Higher Laminar Flame Speed Research Articles

Spark Timing Optimization through Co-Simulation Analysis in a Spark Ignition Engine

The automotive industry is experiencing radical changes under the pressure of institutions that are increasingly reducing the limits on CO2 and pollutant emissions from road vehicles powered by internal combustion engines (ICEs). A way to decarbonize the transport sector without disrupting current automotive production is the adoption of alternative fuels for internal combustion engines (ICEs). Hydrogen is very attractive, thanks to the zero-carbon content and very high laminar flame speed, allowing for extending the lean burn limit. Other alternative fuels are methanol and ethanol. This work deals with the conversion of a small-sized passenger car powered by a three-cylinder spark ignition (SI) engine for the use of alternative fuels. In particular, the spark timing has been optimized to improve the fuel economy under every operating condition. The optimization procedure is based on the MATLAB/Simulink® R2024a-GT-Power co-simulation analysis and minimizes the fuel consumption by varying the spark timing independently for each cylinder. In particular, at full load, the algorithm reduces the spark timing only for the cylinder in which knock is detected, reducing fuel consumption by about 2% compared to the base calibration. This approach will be adopted in future activities to understand how the use of alternative fuels affects the ignition control strategy.

Stabilization regimes and flame structure at the flame base of a swirled lean premixed hydrogen–air injector with a pure hydrogen pilot injection

Hydrogen (H2) has become a key in the decarbonation process of the aeronautical field and new gaseous injection systems must be designed to suit hydrogen’s specific properties (high laminar flame speed, high adiabatic temperature, large flammability limits and fast diffusivity (Le<1)). In this study, Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) are performed for a co-axial injector including an annular premixed hydrogen–air, swirled injection surrounding a central axial pure hydrogen lance. LES of the injector installed in a full chamber are performed at 12bar and the LES resolution issues associated with these high pressures are presented. LES reveals that the flame is stabilized on the rim separating the premixed and the pure hydrogen streams. The stabilization is produced by a structure called RSE for Rim Stabilized Edge Flame. To avoid resolution issues of the LES, the RSE flame structure is analyzed using CANTERA 1D flames and a 2D DNS of the attachment region. Extinction limits are described and implications for flame stability are discussed.

The Ammonia Combustion Engine for Future Power Generation Applications

Energy storage is one of the big challenges for the energy transition and ammonia as a hydrogen carrier is considered a promising candidate for efficient and medium‐ to long‐term chemical storage. Gas engines are well suited for power generation using ammonia as a carbon‐free fuel, making it unnecessary to convert ammonia back to hydrogen for the utilization in the power plant. This study investigates operating strategies for a prechamber combustion concept for a large‐bore gas engine suitable for power generation applications. To overcome the high ignition energy and low laminar flame speed of ammonia, different quantities of hydrogen as a promoting agent to improve the combustion properties of ammonia have been employed in the experimental investigations on a single‐cylinder research engine. The combustion concept shows a wide operating range with stable and robust combustion from 0.3 to 2.5 MPa indicated mean effective pressure and will be transferred to a multicylinder engine for the first combined heat and power demonstration.

Kinetic insight into composition effect on the laminar flame speeds of cracked endothermic hydrocarbon fuel for hypersonic vehicles

Hydrocarbon fuels are promising energy sources and coolants for hypersonic vehicles. A clear understanding on the oxidation mechanism discrepancies of different cracked endothermic hydrocarbon fuels is essential for the performance and thermal management of hypersonic vehicles. Kinetic analyses on the pathway flux, sensitivity, and producing rate were performed to reveal the intrinsic relationships between the composition variation and laminar flame speeds of different pyrolysis products. A skeletal mechanism containing 68 species and 326 reactions was developed and validated by the targeted properties of the pyrolysis products. Analyses showed the flame speed discrepancies were resulted from the kinetic effect of the composition variations. The main oxidation pathways and consumption rates of the key intermediates were hardly changed with composition variation in the pyrolysis products. Sensitivity analyses found the flame speeds of pyrolysis products were mainly affected by the H/O/OH addition reactions of C0–C2 species. Their sensitivity coefficients were independent of the composition variations. Analyses on the rate of product (ROP) indicated the composition variation has a significant effect on the ROPs of H/O/OH radicals. Pyrolysis products including compositions of higher H/C ratios can produce higher contents of H, O, and OH radicals for the flame, resulting in higher laminar flame speeds.

