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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]

Highlights • A CFD model of H 2 -gasoline fueled rotary engine was established and validated. • H 2 was direct injected in the combustion chamber under different strategies. • In-cylinder pressure increased with retarded HIT and extended HID. • The lowest CO emission was achieved in HIT of 110 °CA BTDC, HID of 40 °CA. • Higher NO x emissions were acquired in retarded HIT and extended HID. Abstract In Wankel rotary engine (WRE), high mainstream velocity in combustion chamber blocks flame propagating to the end of combustion chamber, which causes high emissions and low combustion efficiency due to unburned mixtures. To solve this problem, a three-dimensional dynamic simulation model of a hydrogen-gasoline blends fueled WRE is built and validated. The in-cylinder mixture formation and combustion process are investigated under different hydrogen injection timing (HIT) and duration (HID) conditions. The study results show that the concentration of hydrogen distributed between the spark plug region and rear combustion chamber increases with retarded HIT and extended HID. Faster flame speeds are obtained for HIT of 110 °CA BTDC and HID of 40 °CA. At a fixed HID of 20 °CA, compared with HITs of 210 and 160 °CA BTDC, the peak in-cylinder pressures for HIT of 110 °CA BTDC are increased by 14.3% and 6.8%, respectively. At a fixed HIT of 110 °CA BTDC, the peak in-cylinder pressure in HID of 40 °CA is 52.3% and 9.21% higher than HIDs of 20 and 30 °CA. The highest in-cylinder pressure is achieved with the hydrogen injection strategy that HIT of 110 °CA BTDC, HID of 40 °CA. However, as the temperature increases with pressure, nitrogen oxide emissions are also increased with retarded HIT and extended HID obviously. Considering the lowest carbon monoxide is achieved and the unburned zone in the rear region of combustion chamber is eliminated in HIT of 110 °CA BTDC, HID of 40 °CA. The hydrogen direct injection strategy that HIT of 110 °CA BTDC, HID of 40 °CA acquires the best engine performance in this research. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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-ε 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. [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]

Highlights • A CFD model of gasoline/H 2 fueled Wankel engine was established and verified. • The ignition mode consisted of synchronous ignition and asynchronous ignition. • H 2 was direct injected in the combustion chamber under varying twin-spark timing. • The optimal combustion was achieved when the TSP ignited asynchronously. • There was no increase in NO x generation with the enhanced combustion rate. Abstract A Wankel engine with hydrogen direct-injection enrichment is recognized as an attractive method to enhance combustion efficiency. In this paper, on the basis of the chemical kinetic mechanisms, a three-dimensional simulation model was established and validated by the measured results. The ignition and combustion processes in a gasoline Wankel engine with hydrogen direct-injection enrichment were implemented for numerical simulation. The influence of twin-spark timing was firstly analyzed by synchronous ignition. Further investigation was then conducted to simulate how to effect the combustion by asynchronous ignition based on the favorable synchronous ignition timing. Results showed that, the average flow velocities were 18.8, 20.7, 23.6, and 25.4 m/s for the spark timings (ST) of 45, 35, 25, and 15°CA BTDC respectively. With retarded ST, the rich equivalence ratio approached to twin-spark plug continually. For synchronous ignition, at a larger advance of ST, the duration between the timing of the vortex dissipation and ST was longer, and the ignition delay was more prolonged. As the ST advanced, the combustion rate was enhanced with the increment in chamber temperature, the reactants (C 8 H 18 , C 7 H 16 , and H 2) consumption and intermediates (H 2 O 2 , OH, and CH 2 O) generation were accelerated, the maximum H 2 O 2 and CH 2 O decreased whereas the maximum OH increased, and nitrogen oxides (NO x) and carbon monoxide (CO) increased sequentially. For asynchronous ignition, the accelerated flame front was in accordance with the rotating direction of the rotor while the opposite direction was inhibited during the combustion process. An earlier ignition of leading-spark plug (L-plug) or trailing-spark plug (T-plug) provided the higher flame speed and faster H 2 consumption, which contributed to the higher in-cylinder pressure, combustion temperature, and NO x and CO production. The preferable ignition and combustion characteristics was realized when the L-plug angle was 25°CA BTDC, and the T-plug angle was 35°CA BTDC. Compared with the favorable synchronous ignition timing, the flame propagation and H 2 consumption were expedited, the peak pressure increased by 2.8%, the corresponding crank position advanced by 3°CA and there was no increase in NO x and CO generation. Therefore, it was recommended in engineering applications that the ST of T-plug advanced appropriately while the ST of L-plug remained constant. [ABSTRACT FROM AUTHOR]

Highlights • A CFD model of gasoline/H 2 fueled rotary engine was established and validated. • The ignition system consisted of leading-spark plug and trailing-spark plug. • H 2 was direct injected in the combustion chamber under varying DSP arrangements. • The preferable combustion was achieved when the DSP positioned symmetrically. • CO emission notably reduced and NO x emission slightly increased. Abstract A gasoline rotary engine enriched with directly injected hydrogen could be regarded as an appealing option to improve combustion efficiency while reducing emissions. In this paper, a three-dimensional dynamic simulation model coupling with the chemical kinetic mechanisms was established using CONVERGE software and validated by the experimental data. The numerical model was implemented for evaluating the influences of varying duel-spark plug locations on flow field distributions, combustion processes and major emissions formation of a gasoline rotary engine with hydrogen direct injection enrichment. Simulation results showed that, a mainstream flow field formed during the end period of the compression stoke whose direction was identical to the rotor rotating direction. The variation of trailing-spark plug location resulted in slight influence on the mean flow velocity as well as notable impact on hydrogen concentration and turbulent kinetic energy distribution. It was conducive to initial ignition when the local equivalence ratio was intensified near the duel-spark plug on the leading side of the combustion chamber. Higher hydrogen concentration near spark plugs could effectively improve the combustion intensity. The preferable combustion and emission performance were achieved when the duel-spark plug positioned symmetrically with respect to the minor axis. Compared with the scheme of the closest duel-spark plug, the peak pressure increased by 24.4% and corresponding crank position advanced by 9.2°CA. Despite nitric oxides emissions increased slightly, carbon monoxide emission notably reduced by 57.6% versus the scheme of the farthest duel-spark plug. When the leading-spark plug remained constant, the trailing-spark plug arranged on the major axis of cylinder wall and kept an appropriate offset from the minor axis was recommended in engineering applications. [ABSTRACT FROM AUTHOR]

