Ultra-lean Mixture Research Articles

Numerical study on the inner spray and combustion of active pre-chamber jet ignition for achieving higher performance of a hybrid engine under ultra-lean conditions

The mechanism of active pre-chamber (APC) inner fuel-air mixing, initial flame kernel formation, and jet flame evolution is still not clear. In this study, different APC inner geometry structures coulped with different mixture status were designed to synthetically analyze the inner gas flow, spray mixing, and hot jet flame evolution. The results showed that the APCs with small cone angles of 45° and 50° could promote the fuel-air mixing and spray atomization in a wider APC top space compared to APCs with large cone angles of 60° and 65°. An appropriate APC cone angle of 55° could reduce rich mixture zone and facilitate rapid flame kernel formation, fast flame propagation, and high jet activity, which was good for igniting the in-cylinder ultra-lean mixture. APCs with real fuel injection showed one heat release rate (HRR) peak, while APCs filled with homogeneous mixture showed the typical two peaks in the HRR profiles, the suitable mixture in APC duct section burned and formed the second HRR peak. The short thick APC induced the rich mixture distributed around the spark plug to form the initial flame kernel and showed a faster concentrated combustion process along with a higher pressure difference between APC and MC.

Controllability of Pre-Chamber Induced Homogeneous Charge Compression Ignition and Performance Comparison with Pre-Chamber Spark Ignition and Homogeneous Charge Compression Ignition

This paper presents an experimental and numerical evaluation of the pre-chamber induced HCCI combustion concept (PC-HCCI) in terms of engine performance, emissions, and controllability. In this concept, a spark-initiated combustion in the pre-chamber is utilized to trigger the kinetically controlled combustion of an ultra-lean mixture in the main combustion chamber. The experimental measurements were performed on a single-cylinder engine with a custom-made active pre-chamber. A high compression ratio of 17.5 was used, which limits the maximum achievable engine load due to high knocking tendency but enables both standard PCSI combustion (flame propagation) at very high dilution levels and HCCI combustion at reasonable intake temperatures. The analysis of combustion characteristics and the resulting performance is performed at indicated mean effective pressures (IMEPs) of 3.5 and 3.0 bars, and three different intake temperatures of 80 °C, 90 °C, and 100 °C. The variation in engine load was achieved by adjusting the excess air ratio in the main chamber. On each combination of intake temperature and engine load, a spark sweep and an injected PC fuel mass sweep were performed to obtain the highest indicated efficiency while satisfying the restrictions in terms of combustion stability and knock intensity. It was shown that, unlike in a conventional HCCI engine, the combustion phasing can be directly and reliably controlled by adjusting either spark timing or the reactivity of the pre-chamber mixture, ensuring adequate combustion stability and eliminating potential misfires. A similar indicated efficiency as with conventional HCCI combustion was obtained, while the NOx emissions, although slightly elevated, are still insignificant. Compared to PCSI combustion at the same engine load, a 4-percentage-point increase in indicated efficiency and two times lower NOx emissions were achieved. Compared to the most efficient PCSI operating point, it was 1 percentage point lower, indicating that efficiency was achieved, but the specific NOx emissions are reduced by approximately 70%. Most importantly, very similar performance was obtained with significant variations in intake temperature, proving the reliability and adaptability of this combustion concept.

Evaluation of the turbulent hot jet flame characteristics for achieving high thermal efficiency of hybrid engine

