combustion chamber engine exhibits a tumble intake pattern around the cylinder axis perpendicular to the cylinder axis. The TKE reaches its maximum value 20 CAD before the top dead center and then rapidly decreases. The use of a three-injection strategy can form a uniform stratified mixture at the ignition moment. The fuel air equivalence ratio in the ignition area of the pre-combustion chamber is 0.8–0.9, slightly higher than the average fuel air equivalence ratio of 0.75.

Estimation of the Global Equivalence Ratio of a Swirl Combustor from a Small Number of Absorption Spectra Using Machine Learning.

A new optical diagnostic method that predicts the global fuel-air equivalence ratio of a swirl combustor using absorption spectra from only three optical paths is proposed here. Under normal operation, the global equivalence ratio and total flow rate determine the temperature and concentration fields of the combustor, which subsequently determine the absorption spectra of any combustion species. Therefore, spectra, as the fingerprint for a produced combustion field, were employed to predict the global equivalence ratio, one of the key operational parameters, in this study. Specifically, absorption spectra of water vapor at wavenumbers around 7444.36, 7185.6, and 6805.6 cm-1 measured at three different downstream locations of the combustor were used to predict the global equivalence ratio. As it is difficult to find analytical relationships between the spectra and produced combustion fields, a predictive model was a data-driven acquisition. The absorption spectra as an input were first feature-extracted through stacked convolutional autoencoders and then a dense neural network was used for regression prediction between the feature scores and the global equivalence ratio. The model could predict the equivalence ratio with an absolute error of ±0.025 with a probability of 96%, and a gradient-weighted regression activation mapping analysis revealed that the model leverages not only the peak intensities but also the variations in the shape of absorption lines for its predictions.

Comparative Analysis Of Cycle-To-Cycle Variabilities And Combustion Development In An Optical Spark-Ignition Engine Fueled By Pure Hydrogen And Propane: Insights From Chemiluminescence and PI

Hydrogen, derived from renewable sources and devoid of carbon emissions, is a pivotal energy carrier for the future. Representing a viable substitute for fossil fuels in internal combustion engines (ICEs), contemporary studies advocate using very-lean and ultra-lean hydrogen-air mixtures, with a fuel-air equivalence ratio below 0.5, as a potent strategy to reduce NOx emissions in hydrogen-fueled ICEs. An experimental setup with an optical spark-ignition engine was devised to investigate the primary factors influencing cycle-to-cycle variations in H2ICEs by optimizing mixture homogeneity and focusing the study on flame interaction with in-cylinder aerodynamics. Simultaneous in-cylinder pressure, chemiluminescence, and PIV analyses were performed to characterize and compare the behaviors of ultra-lean hydrogen-air and propane-air flames, assessing flame development and cyclic variation features. Results indicate that the flame development of hydrogen and propane is significantly different. Propane exhibited increased in-cylinder pressure trace variability at lean and rich flammability limits with a high flame development difference between fast and slow cycles. In contrast, hydrogen-air mixtures under various equivalence ratios (from very-lean to ultra-lean) presented much more stable in-cylinder pressures. Furthermore, fast propane flames were mainly advected to the cylinder's symmetrical axes, while fast hydrogen flames started further away from the spark plug. Horizontal and vertical PIV measurements showed a single structure flow field over the studied conditions with mainly intensity variations over the measured cycles. Fast flames were associated with more intense tumble motion. The differences between fast and slow flames for hydrogen are attributed to the early flame development. However, after initial differences in flame development over several crank angles, there are similarities in all cycles, suggesting that the low variability in in-cylinder pressures is mostly due to the robustness of hydrogen flame ignition and its independence from in-cylinder flow small variations at the early development phase.

