Flow Mechanism And Control Of Two In-series Turbines For Turbocompound Internal Combustion Engine

Recovering waste heat from engine exhaust by turbocompounding is a main technical solution and research direction for engine energy saving and emission reduction in the future. It is also the state of art research issue in the area of engine fluids mechanics. In a turbocompound engine, the exhaust gas experiences a further expansion in a power turbine. The power turbine transfers the energy from the exhaust into effective work. Therefore, turbocompounding can effectively improve the fuel conomy and reduce CO2 emission of the internal combustion engine. The flow coupling effects between the charging turbine and power turbine exert significant impacts on the performance of the power turbine and engine cycle. The flow coupling effect is one of the important unsolved problems in the turbocompounding research area.The power turbine used to design independently in the turbocompounding research. Therefore, it failed to consider the flow coupling effects of the two turbines. This thesis focuses on the flow mechanism between two turbines and how it influences the performance. Flow control methods are proposed to improve the performance.Throughflow model for the turbine systems is established to consider the effects of coupling design parameter of the two turbines. The coupling analysis method for the engine turbocompound cycle is developed. The thesis studies the impacts of two turbines area ratio on the turbocompound engine performance under overall conditions. It is found that the area ratio exerts important influences on the exhaust energy distribution between the charging turbine and power turbine. Hence, the area ratio is a key parameter determining the turbocompound engine performance under overall conditions. Variable geometry charging turbine is proposed to adjust the area ratio, which will be increased as the engine speed decreases. The proposed flow control method improves the turbocompound engine performance under overall conditions.Three-dimensional steady CFD method is used to study the complex flow interaction between the two turbines and its impacts on the power turbine performance. It is found that the strong swirl flow at the turbocharger turbineexit has great impact on the power turbine performance. Due to the swirl flow,the attack angle at the inlet of the power turbine vane will increase, resulting in larger second flow loss in the vane. Counter-rotating turbine configuration is proposed to alleviate the swirl effects, in order to reduce the second flow loss in the power turbine vane. Calculation results show that the power turbine efficiency is improved by 3.8%.Three-dimensional unsteady CFD method is applied to study the effects of pulsating flow on the flow mechanism of two turbines. It is revealed that the pulsating flow exerts stronger impacts on the downstream power turbine, compared with charging turbine. The flow attack angle near the tip of the power turbine rotor changes greater during a pulse cycle, resulting in significantly lower turbine efficiency. Blunt leading edge design of the rotor tip is proposed to make the power turbine more adaptive to the pulsating flow, resulting in higher performance under pulsating flow conditions. The calculation result shows that the cycle-averaged efficiency of the power turbine rotor is improved by 1.64%.The turbocompound systems are designed and manufactured based on Dongfeng Motor’s DCI11 engine. The turbocompound internal combustion engine with VGT is developed. The engine overall performance test is carried out and the experimental results show that the BSFC reduction under full-load conditions is up to 7.8%. The brake torque at low engine speed is improved by15.3%.