Laser ignition and flame propagation of methanol-air mixture in a constant volume combustion chamber

ABSTRACT The combustion performance of laser-ignited methanol-air mixture in a constant volume combustion chamber was investigated using the Schlieren photography technique. The experimentation was conducted at 5, 7, 9, and 11 MPa fuel injection pressure and 363 K initial chamber temperature using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser of 1064 nm wavelength. The gasoline direct injection (GDI) system was used for injecting the fuel at different pressures inside the chamber. The laser beam is focused into the chamber using a 150 mm focal length lens, and laser energy of 200 mJ was used for all experimentation. The Schlieren system was used for capturing flame propagation of methanol-air mixture for different equivalence ratios. The pressure-time history for equivalence ratio (ϕ) of 0.8, 0.9, and 1.0 was plotted and analyzed at different chamber filling pressures. Flame propagation was found faster for ϕ of 1.0 due to higher laminar velocity than 0.8 and 0.9. Flame visualization showed that the flame attains the ellipsoid shape during the early stage of ignition. It generates two lobes in the vertical direction and then again expands in a horizontal direction toward the ignition source. For all equivalence ratios, the flame propagation toward the incoming laser source (X-) was observed faster compared to the flame propagation in the direction and perpendicular to the laser beam direction. For all equivalence ratios, higher laminar flame speed was observed at 5 MPa injection pressure; however, it decreases as the injection pressure increases. At ϕ of 1.0 and 11 MPa injection pressure, the maximum peak pressure of 0.67 MPa and combustion duration of 19.3 ms were noted. At 9 MPa injection pressure and ϕ of 1.0, the peak pressure and combustion duration was found as 0.61 MPa and 19.4 ms, respectively. At ϕ of 0.8, the peak pressure and combustion duration were observed 0.27 MPa with 34.6 ms duration and 0.47 MPa with 36.9 ms duration at 5 MPa and 11 MPa injection pressures, respectively.

Numerical Investigation of a Heavy-Duty Compression Ignition Engine Converted to Ammonia Spark-Ignition Operation

Abstract Global decarbonization requires the increased use of zero-carbon fuels. Compared to hydrogen, ammonia is easier to store, transport, and produce. In addition, products of complete combustion of ammonia are water and nitrogen. Therefore, ammonia is an ideal green fuel for internal combustion engines. Drawbacks relate to the high ignition energy and low laminar flame speed of ammonia. This three-dimensional numerical study investigated the potential of converting existing diesel engines to ammonia spark ignition operation. Results indicated a slower kernel inception process, but the speed of the fully developed turbulent flame was enough to complete the bulk combustion process despite the lower laminar flame speed. The problem with pure ammonia operation was the reduced combustion efficiency and the high level of unburned ammonia emissions since the slow spark inception process can be compensated by a larger compression ratio. The results also suggested that emissions formation and subsequent oxidation were a more complex phenomenon. For example, lean ammonia combustion produced higher nitrogen oxides (NOX) concentrations due to the nitrogen in the fuel, despite the lower combustion temperature. Moreover, advancing spark timing reduced the NOX emissions, which was opposite to the traditional spark ignition engines. Additionally, the ammonia in engine crevices that escaped the late oxidation process was the main source of nitrous oxide (N2O) species in the exhaust gas that usually do not appear in traditional engines. Overall, all these results suggested that more fundamental research on ammonia combustion is needed to understand its use in efficient, decarbonized engines.