Optimized Emission Analysis in Hydrogen Internal Combustion Engines: Fourier Transform Infrared Spectroscopy Innovations and Exhaust Humidity Analysis

<div>In today’s landscape, environmental protection and nature conservation have become paramount across industries, spurring the ever-increasing aspect of decarbonization. Regulatory measures in transportation have shifted focus away from combustion engines, making way for electric mobility, particularly in smaller engines. However, larger applications like ships and stationary power generation face limitations, not enabling an analogous shift to electrification. Instead, the emphasis shifted to zero-carbon fuel alternatives such as hydrogen and ammonia. In addition to minimal carbon-containing emissions due to incineration of lubricating oil, hydrogen combustion with air results in nitrogen oxide emissions, still necessitating quantification for engine operation compliance with legal regulations. A commonly used multicomponent exhaust gas analyzer on FTIR principle can suffer from higher volumetric water shares in the exhaust gas of the hydrogen engine, influencing the emission analysis. This concern prompted the development of a new evaluation approach for hydrogen operation, analyzing unique wavelength bands for hydrogen operation while considering the higher volumetric water shares in the exhaust gas of a hydrogen engine and its missing carbonaceous emissions. The method’s capability of providing more credible results for hydrogen-powered engines is demonstrated by assessing the newly introduced hydrogen method through variations of the indicated mean effective pressure, the air–fuel equivalence ratio, and the intake air humidity. Presuming minimal CO<sub>2</sub> emissions, the method allows a more realistic allocation of absorption spectra to other emissions. In addition to investigations on the new hydrogen evaluation method, a model for calculating the volumetric water share in the hydrogen engine’s exhaust gas is presented. By comparing the theoretical to the measured water share, the hydrogen emissions of the engine can be calculated without the need for additional hydrogen slip measurement.</div>

Potential Analysis of Defossilized Operation of a Heavy-Duty Dual-Fuel Engine Utilizing Dimethyl Carbonate/Methyl Formate as Primary and Poly Oxymethylene Dimethyl Ether as Pilot Fuel

<div>This study demonstrates the defossilized operation of a heavy-duty port-fuel-injected dual-fuel engine and highlights its potential benefits with minimal retrofitting effort. The investigation focuses on the optical characterization of the in-cylinder processes, ranging from mixture formation, ignition, and combustion, on a fully optically accessible single-cylinder research engine. The article revisits selected operating conditions in a thermodynamic configuration combined with Fourier transform infrared spectroscopy.</div> <div>One approach is to quickly diminish fossil fuel use by retrofitting present engines with decarbonized or defossilized alternatives. As both fuels are oxygenated, a considerable change in the overall ignition limits, air–fuel equivalence ratio, burning rate, and resistance against undesired pre-ignition or knocking is expected, with dire need of characterization.</div> <div>Two simultaneous high-speed recording channels granted cycle-resolved access to the natural flame luminosity, which was recorded in red/green/blue and OH chemiluminescence.</div> <div>Selected conditions were investigated in more detail with the simultaneous application of planar laser-induced fluorescence of OH and HCHO and recording natural flame luminescence in a cycle-averaged manner.</div> <div>Poly oxymethylene dimethyl ether was used as pilot fuel, building on prior investigations. The mixture of 65 vol% Dimethyl Carbonate and 35 vol% Methyl Formate with prior verification on a passenger-car-sized engine substitutes synthetic natural gas in this study.</div> <div>Thermodynamically, the increased compression ratio up to 17.6 resulted in feasible operation and increased indicated efficiency. On the lower compression ratio of 15.48, a more comprehensive range of applicable air–fuel equivalence ratios and increased degrees of freedom regarding the pilot’s total energy share are observed compared to the base configuration with natural gas and EN590 as pilot fuel.</div> <div>The air–fuel equivalence ratio sweep from λ = 1.0–2.0 revealed predominantly premixed and high-temperature heat release via OH*. The temporal and spatial evolution shifts while leaning out the mixture with increasing gradients on the radial distribution and decouples for lean mixtures from the initial spray trajectory.</div>

A numerical procedure to obtain ideal piston trajectories and key design parameters for a hydrogen-fuelled resonating free piston generator

