Lean Combustion Limit Research Articles

Investigation on the lean-combustion characteristics of non-uniform orifice pre-chamber spark plug in low engine speed working conditions compared with high energy spark ignition

For a long time, pre-chamber jet ignition has been an effective method to achieve stable lean-combustion of the engine. However, due to the lack of an additional fuel injector, the passive pre-chamber easily leads to unstable combustion and even misfires during the engine’s low-speed working conditions. This study used simulation and optical single-cylinder engine visualization experiments to investigate the ignition and combustion performance of the pre-chamber spark plug (PCSP) ignition system and different orientations of the scavenging jet nozzle in the cylinder. The results indicate that the PCSP at low speed (1200 r/min) can improve the lean-combustion load performance by up to 6.7% compared with traditional high-energy spark ignition but cannot significantly improve the lean combustion limit and stability. In addition, under each λ condition, the scavenging jet nozzle face toward one of the intake valves (IV2) is most advantageous. The effect of lean combustion on reducing NO began to manifest after λ > 1.3 and achieved the best at 1.6. This kind of jet ignition pre-chamber provides a more stable ignition solution than high-energy spark ignition for low speed and medium/low load aspects.

Volumetric Image Analysis Of Pulsed Non-Thermal Plasma Produced By Microwave

We have been developing microwave pulsed mode ignition system using 2.45 GHz microwaves. This technology has shown improvements in the lean combustion limit and fuel efficiency using a real engine with propane using a microwave discharged igniter (MDI). However, the MDI’s expensive parts are a major drawback when compared to common igniters. For practicality, we shifted to a more economical flat spiral igniter to generate plasma enabling stable and sustained non-equilibrium plasma in the atmosphere. Incorporating semiconductor elements, we miniaturized the microwave generator system, enhancing compactness and increasing its efficiency. Initial plasma was generated by the partial energy from the microwaves (1 mJ) and the sustained plasma was maintained in the atmosphere by the rest of the microwave’s energy. We varied long to short-pulsed modes, changed microwave pulse widths, increasing repetition rates which affecting plasma volume and stability. Our findings highlight the significance of microwave input patterns and the transition from thermal to non-equilibrium plasma. Future discussions should focus on the ignition mechanisms of various fuels and the transition from plasma source to initialization region. Our work contributes to the understanding and practical application of non-thermal plasma in combustion systems.

Influence of Ambient Pressure on the Jet-Ignition Combustion Performance and Flame Propagation Characteristics of Gasoline in a Constant Volume Combustion Chamber

Based on a constant volume combustion chamber, the initial ambient pressure effect on gasoline combustion performance and flame propagation of the active and passive pre-chamber ignition systems were studied. Compared with the passive pre-chamber, the active pre-chamber can significantly expand the lean combustion limit from 1.2 to 1.5, enhance the ignition and combustion performance, and speed up the combustion of the main combustion chamber. At slightly lean combustion conditions, the heat release performance has a positive correlation with the initial ambient pressure. As the premixed λ increases, the heat release performance and the initial ambient pressure begin to become negatively correlated. Ignition delay can be decreased by approximately 50% at an initial pressure of 0.75 MPa, premixed lambda of 1.2 with APC compared with PPC. The high ambient pressure (1.0 MPa) in the constant volume chamber greatly reduces the mixture entrainment ability of the jet flame at lean combustion conditions, thereby reducing the combustion speed and peak heat release rate.

Study on ignition mode transition of methanol pre-chamber jet ignition system controlled by boundary condition parameters

Optimizing boundary condition parameters is an effective means to substantially improve the combustion performance of methanol pre-chamber turbulent jet ignition systems. In this study, an optically accessible constant-volume combustion chamber was employed to delve deeply into the impacts of critical boundary condition parameters on the combustion performance of the pre-chamber jet ignition system. The findings suggest that adjusting the pre-chamber boundary condition parameters can lead to various jet ignition modes, including jet flame ignition, jet direct ignition, jet delayed ignition, jet wake ignition, and dual jet ignition. Different jet ignition modes result in variations in ignition timing, ignition location, combustion rate, and lean burn limits. Under identical initial temperature and pressure conditions, compared to jet flame ignition with the lowest combustion velocity and jet wake ignition with almost no lean burn capability, dual jet ignition can reduce the combustion duration by about 50% and elevate the lean burn limit by more than double. Unlike the patterns observed in traditional spark ignition systems, within the range of experimental tests, lowering the temperature and increasing the pressure significantly improved the lean combustion limit of the pre-chamber system and reduced the probability of jet wake ignition. This performance enhancement is attributed to the increase in the density of the mixture in the main combustion chamber.

