Study On Flow Characteristics Of An Asymmetric Twin-scroll Turbine And Its Effects On Engine Performance

Asymmetric twin-scroll turbocharging consists of two turbine scrolls that exhibit different critical areas.To be specific,the large scroll is capable of effectively remedy the pump loss and fuel economy of the engine,while the small scroll contributes to exhaust gas recirculation(EGR).Accordingly,the contradiction between NO_x emission and pump loss of the engine can be effectively balanced.Asymmetric twin-scroll turbocharging has been considered an effective technology to reduce progressively aggregated internal combustion engine emissions and satisfy fuel economy regulations.Even under the stable inlet flow,the flow characteristics of different turbine scrolls also exhibit asymmetry and complexity,which will be more instantaneous at the engine pulse exhaust state.For the mentioned reason,it is scientifically and practically significant to study on flow characteristics in asymmetric twin-scroll turbines and their matching theory with engines.The major research work and innovation points here are presented below:1.The one-dimensional(1D)matching model and exhaust pulse decoupling model of asymmetric twin-scroll turbine were built and then verified experimentally.The model comprises an efficiency prediction model for an asymmetric twin-scroll turbine,as well as an inlet and exhaust model in terms of an EGR engine.The matching calculation of a 9LEGR National?emission engine was conducted with the 1D matching model.The calculated results were consistent with the experimental data of the engine.Besides,the flow parameters were deviated within 7%,and the deviation of efficiency was within 3%.The pulse decoupling model was built by complying with the plus exhaust flow at the twin-scroll inlets and the flow capacity curves of the asymmetric twin-scroll turbine at different states.With the use of this model,an analysis can be achieved on the pressure intensity,flow parameters and energy of pulse exhaust at the inlet of twin-scroll Furthermore,the results were verified by using a 12.5L EGR National?emission engine,with a mass flow deviation less than 5%.2.The flow test experiments of multiple asymmetric twin-scroll turbines were performed on the turbine performance test rig.Moreover,the flow characteristics of asymmetric tiwn-scroll turbines were characterized.The flow parameters,expansion ratio and turbine efficiency of the turbine were analyzed by regulating the asymmetry(ASY),twin-scroll inlet pressure ratio(SPR)and twin-scroll inlet flow parameter ratio(SMR).As indicated from the results,ASY slightly impacted the performance of turbines at the equal admission state and more significantly impacted the performance at the unequal admission state.ASY was increased,and the flow capacity and efficiency of asymmetric twin-scroll turbines in the constant SPR increased.In addition,the single admission flow parameter ratio(SSMR)of small and large scrolls in the constant ASY displayed an approximately low-slope linear relationship with the expansion ratio,and the mean value of SSMR had a linear relationship with the ASY approximation.The expansion ratio of large and small scroll turbines under constant SMR was nearly exponential to lg(SPR),and SMR critically controlled the flow similarity of asymmetric twin-scroll turbines.3.The flow parameters of the turbine were tested on the turbine performance experimental rig for asymmetric twin-scroll turbines with different flow control systems,and the flow control characteristics of the turbine were characterized.By comparing different flow control systems on the engine performance test rig,the effects of different flow control systems on engine performance and emission was studied.As indicated from the results,the opening of bypass valve showed an approximately logarithmic function relationship with the ratio of bypass flow parameters,the bypass ability displayed a strong correlation with the aperture of bypass valve,and the maximum ratio of bypass flow parameters and bypass valve aperture had an approximately power function relationship.As the balance valve opened more significantly,the flow capacity and efficiency of the asymmetric twin-scroll turbine were improved,and the flow state could be close from partial admission to equal admission state.Furthermore,the balance valve exhibited the EGR rate regulation ability;the smaller the ASY,the wider the regulation range of the balance valve opening on the EGR rate would be(the regulation range of the turbine flow parameter by the balance valve with ASY=0.36 was nearly 167%higher than that with ASY=0.5).The operation efficiency of the unsymmetrical twin-scroll turbine with balance valve significantly exceeded that of the bypass valve turbine under the full load engine condition,which could range from 72 to77.5%.Besides,as compared with the large scroll bypass valve,the air fuel ratio of the balance valve scheme in the high speed range of the engine was elevated by about 2?10%,and the specific fuel consumption was reduced by nearly 1.5?6%.4.By performing the experiment of multiple asymmetric twin-scroll turbines on the engine test rig,the mutual effect of exhaust pulse and asymmetric twin-scroll turbines was studied.Thus,the exhaust pulse intensity at the inlet of the twin-scroll was suggested to decrease with the increase in the engine speed,and the trend of the isentropic power distribution for the small and large scroll turbines complied with that of the exhaust pressure pulse,with high power and low power in the high pressure area and the low pressure area,respectively.With the increase in ASY,the pulse intensity and the cycling average isentropic turbine power at the small scroll inlet were reduced,while the large scroll inlet increased.And the total cycling average isentropic turbine power was maintained to be equivalent.With the increase in ASY,the cycling average turbine efficiency,engine air fuel ratio and engine fuel economy showed the increase.As the balance valve opened more significantly,the pressure and flow parameter fluctuations at the twin-scroll inlet were progressively balanced,and the flow parameter fluctuation range was noticeably broadened.The cycling average isentropic turbine power of the large and small scroll turbines decreased and increased,respectively.Whereas,the exhaust pulse intensity would be weakened at the twin-scroll inlet,and the maintenance of the exhaust energy would be inhibited before the turbine.5.A matching method was proposed for asymmetric twin-scroll turbines based on exhaust pulse energy weight.As revealed from the experimental results,the turbine efficiency deviation index decreased as the engine speed was up-regulated,and the effect of engine pulse in the region of low speed and medium speed should be considered.The matching of pulse energy weight under the key operation conditions of the engine could more fully utilize the pulse energy of exhaust.To enable the energy matching point to run in the optimal efficiency region of the turbine,two optimal regulation strategies were proposed,i.e.,the regulation of the twin-scroll and turbine throat area ratio,so the expansion ratio of the energy matching point was close to that under the optimal efficiency of the turbine;the regulation of the turbine operation speed,so the turbine speed parameters under the key operation conditions of the engine ran at the optimal efficiency state.Lastly,the comparison experiment of the mentioned two regulation strategies was performed on a12.5L National?emission EGR engine.As indicated from the results,under the identical NOx condition,the turbine operation efficiency and engine fuel economy in the medium and low speed ranges of the engine could be improved by nearly 4%and 1%,respectively.In this study,the theoretical guidance and solutions were provided for the engineering development and application of asymmetric twin-scroll turbines.Moreover,the research methods and conclusions proposed here can be referenced for the development of other relevant power systems.