A new numerical procedure is presented to enable a hydrogen-fuelled two-stroke resonating free-piston generator target ideal Otto cycles to provide tracking paths for feedback control to maximise efficiency. Piston trajectories and design parameters are obtained for specified power, compression-ratio, and air-fuel-ratio. These parameters include scavenge port locations, heat release position, equivalent bottom-dead-centre position, and an electrical machine constant which is generally power-switched to take two values within a cycle. The main objectives of the paper are to develop the numerical procedure, and to demonstrate its use in finding a generator with maximum efficiency, and a generator with a single electrical machine constant, avoiding in-cycle power switching. To develop the procedure, a multi-physics generator model provides kinematics to an energy equation, culminating in a novel two-stage procedure. Stage-1 involves iteration and minimisation; Stage-2 just involves minimisation. Testing by simulation involves a 14-kW generator case study with compression ratio of 9, lean air-fuel equivalence ratio of 0.4, plus friction and electrical machine losses. An overall maximum generator efficiency of 54.9% is obtained using the procedure, compared to 51.3% without power switching. Computationally, the two-stage procedure proves highly efficient, typically taking around 11 min in total to complete on a desktop computer. The novelty of the procedure is its ability to optimally design and operate a hydrogen-fuelled resonating free-piston generator to meet a target specification, thereby offering a potentially inexpensive way to decarbonize transport, particularly in heavy duty applications.

Experimental Investigation of a Pulsation Reactor via Optical Methods

Material treatment in pulsation reactors (PRs) offers the potential to synthesize powdery products with desirable properties, such as nano-sized particles and high specific surface areas, on an industrial scale. These exceptional material characteristics arise from specific process parameters within PRs, characterized by the periodically varying conditions and the resulting enhanced heat and mass transfer between the medium and the particulate material. Understanding flame behavior and the re-ignition mechanism is crucial to controlling the efficiency and stability of the pulse combustion process. In order to accomplish this objective, an investigation was conducted into flame behavior within the combustion chamber of a Helmholtz-type pulsation reactor. The study was focused on primarily analyzing the flame propagation process and examining flame velocity throughout the operational cycle of the reactor. Two optical methods—natural flame luminosity (NFL) and particle image velocimetry (PIV)—were applied in related experiments. An analysis of the NFL measurement data revealed a correlation between the intensity of light emitted by the pulsed flame and the air-fuel equivalence ratio (range from 0.89 to 2.08 in this study). It is observed that a lower air-fuel equivalence ratio leads to higher flame luminosity in the PR. In addition, in order to study the parameters related to system stability and energy transfer efficiency, this study also focuses on the local velocity field measurement method and an example of a fluid flow result in a combustion chamber by using a phase-locked PIV measurement system upgraded from a classic PIV system. The presented results herein contribute to the characterization of flame propagation within a pulsation reactor, as well as in pulsatile flows over one working cycle in a broader context, with flow velocity in the center of the combustion chamber ranging from 1.5 m/s to 5 m/s. Furthermore, this study offers insights into the applicable experimental methodologies for investigating the intricate interplay between flames and flows within combustion processes.

Early flame development characterization of ultra-lean hydrogen–air flames in an optical spark-ignition engine

Hydrogen-fueled internal combustion engines (H2ICEs) represent a promising technology for decarbonizing the transport sector. In pursuit of this goal, research is needed on lean hydrogen–air combustion under engine conditions aiming to mitigate NOx emissions. However, there is a limited amount of experimental data focusing on flame development at high pressure and temperature, especially in ultra-lean mixtures, which are recently considered to play an important role for future H2ICEs. This study investigates the behavior of very-lean to ultra-lean hydrogen–air flames (0.20 <ϕ< 0.55) in an optical spark-ignition engine, focusing on the early flame development phase. Fuel–air equivalence ratio and engine speed (900 <N< 1500 rpm) effects on hydrogen combustion are addressed with pressure-based heat release analysis and chemiluminescence flame imaging techniques. The findings revealed an intrinsic association between these two approaches. Hydrogen flames development measured before 2% of Mass Fraction Burned (MFB) strongly correlates with pressure-based quantities up to 50% MFB, underscoring the critical role of understanding the early flame phase for predicting total combustion duration and in-cylinder pressure traces. A model for hydrogen turbulent flames in the initial combustion stages is formulated based on the Zimont equation and the experimental data. An equation for 0D/1D combustion duration prediction up to 10% MFB is proposed, demonstrating its validity from the wrinkled flame to the well-stirred reactor premixed combustion regimes. Additionally, flame chemiluminescence images for each zone within the Peters–Borghi diagram are presented, offering valuable insights into the diverse characteristics of these flames.

