Lean-burn Conditions Research Articles

Experimental Study on the Effect of Ammonia on Combustion and Emission Characteristics of a Spark Ignition Engine Fueled with Hydrogen.

Using hydrogen and its compounds as fuel is one of the key ways to achieve zero carbon emissions in internal combustion engines. Due to the difference in fuel properties of hydrogen and ammonia, mixing the two can make up for each other's shortcomings in combustion performance. Therefore, this paper studies the effects of ammonia addition on the combustion and emission characteristics of a SI commercial hydrogen engine, and studies the differences in these effects under different excess air ratios. The results indicate that increasing the ammonia mass ratio lengthens both flame development and combustion duration, causing a delay in the combustion center. Under stoichiometric conditions, combustion instability increases, peak cylinder pressure and heat release rate decrease significantly, and exhaust temperatures rise. With a higher ammonia mass ratio, brake-specific fuel consumption first decreases and then increases, reaching a minimum at a 0.4-0.6 ammonia mass ratio. Unburned ammonia and N2O emissions gradually increase, while NO X emissions initially rise and then fall, peaking at a low ammonia mass ratio. From the perspective of balancing emissions and fuel economy, for hydrogen-ammonia mixed combustion engines, a 0.6 ammonia mass ratio is recommended under stoichiometric conditions, while lean burn conditions allow for a higher ammonia mass ratio.

Experimental investigation on ammonia combustion ignited by methanol-enriched active pre-chamber in an optical engine

To enhance the ignitability and burning rate of ammonia-fueled spark ignition engines, it is an effective way to adopt active pre-chamber (PC) ignition with high-reactivity fuel. Therefore, in this paper, the ammonia combustion characteristics using methanol-enriched active PC ignition were investigated, based on optical engine experiments with simultaneous in-cylinder flame imaging and pressure measurement. Results show that the overall combustion process is characterized by a three-stage heat release: (I) jet flame controlled combustion; (II) ammonia flame propagation; (III) post jet induced combustion. In stage I, methanol active PC jets with high-momentum and high-reactivity are discharged to ignite MC ammonia mixture, contributing to the first peak of heat release rate. In stage II, ammonia turbulent flame propagation dominates the combustion. In stage III, post jets induce the third peak of heat release rate. In this way, with a minimal methanol energy ratio (ER) of lower than 10 %, stable and fast ammonia combustion is achieved under stoichiometric (λMC = 1) and lean-burn (λMC = 1.2) condition, with cyclic variation of indicated mean effective pressure (IMEP) lower than 5 %, flame probability higher than 80 % and maximum global flame speed of about 4 m/s. A highest IEMP of 0.72 MPa and a peak indicated thermal efficiency (ηit) of 30 % is obtained at λMC = 1 and λMC = 1.2, respectively. However, at the rich limit of λMC = 0.9 and lean limit of λMC = 1.4, the quenching of jet flames leads to weak ignition and unstable combustion, which is identified by reduced flame probability distribution and increased cyclic variation of IMEP. In addition, to ensure the cyclic combustion stability, there is also an operating range of methanol ER. At λMC = 1.2, unstable ignition and deteriorated combustion occur below the lower limit of ER = 6 % and above the upper limit of ER = 17 %. This paper can provide data support and operation strategy for the combustion improvements of ammonia-methanol active jet ignition engines.

Performance assessment of direct injection stoichiometric and lean burn compressed natural gas engine over port fuel gasoline and compressed natural gas engine

ABSTRACT A direct injection (DI) CNG engine was developed and evaluated under stoichiometric and lean burn conditions, emphasising high compression ratios (CRs). A comparative analysis of PFI gasoline and PFI CNG engines was conducted to provide comprehensive insights. Results showed that the DI CNG engine exhibited a shorter combustion duration, indicating higher combustion speed, and enhanced combustion stability, particularly at low load and speed conditions, with a reduced coefficient of variation of indicated mean effective pressure (COVIMEP) below 1%. The DI CNG engine demonstrated a relative increase in net indicated thermal efficiency by 4–5% compared to the PFI CNG engine. Furthermore, CNG engines operating at a high compression ratio of 16 could run at high loads without encountering knocking issues. Lean burn operation, combined with high compression ratios, improved thermal efficiency while minimising emissions; however, challenges such as combustion instability and increased emissions under lean burn conditions were observed.

