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Highlights • A CFD model of H 2 -gasoline fueled rotary engine was established and validated. • The CFD model was modified by installing the compound intake ports. • The mean in-cylinder pressure and indicated thermal efficiency were increased. • Combining compound intake with HDI obtained a better engine performance. • CO emission was decreased, but NOx emissions were increased. Abstract The compound intake possesses large port area. Applying it in Wankel rotary engine (WRE) could increase volumetric efficiency and reduce pumping loss. To investigate the effect of the compound intake on the mixture formation and combustion process in gasoline WREs with hydrogen port and direct injected (HPI and HDI) enrichment, a three-dimensional computational fluid dynamics (CFD) model was established and validated. Investigation results showed that the peripheral-ported intake flow plays a leading role in the compound-ported WRE. Because the peripheral-ported intake flow has the same direction with the rotor movement, the formation time of mainstream flow field is shortened and the mean flow speed is increased, the flame propagation in the same direction with mainstream flow field is accelerated. Meanwhile, the improvements in volumetric efficiency and quality of in-charged mixtures result in the rise of temperature and pressure at spark timing, which could also improve the initial thermal conditions. Therefore, the compound intake effectively accelerates the combustion process in WRE. In HPI conditions, compared with the side-ported WRE, the peak in-cylinder pressure and indicated thermal efficiency in compound-ported WRE are respectively increased by 4.5% and 3.9%. In HDI conditions, the flame could propagate to the rear combustion chamber and eliminate the unburned zone, the maximum in-cylinder pressure and indicated thermal efficiency in side-ported WRE with HDI are 84.7% and 6.6% higher than those of in side-ported WRE with HPI. At the same time, the compound intake could further increase the in-cylinder pressure and indicated thermal efficiency by 6.8% and 2.5%, respectively. However, the improved combustion performance increases the in-cylinder temperature, which provides a suitable thermal-atmosphere for nitrogen oxide (NO x) formation. The compound intake increases mass fractions of NO x emissions from 0.111% to 0.141% in HPI conditions and those from 0.268% to 0.284% in HDI conditions. Considering the compound intake promotes the combustion characteristics and effectively decreases carbon monoxide (CO) emission from 0.043% to 0.033% in HPI conditions, and from 0.008% to 0.003% in HDI conditions, it is a feasible way to improve the performance of WREs. Especially, combining it with HDI could obtain a better engine performance under low engine speed and part load conditions. [ABSTRACT FROM AUTHOR]

This paper aims to reveal the effect of intake closing timing, and engine speed on the flow field, flame propagation, combustion characteristics, and emissions formations of a small-scaled side ported hydrogen-fueled Wankel rotary engine. For this reason, a three-dimensional dynamic simulation model was established using a reasonable turbulent model coupled with a kinetic reaction mechanism and validated by the experimental data. Simulation results show that an earlier intake closing timing increases the volumetric efficiency and increases intake loss. The increasing speed improves the volumetric efficiency while causing a higher intake loss. There are two peaks of the turbulence kinetic energy during the intake stroke. The first one is caused by the airflow hitting the wall, and the other is due to the backflow. During the flame development period, due to the strong unidirectional flow in the combustion chamber, which is not conducive to flame propagation backward. This phenomenon is more pronounced at higher engine speeds, resulting in a merged flame front not exceeding the trailing spark plug. The lean combustion leads to a lower in-cylinder combustion temperature, deteriorating the NOx generation environment. This paper provides a feasible method for matching the operating conditions and intake system. • The shape and area of the intake port varies with the intake closing timing. • Dual-spark plug flame propagation pattern is proposed. • The leakage of gas to adjacent cylinder is reduced at 8000 rpm. [ABSTRACT FROM AUTHOR]

Ammonia and hydrogen are two carbon-free alternative fuels for engines. Considering that there are few studies on improving an ammonia-hydrogen engine's lean combustion performance, this investigation based on an engine with the Miller cycle employs hydrogen direct injection (HDI) and NH 3 port injection and then proposes an adjustment strategy to vary the start of injection (SOI) of HDI for affecting the engine combustion process. When ammonia volume share (AVS) is greater than 50%, the engine shows poor performance. Nevertheless, the stratification effect of hydrogen jets may be used to elevate the hydrogen concentration around spark plug by properly delaying SOI and then creating the locally rich mixture with a high hydrogen share, which prominently promotes the flame kernel formation, shortens CA0-90, reduces C o V P m a x , and enables the engine to reach approximately 36% BTE. It is notable that very late SOI can slightly deteriorate the heat release process and lower engine performance. • Ammonia-hydrogen engines have poor combustion process under lean conditions. • A strategy to improve the lean combustion performance of the engine is proposed. • Effects of NH 3 –H 2 premixing and H 2 stratification effect on the engine are balanced. • Properly delaying SOI enhances the ignition and combustion process of the engine. [ABSTRACT FROM AUTHOR]

