A review of heat transfer in turbochargers

The conventional powertrain has seen a continuous wave of energy optimization, focusing heavily on boosting and engine downsizing. This trend is pushing OEMs to consider turbocharging as a premium solution for exhaust energy recovery. Turbocharger is an established, economically viable solution which recovers waste energy from the exhaust gasses, and in the process providing higher pressure and mass of air to the engine. However, a turbocharger has to be carefully matched to the engine. The process of matching a turbocharger to an engine is implemented in the early stages of design, through air system simulations. In these simulations, a turbocharger component is represented largely by performance maps and it serves as a boundary condition to the engine. The thermodynamic parameters of a turbocharger are calculated through the performance maps which are usually generated experimentally in gas test stands and used as look-up table in the engine models. Thus, the operational of the engine is dictated by the air flow thermodynamic parameters (pressure, temperature and mass flow) from the turbocharger compressor; this in turn will determine the thermodynamic parameters for the exhaust gas entering the turbocharger turbine. The importance and its sensitivity dictate that any heat transfer affecting the experiments to acquire the performance maps will cause errors in the characterization of a turbocharger. This will consequently lead to inaccurate predictions from the engine model if the heat transfer effects are not properly accounted for. The current paper provides a comprehensive review on how the industry and academics are addressing the heat transfer issue through advancing researches. The review begins by defining the main issues related with heat transfer in turbochargers and the state-of-the-art research looking into it. The paper also provides some inputs and recommendations on the research areas which should be further investigated in the years to come.