Characteristics of Fuel-Borne Nitrogen Pollutants in Hydrogen-Ammonia Mixtures Using Argon-Oxygen Atmosphere Under Engine Conditions

Abstract Amid climate change, reducing reliance on fossil fuels and transitioning to renewable energy sources is crucial. Ammonia, a carbon-free and renewable fuel, shows significant potential as an alternative energy source. By incorporating hydrogen as an additive, its flammability can be enhanced to suit existing spark ignition engines. However, understanding the characteristics of nitrogen pollutant emissions (i.e., NOx, which includes NO and NO2, and N2O) from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. However, understanding the characteristics of nitrogen pollutant emissions from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. Therefore, a comprehensive understanding of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion is essential. This study focuses on investigating fuel-borne nitrogen pollutants in an argon-oxygen atmosphere, thereby eliminating nitrogen from the oxidizer and its role in thermal NOx formation. The research examines the formation and evolution of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion under engine-like conditions. Results indicate that fuel-borne nitrogen pollutants act as intermediates, potentially originating from chemical equilibrium. While fuel NO predominantly forms in the burning zone, it undergoes reduction in the burned zone. N2O, absent in thermal NOx mechanisms, shows significant concentrations in the burning zone and is mostly converted to N2, leading to limited N2O in the final fuel-borne nitrogen pollutant concentration. Lean burn conditions, hydrogen addition, and oxyfuel combustion promote fuel NOx formation. Additionally, the equivalence ratio affects the ammonia-hydrogen premixed flame structure due to the de-NOx effect of ammonia. Overall, these findings highlight that fuel-borne nitrogen pollutant mechanisms differ from thermal NOx mechanisms, necessitating specially designed reduction technologies for clean spark ignition engines.

Chemical kinetic study of methane blended with ammonia cracked gas at elevated temperature and pressure

Ammonia (NH3) on-line cracking to produce hydrogen (H2) can avoid direct ammonia combustion and is an important method to enhance combustion performance and control nitrogen oxide (NOx) emissions. This work aims to study the effects of various ammonia energy ratios (ENH3) and cracking ratios (CNH3) on the combustion and NO emission characteristics of methane (CH4) blended with ammonia cracked gas under lean-burn condition. The results showed that methane co-combustion and ammonia cracking significantly enhance the laminar burning velocity (LBV). The chemical effect plays a dominant role in the enhancement of LBV, but its relative contribution gradually decreases with increasing ENH3 and CNH3. A linear relationship between LBV and (O+H+OH+NH2 + CH3)max was observed, with both the slope and intercept being functions of CNH3. At an ENH3 of 20 %, NO emission decrease with increasing CNH3. At an ENH3 of 80 %, NO emission first increase and then decrease. The percentage of thermal NO is less than 1 % under ambient condition and could increase to as much as 22 % under elevated condition (500 K, 10 atm). Additionally, there was a quadratic polynomial relationship between (O+H+OH+NH2)max and the final NO mole fraction within an equivalence ratio range from 0.4 to 0.8. The second-order coefficient exhibits a linear correlation with CNH3, while the first-order and constant coefficient show a quadratic polynomial correlation with CNH3.

Co-optimization and prediction of high-efficiency combustion and zero-carbon emission at part load in the hydrogen direct injection engine based on VVT, split injection and ANN

Hydrogen fuel holds great promise for accelerating the energy sector's decarbonization efforts. However, hydrogen's unique properties pose challenges for hydrogen direct injection (HDI) engines, such as differences in airflow exchange capabilities for very low density and reduced combustion rates due to lean burn conditions. This study investigates the impact of combined variable valve timing (VVT) and split injection strategies on HDI engine combustion, efficiency, and emissions. Using Artificial Neural Network (ANN) models and experimental data, predictive models for Brake Thermal Efficiency (BTE) and Nitrogen Oxide (NOx) emissions were developed. Adjusting intake and exhaust valve timings improved BTE by up to 2.5 % and 1.8 %, respectively, while reducing NOx emissions by 48.9 % and 31.1 % under specific conditions. Optimizing split injection alongside VVT further increased BTE by 1.2 % and 0.9 %, but increased NOx emissions by 29.9 % and 26.5 %. The flexible application of VVT and split injection strategies aims to balance efficiency gains with emissions control. The ANN-based models demonstrated high accuracy (R2 > 0.974), proving reliable substitutes for real-engine testing in certain conditions. In conclusion, this research highlights the potential of VVT and split injection strategies to enhance HDI engine performance while mitigating environmental impact, supporting the transition to sustainable energy solutions.

