End-gas Autoignition Research Articles

An insight into direct water injection applied on the hydrogen-enriched rotary engine

The attractiveness of direct water injection advantages has made a comeback in the minds of designers due to the potential knock mitigation and lower NOx emissions. The current work established and validated an appropriate 3D model for hydrogen-enriched Wankel engines with direct water injection, and the used CFD simulation was mainly a consideration of combustion and flow phenomena inside the engine and securing the gained knowledge. The determinations of the knock operation and position were investigated, and the intrinsic mechanism of end-gas auto-ignition was clarified. Then, the effects of water injection strategies on knock mitigation and NOx reduction were analyzed in detail. Results showed that the end-gas auto-ignition caused the drastic variation of the local velocity field and the severe pressure oscillation in the vicinity of these regions, which led to knocking combustion. The highest knock propensity occurred at the trailing part of the recess, especially on the opposite side of the intake port. The pressure oscillation of water-enriched schemes was smaller than water-free schemes, and the knock intensity gradually decreased with the advance of water injection timing. When the injection timing was 250°EA, the propensity of end-gas auto-ignition was weakened obviously due to the reduced local temperature and pressure within the rotor chamber, and NOx formation was lower than the other schemes. The local pressure fluctuation was decreased by increasing water addition, whose maximum decrement occurred at the trailing part of the recess. The knock intensity gradually decreased as the amount of water injection increased. When the water injection ratio of 40%, there was no obvious auto-ignition occurrence, and NOx formation reduced significantly at the exhaust moment, which confirmed direct water injection was one of the most promising and effective methods to control knock propensity and NOx emissions simultaneously. In addition, the water droplets of a smaller water injection ratio (10%) scheme spread faster than the water-free scheme after spontaneous combustion, which could reach the fuel surface to present azeotropic phenomena and then enhance the combustion. This result shall give insights into the feasibility of a proper water addition for improving the thermal efficiency of the Wankel engine regime.

Analysis of pressure oscillations and wall heat flux due to hydrogen auto-ignition in a confined domain

This study investigates the impact of the near-wall temperature gradient on hydrogen auto-ignition characteristics using one-dimensional (1D) fully resolved simulations. Ten cases are simulated, one featuring normal combustion and the other nine simulating auto-ignitive combustion with different initial pressures, equivalence ratios, and near-wall temperature gradients. The simulations show that the near-wall temperature gradient greatly affects the onset and intensity of the auto-ignition event. For cases with the initial conditions of 833.3 K and 15 bar, a small near-wall temperature gradient delays the timing of auto-ignition and places the auto-ignition kernel further away from the wall, facilitating deflagration-to-detonation transition of the auto-ignitive flame. This leads to a large increase in pressure oscillations within the domain and heat flux to the wall. When the initial conditions are changed to 900 K and 20 bar, the magnitude of the near-wall temperature gradient also affects the number of auto-ignition events, leading to a significant impact on the wall heat flux. The results suggest that an accurate modeling of the near-wall temperature gradient is necessary for the simulations of hydrogen end-gas auto-ignition. This requires special considerations in the near-wall region and a careful selection of the wall heat transfer model in Computational Fluid Dynamics (CFD) tools, such as Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES).

A numerical study on knock combustion suppression using targeted fuel injection in an SI engine

A novel concept to suppress knock combustion using the targeted fuel injection in a spark-ignition (SI) engine is proposed, which utilizes a customized injector to inject a small portion of the total fuel toward the end-gas region in the late compression stroke so that charge cooling effect by fuel vaporization is imposed on the local mixtures. CFD studies were carried out to evaluate the effects of various targeted fuel injection strategies on the processes of charge cooling, air-fuel mixing and combustion in a 1.2-L gasoline SI engine with a high compression ratio. A knock prediction model adopting a level set G-equation model coupled with a transported Livengood-Wu integral correlation was applied to predict the knock event by capturing the occurrence of end-gas auto-ignition. It was found that the local gas temperatures in the sprayed region decreased and knock combustion tendency was suppressed indeed, and the degree of the reduction in local gas temperature and knock tendency was affected by the selected targeted injection strategy. However, the results also indicated that excessive fuels injected could result in worsen air-fuel mixing that led to incomplete combustion. Hence, the injection strategy needs to be optimized by balancing the knock tendency and combustion efficiency. The study further shown that the spark timing could be advanced by 3.3° crank angle in a particular injection strategy case by comparing with the non-targeted-injection case, which demonstrated the effectiveness of the proposed concept.