Investigations on a Spark Ignition Engine, Part II: Impact of Hydrogen Addition and Compression Ratio on a Biogas-Fuelled Engine

Performance, emissions and combustion were studied using a spark ignition engine modified from a single-cylinder diesel engine with varying compression ratios and hydrogen addition. When the compression ratio increases, there is a slight increase in brake power and thermal efficiency. There is a small increase in brake power and brake thermal efficiency when the compression ratio is above 13:1. Increased emissions of nitric oxide, hydrocarbon, and carbon monoxide were observed at compression ratios above 13:1. Leaner mixtures, brake power and brake thermal efficiency are lowered. Spark timing is retarded as the compression ratio rises which results in high peak pressure and rate of heat release. At a compression ratio of 13:1, a small amount of hydrogen (15% on an energy basis) was added along with biogas. The lean limit of biogas combustion and the combustion rate are greatly increased by the addition of hydrogen. In addition, there was a rise in brake power and thermal efficiency. The levels of hydrocarbons were significantly reduced. Nitric oxide emissions did not rise as a result of the use of the retarded ignition timing. The effect of two throttle conditions on spark ignition engines using biogas as fuel is presented as Part I of this study.

Optical investigation of the influence of high-reactivity iso-octane turbulent jet ignition on the combustion characteristics of ammonia/air mixtures

Ammonia has attracted considerable attention as a zero-carbon fuel; however, challenges such as ignition difficulty and slow flame propagation persist. These challenges can be addressed by employing a highly reactive fuel in the pre-chamber to initiate the ignition of ammonia. This work aims to experimentally investigate the influence of pre-chamber fuel composition and concentration on ammonia combustion characteristics across various equivalence ratios. The results are as follows: elevating iso-octane equivalence ratios in the pre-chamber enhances stoichiometric ammonia combustion characteristics, reducing ignition delay and burn duration. However, excessive values result in significant delays and a decrease in flame speed. At an iso-octane equivalence ratio of 0.8, the main chamber exhibited the shortest combustion duration. By utilizing the optimal pre-chamber total equivalence ratios of 2.0 and 2.3, the ammonia lean combustion limit was achieved at an excess air ratio of 1.89 under experimental conditions (0.8 MPa, 440 K). As main chamber equivalence ratios decreased, ignition delay shortened. The pre-chamber total equivalence ratios exert a more pronounced impact on the main chamber ignition delay. In contrast, the main chamber equivalence ratios substantially influence the middle and later stages of combustion.

The effect of the ceramic foam cell structure on the combustion performance of porous media burner: experimental study

To investigate the effect of the ceramic foam cell structure on the combustion characteristics, a porous media burner(PMB) composed of foamed ceramic plates with Kelvin and WP cell models as structural features was designed and developed. The combustion chamber temperature, lean combustion limit, radiation efficiency and flue gas emission characteristics of the burner were analysed in the combustion of methane. The results show that with the same pore density, skeleton diameter and operating parameters, the combustion chamber temperature and radiation efficiency of the WP model are higher than those of the Kelvin model, while the CO and NOx emissions of WP model have an opposite tendency. When the ceramic foams are composed by WP cell model, under the same pore density and operating parameters, the burner has better combustion performance when the skeleton is thin. The combustion chamber temperature and radiation efficiency are higher, CO and NOx emissions are lower, and the lean combustion limit equivalent ratio is lower.