Turbulent jet ignition with active pre-chamber (APC) could enhance the ultra-lean combustion stability thus improving the engine's thermal efficiency. Different APCs structures are analyzed in previous studies, but the lack of quantitative evaluation criteria makes it difficult to optimize the APC under various engine conditions. This paper aims to investigate the characteristic of APC hot jets and their effects on ignition capability and engine combustion performance from perspectives of jet velocity, temperature, turbulent kinetic energy (TKE), and turbulent intensity. The quantitatively evaluate method of hot jet activity based on the key intermediate species during the chemical reactions was proposed. Different APC jet flames and corresponding engine main chamber (MC) combustion performance were discussed by adjusting the APC nozzle diameter and number. The result showed that the jet velocity in a certain range of 450 to 700 m/s has a positive effect on igniting the ultra-lean mixture inside MC. The jet velocity mainly affects the ignition position of jet flame in MC, a higher jet velocity induces the ignition to occur near the cylinder wall and increases the thermal diffusion and heat losses, leading to the lower ignition ability. The jet temperature shows a relatively lower effect on igniting the ultra-lean mixture as the maximum jet flame temperature is over 2250 K. The jet TKE and velocity exhibit a strong correlation in engine performance. The jet turbulence intensity rather than jet TKE strongly influences igniting the ultra-lean mixture. The jet turbulence intensity in the range of 0.025 to 0.035 is beneficial to ignition ability and engine combustion performance. The 4-nozzle APC produces a relatively high jet activity. It’s confirmed that the jet activity higher than 0.055 is sufficient for igniting the ultra-lean mixture. The 4-nozzle APCs can achieve higher engine-indicated thermal efficiency (ITE) as compared to 6-nozzle APCs. The peak ITE of 51.6% is achieved by the 4-nozzle APC with an orifice diameter of 1.8 mm that meets all the good jet characteristics that can produce strong ignition capability.

Prediction of the mean turbulence intensity with a thermodynamic model for CNG and gasoline fuels

Combustion is the main parameter that affects efficiency and exhaust gas emissions in internal combustion engines. In this study, the burning speed of gasoline and CNG were investigated quantitatively by calculating the mean value of turbulent consumption speed instead of qualitatively as is usually done. A three-zone quasi-dimensional thermodynamic model based on the measured cylinder pressure was created to calculate the mean value of turbulent consumption speed and turbulence intensity. The mass flow rate of air was kept constant in all experiments as much as possible, and the spark advance was kept constant at each relative air/fuel ratio. Thus, the effect of fuel and combustion chamber design on the consumption speed and turbulence intensity was directly determined. MR shape reached the highest consumption speed and turbulence intensity in all conditions. In the flat geometry, without any bowl, speed continuously decreased differently from the other designs. Natural gas burned clearly faster in the ultra-lean mixture. The increase in turbulence intensity has different effects on CNG and gasoline. The highest value of the mean turbulence intensity was calculated as approximately 3.4 m/s in the MR design. In the ultra-lean mixture, although the mass flow rate of air was constant, the mean value of the turbulence intensity changed in the same combustion chamber. Therefore, it has been determined that the combustion process affects the turbulence intensity. Using the quasi-dimensional thermodynamic model, mean values of the turbulent burning speeds and turbulence intensity might be calculated without having any optical observation and CFD analysis.

Numerical simulation of the mixture distribution and its influence on the performance of a hydrogen direct injection engine under an ultra-lean mixture condition

In this study, a three-dimensional numerical model of a hydrogen direct-injection engine was established, and the combustion model was verified by experimental data. The influence of the injection timing and nozzle diameter on ultra-lean combustion was evaluated. The results suggest that, with the delay in the injection timing, the mixture concentration near the spark plug and combustion speed gradually increase. The maximum thermal efficiency increased from 47.44% to 49.87%. The combustion duration and ignition lag are shortened from 19.15°CA to 11.15°CA to 16.13°CA and 5.92°CA, respectively. As the nozzle diameter increased, the injection duration was shortened, and the mixture distribution area became more concentrated. Furthermore, under ultra-lean combustion, the combustion rate is more sensitive to the distribution of the mixture. Appropriately increasing the equivalence ratio near the spark plug can significantly shorten the ignition lag and combustion duration and obtain a higher thermal efficiency.

Detecting the Flame Front Evolution in Spark-Ignition Engine under Lean Condition Using the Mask R-CNN Approach

In the wake of previous works, the authors propose a new approach for the identification and evolution of the flame front in an optical SI engine. Currently, it is an essential prerogative to characterize the capability of innovative igniters to guarantee earlier flame development in critical operating conditions, such as ultra-lean mixture, towards which automotive research is moving to deal with the ever more stringent regulations on pollutant emissions. The core of the new approach lies in the R-CNN Mask method. The latter consists of a conceptually simple and general framework for object instance segmentation. It can efficiently detect objects contained in an image while simultaneously generating a high-quality segmentation mask for each instance. In particular, the aim this work is to develop an automatized algorithm for detecting, as objectively as possible, the flame front evolution of lean/ultra-lean mixtures ignited by low-temperature plasma-based ignition systems. The capability of the Mask R-CNN algorithm to automatically estimate the binarized area, without setting a defined binarized threshold, allows us to perform an analysis of the flame front evolution completely independent from the user interpretation. Mask R-CNN can detect the kernel in advance and can identify events as regular combustions instead of misfires or anomalies if compared to other traditional approaches. These features make the proposed method the most suitable option to analysis the real behavior of the innovative ignition systems at critical operating conditions.