Comparison of combustion duration and end-gas autoignition in inwardly and outwardly propagating flames induced by different ignition configurations

Recently, multiple spark ignition has received great attention since it helps to increase thermal efficiency and to reduce misfire in engines. Multiple spark ignition also affects the combustion duration and thereby it can be used for knock control. However, previous studies reported opposite trends in terms of how multiple spark ignition affects engine knock. This work aims to assess and interpret the influence of flame propagation direction induced by different ignition configurations on combustion duration and end-gas autoignition/engine knock. Two simplified and idealised ignition configurations are studied theoretically and numerically. One is with infinite number of sparks at side circular wall, which induces an inwardly propagating flame (IPF); and the other is with a single central spark, which induces an outwardly propagating flame (OPF). In the asymptotic theoretical analysis, the canonical 1D formulations for IPF and OPF are reduced to 0D model. Based on the 0D model, OPF and IPF at different initial temperatures are studied and compared. Counterintuitively, it is found that the combustion duration of OPF is shorter than that of IPF when there is no end-gas autoignition. On the other hand, the combustion duration of IPF is shorter than that of OPF when end-gas autoignition occurs. Furthermore, end-gas autoignition is found to be more prone to occur in IPF than OPF. These interesting observations are interpreted through assessing the ignition delay time and different components of the absolute flame propagation speed. The theoretical results are validated by transient simulations considering detailed chemistry and transport which are conducted for IPF and OPF in an iso-octane/air mixture at different initial temperatures and pressures. Both theoretical and numerical results suggest that compared to infinite number of ignition sparks at side wall, the single central ignition has the advantages in shortening the combustion duration and reducing the tendency of end-gas autoignition.

Hydrogen pre-chamber combustion at lean-burn conditions on a heavy-duty diesel engine: A computational study

The combustion of hydrogen generates almost no harmful emissions except for nitric oxides (NOx), which could be effectively eliminated under lean-burn conditions. In particular, the pre-chamber combustion concept has the potential to further extend the lean-burn limit. To pursue a sustainable mode of the powertrain in future transportation, this work intended to investigate the hydrogen pre-chamber combustion concept on a heavy-duty diesel metal engine under lean-burn conditions. By using the CFD modeling method, the pathway to achieving higher engine efficiency along with lower NOx emissions were explored. The results demonstrated that the lean-burn passive pre-chamber combustion effectively restrained the heat release process within the pre-chamber and yielded negligible NOx emissions while degrading complete combustion and combustion stability as a trade-off. To extend the engine operating range, an adequate amount of fuel was introduced into the pre-chamber. Compared to methane, the active pre-chamber combustion of hydrogen exhibited a significantly higher tendency to induce end-gas autoignition owing to its faster heat release within the pre-chamber. The pressure rise rate within the pre-chamber was successfully controlled by adjusting the pre-chamber throat diameter. Higher thermal efficiency was achieved with an increased compression ratio at the same spark ignition timing because of the advanced combustion phasing. However, the operating range was narrowed for the limit of end-gas autoignition and high pressure rise rate.