Physical and Chemical Features of Hydrogen Combustion and Their Influence on the Characteristics of Gas Turbine Combustion Chambers

Hydrogen plays a key role in the transition to a carbon-free economy. Substitution of hydrocarbon fuel with hydrogen in gas turbine engines and power plants is an area of growing interest. This review discusses the combustion features of adding hydrogen as well as its influence on the characteristics of gas turbine combustion chambers as compared with methane. The paper presents the studies into pure hydrogen or methane and methane–hydrogen mixtures with various hydrogen contents. Hydrogen combustion shows a smaller ignition delay time and higher laminar flame speed with a shift in its maximum value to a rich mixture, which has a significant effect on the flashback inside the burner premixer, especially at elevated air temperatures. Another feature is an increased temperature of the flame, which can lead to an increased rate of nitrogen oxide formation. However, wider combustion concentration ranges contribute to the stable combustion of hydrogen at temperatures lower than those of methane. Along with this, it has been shown that even at the same adiabatic temperature, more nitrogen oxides are formed in a hydrogen flame than in a methane flame, which indicates another mechanism for NOx formation in addition to the Zeldovich mechanism. The article also summarizes some of the results of the studies into the effects of hydrogen on thermoacoustic instability, which depends on the inherent nature of pulsations during methane combustion. The presented data will be useful both to engineers who are engaged in solving the problems of designing hydrogen combustion devices and to scientists in this field of study.

On the Flame Shape in a Premixed Swirl Stabilised Burner and its Dependence on the Laminar Flame Speed

Gas turbines for power generation are optimised to run with fossil fuels but as a response to tighter pollutant regulations and to enable the use of renewable fuels there is a great interest in improving fuel flexibility. One interesting renewable fuel is syngas from biomass gasification but its properties vary depending on the feedstock and gasification principle, and are significantly different from conventional fuels. This paper aims to give an overview of the differences in combustion behaviour by comparing numerical solutions with methane and several different synthesis gas compositions. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for comparison. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model against experimental data for methane combustion followed by simulations with four syngas mixtures. The validated model was used to compare the flame shape and other characteristics of the flow between methane, 40% hydrogen enriched methane and four typical syngas compositions. It was found that the syngas cases experience lower swirl intensity due to high axial velocities that weakens the inner recirculation zone. Moreover, the higher laminar flame speed of the syngas cases has a strong effect on the flame front shape by bending it away from the axial direction, by making it shorter and by increasing the curvature of the flame front. A hypothesis that the flame shape and position is primarily governed by the laminar flame speed is supported by the almost identical flame shapes for bark powder syngas and 40% hydrogen enriched methane. These gas mixtures have almost identical laminar flame speeds for the relevant equivalence ratios but the heating value of the syngas is more than a factor of 3 smaller than that of the hydrogen enriched methane. The syngas compositions used are representative of practical gasification processes and biomass feedstocks. The demonstrated strong correlation between laminar flame speed and flame shape could be used as a rule of thumb to quickly judge whether the flame might come in contact with the structure or in other ways be detrimental to the function of the combustion system.

Experimental and numerical investigation of performance of an ethanol-gasoline dual-injection engine

Experiments and simulations were performed to investigate the effect of ethanol direct injection plus gasoline port injection (EDI + GPI) on engine performance. Gasoline direct injection plus GPI (GDI + GPI) was also tested as a reference to EDI + GPI. The experimental results showed that volumetric efficiency increased with the raise of direct injection ratio in both EDI + GPI and GDI + GPI conditions. The volumetric efficiency and IMEP of EDI + GPI were greater than that of GDI + GPI, due to the stronger charge cooling effect of EDI. Combustion process was improved by EDI when ethanol energy ratio (EER) was less than 42%, however further increase of EER led to the deterioration of combustion process. Simulation results showed that ethanol's high laminar flame speed played a dominate role to the improvement of combustion process. Although EDI negatively affected the equivalence ratio around spark plug, this disadvantage was offset by the high laminar flame speed of ethanol, resulting in shorter initial and major combustion durations. Simulation results also found that combustion process was deteriorated when EER was greater than 42%, which was mainly due to over-cooling and poor mixing of EDI. Regarding emissions, NO decreased while CO and HC increased with the raise of both EDI and GDI ratios.

