Hybrid Propulsion Efficiency Increment through Exhaust Energy Recovery—Part 2: Numerical Simulation Results

Abstract

The efficiency of hybrid electric vehicles may be substantially increased if the energy of exhaust gases, which do not complete the expansion inside the cylinder of the internal combustion engine, is efficiently recovered using a properly designed turbo-generator and employed for vehicle propulsion. Previous studies, carried out by the same authors of this work, showed a potential hybrid vehicle fuel efficiency increment up to 15% employing a 20 kW turbine on a 100 HP-rated power thermal unit. The innovative thermal unit proposed here is composed of a supercharged engine endowed with a properly designed turbo-generator, which comprises two fundamental elements: an exhaust gas turbine expressly designed and optimized for the application, and a suitable electric generator necessary to convert the recovered energy into electric energy, which can be stored in the on-board energy storage system of the vehicle. In this two-part work, the realistic efficiency of the innovative thermal unit for hybrid vehicles is evaluated and compared to a traditional turbocharged engine. In Part 1, the authors presented a model for the prediction of the efficiency of a dedicated radial turbine, based on a simple but effective mean-line approach; the same paper also reports a design algorithm, which, thanks to some assumptions and approximations, allows fast determination of the right turbine geometry for a given design operating condition. It is worth pointing out that, being optimized for quasi-steady power production, the exhaust gas turbine here considered is quite different from the ones commonly employed for turbocharging applications; for this reason, and in consideration of the required power size, such a turbine is not available on the market, nor has its development been previously carried out in the scientific literature. In this paper, Part 2, a radial turbine geometry is defined for the thermal unit previously calculated, employing the design algorithm described in Part 1; the realistic energetic advantages that could be achieved by the implementation of the turbo-generator on a hybrid propulsion system are evaluated through the performance prediction model under different operating conditions of the thermal unit. As an overall result, it was estimated that, compared to a reference traditional turbocharged engine, the turbo-compound system could gain vehicle efficiency improvement between 3.1% and 17.9%, according to the output power delivered, with an average efficiency increment of 10.9% evaluated on the whole operating range.

Hybrid Propulsion Efficiency Increment through Exhaust Energy Recovery—Part 1: Radial Turbine Modelling and Design

“…The design algorithm shares the same structure, but is characterized by a number of assumptions and simplifications that allow a rapid definition of the turbine geometry suitable for the task considered. Both the algorithm and the performance prediction model have been employed in Part 2 [12] to define the turbine geometry according to boundary conditions obtained on a previously studied hybrid propulsion system, and to trace the efficiency map of the turbine. The realistic energetic advantages of the turbocompound engine were hence evaluated [12] on the basis of the realistic turbine efficiencies predicted by the model, and revealed to range between 3.1% and 17.9% depending on the output power level, with a 10.9% average efficiency increment of the realistic turbocompound engine on its best efficiency curve.…”

“…where r 5s and r 5h are the shroud and hub radii at rotor exit, respectively (see Figure 3), N R the number of blades in the rotor, and t the blade thickness. In the calculation performed on the rotor, the outlet static pressure P 5 is known, being part of the boundary conditions adopted (as resumed in Part 2 [12]); the isentropic enthalpy drop in the rotor can be hence calculated as:…”

“…In [10,11], the implementation of an auxiliary turbogenerator in parallel with the main turbocharger allowed for attaining efficiency improvement up to 9%. In these two papers (see also Part 2 [12]), the authors investigated a different arrangement, which is particularly suitable for hybrid propulsion architectures.…”

“…Moreover, both the generator and the battery-charging efficiency can also be considered the same for the comparative hybrid propulsion system equipped with the traditional turbocharged engine; on account of this, the authors decided to fairly base the comparison on the overall mechanical output power produced. The aim of the work reported in these two papers is to provide a more precise and reliable estimation of the performances of the separated electric compound system by evaluating the turbine efficiency in accordance with its operating condition [12]; this should improve the reliability of the results obtained, which would not be influenced any more by the simplifying assumption used in the preliminary studies, for which the turbine efficiency was assumed to be constant. To this end, the turbine was analysed and designed by means of a simple yet effective mean-line-based approach.…”

Review of the Integration of Hybrid Electric Turbochargers for Mass-Produced Road Vehicles

“…The electric motor assists in capturing and storing excess energy during braking or deceleration, which can then be utilized to power auxiliary systems or provide additional propulsion during acceleration. Research [59,[65][66][67] indicates that this regenerative capability not only enhances energy efficiency but also extends the lifespan of the hybrid vehicle's battery. The seamless integration of HETs with energy management systems ensures that the recovered energy is judiciously distributed, optimizing overall powertrain efficiency.…”