The present paper introduced an experiment survey focusing on exploring the impact of hydrogen enrichment on improving the lean-burn performance of an n-butanol rotary engine. During the test, the engine speed and intake pressure were roughly set at 4000 rpm and 35 kPa, respectively. A constant spark advance of 45 °CA was adopted through the test. Hydrogen volumetric fraction of the total intake was severally kept at 0% and 3%. The testing results manifested that the brake thermal efficiency and peak chamber temperature were heightened with hydrogen addition. Besides, the ignition delay and rapid combustion duration were both reduced with the hydrogen additive. Engine running stability gained an improvement by hydrogen supplement. Moreover, HC and CO emissions from the original n-butanol rotary engine were reduced after hydrogen adding. NOx emissions were increased with hydrogen enrichment while reduced with the increase of excess air ratio. This indicated that NOx emissions from both the n-butanol and hydrogen-blended n-butanol rotary engines could be reduced by lean combustion strategies. [ABSTRACT FROM AUTHOR]

This paper aimed to study the effect of spark timing on performance of a hydrogen-gasoline dual-fuel rotary engine. For this aim, a modified rotary engine equipped with a dual-fuel port injection system was developed. An electronic management module (ECM) was specially made to command the fuel injection, excess air ratio and hydrogen volumetric fraction. In this study, the engine was operated at 4500 rpm with a manifold absolute pressure (MAP) of 35 kPa. Hydrogen volumetric percentage of total intake was kept at 0%, 3% and 6%, severally. When the hydrogen volumetric percentage was changed, the gasoline fraction was also adjusted to keep the mixture at the stoichiometric. For a specified hydrogen volumetric fraction, the ignition timing was varied from 24 to 42 °CA BTDC (before top dead center) with a fixed interval of 2 °CA. Experimental results showed that for a specific hydrogen volumetric percentage, the peak combustion pressure and chamber temperature were increased, brake thermal efficiency was first increased and then decreased with the increase of spark advance. Advancing spark timing caused the increased flame development period, the decreased flame propagation period and exhaust temperature. Cyclic variation was initially weaken and then deteriorated after raising spark advance. HC and NOx emissions were reduced after retarding spark timing. Spark timing had little effect on CO emission. [ABSTRACT FROM AUTHOR]

• The characteristics of a rotary engine are predicted by support vector machine model. • The coefficient of determination of the optimized models is greater than 0.98. • The effects of parameters on performance, combustion and emissions are studied. • The optimal thermal efficiency and nitrogen oxides are 18.12 % and 248.58 ppm. The purpose of current research was to implement an intelligent regression model and multi-objective optimization of performance, combustion and emissions characteristics for a hydrogen-enriched gasoline rotary engine. The brake thermal efficiency (BTE), fuel energy flow rate (E f), nitrogen oxides (NO X), carbon monoxide (CO) and hydrocarbon (HC) were predicted by intelligent regression model with hydrogen volume fraction (α H 2 ), excess air ratio (λ) and ignition timing (IT) as independent variables. The intelligent regression models were based on support vector machine (SVM) and optimized by the genetic algorithm (GA) to obtain the optimal parameters of the regression model. The data for training the SVM model were derived from the experimental results of a hydrogen-enriched rotary engine, in which the speed was kept constant at 4500 r/min, the absolute manifold pressure remained at 35 KPa, the variation of α H 2 , λ and IT were 0–6%, 1–1.3 and 24–42 °CA before top dead center (bTDC), respectively. After optimized by GA, the coefficient of determination of BTE, E f , NO X , CO and HC between the SVM model and the corresponding data were greater than 0.98, and the mean absolute percentage error were <1%. The performance, combustion, and emissions characteristics including BTE, E f , NO X , CO and HC were considered for multi-objective optimization to obtain higher performance and lower emissions, and were solved using the non-dominated sorting genetic algorithm II. For this study, when the Pareto-optimal solutions were obtained, the optimal operating parameters were further obtained by limiting the performance and emissions parameters with the α H 2 of 5.06%, λ of 1.09%, and IT of 34.27 °CA bTDC. [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]

• Implementation of oxygen enriched air on rotary engine combustion was performed. • Thorough 3D CFD models of rotary engines was built and verified by experiment data. • Increasing oxygen content stimulated the growth of initial flame and burned volume. • Oxygen enrichment markedly reduced the formations of CO, HC, and soot emissions. • The optimum combustion behavior can be achieved by α O2 = 30% and λ = 1.1 operation. 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. [ABSTRACT FROM AUTHOR]

• A compound intake rotary engine model was established and validated. • The effects of intake modes on mixture formation and combustion was studied. • H2/gasoline/air in peripheral port and air in side port has best performance. • The mean in-cylinder pressure and temperature of S-HGP was increased. • The CO emission of S-HGP was reduced, but NOx emissions were increased. 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 H 2 , 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. [ABSTRACT FROM AUTHOR]