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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]

To investigate the property of the promising and eco-friendly hydrogen-fueled rotary engine, the effect of excess air ratio on the combustion and emission characteristic of it was explored by experiment. The test was conducted under 1500 rpm and 5 CAD ADTC ignition timing. The test results demonstrated that with the decrease of excess air ratio from 2 to 0.85, the thermal efficiency of the hydrogen-fueled rotary engine increases first and then decreases. Besides, increasing MAP is beneficial to improve thermal efficiency. Among the tested condition, the highest brake thermal efficiency is realized when the rotary engine operates at 1.4 excess air ratio and 88 kPa MAP, about 18.34%. And the excellent HC and NO emissions can be obtained at the highest efficiency point. Besides, with the decrease of excess air ratio and the increase of load, the stability and flame development period gradually decrease. With a decreased excess air ratio, the flame propagation period decrease first and then increases, whereas work capacity and thermal efficiency increase first and then decrease. For NO emission, it will increase sharply near the equivalent ratio and gradually decrease after rich combustion. Also, according to the analytical model, it is found that the power performance of the rotary engine depends on the trade-off relationship of in-cylinder pressure and its angle of action. • A rotary engine was modified to run on hydrogen. • The effects of excess air ratio on combustion and emission were studied. • The best efficiency is obtained when the excess air ratio equals 1.4. • An analytical model is presented and the reasonability also be proved. • The emission corresponding to the best efficiency point is excellent. [ABSTRACT FROM AUTHOR]

• The application of O 2 enrichment in a hydrogen Wankel rotary engine is assessed. • O 2 enrichment can improve power output with penalties in efficiency and emission. • O 2 -enriched level can effectively accelerate the heat release process. • O 2 enrichment will lead to knock, backfire and pre-ignition. • Low O 2 -enriched levels has higher application value than high O 2 -enriched levels. Hydrogen-fueled Wankel rotary engines (WREs) with excellent dynamic and emission characteristics have the potential to become a substitute for current fossil-fueled reciprocating piston engines. Oxygen enrichment can be employed to solve the lower power of hydrogen-fueled internal combustion engines, however, it has not been applied in hydrogen-fueled WRE at present. Hence, the goal of this work is to experimentally assess the application of oxygen enrichment in the hydrogen-fueled WRE. This work is conducted at 1500 r/min, wide-open throttle and 2.0 excess air ratio. The work found that within the test range, oxygen enrichment can effectively improve the brake torque of hydrogen-fueled WRE, however, penalize the brake thermal efficiency and NO emission. When the oxygen fraction in air increases from 21% to 29.1%, the brake torque is increased by about 45% while the brake thermal efficiency is lowered by about 8.5% and NO emission deteriorates by three orders of magnitude. Oxygen enrichment can accelerate flame development and propagation process. Besides, as the oxygen-enriched level is increased, abnormal combustion, including knock, backfire and pre-ignition, began to occur and affected the normal operation of hydrogen-fueled WRE. In general, compared with the high oxygen-enriched level, which has severe penalties on performance except for power output, low oxygen-enriched level has higher application potential. [ABSTRACT FROM AUTHOR]

Variable valve timing (VVT) and Miller cycle are advanced technologies employed to optimize engine performance by improving airflow exchange, which are seldom investigated based on the direct-injection (DI) hydrogen engine. The objective of this study is to assess the effects of intake valve closing (IVC) and exhaust valve opening (EVO) timing on the gas exchange performance, combustion, and emissions of a DI hydrogen engine, after which a synergistic control strategy of IVC and EVO timing is proposed. This work is conducted under wide-open throttle and 1500 rpm. The results indicate that the synergistic control of IVC and EVO timing can increase volumetric efficiency by more than 40%, enhance gas exchange performance, shorten combustion duration, and reduce cyclic variation, resulting in approximately 43.15% brake thermal efficiency. Furthermore, brake mean effective pressure can be increased by more than 60% and NO emissions are controlled to less than 20 ppm by optimizing valve timings. • A synergistic control strategy of intake and exhaust VVT is proposed. • The airflow exchange and combustion of a DI hydrogen engine are optimized. • The regulation of valve timings can improve gas exchange performance. • The engine can reach 43.15% maximum brake thermal efficiency. [ABSTRACT FROM AUTHOR]

