Realistic Steady State Performance of an Electric Turbo-Compound Engine for Hybrid Propulsion System

The efficiency of Hybrid Electric Vehicles (HEVs) may be substantially increased if the unexpanded exhaust gas energy is efficiently recovered and employed for vehicle propulsion. This can be accomplished employing a properly designed exhaust gas turbine connected to a suitable generator whose output electric energy is stored in the vehicle storage system; a new hybrid propulsion system is hence delineated, where the power delivered by the main engine is combined to the power produced by the exhaust gas turbogenerator: previous studies, carried out under some simplifying assumptions, showed potential vehicle efficiency increments up to 15% with respect to a traditional turbocharged engine. Given the power target of the required exhaust gas turbo-generator, no commercial or reference product could be considered: on account of this, in the preliminary evaluations, the turbine efficiency was assumed constant. In this paper instead the authors present the result of new evaluations performed by adequately considering the real efficiency of the exhaust gas turbine, which was designed and calculated by means of simple yet effective 1D approach, and validated by means of 3D CFD analysis. The 1D design and evaluation methodology, characterized by short calculation time, revealed sufficiently accurate compared to the result obtained by the time-consuming CFD simulations; the exhaust gas turbine efficiency was hence calculated for each required operating condition and used to compute the expected real efficiency of the compound engine; as a final result, it was found that, compared to a traditional turbocharged engine of the same rated power (73.5 kW), the realistic compound engine exhibit efficiency increments between 5% and 15%, depending on the output power.