Driven by issues such as global warming, there is currently great interest and significance in developing zero carbon-emission internal combustion engines. Limited by structural and sealing requirements, there are no experimental studies related to combustion velocity measurement of the hydrogen-fueled Wankel rotary engine (HWRE) with excellent power performance, which is important to investigate its knock characteristic. In this work, a combustion velocity measuring method of dual spark plug HWRE is proposed, besides the obtained combustion, flow and flame velocities are correlated with the knock to explore their influence on the HWRE knock characteristics. The study found that adopting dual spark plugs in HWRE is necessary to reduce the negative impact of the flame not being able to propagate against the flow field. Besides, excess air ratio affects the combustion velocity of HWRE by changing the flame velocity, while the engine speed affects the combustion velocity of HWRE by changing the flow velocity, which basically does not affect the flame velocity. The knock level of HWRE is influenced by its flame velocity and flow velocity, both of which are positively correlated to the knock intensity and the latter has a more significant effect. • A method of measuring Wankel rotary engine combustion velocity is proposed. • Quantifying the combustion, flow and flame velocity of hydrogen Wankel rotary engine. • Investigating the influence of excess air ratio and engine speed. • Quantifying the relationship between knock intensity and above three velocities. [ABSTRACT FROM AUTHOR]

• Combustion characteristics of NH 3 /H 2 /air with SI and TJI methods were compared. • Effect of fuel composition and equivalence ratio was evaluated under TJI conditions. • Different ignition mechanisms of NH 3 /H 2 /air were found. • Effect of orifice number on ignition and combustion characteristics was evaluated. The mixture of ammonia (NH 3) and hydrogen (H 2) is the potential alternative fuel for internal combustion engines (ICEs). The turbulent jet ignition (TJI) system can provide high ignition energy and turbulent disturbance to promote the combustion of the mixture for NH 3 /H 2 ICEs. Therefore, the combustion characteristics of NH 3 /H 2 /air under passive TJI conditions were experimentally studied in the present study. The experiment was conducted in a constant volume combustion bomb, and the effect of fuel composition and equivalence ratio was investigated. The experimental results show that compared to spark ignition, the combustion of NH 3 /H 2 can be effectively improved by using TJI, especially under high NH 3 fraction conditions. The addition of H 2 has a significant positive effect on the ignition and combustion performance. With the addition of H 2 , the ignition delay and combustion duration decrease evidently. And the H 2 addition is beneficial for improving the ignition mechanism and achieving flame ignition. In addition, poor ignition performance may occur under rich conditions due to the fluid-dynamic quenching of the jet. However, the high flame propagation rate on the rich side still leads to lower combustion duration. Moreover, increasing the orifices number appropriately can enhance the ignition and combustion performance. The jet strength is weakened by the increased total orifice area and the ignition of NH 3 /H 2 can be improved. [ABSTRACT FROM AUTHOR]

Recently, ammonia is considered to be an energy medium for hydrogen or energy storage and ammonia/hydrogen can be a potential fuel blend for the spark-ignition engine. Nevertheless, the combustion properties of ammonia/hydrogen mixtures could be distinctive under different initial conditions. In this study, several essential combustion properties, including laminar burning velocity, minimum ignition energy, NOx and ammonia emissions, combustion efficiency, and mixture heating values of ammonia/hydrogen/air premixed combustion were extensively studied under a wide range of equivalence ratios (ϕ), hydrogen fractions (α) and different compression ratio using one-dimensional laminar planar flame and compared with stoichiometric methane, methanol, and ethanol combustion. The behavior of NOx generation changing with ϕ and α was analyzed in detail. Results showed that most properties of ammonia/hydrogen combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions, except for the mixture heating value is slightly lower in all conditions. Besides, the NO mole fraction non-monotonically changes with ϕ and α due to the competition between the effects caused by the reduction of nitrogen-atom species and the enrichment of H/O radicals. The NO mole fraction of stoichiometric ammonia/hydrogen could be even lower than that of hydrocarbons. Considering the properties of hydrocarbon fuels as a reference, promising working conditions for ammonia/hydrogen mixtures are ϕ from 1.0 to 1.05 and α from 40% to 60%. Besides, a high compression ratio is more suitable for ammonia/hydrogen mixtures due to the excellent knock resistance ability of NH 3 /H 2 and better improvement than hydrocarbon fuels under high compression ratio. • Combustion properties of premixed NH 3 /H 2 /air were assessed under engine conditions. • One dimensional steady flame was simulated under different initial conditions. • NOx generation of NH 3 /H 2 /air combustion is analyzed in detail. • Most combustion properties, including NOx are comparable to light hydrocarbon fuels. • Potential working conditions are suggested based on the analysis. [ABSTRACT FROM AUTHOR]