Effects of natural gas composition and compression ratio on the thermodynamic and combustion characteristics of a heavy-duty lean-burn SI engine fueled with liquefied natural gas

In this study, the effects of natural gas composition and compression ratio on thermodynamic and combustion characteristics were comprehensively investigated using a spark ignition liquefied natural gas engine modified from a heavy-duty compression ignition engine. LNG fuel samples with two different methane compositions and a series of compression ratios were tested and analyzed with the same boundary conditions at a lean burn condition of 1.38 excess air/fuel ratio. The results indicated that the in-cylinder pressures of the heavy-duty spark ignition engine increased with increasing compression ratio (CR), while the magnitude of the increase of the peak combustion pressure firstly increased, reached a peak value at the compression ratio of 14 and then decreased. However, the heat release phasing advanced with increasing CR, the ignition delay and the combustion duration shortened with the 50% combustion location advancing toward the top dead center with increasing CR. Apart from that, the cycle-to-cycle variations of the combustion duration and peak combustion pressure were decreased with increasing CR. The peak combustion pressure declined with the prolonged the combustion duration. In addition, the location of the maximum pressure rise rate advanced and the coefficient of variation of the indicated mean effective pressure decreased with increasing CR. Consequentially, the brake thermal efficiency increased and the brake specific fuel consumption decreased with the increase of the CR. The laminar flame speed of LNG 1-air was slightly higher than that LNG 2-air due to the higher ethane concentration and its higher laminar flame speed.

Effect of the turbulence intensity on knocking tendency in a SI engine with high compression ratio using biogas and blends with natural gas, propane and hydrogen

This research presents the test results carried out in a diesel engine converted to spark ignition (SI) using gaseous fuels, applying a geometry change of the pistons combustion chamber (GCPCC) to increase the turbulence intensity during the combustion process; with similar compression ratio (CR) of the original diesel engine; the increase in turbulence intensity was planned to rise turbulent flame speed of biogas, to compensate its low laminar flame speed. The research present the test to evaluate the effect of increase turbulence intensity on knocking tendency; using fuel blends of biogas with natural gas, propane and hydrogen; for each fuel blend the maximum output power was measured just into the knocking threshold before and after GCPCC; spark timing (ST) was adjusted for optimum generating efficiency at the knocking threshold. Turbulence intensity with GCPCC was estimated using Fluent 13, with 3D Combustion Fluid Dynamics (CFD) numerical simulations; 12 combustion chamber geometries were simulated in motoring conditions; the selected geometry had the greatest simulated turbulent kinetic energy (TKE) and Reynolds number (Re) during combustion. The increased turbulence intensity was measured indirectly through the periods of combustion duration to mass fraction burn 0–5%, 0–50% and 0–90%; for almost all the fuel blends the increased turbulence intensity of the engine, increased the knocking tendency requiring to reduce the maximum output power to keep engine operation just into the knocking threshold. Biogas was the only fuel without power derating by the conditions of higher pressure and higher turbulence during combustion by GCPCC and improve its generating efficiency. Peak pressure, heat release rate, mean effective pressure and exhaust temperature were lower after GCPCC. Tests results indicated that knocking tendency was increased because of the higher turbulent flame speed; fuel blends with high laminar flame speed and low methane number (MN) had higher knocking tendency and lower output power.

Numerical optimization of methane-based fuel blends under engine-relevant conditions using a multi-objective genetic algorithm