Influences of the control parameters and spark plug configurations on the performance of a natural gas spark-ignition engine

Lean-burn and exhaust gas recirculation (EGR) strategies have been used in natural gas spark ignition (SI) engines for meeting the increasingly stringent emission standards. However, the combustion rate of the natural gas (NG) decreases under the lean or ultra-lean and EGR operating conditions. In this paper, a simulation model based on the two-zone combustion modelling approach is established and validated with the experimental data. Then, configuration with different lambdas and EGR ratios, and twin-spark plugs are simulated by using this calibrated model to predict the performance of the NG SI engine. The results indicate that the EGR strategy is more effective in decreasing NOx emissions than that of the lean-burn strategy due to its thermal, dilution, and chemical effects of the EGR components. In addition, the formations of the HC emissions are amplified due to the dilution and chemical effects of the EGR components. Furthermore, under the lambda value of 1.5, the 50% mass fraction burned of the NG SI engine is located at 21.77 °CA after the top dead center (TDC) when the single-spark plug configuration is used, while it is located at 16.58 °CA after the TDC when the twin-spark plugs configuration is used. Besides, compared to the single-spark configuration, using the twin-spark plugs configuration in the NG SI engine is more conducive to decreasing the ignition delay period and combustion duration. Overall, it is verified that the use of twin-spark plugs configuration can effectively extend the lean-burn limit and enhance the combustion rate for the NG SI engine, particularly under the ultra-lean mixture.

Numerical investigation of a fueled pre-chamber spark-ignition natural gas engine

Pre-chamber spark-ignition (PCSI) is a leading advanced ignition concept for internal combustion engines with the potential to enable diesel-like efficiency in medium-duty/heavy-duty (MD/HD) natural gas (NG) engines. By leveraging distributed ignition sources from multiple turbulent jets, the PCSI technology can deliver extremely short combustion duration in ultra-lean mixtures and significantly improve the engine thermal efficiency. However, in the automotive industry there is a lack of adequate science base and predictive simulation tools required for commercial development of PCSI engines. In this study, Reynolds-Average Navier-Stokes simulations are carried out to describe the combustion process in lean-burn NG engines, focusing on the combustion modeling approach. Two combustion models, multi-zone well-stirred reactor (MZ-WSR) and G-equation, are used to simulate the combustion process in an MD NG engine equipped with a fueled-PCSI system for four operating conditions close to the lean operating limit. A skeletal chemical mechanism and a laminar flame speed tabulation are used to compute the combustion accurately. Simulation results are compared with experimental data regarding measured cylinder pressure, heat release rate, and combustion duration. By dividing the PCSI combustion process into four distinct phases, the difference between the two models’ results for each phase is analyzed in detail. The MZ-WSR model overestimates the combustion duration for early flame kernel growth in the pre-chamber due to the lack of a specific formulation to take turbulence-chemistry interaction into account. Despite the prolonged combustion duration and low pressure built-up inside the pre-chamber, the model matches the combustion rate in the main-chamber. In contrast, the G-equation model delivers good agreements for the pre-chamber combustion and turbulent jet-driven combustion processes. However, the model starts to underestimate the combustion rate in the main-chamber, especially under ultra-lean mixture conditions. Finally, improvements are needed for both models to simulate the later combustion stage that occurred in the near-wall regions.