Ignition behaviors of primary reference fuels in a rapid compression machine under vortex-existing/minimized conditions

Knock is one of the main obstacles to improving the thermal efficiency of spark-ignition internal combustion engines. Although knock has been widely studied and accepted as a result of end-gas auto-ignition, the fundamentals regarding auto-ignition behaviors are still not fully revealed. In this study, the ignition behaviors of primary reference fuels were investigated in an optical rapid compression machine equipped with a quartz combustion chamber allowing for visualizing the combustion process from the lateral view. By combining both the lateral view and the top view photography, ignition behaviors under vortex-existing conditions with a creviced piston and vortex-minimized conditions with a flat piston were comprehensively analyzed to reveal the impact of the vortex on the ignition behaviors of PRF fuels. The influence of fuel reactivity was also investigated. The results showed that mild ignition was prevalent under large Da* numbers. The occurrence of mild ignition was closely related to the ignition delay time of the mixture, and the critical ignition delay time was not fixed but decreased with increasing initial temperature. The propensity of mild ignition could be boosted under vortex-existing conditions due to the increasing hotspot formation probability. Vortices were demonstrated to be capable of mitigating knock intensity via 1) the buffer effect of surrounding burned regions on the shock waves generated from surrounded unburnt pockets; 2) a larger burned mass fraction at the instant of the final ignition under vortex-existing conditions. The results also showed that strong ignition was more likely to occur under vortex-minimized conditions. Besides, higher fuel reactivity also could increase the probability of strong ignition occurrence. Compared with the creviced piston, the use of the flat piston could shift the ignition regime towards regions with higher Ret and lower Dat in the Dat-Ret diagram, where the strong ignition is less pronounced.

Detonation development in PRF/air mixtures under engine-relevant conditions

The development of advanced boosted internal combustion engines (ICEs) is constrained by super-knock which is closely associated with end gas autoignition and detonation development. The present study numerically investigates the transient autoignition and detonation development processes under engine-relevant conditions for primary reference fuel (PRF) consisting of n-heptane and isooctane. The effects of PRF composition are systematically examined. By considering the transient local sound speed rather than its initial value, a new non-dimensional parameter is proposed to assess the transient chemical-acoustic interaction and to quantify the autoignition modes. Two detonation sub-modes, normal and over-driven detonation, are identified and the corresponding mechanisms are interpreted. For the over-driven detonation, there exist two developing regimes with weak/strong chemical-acoustic coupling and slow/rapid pressure enhancement. It is found that the maximum pressure caused by autoignition decreases with the blending ratio of isooctane, mainly due to the increase in excitation time. Besides, the strongest detonation induced by hot spot usually occurs within the over-driven detonation sub-regime. Its condition can be well quantified by the new non-dimensional parameter proposed in work and its strength is determined by the ratio of hot spot acoustic time to excitation time. The deviation of transient autoignition front propagation from prediction based on homogenous ignition is mainly attributed to the non-uniform compression effect caused by gradually enhanced pressure wave, while the influence of heat conduction and mass diffusion is negligible. The initial expansion stage dominating the induction period of local autoignition is greatly influenced by the compression of pressure wave. Therefore, the continuously enhanced pressure wave non-uniformly changes the local ignition delay (i.e. reduces its spatial gradient) within the hot spot and thereby accelerates the autoignition front propagation. The relationship among the parameters quantifying the detonation propensity is assessed and interpreted. The present study provides helpful understanding of detonation development under engine conditions.

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.

Numerical study on knock characteristics and mechanism of a heavy duty natural gas/diesel RCCI engine

In this paper, the knock phenomenon of reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel was numerically studied. The knock mechanism of the RCCI engine is explained and the strategy of suppressing knock is put forward. The knock characteristics were studied by setting monitoring points in different spaces positions of the cylinder. The results show that the pressure oscillation amplitude at the center and edge of the cylinder is large under the high load condition of RCCI engine. In addition, the knock mechanism was studied by using pressure difference method, maximum amplitude of pressure oscillation, important components, temperature isosurface, pressure fluctuation and heat release rate. The results show that the knock of RCCI engine is mainly caused by the end-gas auto-ignition. The pressure difference results show that the characteristic frequency is consistent with the natural resonance mode (0,1) of the cylindrical combustion chamber. On this basis, the effects of pilot oil injection timing and compression ratio on engine knock are further studied. It is confirmed that diesel knock and end-gas knock may exist simultaneously in the same cycle when RCCI engine knock occurs. And diesel knock occurs before top dead center, and end-gas knock occurs after top dead center. Proper adjustment of pilot oil injection timing and reduction of compression ratio can effectively suppress engine knock.