The objective of this work is to examine in a systematic way, how conflicting requirements such as maximum ignition delay time and laminar flame speed can be met by adding gaseous components to methane in order to obtain the optimal fuel blend under engine-relevant conditions. Low-dimensional models are coupled with a multi-objective optimization algorithm in order to compute optimal methane/hydrogen, methane/syngas and methane/propane/syngas blend compositions that maximize simultaneously the ignition delay time, the laminar flame speed and the Wobbe number. The non-dominated sorting genetic algorithm (NSGA-II) is used to generate a set of Pareto solutions, and the best compromise solutions are then determined by the technique for order preference by similarity to ideal solution (TOPSIS).It was found that the GRI-Mech 3.0 mechanism could not accurately predict ignition properties of methane-based fuel blends under engine-relevant conditions. The optimization results revealed that initial conditions have a significant effect on the optimal fuel blend composition. For methane/hydrogen and methane/syngas blends, pure methane was the optimal fuel at high temperatures and low equivalence ratios, while high hydrogen contents were beneficial at lower temperatures. When the ignition delay time is of higher importance, the optimal composition shifted towards higher carbon monoxide contents. Blends with higher hydrogen and syngas contents resulted in reduced ignition delay times and higher laminar flame speeds. Regarding the methane/propane/syngas blend, the presence of propane in the optimal blend was found to be more favorable than hydrogen and carbon monoxide to satisfy the objectives.

Experimental and numerical study on the adoption of split injection strategies to improve air-butanol mixture formation in a DISI optical engine

Gasoline replacement with alternative non-fossil fuels compatible with existing units is widely promoted around the world to reduce the dependency on oil-based products by adopting domestic renewable sources. In this context, the possibility to obtain bio-alcohols from non-edible residues of food and plants is particularly attractive for gasoline replacement in SI (Spark Ignition) engines. Such bio-fuels are characterized by higher laminar flame speed (LFS) and octane rating, resulting in improved thermal efficiency and reduced regulated emissions. Low-carbon alcohols (e.g. ethanol) are disadvantageous as gasoline replacement due to poor energy density and high corrosive action on distribution pipelines, whereas high-carbon ones (e.g. n-butanol) are particularly promising candidates thanks to the physical properties and the energy density closer to those of gasoline. High latent heat of vaporization and low saturation pressure are the most relevant weaknesses of n-butanol related to gasoline replacement in DISI (Direct Injection SI) power units. On equal injection pressure and phasing, the slow evaporation rate of n-butanol leads to poor mixture preparation and larger fuel deposits. In particular, this is emphasized by low charge and wall temperatures during part load operation, reducing combustion efficiency and promoting the formation of pollutant particles.Split injection is a promising strategy to improve charge preparation contemporary reducing fuel deposits and improving mixture homogeneity, mostly for low-evaporating fuels. In the present work different split injection strategies are tested in an optically accessible SI engine fueled with n-butanol and simulated through CFD with the aim of identifying trends and understanding the root causes behind measured behaviors. CFD simulations help in understanding changes in charge stratification using different injection strategies, allowing to explain both combustion behavior and soot formation tendency from the analysis of fuel distribution. Mixture quality in the spark region and the presence of very rich mixture pockets in the combustion chamber are identified as the most critical aspects that should be optimized when changing the injection strategy; this in turn contributes to avoid slow burn rates or excessive soot production during operation with low evaporating fuels such as n-butanol. A strong correlation between diffusive flames and rich mixture pockets is found in terms of both location and intensity, proving the first order role of fuel deposits formation and mixture homogenization on both combustion development and soot formation.

Experimental and kinetic study of diisobutylene isomers in laminar flames

Laminar flame speeds of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene were studied at initial temperatures of 298–453 K, the equivalence ratios of 0.8–1.6 and initial pressure of 0.1 MPa. The comparison between 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene was also performed in this work. Results show that 2,4,4-trimethyl-2-pentene has faster laminar flame speeds than 2,4,4-trimethyl-1-pentene. The Metcalfe model was validated and modified, and the Modified model can give fairly good prediction at various conditions on laminar flame speeds of both 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. In addition, the chemical kinetic analysis was conducted. The analysis indicates that the discrepancy of laminar flame speeds between two isomers mainly depends on the kinetic effects with the same adiabatic flame temperature. Furthermore, the kinetic analysis show that IC4H8 and DMPD13 are dominant intermediates of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, respectively. The transformation between IC4H8 and IC4H7 consumes various active radicals and has strong negative effect on the global activity, which results in lower mole fraction of H radical of 2,4,4-trimethyl-1-pentene compared with 2,4,4-trimethyl-2-pentene. Therefore, the latter has higher laminar flame speeds than the former. Sensitivity analysis showed that both of them are sensitive to small radical reactions. Besides, 2,4,4-trimethyl-1-pentene is sensitive to isobutene related reactions and 2,4,4-trimethyl-2-pentene is sensitive to DMPD13 related reactions.

