| 1. |
- Chang, Yachao, et al.
(författare)
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Construction and assessment of reduced oxidation mechanisms using global sensitivity analysis and uncertainty analysis
- 2019
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:1, s. 751-761
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Tidskriftsartikel (refereegranskat)abstract
- The global sensitivity analysis is an effective method to assess the performance of a chemical reaction mechanism. In the present study, the uncertainty analysis and the global sensitivity analysis of a detailed chemical mechanism of n-heptane are first performed based on the Monte Carlo method. The source of the prediction uncertainties of the C0-C7 sub-mechanisms and the reaction classes in the fuel-specific sub-mechanism of the detailed mechanism are determined by the global sensitivity analysis. Then, based on the results, a reduced mechanism for n-heptane oxidation is developed. To assess the performance of the reduced mechanism, the nominal model prediction and the frequency distribution of the ignition delay times using the present reduced mechanism are compared with those of the detailed mechanism and two additional reduced mechanisms obtained by the directed relation graph with error propagation (DRGEP) method. The results indicate that the predictions from the present reduced mechanism and the reduced mechanism with 305 species satisfactorily agree that of the detailed mechanism. Furthermore, the discrepancy of the predictions among these mechanisms is discussed based on the Spearman Rank Correlation analysis. It is found that, for the reduced mechanism with 120 species, the optimization of the reaction rate constants significantly improves the nominal model prediction of the ignition delay time, whereas its influence on the range and profile of the frequency distribution is rather weak due to the intrinsic relationship among the reactions being broken in the reduced mechanism.
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| 2. |
- Xu, Guangfu, et al.
(författare)
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Evaluation of variable compression ratio (VCR)and variable valve timing (VVT)strategies in a heavy-duty diesel engine with reactivity controlled compression ignition (RCCI)combustion under a wide load range
- 2019
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 253, s. 114-128
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Tidskriftsartikel (refereegranskat)abstract
- Variable compression ratio (VCR)and variable valve timing (VVT)are two effective strategies to adjust the effective compression ratio, which is beneficial for controlling the combustion process of advanced combustion modes. In this study, systematic evaluation of the two strategies was conducted based on reactivity controlled compression ignition (RCCI)engine in terms of combustion process control, fuel efficiency, and emission characteristics. By coupling an updated KIVA-3V code with the genetic algorithm, the combustion of a heavy-duty RCCI engine with VCR and VVT strategies was respectively optimized, aiming to simultaneously realize high fuel efficiency and low emissions. The optimal VCR and VVT strategies were compared under a wide load range. The results indicate that, at low and mid loads, high effective compression ratio, large premix ratio, and early fuel injection can be utilized to realize Euro 6 nitrogen oxides (NO x )limit with ultra-low soot emissions and low fuel consumption for both VCR and VVT strategies. The increase of load from low to mid narrows the optimal range of exhaust gas recirculation (EGR)rate for VVT strategy whereas the range for VCR strategy is still wide. At high load, compared to VVT strategy, a further decreased effective compression ratio can be utilized for VCR strategy, which allows early fuel injection, leading to the improvements of fuel efficiency and soot emissions. This suggests that the VCR strategy is more practical for high-load operation of RCCI combustion and the commercialization the RCCI engine in the future compared to VVT strategy.
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| 3. |
- Xu, Guangfu, et al.
(författare)
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Potential of reactivity controlled compression ignition (RCCI) combustion coupled with variable valve timing (VVT) strategy for meeting Euro 6 emission regulations and high fuel efficiency in a heavy-duty diesel engine
- 2018
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Ingår i: Energy Conversion and Management. - : Elsevier BV. - 0196-8904. ; 171, s. 683-698
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Tidskriftsartikel (refereegranskat)abstract
- As an effective strategy to control the combustion of advanced combustion modes, the application of variable valve timing (VVT) in reactivity controlled compression ignition (RCCI) combustion was investigated in this study. By coupling KIVA-3V code with genetic algorithm, the combustion of a heavy-duty engine with RCCI combustion combined with VVT strategy was optimized under a wide load range. At each load, six operating parameters including premix ratio (PR), intake valve closing (IVC) timing, start of injection, exhaust gas recirculation rate, intake pressure, and intake temperature were optimized to realize low-emission and high-efficiency combustion. The optimization results indicate that, at low load, high PR coupled with either late IVC or base IVC can be utilized for the realization of high thermal efficiency. At mid load, the base IVC strategy is integrated with high PR, while the late IVC strategy is coupled with low PR. At high load, only the strategy with late IVC and low PR can be used. The strategy with higher PR and earlier IVC timing exhibits better engine performance on thermal efficiency and soot emissions, while the strategy with lower premix ratio and later IVC timing is superior in ringing intensity. By optimizing the RCCI combustion with the VVT strategy, the Euro 6 NOx limit can be met while maintaining ultra-low soot emissions at low and mid load. However, at least one aftertreatment device is required to further eliminate the NOx or soot emissions at high load. Under the whole load conditions, satisfactory fuel consumption can be obtained.