CO 2 dilution is an effective strategy for combustion control. To understand the detailed dilution effects of CO 2 (including physical effects and chemical effects) on CH 4 and H 2 combustion, an open-source CFD package laminarSMOKE was utilized to simulate the transient one-dimensional outwardly spherical flame combustion process in a closed chamber under 0–15% CO 2 dilution mole fraction (α), at 293 K, 1 bar, stoichiometric ratio. The overall CO 2 dilution effects decrease the unstretched/stretched flame propagation speed, maximum combustion pressure and prolong the combustion duration. In most conditions, physical effects play a dominant role and chemical effects amplify the dilution effects. Typically, the maximum combustion pressure of CH 4 -CO 2 -air flame at α = 15% reduces 1.15 bar compared with the undiluted case, the reduction percentage caused by physical and chemical effects is 12% and 1.5%, respectively. In addition, opposite overall effects of CO 2 dilution on Markstein length (L b) of CH 4 flames and H 2 flame are performed. The physical effects of CO 2 dilution increase the L b of CH 4 flames but decrease that of H 2 flames. Chemical effects are similar to physical effects for CH 4 flames, but non-monotonic behavior is performed for H 2 flame due to the combined effects of density ratio and mixture reactivity changes. • Chemical effects of CO2 dilution on laminar spherical flame were numerically study. • Physical effects play leading roles in all changes of combustion properties. • Chemical effects reduce flame propagation speed, maximum pressure, increase combustion duration. • Chemical effects on Markstein length behave differently in CH 4 and H 2 flame. [ABSTRACT FROM AUTHOR]

• A CFD model of gasoline-H 2 blends Wankel engine was established and verified. • H 2 was direct injected in the rotor chamber under varying mass and directions. • Flame spread and combustion pressure decreased with the widened injection angle. • The optimal engine performance was acquired when α H2 of 3% and IA of 45°. • NO x generations still remained lower, though chamber pressure notably rise. To analyze the effects of hydrogen charge concentration (HCC) and injection angle (IA) on the lean combustion in a gasoline Wankel engine, the present work implemented a numerical simulation model coupling with the kinetic mechanisms and validated with the experimental data. Results found that with the increase in HCC, the penetration of hydrogen injection is enlarged and the area of the high-speed jet flow is expanded. The jet-flow area for IAs of 45° or 135° is larger than that of 90°. Changing IA could obtain the hydrogen distribution at different regions of the rotor chamber and IA of 90° acquires the smallest hydrogen-rich region. Increasing IA brings about the reduced flame speed substantially; the flame area for IAs of 45° and 90° expands with the increment of HCC whereas the contrary pattern is witnessed at the IA of 135°. Smaller IA leads to the major burning occurring untimely, which resulting in less work delivery of the engine. The hydrogen consumption for IAs of 45° and 90° increases as HCC is ascendant while that for IA of 135° is just the reverse. Variations in the mixture distribution and turbulence are the intrinsic mechanism of how the HCC and IA reflects the combustion progress. As hydrogen is injected with larger HCC and smaller IA, a relatively richer mixture and higher turbulent kinetic energy are distributed close in the spark ignition region. The peak combustion pressure reduces and its corresponding crank position delays with the widened IA at any HCC. Considering the fuel combustion and nitric oxide formation, as hydrogen volume fraction is 3% and IA is 45°, the engine could realize the optimized performance under the computational condition. An efficient combustion performance may be performed in engineering application if the hydrogen IA is in accordance with the rotor rotating direction at lower HCC. [ABSTRACT FROM AUTHOR]