Exploring the potential of reformed-exhaust gas recirculation (R-EGR) for increased efficiency of methanol fueled SI engines

Methanol is a promising fuel for spark ignition engines because of its high octane number, high octane sensitivity, high heat of vaporization and high laminar flame speed. To further boost the efficiency of methanol engines, the use of waste heat for driving fuel reforming was considered. This study explores the possibility of the reformed-exhaust gas recirculation (R-EGR) concept for increased efficiency of methanol engines. A simple Otto cycle calculation and a more detailed gas dynamic engine simulation are used to evaluate that potential. Both methodologies point to an enhancement in engine efficiency with fuel reforming compared to conventional EGR but not as much as the increase in lower heating value of the reforming product would suggest. A gas dynamic engine simulation shows a shortening of the flame development period and the combustion duration in line with the expected behavior with the hydrogen-rich reformer product gas. However, the heat loss increases with the presence of hydrogen in the reactants. The improvement of brake thermal efficiency is mainly attributed to the reduction of pumping work. The R-EGR concept is also evaluated for ethanol and iso-octane. As the reforming fraction increases, the efficiency of ethanol and iso-octane fueled engines rises faster than for the methanol engines due to a higher enhancement of exergy in their reforming products. At high reforming fractions, the efficiency of the ethanol engine becomes higher than with methanol. However, if the impact of optimal compression ratio for different fuels are considered, the methanol engine is able to produce a higher efficiency than the ethanol engine.

Laminar flame speeds of pentanol isomers: An experimental and modeling study

Long chain alcohols such as 1- and iso-pentanol are foreseen as a suitable replacement for ethanol, due to more favorable physical properties (higher energy density, higher boiling point and lower hygroscopicity). The present study presents high accuracy laminar flame speed measurements for iso-pentanol/air and 1-pentanol/air mixtures, at initial temperatures of 353 K, 433 K and 473 K, 1 bar pressure and equivalence ratios ranging from 0.7 to 1.5. Comparisons with previous measurements from the literature are also presented and the observed deviations are discussed in detail. The updated kinetic mechanism for alcohols combustion from the CRECK group at Politecnico di Milano is discussed and used for modeling purposes. For a more complete validation of the oxidation mechanism at high temperature conditions, modeling results are also compared with shock tube ignition delay times from the literature. This study extends the presently sparse and uncertain experimental database for high molecular weight alcohols oxidation in laminar flames, providing high accuracy and reliable experimental data of use for alcohols oxidation mechanism development and improvement.

ABE燃料在气道喷射汽油机中的性能与排放特性的研究

ABE (丙酮-丁醇-乙醇，Acetone-Butanol-Ethanol，ABE)是发酵生成丁醇过程中的中间产物，来源广泛，价格低廉。本研究的目的是揭示ABE燃料在火花点燃发动机中的燃烧特性，并且希望证实它有潜力成为内燃机中替代燃料。丙酮，正丁醇和乙醇以3:6:1的体积比混合，然后与无酒精汽油以不同比例混合成不同的燃料。这些混合燃料在一台进气道喷射式汽油机上进行测试，它们的性能通过测量其气缸压力以及排放物的含量来进行评价。随着ABE成分的增加，有效燃油消耗率会有所增加，但NOx排放减少。若只使用ABE燃料，NOx排放可以几乎为零。 ABE (Acetone-Butanol-Ethanol, ABE) fermentation is one of the major methods to produce bio- butanol. The goal of this study is to investigate the combustion characteristics of the intermediate product in the production of butanol, namely ABE in a spark-ignited engine and hence to make an attempt to affirm its potential used as an alternative fuel for ICEs. Acetone, n-butanol and ethanol were blended in a 3:6:1 volume ratio and then blended with pure ethanol-free gasoline with dif-ferent ratios to create various fuel blends. These blends were tested in a port-fuel injected spark ignited engine and their performance was evaluated through measurements of in-cylinder pressure, and various exhaust emissions. ABE is combusted faster than gasoline and n-butanol due to its higher laminar flame speed. Brake Special Fuel Consumption (BSFC) was seen to increase and NOx was seen to decrease with ABE fraction increasing, with pure ABE achieving near-zero NOx.