Effects of Lean Burn on Combustion and Emissions of a DISI Engine Fueled with Methanol–Gasoline Blends

Methanol has significant potential as an alternative fuel for internal combustion engines. Using methanol–gasoline blends with lean-burn technology in traditional spark-ignition engines can enhance fuel economy and reduce emissions. This paper investigates the effects of lean burn on the combustion and emissions in a commercial direct-injection gasoline engine fueled with methanol–gasoline blends. The lean-burn mode is adjusted by controlling the injection strategy. The results show that homogeneous lean burn (HLB) has earlier combustion phase and better power performance when the excess air ratio (λ) is less than 1.3, while its combustion phase extends more than stratified lean burn (SLB) when λ exceeds 1.4. Both lean-burn modes achieve optimal fuel economy at λ = 1.2–1.3. Under stable conditions, BSFC decreases with higher methanol blending ratios, with SLB being more economical at low blending ratios and HLB at higher ratios. The lowest HC and particulate matter emissions for both modes are achieved around λ = 1.3. SLB has lower NOX emissions when λ < 1.3, while HLB shows lower NOX emissions when λ > 1.3. The particulate size distribution is bimodal for blending lean-burn conditions, with SLB having the highest nucleation mode peak and HLB the highest accumulation mode peak. M20 (20% volume of methanol) corresponds to the highest particle emissions under lean-burn conditions. This study can provide a deeper understanding of methanol–gasoline blending lean burn, and provide a reference for emission control of spark-ignition engines.

Impact of Lean-Burn Combustor Flow on Nozzle Guide Vane Performance

Abstract In this paper we investigate the impact of lean-burn-representative swirl and temperature distortion on the aerothermal performance of fully-cooled high-pressure nozzle guide vanes (NGVs) from a modern aero-engine. Experiments were carried out in the Engine Component AeroThermal (ECAT) facility at the University of Oxford. This is a fully-annular warm-flow engine parts facility, designed to operate at engine-representative conditions of Reynolds and Mach number. Inlet profiles of swirl, turbulence, and non-dimensional total temperature were generated using a non-reacting combustor simulator. The NGV outlet flow was experimentally characterized at three downstream planes in experiments with and without lean-burn-representative inlet conditions. Area-survey measurements included distributions of whirl angle, kinetic energy (KE) loss, and non-dimensional total temperature. Experimental data is compared to computational fluid dynamics (CFD) simulations. Fully-featured NGV geometry (including film cooling holes and internal passages) was used, to account for internal cooling flow redistribution resulting from altered external loading. We show that lean-burn inlet conditions result in significant surface flow redistribution, relatively high levels of residual swirl in the downstream flow, and a small increase of integrated KE loss.

NOx generation in marine dual-fuel engines: Effects of ammonia blending ratio and spark ignition timing

