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In this paper, the effects of direct water injection (WI) on characteristics of combustion and emission for a hydrogen (H2)-fueled spark ignition (SI) engine were experimentally investigated. The experiments conducted under different amounts of water injection (AWI) and varied water injection timing (WIT). The experimental results showed that in-cylinder pressure decreased, indicated thermal efficiency (ITE) increased, and the flame development (CA0-10) and propagation (CA10-90) periods prolonged when AWI raised. When AIW grew to 4.5 mg/cycle, Nitrogen oxides (NOx) expelled from the original engine decreased by 53.7% when excess air ratio (λ) was 1.15. Early WIT had positive effects on the reduction of NOx emissions. When WIT retarded, in-cylinder pressure increased, ITE decreased and CA0-10 and CA10-90 shortened, NOx emissions rapidly 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 NOx formation was obtained by increments of HND and HIP, and an inverse correlation can be seen between soot, CO, HC generations and NOx 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 NOx 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.

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.

The present study established a computational fluid dynamics model and numerically investigated the mixture formation and combustion processes of a gasoline rotary engine enriched by the direct injected hydrogen at three injection positions. It is found that the special flow field inside rotary engine combustion chamber has strong interactions with the hydrogen jet-flow, and the hydrogen distribution could further influence the charge combustion. At the end stage of compression stroke, the mainstream field whose direction same to rotor movement is formed, and a vortex with high vorticity existed in the front of combustion chamber. With the influences of mainstream field and vortex, the hydrogen almost fills half of combustion chamber and properly rich hydrogen region distribution is obtained near the spark plug with the injection position close to the spark plug. In addition, proper equivalence ratio distribution could enhance the flame propagation and mixtures combustion completeness in rear region of combustion chamber. Compared with injection position far away from the spark plug, the peak in-cylinder pressure of the injection position close to the spark plug increases by 18.3%, and carbon monoxide emission is decreased obviously, but nitrogen monoxide emission increases due to higher in-cylinder temperature.

A CFD model of hydrogen-enriched gasoline Wankel rotary engine is established and validated. The mixture formation and combustion processes are investigated under both hydrogen port and direct injection enrichment conditions with different hydrogen addition levels. It was found that, under hydrogen port injection enrichment conditions, the peak in-cylinder pressure of 6% is 8.4% higher that of 3%. Meanwhile, under hydrogen direct injection enrichment conditions, the peak in-cylinder pressure of 3% hydrogen direct injection enrichment is 15.3% higher than that of 3% hydrogen port injection enrichment. It was also found that the mainstream field with same direction of rotor rotation is formed during the compression stroke and a vortex with high vorticity existed in the combustion chamber. With the effects of mainstream field and vortex, the hydrogen fills more than half volume of the entire combustion chamber under 4%, 5% and 6% HDIE conditions. At the same time, a local low velocity zone is formed around the vortex because the hydrogen direct injection flow slows down the dissipation of vortex. According to the analysis by PREMIX, the fastest flame speed is achieved at equivalence ratios around 1.4. The proper rich hydrogen region distribution is obtained under 4%, 5% and 6% hydrogen direct injection enrichment conditions. Both the enhanced flame propagation by proper equivalence ratio distribution and the reduced local mainstream flow velocity by vortex existence contribute to solving the unburned zone problem. Compared with 6% hydrogen port injection enrichment, the maximum value of in-cylinder pressure of 4%, 5% and 6% hydrogen direct injection enrichment is improved by 59.6%, 88.4% and 91.6%, respectively. However, carbon monoxide and nitrogen oxide emissions are increased obviously. As a relatively low carbon monoxide emission is obtained with 4% case under hydrogen direct injection enrichment conditions and the problem of unburned zone in the rear region of combustion chamber is solved. The hydrogen volume fraction of 4% with direct injection enrichment achieves the best engine performance in this study.