Intensification of ultra-lean catalytic combustion of methane in microreactors by boundary layer interruptions – A computational study

Oxidation of an ultra-lean mixture of methane and air over a platinum catalyst at the constant temperature of 1000 K is investigated numerically in several microreactor configurations featuring different hydrodynamics. These include a straight microchannel with the catalyst coated on the walls and a few wavy microchannels with continuous and discretised catalytic coating. The surface generated CO2 is selected as the indicator of catalytic activity and is evaluated along the catalytic surfaces and the outlet of reactor. It is shown that separation and reattachment of the boundary layer significantly alters the catalytic activity by modifying the structure of concentration boundary layer. Comparison of a strategically coated wavy microchannel with a straight microchannel, with accounting for the residence time, yields an increase of up to 400% in the production rate of CO2. It is argued that the observed hydrodynamic effects upon catalytic activities could help designing highly improved catalytic microreactors.

Prediction of Ultra-Lean Spark Ignition Engine Performances by Quasi-Dimensional Combustion Model With a Refined Laminar Flame Speed Correlation

AbstractThe turbulent combustion in gasoline engines is highly dependent on laminar flame speed SL. A major issue of the quasi-dimensional (QD) combustion model is an accurate prediction of the SL, which is unstable under low engine speeds and ultra-lean mixture. This work investigates the applicability of the combustion model with a refined SL correlation for evaluating the combustion characteristics of a high-tumble port gasoline engine operated under ultra-lean mixtures. The SL correlation is modified and validated for a five-component gasoline surrogate. Predicted SL values from the conventional and refined functions are compared with measurements taken from a constant-volume chamber under micro-gravity conditions. The SL data are measured at reference and elevated conditions. The results show that the conventional SL overpredicts the flame speeds under all conditions. Moreover, the conventional model predicts negative SL at equivalence ratio ϕ ≤ 0.3 and ϕ ≥ 1.9, while the revised SL is well validated against the measurements. The improved SL correlation is incorporated into the QD combustion model by a user-defined function. The engine data are measured at 1000–2000 rpm under engine load net indicated mean effective pressure (IMEPn) = 0.4–0.8 MPa and ϕ = 0.5. The predicted engine performances and combustions are well validated with the measured data, and the model sensitivity analysis also shows a good agreement with the engine experiments under cycle-by-cycle variations.

Study on lean burn limits and burning characteristics of n-heptane with effects of hydrogen enrichment

Lean combustion can be a more effective method to enhance the thermal efficiency of diesel engine, while also face incomplete-combustion and long-combustion duration issues at ultra-lean conditions. Effects of hydrogen, known with wider flame limits and greater burning velocity, on lean burn limits and burning characteristics of n-heptane were studied fundamentally based on optical experiments and chemical kinetics analysis. The results showed that the laminar flame appeared spherically from λ = 0.8 to λ = 1.5 by spark ignition energy of 37 mJ, whereas leaner mixture, λ ≥ 1.6, could only be ignited by 3000 mJ. However, the low ignition energy failed to burn the ultra-lean mixture, while high ignition energy disturbed the flame morphology. The buoyancy appeared near lean-limits due to lower burning velocity and rapid increase of thermal radiation losses. Lean burn limits of n-heptane increased from λ = 1.9 to λ = 4.2 at 393K, λ = 2.0 to λ = 4.3 at 433K and λ = 2.1 to λ = 4.4 at 473K under H2 addition from 0% to 90%. The increasing impact became more significant at H2 ≥ 70%. Besides, as lifting initial temperature, 393–473K, the lean-limits increased 0.2 λ only. With increase of H2, H increased faster than O and OH because more H2 reacted with both O and OH to produce H. CH2O became prominent among C–H–O species, which reacted with H increased rapidly under H2 addition. Consequently, H and CH2O played a substantial role to improve the lean burn limits and burning velocity of n-heptane under hydrogen addition.

Large-scale Dynamics of Ultra-lean Hydrogen-air Flame Kernels in Terrestrial Gravity Conditions

