Whole engine efficiency optimization of two-stage VNT series-parallel system for diesel engines

A two-stage turbine series-parallel system based on VNT(variable nozzle turbine) was proposed to recover waste heat from the diesel engine. The system adopted valves to control the system and accomplish the transformation of the series or parallel working modes. And the system was established under the concept of parallel and series working modes separately, and effective diameter was chosen as the parameter to define the flow capability of the turbine. Besides, the influences of the effective diameters of the two-stage turbines on the engine power, the power turbine power and the turbocharger performance were analyzed via GT-Power, and the effective diameters of the two-stage turbines were matched according to the engine working condition. And the series or parallel working mode was controlled by the maximum value of the whole engine efficiency at the specific working condition of the diesel engine. Results showed that through the proper optimization and combination of the two-stage VNT and series-parallel working mode, the system could recover waste heat effectively and improve the whole engine efficiency by 5.1% during the back tracking period.