Effect of recess shape on combustion performance and knocking characteristics for a downsized gasoline rotary engine

The combustion of a downsized boosted spark ignition rotary engine (RE) is significantly affected by the rotor recess shape owing to the influence this structure has on the flame propagation and the turbulence flow. However, comprehensive assessments on both knocking and combustion performance of an RE for different recess shapes are generally lacking. Therefore, a three-dimensional dynamic simulating model coupling with the suitable turbulent model and the reduced chemical kinetic mechanism was established and validated. The numerical model was implemented for evaluating the influences of varying recess shapes on combustion performance and knocking characteristics of a downsized RE. Results showed that the end-gas auto-ignition of the RE mainly occurred in the trailing side of the elongated chamber due to the unidirectional flow field and a long distance away from spark plugs. It should be noted that the present results indicated that the knocking intensity was directly proportional to heat release rate. Moreover, the size and quantity of the vortex caused by the recess shape during the combustion process were the fundamental reason for determining the knocking and performance of the RE. Importantly, suppressing the formation of the vortexes in the elongated chamber during the combustion process could significantly reduce the knocking intensity of the RE, and then improved engine performance. In addition, under the present operating conditions, the elongated chamber structure with the leading deep recess was found to reduce the knocking intensity by 75% compared to that with the maximum knocking intensity, which increased indicated mean effective pressure and indicated thermal efficiency of the downsized RE. Therefore, considering knocking characteristics and combustion performance, the elongated chamber structure with the leading deep recess was the best choice for the RE used in boosted conditions. • Effect of recess shape on knocking combustion of a SI RE was analyzed numerically. • The autoignition of the RE occurred in the trailing side of the elongated chamber. • The KI was directly proportional to the heat release rate. • Suppressing vortex formation in the combustion phase significantly reduced the KI. • The LDR dramatically suppressed knocking and then enhanced engine performance.

Experimental and Numerical Study on the Effect of Nitric Oxide on Autoignition and Knock in a Direct-Injection Spark-Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO. Experiments were conducted to measure the responses of KL-CA50 and trace-autoignition CA50, the latter being indicative of CA50 at which end-gas autoignition starts to become measurable from the apparent heat-release rate. Chemical-kinetics simulations were conducted to reveal the role of NO for end-gas autoignition, with a specific focus on sequential autoignition in a thermally stratified end-gas.</div><div class="htmlview paragraph">The simulation results reveal that the magnitude of low-temperature heat release (LTHR) generally increases with NO. However, the relative importance of NO for enhancing LTHR diminishes when the LTHR inherent to a fuel’s chemistry is strong, such as at lower temperatures in a thermal boundary layer. This rendered more uniform LTHR within a hypothetical thermal boundary and led to a more sequential (<i>i.e.</i> slower) autoignition event. It was also revealed that a change in compression ratio influences the importance of intermediate-temperature heat release (ITHR) due to changes of the temperature-pressure history of the end-gas. Together with the condition where end-gas autoignition occurs more sequentially, the shorter time spent in LTHR and ITHR regime can counter the increase in autoignition reactivity at high NO levels and allow KL-CA50 to advance.</div></div>

Ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions

Ethanol as an attractive oxygenated fuel is often blended with gasoline to yield beneficial anti-knocking performance. Due to the distinctions in fuel chemistry, the combustion behavior of gasoline-ethanol blends is not fully understood. This study investigated ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions covering the negative temperature coefficient (NTC) region. Three ethanol blending levels (i.e., E10, E20, and E30 on a volume basis) were introduced into gasoline surrogates. Characteristic timescale analysis for homogeneous auto-ignition was conducted, and localized hotspot auto-ignition and reaction wave propagation was investigated for different gasoline-ethanol blends. Meanwhile, ethanol blending effects on detonation development were comparatively explored. The results show that ethanol addition significantly suppresses fuel reactivity for homogeneous scenarios, especially in low-temperature regions, manifesting prolonged timescales and diminishing NTC behavior. Despite this, different reaction wave propagation modes can be identified during transient auto-ignition and reaction wave propagation, including deflagration, detonation development, and thermal explosion. The detonation wave is always at an underdeveloped status under engine-relevant conditions, and the induced pressure characteristics are sensitive to initial temperatures, hotspot sizes, and fuel reactivity. Meanwhile, ethanol addition tends to promote transient detonation development by elevating the reaction front propagation speed inside the hotspot and enhancing the deflagration to detonation transition outside the hotspot. Such observations are not contrary to that ethanol yields beneficial anti-knocking performance because only chemical effects of ethanol blending are addressed, without considering vaporization cooling effects. The present work demonstrates that ethanol addition globally suppresses engine knocking, but it may induce stronger knocking intensity once end-gas auto-ignition and detonation development are initiated. The current study provides useful insights into ethanol perturbative effects on auto-ignition and reaction wave propagation from chemical perspectives.

On knocking combustion development of oxygenated gasoline fuels in a cooperative fuel research engine

Fuel octane number has long been used as an indicator for anti-knock capability of gasolines. However, whether octane number could capture the real knocking combustion process in practical engines remains unknown. In this work, the knocking combustion characteristics of methanol/PRF (Primary Reference Fuels), ethanol/PRF, and iso-propanol/PRF fuel blends with either the same research octane number (RON) or the motor octane number (MON) were studied using a three-dimensional numerical model of a Cooperative Fuel Research (CFR) engine. It is observed that the three fuel blends with the same octane rating present different knocking combustion characteristics. The methanol/PRF blend shows an advanced knock onset and higher knock intensity compared to the ethanol/PRF and iso-propanol/PRF blends at the RON test condition, while the iso-propanol/PRF blend is observed to have the earliest knock onset at the MON test condition. It is discovered that even with the same octane number, deviations in the end-gas auto-ignition and flame characteristics of the three fuel blends lead to different knock onsets, which in turn influences their knock intensity.

Experimental observation on the end-gas autoignition and detonation affected by chemical reactivity in confined space

The deflagration-to-detonation transition remains one of the most interesting and mysterious physical phenomena in the combustion of energetic materials, which contains substantial complicated and nonlinear characteristics. In the present work, the effect of the chemical reactivity of different fuels and diluent gases on the end-gas autoignition and detonation development in a confined space was investigated. Five fuels (hydrogen, methane, iso-octane, n-heptane, and PRF50) and three diluent gases (argon, nitrogen, and carbon dioxide) were used to change the chemical reactivity. The results showed that both the chemical reactivity and shock wave had a significant influence on the end-gas autoignition and detonation development. For mixtures with different diluent gases, it was observed that the transition thresholds (denoted by critical oxygen fraction) increased in the order of argon, nitrogen, and carbon dioxide. Different detonation modes with varying shock compressions were observed under different diluents for n-heptane. Although the flame propagation of different fuels differs at 21% oxygen fraction, end-gas autoignition and detonation development processes can still be observed in all kinds of fuels when the oxygen fraction was elevated to a certain value. The transition thresholds increased in the order of hydrogen, n-heptane, PRF50, iso-octane, and methane. Further analysis revealed that the fuel with a shorter ignition delay usually required a lower flame tip velocity, accomplished with a delayed occurrence of detonation. In addition, the transition threshold was determined by the chemical reactivity and flame speed.