Characteristics of multi-cycle two-phase pulse detonation waves traveling near the lean combustion limit

The need for high combustion efficiency in two-phase pulse detonation engines necessitates the implementation of a lean combustion concept. However, there have been no research initiatives attempting to conduct two-phase pulse detonation in a lean combustion environment due to the highly sensitive nature of the deflagration-to-detonation transition toward the reactivity of the reactant composition. The present study explores methods to realize lean combustion organization in two-phase pulse detonation through the incorporation of secondary air injection. Valveless pulse detonation operation based on gasoline was carried out, while the frequency varies from 20 to 100 Hz. The initiation and propagation characteristics of the pulse detonation wave are influenced first by the equivalence ratio of the detonation initiation section and then by the equivalence ratio of the detonation propagation section. Furthermore, secondary air injection enabled a reduction in the minimum global equivalence ratio for the stable operation of multi-cycle two-phase pulse detonation waves to 0.38, while maintaining an 80% detonation rate.

Fundamental Experiment of Ignition and Flame Development in Turbulent Jet Ignition Ignited Methane Jet Flame

ABSTRACT In heavy-duty natural gas engines, diesel is usually injected before natural gas to initiate methane combustion. However, such an approach requires additional fuel and precise injection strategies. In the present work, an alternative approach of pre-chamber turbulent jet ignition (TJI) ignited methane jet flame is performed experimentally. The underlying mechanism of TJI-HPDI jet combustion is investigated in an optically visualized constant-volume combustion chamber with an active pre-chamber. It is found that with the increase of the injection-ignition delay (ti), the combustion process undergoes several transitions. In addition, the success ignition region and ignition failure region separated by ti and T MI are discerned on the ti-T MI diagram. As ti increases, there is an initial rise in both the mean flame front velocity and peak pressure, followed by a subsequent decrease This behavior highlights the TJI-HPDI system’s notable advantages in expanding the lean combustion limit and enhancing overall combustion characteristics compared to premixed combustion. These advantages can be attributed to the mixture stratification and turbulence. Furthermore, pre-chambers fueled with hydrogen can further improve the intensity and energy of the turbulent hot jet, which further decreases the critical injection-ignition delay to initiate global flame propagation in the TJI-HPDI system. The present work deals with real engineering technologies, and it will provide new insights into the design and control of TJI-HPDI engines, especially for heavy-duty natural gas engines.

Experimental study of the ignition chamber with accelerating cavity applying to methanol lean combustion

Owing to the high laminar flame velocity and broad flammability limit of methanol (CH3OH), it is ideal for lean burning. How to obtain high energy and reliable ignition is one of the biggest challenges with lean or ultra-low combustion. To gain good ultra-lean burn performance, one solution is utilizing an turbulent jet ignition (TJI)system. Moreover, the methanol can be directly used in the jet ignition chamber for its low vaporization temperature and volatile characteristics. To enhance the methanol jet ignition performance, the two-stage accelerating cavity was applied to accelerate the combustion and extend the lean-burn limit in this research. In a optical experimental apparatus, jet ignition and combustion process of premixed methanol in the main combustion chamber (MCC) were investigated using the shadowgraph and transient pressure data acquisition methods. The experiment results showed that the pressure of the MCC emerged an obvious two-stage rising process to use the two-stage accelerating cavity (SAC). In addition, when the SAC was applied in the ignition chamber (IC), the proportion and peak value of the first stage heat release in the MCC were clearly higher than those of the single straight hole (SSH). Moreover, the ignition chamber compounded with SAC exhibited a short ignition delay and long jet ignition duration. Compared to SSH, the methanol combustion duration in the MCC by using SAC shortened by approximately 28% under both stoichiometric and lean conditions. In other words, The SAC accelerated the combustion process of methanol. More significantly, it was found that the proper secondary acceleration hole diameter (SAHD) play an crucial role in the jet ignition and combustion process, especially under ultra-lean conditions. In addition, by utilizing SAC-3-4.5, the combustion duration of was shortened by 41.9% than that of using SAC-3-3 at the equivalence ratio of 0.6. Finally, the ultra-lean burn limit could be extended to 0.35 by using SAC-3-4.5, which greatly expanded the methanol lean combustion limit.