Abstract The influences of wall are important in practical combustion devices. In present study, the propagating processes of near wall ignited laminar methane/hydrogen/air flame were explored under different hydrogen fractions in a constant volume combustion vessel mimicking engine geometry. Results showed that both effects of heat losses and wall compression cause difference of local flame speed at different directions. The flow inside burned zone induced by compression accelerates local flame speed at direction opposing to the wall, makes the local flame speed higher than freely propagating laminar flame speed. Meanwhile, flame shape changing process was quantified by fitted ellipses. It was found that flame shapes are strongly affected by the wall compression but not obviously influenced by hydrogen addition. Hydrogen addition exacerbated flame instabilities, notably improved the local and global flame speeds due to both increase of laminar flame speed and flow velocity inside burned zone. The maximum local speed increase from 258 cm/s for 20% hydrogen fraction to 695 cm/s for 80% hydrogen fraction. Maximum combustion pressure and maximum pressure rise rate were slightly increased by hydrogen addition. On contrary, the combustion duration notably decreased nearly 3 times when hydrogen fraction increased from 20% to 80%. Highlights • Near wall ignited lean CH 4 /H 2 /air flame were studied in a constant volume chamber. • Compression induced flow inside burned zone played a role for near wall flame. • Hydrogen addition enhances the flame propagation and compression induced flow. • Effects of heat transfer are more obvious for later combustion stage. [ABSTRACT FROM AUTHOR]

In this study, a double spark plugs direct injection rotary engine is used to investigate the effects of synchronous and asynchronous changes of ignition timing on combustion and emission performance. The three-dimensional dynamic model was established by CONVERGE software and validated by the experimental data. The results indicate that setting the fuel injector toward the spark plug can reduce the fuel distribution at the tail of the combustion chamber, and the in-cylinder mixture inhomogeneity index gradually increases with the advance of the ignition timing. Compared with the synchronous change of ignition timing, advancing the ignition timing of the tailing spark plug can promote the combustion process. When the ignition timing of the leading spark plug is too early, the flame front will be irregular. With the advance of ignition timing, CA 0–10 corresponding to synchronous and asynchronous change all gradually increase, and the change of CA 10–90 is more stable than that of CA 0–10. Whether the ignition timing is changed synchronously or asynchronously, with the advance of the ignition time, the in-cylinder pressure shows an upward trend. The mean in-cylinder pressure corresponding to the synchronous change with the same change amplitude is slightly higher than that of asynchronous change. The peak pressure of 16.4% higher than that of the original engine can be obtained by adopting the ignition strategy of the synchronous advance of 30° CA. Under this ignition strategy, the temperature and NOx mole fraction in the combustion chamber are all the lowest. • A direct injection rotary engine CFD model was established and validated. • The combined effects of direct injection and ignition strategy were investigated. • The mixture formation, combustion, and emissions process were analyzed. • The in-cylinder inhomogeneity index increases with the advance of ignition timing. • The synchronous advance of ignition time by 30° CA can increase the pressure by 16.40%. [ABSTRACT FROM AUTHOR]

Hydrogen-fueled Wankel rotary engine (HWRE), as a high power density and eco-friendly internal combustion engine, has the potential to become an alternative for gasoline-fueled piston engines. Cooled EGR, as an effective means of improving engine performance, is less studied based on HWRE. However, due to the different operating way and structure, the flame development and propagation of WRE are significantly different from those of the piston engine, so may the effect of cooled EGR. Hence, the goal of present work is to analyze the effect of cooled EGR on the combustion characteristics of HWRE. This work is conducted under 1500 r/min and wide-open throttle conditions. The results show that when the ignition timing and excess air ratio are fixed at 5°CA ATDC and 1, the cooled EGR level has a significant influence on the combustion process and operating stability. In addition, when maximum brake torque CA50 is employed, within test range, whether stoichiometric or lean combustion, both the brake torque and brake thermal efficiency are monotonous to the cooled EGR level. And cooled EGR can achieve high brake thermal efficiency compared with lean combustion at the same brake torque. Compared with the hydrogen-fueled piston engine, HWRE allows for a higher cooled EGR level whether in terms of efficiency or power output considerations. In general, the cooled EGR can be used as an excellent load control means to achieve high efficiency of HWRE. • The effect of cooled EGR on combustion in hydrogen Wankel rotary engine is analyzed. • Cooled EGR has significant impacts on the combustion characteristic of hydrogen WRE. • Cooled EGR can be employed as an excellent means of load control of hydrogen WRE. • Cooled EGR is more suitable for hydrogen WRE than the hydrogen piston engine. [ABSTRACT FROM AUTHOR]