Ammonia application in marine engines can effectively mitigate greenhouse gas emissions. However, engines powered by ammonia produce considerably higher levels of NOx emissions compared to conventional fuel engines. In the present study, a combustion model for the marine dual-fuel engine fueled with natural gas and ammonia has been established using the CONVERGE software. The nitrogen element-tracking method is employed in the engine combustion simulation. This study aims to investigate the effects of ammonia blending ratio (XNH3 = 0% ∼ 50%) and spark ignition timing (SIT = −20 °CA ATDC ∼ −40 °CA ATDC) on the emission characteristics and spatiotemporal distribution of nitrogen-based pollutants under lean-burn and stoichiometric-burn conditions. The results indicate that as the XNH3 increases, total NOx emissions decrease at stoichiometric-burn conditions, while it initially increases and then decreases at lean-burn conditions, reaching a peak of 3760 ppm at XNH3 = 30%. Besides, fuel NO emissions are predominant at lean-burn conditions, whereas at stoichiometric-burn conditions, thermal N*O emissions are predominant. Additionally, as the SIT advances, total NOx emissions initially decrease and then remain unchanged, reaching a minimum at −35 °CA ATDC, and it gradually increases at lean-burn combustions. Furthermore, the spatial distribution of NO at CA50 illustrates that fuel NO is primarily concentrated at the flame front, while thermal N*O predominates in the high-temperature burned region. N2O is observed in the low-temperature region near the cylinder wall during the expansion stroke. Moreover, fuel NO is primarily produced by ammonia's decomposition with OH radicals. There are three dominant reactions for the formation of NO: HNO→NO, NH→NO, and N→NO. The coupling mechanism of carbon and nitrogen elements is achieved by H2CN and HCN radicals.

The formation process of the gaseous pollutant emissions in pure ammonia fueled engines with pre-chamber jet ignition

Ammonia, as a carbon-free fuel, can serve as an alternative fuel for future engines, which is consistent with the requirements of national carbon reduction policies. However, the higher concentrations of NOx, unburned ammonia and hydrogen emissions of ammonia-fueled engines need to be controlled seriously to meet the increasingly stringent emission regulations. In this study, a pre-chamber jet ignition single-cylinder engine with port pure ammonia injection was carried. The formation process of gaseous pollutants during engine operation was investigated using CFD simulations in the equivalence ratio range of 0.8–1.3. And, the study also discusses different methods for reducing gaseous pollutant emissions. The results reveal that under lean burn conditions, the ITE can exceed 49%, and unburned ammonia and hydrogen emissions are extremely low. However, the NO emission concentration is relatively high, reaching above 1500 × 10−6, and primarily dominated by thermal NO. N2O is primarily distributed near the flame front, The low temperature at the cylinder walls decelerates the pyrolysis of N2O, coupled with the influence of secondary combustion, both leading to higher N2O emission concentrations, with maximum value reaching 78.6 × 10−6. NO2 emissions are primarily generated in the burned regions. Under rich burn conditions, the ITE significantly decreases, with an ITE of only 34.6% at an equivalence ratio of 1.3. The unburned ammonia emissions rise with an increase in the equivalence ratio, and hydrogen emissions are mainly generated through the pyrolysis of residual ammonia in the burned region, so that the elevated unburned ammonia emissions could lead to an increase in hydrogen emissions. Fuel NO is dominant in NO emissions, and the reduction reaction of unburned ammonia results in extremely low emission concentrations, and also leading to the low levels of N2O and NO2 emissions. According to a comprehensive evaluation of pollutant emissions and engine thermal efficiency, stoichiometric condition is deemed optimal for engine operation, as it can achieve a balance between ITE and gaseous pollutant emissions. Moreover, the adoption of water injection technology can further reduce NOx emissions. Furthermore, the EGR technology and direct liquid ammonia injection will also achieve lower NOx emissions. However, it is insufficient to meet the national emission regulations that only controlling gaseous pollutant emissions during the combustion, and development of the special aftertreatment systems for ammonia fueled engines should not be neglected.

The effect of CO content on CH4/CO/H2 rotating detonation wave propagation characteristics

