| With the exacerbation of energy and environmental problems,seeking for efficient and clean alternative fuel engines has gradually become a hot topic in the internal combustion engine industry.Diesel/CNG dual-fuel engines stand out in kinds of alternative fuel engines due to their good power,economy and environmental protection.At present,DOC is one of the main measures to control and optimize the emission performance of CNG engines so that CNG engines can meet the increasingly stringent emission standards.Due to the separation of gas flow,pressure loss and other phenomena in the catalyst,uniformity of velocity is reduced,which affects the conversion efficiency of the DOC.Therefore,the optimization of structural parameters for the flow and pressure characteristics is of great significance.In this paper,Pro/E software was used to establish the three-dimensional modeling of the DOC.Hypermesh and AVL-FIRE software were used to establish the numerical model of the DOC,and the accuracy of the model was verified by experiment.On the basis of verifying the reliability of the model,numerical simulation of the flow field inside the catalytic converter with different structural parameters was carried out.The influences of each parameter on the flow velocity distribution and the pressure loss of the DOC were analyzed.The simulation results found that under the same exhaust flow rate,reducing the diameter ratio of DOC properly was beneficial to reducing the phenomenon of velocity separation in the expanding tube of DOC,decreasing the pressure loss,and improving the uniformity of velocity.The radial velocity in the center of the catalyst was higher,and edge of the catalyst showed a gradual decrease in velocity.With the increase of CPSI,velocity in the central area decreased and the velocity in the edge area increased,increasing the uniformity of DOC radial velocity.At the same time,the total pressure drop in the DOC increased with the increase of CPSI.Compared with the single-carrier DOC,the dual-carrier DOC showed a significant decrease in the gradient distribution of radial velocity and produced a small amount of vortex phenomenon between the two carriers.The uniformity of velocity of the rear carrier of the dual-carrier catalyst was better than that of the front carrier.And as the carrier spacing increased,the uniformity of velocity of the front carrier deteriorates,while that of the rear carrier increased.With the increase of the inlet velocity of the DOC,the velocity differentiation of the carrier center area and edge area gradually increased,and total pressure loss of the DOC increased accordingly.Through the bench test,the emission performance of the engine in pure diesel mode and diesel/CNG dual-fuel mode were analyzed and the effect of the DOC on the emission performance of dual-fuel engine was verified.The ESC experiment results showed that both HC and CO emission of dual-fuel engines were higher than that of pure diesel mode,and emission values decreased with the increase of engine load.Under medium and high load conditions,the NO_X emission of dual-fuel engine was higher than those of pure diesel mode and increased with the load.Under the ESC experiment,the maximum reduction of HC and NO_X emission reached 93%and 92%separately,and the CO emission reduced by 90%on the basis of the original low emission when the dual-fuel engine was equipped with the combination of DOC and SCR.During the ETC experiment,the emission values of HC,CO and NO_X increased obviously at low speed of urban road condition,and HC emission increased significantly under the accelerated conditions of rural road.Under other conditions,the emission values of exhaust were stable at low levels.In contrast to the ETC emission limits of national V emission standard,the emission values for diesel/CNG dual-fuel engines could meet the emission standard. |
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