The Lean-Burn Limit Extending Experiment on Gasoline Engine with Dual Injection Strategy and High Power Ignition System

In the context of the energy crisis and global warming, improving thermal efficiency is the most important issue in research on gasoline engines, and lean mixture combustion strategy is becoming the most promising method. Thus, a high compression ratio, a high-power interval ignition system, and a stratified combustion scheme achieved via dual injection were novelly adopted in a single cylinder gasoline engine in this study. The results show that the lean combustion limit could be literally extended and improved thermal efficiency was observed under the ultra-lean condition. Meanwhile, reverse combustion performance trends were observed by altering the second injection proportion from 30% to 45% under the lean condition (λ = 1.6) and ultra-lean condition (λ = 1.9). This was related to a combustion velocity change caused by great concentration gradient at the middle and end combustion stage. Finally, according to research on the effects of altering the timing of the second injection, it is clear that the dual injection strategy is an ideal method for realizing operation under the lean condition (λ = 1.6). But for the operation under the ultra-lean condition (λ = 1.9), more injection times and suitable air flow organization are needed to enhance the robustness of mixture distribution.

Optical Study on Spark Plug Gap in Extending Methane Lean Combustion Limits under High Ignition Energy Conditions

<div>Lean combustion has the potential to achieve high thermal efficiency for internal combustion engines. However, natural gas (NG) engines often suffer from slow burning rates and large cyclic variations when adopting lean combustion. In this study, the effects of spark plug gaps (SPGs) on methane lean combustion are optically investigated under high ignition energy conditions. Synchronization measurements of in-cylinder pressure and high-speed photography are performed for combustion analysis. The results show that large SPGs with high ignition energy exhibit great improvement in engine combustion stability and power capability. Under ultra-lean conditions, a large SPG with a high ignition energy of 150–200 mJ can extend the lean limit to 1.55. Combustion images indicate that this is contributed by the enlarged initial flame kernel, which promotes early flame propagation. Besides, an empirical criterion is adopted to quantify the underlying mechanism, and the results confirm that a more stable early flame development with a faster burning rate can be obtained by a larger SPG and higher ignition energy under lean conditions. Therefore, a large SPG is an effective way to improve combustion stability and thermal efficiency for NG engines.</div>

Investigation Effects of Pre-chamber Volume Variations on the Performance of a Natural Gas Lean-Burn Engine

<div>The pre-chamber ignition system accelerates combustion efficiently by supplying multiple ignition points, high ignition energy, and strong turbulent disturbance. This system expands the lean combustion limit and improves combustion stability on natural gas engines. This work studied the effects of pre-chamber volume variations on combustion, performance, and emission behaviors of a natural gas lean-burn engine under experimental and numerical methods. Results show an increase in the pre-chamber volume from 0.3% to 4.4% of compression volume can increase the in-cylinder pressure in single-stage combustion. The energy and exergy efficiency of the engine Model-1.3% increased up to 43.7% and 41.9%, respectively, which are the highest values among the prepared models. Simultaneously, the model heat loss with the maximum pre-chamber volume was two times higher than the minimum pre-chamber volume. The exhaust gas exergy of Model-1.3% and Model-2.1% are the lowest values, approximately 29% of the fuel exergy. Increasing the volume of the pre-chamber from 1.3% of compression volume led to a reduction in emissions. An increase in the pre-chamber volume increases the exit velocity of gases from the pre-chamber bores, and turbulence in the entrance, exit, and inside the bores, which generates uniform and isothermal heat across the cylinder. Therefore, a pre-chamber with a compression volume below 5% stabilizes the combustion. Under stable working conditions, a pre-chamber volume can be defined such that the spark ignition (SI) engine would have the highest energy and exergy efficiencies with the lowest emission.</div>

Dependence of polyethylene combustion dynamics in a 1 m<sup>3</sup> chamber on particle size