Rotating detonation experiments were conducted using CH4/CO/H2 (methane/carbon monoxide/hydrogen) mixtures with varying CO contents, the modes of rotating detonation wave (RDW) propagation in the mixtures were analyzed, and the impact of CO content on the propagation characteristics of the RDW in the gas mixture was compared. Three propagation modes of RDW were observed: sawtooth wave mode, mixed mode, and single wave mode. An increase in the CO content resulted in an upward shift in the range of working equivalence ratios for different gas mixtures. Additionally, the propagation modes of the same gas mixture change with increased fuel flow rate. When the equivalence ratio is below 1.13, it is observed that the gas mixture with the lowest CO content exhibits the highest RDW velocity and the shortest time required to establish RDW. This was attributed to the higher content of oxygen-containing functional groups, such as OH (hydroxyl), HO2 (peroxyhydroxyl), and O (oxygen atom), which were present under lean combustion conditions, along with the highest mass content of H2 in the gas mixture with the lowest CO content. Conversely, for equivalence ratios above 1.13, it is observed that the gas mixture with the highest CO content exhibits the highest propagation velocity and the shortest time required to establish RDW. This was attributed to the lowest mass content of CH4 and H2 in the gas mixture with the highest CO content at the same equivalence ratio, along with the inhibitory effect of elevated CO content on CH4 consumption under fuel-rich combustion conditions. The increase in the CO content resulted in maximum propagation velocities of the detonation wave being achieved for the three gas mixtures at equivalence ratios of 0.91, 1.09, and 1.19, with corresponding velocities of 1136.7, 1108.7, and 1113.2 m/s, and the shortest times required to establish RDW were measured at 1.5, 1.1, and 0.8 ms for the respective mixtures.

Effect of injection strategy on the hydrogen mixture distribution and combustion of the hydrogen-fueled engine with passive pre-chamber ignition under lean burn condition

Turbulent jet ignition (TJI) effectively achieves lean and stable combustion in hydrogen engines. However, research on injection strategies for TJI hydrogen engines is still lacking. In this study, experimental methods investigated the effects of single and split injection strategies on the combustion and emissions of TJI hydrogen engines under medium loads. The numerical simulation reveals the operational characteristics of fuel distribution, ignition capability, and energy conversion in the pre-chamber at different injection times in a single injection strategy. This work is conducted at 1600 rpm with a manifold absolute pressure of 70 kPa. The experimental results indicate that the delay of the start of injection (SOI) can enhance the performance of the TJI hydrogen engine. However, as SOI approaches TDC, emissions worsen, and combustion stability decreases. When SOI is 90°CA BTDC, the brake mean effective pressure (BMEP) and brake thermal efficiency (BTE) can achieve 3.1 bar and 32.9 %, with the coefficient of variation of the IMEP (COVIMEP) of 1.6 % and lower emissions. For the split injection strategy, delaying the secondary end of injection (SEOI) can increase BMEP and BTE. As SEOI gradually delays the TDC, emissions deteriorate sharply. Under the split injection strategy, COVIMEP is higher, which is unfavorable for hydrogen engine applications.

Experimental study on combustion instability characteristics in a pre-chamber natural gas engine under lean burn conditions

Pre-chamber jet ignition is a key technology for future high-efficiency natural gas (NG) engines. It can achieve fast and stable combustion through excellent ignition performance. However, there are still some challenges, such as high combustion instability near the lean burn limit and the narrow engine operating range. Therefore, this paper investigates the combustion instability of a pre-chamber NG engine under ultra-diluted conditions by experimental method. At two engine loads, experiments are carried out with different jet ignition intensity schemes to study the effect of jet ignition intensity on the cyclic combustion variations. Then, the combustion instability characteristics of the pre-chamber NG engine are studied by cyclic variation analysis and phase space reconstruction. The results show that with the increase in the jet ignition intensity, the cyclic combustion variations decrease, and the cyclic variation coefficient of the indicated mean effective pressure decreases to below 2%. The lean burn limit of the pre-chamber natural gas engine is extended to an excess air ratio of 2.0. The operation instability of the pre-chamber NG engine is mainly due to cyclic variations in the ignition and combustion process. The nonlinear dynamic analysis shows that the combustion process in the lean burn pre-chamber NG engine behaves with chaotic characteristics under the operating conditions of low jet ignition intensity. As the jet ignition intensity increases, the combustion stability is improved and the cycle-to-cycle variations change from fairly deterministic to more stochastic behavior. The chaotic characteristics of the combustion process become weaker. In conclusion, it is of great importance to generate stable and high ignition intensity jets for reducing combustion instability and improving combustion efficiency in lean burn pre-chamber NG engines.

Numerical Investigation of the Operating Characteristics of the Passive and Active Prechamber Jet Ignition in a Natural Gas Engine.