Numerical modelling and simulation of hydrogen and air mixing to prevent ignition delay and flashback

Practically all residential and commercial gas appliances installed within the EU today were designed for operation with natural gas. A clean and efficient solution for heating and hot water generation is the combustion of hydrogen in case of a gas condensing boiler. Safe and stable combustion of hydrogen is a complex issue, and several influence parameters must be understood for the safe design of hydrogen capable gas condensing boilers. In premixed hydrogen-air combustion, there are two physical problems that should be avoided at all costs: flashback through the burner to the premixing duct and ignition delay in the combustion chamber. Well mixing of reactants is therefore very important to achieve stable and efficient hydrogen combustion. To evaluate the influence of commercialized mixing stages (fan-venturi combination), the impact of the rotational velocity on the degree of mixing in case of a venturi-fan combination used in domestic gas condensing boilers is presented in this paper. Transient, three-dimensional simulations with different turbulence modelling approaches were performed to assess the degree of mixing upstream the hydrogen capable multi-hole burner. It is shown that the lower angular velocities produce better mixing. It can also be assumed that a local variation of up to 17% in the adiabatic flame speed can be expected due to the mixing processes as the consequence of the local air-fuel equivalence ratio variation.

Pellet Combustion in Stove: Perfomance and Emissions Statistical Approach

Small scale stoves, producing heat and hot water, are suited for domestic purposes. A pilot 24 kWt fixed bed pellet stove has been developed for the study of pellet combustion in laboratory. Its design and construction was done in order to permit its maximum flexibility for the simulation of different stove configurations and operational circumstances. Temperatures, pressures, flows and gas compositions are measured and analysed. An extensive study of different load conditions is presented through the application of an experiment design technique and a subsequent statistical analysis of the results. It is shown that the main features of the plant are controlled mainly by fuel feed rate and by air-fuel equivalence ratio, being very low influenced by the temperature of the water. Very good correlation between the studied parameters have been found together with a great agreement between the predicted and the measured values.

Control device for improving swirl flame stabilization.

Aiming to improve the stabilization of unstable swirling turbulent premixed flames, an actively controlled swirler and electrical hardware and control software are developed, implemented, and tested in the present study. Stereoscopic particle image velocimetry is performed to calculate the swirl number and study the flame stabilization. A mixture of methane and air with a mean bulk flow velocity of 5.0m/s and a fuel-air equivalence ratio of 0.6 is examined. This test condition, along with an original swirler vane angle of 30°, led to the initial anchoring of an unstable premixed flame at the burner exhaust. The developed actuation mechanism allows for changing the swirler vane angle from 30° to 60° and back to 30°. Increasing the vane angle from 30° to 60° increases the swirl number to relatively large values, which leads to the formation of a recirculation zone, a downward velocity along the burner centerline, and, as a result, the stabilization of an M-shaped flame. After the vane angle is reduced to 30°, the swirl number decreases but remains relatively large compared to its original value. As a result, the recirculation zone is present at the end of the actuation process and a V-shaped flame is stabilized. Improving the stabilization of the swirl flames is made possible because of the control apparatus and the method developed in the present study. The apparatus and technique designed and tested in this study facilitate the development of robust tools for improving the stabilization of swirling premixed flames in gas turbine combustors.

Environmental and second law analysis of a turbojet engine operating with different fuels

Fuels such as kerosene, diesel No. 2, and JP-4 are extensively utilized in aviation engines. This study conducts a comprehensive comparison of these fuels within the context of a turbojet engine modeled using GasTurb. While key operational parameters like Mach numbers and total pressure ratios are held constant, variations are introduced in the fuel-air equivalence ratio (ϕ) and the pressure ratio of the high-pressure compressor in the turbojet engine. The results of these analyses show that kerosene exhibits the most favorable operational range, resulting in the lowest production of nitrogen oxides (NO). Conversely, diesel fuel displays a broader operational range with higher NO production. Moreover, diesel fuel yields greater CO2-equivalent emissions during combustion due to elevated levels of both CO2 and NO emissions. On the other hand, kerosene consistently produces the lowest CO2-equivalent emissions across all scenarios. Despite kerosene exhibiting relatively lower exergy efficiency and the highest exergy destruction among the three fuels, it emerges with the lowest total and specific environmental pollution costs. By analyzing a range of parameters and incorporating environmental considerations, this study identifies kerosene as the optimal choice for minimizing emissions and mitigating the environmental impact in aviation applications.