Introduction. The results of a standard study on the explosion hazard of polyethylene air suspensions (PES) can contribute to the theory of turbulent combustion. For example, analysis of polydispersity data and values of the PES lean combustion limit in a 1 m3 chamber helped to identify the maximum size of explosive particles d*m,t ≈ 100 µm (Poletaev, 2014). In this work, a relationship was obtained between the dynamics of PES combustion in a 1 m3 chamber and the average particle size of the suspension, which is understood as the average particle size of its explosive fraction d*50.Initial data. Well-known findings of a study on the explosion of 28 polyethylene specimens in a 1 m3 chamber were used. Continuous functions of specimen particles distribution by size, necessary for calculating d*50, were represented using the Rosin-Rammler distribution.Combustion dynamics. The dynamics of PES turbulent combustion in a 1 m3 chamber is described by the maximum rate of air suspension burnout Ub. Ub was calculated according to the formula (Kumar, 1992) intended for gas-air mixtures by substituting PES explosion parameters into this formula.Results and its discussion. The graph, describing the dependence of the complex d*50Ub on d*50, is provided. The averaged value of the complex (≈ 45 µm · (m/s)) is constant in the range 40 µm < d*50 < 90 µm. The latter is typical for the product of the particle size and the normal velocity of laminar flame in liquid aerosols (Myers, 1986), which indicates similarity between the effect of particle dispersion and dynamics of turbulent and laminar combustion of the aforementioned heterogeneous mixtures.Conclusions. The dispersive capacity of an explosive polydisperse polyethylene specimen is determined by the average particle size of the explosive fraction of the specimen d*50. The similarity of combustion patterns indicates the proximity of propagation mechanisms typical for turbulent flame, typical for PES, and laminar flame, typical for liquid aerosols.

A methodology to initialize tumble flow fields for fast 3D-CFD simulations of pent-roof SI engines

The demand of powertrain technologies able to efficiently employ several non-fossil fuels types, as bio or synthetic ones, compels to develop simplified strategies to accelerate the engines industrial optimization. High-tumble ultra-lean SI engines are currently an attractive option, thanks to their fuel flexibility and the potential of extending the lean combustion limit with novel ignition strategies. This work presents a methodology to initialize tumble flow fields at closed-valves inside pent-roof SI engines, without the need of simulating the gas exchange process. First, assuming an elliptical-shaped vortex, a velocity field is initialized into the cylinder according to the target tumble ratio at inlet valve closing (IVC). Then, the flow field is evolved at fixed piston position for a time duration proportional to the integral time scale of turbulence. This process is essential to adapt the vortex morphology to the surrounding geometrical details, despite some kinetic energy is lost due to frictions. To compensate this last effect, the velocity field is re-scaled at the end of the process to match the initial tumble ratio. Finally, the parameters affecting the tumble initialization are calibrated by comparing the development of the flow-field during the compression stroke with the results from a full-cycle simulation in a pent-roof SI engine. The methodology is validated on the Darmstadt optical engine. First, the numerical velocity fields are compared against PIV measurements at different crank-angles. Then, the combustion prediction is also evaluated, to assess the quality of the initialized tumble flow field. The achieved results are satisfactory, demonstrating the industrial applicability of the presented methodology.

Effect of hydrogen direct injection on natural gas/hydrogen engine performance under high compression-ratio conditions

In traffic transportation, the use of low-carbon fuels is the key to being carbon-neutral. Hydrogen-enhanced natural gas gets more and more attention, but practical engines fueled with it often suffer from low engine power output. In this study, the inner mechanism of hydrogen direct injection on methane combustion was optically studied based on a dual-fuel supply system. Simultaneous pressure acquisition and high-speed direct photography were used to analyze engine performance and flame characteristics. The results show that lean combustion can improve methane engine's thermal efficiency, but is limited by cyclic variations under high excess air coefficient conditions. Hydrogen addition mainly acts as an ignition promoter for methane lean combustion, as a result, the lean combustion limit and thermal efficiency can be improved. As for hydrogen injection timing, late injection can increase the in-cylinder turbulence intensity but also the inhomogeneity, so a suitable injection timing is needed for improving the engine's performance. Besides, late hydrogen injection is more effective under lean conditions because of the reduced mixture inhomogeneity. The current study shall give some insights into the controlling strategies for natural gas/hydrogen engines.