Prechamber jet ignition is a promising technology that enables stable ignition and fast combustion by combining thermal effects, chemical kinetics, and turbulent disturbance. The development and application of the prechamber ignition require a comprehensive and in-depth understanding of the operating characteristics of the prechamber ignition in the real engine working cycle. Therefore, numerical simulations are conducted to explore the operating performance of the prechamber ignition applied to a large-bore natural gas engine in this study. The differences between the passive prechamber (PPRE) and active prechamber (APRE) near the lean burn limit are compared. The results show that the jet ignition performance of the PPRE is hampered by the high residual gas coefficient and lean mixture in the prechamber under lean burn conditions. The ignition mode of the PPRE is similar to torch ignition, and the combustion process in the main chamber is mainly turbulent flame propagation. The ignition mechanism of the APRE is flame jet ignition. The main chamber combustion process presents a two-stage heat release characteristic, which can be subdivided into three phases: the initial flame development phase, the rapid combustion phase, and the late combustion phase. The heat release rate during the initial flame development phase depends on the physical and chemical properties of the prechamber jet and the mixture conditions in the main chamber. During the rapid combustion phase, the flame propagation along the radial direction of the jet largely depends on the time scale of the chemical reaction. The heat release rate depends on the coverage area of the jet, the jet residual momentum, and the turbulent flame speed. During the late combustion phase, the flame propagation is mainly affected by the turbulent flame speed. These results provide theoretical guidance for the subsequent application of prechamber ignition systems in various powertrains.

Jet ignition characteristics of ammonia-hydrogen passive pre-chamber: Emphasis on equivalence ratio and hydrogen/ammonia ratio

Ammonia-hydrogen blend fuels offer the potential for zero-carbon emissions in internal combustion (IC) engines. To improve combustion performance, considerable attention has been focused on pre-chamber jet ignition technologies. Passive pre-chamber featuring a simple structure can be installed in the spark plug hole without modifying the cylinder head. However, the jet ignition characteristics of ammonia-hydrogen passive pre-chambers are not fully understood, and there is a lack of visualized evidence on how critical parameters (e.g., equivalence ratio and hydrogen/ammonia ratio) impact ignition processes. This work investigated the effects of equivalence ratio and hydrogen/ammonia ratio on ammonia-hydrogen jet ignition characteristics in a rapid compression machine with passive pre-chamber. High-speed photography and instantaneous pressure acquisition were employed to capture detailed ignition and combustion evolutions. Results show that under low hydrogen/ammonia ratio conditions, there exists a combustion regime with noticeable fluctuations in jet penetration, indicating degenerate ignition performance. As the hydrogen/ammonia ratio increases from 0 % to 20 %, the ignition delay time of mixtures in the main chamber is reduced by 70 %, suggesting an improved overall ignition performance. Regarding the equivalence ratio, more pronounced effects on jet ignition and flame propagation are identified under lean-burning conditions. For mixtures at the hydrogen/ammonia ratio of 20 %, as the equivalence ratio increases from 0.6 to 0.8 and from 0.8 to 1.0, the decay rates for 10 %–90 % combustion duration are 82.5 % and 20.6 %, respectively. Besides, with the increase of equivalence ratio, flame initiation is shifted from the jet head to the side surface. The role of hydrogen/ammonia ratio and equivalence ratio is different, with the former modifying mixture reactivity while the latter affecting ignition energy.

The Suitability of the Three-way Catalyst for Hydrogen Fuelled Engines

This experimental study investigates the palladium/rhodium based three-way catalyst (TWC) in a hydrogen-gasoline dual-fuel spark ignition (SI) engine under stoichiometric and lean conditions. The work focused on lean-burn engine operating conditions with the aim of reducing nitrogen oxides (NOx) emissions during the combustion process, where the TWC is not effective, while improving the thermal efficiency of the engine. Under these lean-burn engine conditions, the combustion promoting properties of hydrogen allowed for maintained engine combustion stability as determined by the cycle-to-cycle variation (COVimep) values even up to ultra lean conditions (λ= 2.0). It was found that by reducing the combustion temperature through the application of lean conditions, engine-out NOx emissions could be reduced or even eliminated, while under these conditions the TWC was effective in reducing engine-out carbon-based gaseous emissions.