Flame Propagation in Blends of R-152a, R-134a, and R-1234yf with Air

ABSTRACT Some new, low-global warming potential refrigerants will be flammable, and the laminar burning velocity is a useful parameter for quantifying fire risk. Laminar burning velocity measurements have been made using a constant volume experiment with dry air and the refrigerant R-152a (CH3CHF2), pure and blended with R-134a (CH2FCF3), or R-1234yf (CF3CFCH2). The resulting burning velocity data deduced from the pressure rise in the chamber are presented for a range of fuel air equivalence ratio and loading of the less flammable refrigerant, for unburned gases at 298 K and 101 kPa as well as at 375 K and 253 kPa. For comparison, the 1-D, planar laminar burning velocity was numerically simulated using a recently developed kinetic mechanism that includes a wide range of refrigerants with air. The predicted burning velocities agree reasonably well with the experimental values, and the numerical results are used to understand the kinetic mechanism of the reaction of the refrigerants. Uncertainties in the experimental data from radiation heat losses as well as extrapolation to ambient conditions are explored.

Relations between the number of spray droplets and chemiluminescence for Jet A-1 flames stabilized in a gas turbine model combustor

The spatial and temporal relations between the flame chemiluminescence and the number of droplets for spray flames stabilized in a gas turbine model combustor are investigated experimentally. Pressure measurements, Interferometric Laser Imaging for Droplet Sizing, shadowgraphy, as well as flame chemiluminescence and Mie scattering imaging are performed. Test conditions corresponding to premixed methane and air, Jet A-1 spray, and Jet A-1 spray in premixed methane and air flames are examined. For all test conditions, the fuel-air equivalence ratio is 0.6, and the fuels and air flow rates are adjusted to generate a fixed nominal power of 10 kW. It is shown that, compared to the Jet A-1 spray flames, the dual-fuel flames feature smaller plenum pressure fluctuations. The droplet sizing suggests that, compared to Jet A-1, the dual-fuel spray is poorly atomized, leading to the generation of large droplets. Spectral analysis shows that, although the tested flames are not thermoacoustically excited, an injector-induced instability exists. The spatial relations between the flame chemiluminescence and the spray number density shows the coupling between these parameters is driven by the spray dynamics for the Jet A-1 flames; however, this coupling is driven by methane combustion for the dual-fuel test condition. A time-lag-based linear-regression model is developed and used to investigate the temporal relations between the flame chemiluminescence and the spray number of droplets for Jet A-1 spray flames. It is obtained that the oscillations of the number of spray droplets lead those of the flame chemiluminescence. The findings suggest that, in addition to the several spray characteristics reported in the literature, the droplets number density plays an important role in elaborating the coupling between the spray and flame chemiluminescence.

Gas-Dynamic Interactions between Pre-Chamber and Main Chamber in Passive Pre-Chamber Ignition Gasoline Engines

&lt;div&gt;Pre-chamber turbulent jet ignition (TJI) is a method of generating distributed ignition sites through multiple high-speed turbulent jets in order to achieve an enhanced burn rate in the engine cylinder when compared to conventional spark plug ignition. To study the gas-dynamic interactions between the two chambers in a gasoline engine, a three-dimensional numerical model was developed using the commercial CFD code CONVERGE. The geometry and parameters of the engine used were based on a modified turbocharged GM four-cylinder 2.0 L GDI gasoline engine. Pre-chambers with nozzle diameters of 0.75 mm and 1.5 mm were used to investigate the effect of pre-chamber geometry on pre-chamber charging, combustion, and jet formation. The local developments of gas temperature and velocity were captured by adaptive mesh refinement, while the turbulence was resolved with the k-epsilon model of the Reynolds averaged Navier–Stokes (RANS) equations. The combustion process was modeled with the extended coherent flamelet model (ECFM). Data from engine experiments were compared with the computed main chamber pressures and heat release rates, and the results show good consistency with the model calculations. The scavenging and air–fuel equivalence ratio (λ) distribution of the pre-chambers improved with the larger nozzle, while the smaller nozzle generated jets with higher velocity, greater turbulence kinetic energy, and longer penetration length. Moreover, after the primary jet formation, secondary pre-chamber charging, combustion, and secondary jet formation were observed.&lt;/div&gt;