• Experimental and numerical study of stoichiometric and lean combustion is conducted. • High-temperature combustion and its duration have crucial effects on burning rates. • The combustion initiation and combustion duration are shortened by H2 enrichment. • H2 addition with lean operation shows higher thermal efficiency and lower emissions. • There is a close correlation between early flame growth and combustion stability. Based on a retrofitted Wankel engine, fundamental experiments and numerical simulations were conducted to investigate the role of the hydrogen addition under stoichiometric and lean combustion conditions. The test-bench recording of in-cylinder pressure, heat release, cycle-to-cycle fluctuation, and exhaust emissions were used to characterize fuel combustion. The verified CFD model clarified the inherent mechanisms behind experimental data. Results showed that the introduction of hydrogen addition caused combustion temperature to increase, the timing taken for active radical formations was significantly advanced, and the peak concentrations increased. This contributed to the increased pressure and the shortened combustion phasing in experiments. There was a proportional correspondence of the early flame growth and cyclic variation. Hydrogen addition has a minimal effect on the turbulent intensity after spark timing. However, adopting a highly diluted mixture caused the enhanced turbulent intensity, which was responsible for improving the stability of the engine. Due to the synergistic effects of the work delivery, cyclic fluctuation, and exhaust temperature, a larger hydrogen addition with stoichiometric operation showed lower higher brake thermal efficiency. Nitrogen oxide emissions increased with hydrogen enrichment and reduced with a higher degree of mixture dilution owing to a close positive correlation between the mass elevated-temperature and NO x formation. Benefits from sufficient oxygen concentration and elevated combustion temperature, the chain transfer process can be accelerated. This explained why CO emissions decreased at a larger hydrogen-enriched and leaner condition in the experimental investigation. Increasing the hydrogen content provided improvement in the desired HC emissions for all operating points. [ABSTRACT FROM AUTHOR]

This study aims to investigate the effects of data points number on the accuracy of flame speed extrapolation using expanding spherical flame, which is a practical issue in high flame speed experimental measurement. Monte Carlo simulations were conducted based on the flame radius data obtained from one-dimensional spherical flame simulation. Given number of points were randomly picked from 70 points to obtain the unstretched flame speed by linear and nonlinear extrapolation under different Lewis number, fuel type, and initial pressures. The results were statistically analyzed and show that the least number for flame speed extrapolation is related to the Lewis number of mixtures. Mixtures with Lewis number smaller than unity need more data points to ensure accurate extrapolating results. In addition, the accuracy of flame speed determination is very sensitive to the error in flame radius when data points are sparse. When 0.1% of Gauss noise was added to flame radius data, additional dozens of points are required to keep the same accuracy. At least 30 data points were suggested to be adopted in the extrapolation of flame speed to reduce the uncertainty from data points number to less than 1%. The effects of the flame radius range on the determination of flame speed again validated by the current study, indicating a wide range of flame radius should be used to avoid uncertainty. It was also shown that the nonlinear extrapolation method needs fewer data points than the linear method to achieve the same accuracy for mixtures. [ABSTRACT FROM AUTHOR]

• A thorough 3D CFD model of the H 2 -gasoline Wankel engine is built up and validated. • Burning-source connection of the twin-plug has a negative influence on flame spread. • Combustion is enhanced in the L-side of the rotor chamber by increased spark-energy. • The improvement of ignitability was gained in the H 2 -enriched ignition environment. • Maximum mass fraction of the temperature region is closely related to NO x formation. Due to the elongated shape of the combustion chamber and strong one-way flow characteristic, changing the location of enhanced combustion is an effective means to ameliorate the indicated efficiency of the Wankel engine. For this purpose, strengthening combustion can happen by allocating spark-energy in the desired location along with a favorable ignition environment. In this study, three-dimensional CFD simulations were implemented and the numerical results were compared with existing measured data, in which ignition environment variations were achieved by hydrogen enrichment within the intake manifold of a spark-ignition Wankel engine. Based on CONVERGE code with the SAGE detailed chemistry solver, the twin-plug engine model was then used to assess the location of strengthened combustion employing a spark-energy allocation from the perspective of species evolution, combustion characteristics, and emissions formation. The results indicate that enhancing spark-energy at the leading side of the rotor chamber has an overall higher burned volume rate and faster species evolution due to the later connection of two burning sources prior to quenching. Increasing the hydrogen content in the ignition environment causes richer active radicals and higher turbulence, ultimately promoting flame propagation and combustion intensity. A larger T-plug spark-energy decreases the indicated mean effective pressure, heat release rate, and thermal efficiency because of the intensified wall heat flux and the lower mass burning rate at the leading side. There is no doubt that the introduction of hydrogen enrichment presents a positive effect on both thermal efficiency and specific fuel consumption at the price of a slighter increase of nitrogen oxide formation due to higher mass fraction of high-temperature regions. It is recommended that enhancing local mass burning rate in the leading part of the rotor chamber under the hydrogen-enriched ignition environment can provide a great potential in the quest towards high-efficiency Wankel rotary engine. [ABSTRACT FROM AUTHOR]