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 the effects of intake valve timing and lift on the combustion and emission performance of ethanol, N-butanol, and gasoline engine under stoichiometric combustion and lean burn conditions

To achieve carbon neutrality goals, renewable alcohols and new technologies are receiving increased attention. Variable valve technology can effectively improve engine efficiency. However, there is limited research on valve strategies suitable for alcohol. To further promote the efficient and clean application of alcohol, this article studies the effects of intake valve timing and lift on performance of ethanol, n-butanol, and gasoline engines under stoichiometric combustion and lean burn. It is found that the combustion process of ethanol and n-butanol is prolonged by advancing intake valve timing, while gasoline is the opposite. Meanwhile, the improvement of equivalent brake-specific fuel consumption (ESFC) is the most significant when fueled with gasoline, improving by 6.9 %. The improvement of BSNOx is the highest when fueled with ethanol, improving by 42.07 %. Adjusting intake valve lift has a weaker impact on economy than valve timing. However, when adjusting the combination of throttle, valve lift and timing, the ESFC of ethanol, n-butanol, and gasoline can be reduced by 6.79 %, 4.35 %, and 10.98 %. Furthermore, combining valve timing and lean burn can further improve economy and emissions. The ESFC of ethanol, n-butanol, and gasoline can be decreased by 12.73 %, 6.87 %, and 11.96 %, while BSNOx decrease by 93.69 %, 74.86 % and 61.65 %.

Experimental and simulation research on the lean combustion characteristics of direct-injection hydrogen engine

At a constant engine speed of 2000 r/min and 5 bar gross indicated mean effective pressure, experiments were carried out on a 1.5 L engine with a variable geometry turbocharger to examine the combustion and emission characteristics of a direct injection hydrogen engine under lean combustion conditions. Additionally, a CFD simulation model was established to investigate the formation process of the air-fuel mixture and the generation of nitrogen oxides (NOx). The findings demonstrated that the uniformity of the hydrogen-air mixture in the cylinder descended with a delay in SOI (start of injection), and that the thermal efficiency often rose initially before declining. When SOI was set at −80°CA ATDC, the engine achieved maximum indicated thermal efficiency, brake thermal efficiency, and output torque values of 40.8%, 36.22%, and 61.2 Nm, respectively. However, as the SOI was delayed, the combustion variability ascended, and when the SOI was delayed to −20°CA ATDC, there was a significant chance that abnormal phenomena like pre-ignition would occur. BTE achieves a maximum of 36.38% when λ is 2.2 and SOI is −40°CA ATDC. Under lean combustion conditions, temperature was a major factor in the generation of NOx, and homogenous combustion can substantially reduce NOx emissions. NOx emissions are always less than 100 ppm when SOI is before −80°CA ATDC.

Performance Evaluation of Low-Concentration Methane Combustion in a Four-Layer Gradually-Varied Porous Burner

ABSTRACT Porous media combustion (PMC) has made a comeback as a practical technology for the utilization of low-concentration methane (LCM) from coal mining. However, the conventional direct-fired PM burner still suffers from the rampart of flame stability and combustion efficiency not addressing industrial demands. Herein, a four-layer gradually-varied porous burner was innovatively developed for LCM combustion to investigate combustion performance under lean combustion conditions (CH4 volume fraction below 5%). The methane conversion, pollutant emissions, and flue gas temperature were also evaluated in detail. The results indicated that the burner offered a favorable combustion resistance due to the gradually-varied PM arrangement to heighten combustion stability with subtle temperature fluctuation and flame migration. The flammability limit of LCM was extended to the lowest equivalence ratio of 0.43 with stationary combustion at 240°C for 120 min. The energy efficiencies of the LCM combustion under lean combustion conditions were greatly boosted with the highest combustion and thermal efficiencies attained at 99.52% and 70.05%, respectively. The maximum 99.93% CH4 conversion was acquired at an equivalence ratio of 0.45 and a flow rate of 80 L/min. LCM combustion in the burner achieved extremely low pollutant emission levels and the overall CO and NOx emissions were 58.04 ppm and below 23 ppm respectively under the experimental conditions. In addition, the high-quality flue gas with an average temperature of more than 516°C was detected in the operating process, which allowed the available heat utilization at the coal mine scenes or in other industries.