Trends of Syngas as a Fuel in Internal Combustion Engines

Syngas from biomass and solid waste is a carbon-neutral fuel believed to be a promising fuel for future engines. It was widely used for spark-ignition engines in the WWII era before being replaced with gasoline. In this paper, the technological development, success, and challenges for application of syngas in power generating plants, the trends of engine technologies, and the potential of this fuel in the current engine technology are highlighted. Products of gasification vary with the variation of input parameters. Therefore, three different syngases selected from the two major gasification product categories are used as case studies. Their fuel properties are compared to those of CNG and hydrogen and the effects on the performance and emissions are studied. Syngases have very low stoichiometric air-fuel ratio; as a result they are not suitable for stoichiometric application. Besides, syngases have higher laminar flame speed as compared to CNG. Therefore, stratification under lean operation should be used in order to keep their performance and emissions of NOx comparable to CNG counterpart. However, late injection stratification leads to injection duration limitation leading to restriction of output power and torque. Therefore, proper optimization of major engine variables should be done in the current engine technology.

The Effect of Intake Valve Deactivation on Lean Stratified Charge Combustion at an Idling Condition of a Spark Ignition Direct Injection Engine

A combined experimental and analytical study was carried out to understand the improvement in combustion performance of a four-valve spark ignition direct injection (SIDI) wall-guided engine operating at lean, stratified idle with enhanced in-cylinder charge motion by deactivating one of the two intake valves. A fully warmed-up engine was operated at low speed, light load by injecting the fuel from a pressure-swirl injector during the compression stroke to produce a stratified fuel cloud surrounding the spark plug at the time of ignition. Steady state flow-bench measurements and computational fluid dynamics (CFD) calculations showed that valve deactivation primarily increased the in-cylinder swirl intensity as compared with opening both intake valves. Engine dynamometer measurements showed an increase in charge motion led to improved combustion stability, increased combustion efficiency, lower fuel consumption, and higher dilution tolerance. A CFD study was conducted using in-house models of spray and combustion to simulate the engine operating with and without valve deactivation. The computations demonstrated that the improved combustion was primarily driven by higher laminar flame speeds through enhanced mixing of internal residual gases, better containment of the fuel cloud within the piston bowl, and higher postflame diffusion burn rates during the initial, main, and late stages of the combustion process, respectively.

Statistics and dynamics of turbulence–flame alignment in premixed combustion

The geometric alignment of turbulent strain-rate structures with premixed flames greatly influences the results of the turbulence–flame interaction. Here, the statistics and dynamics of this alignment are experimentally investigated in turbulent premixed Bunsen flames using high-repetition-rate stereoscopic particle image velocimetry. In all cases, the statistics showed that the most extensive principal strain-rate associated with the turbulence preferentially aligned such that it was more perpendicular than parallel to the flame surface normal direction. The mean turbulence–flame alignment differed between the flames, with the stronger flames (higher laminar flame speed) exhibiting stronger preferential alignment. Furthermore, the preferential alignment was greatest on the reactant side of the mean flame brush. To understand these differences, individual structures of fluid-dynamic strain-rate were tracked through time in a Lagrangian manner (i.e., by following the fluid elements). It was found that the flame surface affected the orientation of the turbulence structures, with the majority of structures rotating as they approached the flame such that their most extensive principal strain-rate was perpendicular to the flame normal. The maximum change in turbulent structure orientation was found to decrease with the strength of the structure, increase with the strength of the flame, and exhibit similar trends when the structure strength and flame strength were represented by a Karlovitz number. The mean change in orientation decreased from the unburnt to burnt side of the flame brush and appears to be influenced by the overall flame shape.