1 Copyright © 2011 by ASME Proceedings of the ASME Turbo Expo 2011: Power for Land, Sea and Air June 6-10, 2011, Vancouver, Canada GT2011- 45541 A HIGH PERFORMANCE LOW PRESSURE RATIO TURBINE FOR ENGINE ELECTRIC TURBOCOMPOUNDING Aman M.I. Mamat, Muhamad H. Padzillah, Alessandro Romagnoli and Ricardo F. Martinez-Botas Mechanical Engineering Department Imperial College London United Kingdom ABSTRACT In order to enhance energy extraction from the exhaust gases of a highly boosted downsized engine, an electric turbo- compounding unit can be fitted downstream of the main turbocharger. The extra energy made available to the vehicle can be used to feed batteries which can supply energy to electric units like superchargers, start and stop systems or other electric units. The current research focuses on the design of a turbine for a 1.0 litre gasoline engine which aims to reduce the CO 2 emissions of a "cost-effective, ultra-efficient gasoline engine in small and large family car segment". A 1-D engine simulation showed that a 3% improvement in brake specific fuel consumption (BSFC) can be expected with the use of an electric turbocompounding. However, the low pressure available to the exhaust gases expanded in the main turbocharger and the constant rotational speed required by the electric motor, motivated to design a new turbine which gives a high performance at lower pressures. Accordingly, a new turbine design was developed to recover energy of discharged exhaust gases at low pressure ratios (1.05 - 1.3) and to drive a small electric generator with a maximum power output of 1.0 kW. The design operating conditions were fixed at 50,000 rpm with a pressure ratio of 1.1. Commercially available turbines are not suitable for this purpose due to the very low efficiencies experienced when operating in these pressure ranges. The low pressure turbine design was carried out through a conventional non-dimensional mixed-flow turbine design method. The design procedure started with the establishment of 2-D configurations and was followed by the 3-D radial fibre blade design. A vane-less turbine volute was designed based on the knowledge of the rotor inlet flow direction and the magnitude of the absolute speed. The overall dimensions of the volute design were defined by the area-to-radius ratios at each respective volute circumferential azimuth angle. Subsequently, a comprehensive steady-state turbine performance analysis was performed by mean of Computational Fluid Dynamics (CFD) and it was found that a maximum of 76% of total-static efficiency,  t-s can be achieved at design speed. Keywords: Energy Recovery, Mixed-flow Turbine, Turbocompounding, Low Pressure, Performance Prediction, Flow Field NOMENCLATURE A Area [ m 2 ] b Blade Height [m] BMEP Brake Mean Effective Pressure [bar] BSFC Brake Specific Fuel Consumption [kg/kW/hr] C Absolute Flow Velocity [ m/s ] C P Specific Heat at Constant Pressure [ kJ/kg.K] h Specific Enthalpy [ kJ/kg ] K Thermal Conductivity k Specific Heat Ratio l Length [m]  Mass Flow Rate [ kg/s ] M Mach Number MFP Mass Flow Parameter        P Pressure [ Pa ] PR Pressure Ratio R Gas Constant [ kJ/kg.K] r Radius [ m ] S Entropy [ kJ/kg.K] T Temperature [ K ] U Rotor Velocity [ m/s ] u Velocity [ m/s ] v Exit Hub-to-shroud radius ratio VR Velocity Ratio W Relative Flow Velocity [ m/s ]   Power [ kW ] Z Blade Number α Absolute Flow Angle