• A CFD model of spark-ignited gasoline rotary engine was established and verified. • H 2 was direct injected to chamber by varying HIPs and HNDs in the critical state. • Flame spread, combustion pressure and HRR decreased with the increased HND and HIP. • The optimal engine performance was acquired when HND of 1 mm and HIP of 0.4 MPa. • Soot, CO and HC generations tangibly reduced, though NO x generation is increased. A three-dimensional simulation on flow behavior and lean combustion in a hydrogen direct-injection gasoline rotary engine was implemented. The combined influence of hydrogen injection pressure (HIP) and nozzle diameter (HND) was numerically analyzed in the critical injection state. Results showed that with the increments of HIP and HND, further penetration and larger dispersion angle of jet-flow were presented. Increasing HIP led to larger jet-flow area and more jet-wall interaction, as well as greater deflection degree of hydrogen-flow after crashing upon the rotor surface. High-speed region expanded, while more jet-wall impingement occurred with the increment of HND. The spark ignited earlier and flame propagated faster as the HND modified smaller. As the HIP increased, the flame speed decreased slightly. The difference in the local Φ distribution and in-cylinder velocity field were the intrinsic mechanism of the HND impact, and the variation in the mixture concentration and TKE magnitude were the root causes of the HIP effect. The pressure and HRR traces were shifted to the downward clearly, while the peak positions were forming later as the HND extended larger. No perceptible difference in the peak positions of pressure and HRR were obtained with the HIP decrease, although pressure tended to increase along with heat release retard to be shortened. The combustion rate decreased with increments of HND and HIP, the timings of the initial consumption were delayed, and a proportional correspondence of reactants and intermediates was witnessed. The reduced NO x formation was obtained by increments of HND and HIP, and an inverse correlation can be seen between soot, CO, HC generations and NO x concentration. The scheme of 0.4 MPa HIP and 1 mm HND was the optimum configuration. Compared with a larger HND and higher HIP configuration, the peak pressure and maximum HRR were increased by 24.1% and 40.5%, and the productions of soot, CO, and HC were reduced by 55.7%, 61.5%, and 54.7%, respectively, though the NO x generation was increased to some extent. It was recommended in practice operations that the combination of lower HIP and smaller HND left a positive influence on improving the combustion characteristics of Wankel engines. [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]

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]

[Display omitted] • 1-D Wankel rotary engine model considering gas leakage is developed. • The effect of different leakage patterns on volumetric efficiency is discussed in detail. • The influence of λ , IPC, and EPO are investigated by using parametric analysis. • Spearman Rank Correlation coefficient is applied to calculate the statistical relevance. The use of hydrogen energy is considered an essential research direction for carbon neutrality. Hydrogen Wankel rotary engines (WREs) are of increasing interest to researchers due to their excellent power and emissions characteristics. For WREs, the intake and exhaust phases significantly influence performance and emissions. This study focuses on the implementation of a one-dimensional (1-D) model of a hydrogen WRE, and studies the effects of intake port closing (IPC) timing and exhaust port opening (EPO) timing on the performance and emissions. The 1-D model is implemented in the AVL BOOST modeling environment and verified with experimental data at different excess air ratios (λ). The effects of leakage and crevice are also considered in the modeling process. The simulation results demonstrated that the indicated mean effective pressure (IMEP) decreased by approximately 14% due to leakage and crevice compared with the ideal model. The volumetric efficiency (VE) reduction caused by the crevice was less than the leakage effect. Meanwhile, VE improved with the increasing λ. As the IPC advanced, the gas reflux was reduced, which increased the in-cylinder mass and IMEP. However, the relative NOx emissions increased due to the higher temperature. The variation caused by EPO timing in performance and emissions was reversed. The VE first increased and then decreased with the delayed EPO timing. This paper provides a viable solution with more practical guidance for optimizing the performance and emissions of hydrogen WRE. [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]