Influence of laser ignition on characteristics of an engine for hydrogen enriched CNG and iso-octane usage

The study has focused on determining the laser plug effects on engine characteristics and the laser plug usage results have compared with spark plug usage. The laser ignition technique is a type of new ignition technique and an important solution that can make combustion systems more efficient. The testing of an engine with a laser plug is the novelty of the study and the tests were carried out with reference to equivalence ratio and plug power ranges. The behaviors of the engine at full load were examined so experimentally for both ignition techniques at hydrogen enriched CNG and iso-octane mixture usage. The tests were carried out for variations of 0.4–2.0 equivalence ratio and 20–120 W plug power. A mixture that 90% iso-octane and 10% HCNG in mass was used at two ignition modes in tests for 3300 rpm maximum engine torque speed. Also, the flame formation and propagation for both ignition techniques were detected via a high-speed camera. The tests have shown the laser ignition leads to more energy consumption in the rich mixture conditions and also, less energy is required in the lean conditions. The laser ignition discharge has extended the engine's lean combustion limits via a small energy input at the tests. The high-speed camera images have shown that the laser ignition reduces the Kernel flame formation and propagation time. The laser ignition technique was produced less NOx than the conventional spark ignition method.

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ϕ = 0.6–1.9, mixed hydrogen: ϕ = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.

Effects of pre-chamber jet ignition on knock and combustion characteristics in a spark ignition engine fueled with kerosene

Pre-chamber jet ignition (PJI) is effective in accelerating combustion, expanding lean combustion limit and improving combustion stability on gas fuel and volatile liquid fuels. However, the effects of PJI on kerosene, which is less volatile and is more prone to knock than gasoline, are rarely discussed. In this work, PJI was applied to ignite kerosene in a direct injection dual spark plugs ignition (DSPI) engine and its effects on knock intensity, knock randomness, heat release rate, combustion phase and combustion stability were studied. Experiments were conducted with compression ratios (CR) of 6 and 7 respectively. Results show that PJI cannot improve the available CR of kerosene. At the early stage of PJI combustion process, a sudden increase and a fluctuation of heat release rate can be observed, which is coincident with the multi point ignition and highly turbulent flame caused by the rapid jet flow. PJI can improve the heat release rate, advance the combustion phase and improve the combustion stability of kerosene, but can also deteriorate the knock intensity without ignition timing adjustment. Due to the improved combustion stability, the randomness of knock is reduced. As a result, advanced combustion phase and reduced knock randomness lead to a better output and fuel consumption performance of PJI under critical knock condition than that of DSPI. The result shows that PJI.

Exploring the lean limit operation and fuel consumption improvement of a homogeneous charge pre-chamber torch ignition system in an SI engine fueled with a gasoline-bioethanol blend

The transportation sector faces a big energy challenge as it looks for more advanced technologies and renewable sources, in which pre-chamber ignition systems and bioethanol have gained worldwide recognition. In this manuscript, a homogenous torch ignition (HTI) system was experimentally analyzed in a commercial four-cylinder engine equipped with port-fuel injection and fueled with a gasoline-bioethanol blend with 25% of bioethanol by volume (E25). Tests were performed at a wide range of speeds and loads for stoichiometric and lean mixtures. Through in-cylinder pressure measurement, a combustion analysis was carefully carried out. Results show that the HTI system leads to a much faster combustion rate and low engine cycle-to-cycle variation due to enhanced ignition energy and turbulence levels in the main chamber, and is able to extend the lean combustion limit with acceptable combustion covariance up to λ = 1.30. Moreover, improved combustion phasing combined with reduced pumping losses and lower heat transfer to the cooling system decreased fuel consumption. BSFC was improved up to 8.4%, 12.1% and 10.2% for engine speeds of 2500, 3500 and 4500 rpm, respectively, for the 3.5 and 4.6 BMEP conditions at λ = 1.20, while the higher load (5.8 bar BMEP) was improved up to 8.6% at λ = 1.30.