Effect of fuel composition and EGR on spark-ignited engine combustion with LPG fueling: Experimental and numerical investigation

This paper presents an experimental and numerical investigation of a spark-ignited (SI) cooperative fuel research (CFR) engine fueled with different liquefied petroleum gas (LPG) fuels and exhaust gas recirculation (EGR). The effects of LPG fuel composition on engine combustion characteristics are initially evaluated at two different compression ratios (CR). Results show normal combustion at CR 7 and heavy knocking combustion at CR 10 for all the tested fuels, with a more substantial impact for the LPG fuel with high proportions of n-butane species. The Livengood-Wu (LW) integral method is then used to analyze the knock occurrence risk of individual fuel based on the reactivity of the tested fuels. The introduction of EGR then demonstrates the potential of knock intensity reduction below the borderline knock limit. A zonal-based kinetic interactions study is also performed to understand the knock mitigation effectiveness of EGR over the pressure–temperature domain relevant to SI engine operation. Finally, a multidimensional, computational fluid dynamics (CFD) simulation model is shown to predict the LPG combustion characteristics and presents the evolution of in-cylinder temperature and chemical species to demonstrate the development of end-gas autoignition events without and with EGR.

Effect of active ignition on the end-gas autoignition with detonation combustion in confined space

In spark-ignition engines, the presence of knock significantly prevents the engines from further improving thermal efficiency. In this work, the effect of active ignition in the end-gas region on the end-gas autoignition with detonation combustion was optically observed in the self-designed constant volume combustion bomb (CVCB-TJU). The different combustion modes together with pressure oscillation were presented. The results indicate that advancing the ignition timing of the end-gas spark can reduce the pressure oscillation intensity and even suppress the occurrence of autoignition. In addition, the present work observed a statistically positive relationship between unburned mass fraction (UMF) and detonation intensity for the knock induced by developing detonation. The underlying influencing mechanism of end-gas ignition on auto-ignition was presented. The end-gas ignition influences the occurrence and intensity of detonation and pressure oscillation by two means: 1) reducing the UMF burned by autoignition, 2) influencing the flame acceleration, thereby reducing the flame tip velocity, and the subsequent shock intensity. Furthermore, the burned mass fraction (BMF) consumed by end-gas flame when jet flames emerge is applied to substitute the ignition timing when measuring its influence at different oxygen concentrations of 29% and 33%. It is found there exists a critical BMF above which the autoignition would be suppressed. And this critical value increases with increased oxygen concentration.

Influence of gasoline fuel formulation on lean autoignition in a mixed-mode-combustion (deflagration/autoignition) engine

Stoichiometric spark-ignition engines suffer efficiency penalties due to throttling losses at low loads, a low specific-heat ratio of the stoichiometric working fluid, and limits on compression ratio due to end-gas autoignition leading to undesirable knocking. Mixed-Mode Combustion (MMC) mitigates these shortcomings by using a lean working fluid where a spark-initiated pilot-stabilized deflagrative flame front is followed by controlled end-gas autoignition. This MMC study investigates the effects of initial conditions (intake air temperature, intake pressure, equivalence ratio, and intake oxygen fraction) on autoignition tendency of four gasoline-range fuels with varying properties and composition. The use of fuels with varying octane sensitivity (S) allowed exploring the importance of low-temperature heat release in triggering autoignition. Fuels with high S were less reactive for conditions that promote low-temperature chemistry (operation at high intake air pressure or without N2 dilution). Conversely, an Alkylate fuel with low S showed a greater autoignition resistance at operating conditions that were unfavorable for low-temperature chemistry. Next, the effect of residual gas composition on autoignition tendency of fuels was examined with a chemical-kinetics model. Among the various molecules in the residual gas, nitric oxide (NO) enhanced the low-temperature chemistry and increased the autoignition tendency most significantly. The fuels’ autoignition response to increasing NO amount corroborates the experimental observations. Next, the sequential autoignition of the end-gas was assessed to be less impacted by thermal stratification because of lean mixtures showing relatively less low-temperature chemistry, when compared to stoichiometric mixtures. Next, the effect of changing equivalence ratio on the autoignition was found to be similar for all fuels, regardless of their S. With changing intake air temperature, the response of fuels’ autoignition tendency depended on the dilution level used. At high dilution (i.e. low intake [O2]), fuels’ reactivity increased with increasing intake air temperature. In contrast, for operation without dilution, the autoignition tendency of the low-S Alkylate fuel decreased with increasing intake air temperature, while that of high-S High Cycloalkane fuel still increased with increasing intake air temperature. In conclusion, conventional octane metrics (RON and MON) have utility in assessing the autoignition tendency under lean MMC operation. Moreover, the fuel requirements for MMC align with that of stoichiometric operation: i.e., high RON and high S fuels are desirable for stable non-knocking operation.