Design, construction and validation of a simple, low-cost phi meter

The best correlation between the yields of the major toxicants in fire smoke and the fire condition is obtained by expressing the ventilation in terms of the equivalence ratio, phi. Phi meters allow this fuel-air equivalence ratio to be determined during a large-scale fire test. The original design used a platinum catalyst, a large furnace, and required pure oxygen. This work aimed to make a simple, low-cost device which could monitor the equivalence ratio during a large-scale fire test. The current design produces the same quality of measurement with a much smaller footprint and throughput. Benefits include enhanced portability, reduction in sample gas cleaning and drying requirements and lower cost. The work showed that normal furnace components (alumina and silica) provide suitably catalytic surfaces at 900 °C eliminating the requirement for platinum catalysts and the use of pure oxygen. The ISO/TS 19700 steady state tube furnace (SSTF) was used to validate the phi meter measurements as it can both pre-set and independently quantify the equivalence ratio during a test. Long sample collection times were overcome with a larger sampling pump and effluent being split between the phi meter furnace and the exhaust. It is hoped that this simpler, optimized apparatus will encourage more widespread use and lead to better prediction of smoke toxicity.

Effect of 2-Butanone addition on laminar burning velocity of gasoline XP95 at higher mixture temperatures

2-Butanone, a product of pyrolysis of biomass, micro-biological fermentation of agricultural waste, possesses several advantages as a next generation biofuel for spark ignition engines. Very little is known about its combustion behavior due to non-availability of detailed combustion measurements. In this study, the laminar burning velocities (LBV) of 2-Butanone were measured and compared with benchmarked ethanol and gasoline using the externally heated diverging channel method. The measurements were conducted for 2-Butanone (B), premium gasoline (XP95) and their three different fuel blends (volume fraction: 10% B + 90% XP95, 30% B + 70% XP95, and 50% B + 50% XP95) for fuel-air equivalence ratios (ϕ) = 0.6–1.4, at elevated mixture temperatures of 300–620 K, and atmospheric pressure (1 bar). The present measurements of 2-Butanone show a good match with the values reported in literature and exhibit a good consistency with the predictions of Hemken 2017 reaction mechanism. The sensitivity analysis reveals that the inhibition effect of the key reaction R43: CH3 + H (+M) ⇔ CH4 (+M) is reduced by 34%, when the mixture temperature increased from 305 K to 620 K at ϕ = 1.1 and increased by 39% with an increase in mixture equivalence ratio from 0.7 to 1.3 at 620 K. The LBV of 2-Butanone shows a slight difference with ethanol and gasoline for leaner mixtures (ϕ = 0.6, 0.7). However, with increase in mixture strength (ϕ ≥ 0.8), the LBV becomes significantly higher than that of gasoline XP95 and slightly lower than ethanol regardless of the mixture temperature. With addition of 2-Butanone in gasoline XP95, the LBV improved by 14%, 24.6%, and 36.1% for 10% B + 90% XP95, 30% B + 70% XP95, and 50% B + 50% XP95 blends respectively, at ϕ = 1.0, 620 K. From the improved flame behavior of 2- Butanone under typical engine operating conditions against benchmarked ethanol, makes it a suitable biofuel substitute for gasoline in a sustainable transportation application.

