Model based evaluation of a turbocharged engine exhaust heat recovery by auxiliary turbine

Recent developments in terms of vehicle emissions and fuel consumption regulations have pushed the transportation sector toward electrification and hybridization of powertrains. In order to meet these environmental targets, a transition era, where the internal combustion engines (ICEs) still play a crucial role, is approaching, and technologies already mature should be rapidly applied to reduce fuel consumption. Waste heat recovery from exhaust gases has been studied for decades and can now be readly applied to existing engines, particularly in hybrid configurations, where the recovered energy is converted into electrical form and used on board. It is particularly important for heavy duty transportation, where the potential is enormous and the difficulties to fully electrify the powertrain are higher. Among various technologies proposed for waste heat recovery, the use of additional turbine in the engine exhaust is surely one of the most mature, although there are not many applications in the market. In this work, a 3.0 L turbocharged diesel engine, used to equip commercial and heavy duty vehicles, is considered. Having the opportunity to test the engine at representative working points, the potential of exhaust heat recovery using an additional turbine has been extensively explored. Data from the engine test bench has been used as a boundary condition to develop a mathematical model of the exhaust line and, in particular, the turbocharger. This is controlled by a variable geometry inlet nozzle, opening and closing a pre-statoric row of vanes on the turbine, in order to match the instantaneous requests of the compressor placed on the same shaft. Hence, a model of an auxiliary turbine has been derived and used to evaluate the possible energy recovery when it is placed downstream of the existing one (so-called series turbocompound). Results in terms of mechanical power produced (and possible electrical conversion) have been assessed, considering the real turbine behavior (expansion ratio vs. flow rate vs. revolution speed vs. efficiency) and has an average value of 6% in a wide range of operating conditions of the engine. The new thermodynamic conditions of the exhaust line have been re-evaluated, calculating the new equilibrium of the turbocharger when the additional turbine is introduced and its related backpressure increase on the exhaust, which can reduce the net power recovered value. In fact, the backpressure effect on the engine increases the specific fuel consumption, which should be compared to the benefit of the additional power produced by the turbocompound. Environmental considerations, such as the amount of CO2 avoided thanks to the recovery section, have been also calculated, showing very interesting results (averagely 45 g/kWh) considering emissions reduction policies worldwide and the electrification of fleets.