• Hydrogen two-stage direct-injection is implemented on a gasoline SI rotary engine. • Injecting proper hydrogen in the second pulse after spark-ignition is investigated. • Two-stage injection strategies can realize desirable stratified conditions of hydrogen. • High thermal efficiency and good knock resistance are the benefits of the HTDI. • The HTDI allows significant CO and THC reduction with a slight NOx penalty rate. Hydrogen two-stage direct-injection enrichment is a novel injection strategy to utilize hydrogen more efficiently and effectively in gasoline rotary engines by inheriting the merits of hydrogen and direct injection simultaneously, such as flexible control, efficiency improvement, and emissions reduction. Based on the CONVERGE code with detailed chemistry solvers, a full-cycle CFD modeling including hydrogen jet-flow and combustion processes was presented and validated by experimental data. To understand the role of hydrogen two-stage injection in improving engine performance at part-load and lean-burn regime, six different injection arrangements for which two-stage injection strategies with variable hydrogen amount for each pulse (up to 30% for post-injection) had considered. The different effects on species evolution, combustion characteristics, knock propensity, and emissions formation were analyzed step-by-step. The simulation results showed that compared with direct-injected hydrogen for single-pulse, injecting adequate hydrogen in the second pulse after spark-ignition onwards performed a significant beneficial effect on the mixture stratification and flame propagation, especially for the trailing part of the rotor chamber, which contributed to the improvement in both combustion characteristics and thermal efficiency. The assessment of knock propensity demonstrated that the two-stage direct-injected hydrogen had the potential of mitigating the knock under lean operations. Using split injection accompanied by an optimized hydrogen allocation strategy allowed substantial unburned hydrocarbon and carbon monoxide reductions with a slight nitrogen oxide penalty rate due to the elevated combustion temperature. [ABSTRACT FROM AUTHOR]

To achieve more efficient and robust combustion, split direct-injected hydrogen as a new injection strategy for rotary engines was used to clarify the influences of various injection parameters, including hydrogen amount for each pulse, the dwell between pulses, and secondary injection pulse-width, on combustion performance and emissions formation by CFD modeling. The results of this investigation confirmed the benefit of split direct-injection as a useful means to govern mixture stratification which enabled the combustion more effective and rapid compared to single-injection mode. Enhanced combustion could be achieved by increasing the mass fraction of post-injection with an optimized dwell between injections. Further investigation on secondary injection pulse-width demonstrated that engine performance was substantially improved with a wider second injection pulse. From one side, as the secondary injection pulse-width was increased, the surrounding mixture distribution of the spark-plug location was richer, which was confirmed as a beneficial inhomogeneous circumstance of hydrogen to accelerate the burning rate. From another side, strong tumble level and turbulence intensity were of significant importance for facilitating the combustion process and getting optimal thermal efficiency. For emissions formation, split direct-injected hydrogen made a near-zero HC and CO emissions at the price of a comparable increase of NOx emissions. • Hydrogen split direct-injection is implemented on a gasoline SI Wankel engine. • Analyzing three factors for amelioration including first, post-injection, and dwell. • Split injection strategies can realize desirable stratified conditions of hydrogen. • Injecting proper hydrogen in each pulse with an adequate dwell is of importance. • Thermal efficiency can be notably improved benefits from the wide second injection. [ABSTRACT FROM AUTHOR]

• A simulation model of the hydrogen-enriched Wankel engine was built and validated. • Single-spark ignition strategies consist of split ignition, high-energy ignition. • Twin-spark ignition strategies cover asynchronous ignition and energy allocation. • The preferable arrangement of the twin spark plug was T/L = 1.5. • Optimal combustion was gained as the leading plug was ignited earlier and stronger. For the hydrogen-enriched spark-ignited Wankel engine, the optimization of ignition strategy is conducive to improve combustion performance and specifically effective to lessen the unburned region due to the elongated rotor chamber. In this paper, the role of the number of the ignition source, twin-spark plug location, asynchronous ignition, and energy allocation in improving lean combustion was investigated through the three-dimensional computational fluid dynamics model coupling with kinetic mechanisms. The model was validated by experiment, and good agreements between measured and predicted combustion pressure and the heat release rate was obtained. Results showed that the improvements of engine combustion were limited by single-spark ignition strategies, and the twin-spark ignition configuration was capable of enhancing combustion efficiency drastically. The arrangement of the twin-spark plug determined the space for flame development, and it was favorable for the trailing plug to stand a greater offset from the minor axis of the engine. An earlier leading-spark ignition enabled flame propagation faster and occurred quenching rapidly, which contributed to higher pressure-output and better heat-release. The higher energy of leading-spark ignition made the mixture consumption faster, combustion pressure higher, and combustion duration shorter. The optimum strategy on combustion was expressed as follows: the location of trailing-spark plug is offset from the minor axis by 20.7 mm; the spark timing and discharge energy of leading-spark plug is 325°EA and 0.03 J, respectively; and those of trailing-spark plug is 335°EA and 0.01 J. It was recommended that the leading-spark ignition was set earlier and stronger for practical operations. [ABSTRACT FROM AUTHOR]