A novel multiple spark ignition strategy to achieve pure ammonia combustion in an optical spark-ignition engine

Ammonia is an attractive carbon-free fuel that has the potential to reduce the need for conventional hydrocarbons (HC) and reduce emissions of undesirable pollutants such as CO, CO2, particulates, and unburned hydrocarbons. However, ammonia has different combustion characteristics than conventional HC fuel. Ammonia is difficult to ignite and has a low combustion rate, resulting in large cyclic variations. In addition, nitrogen oxides (NOx) emissions in the exhaust tract are a major challenge when using ammonia as a fuel directly in engines. In this study, the effect of multiple spark ignition sites on the combustion of pure ammonia in an optical spark ignition engine (SI) was investigated. The experiment was conducted with four spark plugs mounted equidistantly on a special metal liner and one spark plug fitted at the top of the cylinder head. The multiple flames emitted from the different spark ignition sites were captured by natural flame luminosity (NFL) imaging. In the results, the conventional single spark ignition is compared with multiple ignitions. It was found that single spark ignition resulted in lower in-cylinder pressure, longer combustion duration, and higher combustion instability due to the poor ammonia fuel combustion rate. However, firing multiple spark plugs significantly improved combustion stability, increased engine power, and shortened the combustion period under the same operating conditions. In addition, the flame kernels produced by multiple ignition sites resulted in higher NOx emissions in the exhaust tract due to the higher temperatures in the cylinder. In addition, this study also investigated the effect of three different air–fuel equivalence ratios (λ) of 1.0, 1.2, and 1.4 on the combustion characteristics of ammonia fuel. The maximum NOx level was obtained for λ: 1.2 because the excess air in the mixture oxidizes the ammonia and provides abundant oxygen to generate more NOx. Further reducing the ammonia fuel and increasing the excess air to λ: 1.4 dramatically reduced NOx emissions due to the lower pressures and temperatures in the cylinder resulting from the longer combustion period.

Impact of H2/CH4 blends on the flexibility of micromix burners applied to industrial combustion systems

The present paper investigates the feasibility of using H2/CH4 fuel blends in micromix-type burners applied to industrial combustion systems. The micromix burner concept, characterised by the formation of miniaturised and compact turbulent diffusion flames, was developed for gas turbine hydrogen burners showing low NOx emissions (below 10 ppm) without flashback risk, which represent the main issues when using pure hydrogen or hydrogen enriched natural gas blends as fuel, making it a promising concept to be applied in industrial burners. The study was carried out through numerical CFD simulations, accounting for detailed chemistry calculations of turbulent micromix flames and previously validated through experimental measurements in a laboratory-scale micromix burner prototype. The resulting flow, temperature and exhaust emission characteristics for three H2/CH4 fuel blends with H2 content of 90, 75 and 60% respectively were analysed and discussed for air-fuel equivalence ratios at λ=1.8 and 1.6 (lower than the well-characterised air-fuel equivalence ratios in micromix gas turbine burners at λ=2.5 and closer to current industrial burners), considering an energy density of 14 MW/m2 bar. Numerical results showed low fuel flexibility for industrial-scale micromix burners, with still low NOx emissions (12–85 ppm) but relatively high CO emissions (448–4970 ppm) for the considered blends and λ values. The lowest CO emissions were given together with jet penetration phenomena, ruling out the feasibility of these design points due to the greater importance of the latter phenomenon.

Simulation Investigation on the Effects of EGR Ratio and Nozzle Angle on In-Cylinder Combustion and Pollutant Generation of PODE/Methanol Blends

ABSTRACT In order to explore effects of coal-based oxygenated fuels polyoxymethylene dimethyl ether (PODE) and methanol on pollutant emissions of diesel engines, in-cylinder combustion numerical model was developed by coupling PODE/methanol reaction mechanism with CONVERGE software. The influences of exhaust gas recirculation (EGR) and nozzle angle on in-cylinder combustion and pollutant generation of PODE/methanol blends were studied. Results show that with the increase of EGR ratio, the fuel-air equivalence ratio increases, the in-cylinder pressure decreases, and the ignition delay period is prolonged. The generation range and rate of NOx gradually decrease, and the final NOx generation decreases by 85.1% at EGR = 20%. However, the peak soot generation rate and the final soot generation increase. With the increase of nozzle angle, the peak in-cylinder pressure and the overall heat release rate increase. When the nozzle angle is adjusted from 148° to 164°, the rich area of fuel-air equivalent ratio in the piston pit decreases, the high temperature area in the piston pit almost disappears. The final in-cylinder NOx generation has been reduced by 29.3%. The soot distribution gradually moves to the upper layer of combustion chamber, and the peak soot generation rate decreases, while the final soot generation increases by almost 49.5%.