ABSTRACT The paper discusses the results of an experimental study on combustion development in a large volume filled with ultra-lean hydrogen-air mixture under terrestrial gravity conditions. For the first time, characteristicflame ball evolutionIn particular, ittheBesides, the lateral expansion of the flame ball plays an important role, defining the changes in curvature radius and stretching of the flame. Due to this, flame breaks up and secondary flame kernels arise. So, the developed structure of the ultra-lean flame occurs to be much more complex than earlier registered cap-shaped flames. Large-scale dynamics of the flame ball is of paramount interest for understanding the stability of near-limit flames and for estimation of hazards related to the development of flame balls under real conditions. Studied processes represent a possible way of energy transfer from the spatial areas filled with less reactive mixtures toward areas with high mixture reactivity. Such a phenomenon stages of the during its convective upward motion on large spatial scales are described. The physical mechanisms defining the particularities of ultra-lean flame evolution subjected to the natural convection are proposed. is shown that the key role in the process of flame propagation in the ultra-lean mixture belongs to the convective rise of the hot combustion products in the gravity field. Herewith, velocity of the flame upward motion occurs to be much higher than the laminar burning velocity in the considered ultra-lean hydrogen-air mixture. may become a reason for the development of emergencies and should be thoroughly investigated and taken into account while designing reliable fire and explosion safety systems. Experimental results are compared with previously obtained numerical data that allowed to demonstrate the correctness of the mathematical modeling and physical mechanisms identified on its basis.

Reducing NOx emissions by adding hydrogen-rich synthesis gas generated by a plasma-assisted fuel reformer using Saudi Arabian market gasoline and ethanol for different air/fuel mixtures

Environmental contamination poses a real threat to the environment and all organisms. Air pollution has increased markedly due to an increase in human activities and petroleum use for electricity generation, transportation, and industrial applications. Internal combustion engines play a significant role in society’s health and power requirements. However, automobiles are the main source of pollution and NOX emissions. This work presents a study of the performance and exhaust emissions of an internal combustion engine fuelled by gasoline available in the Saudi Arabian market, RON91/RON95, with an admixture of syngas and 5% by volume pure ethanol (E5) in the presence of different ultra-lean mixture regimes, including λ=1 for a stoichiometric mixture. The studied ranges were λ=1.13, λ=1.26, λ=1.43, and λ=1.67. An entirely automated engine and plasma converter system was developed for feeding the same type of fuel. The engine was modified for a more efficient operation by introducing a plasma-based fuel reformer. Syngas was produced through the partial oxidation of gasoline with air in a plasma-assisted fuel reformer in the presence of steam to reduce the amount of soot formed in the plasma reactor. The fuel consumption and related emissions were measured. The experimental results demonstrated a significant total reduction of NOx emissions compared with those from the original engine. The most obvious reduction (approximately 50%) of harmful pollution was observed under lean conditions, and the total gasoline consumption (including the gasoline required for the plasma-assisted converter) slightly increased. The results also showed that the NOx content for these new blends was lower using E5-gasoline 91 than that using E5-gasoline 95 and was generally lower using E5-gasoline 91 and syngas than that using E5-gasoline 95 and syngas.

Investigation of Burn Duration and NO Emission in Lean Mixture with CNG and Gasoline

The results of experiments performed by gasoline and natural gas fuels in a single cylinder research engine were evaluated in this study. The main objective of this study is to compare exhaust gas emissions, efficiency, and burn durations for both fuels in stoichiometric and lean mixture. At the same time, cycle to cycle variation in these operating conditions should not exceed an acceptable value. In the ultra-lean mixture, gasoline fuel exceeded this determined limit before Compressed Natural Gas (CNG). Therefore, the reduction in NO was restricted by cyclic variations. In combustion analysis, although the burn duration of the gasoline in stoichiometric conditions was shorter than CNG, this situation reversed in favor of CNG in the ultra-lean mixtures. Contrary to some studies in the literature, the spark advance and ignition delay for CNG were the same or shorter than gasoline in this study. The primary reasons for this change are the high compression ratio and the different combustion chamber geometry. The increase in turbulence intensity has different effects on CNG and gasoline. As a result, it has been observed that NO emissions can meet the limits without a loss of efficiency for this engine operated with CNG under the ultra-lean mixture.

