Characterisation of a low pressure turbine for turbocompounding applications in a heavily downsized mild-hybrid gasoline engine

Abstract

This paper describes the outcomes of a study carried out on a novel exhaust energy recovery system for a heavily downsized gasoline engine. The investigation was carried out based on a 1.0 L turbocharged gasoline engine offering the same performance as a 2.0 L model, with the aim of reducing CO2 emissions from 169 g/km to 99 g/km. This is to be obtained by the synergistic application of electrified boosting, stop/start technology, regenerative braking, torque assist and exhaust energy recovery systems.The research focussed on the design of a high performance LPT (Low Pressure Turbine) to recover latent energy left in the exhaust gas after expanding in the main turbocharger turbine. The LPT is meant to be coupled to an electric generator whose combination is usually referred as electric turbocompounding. Given the small engine size and therefore the low mass flow rate of the exhaust gas, the design operating conditions were fixed at 50,000 rpm with an optimum pressure ratio of PR ≈ 1.1. Commercially available turbines are not suitable for this purpose due to the very low efficiencies (less than 40%) experienced when operating in such low pressure ratios range. The design was accomplished by following a number of steps which go from meanline loss model development, full 3-D CFD (Computational Fluid Dynamics) analysis (using ANSYS CFX), prototype manufacturing and steady-state testing. The test results, that were conducted by using the Imperial College London cold-flow test facility, showed good agreement with CFD analysis with an efficiency greater than 70%.The steady maps obtained from testing were then input into a 1-D engine model including the electric turbocompounding. Three different arrangements were considered for the turbocompounding: (1) pre-catalyst, (2) post-catalyst and (3) in the wastegate of the main turbocharger. Two different scenarios were investigated in which the extra energy recovered by the turbocompounding unit is either fully regenerated into the engine crankshaft or stored and not re-used. The BSFC (brake specific fuel consumption) and the BMEP (brake mean effective pressure) were calculated and compared with the baseline engine under full and part load conditions. At full load, the analysis was performed for several engine speeds (from 1000 rpm to 6000 rpm at steps of 500 rpm each). The results showed that the extra energy recovered by the turbocompounding device is offering a significant benefit on engine performance; the post-catalyst solution offers the best compromise in terms of BSFC and BMEP with an improvement of 2.41% and 2.21% respectively. At part load engine instead, the analysis was performed for three engine speeds (1500 rpm, 2000 rpm and 4000 rpm) for the post-catalyst position only. Despite the increment in pumping work due to the presence of the LPT, no significant penalty in BSFC was calculated when no energy is returned into the engine whereas an improvement of ≈2.64% was found in the other case.