Cycle-resolved visualization of lubricant-induced abnormal combustion in an optical natural gas/hydrogen engine

The carbon-free policies and the shortage of crude oil have made engines fueled with natural gas get more and more attention, but actual engines fueled with natural gas often suffer from low engine power output and thermal efficiency. Hydrogen addition and high boosting are effective ways to improve natural gas engines’ efficiency and power output, however, abnormal combustion may occur and prevent the further increase in engine performance. In this study, the inner mechanism of lubricant-induced abnormal combustion was studied based on an optical natural gas/hydrogen engine. The test fuel was chosen as M70H30 and the in-cylinder lubricant was achieved by smearing the piston with excessive lubricating oil. Simultaneous pressure acquisition and high-speed direct photography were used to study flame propagation and abnormal combustion processes. The results show that the lubricant-induced auto-ignition occurred randomly, while no knock occurs due to the low energy density of M70H30. Besides, the abnormal combustion is observed to be directly related to the retained hot spot, and an early auto-ignition always results in too much negative work and low thermal efficiency. Comparing the location of abnormal combustion, it is found that the effect of auto-ignition on combustion also strongly depends on the location of hot spots. Furthermore, the comparison between end-gas auto-ignition cycle and normal cycle implied that non-knocking end-gas auto-ignition is favorable for improving natural gas/hydrogen engines’ efficiency, because the heat release is more concentrated. The current study shall give insights into understanding the abnormal combustion in high-power natural gas/hydrogen engines.

Flame propagation mode transition of premixed syngas-air mixtures in a closed duct

Knowledge regarding the transformation of the flame propagation dynamics is of importance to predict the safety issues of explosion disasters. The current work experimentally investigated the premixed syngas-air flame propagating in a 2 m duct with a set of obstacles, focusing on the flame propagation mode transition with the change of hydrogen volume fraction (φ). Three modes were identified as the φ increases: steady flame propagation, oscillating flame propagation, and end-gas autoignition. The steady flame propagation mode occurred when φ = 0, where the slow oxidation of pure CO determines its propagation feature. The flame became unsteady and oscillated violently with a slight addition of hydrogen. Flame oscillation resulted from its interaction with pressure wave, and the strength of the pressure wave directly impacted the severity of oscillation. The autoignition appeared when φ ≥ 0.4, resulting from the secondary reflected pressure wave interacted with the region that the thermodynamic state was highly disturbed. What is more, the autoignition kernel migrated from the region ahead of the flame front to the end-wall region when φ ≥ 0.6. The reason is that syngas with higher φ has a lower temperature threshold for explosion and a shorter ignition delay.

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H 2 addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH 2 and H 2 NO are most important nitrogen-containing species, and R49 (NH 2 +NO<=>N 2 +H 2 O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H 2 content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas. • Detailed simulations on end-gas knocking combustion are performed. • Ammonia and hydrogen binary fuel blends with different ratios are considered. • Different end-gas combustion modes are observed from parametric studies. • Evolutions of thermochemical states in end-gas combustion are discussed. • Knocking intensity and timing in ammonia/hydrogen blending fuel are studied.