Combustion Instabilities of Ultra-Lean Premixed Mixtures by Prechamber Turbulent Jet Ignition

Unstable combustion was observed in a dual-chamber combustor that mimics heavy-duty gas engines utilizing prechamber jet ignition. Prechamber combustion generated a hot turbulent jet that ignited the ultra-lean mixture in the main chamber. Thermoacoustic combustion instability was initiated for main-chamber equivalence ratio, , and grew severe in the lean-burn regime, . Simultaneous schlieren and chemiluminescence were used to visualize the unstable flame propagation and to characterize the instability. Classical acoustic resonator analysis and pressure spectra showed the longitudinal mode of acoustic disturbance as the primary instability mode for all equivalence ratios. However, a leaner flame was affected more due to stronger coupling between heat release and the pressure field perturbation. Transverse and mixed modes were observed at lean conditions, . A phase-resolved measurement of strain rates along flame edge showed an oscillating strain along pressure perturbation cycle. To model the instability, 3D linearized Euler equations were solved in the frequency domain coupled with combustion response models. The physical significance of the instability modes and their behavior were identified using dynamic mode decomposition. Based on the experimental and modeling results, a mechanism for thermoacoustic instability was proposed for ultra-lean premixed ignition by a hot turbulent jet.

Large-scale flame structures in ultra-lean hydrogen-air mixtures

The paper discusses the peculiarities of flame propagation in the ultra-lean hydrogen-air mixture. Numerical analysis of the problem shows the possibility of the stable self-sustained flame ball existence in unconfined space on sufficiently large spatial scales. The structure of the flame ball is determined by the convection processes related to the hot products rising in the terrestrial gravity field. It is shown that the structure of the flame ball corresponds to the axisymmetric structures of the gaseous bubble in the liquid. In addition to the stable flame core, there are satellite burning kernels separated from the original flameball and developing inside the thermal wake behind the propagating flame ball. The effective area of burning expands with time due to flame ball and satellite kernels development. Both stable flame ball existence in the ultra-lean mixture and increase in the burning area indicate the possibility of transition to rapid deflagrative combustion as soon as the flame ball enters the region filled with hydrogen-air mixture of the richer composition. Such a scenario is intrinsic to the natural spatial distribution of hydrogen in the conditions of terrestrial gravity and therefore it is crucial to take it into account in elaborating risk assessments techniques and prevention measures.

Natural Gas Partially Stratified Lean Combustion: Analysis of the Enhancing Mechanisms into a Constant Volume Combustion Chamber

The use of ultra-lean mixture natural gas fueled Internal Combustion Engines has been widely accepted to improve thermal efficiency and reduce exhaust emissions. This is mainly due to the intrinsic features of natural gas such as the wide availability and lower carbon content. Other advantages may occur running the engine under lean operating condition, as for example the lower flame temperature, the higher volumetric efficiency and compression ratios achievable, together with the opportunity of load control by varying the mixture equivalent ratio. Issues related to combustion stability and increase of CoV may however arise at leaner conditions.A comprehensive analysis of natural gas ultra-lean combustion process has been presented in this work using local charge stratification (Partially Stratified Charge combustion) to stabilize the process and limit the above-mentioned issues. A detailed analysis of the main sub-phases of the Partially Stratified Charge combustion process has been performed using a LES approach to highlight the main driving mechanisms of this combustion strategy. The accuracy of the numerical results has been evaluated by means of statistical and grid independence analysis. The effects of the local conditions around the spark plug on the propagation of the subsequent combustion event have demonstrated to play a key role on the efficiency of ignition and combustion sub-processes. A trade-off between the competing mechanisms of heat losses generated by heat convection of the gaseous jet and decay of the turbulent kinetic energy enhancing the process has been highlighted by the numerical results.Results show that a one-equation closure model is able to predict accurately the evolution of the injection process, providing a more reliable description of the local distribution of the fuel-oxidizer mixture and turbulent kinetic energy. Very good agreement between numerical results and experimental data has been obtained for the simulation of the combustion process. In particular, the numerical framework here proposed has been able to correctly capture two fundamental aspects of the PSC combustion, such as the interaction between the fuel jet and the flame evolution, which results in an elongated and strongly corrugated flame plumb, and the quenching due to the local high velocity field around the spark plug. These phenomena are crucial to control the combustion stability and the cycle-by-cycle variability in lean burn Internal Combustion Engines.