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| 4. |
- Xu, Leilei, et al.
(författare)
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A skeletal chemical kinetic mechanism for ammonia/n-heptane combustion
- 2023
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 331
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Tidskriftsartikel (refereegranskat)abstract
- Progressively stricter pollutant emission targets in international agreements have shifted the focus of combustion research to low carbon fuels. Ammonia is recognized as one of the promising energy vectors for next-generation power production. Due to the low flame speed and high auto-ignition temperature, ammonia is often burned with a high reactivity pilot fuel (e.g. diesel). However, chemical kinetic mechanisms describing the combustion of ammonia and large hydrocarbon fuels (such as n-heptane, a surrogate of diesel) are less developed. In this work, a skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed using a joint decoupling methodology and optimization algorithm. A sensitivity analysis of the ignition delay times of the ammonia/n-heptane mixture is performed to identify the dominant reactions. A genetic algorithm is used to optimize the mechanism further. The final skeletal mechanism is made up of 69 species and 389 reactions. The skeletal ammonia/n-heptane mechanism is validated against the experimental data for combustion of pure ammonia, ammonia/hydrogen and ammonia/n-heptane mixtures, including the global combustion characteristic parameters such as ignition delay times measured in shock tubes or rapid compression machines, laminar burning velocities measured in heat flux burners or spherical flame vessels, and species data measured in jet-stirred reactors. Comparing the results from the skeletal mechanism with those from other mechanisms from the literature is conducted to evaluate the mechanism further. The present skeletal mechanism can well predict the combustion processes for a wide range of conditions, and the mechanism is computationally efficient, showing good potential to model ammonia/n-heptane combustion with good accuracy and efficiency.
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| 5. |
- Xu, Zhen, et al.
(författare)
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Computational optimization of fuel supply, syngas composition, and intake conditions for a syngas/diesel RCCI engine
- 2018
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 234, s. 120-134
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Tidskriftsartikel (refereegranskat)abstract
- By utilizing the promising alternative fuel of syngas, and the syngas/diesel dual-fuel reactivity controlled compression ignition (RCCI) is a potential combustion strategy for internal combustion engines. However, the optimal operating parameters for syngas/diesel RCCI engines under wide operating conditions have not been investigated. In this study, the operating parameters include fuel supply, syngas composition, and intake conditions of a syngas/diesel RCCI engine were optimized under wide load by integrating the KIVA-3V code and the non-dominated sort genetic algorithm II (NSGA-II). The results indicated that nitrogen oxides (NOx) emissions can be controlled in considerably low levels, and the efficient combustion of the premixed syngas in the squish region can be realized with high premix ratio and early pilot injection of diesel. Equivalent indicated specific fuel consumption (EISFC) and ringing intensity (RI) are the major issues for the optimization at low and mid load, respectively. The double diesel injection strategy with the relatively late main injection timing is an effective way to both improve combustion efficiency at the low load and reduce RI at the mid load. For the double diesel injection, the ratio of pilot injection is controlled in a narrow range to provide sufficient high reactivity fuel in the piston bowl and to avoid the local high-temperature combustion region simultaneously. With the restrictions of EISFC and RI, the optimal H2 fraction in the syngas is 60–80%. Based on the optimal fuel supply and intake conditions, a syngas with 75% H2 and the diluent factor C of 0.8 is capable of realizing the high efficiency, moderate combustion, and low emissions for the RCCI engine at full load range.
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