The study aims to numerically investigate the combustion and emissions formation processes in a spark-ignition rotary engine fueled with hydrogen-gasoline blends. The Renormalization Group k-e turbulence coupled with a skeletal primary reference fuel mechanism were adopted to simulate the engine working process under 0%, 2% and 4% hydrogen volume fractions in CONVERGE software. The flow field variation and detailed combustion processes were analyzed and discussed. Results showed that a mainstream flow field along with the rotor movement was formed during the compression stroke and sustained in the combustion chamber until the exhaust valve opening. The center of the burned zone would move in same direction with the mainstream flow. Meanwhile, due to the effect of the mainstream flow, the flame propagation in direction of the mainstream flow was expedited. While the flame propagation in contrary direction was retarded, consequently, the unburned mixtures at rear region of combustion chamber suffered incomplete combustion. The distributions of nitric oxide and carbon monoxide emissions were also dominantly affected by the flow field in the combustion chamber. After hydrogen addition, the increased OH, H and O radicals concentrations combined with the intense flow flied accelerated the combustion process, which resulted in the improvement and advancement of in-cylinder pressure and temperature. Compared with original gasoline case, the peak in-cylinder pressure was increased by 9.1% and 13.7% with 2% and 4% hydrogen blends, respectively. With hydrogen enrichment, the increased in-cylinder temperature promoted the formation of nitric oxide emission. Besides, Carbon monoxide emission was decreased with the increase of hydrogen addition fraction.

In this paper, the effects of direct water injection (WI) on characteristics of combustion and emissions for a hydrogen (H2)-fueled spark ignition (SI) engine were experimentally investigated. The experiments were conducted under different excess air ratios (λ) and ignition timing (IT). In each test, the water to air ratio (α) was varied, and the results were compared with baseline data without WI. The experimental results showed that in-cylinder pressure decreased, indicated thermal efficiency increased, and the flame development and propagation periods prolonged when WI was used. When α increased to 1.87%, NOx emissions expelled from the original engine separately decreased by 36.9% and 55.4% when IT were 5°CA BTDC and 15°CA BTDC at λ of 1.15.

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.

The attractiveness of oxygen-enriched combustion advantages are making a comeback in the implementation of the rotary engine concept. In this connection, a numerical investigation on influences of intake oxygen enrichment combined with excess air ratio on combustion behavior of the rotary engine needs to be clarified. A three-dimensional computational fluid dynamics simulations coupling with chemical kinetic mechanisms was performed in this study, and the numerical results had been validated with existing experimental data. The species evolution, combustion characteristics, and pollutant formation were monitored to assess the oxygen-enriched combustion of the rotary engine. Results showed that there was a significant influence of intake oxygen enrichment for improving flame development and burned volume due to the reasonable species concentration and intense turbulence intensity. Increasing oxygen enrichment led to the enhanced peak pressure and advanced corresponding position, resulted in a rapid combustion period for improving heat-release efficiency and combustion efficiency, meanwhile made a higher indicated mean effective pressure at the price of a smaller increment of nitrogen oxide formation. The substantial reductions of carbon monoxide, unburned hydrocarbon, and soot were recorded, which is attributed to the higher-oxygen mixture with suitable lean operations. Considering combustion characteristics and emissions performance, the operation of intake oxygen volume of 30% accompanying the excess air ratio of 1.1 is the best choice for the engine used in the current study.

This work aimed to realize stratified mixtures distribution in a hydrogen-enriched compound-intake gasoline Wankel engine by utilizing the difference of gas velocity from different intake ports. A three-dimension computational fluid dynamics (CFD) WRE model was established to investigate the mixture formation and combustion process under five different intake modes, i.e. the way of the binary fuel enters the combustion chamber through different intake ports (side intake port, peripheral intake port or both port). The intake mode S-HGP (the mixtures of H2, gasoline and air are in the peripheral intake port and pure air is in the side intake port) realized an optimal equivalence ratio distribution, in which condition, the most fuels gather near the spark plug and the minority fuel in the rear region of the combustion chamber. Since the unburned mixtures always exist in the rear region of the chamber due to the mainstream in the WRE, the stratified distribution of S-HGP could consume most of the fuel. Because the faster flame propagation release a large amount of heats near top dead center (TDC), S-HGP possesses the highest average in-cylinder pressure under five intake modes, is about 1.264 times than the worst intake mode. Moreover, S-HGP has the maximum average in-cylinder temperature, the ideal thermal condition and moderate lean combustion get the lowest CO emission, comparing with the intake mode performing the highest CO emissions, the CO emission of S-HGP is decreased by 745% near the exhaust valve opening timing (210 °CA ATDC). However, owing to the higher temperature inside cylinder, S-HGP has slightly high NO emission. In conclusion, the results mean that compound intake combined with intake modes control is an effective method for fuel distribution optimization besides the fuel direct injection.