Experimental Study on the Effect of Hydrogen Enrichment of Methane on the Stability and Emission of Nonpremixed Swirl Stabilized Combustor

An ultra lean mixture (ϕ ≤ 0.5) of methane–hydrogen–air was experimentally investigated to explore the effect of fuel flexibility on the flame stability and emission of a nonpremixed swirl stabilized combustor. In order to isolate the effect of hydrogen addition to methane, experiments were carried out at fixed fuel energy input to the combustor while increasing the hydrogen content from 0% up to 50% in the methane–hydrogen mixture on volume basis. The combustor fuel energy was then increased up to the range of typical gas turbine combustors. Equivalence ratio sweep was carried out to determine the lean stability limit of the combustor. Results show that the hydrogen content in the fuel mixture and fuel energy input have a coupled effect on the combustor lean blow off velocity (LBV), temperature and emissions. The LBV increases by ∼103% with the addition of 30% H2. On the other hand, the LBV increases by ∼20% as the fuel energy increases from 1.83 MW/m3 to 2.75 MW/m3. Burning under ultra lean condition serves two purposes. (1) The excess air supplied reduces the overall combustor temperature with its ensuing effect on low NOx formation. (2) It increases the overall combustor volume flow rate which reduces the residence time for NOx formation. The axial temperature profile presented along with the emission data can serve as basis for the validation of numerical models. This would give more insight onto the effect of hydrogen on the turbulence level and how it would improve the localized extinction of methane in a cost-effective way.

High power performance with zero NOx emission in a hydrogen-fueled spark ignition engine by valve timing and lean boosting

For a hydrogen engine to be successfully commercialized in a short period of time, it must have power on the same level as gasoline engines, almost no NOx emissions, and high efficiency. This simultaneous achievement would be possible with the use of low temperature combustion, which supercharges an ultra-lean mixture, because NOx from a hydrogen engine is thermally produced. However, increasing the energy input for higher output produces backfire, which is a significant problem for hydrogen-fueled engines with external mixture, and why the simultaneous achievement of high output and low-exhaust is not easily accomplished. This research investigated the potential of simultaneously achieving high output, similar to the output of a gasoline engine, without backfire, by using a complex valve timing variation and lean boosting. The study achieved almost zero NOx emission and high efficiency in a single cylinder engine built for research purposes with a valve timing variation system.Experimental results revealed that retarding the intake valve opening timing could control the backfire generated when increasing the output of the hydrogen engine, and the method above is also effective with lean boosting. Stable combustion in the lean region of Φ=0.2, which can reduce temperature below the NOx generation temperature, was possible due to the large ultra-lean limit of hydrogen. In addition, the exhaust was almost NOx free due to the low temperature combustion, by supercharging the ultra-lean mixture under high power operation similar to that of gasoline.The above results verified that almost pollution-free emission without backfire, and simultaneous high power and efficiency could be achieved in a hydrogen engine through a method of intake valve timing retardation and lean overcharging.

Parametric study of instantaneous heat transfer based on multidimensional model in direct-injection hydrogen-fueled engine

This paper presents a parametric study on instantaneous heat transfer of a direct-injection hydrogen-fueled engine using a multidimensional model. A simplified single-step mechanism was considered for estimating the reaction rate of hydrogen oxidation. The modified wall-function was used for resolving the near-wall transport. An arbitrary Lagrangian–Eulerian algorithm was adopted for solving the governing equations. Experimental measurements were implemented to verify the developed model. They show that the instantaneous heat-transfer model is sufficiently accurate. The influence of engine speed, equivalence ratio, and the start of injection timing were investigated. The flow fields appeared to have greater size vectors and coarser distribution with an increase of engine speed. A heterogeneous distribution was obtained for an ultra-lean mixture condition (φ ≤ 0.5), which decreased with an increase of equivalence ratio. There was no pronounced influence of the start of injection on the flow field pattern and mixture homogeneity. Thermal field analysis was used to demonstrate trends in the instantaneous heat transfer. It was observed that there was a crucial distinction between the lean and ultra-lean mixtures as well as the engine speed. Furthermore, a non-uniform behavior was found for the impact of the equivalence ratio on temperature distribution. It is clear that the developed models are powerful tools for estimating the heat transfer of the hydrogen-fueled engine. The developed predictive correlation is highly accurate in predicting the heat transfer of the hydrogen-fueled engine, focusing on the equivalence ratio as a governing variable.